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Fabry-perot modes enhanced pump-probe coupling in gold micro-disk patterned ruby thin film
Optical Materials 72 (2017) 375e379
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabry-perot modes enhanced pump-probe coupling in gold microdisk patterned ruby thin film Satchi Kumari a, *, Alika Khare b, Reema Gupta c, Monika Tomar c, Vinay Gupta c a
Centre for Applied Physics, Central University of Jharkhand, Ranchi, 835205, India Indian Institute of Technology, Guwahati, 781039, India c Department of Physics & Astrophysics, University of Delhi, 110007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2016 Received in revised form 4 June 2017 Accepted 7 June 2017
Enhanced pump-probe coupling has been experimentally observed in epitaxial Ruby thin film patterned with equidistant gold micro-disks (~100 mm), as compared to pure Ruby film. This has been attributed to Fabry-Perot and surface plasmon modes in Ruby/gold film. In case of S polarized pump and probe beam, Fabry-Perot modes leads to a two-wave mixing gain of ~1.35. Moreover gain was ~1.62 in P-polarization case, it has been attributed to coupling of Fabry-Perot and surface plasmon modes. Enhanced coupling for P-polarization can lead to improved nonlinear response in the thin film geometry. It can find applications in thin film based compact photonic devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thin-film Fabry-perot Polarization Surface plasmon
1. Introduction Ruby is a well-known optical material for nonlinear optics [1e4]. Its long fluorescence life time (~3 ms) and high quantum efficiency makes it suitable for generation of various nonlinear optical (NLO) processes. It has been known to exhibit many interesting NLO phenomenon viz. self-phase modulation, hole burning, saturable absorption, degenerate and non-degenerate two-wave mixing, optical delay (slow & fast light) [5e11]. These nonlinear processes has been primarily studied in case of bulk Ruby crystals. The thin film of Ruby exhibiting above mentioned properties can find application in the form of compact photonic devices for variable optical delay lines, optical data storage, all optical signal processing, high precession spectroscopy and non-linear optics. Optical quality thin film of Ruby (~3.5 mm) has been successfully grown on sapphire substrate using Pulsed laser deposition (PLD) technique [12e14]. Since sapphire epitaxially matches with Ruby, the grown film was found to be highly C-axis oriented [12e15]. Moreover sapphire has excellent optical properties making it suitable candidate for photonic applications. The open and closed Z scan measurement of non-linear coefficients has confirmed
Abbreviations: NLO, Non-linear optical; SP, Surface plasmon. * Corresponding author. E-mail address: [email protected] (S. Kumari). http://dx.doi.org/10.1016/j.optmat.2017.06.015 0925-3467/© 2017 Elsevier B.V. All rights reserved.
nonlinear behavior of epitaxial Ruby film [14]. The value of nonlinear refractive index (n2 ~ 3.1 105 m2/W) for Ruby thin film is three orders of magnitude higher compared to crystal [4,12]. The optical delay measurements of Ruby thin film using pump-probe technique [12] shows a delay of ~12 ns [12]. The observed delay (~few nanoseconds) has been much lower as compared to millisecond delay in case of Ruby crystal due to the poor coupling of pump and probe beams inside the thin film geometry [6e8]. This poor coupling results from smaller interaction length in case of thin film geometry in comparison to Ruby crystal/rod. Hence, for achieving significant optical delay the pump-probe coupling inside thin film geometry should be enhanced. This issue has been addressed in the present manuscript. The light matter interaction inside the thin film geometry is enhanced via Fabry-Perot and surface plasmon (SP) modes. Fabry-Perot cavities allow efficient storage of electromagnetic energy [16e22]. Confinement of light in small mode for sufficiently large time leads to strong light matter interactions and can help to enhance weak physical processes [16]. Further, SP modes like Fabry-Perot modes also allows strong coupling and enhances various nonlinear processes viz. SHG, fluorescence, and surface enhanced Raman scattering. The Fabry-Perot modes and SP modes can be supported by 1D grating structures [19]. Keeping in view the possibility of enhancing light matter interaction using engineered nanostructures, in the present work we have explored pump-probe
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S. Kumari et al. / Optical Materials 72 (2017) 375e379
coupling in gold micro-disk patterned Ruby film. 2. Experimental details Epitaxial Ruby thin film of thickness ~3.5 mm was grown on sapphire substrate using pulsed laser deposition technique [13,14]. The Gold micro-disks of thickness ~40 nm and diameter ~100 mm were patterned onto the Ruby film using thermal evaporation technique and shadow masking. The distance between consecutive dots was ~200 mm as shown in optical micrograph, Fig. 1(a). Fig. 1(b) shows the schematic of the cross-section of sample along with incident rays and reflected rays giving rise to interference pattern. The schematic of pump-probe set-up used to study coupling in Ruby and Ruby/gold film is shown in Fig. 1(c). Pump and probe beams were launched onto the Ruby & Ruby/gold film at a beam crossing angle of 31 as shown in Fig. 1(b). Diode pumped solid state laser (DPSSL) having wavelength 532 nm was used to derive both pump and probe beam. Excitation wavelength of 532 nm was selected keeping in view green U-band absorption of Ruby [12]. Gold micro-disk was selected over other nobel metals like silver and aluminum due to its higher stability against oxidation. Although gold shows inter-band transitions in the green wavelength range causing some absorption losses, but shows reflectivity of 59.4% enabling coupling of localized plasmon modes to metal disk arrays [23]. Polarizing cube beam splitter was used to separate the S and P polarized components. The polarization of laser beam was changed by changing the orientation of the beam splitter. The angle of incidence (q) between probe beam and the normal to the surface is varied by rotating the sample using goniometer. 3. Results and discussion Fig. 2(a) shows the absorption spectra of pure Ruby and Ruby/ gold film. The absorption of patterned Ruby thin film was found to be large as compared to pure Ruby film. The enhanced absorption can be attributed to surface plasmon (SP) modes. The SP modes allow the absorption of more number of photons in the Ruby/gold film. We see change in the absorption spectra but not as a sharp peak. Such broad resonant absorption peak arises from large
Fig. 2. (a) Absorption and (b) photoluminescence spectra of Ruby and Ruby/gold sample.
diameter of nano disk. Absorption peaks has been observed to broaden with increasing diameter for aluminum metal disks by
Fig. 1. (a) Optical micrograph of Ruby/Gold film. (b) Schematic of Pump-Probe studies in Ruby/gold film. (c) Experimental set-up.
S. Kumari et al. / Optical Materials 72 (2017) 375e379
Naomi halas and group [24]. Fig. 2(b) shows the corresponding photoluminescence from Ruby and Ruby gold film. The Ruby film shows the characteristic R-line fluorescence along with vibronic side bands [13]. The R-line fluorescence (observed in the wavelength range 690e695 nm) has been quenched in case of Ruby/gold film and vibronic side bands have been enhanced, see inset of Fig. 2(b). This shows that the non-radiative relaxation process is
Fig. 3. Transmission from (a) pure Ruby thin film and (b) Ruby/gold thin film in pump on and off conditions for S-polarization (c) Gain in case of Ruby and Ruby/gold thin film for S-polarization.
377
dominant in Ruby/gold film. Further, confirming the enhanced absorption of radiation inside the Ruby/gold film (see Fig. 2(a)). Fig. 3(a) shows the transmission from pure Ruby film and Fig. 3(b) from Ruby/gold film in pump on and off conditions for S-polarization as a function of film orientation (q). In case of pure Ruby film no systematic variation was observed w. r. t. the incidence angle as shown in Fig. 3(a). While film patterned with metal disk array shows modes in the transmission spectra as shown in Fig. 3(b). The gain in pure Ruby film and Ruby/gold film for S-polarization is shown in Fig. 3(c). It was estimated from the ratio of transmittance in pump off & pump on condition as reported earlier [25]. Gain was found to be large in case of patterned Ruby film as compared to pure Ruby as shown in Fig. 3(c) and maximum gain was observed at incidence angle of ~31. Under the same experimental condition Fabry-Perot modes were generated in the Ruby/gold film as compared to bare Ruby film. Fabry-perot modes are resonance modes generated due to interference of light multiply reflected from two reflecting surfaces such as gold and Ruby/Sapphire/air interface in our case. The observed modes were broad and almost identical in pump on & pump off conditions. In pump on condition it shows shift and amplification. This shows enhanced coupling due to combined effect of metal disk induced Fabry-Perot modes. Further, we performed measurement with changed polarization to distinguish between the roles of Fabry-Perot modes and SP modes. Fig. 4(a) and (b) shows the transmission spectra for Ppolarized light for Ruby and Ruby/gold film respectively. Similar to the case of S-polarization no modes were observed in pure Ruby case. In case of P-polarized light, the transmission spectra of patterned Ruby film shows sharp and more number of modes as compared to S-polarization light. These modes are formed due to combined effect of SP and Fabry-Perot interferes in case of P-polarization. Surface plasmons gives rise to slanted background while difference in the boundary conditions for S-polarized and Ppolarized light (because at metal interface the electric field component has to be zero due to finite penetration depth of electromagnetic waves) leads to observed difference in number of modes and their sharpness for the two polarization states. The modes were identical in both the pump on and pump off conditions. The modes in P-polarization case are sharp as compared to Spolarization and there is significant effect of pump on the shift and broadening of these modes as compared to S-polarization, Figs. 3(b) and 4(b). Further the decreased intensity also confirmed second player in case of P-polarized measurements. The gain in pure Ruby film and Ruby/gold film for P-polarization is shown in Fig. 4(c). It was estimated from the ratio of transmittance in pump off & pump on condition. For the P-polarized light as well, the gain was found to be large in case of patterned Ruby film as compared to pure Ruby as shown in Fig. 4(c). P-polarized light showed higher gain as compared to S-polarized light and it was maximum at an incidence angle of ~31. This enhanced coupling in Ruby/gold film in case of Ppolarization is accounted to combined effect of Fabry-Perot and surface plasmon induced modes in the Ruby/gold film. The cavity modes present in the Ruby/gold film traps the electromagnetic energy for sufficiently long time as compared to pure Ruby film. This multiple reflection enhances the interaction process and thus the energy transfer from pump to probe increases significantly. Further this process is more dominant for P-polarized light as compared to S-polarization. Fig. 5(a) and (b) shows the fitting of observed variation in transmission with film orientation i.e incident angle for S-polarized light and P-polarized light respectively in pump on condition. For the Fabry perot interference the variation in transmitted in1 tensity is expressed as IIo ¼ , Where d ¼ 4lp h cos qr ¼ 2 1þF sin d=2
K cos qr ;
K ¼ 4lp h .qr is the angle of refraction (in radians) and is
378
S. Kumari et al. / Optical Materials 72 (2017) 375e379
Fig. 5. Fitting of experimental curves in the Pump on Conditions for Ruby/Gold film.
propagation and coupling losses in beam path. F is known as coefficient of fineness and is expressed in terms of reflectivity (R) as F ¼ 4R 2 . The positive and negative portions of the pattern was ð1RÞ
Fig. 4. Transmission from (a) pure ruby thin film and (b) ruby/gold thin film in pump on and off conditions for P-polarization. (c) Gain in case of ruby and ruby/gold thin film for P-polarization.
related to angle of incidence ðfÞ by a constant a using Snell's law (n1 sin f ¼ n2 sin qr ). The experimentally observed transmission 1 pattern was fitted with the equation I ¼ A þ mf. 2 1þF sin ðafÞ
Where A is constant taking account of detector efficiency,
fitted separately due to difference in background and obtained average values are reported. The fitting for S-polarized light requires A ¼ 36.1, F ¼ 0.277, a ¼ e11.16 and slope m ¼ e8.3. Fitting for P-polarized light gives A ¼ 25.53, F ¼ e0.229, a ¼ 17.24 with m ¼ 19.8. In case of P-polarized light, the transmitted light follows closely the Fabry perot interference pattern along with a slanting background arising from surface plasmon modes; confirming origin of oscillating intensity from fabry-perot modes. From continuity of vector components of electric and magnetic fields at the single interface, we know that SPP only exist for the P-polarized light (TM polarization) [26]. In case of S-polarized light as well, the variation of transmitted intensity with angle of incidence also follow the expression of fabry-perot interference with nearly flat background with angle variation due to absence of any SPP modes in case of Spolarized light. This confirms that the multiple reflections experienced by light trapped between gold disks and substrate in between film results into enhanced pump-probe energy transfer i.e increased interaction length. Additionally, the light confined as plasmonic modes also contributes towards observed increased interaction length in case of P-polarization resulting into observed high gain as compared to S-polarized light.
S. Kumari et al. / Optical Materials 72 (2017) 375e379
4. Conclusion In summary, enhanced pump-probe coupling has been demonstrated in gold micro-disks patterned epitaxial Ruby thin film. The two-wave coupling gain has been studied for S and P polarization of laser beam. In case of P-polarized light, the FabryPerot modes along with surface SPP modes accounts for observed enhanced energy transfer in pump-probe process. While, in case of S-polarized light Fabry-Perot alone plays towards energy transfer during pump-probe process. We have shown that by patterning metal micro-disk arrays on Ruby films the energy transfer in pumpprobe process can be enhanced through Fabry-perot and SPP modes. Such a novel approach is highly useful to enhance the lightmatter interaction for various nonlinear device applications on a chip operating at lower pump power. Acknowledgements This work was supported in part by Department of science and technology Govt. of India IFA12-PH-48 (INSPIRE). The corresponding author would like to thank Prof. R. P. Singh (PRL, Ahmedabad) for fruitful discussion and suggestions towards the improvement of manuscript. References [1] I. McMicheal, P. Yeh, P. Beckwith, Nondegenerate two-wave mixing in ruby, Opt. Lett. 13 (1988) 500. [2] H.K. Lee, S.S. Lee, Measurements of the anisotropic nonlinear refractive-index coefficients of ruby, Opt. Lett. 15 (1990) 54. [3] S.A. Boothroyd, J. Chrostowski, M.S. O'Sullivan, Two-wave mixing by phase and absorption gratings in saturable absorbers, J. Opt. Soc. Am. B 6 (1989) 766. [4] L.C. Oliveira, S.C. Zilio, Single-beam time resolved Z-scan measurement of slow absorbers, Appl. Phys. Lett. 65 (1994) 2121. [5] H. Riesen, A.K. Rebane, A. Szabo, I. Carceller, Slowing light down by low magnetic fields: pulse delay by transient spectral hole-burning in ruby, Opt. Exp. 20 (2012) 19039.
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[6] L. Cerdan, R. Weigand, J.M.G. Perez, Am. J. Phys. 76 (2008) 826. [7] F. Gao, J. Xu, G. Zhang, F. Bo, H. Liu, Paraxial energy transport of a focused Gaussian beam in ruby with nondegenerate two-wave couplinglike mechanism, Appl. Phys. Lett. 92 (2008) 021121. [8] M.S. Bigelow, N.N. Lepeshkin, R.W. Boyd, Observation of ultraslow light propagation in a ruby crystal at room temperature, Phys. Rev. Lett. 90 (2003) 113903. [9] Q. Yang, J.T. Seo, B. Tabibi, H. Wang, Slow light and superluminality in Kerr media without a pump, Phys. Rev. Lett. 95 (2005) 063902. [10] F. Arrieta-Ý~ anez, O.G. Calderon, S. Melle, Slow and fast light based on coherent population oscillations in erbium-doped fibres, J. Opt. 12 (2010) 104002. [11] B. Macke, B. Segard, Slow light in saturable absorber, Phys. Rev. A 78 (2008) 013817. [12] Satchi kumari, Alika Khare, Optical and structural characterization of pulsed laser deposited ruby thin films for temperature sensing application, Appl. Surf. Sci. 265 (2013) 180e186. [13] Satchi kumari, Alika khare, Epitaxial ruby thin film based photonic sensor for temperature measurement, Rev. Sci. Instrum. 82 (2011) 066106. [14] Satchi Kumari, Alika Khare, Studies on nonlinear response of epitaxial ruby thin film, IEEE J. Quant. Electron. 50 (2014) 645e650. [15] H. Yao, C.H. Yan, S.P. Denbaars, J.M. Zavada, Mat. Res. Soc. Symp. Proc. 512 (1998) 411. [16] B.S. Dennis, V. Aksyuk, M.I. Haftel, S.T. Koev, Enhanced coupling between light and surface plasmons by nano-structured fabry-perot resonator, J. Appl. Phys. 110 (2011) 066102. [17] V.J. Sorger, R.F. Oulton, J. Yao, G. Bartal, X. Zhang, Plasmonic fabry-perot nanocavity, Nano Lett. 9 (2009) 3489. [18] M. Kuttge, E. J. R. Vesseur, and A. polman, “Fabry-perot resonator for surface polaritons probed by cathodoluminescence. [19] A. Artar, A.A. Yanik, H. Altug, Fabry-perot nanocavities in multilayered plasmonic crystals for enhanced biosensing, Appl. Phys. Lett. 95 (2009) 051105. [20] D. Crouse, P. Keshavareddy, Polarization independent enhanced optical transmission in one-dimensional gratings and device application, Opt. Express 15 (2006) 1415. [21] T. Gao, E. Stevens, J.-K. Lee, P.W. Leu, Deigning metal hemispheres on silicon ultrathin film solar cells for plasmonic light trapping, Opt. Lett. 39 (2014) 4647. [22] M. Yang, Z. Fu, F. Lin, X. Zhu, Incident angle dependence of absorption enhancement in plasmonic solar cells, Opt. Express 19 (2011) 763. [23] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370. [24] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum for plasmonics, ACS Nano 8 (2014) 834e840. [25] Satchi Kumari, Alika Khare, IEEE J. Quant. Electron. 47 (2011) 972. [26] S. Maier, Plasmonics Fundamentals and Applications, Springer.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabry-perot modes enhanced pump-probe coupling in gold microdisk patterned ruby thin film Satchi Kumari a, *, Alika Khare b, Reema Gupta c, Monika Tomar c, Vinay Gupta c a
Centre for Applied Physics, Central University of Jharkhand, Ranchi, 835205, India Indian Institute of Technology, Guwahati, 781039, India c Department of Physics & Astrophysics, University of Delhi, 110007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2016 Received in revised form 4 June 2017 Accepted 7 June 2017
Enhanced pump-probe coupling has been experimentally observed in epitaxial Ruby thin film patterned with equidistant gold micro-disks (~100 mm), as compared to pure Ruby film. This has been attributed to Fabry-Perot and surface plasmon modes in Ruby/gold film. In case of S polarized pump and probe beam, Fabry-Perot modes leads to a two-wave mixing gain of ~1.35. Moreover gain was ~1.62 in P-polarization case, it has been attributed to coupling of Fabry-Perot and surface plasmon modes. Enhanced coupling for P-polarization can lead to improved nonlinear response in the thin film geometry. It can find applications in thin film based compact photonic devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thin-film Fabry-perot Polarization Surface plasmon
1. Introduction Ruby is a well-known optical material for nonlinear optics [1e4]. Its long fluorescence life time (~3 ms) and high quantum efficiency makes it suitable for generation of various nonlinear optical (NLO) processes. It has been known to exhibit many interesting NLO phenomenon viz. self-phase modulation, hole burning, saturable absorption, degenerate and non-degenerate two-wave mixing, optical delay (slow & fast light) [5e11]. These nonlinear processes has been primarily studied in case of bulk Ruby crystals. The thin film of Ruby exhibiting above mentioned properties can find application in the form of compact photonic devices for variable optical delay lines, optical data storage, all optical signal processing, high precession spectroscopy and non-linear optics. Optical quality thin film of Ruby (~3.5 mm) has been successfully grown on sapphire substrate using Pulsed laser deposition (PLD) technique [12e14]. Since sapphire epitaxially matches with Ruby, the grown film was found to be highly C-axis oriented [12e15]. Moreover sapphire has excellent optical properties making it suitable candidate for photonic applications. The open and closed Z scan measurement of non-linear coefficients has confirmed
Abbreviations: NLO, Non-linear optical; SP, Surface plasmon. * Corresponding author. E-mail address: [email protected] (S. Kumari). http://dx.doi.org/10.1016/j.optmat.2017.06.015 0925-3467/© 2017 Elsevier B.V. All rights reserved.
nonlinear behavior of epitaxial Ruby film [14]. The value of nonlinear refractive index (n2 ~ 3.1 105 m2/W) for Ruby thin film is three orders of magnitude higher compared to crystal [4,12]. The optical delay measurements of Ruby thin film using pump-probe technique [12] shows a delay of ~12 ns [12]. The observed delay (~few nanoseconds) has been much lower as compared to millisecond delay in case of Ruby crystal due to the poor coupling of pump and probe beams inside the thin film geometry [6e8]. This poor coupling results from smaller interaction length in case of thin film geometry in comparison to Ruby crystal/rod. Hence, for achieving significant optical delay the pump-probe coupling inside thin film geometry should be enhanced. This issue has been addressed in the present manuscript. The light matter interaction inside the thin film geometry is enhanced via Fabry-Perot and surface plasmon (SP) modes. Fabry-Perot cavities allow efficient storage of electromagnetic energy [16e22]. Confinement of light in small mode for sufficiently large time leads to strong light matter interactions and can help to enhance weak physical processes [16]. Further, SP modes like Fabry-Perot modes also allows strong coupling and enhances various nonlinear processes viz. SHG, fluorescence, and surface enhanced Raman scattering. The Fabry-Perot modes and SP modes can be supported by 1D grating structures [19]. Keeping in view the possibility of enhancing light matter interaction using engineered nanostructures, in the present work we have explored pump-probe
376
S. Kumari et al. / Optical Materials 72 (2017) 375e379
coupling in gold micro-disk patterned Ruby film. 2. Experimental details Epitaxial Ruby thin film of thickness ~3.5 mm was grown on sapphire substrate using pulsed laser deposition technique [13,14]. The Gold micro-disks of thickness ~40 nm and diameter ~100 mm were patterned onto the Ruby film using thermal evaporation technique and shadow masking. The distance between consecutive dots was ~200 mm as shown in optical micrograph, Fig. 1(a). Fig. 1(b) shows the schematic of the cross-section of sample along with incident rays and reflected rays giving rise to interference pattern. The schematic of pump-probe set-up used to study coupling in Ruby and Ruby/gold film is shown in Fig. 1(c). Pump and probe beams were launched onto the Ruby & Ruby/gold film at a beam crossing angle of 31 as shown in Fig. 1(b). Diode pumped solid state laser (DPSSL) having wavelength 532 nm was used to derive both pump and probe beam. Excitation wavelength of 532 nm was selected keeping in view green U-band absorption of Ruby [12]. Gold micro-disk was selected over other nobel metals like silver and aluminum due to its higher stability against oxidation. Although gold shows inter-band transitions in the green wavelength range causing some absorption losses, but shows reflectivity of 59.4% enabling coupling of localized plasmon modes to metal disk arrays [23]. Polarizing cube beam splitter was used to separate the S and P polarized components. The polarization of laser beam was changed by changing the orientation of the beam splitter. The angle of incidence (q) between probe beam and the normal to the surface is varied by rotating the sample using goniometer. 3. Results and discussion Fig. 2(a) shows the absorption spectra of pure Ruby and Ruby/ gold film. The absorption of patterned Ruby thin film was found to be large as compared to pure Ruby film. The enhanced absorption can be attributed to surface plasmon (SP) modes. The SP modes allow the absorption of more number of photons in the Ruby/gold film. We see change in the absorption spectra but not as a sharp peak. Such broad resonant absorption peak arises from large
Fig. 2. (a) Absorption and (b) photoluminescence spectra of Ruby and Ruby/gold sample.
diameter of nano disk. Absorption peaks has been observed to broaden with increasing diameter for aluminum metal disks by
Fig. 1. (a) Optical micrograph of Ruby/Gold film. (b) Schematic of Pump-Probe studies in Ruby/gold film. (c) Experimental set-up.
S. Kumari et al. / Optical Materials 72 (2017) 375e379
Naomi halas and group [24]. Fig. 2(b) shows the corresponding photoluminescence from Ruby and Ruby gold film. The Ruby film shows the characteristic R-line fluorescence along with vibronic side bands [13]. The R-line fluorescence (observed in the wavelength range 690e695 nm) has been quenched in case of Ruby/gold film and vibronic side bands have been enhanced, see inset of Fig. 2(b). This shows that the non-radiative relaxation process is
Fig. 3. Transmission from (a) pure Ruby thin film and (b) Ruby/gold thin film in pump on and off conditions for S-polarization (c) Gain in case of Ruby and Ruby/gold thin film for S-polarization.
377
dominant in Ruby/gold film. Further, confirming the enhanced absorption of radiation inside the Ruby/gold film (see Fig. 2(a)). Fig. 3(a) shows the transmission from pure Ruby film and Fig. 3(b) from Ruby/gold film in pump on and off conditions for S-polarization as a function of film orientation (q). In case of pure Ruby film no systematic variation was observed w. r. t. the incidence angle as shown in Fig. 3(a). While film patterned with metal disk array shows modes in the transmission spectra as shown in Fig. 3(b). The gain in pure Ruby film and Ruby/gold film for S-polarization is shown in Fig. 3(c). It was estimated from the ratio of transmittance in pump off & pump on condition as reported earlier [25]. Gain was found to be large in case of patterned Ruby film as compared to pure Ruby as shown in Fig. 3(c) and maximum gain was observed at incidence angle of ~31. Under the same experimental condition Fabry-Perot modes were generated in the Ruby/gold film as compared to bare Ruby film. Fabry-perot modes are resonance modes generated due to interference of light multiply reflected from two reflecting surfaces such as gold and Ruby/Sapphire/air interface in our case. The observed modes were broad and almost identical in pump on & pump off conditions. In pump on condition it shows shift and amplification. This shows enhanced coupling due to combined effect of metal disk induced Fabry-Perot modes. Further, we performed measurement with changed polarization to distinguish between the roles of Fabry-Perot modes and SP modes. Fig. 4(a) and (b) shows the transmission spectra for Ppolarized light for Ruby and Ruby/gold film respectively. Similar to the case of S-polarization no modes were observed in pure Ruby case. In case of P-polarized light, the transmission spectra of patterned Ruby film shows sharp and more number of modes as compared to S-polarization light. These modes are formed due to combined effect of SP and Fabry-Perot interferes in case of P-polarization. Surface plasmons gives rise to slanted background while difference in the boundary conditions for S-polarized and Ppolarized light (because at metal interface the electric field component has to be zero due to finite penetration depth of electromagnetic waves) leads to observed difference in number of modes and their sharpness for the two polarization states. The modes were identical in both the pump on and pump off conditions. The modes in P-polarization case are sharp as compared to Spolarization and there is significant effect of pump on the shift and broadening of these modes as compared to S-polarization, Figs. 3(b) and 4(b). Further the decreased intensity also confirmed second player in case of P-polarized measurements. The gain in pure Ruby film and Ruby/gold film for P-polarization is shown in Fig. 4(c). It was estimated from the ratio of transmittance in pump off & pump on condition. For the P-polarized light as well, the gain was found to be large in case of patterned Ruby film as compared to pure Ruby as shown in Fig. 4(c). P-polarized light showed higher gain as compared to S-polarized light and it was maximum at an incidence angle of ~31. This enhanced coupling in Ruby/gold film in case of Ppolarization is accounted to combined effect of Fabry-Perot and surface plasmon induced modes in the Ruby/gold film. The cavity modes present in the Ruby/gold film traps the electromagnetic energy for sufficiently long time as compared to pure Ruby film. This multiple reflection enhances the interaction process and thus the energy transfer from pump to probe increases significantly. Further this process is more dominant for P-polarized light as compared to S-polarization. Fig. 5(a) and (b) shows the fitting of observed variation in transmission with film orientation i.e incident angle for S-polarized light and P-polarized light respectively in pump on condition. For the Fabry perot interference the variation in transmitted in1 tensity is expressed as IIo ¼ , Where d ¼ 4lp h cos qr ¼ 2 1þF sin d=2
K cos qr ;
K ¼ 4lp h .qr is the angle of refraction (in radians) and is
378
S. Kumari et al. / Optical Materials 72 (2017) 375e379
Fig. 5. Fitting of experimental curves in the Pump on Conditions for Ruby/Gold film.
propagation and coupling losses in beam path. F is known as coefficient of fineness and is expressed in terms of reflectivity (R) as F ¼ 4R 2 . The positive and negative portions of the pattern was ð1RÞ
Fig. 4. Transmission from (a) pure ruby thin film and (b) ruby/gold thin film in pump on and off conditions for P-polarization. (c) Gain in case of ruby and ruby/gold thin film for P-polarization.
related to angle of incidence ðfÞ by a constant a using Snell's law (n1 sin f ¼ n2 sin qr ). The experimentally observed transmission 1 pattern was fitted with the equation I ¼ A þ mf. 2 1þF sin ðafÞ
Where A is constant taking account of detector efficiency,
fitted separately due to difference in background and obtained average values are reported. The fitting for S-polarized light requires A ¼ 36.1, F ¼ 0.277, a ¼ e11.16 and slope m ¼ e8.3. Fitting for P-polarized light gives A ¼ 25.53, F ¼ e0.229, a ¼ 17.24 with m ¼ 19.8. In case of P-polarized light, the transmitted light follows closely the Fabry perot interference pattern along with a slanting background arising from surface plasmon modes; confirming origin of oscillating intensity from fabry-perot modes. From continuity of vector components of electric and magnetic fields at the single interface, we know that SPP only exist for the P-polarized light (TM polarization) [26]. In case of S-polarized light as well, the variation of transmitted intensity with angle of incidence also follow the expression of fabry-perot interference with nearly flat background with angle variation due to absence of any SPP modes in case of Spolarized light. This confirms that the multiple reflections experienced by light trapped between gold disks and substrate in between film results into enhanced pump-probe energy transfer i.e increased interaction length. Additionally, the light confined as plasmonic modes also contributes towards observed increased interaction length in case of P-polarization resulting into observed high gain as compared to S-polarized light.
S. Kumari et al. / Optical Materials 72 (2017) 375e379
4. Conclusion In summary, enhanced pump-probe coupling has been demonstrated in gold micro-disks patterned epitaxial Ruby thin film. The two-wave coupling gain has been studied for S and P polarization of laser beam. In case of P-polarized light, the FabryPerot modes along with surface SPP modes accounts for observed enhanced energy transfer in pump-probe process. While, in case of S-polarized light Fabry-Perot alone plays towards energy transfer during pump-probe process. We have shown that by patterning metal micro-disk arrays on Ruby films the energy transfer in pumpprobe process can be enhanced through Fabry-perot and SPP modes. Such a novel approach is highly useful to enhance the lightmatter interaction for various nonlinear device applications on a chip operating at lower pump power. Acknowledgements This work was supported in part by Department of science and technology Govt. of India IFA12-PH-48 (INSPIRE). The corresponding author would like to thank Prof. R. P. Singh (PRL, Ahmedabad) for fruitful discussion and suggestions towards the improvement of manuscript. References [1] I. McMicheal, P. Yeh, P. Beckwith, Nondegenerate two-wave mixing in ruby, Opt. Lett. 13 (1988) 500. [2] H.K. Lee, S.S. Lee, Measurements of the anisotropic nonlinear refractive-index coefficients of ruby, Opt. Lett. 15 (1990) 54. [3] S.A. Boothroyd, J. Chrostowski, M.S. O'Sullivan, Two-wave mixing by phase and absorption gratings in saturable absorbers, J. Opt. Soc. Am. B 6 (1989) 766. [4] L.C. Oliveira, S.C. Zilio, Single-beam time resolved Z-scan measurement of slow absorbers, Appl. Phys. Lett. 65 (1994) 2121. [5] H. Riesen, A.K. Rebane, A. Szabo, I. Carceller, Slowing light down by low magnetic fields: pulse delay by transient spectral hole-burning in ruby, Opt. Exp. 20 (2012) 19039.
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