GaN晶体在飞秒紫外波段激发下的可变非线性吸收效应和光动力学过程研究
|
摘要
采用Z扫描和泵浦-探测技术研究了GaN薄膜在370nm时的非线性光学效应和非线性光动力学过程。首先,基于GaN薄膜的透射光谱,结合线性光学理论分析得到了其在370nm的线性折射率n_0、线性吸收系数α_0、光学带隙E_g等线性光学性质。采用飞秒激光Z扫描技术,得到了不同光强激发下的Z扫描实验响应结果,结合非线性光学理论提取出GaN薄膜可变的光学非线性吸收效应。在激发光子能量接近GaN带隙情况下,低光强时材料表现为饱和吸收而高光强时为反饱和吸收,这是因为低光强下单光子吸收占主导而高光强下以单光子感应自由载流子吸收为主。闭孔Z扫描测量得到了GaN薄膜的三阶非线性折射系数为n_2=-(1.0±0.1)×10~(-3) cm~2·GW-1,它几乎比传统非线性介质的高出一个数量级。为了探究上述非线性过程的动力学弛豫时间以及进一步探究GaN薄膜非线性光动力学过程的深层物理机制,采用了交叉偏振飞秒退相泵浦探测技术观察GaN薄膜的光激发载流子动力学弛豫过程。实验结果表明,在低光强下,饱和吸收效应来源于瞬态单光子吸收,高光强下单光子感应自由载流子吸收为非瞬态光动力学过程,其自由载流子弛豫时间约为17ps。该工作将为GaN薄膜在紫外非线性纳米器件应用以及GaN薄膜非线性过程的机制分析理解提供新的思路。
In this paper,by employing conventional femto-second Z-scan and pump-probe measurements,the nonlinear optical properties and photoexcited carrier dynamics relaxation time of the GaN film were investigated.Based on the oscillation transmittance spectrum of GaN film and linear optical theories,the linear absorption coefficients(α_0),linear refractive index(n_0)and optical bandgap(E_g)were obtained at near ultraviolet wavelength of 370 nm.Variable nonlinear absorption effects were observed with the open-aperture(OA)Z-scan experiments at different levels of laser excitation intensities.It is found that the GaN film shows saturable absorption at low intensities and reverse saturable absorption under the excitation of intense irradiances when the photon energy is close to the band gap of GaN film.It is believed that the observed nonlinear absorption originates from the one-photon absorption at low intensities whereas one-photon induced free carrier absorption dominates the nonlinear response at high intensities.With the carry out of closed-aperture(CA)Z-scan experiments,a large nonlinear refractive index in GaN was obtained,which was one orders of magnitude larger than that of conventional nonlinear media.To identify ultrafast carrier dynamic relaxation time of the observed nonlinearities and to get an insight of the physical mechanism,the cross-polarized femtosecond degenerate pump-probe measurement was performed at a near-ultraviolet wavelength of 370 nm.The results indicate that the saturable absorption originates from the instantaneous one-photon absorption process at low intensities,while one-photon absorption induced free carrier absorption dominates the non-instantaneous nonlinear absorption process at high intensities with the free carriers dynamic relaxation time ~17 ps.This work provides significant insight to the application of GaN in nonlinear optical ultraviolet photonics devices and the understanding of nonlinear response mechanism in GaN films.
In this paper,by employing conventional femto-second Z-scan and pump-probe measurements,the nonlinear optical properties and photoexcited carrier dynamics relaxation time of the GaN film were investigated.Based on the oscillation transmittance spectrum of GaN film and linear optical theories,the linear absorption coefficients(α_0),linear refractive index(n_0)and optical bandgap(E_g)were obtained at near ultraviolet wavelength of 370 nm.Variable nonlinear absorption effects were observed with the open-aperture(OA)Z-scan experiments at different levels of laser excitation intensities.It is found that the GaN film shows saturable absorption at low intensities and reverse saturable absorption under the excitation of intense irradiances when the photon energy is close to the band gap of GaN film.It is believed that the observed nonlinear absorption originates from the one-photon absorption at low intensities whereas one-photon induced free carrier absorption dominates the nonlinear response at high intensities.With the carry out of closed-aperture(CA)Z-scan experiments,a large nonlinear refractive index in GaN was obtained,which was one orders of magnitude larger than that of conventional nonlinear media.To identify ultrafast carrier dynamic relaxation time of the observed nonlinearities and to get an insight of the physical mechanism,the cross-polarized femtosecond degenerate pump-probe measurement was performed at a near-ultraviolet wavelength of 370 nm.The results indicate that the saturable absorption originates from the instantaneous one-photon absorption process at low intensities,while one-photon absorption induced free carrier absorption dominates the non-instantaneous nonlinear absorption process at high intensities with the free carriers dynamic relaxation time ~17 ps.This work provides significant insight to the application of GaN in nonlinear optical ultraviolet photonics devices and the understanding of nonlinear response mechanism in GaN films.
引文
[1]Marquardt O,Geelhaar L,Brandt O.Nano Letters,2015,15:4289.
[2]Kim J H,Hwang S M,Seo Y G,et al.Sci.Technol.,2013,28:085007.
[3]Lugani L,Carlin J,Marcel A,et al.J.Appl.Phys.,2013,113:214503.
[4]Huang J,Xu K,et al.Nanoscale Res.Lett.,2012,7:150.
[5]Zhang G,Zhang M,Ye X,et al.Adv.Mat,2013,26:805.
[6]Davies M J,Dawson P,Massabuau F.et al.Appl.Phys.Lett.,2014,105:092106.
[7]Chang J R,Chang S P,Li Y J,et al.Appl.Phys.Lett.,2012,100:261103.
[8]Gu B,Liu D H,et al.Appl.Phys.B,2014,117:1141.
[9]Wang Y,Liu S,Yuan J,et al.Sci.Rep.,2016,6:33070.
[10]Wang Y,Mu H,Li X,et al.Appl.Phys Lett.,2016,108:221901.
[11]Jin Z M,Xu Y,et al.Appl.Phys.Lett.,2012,100:071105.
[12]Huang Y L,Sun C K,et al.Appl.Phys.Lett.,1999,75:3524.
[13]Fang Y,Xiao Z G,et al.Appl.Phys.Lett.,2015,106:251903.
[14]Yang C Y,Chia C T,et al.Appl.Phys.Lett.,2014,105:212105.
[15]Wang Y,Huang G,Mu H,et al.Appl.Phys.Lett,2015,107:091905.
[16]Yang Z,Zhang X,et al.Acta Phys.Sin.,2015,64:177901.
[17]Gao Y C,Zhang X R,et al.Optics Commun.,2005,251:429.
[18]Tauc J,Grigorovici R,Vancu A.Phys.Stat.Sol.,1996,15:627.
[19]Gu B,Ji W,Huang X Q.Appl.Opt.,2008,47:1187.
[20]Zhang F,Han X Y.J.Modern Opt.,2014,61:1360.
[21]Gu B,Wang Y,Wang J,et al.J.Opt.Express,2009,17:10970.
[22]Chen A P,Long H,Wang K,et al.Acta Phys.Sin.,2009,58:1.
[23]Anderson K,LaGasse M J,et al.Appl.Phys.Lett.,1990,56:1834.
[24]Lin W Z,Schoenlein R W,et al.IEEE J.Quantum Elect.,1998,24:267.
[25]Fang Y,Yang J Y,et al.Appl.Phys.Lett.,2014,105:161909.
[2]Kim J H,Hwang S M,Seo Y G,et al.Sci.Technol.,2013,28:085007.
[3]Lugani L,Carlin J,Marcel A,et al.J.Appl.Phys.,2013,113:214503.
[4]Huang J,Xu K,et al.Nanoscale Res.Lett.,2012,7:150.
[5]Zhang G,Zhang M,Ye X,et al.Adv.Mat,2013,26:805.
[6]Davies M J,Dawson P,Massabuau F.et al.Appl.Phys.Lett.,2014,105:092106.
[7]Chang J R,Chang S P,Li Y J,et al.Appl.Phys.Lett.,2012,100:261103.
[8]Gu B,Liu D H,et al.Appl.Phys.B,2014,117:1141.
[9]Wang Y,Liu S,Yuan J,et al.Sci.Rep.,2016,6:33070.
[10]Wang Y,Mu H,Li X,et al.Appl.Phys Lett.,2016,108:221901.
[11]Jin Z M,Xu Y,et al.Appl.Phys.Lett.,2012,100:071105.
[12]Huang Y L,Sun C K,et al.Appl.Phys.Lett.,1999,75:3524.
[13]Fang Y,Xiao Z G,et al.Appl.Phys.Lett.,2015,106:251903.
[14]Yang C Y,Chia C T,et al.Appl.Phys.Lett.,2014,105:212105.
[15]Wang Y,Huang G,Mu H,et al.Appl.Phys.Lett,2015,107:091905.
[16]Yang Z,Zhang X,et al.Acta Phys.Sin.,2015,64:177901.
[17]Gao Y C,Zhang X R,et al.Optics Commun.,2005,251:429.
[18]Tauc J,Grigorovici R,Vancu A.Phys.Stat.Sol.,1996,15:627.
[19]Gu B,Ji W,Huang X Q.Appl.Opt.,2008,47:1187.
[20]Zhang F,Han X Y.J.Modern Opt.,2014,61:1360.
[21]Gu B,Wang Y,Wang J,et al.J.Opt.Express,2009,17:10970.
[22]Chen A P,Long H,Wang K,et al.Acta Phys.Sin.,2009,58:1.
[23]Anderson K,LaGasse M J,et al.Appl.Phys.Lett.,1990,56:1834.
[24]Lin W Z,Schoenlein R W,et al.IEEE J.Quantum Elect.,1998,24:267.
[25]Fang Y,Yang J Y,et al.Appl.Phys.Lett.,2014,105:161909.