RAPID COMMUNICATIONS PHYSICAL REVIEW B 83, 201103(R) (2011) Enhanced optical generation and detection of acoustic nanowaves in microcavities N. D. Lanzillotti-Kimura, 1,* A. Fainstein, 1 B. Perrin, 2 B. Jusserand, 2 L. Largeau, 3 O. Mauguin, 3 and A. Lemaitre 3 1 Centro At´ omico Bariloche & Instituto Balseiro, C.N.E.A., R8402AGP S. C. de Bariloche, R´ ıo Negro, Argentina 2 Institut des NanoSciences de Paris, UMR 7588 CNRS - Universit´ e Paris VI, 75252 Paris cedex 05, France 3 Laboratoire de Photonique et de Nanostructure, CNRS, Route de Nozay, 91460 Marcoussis, France (Received 5 March 2011; revised manuscript received 14 April 2011; published 25 May 2011) The enhancement of the ultrafast coherent generation and detection of acoustic phonons in an optical microcavity is experimentally studied. We report pump-probe terahertz ultrasonic experiments in an optical microcavity as a function of laser energy and probe incidence angle. By tuning the laser wavelength to resonate with the microcavity mode at normal incidence and simultaneously varying the incidence angle of the probe beam we achieve a double optical enhancement. Under this condition both the coherent generation and detection of acoustic nanowaves are strongly enhanced. We demonstrate the use of optical microcavities as a promising tool to study acoustic phonons in reduced dimensions with increased sensitivity. DOI: 10.1103/PhysRevB.83.201103 PACS number(s): 78.67.Pt, 63.22.Np, 78.20.hb, 78.20.hc An optical microcavity confines the electromagnetic field both spectrally and spatially, inducing strong changes in the light–matter interaction and giving rise to novel physical phenomena and devices. 1–5 In the case of planar semicon- ductor optical microcavities, two distributed Bragg reflectors (DBRs) enclose an optical spacer. 5 Optical microcavities have been the subject of very active research during the last 15 years, and have been used to study the modification of the photonic lifetimes, 6 parametric oscillations, 3 cavity polariton Bose-Einstein condensates, 7,8 the polariton laser, 9,10 and amplification of Raman scattering signals, 11–13 among others. Here we experimentally study the signal enhancement in coherent phonon generation using optical microcavities. The photonic confinement and amplification have been used in these high-Q resonators to amplify the optical generation of incoherent phonons through Raman processes and to prove effects in the phonon physics and dynamics in semiconductor nanostructures. 11–17 The optical resonances can be comple- mented with electronic resonances giving rise to amplified Raman cross sections of up to 10 7 . On the contrary, the use of optical confinement for the enhanced coherent generation of acoustic phonons (in contrast with incoherent generation by spontaneous Raman scattering) is a concept that has been rarely treated up to now. The realization of a monochromatic, coherent, and intense source of ultrahigh frequency acoustic phonons 18–20 is only one of its potential applications. In coherent phonon generation experiments a femtosecond laser pulse (pump) generates coherent subterahertz phonons in a structure. These phonons modulate the optical properties of the sample. Finally, a second, delayed, and less intense laser pulse (probe) detects the time-dependent optical reflectivity. 21 The coherent generation of acoustic phonons using ultrafast lasers is characterized by a low efficiency in the light–hypersound transduction. In a previous work we demonstrated that planar optical microcavities can be used in coherent generation experiments; 22 we showed indications of signal enhancement as the optical resonance was approached and the modification of the selection rules in the phonon generation-detection process. Maris and co-workers 23 proposed a similar scheme using external microcavities to study systems with small photoelastic constants, presenting an alternative tool to the interferometric detection introduced by Perrin et al. 24 The modulation of optical microcavity modes, on the other hand, has been studied by the injection of hypersound pulses in both the polaritonic and the optic regimes. 25–27 Similarly, surface acoustic waves have been used to modulate optical microcavities and to control cavity polaritons. 28–30 In this work we present results of coherent phonon generation experiments in an acoustic nanocavity embedded in an optical microcavity under optical resonance conditions. It has been theoretically shown that the maximum amplification of the coherent generation and detection using microcavities cannot be reached simultaneously when the pump and probe beams have the same wavelength and angle of incidence. 31 In fact, when the laser is tuned with the cavity mode, although the coherent phonon generation efficiency reaches its maximum, the detection is an absolute zero. 32 Taking advantage of the light dispersion in microcavities, we introduce a strategy to achieve simultaneously the maximum amplification of both the generation and the detection of coherent acoustic phonons. We analyze the dependence of the microcavity efficiency on the laser wavelength and angle of incidence. The studied sample consists of a planar semiconductor optical microcavity grown on a 001 oriented GaAs substrate by molecular beam epitaxy. A scheme of the sample can be found in Fig. 1(a). The optical microcavity is composed of two distributed Bragg reflectors (DBRs) enclosing a spacer. The top (bottom) DBR is composed of 3 (10) periods of Ga 0.8 Al 0.2 As/AlAs 55.39/64.19 nm, corresponding to a (λ l /4, λ l /4) multilayer. Here λ l is the resonant wavelength of the microcavity. We choose a different number of periods in the DBRs to get a symmetric cavity mode by compensating the difference between the indices of refraction of air and the GaAs substrate. The Q factor of the optical microcavity is around 60, allowing 80-fs light pulses to enter in the resonator mode without being filtered. An acoustic nanocavity 14,17,20,33 acts as the 2λ l spacer of the optical microcavity. The coherent generation and detection of the phonons of the latter structure will be the subject of the present study. The acoustic nanocavity is formed by two phonon mirrors (PM); each reflector is formed by 13 bilayers of GaAs/AlAs 5.75/2.27 nm, corresponding to (3λ ph /4, λ ph /4) stacks. 34 A 3λ ph /2 GaAs layer constitutes the spacer between the two PMs. Here λ ph is the acoustic resonant wavelength. The electronic 201103-1 1098-0121/2011/83(20)/201103(4) ©2011 American Physical Society