Femtosecond laser writing over silver nanoparticles system embedded in silica using nonlinear microscopy

Optical Materials 36 (2014) 682–686

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Femtosecond laser writing over silver nanoparticles system embedded in silica using nonlinear microscopy Jacob Licea-Rodríguez a, Israel Rocha-Mendoza a,⇑, Raúl Rangel-Rojo a, Luis Rodríguez-Fernández b, Alicia Oliver b a b

Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, 22860 Ensenada, Baja California, México Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica S/N, Ciudad Universitaria, Distrito Federal, México

a r t i c l e

i n f o

Article history: Received 10 September 2013 Received in revised form 5 November 2013 Accepted 6 November 2013 Available online 26 November 2013 Keywords: Nonlinear optical materials Nonlinear microscopy Surface plasmons

a b s t r a c t We present results for the induction and monitoring of structural modification of a composite consisting of elongated silver nanoparticles films embedded in silica using ultrafast femtosecond laser irradiation and second harmonic generation imaging, respectively. Waveguide-like patterns are written and characterized under a laser scanning nonlinear microscope system by simply changing the laser fluence in the sample; switching in this way between two different physical processes occurring only within the composite film: second harmonic generation and laser induced nanoparticles removal. A study of the nanoparticles damage process as a function of the laser energy, polarization and scan velocity is also presented and discussed. The use of the non-irradiated zone between two written channels is proposed as a potential linear and nonlinear optical waveguide. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Composite materials containing metallic nanoparticles are considered the basis for designing new photonic media in optoelectronic and nonlinear optics [1,2]. In particular, silver nanoparticles (Ag-NPs) embedded in various dielectric materials such as MgO and SiO2 have generated considerable interest due to their strong coupling to incident electromagnetic fields [3] and nonlinear optical properties [4,5]. By means of second harmonic generation [4] and z-scan [5] experiments, respectively, the second- and third-order nonlinearities of both randomly positioned spherical and elongated (but aligned in a preferred direction) Ag-NPs embedded in silica have been recently studied in our group. For the case of the elongated Ag-NPs, the linear and nonlinear optical properties strongly depend on the nanoparticle geometry, size, distribution, and with the incident light polarization as well; making this material attractive for potential linear and nonlinear optical waveguides fabrication. A reliable method for elongated Ag-NPs fabrication is metal ion implantation in dielectric matrices allowing good control of the NPs size, concentration and implantation depth [6,7]. Despite the fact that the method has been useful for preparing thin layers of NPs embedded in silica, fabricating waveguide patterns is still a challenge. An alternate and easier possibility for producing waveguide patterns is through direct writing using femtosecond (fs) pulses via a nonlinear microscope system. ⇑ Corresponding author. Tel.: +52 6461750500; fax: +52 6161750553. E-mail address: [email protected] (I. Rocha-Mendoza). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.11.012

This approach has been already used for waveguide fabrication and characterization in second order nonlinear materials such as crystalline oxide substrates like lithium niobate [8], and lithium tantalate [9] but the energies utilized are still rather high, in the order of hundreds of nJ employing the use of costly fs amplifier systems. In addition, fs pulses at the kHz repetition rate have been used to produce embedded Ag clusters in photosensitive glass Foturan for 3D microstructure fabrication [10–12]. In this work, we use a nonlinear microscope system to write and characterize waveguide-like patterns in samples containing randomly placed but highly aligned elongated Ag-NPs embedded in glass using fs pulses with energies less than a couple of nJ easily obtained with our oscillator system, avoiding in this way structural modification of the bulk material due to laser induced heating processes as is commonly reported in literature for waveguide fabrication [13–15]. With our nonlinear microscope system different waveguiding photonic structures patterns and even Bragg gratings could be written and monitored as well. 2. Materials and methods 2.1. Sample preparation The double implantation procedure to prepare the samples with elongated metallic NPs is described elsewhere [5,7]. First, 2 MeV Ag2+ ions (of 2:4  1017 Ag/cm2 of fluence) are implanted into host matrices of high-purity silica glass at room temperature. The samples are thermally annealed afterwards at 600  C in a

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50%N2 + 50%H2 reducing atmosphere. This implanting and subsequent annealing, results in an embedded film containing spherical-like Ag-NPs of around 6 nm in diameter. Then a second ion implantation process is performed irradiating the sample at an angle of 45 with respect to its surface normal, with 8 MeV Si ions, and around 1  1016 Si/cm2 of fluence. The end result is a 0:5 lm thick layer, at 1 lm depth inside the silica matrix, containing randomly placed elongated NPs aligned in the direction of the second ion implantation process. The elongated Ag-NPs are actually shaped as prolate spheroids, with an average minor axis diameter of 5 nm, and an aspect ratio of 1:7.

Ag-NPs long axes point in the direction we label as x. The laser linear polarization is also set in the x direction for optimum SHG signal [4]. The signal is forward collected using the microscope condenser and a second lens in a telescope arrangement to de-scan the laser beam. The signal is filtered using a pass band filter (FF02 – 435/40; Semrock) and detected using a photomultiplier tube, PMT (R2557; Hamamatsu), and a preamplifier module (70710; Oriel). The raster scanning and signal detection are synchronized by means of a data acquisition card (BNC2110; National Instruments) and an interface program written in NI LabWindows™/CVI using a personal computer. 2.3. Laser writing

2.2. Second harmonic generation microscopy We perform second harmonic generation (SHG) microscopy using a custom made laser scanning optical system, as described elsewhere [16–18]. The system consists on a Titanium–Sapphire (NJA4; Clark:MRX) laser oscillator, pumped with a 5 W laser at 532 nm (Millennia Vs; Spectra Physics), delivering 830 nm ultrashort 88 fs pulses at a 94 MHz repetition rate, and 350 mW of average output power which is controlled using a half-wave plate and a polarizer. The output beam is expanded and collimated to fill the area of two galvanometric scan mirrors (6230H, Cambridge Technology). A telescope arrangement is used afterwards to couple the beam to an inverted microscope (Eclipse TE2000U; Nikon) in order that the beam fills the back aperture of the objective lens. The scan mirrors are closely scanned to optimize telecentricity and to pivot the laser-beam in the objective back aperture. For this experiment a conventional 10 (NA = 0:30) microscope objective is used to focus the fs pulses at the sample which is mounted on the two-dimensional motion microscope stage. The ratio between the beam diameter (DBeam) and the objective back-aperture diameter (DBA) at 1/e2 is DBeam=DBA ¼ 0:31, resulting in computed lateral and axial linear resolutions of approximately 2:1 and 34 lm, respectively [18]. The measured lateral resolution of our system is 3:4 lm (then a radius of 1:7 lm) which is used here to estimate the fluence. Fig. 1 depicts only the excitation and collection beams of the nonlinear microscope, where the orientation of the NPs is shown in the laboratory coordinate xyz system. The NPs long axes are tilted 45 with respect to the substrate normal and lay on the xz plane. When viewed from the front, the projection of the elongated

The laser writing of patterns over the Ag-NPs is performed in three straightforward steps. Firstly, the plane of the embedded composite is tracked measuring the SHG signal originated in the Ag-NPs. The signal is maximized while imaging the sample by rotating the laser polarization and adjusting the focus. Imaging is performed using an average pulse energy of 0.37 nJ to avoid damaging the sample as will be shown later. Secondly, line patterns are written using higher average pulse energies to damage the NPs composite; the energy per pulse employed is 1.6 nJ (fluence of 14:6 mJ/cm2). Writing is performed either by line scanning the beam over the sample, using the galvanometric mirrors or, for a longer scale (millimeters), by manually translating back and forth the sample over the laser focal spot moving the microscope stage. Finally, the written patterns are evaluated by means of SHG imaging as in the first step. 3. Results and discussion Fig. 2a shows the plasmonic absorption spectra of our samples using linearly polarized light at the two mutually orthogonal s- and p-polarizations as taken in [4]; the form of the spectra depends on the light polarization, and the angle of incidence. The absorption spectra is plotted measuring the transmitted light over the sample and using the definition aL ¼ lnð1=TÞ [19]; where L is the sample

wavelength [nm] 3

400 500 600 700 800 900

αL

2

1 polarization p s

normalized intensity

(a)

1

(c)

s

(b)

p y x 250μm

(d)

p

250μm

Fig. 1. Experimental layout showing the exciting/writing beam and SHG signal collection over elongated Ag-NPs.

Fig. 2. (a) Plasmon absorption spectra of elongated Ag-NPs with s-(red) and p-polarized (black) light. The laser (dotted) and shg (dashed) spectra are also illustrated. (b) SHG image with p-polarized excitation. Transmission images with s- (c) and p-polarized light (d), respectively. The NPs long axis projection yields along the x-axis direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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signal [arb. un.]

104

(a) m=2 m=3 1 scan 2 scans 5 scans 100

101

signal [arb. un.]

2

2.2 mJ/cm 2 4.7 mJ/cm 2 11.7 mJ/cm

(b)

10k 5k 0 -90

3

2

x

100 μm

0

90

180

270

polarization [deg] Fig. 4. Polar dependance of the SHG signal taken after the laser writing process at different fluences, dwell time and scan repetitions. The inset shows a SHG image taken at 2 mJ/cm2 showing different irradiated zones labeled as follows: No laser damage induced (1); laser writing at 4:7 mJ/cm2 fluence, 1 ms/lm dewell time and one scan repetition (2); and laser writing at 4:7 mJ/cm2 fluence, 1 ms/lm dewell time and five scan repetition (3); laser damage at the maximum energy fluence in our system 14:6 mJ/cm2 until SHG signal disappearance (4).

pulses 20k

0

100k 2

2.2 mJ/cm 2 4.7 mJ/cm 2 11.7 mJ/cm

15k

10k

5k

0 0.0

0.5

1.0

1.5

Fig. 5. PMT signal as function of the dwell time per micron at different fluences as used in Fig. 3. Open squares, circles and triangles indicate one, two and five scan repetitions, respectively.

fluence [mJ/cm2]

15k

y

-90

1 2 3 4

4

dwell time/microns [ms/μm]

103

102

1

1000

shg [arb. un.]

width (in our case 1 mm) and T is the transmittance of the sample. At normal incidence, a single plasmon resonance at around 365 nm is obtained for s-polarization (red) and is associated with the NPs size along the short-axis, while two spectrally separated resonances are obtained for p-polarization (black). It is well know that for elongated NPs, the surface plasmon resonance splits into two [20]. The wide resonance at 570 nm is due to the broad distribution of NPs elongation lengths. The 365 nm resonance is also present and is attributed to the actual fraction of remanent spherical NPs. The IR laser spectrum is also shown in Fig. 2a, where it is clearly seen that there is significant absorption for p-polarization (along the NPs long axis). Fig. 2b shows the SHG image taken before laser writing over the sample in order to identify the NPs plane. After the writing process, transmission images are taken under the same focusing conditions in the microscope by polarizing the light from the halogen lamp. Note that when the transmission image is taken under s-polarized light (see Fig. 2c), neither the modified zone nor the rest of the composite absorbs light and no damage is appreciated in the sample. However, when p-polarized light is used (see Fig. 2d) the modified region becomes visible due to the bleaching of the plasmonic absorption of the composite; having higher transmission in the written channel zone means that an evident structural modification of the NPs composite has occurred. To investigate the laser pulse energy required to modify the Ag-NPs, the PMT signal was studied as a function of the laser fluence, polarization and scan velocity. The results are shown in Figs. 3–5, where each data point corresponds to the averaged PMT signal of a single scanned image of around 250 lm2. Five consecutive images are taken in the same area at a fixed pulse energy before moving the laser to scan the adjacent area and increasing the laser energy. The laser energy was increased from 0:1 to 1:5 nJ. In Figs. 3 and 5, open squares, open circles and open triangles, stand for the averaged PMT signal after one, two and five scan repetitions, respectively. The different slopes traced in Fig. 3a are meant to indicate that two different physical processes occur in the Ag-NPs as the input laser fluence is increased. Below 2 mJ/cm2 the signal is adjusted to a quadratic slope corresponding to the second order nonlinear optical process originated in the

signal [arb. un.]

684

-45

0

45

90

polarization [deg] Fig. 3. PMT signal as function of the laser fluence (a) and polarization angle (b). Arrows in (a) indicate the fluences used in figures (b), as labeled. Open squares, circles and triangles indicate one, two and five scan repetitions, respectively.

composite. The origin of the optical second order process in the same Ag-NPs composites was demonstrated experimentally and modeled earlier in [4], and will not be further discussed here; in short the macroscopic second-order susceptibility of the composite layer is originated from the coherent summation of the cylindrically symmetric hyperpolarizabilities associated to each NP. For laser fluences between 2 and 3:5 mJ/cm2 the slope of the signal increases indicating that other physical processes starts taking place. This change of slope seems to signal the onset of the laser damage process to the NPs. The perceived increase in slope is probably due to luminescence from sparks or even plasma emission observable by the naked eye, which is not filtered out completely by the microscope system. For laser fluences higher than 3:5 mJ/cm2 a signal decay is clearly observed for subsequent scan repetitions, and at 6 mJ/cm2 such signal bleaching is even more evident, we associate this signal decay to a considerable NPs removal. Otherwise, in Fig. 3b is shown that regardless of the fluence deposited in the composite the signal depends on the input laser polarization, reaching its maximum when the laser is p-polarized, i.e. polarization parallel to the NPs long axis. This indicates that the damage

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process is related with the plasmonic absorption of elongated Ag-NPs. Laser damage of embedded spherical Ag-NPs has been reported before numerically and experimentally using ns pulses [21]. In that work, laser induced heating of the NPs up to the point of dissolution was demonstrated to be the mechanism responsible for the composite damage, taking place for 308 nm pulses from an excimer laser with 55 ns duration. In such case, the Ag-NPs were responsible for the laser absorption due to remaining plasmon resonance at 400 nm and the subsequent dissolution took place when the NPs melting temperature was reached. In contrast, we are using IR fs pulses in our experiments, i.e. a pulse length shorter than the thermalization time of the NPs which is in the picosecond range [22,23] and the pulse energy is too low to generate measurable temperature increases. The use of a high repetition rate (100 MHz) pulse train, can however produce a significant cumulative temperature change and the melting temperature of Ag-NPs is lower than that of either bulk silver, or silica, being in the order of 400–600  C for Ag-NPs with 2–3 nm radius, respectively [24]. For such a high repetition pulse train, energy absorption will be counteracted by thermal diffusion to reach a steady state. For the present conditions, a thermal induced redissolution process for the damage of the Ag-NPs cannot be ruled out, and could be probably present to a certain extent. However, the high peak irradiance values employed in the experiments, up to Ipeak ¼ 200 GW=cm2 , imply that nonlinear multiphoton absorption processes are also present. The observation of white light luminescence in our experiments, at the highest input irradiances, indicates the formation of plasma produced by such multiphoton processes. Since these processes happen in the duration of a single pulse, they can take place before thermal dissolution sets in. In addition, the strong polarization dependence observed in Fig. 3b, suggests that the damage could be triggered by the IR plasmonic resonance of the elongated NPs. Since there is no significant absorption in the IR for the spherical NPs [20], no damage could be induced, at least up to the highest pulse energy of our fs oscillator. However, we do not preclude the possibility of achieving damage at higher fluences for the spherical NPs. Note that to observe damage in the samples with elongated NPs using UV ns pulses, considerably higher input fluences, implying higher thermal loads, are required [21] than the used here with IR fs pulses. Therefore, what we have is an irradiance dependent multi-photon absorption process. In fact, a multiphoton process using fs pulses to reduce silver ions Ag+ to Ag clusters embedded in Foturan glass (a doped glass with silver, cerium and antimony) has been used by Masuda et al. [12] to generate 3D microstructures. In this sense, it is possible that the inverse process takes place where the Ag-NPs are reduced to Ag+, consistently with the ns redissolution studies [21]. Other possible explanation for the NPs damage could be Coulomb fragmentation, as previously discussed in [25], however, in order to see such effects, higher excitation energies (two orders of magnitude higher than the one used here) are needed to have a significant amount of electrons ejected from the nanoparticles, creating in that way highly charged particles and inducing the so called Coulomb explosion. Regarding as to what happens to the NPs under such strong irradiation, it is worth mentioning that since the NPs containing layer is buried in the silica host, surface ablation is not possible, and indeed is not observed. Whether by thermal or multiphoton absorption processes, the end result is most likely to be silver redissolution into the glass matrix, resulting in the disappearance of the NPs. This is supported by the fact that the SHG signal decreases considerably in the irradiated zones and possibly would go to zero as the writing energy is increased above 1:6 nJ, the maximum energy used here, or the irradiation exposure time (at the maximum energy) is left in the sample until SHG disappearance. This is confirmed in Fig. 4, where it is observed that the laser polarization dependence of the SHG signals

remain anisotropic after the laser writing process at different fluences. Since the polar dependence of the SHG signal for spherical Ag-NPs is isotropic [4], we therefore assume that no break-up of large elongated NPs into smaller, more spherical Ag-NPs takes place post laser writing process. The influence of the laser scan speed in the damage process is shown in Fig. 5 where the signal is plotted as a function of the dwell time per micron (inverse of the scan speed). For an irradiation fluence of 2:2 mJ/cm2 no significant signal decay is observed after five scan repetitions, indicating that no damage is induced in the NPs within a scanning speed range from 0:8 to 1:6 mm/s (1:3 to 0:6 ms/lm, respectively). However, for a fluence of 4:7 mJ/cm2 and at 0:3 ms/lm (3:3 mm/s), a signal decay can be observed for subsequent scan repetitions indicating that either a morphological change or dissolution of some NPs start taking place. The initial increase in the signal with respect to the dwell time indicates a possible SHG signal contribution from the remaining NPs in the composite. For 11:7 mJ/cm2 the signal is no longer reproducible after a single scan. According to Crespo-Sosa et al. [21], ten pulses of 55 ns at 2 J/cm2 are required to start inducing NPs melting, in our experiments, the laser fluences are three orders of magnitude below and therefore longer laser dwell times per micron are required so that more pulses contribute collectively to induce damage in the NPs. From Figs. 3 and 5, it can be seen that for fluences above 4:7 J/cm2 and 0:3 ms=lm (28  103 pulses/lm), the signal decay is evident and we believe these are the conditions where the NPs damage starts taking place. We stress out that both the low energy fluence deposited in the composite and the scan speed used to write the patterns avoid glass damage due to heat accumulation processes [26]. Finally, the feasibility of the writing/monitoring technique here presented is demonstrated in Fig. 6. A set of five images are taken before (a) and after (b) the writing process, respectively with the aim of displaying the waveguide-like patterns written across the whole 5 mm long sample. In Fig. 6c a close up of the fifth image in Fig. 6b (dashed square) shows the well-defined written channels. A 5 lm width channel of Ag-NPs is left between the two parallel written channels, each around 9 lm wide. The channel width can be easily controlled at the sub-micron scale by laterally shifting the laser beam using the scanning system, being limited to a width dictated by the lateral resolution of the system. Fig. 6d

(a)

(b)

(c)

(d)

Fig. 6. SHG imaging before (a) and after (b) fs laser writing. (c) Zoom of (b) showing the waveguide-like pattern. (d) SHG signal profile of traced line in figure (c).

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shows the SHG signal profile of a single line selected in Fig. 6c; it can be seen that the SHG signal in the irradiated area decays significantly indicating that the composite material has been modified. The NPs implanted region has a considerably higher refractive index than the host (Dn  102 Þ, so waveguiding through the highly nonlinear NPs implanted material is possible. Structural changes like the ones presented here by fs pulse irradiation are therefore a good candidate for direct writing of waveguiding structures, and of other photonic structures such as Bragg gratings for example. In general, with our nonlinear microscope system photonics structures patterns can be written and monitored at the same time. 4. Conclusions In conclusion, we have induced and monitored structural modification of elongated Ag-NPs films embedded in silica using ultrafast fs laser irradiation and second harmonic generation imaging, respectively. Also, we have performed a study of the NPs damage process as a function of the laser polarization, energy, and scan velocity. The damage process is related to the characteristic plasmonic absorption of composites containing elongated Ag-NPs, and start taking place at a scan speed of 3:3 mm/s and an energy fluences above 4:7 mJ/cm2. Other complementary studies, like Raman spectroscopy and SEM imaging should be taken into account in order to have a better understanding about the NPs structural change. The technique is in any case, a good candidate to generate linear and nonlinear optical waveguiding devices, in a versatile fashion. Acknowledgements The authors acknowledge financial support from CONACyT through Grants 155803 and 102937, and valuable technical assistance from Dr. Marcos Plata Sánchez. References [1] P. Chakraborty, Metal nanoclusters in glasses as non-linear photonic materials, Mater. Sci. 33 (9) (1988) 2235–2249. [2] I. Matsui, Nanoparticles for electronic device applications: a brief review, JCEJ 38 (8) (2005) 535–546. [3] A.L. Stepanov, Synthesis of silver nanoparticles in dielectric matrix by ion implantation: a review, Rev. Adv. Mater. Sci. 26 (1/2) (2010) 1–29. [4] I. Rocha-Mendoza, R. Rangel-Rojo, L. Rodríguez-Fernández, A. OliverA, Secondorder nonlinear response of composites containing aligned elongated silver nanoparticles, Opt. Express 19 (22) (2011) 21575–21587. [5] R. Rangel-Rojo, J. McCarthy, H.T. Bookey, A.K. Kar, L. Rodríguez-Fernández, J.C. Cheang-Wong, A. Crespo-Sosa, A. López-Suarez, A. Oliver, V. RodríguezIglesias, H.G. Silva-Pereyra, Anisotropy in the nonlinear absorption of elongated silver nanoparticles in silica, probed by femtosecond pulses, Opt. Commun. 282 (2009) 1909–1912.

[6] S. Vilayurganapathy, A. Devaraj, R. Colby, A. Pandey, T. Varga, V. Shutthanandan, S. Manandhar, P.Z. El-Khoury, A. Kayani, W.P. Hess, S. Theyuthasan, Subsurface synthesis and characterization of Ag nanoparticles embedded in MgO, Nanotechnology 24 (9) (2013) 095707. [7] A. Oliver, J.A. Reyes-Esqueda, J.C. Cheang-Wong, C.E. Román-Velázquez, A. Crespo-Sosa, L. Rodríguez-Fernández, J.A. Seman, C. Noguez, Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation, Phys. Rev. B 74 (24) (2006) 245425. [8] R.R. Thomson, S. Campbell, I.J. Blewett, A.K. Kar, D.T. Reid, Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime, Appl. Phys. Lett. 88 (2006) 111109. [9] B. McMillen, K.P. Chen, H. An, S. Fleming, V. Hartwell, D. Snoke, Waveguiding and nonlinear optical properties of three-dimensional waveguides in LiTaO3 written by high-repetition rate ultrafast laser, Appl. Phys. Lett. 93 (2008) 111106. [10] S. Ho, P.R. Herman, Y. Cheng, K. Sugioka, K.Midorikawa, Direct ultrafast laser writing of buried waveguides in Foturan glass, OSA/CLEO, 2004. CThD6. [11] Y. Cheng, K. Sugioka, M. Masuda, K. Shihoyama, K. Toyoda, K. Midorikawa, Optical gratings embedded in photosensitive glass by photochemical reaction using a femtosecond laser, Opt. Express 11:15 (2003) 1809–1816. [12] M. Masuda, K. Sugioka, Y. Cheng, N. Aoki, M. Kawachi, K. Toyoda, H. Helvajian, K. Midorikawa, 3-D microstructuring inside photosensitive glass by femtosecond laser excitation, Appl. Phys. A 76 (2003) 857–860. [13] K. Miura, J.R. Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Photowritten optical waveguides in various glasses with ultrashort pulse laser, Appl. Phys. Lett. 71 (1997) 3329–3331. [14] R.R. Gattass, E. Mazur, Femtosecond laser micromachining in transparent materials, Nat. Photon. 2 (4) (2008) 219–225. [15] C.B. Schaffer, A. Brodeur, J.F. García, E. Mazur, Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy, Opt. Lett. 26 (2) (2001) 93–95. [16] P.J. Campagnola, M.D. Wei, A. Lewis, L.M. Loew, High resolution non-linear optical microscopy of living cells by second harmonic generation, Biophys. J. 77 (1999) 3341–3349. [17] J. Squier, M. Müller, High resolution nonlinear microscopy: a review of sources and methods for achieving optimal imaging, Rev. Sci. Instrum. 72 (7) (2001) 2855–2867. [18] F. Helmchen, W. Denk, Deep tissue two-photon microscopy, Nat. Methods 2 (12) (2005) 932–940. [19] M. Born, E. Wolf, Principles of Optics, seventh ed., Cambridge University Press, Cambridge, UK, 1999. [20] A.L. González, J.A. Reyes-Esqueda, C. Noguez, Optical properties of elongated noble metal nanopaticles, J. Phys. Chem. C 112 (19) (2008) 7356–7362. [21] A. Crespo-Sosa, P. Schaaf, J.A. Reyes-Esqueda, J.A. Seman-Harutinian, A. Oliver, Excimer laser absorption by metallic nano-particles embedded in silica, J. Phys. D: Appl. Phys. 40 (2007) 1890–1895. [22] C. Voisin, N. Del Fatti, D. Christofilos, F. Vallée, Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles, J. Phys. Chem. B 105 (12) (2001) 2264–2280. [23] S. Stagira, M. Nisoli, S. De Silvestri, A. Stella, P. Tognini, P. Cheyssac, R. Kofman, Ultrafast optical relaxation dynamics in metallic nanoparticles: from bulk-like toward spatial confinement regime, Chem. Phys. 251 (2000) 259–267. [24] R. Kofman, P. Cheyssac, A. Aouaj, Y. Lereah, G. Deutscher, T. Ben-David, J.M. Penisson, A. Bourret, Surface melting enhanced by curvature effects, Surf. Sci. 303 (1994) 231–246. [25] F. Hubenthal, M. Alschinger, M. Bauer, D. Blázquez Sánchez, N. Borg, M. Brezeanu, R. Frese, C. Hendrich, B. Krohn, M. Aeschlimann, F. Träger, Irradiation of supported gold and silver nanoparticles with continuous-wave, nanosecond and femtosecond laser light:a comparative study, in: SPIE Proc., 2005, pp. 224– 235, doi: 10.1117/12.608560.5838. [26] S.M. Eaton, H. Zhang, M.L. Ng, J. Li, W.J. Chen, S. Ho, P.R. Herman, Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides, Opt. Express 16 (13) (2008) 9443–9458.