Direct measurements of multi-photon induced nonlinear lattice dynamics in semiconductors via time-resolved x-ray scattering

Abstract

Nonlinear optical phenomena in semiconductors present several fundamental problems in modern optics that are of great importance for the development of optoelectronic devices. In particular, the details of photo-induced lattice dynamics at early time-scales prior to carrier recombination remain poorly understood. We demonstrate the first integrated measurements of both optical and structural, material-dependent quantities while also inferring the bulk impulsive strain profile by using high spatial-resolution time-resolved x-ray scattering (TRXS) on bulk crystalline gallium arsenide. Our findings reveal distinctive laser-fluence dependent crystal lattice responses, which are not described by previous TRXS experiments or models. The initial linear expansion of the crystal upon laser excitation stagnates at a laser fluence corresponding to the saturation of the free carrier density before resuming expansion in a third regime at higher fluences where two-photon absorption becomes dominant. Our interpretations of the lattice dynamics as nonlinear optical effects are confirmed by numerical simulations and by additional measurements in an n-type semiconductor that allows higher-order nonlinear optical processes to be directly observed as modulations of x-ray diffraction lineshapes.

Measurement of transient strain induced by two-photon excitation

“…However, the stresses by hot phonons often prevail [13]. In GaAs a nonlinear contribution to the fluence dependence of the strain adds to the saturating one-photon absorption [14]. Various time-resolved magneto-optical experiments in metallic magnets demonstrate the possibility to trigger and control magnetization precession [15][16][17][18], using laser-induced strain pulses.…”

Probing Electronic Strain Generation by Separated Electron-Hole Pairs Using Time-Resolved X-ray Scattering

“…In crystal Silicon S el is negative (contractile), whereas in crystal Gallium Arsenide S el is positive (tensile) [6] and therefore also results in expansion similar to thermal strain. In the past two decades, it has become recognized that the deformation potential often plays a dominant role in the lattice dynamics of semiconductor materials following ultrafast laser excitation [7][8][9][10][11]. In many of these studies, such as the one presented here, small transient changes in lattice spacing can be resolved by high-resolution synchrotron X-ray diffraction, and the logarithmic range of timescales probed can be exploited along with depth or material sensitivity to disentangle the transport of heat, sound, and charge in bulk or heterostructure crystals.…”

Measuring femtometer lattice displacements driven by free carrier diffusion in a polycrystalline semiconductor using time-resolved x-ray scattering

“…1,2 The resulting change in interatomic spacing is proportional to the number of free carriers generated at low intensities below the onset of nonlinear absorption. 3 Excess energy from the light can be transferred to the lattice and increases the lattice spacing by creating a thermoelastic stress. In the field of optoelectronic applications, understanding the extent to which the structure or morphology of the material influences carrier and thermal transport is crucial for improving device performance.…”

“…8 Similarly, the onset of twophoton absorption in n-doped GaAs manifests structurally as a one-millidegree rocking curve sideband. 3 For this purpose, improvements to the TRXS method have included increasing the sensitivity (e.g., grazing incidence to improve the x-ray/ laser overlap 9 ), improving the angular resolution (e.g., tripleaxis diffractometry 10 ), and extending the detector dynamic range (e.g., proportional mode detectors used with deep memory oscilloscopes 11 ). However, most practical optoelectronic devices, such as photodetectors, saturable absorbers, high-speed transistors, photovoltaics, and light emitting diodes, are made of polycrystalline semiconductors comprised of micro-to-nanoscale grains.…”

“…3, we use TRXS simulation 3,7 to model transient lattice displacements within 100 nm into the bulk when an electron diffusion coefficient of 44 cm 2 /s is used. However, fitting the numerical modeling to data yields an effective diffusion constant of D eff ¼ 1.00 6 0.13 cm 2 /s.…”