Space-charge wave excitation by superposition of static and moving interference patterns

The optical excitation of space-charge waves (SCW) has been realized experimentally by superposition of static and moving light interference patterns with ${\text{Bi}}_{12}{\text{GeO}}_{20}$ as an example. A clear resonance behavior of the ac detected in an external electric circuit, as well as an inverse dispersion law, is verified, and a low frequency signal due to a modulation of the photoconductivity appears. The results are compared to the classical method using an oscillating light pattern for excitation. The optimized method allows for the detection of the charge carriers participating in SCW formation and yields an excitation of higher quality.

Figure 1

(a) Experimental approach; ${\text{R}}_{1,2}$: reference beams, ${\text{S}}_{1,2}$: signal beams, PM: phase modulator, HV: high-voltage source, ${\text{R}}_{\text{L}}$: loading resistor, D: detection system including lock-in amplifier. (b) Scheme of the experimental setup with an ${\text{Ar}}^{+}{\text{-Kr}}^{+}$-laser at $\lambda =514\text{nm}$ as pump source; LP: half-wave plate and polarizer, BS: beam splitters, PDM: piezo driven mirror, BE: beam expanders, BP: beam splitter plate, M: mirrors, ${\mathbf{E}}_{\mathbf{0}}$: applied electric field.

Figure 2

Dependence of $I_1$ on $\Omega$ for $K=3.6\cdot {10}^3{\text{cm}}^{-1}$ and $E_0=4.3\text{kV}{\text{cm}}^{-1}$ with $\lambda =514\text{nm}$, $W_0=30\text{mW}{\text{cm}}^{-2}$, $m=0.45$, and $\theta =0.3\pi$. The data of static and moving patterns with reversed direction of the moving pattern (△) are normalized to the ac at resonance with the moving pattern and electric field having the same direction.

Figure 3

Dependence of ${\Omega }_{\text{R}}$ on $K$ for $E_0=7.6\text{kV}{\text{cm}}^{-1}$ with $\lambda =514\text{nm}$, $W_0=30\text{mW}{\text{cm}}^{-2}$, $m=0.45$, and $\theta =0.3\pi$. The solid line is a fit of Eq. (29) in Ref. 16 to the data of static and moving patterns with ${\tau }_{\text{M}}\cdot \mu \tau =1.4\cdot {10}^{-14}{\text{m}}^2\text{s}{\text{V}}^{-1}$.

Figure 4

Modulation of the dc current in dependence on the phase modulation $\Omega t$ for $W_0=30\text{mW}{\text{cm}}^{-2}$, $m=0.45$, $K=3.6\cdot {10}^3{\text{cm}}^{-1}$, and $E_0=4.3\text{kV}{\text{cm}}^{-1}$. The time interval between increasing $\Omega t$ and obtaining the current is 5 s. The line is a guide to the eye.

Figure 5

(a) Dependence of $I_1\left({\Omega }_{\text{R}}\right)$ on $K$ for $E_0=7.6\text{kV}{\text{cm}}^{-1}$ with $\lambda =514\text{nm}$, $W_0=30\text{mW}{\text{cm}}^{-2}$, $m=0.45$, and $\theta =0.3\pi$. (b) Dependence of $I_1\left({\Omega }_{\text{R}}\right)$ on $E_0$ for $K=6.1\cdot {10}^3{\text{cm}}^{-1}$ with $\lambda =514\text{nm}$, $W_0=30\text{mW}{\text{cm}}^{-2}$, $m=0.45$, and $\theta =0.3\pi$.