The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process ͑opto-acoustic transformation͒ and the detection process ͑acousto-optic transformation͒. We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.

We investigate terahertz (THz) emission from heavy-ion-irradiated In0.53Ga0.47As photoconductive antennas excited at 1550 nm. The carrier lifetime in the highly irradiated In0.53Ga0.47As layer is less than 200 fs, the steady-state mobility is 490cm2V−1s−1, and the dark resistivity is 3Ωcm. The spectrum of the electric field radiating from the Br+-irradiated In0.53Ga0.47As antenna extends beyond 2 THz. The THz electric field magnitude is shown to saturate at high optical pump fluence, and the saturation fluence level increases with the irradiation dose, indicating that defect center scattering has a significant contribution to the transient mobility.

We report the heterodyne detection and phase locking of a 2.5 THz quantum cascade laser (QCL) using a terahertz frequency comb generated in a GaAs photomixer using a femtosecond fiber laser. With 10 mW emitted by the QCL, the phase-locked signal at the intermediate frequency yields 80 dB of signal-to-noise ratio in a bandwidth of 1 Hz.