Xiuhuan Liu

Pockel’s effect and optical rectification in (111)-cut near-intrinsic silicon crystals

Pockel’s effect and optical rectification are demonstrated in the charge space region of a (111)-cut near-intrinsic silicon crystal by the use of a planar metal-insulator-semiconductor structure. The results show that both Pockel’s effect and optical rectification are so considerable that these effects should be taken into account for designing silicon-based photonic devices. The anisotropy of optical rectification is measured too, and experimental results are in good accordance with the theoretical analysis. These effects can also be used as a tool to investigate the properties of the charge space region of silicon devices in future.

Hemispherical semi-insulating GaAs double-frequency absorption photodetector operating at 1.3μm wavelength

The hemispherical semi-insulating GaAs photodetector operating at 1.3μm is presented. The GaAs hemisphere was used both as a detector and a solid immersion lens to improve the responsivity. The physical mechanism of the detector is attributed to double-frequency absorption (DFA) confirmed by the measured photocurrent quadratically dependent on the incident optical power and nonlinearly dependent on the bias and by the relationship between the photocurrent and the azimuth in agreement with the anisotropy of DFA in GaAs single crystal.

Light emission from cBN crystal synthesized at high pressure and high temperature

The light emission at a wavelength of about 400nm is observed from the nonintentionally doped n-cubic boron nitride crystal, when the avalanche breakdown occurs inside the cBN crystal that is prepared by hexagonal boron nitride at high pressure and high temperature using nitride as catalyst. The measured spectrum has a peak in the blue-violet range. It shows the electronic transition between valleys of the conduction band of the cBN crystal. At the same time, the current-controlled differential negative resistance phenomenon occurs as well.

Measurement of third-order nonlinear optical susceptibility of synthetic diamonds

Diamonds are wide-gap semiconductors possessing excellent physical and chemical properties; thus, they are regarded as very appropriate materials for optoelectronic devices. Based on the Kerr effect, we introduce a simple and feasible method for measuring the third-order nonlinear optical susceptibility of synthetic diamonds. In the experiments, synthetic type I diamond samples and transverse electro-optic modulation systems are utilized. As for the laser with the wavelength of 650 nm, the third-order susceptibility and Kerr coefficient of the diamond samples are obtained at ?(3)1212=2:17×10-23 m2=V2 and S44= 1.93×10-23 m2=V2, respectively.

The DC electric field induced second-order nonlinear susceptibility of silicon crystals

Silicon crystal is a kind of centrosymmetric materials, which has no second-order optical effects at dipole approximation. However, the inversion symmetry of silicon crystals will be broken by the built-in field or the DC electric field applied to it. We theoretically studied the response of the third-order susceptibility to the electric field and deduced the effective second-order susceptibility tensors when the electric fields applied to silicon are along the [111], [110] and [001] directions, respectively. The results show that the forms of the effective second-order susceptibilities of Si are consistent with those of C3V, C2V and C4V symmetry groups of crystals which indicate that silicon crystals should belong to C3V, C2Vand C4V symmetry groups instead of Oh symmetry group, respectively. So silicon crystals will exhibit some second-order nonlinear optical properties corresponding to related symmetric crystals under the corresponding incident electric fields. This research method of the electric field-induced second-order nonlinear optical processes can also be used to other centrosymmetric materials.

Pockels effect and optical rectification induced by the built-in electric field in the space charge regions of surface layers of silicon crystals

In this paper, electric field induced Pockels effect and optical rectification were demonstrated in the space charge regions of surface layers of (001)- and (110)-cut silicon crystals. The Pockels signals were much larger than the Kerr signals. These effects were considerable that they should be taken into account when designing silicon devices. Dependence of the optical rectification on various depth of the silicon crystal was investigated which could be used as a simple and nondestructive method to detect distribution of electric field of silicon devices.

Electro-optic effects induced by the built-in electric field in a ｛001｝-cut silicon crystal

Pockel’s effect and optical rectification induced by the built-in electric field in the space charge region of a silicon surface layer are demonstrated in a ｛001｝-cut high-resistance silicon crystal. The half-wave voltage is about 203 V, deduced by Pockel’s effect. The ratio χzxx(2)/χzzz(2) is calculated to be about 0.942 according to optical rectification. Our comparison with the Kerr signal shows that Pockel’s signal is much stronger. This indicates that these effects are so considerable that they should be taken into account when designing silicon-based photonic devices.

Electroluminescence of cubic boron nitride single crystal flakes with color-zoning

The current-voltage (I-V) characteristics and phenomena of electroluminescence of cubic boron nitride (cBN) single crystal flakes with color-zoning under extremely non-uniform electric fields (ENUEFs) induced by needle-plate electrodes were observed. When a cBN flake with sizes of 0.3×0.3×0.1 mm3 was tightly fixed between the tungsten needle and brass plate electrodes in the atmosphere, the I-V relationship exhibited nonlinearity, and peculiar phenomena of electroluminescence with bright blue-violet light appeared at the bias voltage in a range of 700-1200 V. The current-controlled differential negative resistance was synchronously observed. The electroluminescent phenomena were somewhat different for cases of the needle electrode respectively contacting to the amber and transparent zones. The electroluminescent radiations of cBN flakes biased at voltages with a range of 600-1550 V were also investigated in vacuum. In a vacuum chamber, the green emitting phosphor spread around the cBN flake might be excited by the vacuum ultraviolet (VUV) emission from the cBN crystal, and the green fluorescence was observed by naked eyes. The VUV radiation spectrum with a peak wavelength of 149 nm was measured. In the atmosphere, the blue-violet light emission may be the gas discharge resulted from the air ionization induced by the VUV emission from the cBN crystal under the ENUEF, and the ENUEF subsequently keeps the air discharging. The VUV emission from the cBN crystal under the ENUEF can be caused by the original interband transition and the subsequent intraband transfer for electrons, and the final electron-hole direct recombination.

Photo-and-dark-current-voltage characteristics of normal-incidence GaAs photodetectors with two types of electrode configurations

The characteristics of photo-current-voltage and dark-current-voltage for two-photon-response semi-insulating GaAs photodetectors responding to near-infrared wavelengths of 1.31 μm and 1.55μm are investigated. The semi-insulating GaAs photodetectors were fabricated into hemisphere on whose bottom two types of electrodes were deposited. In experiments, the incident laser was adjusted to travel normally to the photodetector and focus at the center of the bottom so as to improve the nonlinear photo-responsivity markedly. It is observed that the photocurrent dependent on bias exhibits quadratic nonlinearity for both lasers and both electrode configurations, which reflects frequency-doubled absorption responsible for the physical mechanisms of the photodetectors; and the reasonable analysis demonstrates the important role of the electric-field-induced frequency-doubled absorption in two-photon response. Furthermore, it is found that the photocurrent is quite more greater when the electrode positioned at the bottom center of the photodetectors (central electrode for short) is negatively charged than that in the case of it positively charged under the conditions of the identical bias voltage and the same incident optical power; while the dark-current varies in exactly the opposite mode compared to the photocurrent. The aforementioned disparate variations of the photocurrent and the dark-current are well interpreted by the theory of surface band-bending of semi-insulating GaAs, and such variations result in a large ratio of photo-current to dark-current in the case of the central electrode negatively charged. The investigated results also indicate that the optimization of electrode structure is essential to improve the photo-responsivity of the photodetector.

Measurement of electro-optic effects in near-intrinsic silicon

The electro-optic effects in silicon include Kerr effect, plasma dispersion effect, and Franz-Keldysh effect etc.. Silicon does not have the linear electro-optic effect in the bulk because of the inversion symmetry, which restricts the development of the silicon-based optoelectronics and silicon photonics. However, the electric field can destroy the inversion symmetry of silicon, and produce so-called electric-field-induced linear electro-optic effect. In intrinsic or near-intrinsic silicon, these electro-optic effects exist simultaneously. In this paper, a transverse electro-optic modulation system was designed to detect these electro-optic effects. The electric-field-induced linear electro-optic effect was demonstrated in the space charge region of silicon sample and distinguished from Kerr effect based on the different frequency response. The relationship between the linear electro-optic signal and the azimuth angle of the analyzer was measured too, which was used for distinguishing the electric-field-induced linear electro-optic effect from the plasma dispersion effect. The results showed that the electric-field-induced linear electro-optic effect was stronger than Kerr effect and the plasma dispersion effect in the near-intrinsic silicon samples.