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Polarization dependent micro-structuring of silicon with a femtosecond laser
Applied Surface Science 353 (2015) 600–607
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Polarization dependent micro-structuring of silicon with a femtosecond laser H. Al-Khazraji, V.R. Bhardwaj ∗ Department of Physics and Center for Research in Photonics, University of Ottawa, 25 Templeton Street, Ottawa, ON K1N 6N5, Canada
a r t i c l e
i n f o
Article history: Received 25 March 2015 Received in revised form 19 May 2015 Accepted 15 June 2015 Available online 2 July 2015
a b s t r a c t We experimentally demonstrate formation of a sub-micron rim around femtosecond laser ablated crater on silicon whose height and width were sensitive to laser polarization. Except for circularly polarized light we show that the rim height and width were asymmetric – larger along the direction of the laser polarization for n-type and intrinsic silicon, while in p-type silicon the asymmetry was perpendicular. Polarization dependent rim formation is attributed to the transient light–plasma interaction that gives rise to local-field enhancements resulting in an asymmetric electron density and energy deposition. Picoseconds later when the electron energy is transferred to the lattice, the asymmetry is retained in the temperature distribution within the interaction region. The temperature distribution eventually leads to non-symmetric radial outward fluid motion of a thin layer of molten material from the centre of the ablation crater that subsequently re-solidifies on a nanosecond timescale. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
1. Introduction The interaction of femtosecond laser pulses with materials involves several fundamental physical processes such as excitation/energy deposition, melting, and material removal/ modification that are separated in time. Excitation is determined by the duration of the laser pulse typically in the femtosecond range, melting occurs on picosecond timescale, and the final material removal/modification may take up to several nanoseconds. Apart from the duration of the laser pulse, light–matter interactions are often governed by the intensity, wavelength, and polarization of light. In atoms and molecules, the influence of polarization of light was well understood but its role in the interaction with solids is subtle and not clear. Some of the basic mechanisms involved in the interaction depend on the laser polarization that can potentially leave an imprint on the modification/ablation process. For example, multiphoton ionization of crystalline dielectrics was found to depend on sample orientation due to direction dependence of the effective mass of the electron [1]. Intense ultrashort laser pulses rapidly produce high carrier densities forming a plasma that can subsequently interact with incident light leading to polarization-dependent absorption. Experiments on laser-produced plasmas have
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demonstrated sharp differences (up to 50–60%) in absorption of p- and s-polarized light due to resonance absorption [2–4]. In addition, the interaction of linearly polarized light with dense plasma can also lead to local field enhancements akin to that observed in metal nanoparticles [5]. To date, signatures of polarization-dependent phenomena have been observed mostly in the multiple pulse regime both on surfaces and inside bulk materials. Polarization-dependent selforganized three-dimensional (3D) periodic nanostructures have been observed inside fused silica [5,6] and their origin was attributed to local field enhancement arising from light-plasma interaction [5,7,8]. For circularly polarized light, the handedness of the light was imprinted in the form of ordered sub-micron chiral structures [9]. Similarly, complex polarization state of the light in the focal volume was also visualized with sub-wavelength resolution [10]. Polarization-dependent periodic structures (surface ripples) have also been observed on surfaces in the ablation crater of a variety of materials including dielectrics, semiconductors and metals under widely different illumination conditions [11–14]. Wavelength [15–19] and sub-wavelength [20] ripple spacing has been observed and was found to vary with the fluence and the number of laser pulses. The surface ripples have been found to be perpendicular and parallel to the laser polarization in different regions of the ablation crater [21–23]. The ripple formation is described in terms of interference between the incident light and the surface scattering wave [24] or surface plasmon polaritons [25].
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The overall features of the ablation region were also found to be polarization dependent. Ablation of bulk polymer with a single femtosecond laser pulse produced craters elongated along the major axis of the polarization vector [26]. Nearly 40% elongation was observed for linearly and elliptically polarized light in a fluence range of 4–20 J/cm2 , only circularly polarized light produced near circular ablation craters irrespective of pulse energies. Numerical simulation of the interaction of intense light pulses with solids demonstrated that the polarization dependence of the ablation features arise from a local field enhancement during light–plasma interaction. Recently, single shot femtosecond laser ablation of silicon also demonstrated formation of elliptical-shaped craters that were elongated along the major axis of the polarization direction [27]. The morphology of the ablation craters was found to evolve from an ellipse to nearly a circle with increasing fluence. Redistribution of the electric field on the silicon surface due to surface defects was found to play a crucial role in the creation of elliptical-shaped craters in addition to field enhancements. Similar elongation of ablation craters along the major axis of the polarization vector was also observed in multi-pulse ablation of thin metal films [28]. In this paper, we show that single femtosecond pulse irradiation of silicon produces a rim around the ablation crater and that the rim dimensions are polarization dependent. Rim formation around an ablation crater was observed in silicon with ns [29] and fs laser irradiation [30,31], and in glass [32] irradiated with a single femtosecond laser pulse. However, polarization dependent rim formation was never observed. The rim height and width were found to be larger along or perpendicular to the direction of linear laser polarization depending on whether the majority carriers in silicon were electrons or holes, respectively. The polarization dependence of rim dimensions was attributed to local field enhancement that arises when the leading edge of an intense laser pulse creates a free-electron plasma with which the trailing edge of the pulse interacts resulting in an asymmetric electron density distribution. Picoseconds later when the electron energy is transferred to the lattice the asymmetry is preserved in the temperature distribution that eventually causes melting. The non-symmetric radial outward fluid motion of a thin layer of molten material from the centre of the ablation crater that subsequently re-solidifies on a nanosecond timescale leads to rim formation. The field enhancement is parallel (perpendicular) to the laser polarization when electron density is above (below) the critical density of silicon.
2. Experimental methods In our experiments, 800 nm light from a Ti:Sapphire laser system operating at a repetition rate of 1 kHz and producing 45 fs pulses with a maximum energy of 2.5 mJ, was focused on the silicon surface by a 0.25 NA (16×) aspheric microscope objective. The back aperture of the microscope objective was slightly overfilled to minimize alignment errors. The position of the laser focus relative to the sample surface was accurately determined by imaging the back reflected light with a CCD camera at very low pulse energies below the ablation threshold. After locating the surface of silicon, the glass plate at 45◦ , used for directing the back reflected light, was removed in order to avoid distortion of the incident polarization. A thin broadband beam sampler at the output of the laser directed a small fraction of the beam into a single-shot autocorrelator to monitor the pulse duration continuously. The pulses were not pre-chirped and the duration was measured to be 70 fs at the back aperture of the objective after propagating through all the optics. The use of an aspheric objective, where a single asphere replaced the compound lens system of a standard microscope objective, reduced dispersion and spherical aberrations. The laser focal spot size was measured to
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be 1.8 ± 0.1 m using the knife-edge method for the 0.25 NA microscope objective, in close agreement with the diffraction-limited spot size. The laser beam profile, imaged by a CCD camera with a flat diffuser, was nearly gaussian with no hot spots. Single femtosecond laser pulses were selected by operating the laser in an external trigger mode. A combination of half-wave plate and a polarizer were used to vary the pulse energy from 10 nJ to 200 nJ. The pulse energy was varied in steps of 5 nJ up to 120 nJ and in steps of 10 nJ thereafter. A small fraction of the incident beam was reflected on to a calibrated fast photodiode operating in the linear regime to monitor the incident power. The pulse energies incident on the sample after the microscope objective was estimated by taking into account the transmission and reflection losses of all the optics. The polarization of the incident light was varied by a half-waveplate (quarter-waveplate) to obtain linearly (circularly) polarized light. The silicon samples of ≈1 cm2 , obtained from cleaving the wafers, were mounted on three-axis translation stages with a resolution of 50 nm along the lateral dimensions (X, Y) and 100 nm along the axial direction (Z). The impurity concentration in n- and p-type silicon (doped with phosphorus and boron, respectively) was ∼1015 cm−3 and the corresponding resistivities were 4.6 and 13.5 m. The standard (1 0 0) crystal orientation was used for all silicon wafers (intrinsic, n- and p-type). The laser-ablated regions were characterized by a scanning electron microscope (SEM) after gold coating the silicon surface with a thin layer (few nanometers) to make them conductive. All SEM images were taken with the electron beam perpendicular to the sample (zero tilt). Laser induced surface topology was investigated by an Atomic Force Microscope operating in contact mode with a pyramidal tip radius of 10 nm, a spring constant of 0.24 N/m and with front, back, and side angles of 15◦ , 25◦ , and 17.5◦ , respectively. Images were captured with a relative tip speed of 1 m/s. The typical lateral and vertical resolutions were ∼10 nm and 0.1 nm respectively.
3. Results SEM images of Fig. 1 show morphological evolution of microstructures on n-type silicon induced by a single femtosecond pulse of different energies. A rim is formed around the ablation crater that increases in size with pulse energy. The lowest energy at which ablation features were visible under SEM was defined as the threshold value. The ablation threshold was also determined from the semi-logarithmic plot of the squared diameter of the modified region, measured with an SEM, as a function of laser fluence [33]. The single shot ablation threshold of silicon was determined to be 19 nJ corresponding to a peak laser fluence of 0.37 J/cm2 , in good agreement with the published data [30,34–36]. As the pulse energy was increased, the rim height increased up to ∼300 nm above the undamaged surface (bottom-right panel of Fig. 1) while its width varied from 300 to 900 nm (not shown). This resulted in the increased contrast of the ring like structure in the SEM images. The variation of rim dimensions with pulse energy occurred simultaneously with increasing depth of the ablated crater (bottom-right panel of Fig. 1). Similar structures were observed for intrinsic and p-type silicon. Fig. 2 shows three dimensional AFM profile of a micro-structure induced by a single femtosecond pulse of energy 115 nJ. The laser polarization was linear and parallel to the x-axis. The rim height was observed to be nonuniform – higher along the polarization direction than along the orthogonal direction. The dependence of rim height on laser polarization becomes obvious in the 2D colour maps shown in Fig. 3. When the angle of the linearly polarized light was varied the asymmetry in rim height followed the laser polarization (Fig. 3a and c). For circularly
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Fig. 1. SEM images showing surface topography induced by a single femtosecond laser pulse in n-type silicon ((1 0 0) orientation) at different pulse energies. The corresponding peak fluences are 0.88, 1.08, 1.28 and 2.55 J/cm2 . The bottom right panel shows the variation of average rim height and depth of the ablation crater as a function of the laser fluence.
Fig. 5 shows the width asymmetry as a function of laser fluence in intrinsic and extrinsic silicon for linear and circularly polarized light. In n-type and intrinsic silicon the asymmetry was greater than unity for all fluences (Fig. 5a and b) suggesting wider rim width along the laser polarization. In contrast, in p-type silicon the asymmetry was less than unity for all fluences suggesting wider rim width along a direction orthogonal to the laser polarization (Fig. 5c). With circularly polarized light the rim width asymmetry was close to unity at all fluences in n-type silicon (Fig. 5d). Fig. 2. 3D AFM profile of a micro-structure in n-type silicon induced by a linearly polarized single femtosecond pulse of energy 115 nJ (peak fluence of 2.3 J/cm2 ). The laser polarization is along the x-axis and crystal orientation is (1 0 0).
4. Discussion
polarized light the asymmetry in height disappeared (Fig. 3b). The cross-sectional profile of the micro-structure in Fig. 3a, produced with a linearly polarized light, along two orthogonal directions is shown in Fig. 3d. Not only was the height of the rim higher along the laser polarization but also the width, measured at the unmodified surface, was larger. Fig. 4 shows the height asymmetry as a function of laser fluence in intrinsic and extrinsic silicon for linear and circularly polarized light. The asymmetry was defined as the ratio of rim dimensions along the laser polarization to an orthogonal direction. A value of unity suggests uniform rim dimensions along all directions. In n-type silicon the asymmetry was greater than unity (Fig. 4a) suggesting higher rim height along the laser polarization. In p-type silicon the asymmetry was less than unity for all fluences suggesting an opposite behaviour to that of n-type silicon – higher rim height along a direction orthogonal to the laser polarization. In intrinsic silicon, the asymmetry was close to unity at low fluences but at high fluences the behaviour was similar to that of n-type silicon. With circularly polarized light the asymmetry was close to unity for all fluences in n-type silicon (Fig. 4d).
The schematic in Fig. 6 illustrates the processes involved in the interaction of an intense femtosecond pulse with silicon and the subsequent rim formation. The interaction of ultrafast pulses at 800 nm (photon energy of 1.55 eV) with silicon (bandgap of 1.1 eV) is dominated by linear and two-photon absorption [37,38]. Optical response of the material is dominated by generation of free carriers. The absorption processes determine the density, the temperature, and the spatial profile of the excited carrier plasma. Energy absorbed by the free carriers is typically transferred to the lattice on a picosecond time-scale resulting in melting of a thin layer on a nanosecond time-scale. However, with femtosecond pulses nonthermal laser-induced melting can occur on much faster timescales due to disordering caused by plasma induced instabilities when carrier densities are very high [39]. The spatial profile of the carrier density follows the Gaussian intensity distribution of the laser pulse (Fig. 6a) and translates into a lattice temperature distribution that is high in the middle of the interaction region and falls off to the edges (Fig. 6b). The thickness of the molten layer in silicon is a few hundred nanometers, determined primarily by the optical penetration depth represented by ds in the schematic. The surface morphology is
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Fig. 3. AFM colour maps of a n-type silicon modified by a single femtosecond pulse of energy 150 nJ, corresponding to a peak fluence of 2.9 J/cm2 . (a, c) linearly polarized light. Arrows indicate the polarization direction. (b) Circularly polarized light. (d) AFM cross-sections of the structure for linear polarization (panel a) along two orthogonal directions, black (red) is parallel (perpendicular) to the laser polarization. Crystal orientation is (1 0 0). (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
influenced by the spatial velocity distribution of the melt layer (Fig. 6c) which is related to the temperature distribution (Fig. 6b). Based on the SEM and AFM data presented, the molten material moved outward from the centre of the irradiated region. Molten silicon can undergo displacement from the centre to the edges of the crater either due to thermocapillary forces (Marangoni flow) or hydrodynamic forces exerted by the pressure of the plasma on the surface (Fig. 6c). The spatial temperature distribution on the surface affects the melt layer velocity thereby influencing the modifications to the surface morphology. As the molten material is driven to the edges, the melt layer resolidifies when its temperature decreases below the melting temperature of silicon (∼1690 K), thus creating an elevated rim around the ablation crater (Fig. 6d). The Marangoni effect in silicon arises due to surface tension gradients and has been investigated and modelled using fluid mechanics [29,40–42]. Marangoni flow was shown to consist of two distinct components, thermocapillary associated with local temperature gradient and chemicapillary associated with local compositional gradient. In thermocapillary flow the Gaussian beam intensity profile of the laser induces a temperature gradient on the surface that drives material from the hot centre to the cold periphery due to surface tension gradient. This is the case in silicon where the surface tension decreases as the fluid gets hotter. In chemicapillary flow, depletion of a surfactant such as the native oxide layer gives rise to compositional gradient that drives the molten material from the edges towards the central region where surfactant concentration is low due to ablation. In glass, unlike in silicon, the surface tension increases as the fluid gets hotter so the rim formation was explained in terms of
hydrodynamic pressure gradients exerted by the plasma onto the molten material inducing a lateral melt flow to the periphery [32]. Characteristic time scales associated with the Marangoni flow ( M ) and pressure-driven flow ( p ) were defined in reference [32] as Marangoniflow : M ≈
L2 , T Tm hm
Pressure-drivenflow : p ≈
L2 Ppl h2m
(1)
,
(2)
where hm is the average melt depth, L is a typical radial dimension, Tm is the melting temperature, Ppl is the average plasma pressure, is the viscosity and T is the temperature coefficient of surface tension. For a melt depth of hm = 750 nm, a typical crater radius of L = 2 m, T = −0.28 × 10−3 N/mK and an average plasma pressure of ≈100 atm at ns time scale [43] (timescale during which melting occurs) the ratio of the two characteristic times
M p
P hm
≈ O( pl Tm ) ≈ 15, T
suggesting both mechanisms can play a role in moving the fluid in silicon. We now explain the origin of polarization dependence of the rim dimensions in terms of transient plasmonics [5]. Generation of free carriers is the first step of interaction of an ultrafast laser pulse with semiconductors. Electrons are excited from the occupied valence band to the empty conduction band [44]. Excitation of free carriers, dominated by linear and two-photon absorption, leads to a very steep carrier distribution that can be rapidly attained during the leading edge of the pulse at moderately high laser
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Fig. 4. Asymmetry in rim height as a function of peak laser fluence in silicon (a) n-type, (b) intrinsic, (c) p-type and (d) n-type with circular polarization. The asymmetry is the ratio of the dimensions along two orthogonal directions, parallel and perpendicular to the laser polarization. Red line represents an asymmetry of unity. All silicon wafers had the standard (1 0 0) crystal orientation. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Asymmetry in rim width as a function of peak laser fluence in silicon (a) n-type, (b) intrinsic, (c) p-type and (d) n-type with circular polarization. The asymmetry is the ratio of the dimensions along two orthogonal directions, parallel and perpendicular to the laser polarization. Red line represents an asymmetry of unity. All silicon wafers had the standard (1 0 0) crystal orientation. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Schematic showing rim formation in silicon (a–d) and the origin of height asymmetry (a –d ) at different time steps of the interaction with femtosecond light pulses. (a) Radial Gaussian intensity distribution of the light pulse incident on silicon. k is the propagation direction and E is the laser polarization. (b) Spatial profile of the temperature distribution in silicon. ds represents the thickness of the melt region. (c) Displacement of molten silicon from the centre to the edges, indicated by arrows. Spatial profile of the melt layer velocity due to the forces exerted is also shown. (d) Formation of rim around the ablation crater, (a ) Leading edge of the intense pulse creates dense carrier distribution with which the trailing edge of the laser pulse interacts resulting in asymmetric carrier distribution due to local field enhancement. (c ) Asymmetric temperature distribution and displacement of molten silicon indicated by arrows of different length and density. Melt layer velocity along the two lateral directions is also shown highlighting the different distributions. (d ) Formation of rim around the ablation crater with asymmetric heights.
intensities. Interaction of the trailing edge of the pulse with the plasma leads to local field enhancements that results in asymmetric carrier distribution (Fig. 6a ). The field enhancement occurs along or perpendicular to the laser polarization depending on the plasma density [7]. On a picosecond timescale when the absorbed optical energy is transferred from electrons to phonons causing lattice heating the spatial asymmetry is retained in the temperature distribution represented by the lengths of arrows in Fig. 6c . As a result the thermocapillary and hydrodynamic forces that drive the molten silicon are not spatially symmetric resulting in different velocities along the orthogonal directions. Consequently, the rim height and width are larger along or perpendicular to the laser polarization (Fig. 6d ).
Asymmetry of the initial electron kinetic energy distribution due to the laser electric field and the related variation in excitation transfer probability has been recently used to describe polarization dependence of self-organized structure formation upon femtosecond laser ablation [45]. Asymmetric energy deposition and carrier distribution can also arise from polarization dependent absorption of light by plasma apart from local field enhancements. Resonance absorption in plasma was shown to depend on the angle of incidence and often the differences in absorption between s and p polarization were found to be maximum for an angle of incidence in the 35–45◦ range. However, for normal incidence, as is the case in the present experiment, the absorption remained the same for both polarizations [2,4,46,47] and is unlikely to play a role.
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In n-type silicon the majority carriers are electrons. When intense femtosecond pulses are incident on such a material electron densities greater than the critical density in silicon are produced. Such high densities have been reported based on reflectivity measurements in silicon [39]. Interaction of light with a dense plasma is analogous to a metal nanoparticle where collective electron oscillations lead to localized field enhancement along the polarization direction [26]. The enhanced fields lead to further generation of free carriers along the polarization direction resulting in a spatial asymmetry in their density. At low laser fluences, the field enhancement is negligible and hence the asymmetry of the rim height is close to unity (Fig. 4a). The height asymmetry increases with the laser fluence but starts to decrease beyond 2.5 J/cm2 . This is possibly because of the decrease in coupling of optical energy with the plasma that becomes highly reflective. Similar trends were observed in rim width asymmetry (Fig. 5a). In intrinsic silicon, the electron plasma density generated by the femtosecond pulse is lower than in n-type silicon. As a result, higher laser fluences are required to start observing rim height asymmetries (Fig. 4b). In p-type silicon where the majority of carriers are holes, femtosecond laser excitation leads to electron densities lower than the critical density. Interaction of light with an under-dense plasma leads to field enhancements perpendicular to the laser polarization [26]. As a result the rim height and width asymmetries are lower than unity (Figs. 4c and 5c). At very high fluences the height asymmetry tends to be close to unity similar to the behaviour observed in intrinsic silicon at lower fluences. For circularly polarized light the local field enhancements disappear and as a result the height and width asymmetries are close to unity (Figs. 4d and 5d). Polarization dependent single pulse ablation has been previously observed in silicon [27] and in PMMA [26] where the overall features of the ablation crater were found to be elongated along the major axis of the polarization vector. In contrast, in silicon only the rim dimensions were found to be polarization sensitive while the ablation crater remained circular. In PMMA and silicon, the origin of polarization dependence was described by us in terms of local field enhancement during light-plasma interaction. The differences could be in the nonlinearity of the interaction. In silicon the interaction is dominated by one- and two-photon absorption while in PMMA it is a three/four-photon process (bandgap ∼4.6–5.3 eV [48]). So for a given field enhancement the local field effects will be more pronounced in PMMA than in silicon resulting in an asymmetric ablation crater in the former. In silicon, when the laser fluence was increased significantly beyond 4 J/cm2 , we did observe the ablation crater to elongate along the polarization direction.
5. Conclusions We demonstrated polarization sensitive rim formation around the ablation crater in silicon when irradiated by a single femtosecond laser pulse. The rim height and width were found to be larger along or perpendicular to the direction of the laser polarization depending on whether the majority carriers in silicon were electrons or holes, respectively. Results were explained in terms of transient light–plasma interaction that occurs when the leading edge of the laser pulse creates a free-electron plasma with which the trailing edge of the pulse interacts. Polarization sensitivity in such a transient interaction arises due to local field enhancements, which depends on the initial electron density, and impacts the final electron density distribution to be asymmetric. In the subsequent stages of the ablation process the asymmetry is preserved and in silicon where only a thin layer of molten material is created the radial outward motion of the fluid forms the rim.
Our results bring to the fore two key aspects of the nature of light–matter interaction. First, by tightly focusing intense ultrashort light pulses a localized plasma can be created whose density evolves during the pulse. Its subsequent interaction with the light that created it can lead to unique nonlinear phenomena whose signatures can be seen either with a single laser pulse or reinforced by multiple pulses [5]. This interaction with light is akin to transient plasmonics where plasmon excitation around metal nanostructures and films confines light to dimensions below the diffraction limit. In fact, ultrafast active plasmonics has been reported where optical excitation of metal by intense femtosecond laser pulses induced an ultrafast modulation of the plasmonic resonance that can be used as an ultrafast optical switch [49]. Second, it paves the way to study how the fluid motion manifests in the case of multiple pulses and how it can be controlled using structured light (for example hypergeometric-Gaussian modes) to create unique nanostructures. Acknowledgements The authors acknowledge fruitful discussions with Paul Corkum, Thomas Brabec and Lora Ramunno. We thank Prof. Xudong Cao and Yubo Qin in the Department of Biochemical Engineering for allowing us to use their atomic force microscope. V.R.B’s research is funded by Natural Science and Engineering Research Council of Canada, Canada Research Chairs, Canadian Foundation for Innovation and Ontario Ministry of Economic Development and Innovation. H.A. is supported by NSERC-CREATE in Extreme photonics fellowship and by Ontario Graduate Scholarship. References [1] M. Gertsvolf, H. Jean-Ruel, P.P. Rajeev, D.D. Klug, D.M. Rayner, P.B. Corkum, Phys. Rev. Lett. 101 (2008) 243001. [2] J.S. Pearlman, J.J. Thomson, C.E. Max, Phys. Rev. Lett. 38 (1977) 1397. [3] R. Dinger, K. Rohr, H. Weber, Laser Part. Beams 5 (1987) 691. [4] K.R. Manes, V.C. Rupert, J.M. Auerbach, P. Lee, J.E. Swain, Phys. Rev. Lett. 39 (1977) 281. [5] V.R. Bhardwaj, E. Simova, P.P. Rajeev, C. Hnatovsky, R.S. Taylor, D.M. Rayner, P.B. Corkum, Phys. Rev. Lett. 96 (2006) 057404. [6] Y. Shimotsuma, P.F. Kazansky, J. Qiu, K. Hirao, Phys. Rev. Lett. 91 (2003) 247405. [7] P.P. Rajeev, M. Gertsvolf, C. Hnatovsky, E. Simova, R.S. Taylor, P.B. Corkum, D.M. Rayner, V.R. Bhardwaj, J. Phys. B 40 (2007) S273. [8] K.I. Popov, C. McElcheran, K. Briggs, S. Mack, L. Ramunno, Opt. Exp. 19 (2011) 271. [9] R.S. Taylor, E. Simova, C. Hnatovsky, Opt. Lett. 33 (2008) 1312. [10] C. Hnatovsky, V. Shvedov, W. Krolikowski, A. Rode, Phys. Rev. Lett. 106 (2011) 123901. [11] J.F. Young, J.E. Sipe, H.M. van Driel, Phys. Rev. B 30 (1984) 2001. [12] J.F. Young, J.E. Sipe, H.M. van Driel, Opt. Lett. 8 (1983) 431. [13] Z. Guosheng, P.M. Fauchet, A.E. Siegman, Phys. Rev. B 26 (1982) 5366. [14] J.S. Preston, H.M. van Driel, J.E. Sipe, Phys. Rev. B 40 (1989) 3942. [15] H. Varel, M. Wahmer, A. Rosenfeld, D. Ashkenasi, E.E.B. Campbell, Appl. Surf. Sci. 127-128 (1998) 128. [16] A.M. Ozkan, A.P. Malshe, T.A. Railkar, W.D. Brown, M.D. Shirk, P.A. Molian, Appl. Phys. Lett. 75 (1999) 3716. [17] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 76 (2003) 983. [18] A. Borowiec, H.K. Haugen, Appl. Phys. Lett. 82 (2003) 4462. [19] J. Bonse, S. Baudach, J. Kruger, W. Kautek, M. Lenzner, Appl. Phys. A 74 (2002) 19. [20] R. Buividas, M. Mikutis, S. Juodkazis, Prog. Quant. Electron 38 (2014) 119. [21] Y. Han, X. Zhao, S. Qu, Opt. Express 19 (2011) 19150. [22] S. Baudach, J. Bonse, W. Kautek, Appl. Phys. A 69 (1999) S395. [23] F. Costache, S. Kouteva-Arguirova, J. Reif, Appl. Phys. A 79 (2004) 1429. [24] J.F. Young, J.S. Preston, H.M. van Driel, J.E. Sipe, Phys. Rev. B 27 (1983) 1155. [25] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, ACS Nano 3 (2009) 4062. [26] J.-M. Guay, A. Villafranca, F. Baset, K. Popov, L. Ramunno, V.R. Bhardwaj, New J. Phys. 14 (2012) 085010. [27] X. Ji, L. Jian, X. Li, W. Han, Y. Liu, Q. Huang, Y. Lu, Appl. Opt. 53 (2014) 6742. [28] K. Venkatakrishnan, B. Tan, P. Stanley, N.R. Sivakumar, J. Appl. Phys. 92 (2002) 1604. [29] T. Schwarz-Selinger, D.G. Cahill, S.-C. Chen, S.-J. Moon, C.P. Grigoropoulos, Phys. Rev. B 64 (2001) 1555323. [30] A. Borowiec, M. Mackenzie, G.C. Weatherly, H.K. Haugen, Appl. Phys. A 76 (2003) 201. [31] S. Panchatsharam, B. Tan, K. Venkatakrishnan, J. Appl. Phys. 105 (2009) 093103.
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Polarization dependent micro-structuring of silicon with a femtosecond laser H. Al-Khazraji, V.R. Bhardwaj ∗ Department of Physics and Center for Research in Photonics, University of Ottawa, 25 Templeton Street, Ottawa, ON K1N 6N5, Canada
a r t i c l e
i n f o
Article history: Received 25 March 2015 Received in revised form 19 May 2015 Accepted 15 June 2015 Available online 2 July 2015
a b s t r a c t We experimentally demonstrate formation of a sub-micron rim around femtosecond laser ablated crater on silicon whose height and width were sensitive to laser polarization. Except for circularly polarized light we show that the rim height and width were asymmetric – larger along the direction of the laser polarization for n-type and intrinsic silicon, while in p-type silicon the asymmetry was perpendicular. Polarization dependent rim formation is attributed to the transient light–plasma interaction that gives rise to local-field enhancements resulting in an asymmetric electron density and energy deposition. Picoseconds later when the electron energy is transferred to the lattice, the asymmetry is retained in the temperature distribution within the interaction region. The temperature distribution eventually leads to non-symmetric radial outward fluid motion of a thin layer of molten material from the centre of the ablation crater that subsequently re-solidifies on a nanosecond timescale. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
1. Introduction The interaction of femtosecond laser pulses with materials involves several fundamental physical processes such as excitation/energy deposition, melting, and material removal/ modification that are separated in time. Excitation is determined by the duration of the laser pulse typically in the femtosecond range, melting occurs on picosecond timescale, and the final material removal/modification may take up to several nanoseconds. Apart from the duration of the laser pulse, light–matter interactions are often governed by the intensity, wavelength, and polarization of light. In atoms and molecules, the influence of polarization of light was well understood but its role in the interaction with solids is subtle and not clear. Some of the basic mechanisms involved in the interaction depend on the laser polarization that can potentially leave an imprint on the modification/ablation process. For example, multiphoton ionization of crystalline dielectrics was found to depend on sample orientation due to direction dependence of the effective mass of the electron [1]. Intense ultrashort laser pulses rapidly produce high carrier densities forming a plasma that can subsequently interact with incident light leading to polarization-dependent absorption. Experiments on laser-produced plasmas have
∗ Corresponding author. Tel.: +1 613 562 5800x6759; fax: +1 613 562 5190. E-mail address: [email protected] (V.R. Bhardwaj). http://dx.doi.org/10.1016/j.apsusc.2015.06.081 0169-4332/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.
demonstrated sharp differences (up to 50–60%) in absorption of p- and s-polarized light due to resonance absorption [2–4]. In addition, the interaction of linearly polarized light with dense plasma can also lead to local field enhancements akin to that observed in metal nanoparticles [5]. To date, signatures of polarization-dependent phenomena have been observed mostly in the multiple pulse regime both on surfaces and inside bulk materials. Polarization-dependent selforganized three-dimensional (3D) periodic nanostructures have been observed inside fused silica [5,6] and their origin was attributed to local field enhancement arising from light-plasma interaction [5,7,8]. For circularly polarized light, the handedness of the light was imprinted in the form of ordered sub-micron chiral structures [9]. Similarly, complex polarization state of the light in the focal volume was also visualized with sub-wavelength resolution [10]. Polarization-dependent periodic structures (surface ripples) have also been observed on surfaces in the ablation crater of a variety of materials including dielectrics, semiconductors and metals under widely different illumination conditions [11–14]. Wavelength [15–19] and sub-wavelength [20] ripple spacing has been observed and was found to vary with the fluence and the number of laser pulses. The surface ripples have been found to be perpendicular and parallel to the laser polarization in different regions of the ablation crater [21–23]. The ripple formation is described in terms of interference between the incident light and the surface scattering wave [24] or surface plasmon polaritons [25].
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The overall features of the ablation region were also found to be polarization dependent. Ablation of bulk polymer with a single femtosecond laser pulse produced craters elongated along the major axis of the polarization vector [26]. Nearly 40% elongation was observed for linearly and elliptically polarized light in a fluence range of 4–20 J/cm2 , only circularly polarized light produced near circular ablation craters irrespective of pulse energies. Numerical simulation of the interaction of intense light pulses with solids demonstrated that the polarization dependence of the ablation features arise from a local field enhancement during light–plasma interaction. Recently, single shot femtosecond laser ablation of silicon also demonstrated formation of elliptical-shaped craters that were elongated along the major axis of the polarization direction [27]. The morphology of the ablation craters was found to evolve from an ellipse to nearly a circle with increasing fluence. Redistribution of the electric field on the silicon surface due to surface defects was found to play a crucial role in the creation of elliptical-shaped craters in addition to field enhancements. Similar elongation of ablation craters along the major axis of the polarization vector was also observed in multi-pulse ablation of thin metal films [28]. In this paper, we show that single femtosecond pulse irradiation of silicon produces a rim around the ablation crater and that the rim dimensions are polarization dependent. Rim formation around an ablation crater was observed in silicon with ns [29] and fs laser irradiation [30,31], and in glass [32] irradiated with a single femtosecond laser pulse. However, polarization dependent rim formation was never observed. The rim height and width were found to be larger along or perpendicular to the direction of linear laser polarization depending on whether the majority carriers in silicon were electrons or holes, respectively. The polarization dependence of rim dimensions was attributed to local field enhancement that arises when the leading edge of an intense laser pulse creates a free-electron plasma with which the trailing edge of the pulse interacts resulting in an asymmetric electron density distribution. Picoseconds later when the electron energy is transferred to the lattice the asymmetry is preserved in the temperature distribution that eventually causes melting. The non-symmetric radial outward fluid motion of a thin layer of molten material from the centre of the ablation crater that subsequently re-solidifies on a nanosecond timescale leads to rim formation. The field enhancement is parallel (perpendicular) to the laser polarization when electron density is above (below) the critical density of silicon.
2. Experimental methods In our experiments, 800 nm light from a Ti:Sapphire laser system operating at a repetition rate of 1 kHz and producing 45 fs pulses with a maximum energy of 2.5 mJ, was focused on the silicon surface by a 0.25 NA (16×) aspheric microscope objective. The back aperture of the microscope objective was slightly overfilled to minimize alignment errors. The position of the laser focus relative to the sample surface was accurately determined by imaging the back reflected light with a CCD camera at very low pulse energies below the ablation threshold. After locating the surface of silicon, the glass plate at 45◦ , used for directing the back reflected light, was removed in order to avoid distortion of the incident polarization. A thin broadband beam sampler at the output of the laser directed a small fraction of the beam into a single-shot autocorrelator to monitor the pulse duration continuously. The pulses were not pre-chirped and the duration was measured to be 70 fs at the back aperture of the objective after propagating through all the optics. The use of an aspheric objective, where a single asphere replaced the compound lens system of a standard microscope objective, reduced dispersion and spherical aberrations. The laser focal spot size was measured to
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be 1.8 ± 0.1 m using the knife-edge method for the 0.25 NA microscope objective, in close agreement with the diffraction-limited spot size. The laser beam profile, imaged by a CCD camera with a flat diffuser, was nearly gaussian with no hot spots. Single femtosecond laser pulses were selected by operating the laser in an external trigger mode. A combination of half-wave plate and a polarizer were used to vary the pulse energy from 10 nJ to 200 nJ. The pulse energy was varied in steps of 5 nJ up to 120 nJ and in steps of 10 nJ thereafter. A small fraction of the incident beam was reflected on to a calibrated fast photodiode operating in the linear regime to monitor the incident power. The pulse energies incident on the sample after the microscope objective was estimated by taking into account the transmission and reflection losses of all the optics. The polarization of the incident light was varied by a half-waveplate (quarter-waveplate) to obtain linearly (circularly) polarized light. The silicon samples of ≈1 cm2 , obtained from cleaving the wafers, were mounted on three-axis translation stages with a resolution of 50 nm along the lateral dimensions (X, Y) and 100 nm along the axial direction (Z). The impurity concentration in n- and p-type silicon (doped with phosphorus and boron, respectively) was ∼1015 cm−3 and the corresponding resistivities were 4.6 and 13.5 m. The standard (1 0 0) crystal orientation was used for all silicon wafers (intrinsic, n- and p-type). The laser-ablated regions were characterized by a scanning electron microscope (SEM) after gold coating the silicon surface with a thin layer (few nanometers) to make them conductive. All SEM images were taken with the electron beam perpendicular to the sample (zero tilt). Laser induced surface topology was investigated by an Atomic Force Microscope operating in contact mode with a pyramidal tip radius of 10 nm, a spring constant of 0.24 N/m and with front, back, and side angles of 15◦ , 25◦ , and 17.5◦ , respectively. Images were captured with a relative tip speed of 1 m/s. The typical lateral and vertical resolutions were ∼10 nm and 0.1 nm respectively.
3. Results SEM images of Fig. 1 show morphological evolution of microstructures on n-type silicon induced by a single femtosecond pulse of different energies. A rim is formed around the ablation crater that increases in size with pulse energy. The lowest energy at which ablation features were visible under SEM was defined as the threshold value. The ablation threshold was also determined from the semi-logarithmic plot of the squared diameter of the modified region, measured with an SEM, as a function of laser fluence [33]. The single shot ablation threshold of silicon was determined to be 19 nJ corresponding to a peak laser fluence of 0.37 J/cm2 , in good agreement with the published data [30,34–36]. As the pulse energy was increased, the rim height increased up to ∼300 nm above the undamaged surface (bottom-right panel of Fig. 1) while its width varied from 300 to 900 nm (not shown). This resulted in the increased contrast of the ring like structure in the SEM images. The variation of rim dimensions with pulse energy occurred simultaneously with increasing depth of the ablated crater (bottom-right panel of Fig. 1). Similar structures were observed for intrinsic and p-type silicon. Fig. 2 shows three dimensional AFM profile of a micro-structure induced by a single femtosecond pulse of energy 115 nJ. The laser polarization was linear and parallel to the x-axis. The rim height was observed to be nonuniform – higher along the polarization direction than along the orthogonal direction. The dependence of rim height on laser polarization becomes obvious in the 2D colour maps shown in Fig. 3. When the angle of the linearly polarized light was varied the asymmetry in rim height followed the laser polarization (Fig. 3a and c). For circularly
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Fig. 1. SEM images showing surface topography induced by a single femtosecond laser pulse in n-type silicon ((1 0 0) orientation) at different pulse energies. The corresponding peak fluences are 0.88, 1.08, 1.28 and 2.55 J/cm2 . The bottom right panel shows the variation of average rim height and depth of the ablation crater as a function of the laser fluence.
Fig. 5 shows the width asymmetry as a function of laser fluence in intrinsic and extrinsic silicon for linear and circularly polarized light. In n-type and intrinsic silicon the asymmetry was greater than unity for all fluences (Fig. 5a and b) suggesting wider rim width along the laser polarization. In contrast, in p-type silicon the asymmetry was less than unity for all fluences suggesting wider rim width along a direction orthogonal to the laser polarization (Fig. 5c). With circularly polarized light the rim width asymmetry was close to unity at all fluences in n-type silicon (Fig. 5d). Fig. 2. 3D AFM profile of a micro-structure in n-type silicon induced by a linearly polarized single femtosecond pulse of energy 115 nJ (peak fluence of 2.3 J/cm2 ). The laser polarization is along the x-axis and crystal orientation is (1 0 0).
4. Discussion
polarized light the asymmetry in height disappeared (Fig. 3b). The cross-sectional profile of the micro-structure in Fig. 3a, produced with a linearly polarized light, along two orthogonal directions is shown in Fig. 3d. Not only was the height of the rim higher along the laser polarization but also the width, measured at the unmodified surface, was larger. Fig. 4 shows the height asymmetry as a function of laser fluence in intrinsic and extrinsic silicon for linear and circularly polarized light. The asymmetry was defined as the ratio of rim dimensions along the laser polarization to an orthogonal direction. A value of unity suggests uniform rim dimensions along all directions. In n-type silicon the asymmetry was greater than unity (Fig. 4a) suggesting higher rim height along the laser polarization. In p-type silicon the asymmetry was less than unity for all fluences suggesting an opposite behaviour to that of n-type silicon – higher rim height along a direction orthogonal to the laser polarization. In intrinsic silicon, the asymmetry was close to unity at low fluences but at high fluences the behaviour was similar to that of n-type silicon. With circularly polarized light the asymmetry was close to unity for all fluences in n-type silicon (Fig. 4d).
The schematic in Fig. 6 illustrates the processes involved in the interaction of an intense femtosecond pulse with silicon and the subsequent rim formation. The interaction of ultrafast pulses at 800 nm (photon energy of 1.55 eV) with silicon (bandgap of 1.1 eV) is dominated by linear and two-photon absorption [37,38]. Optical response of the material is dominated by generation of free carriers. The absorption processes determine the density, the temperature, and the spatial profile of the excited carrier plasma. Energy absorbed by the free carriers is typically transferred to the lattice on a picosecond time-scale resulting in melting of a thin layer on a nanosecond time-scale. However, with femtosecond pulses nonthermal laser-induced melting can occur on much faster timescales due to disordering caused by plasma induced instabilities when carrier densities are very high [39]. The spatial profile of the carrier density follows the Gaussian intensity distribution of the laser pulse (Fig. 6a) and translates into a lattice temperature distribution that is high in the middle of the interaction region and falls off to the edges (Fig. 6b). The thickness of the molten layer in silicon is a few hundred nanometers, determined primarily by the optical penetration depth represented by ds in the schematic. The surface morphology is
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Fig. 3. AFM colour maps of a n-type silicon modified by a single femtosecond pulse of energy 150 nJ, corresponding to a peak fluence of 2.9 J/cm2 . (a, c) linearly polarized light. Arrows indicate the polarization direction. (b) Circularly polarized light. (d) AFM cross-sections of the structure for linear polarization (panel a) along two orthogonal directions, black (red) is parallel (perpendicular) to the laser polarization. Crystal orientation is (1 0 0). (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
influenced by the spatial velocity distribution of the melt layer (Fig. 6c) which is related to the temperature distribution (Fig. 6b). Based on the SEM and AFM data presented, the molten material moved outward from the centre of the irradiated region. Molten silicon can undergo displacement from the centre to the edges of the crater either due to thermocapillary forces (Marangoni flow) or hydrodynamic forces exerted by the pressure of the plasma on the surface (Fig. 6c). The spatial temperature distribution on the surface affects the melt layer velocity thereby influencing the modifications to the surface morphology. As the molten material is driven to the edges, the melt layer resolidifies when its temperature decreases below the melting temperature of silicon (∼1690 K), thus creating an elevated rim around the ablation crater (Fig. 6d). The Marangoni effect in silicon arises due to surface tension gradients and has been investigated and modelled using fluid mechanics [29,40–42]. Marangoni flow was shown to consist of two distinct components, thermocapillary associated with local temperature gradient and chemicapillary associated with local compositional gradient. In thermocapillary flow the Gaussian beam intensity profile of the laser induces a temperature gradient on the surface that drives material from the hot centre to the cold periphery due to surface tension gradient. This is the case in silicon where the surface tension decreases as the fluid gets hotter. In chemicapillary flow, depletion of a surfactant such as the native oxide layer gives rise to compositional gradient that drives the molten material from the edges towards the central region where surfactant concentration is low due to ablation. In glass, unlike in silicon, the surface tension increases as the fluid gets hotter so the rim formation was explained in terms of
hydrodynamic pressure gradients exerted by the plasma onto the molten material inducing a lateral melt flow to the periphery [32]. Characteristic time scales associated with the Marangoni flow ( M ) and pressure-driven flow ( p ) were defined in reference [32] as Marangoniflow : M ≈
L2 , T Tm hm
Pressure-drivenflow : p ≈
L2 Ppl h2m
(1)
,
(2)
where hm is the average melt depth, L is a typical radial dimension, Tm is the melting temperature, Ppl is the average plasma pressure, is the viscosity and T is the temperature coefficient of surface tension. For a melt depth of hm = 750 nm, a typical crater radius of L = 2 m, T = −0.28 × 10−3 N/mK and an average plasma pressure of ≈100 atm at ns time scale [43] (timescale during which melting occurs) the ratio of the two characteristic times
M p
P hm
≈ O( pl Tm ) ≈ 15, T
suggesting both mechanisms can play a role in moving the fluid in silicon. We now explain the origin of polarization dependence of the rim dimensions in terms of transient plasmonics [5]. Generation of free carriers is the first step of interaction of an ultrafast laser pulse with semiconductors. Electrons are excited from the occupied valence band to the empty conduction band [44]. Excitation of free carriers, dominated by linear and two-photon absorption, leads to a very steep carrier distribution that can be rapidly attained during the leading edge of the pulse at moderately high laser
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Fig. 4. Asymmetry in rim height as a function of peak laser fluence in silicon (a) n-type, (b) intrinsic, (c) p-type and (d) n-type with circular polarization. The asymmetry is the ratio of the dimensions along two orthogonal directions, parallel and perpendicular to the laser polarization. Red line represents an asymmetry of unity. All silicon wafers had the standard (1 0 0) crystal orientation. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Asymmetry in rim width as a function of peak laser fluence in silicon (a) n-type, (b) intrinsic, (c) p-type and (d) n-type with circular polarization. The asymmetry is the ratio of the dimensions along two orthogonal directions, parallel and perpendicular to the laser polarization. Red line represents an asymmetry of unity. All silicon wafers had the standard (1 0 0) crystal orientation. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Schematic showing rim formation in silicon (a–d) and the origin of height asymmetry (a –d ) at different time steps of the interaction with femtosecond light pulses. (a) Radial Gaussian intensity distribution of the light pulse incident on silicon. k is the propagation direction and E is the laser polarization. (b) Spatial profile of the temperature distribution in silicon. ds represents the thickness of the melt region. (c) Displacement of molten silicon from the centre to the edges, indicated by arrows. Spatial profile of the melt layer velocity due to the forces exerted is also shown. (d) Formation of rim around the ablation crater, (a ) Leading edge of the intense pulse creates dense carrier distribution with which the trailing edge of the laser pulse interacts resulting in asymmetric carrier distribution due to local field enhancement. (c ) Asymmetric temperature distribution and displacement of molten silicon indicated by arrows of different length and density. Melt layer velocity along the two lateral directions is also shown highlighting the different distributions. (d ) Formation of rim around the ablation crater with asymmetric heights.
intensities. Interaction of the trailing edge of the pulse with the plasma leads to local field enhancements that results in asymmetric carrier distribution (Fig. 6a ). The field enhancement occurs along or perpendicular to the laser polarization depending on the plasma density [7]. On a picosecond timescale when the absorbed optical energy is transferred from electrons to phonons causing lattice heating the spatial asymmetry is retained in the temperature distribution represented by the lengths of arrows in Fig. 6c . As a result the thermocapillary and hydrodynamic forces that drive the molten silicon are not spatially symmetric resulting in different velocities along the orthogonal directions. Consequently, the rim height and width are larger along or perpendicular to the laser polarization (Fig. 6d ).
Asymmetry of the initial electron kinetic energy distribution due to the laser electric field and the related variation in excitation transfer probability has been recently used to describe polarization dependence of self-organized structure formation upon femtosecond laser ablation [45]. Asymmetric energy deposition and carrier distribution can also arise from polarization dependent absorption of light by plasma apart from local field enhancements. Resonance absorption in plasma was shown to depend on the angle of incidence and often the differences in absorption between s and p polarization were found to be maximum for an angle of incidence in the 35–45◦ range. However, for normal incidence, as is the case in the present experiment, the absorption remained the same for both polarizations [2,4,46,47] and is unlikely to play a role.
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In n-type silicon the majority carriers are electrons. When intense femtosecond pulses are incident on such a material electron densities greater than the critical density in silicon are produced. Such high densities have been reported based on reflectivity measurements in silicon [39]. Interaction of light with a dense plasma is analogous to a metal nanoparticle where collective electron oscillations lead to localized field enhancement along the polarization direction [26]. The enhanced fields lead to further generation of free carriers along the polarization direction resulting in a spatial asymmetry in their density. At low laser fluences, the field enhancement is negligible and hence the asymmetry of the rim height is close to unity (Fig. 4a). The height asymmetry increases with the laser fluence but starts to decrease beyond 2.5 J/cm2 . This is possibly because of the decrease in coupling of optical energy with the plasma that becomes highly reflective. Similar trends were observed in rim width asymmetry (Fig. 5a). In intrinsic silicon, the electron plasma density generated by the femtosecond pulse is lower than in n-type silicon. As a result, higher laser fluences are required to start observing rim height asymmetries (Fig. 4b). In p-type silicon where the majority of carriers are holes, femtosecond laser excitation leads to electron densities lower than the critical density. Interaction of light with an under-dense plasma leads to field enhancements perpendicular to the laser polarization [26]. As a result the rim height and width asymmetries are lower than unity (Figs. 4c and 5c). At very high fluences the height asymmetry tends to be close to unity similar to the behaviour observed in intrinsic silicon at lower fluences. For circularly polarized light the local field enhancements disappear and as a result the height and width asymmetries are close to unity (Figs. 4d and 5d). Polarization dependent single pulse ablation has been previously observed in silicon [27] and in PMMA [26] where the overall features of the ablation crater were found to be elongated along the major axis of the polarization vector. In contrast, in silicon only the rim dimensions were found to be polarization sensitive while the ablation crater remained circular. In PMMA and silicon, the origin of polarization dependence was described by us in terms of local field enhancement during light-plasma interaction. The differences could be in the nonlinearity of the interaction. In silicon the interaction is dominated by one- and two-photon absorption while in PMMA it is a three/four-photon process (bandgap ∼4.6–5.3 eV [48]). So for a given field enhancement the local field effects will be more pronounced in PMMA than in silicon resulting in an asymmetric ablation crater in the former. In silicon, when the laser fluence was increased significantly beyond 4 J/cm2 , we did observe the ablation crater to elongate along the polarization direction.
5. Conclusions We demonstrated polarization sensitive rim formation around the ablation crater in silicon when irradiated by a single femtosecond laser pulse. The rim height and width were found to be larger along or perpendicular to the direction of the laser polarization depending on whether the majority carriers in silicon were electrons or holes, respectively. Results were explained in terms of transient light–plasma interaction that occurs when the leading edge of the laser pulse creates a free-electron plasma with which the trailing edge of the pulse interacts. Polarization sensitivity in such a transient interaction arises due to local field enhancements, which depends on the initial electron density, and impacts the final electron density distribution to be asymmetric. In the subsequent stages of the ablation process the asymmetry is preserved and in silicon where only a thin layer of molten material is created the radial outward motion of the fluid forms the rim.
Our results bring to the fore two key aspects of the nature of light–matter interaction. First, by tightly focusing intense ultrashort light pulses a localized plasma can be created whose density evolves during the pulse. Its subsequent interaction with the light that created it can lead to unique nonlinear phenomena whose signatures can be seen either with a single laser pulse or reinforced by multiple pulses [5]. This interaction with light is akin to transient plasmonics where plasmon excitation around metal nanostructures and films confines light to dimensions below the diffraction limit. In fact, ultrafast active plasmonics has been reported where optical excitation of metal by intense femtosecond laser pulses induced an ultrafast modulation of the plasmonic resonance that can be used as an ultrafast optical switch [49]. Second, it paves the way to study how the fluid motion manifests in the case of multiple pulses and how it can be controlled using structured light (for example hypergeometric-Gaussian modes) to create unique nanostructures. Acknowledgements The authors acknowledge fruitful discussions with Paul Corkum, Thomas Brabec and Lora Ramunno. We thank Prof. Xudong Cao and Yubo Qin in the Department of Biochemical Engineering for allowing us to use their atomic force microscope. V.R.B’s research is funded by Natural Science and Engineering Research Council of Canada, Canada Research Chairs, Canadian Foundation for Innovation and Ontario Ministry of Economic Development and Innovation. H.A. is supported by NSERC-CREATE in Extreme photonics fellowship and by Ontario Graduate Scholarship. References [1] M. Gertsvolf, H. Jean-Ruel, P.P. Rajeev, D.D. Klug, D.M. Rayner, P.B. Corkum, Phys. Rev. Lett. 101 (2008) 243001. [2] J.S. Pearlman, J.J. Thomson, C.E. Max, Phys. Rev. Lett. 38 (1977) 1397. [3] R. Dinger, K. Rohr, H. Weber, Laser Part. Beams 5 (1987) 691. [4] K.R. Manes, V.C. Rupert, J.M. Auerbach, P. Lee, J.E. Swain, Phys. Rev. Lett. 39 (1977) 281. [5] V.R. Bhardwaj, E. Simova, P.P. Rajeev, C. Hnatovsky, R.S. Taylor, D.M. Rayner, P.B. Corkum, Phys. Rev. Lett. 96 (2006) 057404. [6] Y. Shimotsuma, P.F. Kazansky, J. Qiu, K. Hirao, Phys. Rev. Lett. 91 (2003) 247405. [7] P.P. Rajeev, M. Gertsvolf, C. Hnatovsky, E. Simova, R.S. Taylor, P.B. Corkum, D.M. Rayner, V.R. Bhardwaj, J. Phys. B 40 (2007) S273. [8] K.I. Popov, C. McElcheran, K. Briggs, S. Mack, L. Ramunno, Opt. Exp. 19 (2011) 271. [9] R.S. Taylor, E. Simova, C. Hnatovsky, Opt. Lett. 33 (2008) 1312. [10] C. Hnatovsky, V. Shvedov, W. Krolikowski, A. Rode, Phys. Rev. Lett. 106 (2011) 123901. [11] J.F. Young, J.E. Sipe, H.M. van Driel, Phys. Rev. B 30 (1984) 2001. [12] J.F. Young, J.E. Sipe, H.M. van Driel, Opt. Lett. 8 (1983) 431. [13] Z. Guosheng, P.M. Fauchet, A.E. Siegman, Phys. Rev. B 26 (1982) 5366. [14] J.S. Preston, H.M. van Driel, J.E. Sipe, Phys. Rev. B 40 (1989) 3942. [15] H. Varel, M. Wahmer, A. Rosenfeld, D. Ashkenasi, E.E.B. Campbell, Appl. Surf. Sci. 127-128 (1998) 128. [16] A.M. Ozkan, A.P. Malshe, T.A. Railkar, W.D. Brown, M.D. Shirk, P.A. Molian, Appl. Phys. Lett. 75 (1999) 3716. [17] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 76 (2003) 983. [18] A. Borowiec, H.K. Haugen, Appl. Phys. Lett. 82 (2003) 4462. [19] J. Bonse, S. Baudach, J. Kruger, W. Kautek, M. Lenzner, Appl. Phys. A 74 (2002) 19. [20] R. Buividas, M. Mikutis, S. Juodkazis, Prog. Quant. Electron 38 (2014) 119. [21] Y. Han, X. Zhao, S. Qu, Opt. Express 19 (2011) 19150. [22] S. Baudach, J. Bonse, W. Kautek, Appl. Phys. A 69 (1999) S395. [23] F. Costache, S. Kouteva-Arguirova, J. Reif, Appl. Phys. A 79 (2004) 1429. [24] J.F. Young, J.S. Preston, H.M. van Driel, J.E. Sipe, Phys. Rev. B 27 (1983) 1155. [25] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, ACS Nano 3 (2009) 4062. [26] J.-M. Guay, A. Villafranca, F. Baset, K. Popov, L. Ramunno, V.R. Bhardwaj, New J. Phys. 14 (2012) 085010. [27] X. Ji, L. Jian, X. Li, W. Han, Y. Liu, Q. Huang, Y. Lu, Appl. Opt. 53 (2014) 6742. [28] K. Venkatakrishnan, B. Tan, P. Stanley, N.R. Sivakumar, J. Appl. Phys. 92 (2002) 1604. [29] T. Schwarz-Selinger, D.G. Cahill, S.-C. Chen, S.-J. Moon, C.P. Grigoropoulos, Phys. Rev. B 64 (2001) 1555323. [30] A. Borowiec, M. Mackenzie, G.C. Weatherly, H.K. Haugen, Appl. Phys. A 76 (2003) 201. [31] S. Panchatsharam, B. Tan, K. Venkatakrishnan, J. Appl. Phys. 105 (2009) 093103.
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