Optomechanics Using Excitonic Transitions in Semiconductor Heterostructures

Hajime Okamoto1, Takayuki Watanabe1,2, Ryuichi Ohta1, Koji Onomitsu3,
Hideki Gotoh4, Tetsuomi Sogawa4, and Hiroshi Yamaguchi1,2
1Physical Science Laboratory, 2Tohoku University, 3Materials Science Laboratory,
4Optical Science Laboratory

 Optical control of micromechanical resonators has been widely demonstrated via cavity-enhanced radiation pressure or photothermal backaction [1]. Such cavity optomechanics allow highly tunable manipulation of a single mechanical resonator, including vibration amplification and damping (i.e., mode cooling). However, it cannot be straightforwardly extended to integrated mechanical systems because it needs delicate cavity operation, including tapered-fiber access and coupling adjustment. Thus, an alternative cavity-free approach is highly demanded in order to practically apply the optical control capability to integrated micromechanical systems. Here, we present excitonic optomechanics implemented in a compound semiconductor microcantilever. By using opto-piezoelectric stress induced via excitonic transitions, cavity-free control of a micromechanical resonator is achieved [2].
 We used the AlGaAs/GaAs heterostructured cantilever shown in Fig. 1(a). In this system, the optically excited electrons and holes are separated via the built-in electric field [Fig. 1(b)]. This causes piezoelectric (compressive) stress along the longitudinal direction in the GaAs layer, which leads to downward bending of the cantilever [Fig. 1(a)]. This effect depends on the optical absorption and on strain via the deformation potential. Therefore, strain-dependent opto-piezoelectric stress appears around the exciton resonance, where the per-strain change in the optical absorption is maximized. Since this stress acts on the cantilever in a time delay with respect to the optical excitation, it causes a self-feedback effect on the mechanical vibration, where the sign and gain of the self-feedback depend on the slope of the absorption spectrum [Fig. 1(c)]. We achieved both vibration amplification and mode cooling (damping) by detuning the photon energy from the exciton resonance [Fig. 1(d), (e)].

Fig. 1. (a) Schematic drawing of the cantilever and opto-piezoelectric effect. (b) Calculated energy-band diagram, in which the separation of photoexcited electrons and holes is schematically drawn. (c) Photoluminescence excitation (PLE) spectrum in the vicinity of the exciton resonance. (d) Photon-energy dependence of mode temperature (Teff) normalized by the sample temperature T = 9.2 K for the laser power of 1.19 μW. The broken line is a theoretical fit, which depends on the slope of the PLE spectrum. (e) Laser-power dependence of the thermal displacement noise power spectrum at 1.516 eV.