Research On Two-photon Response Of Hemispherical Si-and GaAs-based Photodetectors

As the most important semiconductors, Si and GaAs crystals are commonly used in microelectronics and optoelectronics. Silicon photonics has attracted much attention recently because of its potential applications in the near- and mid-infrared regions especially, and it is a potential candidate for future optoelectronic integration, photonic integration, and all-optical signal processing.When a semiconductor crystal is irradiated by a beam of laser whose photon energy is less than the bandgap of the semiconductor but greater than half of the bandgap, both double-frequency absorption (DFA) and two-photon absorption (TPA) can occur inside the crystal. DFA is a second-order nonlinear optical effect, in which the frequency of the fundamental light is doubled firstly, then one frequency-doubled photon with energy greater than the bandgap is absorbed by the crystal and thus intrinsic transition occurs; while TPA is a third-order nonlinear optical effect, in which two fundamental photons are absorbed simultaneously to induce intrinsic transition accordingly. DFA and TPA are generally defined as two-photon response (TPR) which is proportional to the square of the incident optical intensity. The bandgap of Si crystal is 1.12eV corresponding to a wavelength range of 1.2-2.1μm in TPR, while a wavelength range of 0.88-1.73μm in TPR for GaAs crystal with a bandgap of 1.43 eV. The TPR regions of the two crystals all cover the commonly used wavelengths of 1.3μm and 1.5μm in electrooptical sampling techniques and high-speed optical communications systems. Therefore, the solid immersion lens (SIL) microscopy was connected with the fabrication of TPR photodetectors for the first time to our knowledge, and the Si-based and the GaAs-based hemispherical TPR photodetectors were studied. The centrosymmetric Si crystal belongs to the m3m symmetry group of crystals. We developed the near-intrinsic Si material into a hemispherical photodetector with a radius of 3 mm and a bottom of (110) plane; and concentric circular and annular electrodes with a spacing of ~0.15 mm were made on the bottom. The characteristics of TPR for the hemispherical Si photodetector operating at a wavelength of 1.3μm from a continuous wave (cw) solid laser were studied. In the experiments, the fundamental light was aligned with [ī?ο] orientation and focused at the center of the hemisphere by a focusing lens. First of all, a measurement of photocurrent dependent on the incident optical power was carried out, and the measured result shows a quadratic dependence of the photocurrent on the incident power, the responsivity of the detector was above 2μA/W at 1 V bias, moreover, the responsivity of the hemispherical detector has a potential improvement with a pulse laser. Second, the anisotropy of TPR in Si single crystal was studied. The dependence of the photocurrent on the azimuth of the incident optical field is consistent with the anisotropy of the two-photon absorption in Si crystal. Deduced from the anisotropy of the photocurrent of the photodetector, the ratio of the two independent components of the third-order susceptibility of silicon is obtained to be 0.42. Third, the output photocurrent approached its maximum value with 1 V bias consistent with two-photon absorption in Si crystal also.For non-centrosymmetric GaAs crystal which belongs to the 43m symmetry group of crystals, both DFA and TPA can occur inside it. DFA is a second-order nonlinear optical effect, therefore, the DFA is much more significant than the TPA of third-order nonlinear optical effect in theory. The measure of the semi-insulating GaAs (SI-GaAs) material was similar with that of Si. The responsivity of the detector was above 2 mA/W at 25 V bias, the GaAs hemisphere as SIL improved the response sensitivity approximately five times compared to the SI-GaAs block photodetector. The deviated shape of the original ball lens from the perfect sphere and the thickness error of the SIL reduce to the function of SIL. The dependence of the photoexcited carrier density on the azimuth of the fundamental light polarization is in accordance with the theory of DFA. The detector have a larger constant background current than the dark-current, one contribution to the constant background current in the fitted result is effective DFA consequent upon the third-order dielectric response to the surface field and the applied field, another contribution is two-step-single-photon absorption due to the EL2-like defect level in the mid-gap in SI-GaAs. We have a conclusion through the research about anisotropy of DFA, TPA and electric-field-induced second-harmonic generation (EFISHG) in theory and experiment of Si- and GaAs-based photodetector. The conclusion is that the physical mechanism of the detector is attributed to DFA in GaAs crystal while that is TPA in Si crystal under lower voltage. This laid the foundation of the future research on TPR photodetectors.The research on the TPR photodetectors with high responsivity can facilitate the identification of the physical mechanisms of TPR. Specially, the GaAs-based and Si-based hemispherical photodetectors will be very useful in autocorrelation measurements, high-speed optoelectronics, optical communications systems, and the domain of THz science and technology, etc.