Cavity Optomechnics with 150nm-thick GaAs Membrane

Optomechanical properties of 150nm-thick GaAs membrane are studied for exploring its potential use in membranein- the-middle approach of cavity optomechanics [1, 2]. GaAs posseses many interesting properties [3]; direct band gap transition, high mobility of electrons, and strong piezoelectric properties to name just a few. These extra properties may be found favorable to approach the quantum regime with optomechanical system. Investigating optonanomechanical properties of the semiconductor may also be beneficial for advancing already-mature optoelectrical, electrical, and electromechanical semiconductor integrated devices further. To investigate the optomechanical properties of a fabricated 150nm-thick intrinsic GaAs membrane (1.3mm1.9mm) a 975nm-laser (below band gap) is used to probe the mechanical resonances via beam deflection method. An 810nmlaser (above band gap), which is mode-coupled to a Fabry-P´erot cavity formed between the membrane and a mirror (Finesse: 24) inside a vacuum chamber (10 7Torr), is used to lock the cavity length at the cavity resonant slope and to induce mechanical oscillations by modulating the intensity from the offset level for ring down measurements. We observe the cavity cooling of the mechanical modes when the cavity is locked at the blue-side of the slope. The instability is set in when the cavity is around the red-side of the slope or the cavity input exceed 50W and prevents us from using these conditions for the ring down measurements. The mechanical ring down time, , decreases as the cavity input power is increased from 5W to 45W (cavity cooling). The measured ring down times are extrapolated down to zero cooling power to evaluate the intrinsic mechanical quality factor Q of the membrane, which is found to be 0.5106 for =23.4kHz fundamental mode at room temperature. Up to 150kHz the Q products are measured to be nearly constant (around 31010Hz). From the ratio of the extrapolated intrinsic ring down time to that at 50W the cooling factor can be evaluated for each mode. For the fundamental mode it is found to be as large as 12 meaning that the effective temperture of the mode is cooled down to 25K. We believe that the main cause of the cooling is neither the radiation pressure nor the photothermal effect given the relavant physical parameters. We will report the progress of the experiments aiming to reveal the underlying cavity cooling mechanism of the GaAs membrane.