Picosecond strain pulses generated by a supersonically expanding electron-hole plasma in GaAs

Strain pulses with picosecond duration are generated directly in GaAs by optical excitation from a femtosecond laser. The photons are absorbed in a 15-nm layer near the surface, creating the electron-hole plasma, which diffusively expands into the bulk of the GaAs. At an early time, the drift velocity of the expanding plasma exceeds the speed of longitudinal sound, and the generated strain pulses cannot escape the plasma cloud. Such supersonic generation of strain pulses results in specific temporal and spatial shapes of the generated strain pulses, where the compression part has a much lower amplitude than the tensile part. This phenomenon is studied experimentally at low temperatures and analyzed theoretically based on the wave and diffusion equations for strain and plasma density, respectively. Two mechanisms, deformation potential and thermoelasticity, are responsible for the experimental observations. The relative contributions from these mechanisms are governed by the nonradiative recombination rate in the plasma and depend on the optical excitation density, inducing such nonlinear optoacoustic effects as shortening of the leading strain front and a superlinear/quadratic increase in its amplitude with the rise of pump laser fluence.

Figure 1

The schemes describing the generation of the strain pulses $U\left(t,x\right)$ in the static (a) and supersonic (b) regimes, when the optical excitation is absorbed in a near-surface layer with the thickness $\xi$; (c) the scheme demonstrating the transformation of the generated strain pulse due to dispersion and nonlinear effects after the propagation through the crystalline substrate.

Figure 2

Time evolution of the strain pulses measured for direct excitation of GaAs (a) and 30-nm Al film (b). The excitation density corresponds to the linear propagation regime through the GaAs substrate. The oscillations after the bipolar pulses are due to the dispersion of high-frequency acoustic LA phonons.

Figure 3

Time evolution of strain pulses measured for various excitation densities of GaAs.

Figure 4

Experimental (black curves) and calculated for diffusion coefficient $D$ $=$ 4 cm${}^2$/s [red (dark gray) curves)] strain evolutions of the incident pulses for direct excitation of GaAs (a) and 30-nm Al film (b). The excitation density corresponds to the linear propagation regime in the GaAs substrate. The insets show the strain pulses initially generated in GaAs (a) and Al film (b).

Figure 5

Experimental (solid curves, black) and calculated [dotted curves, red (dark gray)] temporal evolutions of the compressive parts of the strain pulses obtained by the direct excitation of GaAs at various excitation densities, $F_L$. The calculations were performed for a carrier diffusion coefficient $D$ $=$ 4 cm${}^2/$s. The inverse decay rate ${\omega }_{+}^{-1}$, and the strain amplitude ${\eta }_0$ of the compressive part of the generated strain pulse by e-h plasma, indicated in the corresponding panels, have been chosen to provide the best fit for the arrival times of the first and second solitons.

Figure 6

Calculated dependences of the parameters characterizing the strain pulse generation for excitation of GaAs as a function of excitation density: (a) inverse duration of the compressive part of the strain pulse, ${\omega }_{+}$ (dotted curve, black), and recombination rate, ${\omega }_R$ [solid curve, red (dark gray)]; (b) amplitude of the compressive strain, ${\eta }_0$, calculated for deformation potential (dashed curve, black) and thermoelastic [dotted curve, red (dark gray)] generation mechanisms and also for both excitation mechanisms [solid curve, blue (medium gray)]. The symbols are the values obtained from the fitting of the calculated temporal strain pulse evolutions to the experimental data. Square-law behavior is indicated by the dashed-dotted line in (b).