Hemispherical Two-photon Response Photodetectors: Research On The GaAs-based Photodetector And Design For The Si-based Photodetector

When a non-centrosymmetric 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 TPR photodetector, which can act as both the nonlinear mixer and the detector replacing the phase-matched nonlinear crystal and the photomultiplier tube in a conventional autocorrelator, has been an attractive alternative for constructing optical autocorrelator, with advantages of lower cost, compactness, ease of use and a wide wavelength range of operation. So it is preferable to fabricating TPR photodetectors with high response sensitivity used in autocorrelation for measuring ultrashort laser pulses. It is possible to improve the TPR responsivity by adopting solid immersion lens (SIL) microscopy to increase the numerical aperture of the focused system so as to decrease the size of the focused spot. In theory the TPR responsivity can be improved by a factor of n4, where n is the refractive index of the semiconductor.The measurements of ultrashort laser pulses at 1.3μmand 1.5μm typically carried out by optical autocorrelation technique are becoming increasingly important for electrooptical sampling techniques and the characterization of high-speed optical communications systems. The most important semiconductors of GaAs and Si crystals are commonly used in optoelectronics. The bandgap of GaAs crystal is 1.43eV corresponding to a wavelength range of 0.88-1.73μm in TPR, while a wavelength range of 1.2-2.1μmin TPR for Si crystal with a bandgap of 1.12 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 optical communications systems. Therefore, the SIL microscopy was connected with the fabrication of TPR photodetectors for the first time to our knowledge, and the GaAs-based and the Si-based hemispherical TPR photodetectors were studied.For non-centrosymmetric GaAs crystal which belongs to the 43m symmetry group of crystals, both DFA and TPA can occur inside it. We developed the semi-insulating GaAs (SI-GaAs) material into a hemispherical photodetector with a radius of 3 mm and a bottom of (001) plane; and concentric circular and annular electrodes with a spacing of ⁰.5 mm were made on the bottom. The characteristics of TPR for the hemispherical SI-GaAs 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 [001] 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; compared to the SI-GaAs block photodetector with a size of 7×7×3.5mm3, the GaAs hemisphere as SIL improved the response sensitivity approximately five times; the responsivity of the detector was above 2 mA/W at 25 V bias, moreover, the responsivity of the hemispherical detector has a potential improvement with a pulse laser. Next, the anisotropy of DFA in GaAs single crystal was studied. In theory, the DFA effect is demonstrated to be the most significant when the polarization of the fundamental light is along <111> orientation; when the fundamental light is propagating perpendicularly to the (001) bottom of the GaAs hemisphere, the second-harmonic will radiate parallel to the (001) plane; the dependence of the DFA-induced photoexcited carrier density on the azimuth of the fundamental polarization is in accordance with the relationship of N∝1/2d₁₄² E₀^αω(1- cos4α), and it was confirmed by measuring the photocurrent dependent on the azimuth of polarization of the fundamental light; 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. Finally, the nonlinear dependence of the photocurrent on the bias was measured, and no saturation with bias was observed in the experiments. The above results confirm the DFA-based physical mechanism inside the hemispherical SI-GaAs detector.However, for the centrosymmetric Si single crystal which belongs to the m3m symmetry group of crystals, second-order nonlinear optical effects do not exist in the bulk of silicon at dipole approximation. But, there can be nonzero effective second-order susceptibility in silicon if the inversion symmetry is broken by the applied direct electric field. We theoretically studied electric field-induced (EFI) effective second-order (ESO) susceptibilities and EFI double-frequency effect for silicon. Specifically, the forms of the ESO susceptibilities agree with those of the C3 v, C 2vand C 4vsymmetry groups of crystals when the electric fields applied to silicon are along the [111], [110] and [001] directions, respectively. A general deduction of ESO susceptibility for silicon under an arbitrary applied electric field is also proposed. This research method can also be applied to the other centrosymmetric materials. Based on the EFI ESO susceptibilities, one can further study the anisotropy of EFI DFA in the bulk of silicon by the fundamental light propagating perpendicularly to the(110)plane for the case of the electric field along the [111] direction, perpendicularly to the(110) or (001) plane for the case of the electric field along the[110]direction, and the fundamental light propagating along any crystallographic orientation except for the [001] direction for the case of the electric field along the [001] direction, respectively. The theoretical demonstration shows that the EFI DFA is intensively dependent on the applied electric field, and proportional to the square of the amplitude of the electric field. Therefore, in theory, the EFI DFA is much more significant than the third-order nonlinear optical effect TPA. The fabrication of Si hemispherical SIL was presented, and further fabrication as a hemispherical Si detector and the research scheme of TPR characteristics for the future detector were also proposed.The main advantages of hemispherical TPR photodetecters are of ease of fabrication, high TPR sensitivity, and a wide wavelength range of operation. The research on the TPR photodetectors with high responsivity can facilitate the identification of the physical mechanisms of TPR. Especially, the GaAs-based and Si-based hemispherical photodetectors with their TPR wavelengths all covering the most commonly used wavelengths of 1.3μmand 1.5μm will be very useful in autocorrelation measurements, high-speed optoelectronics, optical communications systems, and the domain of THz science and technology , etc.