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(a) Optical micrograph of the cantilever used for the measurement. (b) Energy band diagram of the cantilever along the thickness direction. The energy scale is exaggerated for a clear understanding of the structure. Both surface Fermi levels of the cantilever are pinned in the band gap. The blue area is the conduction band (CB) and the green area is the valence band (VB). Closed circles show electrons excited by photons (red arrow) and open circles represent holes generated in the VB. Orange arrows indicate directions of each carrier’s movement. (c) Schematic illustration of the cantilever and measurement setup. A CW Ti:Sa laser is used as an excitation laser and its spot size on the sample is about 6 μm. A HeNe Doppler interferometer is used to detect cantilever motion. HeNe laser diameter is about 1 μm.
PSDs of the cantilever’s fundamental mode for different excitation laser wavelengths (blue, green, red, and purple curves). The PSD without excitation is also shown (black solid line). The purple curve looks to have a higher noise floor and extra signal peaks other than the resonance peak. These peaks from a signal output voltage limitation of the Doppler vibrometer due to a too large cantilever amplitude; they are not real signals from the cantilever vibration. Besides, the peak amplitude and Q factor are evaluated to be much lower than their actual values because the resonance signal is not modulated properly.
Excitation laser wavelength dependence of the mechanical vibration at the resonance frequency. Three different laser powers (12 (red), 7.3 (green), and 3.6 (blue) μW) are used. Dots are data and solid lines are guides for the eyes. (a) Dependence of Γ. (b) Dependence of peak amplitude. (c) Photoluminescence spectrum, shown for comparison (black solid line).
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