Carrier-Mediated Opto-Mechanical Coupling in GaAs Cantilevers


Hajime Okamoto, Koji Onomitsu, Haruki Sanada*, Hideki Gotoh*,
Tetsuomi Sogawa*, and Hiroshi Yamaguchi
Physical Science Laboratory, *Optical Science Laboratory

 Optically induced dynamic backaction in micromechanical systems has recently become the focus of research [1, 2]. Cavity-induced opto-mechanical coupling via radiation pressure or photothermal stress influences the thermal vibration of the mechanical system, leading to amplification and de-amplification of the vibration modes [1, 2]. The vibration amplification is of great interest because it can lead to the self-oscillation of a micromechanical resonator [1, 2]. The de-amplification is of equal interest because it enables cooling of the vibration modes [1, 2]. In contrast, we have recently observed novel opto-mechanical coupling, which does not require any cavities but is based on optical carrier excitation [3, 4]. Here, we report the carrier-mediated opto-mechanical coupling found in n-GaAs/i-GaAs bilayer cantilevers [Fig. 1(a)].
 The carrier-mediated opto-mechanical coupling is based on the strain-assisted opto-piezoelectric effect, which is associated with the separation of electron-hole pairs due to the built-in electric field. Thermal vibration of the [110]-oriented cantilever is amplified by the optical excitation with the near absorption-edge wavelength (λex = 840 nm at 50 K) and the self-oscillation is induced for the strong excitation (Pex > 10µW) [Fig. 1(b)]. In contrast, for the [-110]-oriented cantilever, the opto-piezoelectric backaction de-amplifies the vibration because the piezoelectric effect is reversed in the 90-degree rotated orientation [Fig. 1(c)]. This opto-piezoelectric backaction is maximized when the laser wavelength matches the optical absorption edge [4]. This is because the strain-induced change in the optical absorption is maximized at the strain-sensitive absorption edge. This carrier-mediated opto-mechanical coupling has an advantage in compatibility with semiconductor opto-electronics and will also provide a tool for studying the fundamental properties of semiconductors, such as carrier dynamics, strain effects, and carrier-related energy relaxation.

[1] I. Favero and K. Karrai, Nature Photon. 3 (2009) 201.
[2] C. H. Metzger and K. Karrai, Nature 432 (2004) 1002.
[3] H. Okamoto et al., Appl. Phys. Express. 2 (2009) 035001.
[4] H. Okamoto et al., Phys. Rev. Lett. 106 (2011) 036801.

Fig. 1. (a) Scanning electron micrograph of the cantilever. The cantilever consists of 100-nm-thick n -GaAs and 200-nm-thick i-GaAs.
The Ti:Sa cw laser beam is focused on the leg where the larger strain results. The thermal vibration is detected with the He:Ne
cw laser beam by laser interferometry. The measurements were done in a vacuum at 50 K. The Ti:Sa laser power dependence
of the displacement noise power spectrum for λex = 840 nm in the (b) [110]-oriented cantilever and (c) [-110]-oriented cantilever.

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