Relaxation processes of the $\mathrm{Ge}\text{−}\mathrm{H}$ stretch modes in hydrogenated amorphous germanium

Infrared transient grating measurements on the $\mathrm{Ge}\text{−}\mathrm{H}$ stretch mode at $1880{\mathrm{cm}}^{-1}$ in hydrogenated amorphous germanium show a long, multiexponential relaxation time having a mean decay constant of $306\mathrm{ps}$. The thermal activation of the decay rate suggests a decay via two $\mathrm{Ge}\text{−}\mathrm{H}$ bending modes with the energy mismatch bridged by three bulk $\mathrm{Ge}\text{−}\mathrm{Ge}$ vibrations. From the probe pulse diffraction efficiency, we have determined a value for the nonlinear refractive index of $n_2=5.9\times {10}^{-4}{\mathrm{cm}}^2∕\mathrm{GW}$. Utilizing the photon echo technique, we determine that the dominant contribution to a vibrational dephasing near room temperature is the elastic scattering of TO modes near $270{\mathrm{cm}}^{-1}$. At $10\mathrm{K}$ additional dephasing from nonequilibrium phonons are readily observable as an excitation dependent contribution.

Figure 1

Infrared absorption spectrum of $a\text{−}\mathrm{Ge}:\mathrm{H}$.

Figure 2

(a) $10\mathrm{K}$ transient grating signals for resonant excitation of the $\mathrm{Ge}\text{−}\mathrm{H}$ stretch modes at $5.32\mathrm{\mu }\mathrm{m}$ Inset: The superimposed transient grating signals at $10\mathrm{K}$ (open circles) and $220\mathrm{K}$ (open squares), (b) the temperature dependent, mean vibrational population decay rates. The dotted line is the decay into $3\mathrm{Ge}\text{−}\mathrm{H}$ bending modes at $560{\mathrm{cm}}^{-1}$ and 1 LA mode at $170{\mathrm{cm}}^{-1}$; the dashed-dotted line is the decay into $3\mathrm{Ge}\text{−}\mathrm{H}$ bending modes and 2 TA modes at $85{\mathrm{cm}}^{-1}$; the dashed line is the decay into 2 bending modes and 3 LO modes at $247{\mathrm{cm}}^{-1}$, while the solid line shows the decay into 2 bending modes, two TO modes at $280{\mathrm{cm}}^{-1}$, and 1 LA mode at $180{\mathrm{cm}}^{-1}$.

Figure 3

(a) $10\mathrm{K}$ photon echo signal for resonant excitation of the $\mathrm{Ge}\text{−}\mathrm{H}$ stretch modes at $5.32\mathrm{\mu }\mathrm{m}$, (b) the $S_p\left(t\right)∕S_g\left(t\right)$ ratio at $100\mathrm{K}$ illustrating single exponential pure dephasing, and (c) the temperature dependence of the pure dephasing rate. The data shown as open triangles are recorded at three times the excitation density of the data shown as open circles. The solid line is a fit to Eq. (2) with $a=9.0\mathrm{GHz}$ and $A=336\mathrm{GHz}$ with elastic scattering from a single vibrational mode having an energy of $270{\mathrm{cm}}^{-1}$.

Figure 4

$10\mathrm{K}$ photon echo signal using a single FELIX micropulse per macropulse.