Opto- and Electro-Mechanical coupling between the depletion and the piezoelectric region of a Gallium Arsenide (GaAs) Micro Torsional Resonator Photodiode

Abstract

In this work, the opto-piezo actuation and piezoelectric actuation in gallium arsenide (GaAs) is experimentally and theoretically verified. Experimentally the response of the respective actuations are measured using the current generated from the inverse piezoelectric effect. The mechanical structure used to generate this current is a micron size torsional resonator fabricated from a GaAs photodiode heterostructure. The photodiode heterostructure is optically and electrically designed as a photovoltaic (PV) cell while mechanically the structure resembles a bimorph. The bimorph design is a result of the PV cell consisting of a pn junction and a heterojunction where the depletion regions have the additional property of being piezoelectric. The opto-piezo actuation results from using the photogenerated voltage to piezoelectrically drive a mechanical structure. Using light modulated at the resonance frequency of the torsional resonator the measured current is shown to linearly increase with intensity. For the electrical actuation case, the torsional resonator is driven using the non- linear response of the pn junction to an applied voltage. The non-linear response results in generation of voltage at the harmonic frequencies of the applied voltage. The voltage generated at twice the applied frequency is given the label 2f and is used to piezoelectrically drive the mechanical structure. The above results for the two methods of actuation are theoretically validated by deriving a model for the expected current. The model predicts the current as a function of the voltage. For the opto-piezo case this voltage is the photovoltage. The photovoltage is determined using the AC PV model. This model is derived using the DC PV model and predicts the AC operation of a photodiode in the 3rd and 4th quadrants to resistive and reactive loads. Using the opto-mechanical coupling coefficient the efficiency of the opto-piezo actuation is compared to opto-thermal actuation and radiation pressure actuation. It is shown that the opto-piezo effect, in general, is several orders of magnitude better than the other two in converting optical energy into mechanical energy. This is an important result because in situations where low optical powers are only available and power, in general, cannot be spared, for e.g. on a satellite, devices that make use of the opto-piezo effect could be used for either actuation or sensing. Generally however, using the opto-piezo effect can lead to either integration of existing photonic devices with mechanical resonators or new photonic devices all together. For e.g. using the opto-piezo effect an adaptable optical correlator can be made which could be used to make artificial intelligent machines.