Coherent Excitation of Optical Phonons in GaAs by Broadband Terahertz Pulses

Abstract

Coherent excitation and control of lattice motion by electromagnetic radiation in optical frequency range has been reported through variety of indirect interaction mechanisms with phonon modes. However, coherent phonon excitation by direct interaction of electromagnetic radiation and nuclei has not been demonstrated experimentally in terahertz (THz) frequency range mainly due to the lack of THz emitters with broad bandwidth suitable for the purpose. We report the experimental observation of coherent phonon excitation and detection in GaAs using ultrafast THz-pump/optical-probe scheme. From the results of THz pump field dependence, pump/probe polarization dependence, and crystal orientation dependence, we attributed THz wave absorption and linear electro-optic effect to the excitation and detection mechanisms of coherent polar TO phonons. Furthermore, the carrier density dependence of the interaction of coherent phonons and free carriers is reported.

Figure 1. Experimental configuration and characterization of the incident THz pulses.

(a) Experimental setup of THz-pump/optical-probe reflectivity and electro-optic reflectivity (TPOP-ΔR and TPOP-ΔR_eo) measurement. WP is Wollaston prism and ITO is indium tin oxide glass. Balanced photodiodes are used for detection of the 800 nm optical probe. (b) Scheme of polarization of THz pump and optical probe in TPOP-ΔR_eo measurement of (110) orientated GaAs. (c) THz autocorrelation signal measured by THz Michelson interferometer. (d) Fourier spectrum of the THz autocorrelation signal. Dashed horizontal line indicates the 10% of the maximum amplitude.

Figure 2. THz induced transient reflectivity change in (100) and (110) intrinsic GaAs.

(a) TPOP-∆R signal in time domain. The labels for (100) i-GaAs indicates the angle between [011] axis and the polarization direction of the pump and probe. For the (110) i-GaAs, [1–10] crystal direction is parallel to the pump and probe polarization. The inset is the oscillatory part subtracted by the exponential background. (b) Fourier transform of the oscillatory signal. The calculated curve is based on equation (8) assuming impulsive time domain THz pulse and normalized to the peak at TO frequency. Slowly decaying background was subtracted prior to Fourier transformation. (c) The calculated curve the response function of the harmonic oscillator, g(ω) and the transmission through nitrogen/GaAs interface, T(ω).

Figure 3. The pump/probe polarization dependence and pump field dependence of TPOP-∆R_eo signals in (110) undoped GaAs.

(a) Time-domain signals at the crystal rotation angle 60/120/180 degrees equivalent to different pump/probe polarizations. (b) The pump/probe polarization dependence of the Fourier component at TO and LO frequencies. The amplitudes are normalized to the maximum points near 120 degrees. The temporal signals correspond to sample rotation angles 60/120/180 degrees, as indicated by black/red/blue arrows in (a) respectively. (c) Dependence of coherent TO phonons amplitude on the peak electric field of the THz pump.

Figure 4. THz-excited cohernent phonons in doped and undoped GaAs.

(a) and (b) are TPOP-ΔR_eo signals and fitting in time domain for undoped and doped (110) GaAs respectively. (c) Shows the relaxation times and oscillation frequencies obtained by fitting the time-domain signals measured at different angles of the samples.

Figure 5. Frequency-dependent transmission coefficients of GaAs with different carrier densities.

Black/red/blue curves are transmission coefficients T(ω) of gas/GaAs interface for different carrier densities of GaAs. Grey part indicates the response function of the harmonic oscillators g(ω).