Excitation and detection of high-frequency coherent acoustic phonons in low-symmetry superlattices P. Walker, R. P. Campion, and A. J. Kent School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom D. Lehmann Institute of Theoretical Physics, Technische Universität Dresden, D-01062 Dresden, Germany Cz. Jasiukiewicz Faculty of Mathematics and Applied Physics, Rzeszow University of Technology, ul. W. Pola 2, PL-35-959 Rzeszow, Poland Received 11 July 2008; published 10 December 2008 The generation and detection of coherent acoustic phonons in GaAs/AlAs superlattices grown on the low- symmetry, 311 and 211, planes of GaAs using femtosecond time-resolved pump-probe measurements is described. Frequencies of the excited phonons are deduced from the oscillations in the surface reflectivity of the probe and are compared with theoretical calculations assuming that Raman scattering is responsible for coherent phonon generation. The measured frequencies and relative intensities of the modes agree well with the theoretically predicted folded bulk acoustic modes and are determined by the period and symmetry of the superlattices. DOI: 10.1103/PhysRevB.78.233307 PACS numbers: 63.20.D-, 63.22.Np, 68.65.Cd Recently, there has been considerable interest in develop- ing usable sources of monochromatic acoustic phonon beams in the terahertz range for applications in phonon spectros- copy of nanostructures and high-speed optical modulation. Generation of high intensity pulses of zone-folded longitudi- nally polarized LA coherent acoustic phonons in GaAs/ AlAs superlattices SLs by ultrafast optical excitation has been demonstrated by a number of groups, see, e.g., Refs. 1 and 2. The frequency of the phonons is determined by the SL period, d SL , and is given approximately by c s / d SL , where c s is the velocity of sound in the SL. It has also been shown that the phonons leak out of the superlattice and propagate over macroscopic distances at low temperature, 3 which is impor- tant for the potential applications mentioned above. How- ever, most work was restricted to SLs grown on 100 GaAs. Superlattices grown on the low-symmetry, e.g., 311 and 211, planes of GaAs allow the possibility of generating also coherent transverse polarized TA acoustic phonon modes using femtosecond excitation. 4 No such possibility exists for 100 SLs due to symmetry considerations. However, owing to the low symmetry, TA modes are Raman active in 311 and 211 SLs as has been shown in Raman-scattering measurements, 5,6 and this could facilitate their generation and/or detection in femtosecond pump-probe measurements. The aim of the work described here was to obtain a more complete theoretical model of the processes of generation and detection of coherent acoustic phonons in low-symmetry SLs by femtosecond optical techniques. This is necessary because the results of conventional Raman-scattering mea- surements and associated theoretical models may not be di- rectly applicable to resonant optical excitation by femtosec- ond pulses. Furthermore, standard Raman-scattering measurements normally probe the backscattering modes only. More specialized variants of the technique are needed to probe forward-scattering modes. In this Brief Report, the symmetry of the phonon displacement field and the scatter- ing line intensities have been analyzed for nonhigh- symmetry directions of the superlattices. For these purposes, we have explicitly included the full acoustic anisotropy in our calculations. We make quantitative comparison of the predictions with the measured frequencies of the quasi-LA QLA and quasi-slow TA QSTA modes, both of which have mixed LA and TA characteristics, and the pure fast TA FTA mode. The SL samples used in the experiments were grown by molecular-beam epitaxy on semi-insulating 311 and 211 GaAs substrates. Both samples contained 40 period GaAs/ AlAs SLs, and the initial design thicknesses of the layers were 28 monolayers ML of GaAs and 18 ML of AlAs. The samples were characterized postgrowth by x-ray diffraction and were found to be of excellent quality, and with layer thicknesses close to the design parameters. The period, d SL , of the 311 sample was measured as 7.89 nm made up from d GaAs =4.82 nm of GaAs and d AlAs =3.07 nm of AlAs, and the period of the 211 SL was measured as 8.24 nm 4.82 nm of GaAs and 3.42 nm of AlAs. Measurements of the coherent SL phonons were made using a conventional reflection pump-probe technique: 100 fs, 10 nJ pulses from a mode-locked Ti:sapphire oscillator repetition rate of 82 MHz were split 90% to pump and 10% to probe. The pump beam was mechanically chopped at 1 kHz and focused to a spot of diameter of about 100 m on the sample which was mounted in an optical access cry- ostat. Allowing for the losses in the optical system, the pump fluence incident on the sample was 0.05 mJ cm -2 . The probe was passed through an optical delay and focused to a spot of about 50 m diameter within the pump spot. The reflected probe was detected by a photodiode and lock-in amplifier using the pump chopper frequency as reference. All measure- ments were made with the sample at a temperature T = 12 K in an optical cryostat. The laser wavelength was  L = 767 nm, resonant with the fundamental gap of the SLs E1- HH1=1.62 eV. Figure 1a shows the change in probe reflectance nor- PHYSICAL REVIEW B 78, 233307 2008 1098-0121/2008/7823/2333074 ©2008 The American Physical Society 233307-1