Investigation on the electric-field-induced Pockels effect and optical rectification in near-intrinsic silicon samples

Based on the classical polarization theory, we studied and specified the physical mechanism of the electric-field-induced (EFI) Pockels effect and optical rectification in the space charge region of a near-intrinsic silicon sample with the planar capacitor structure. Especially, the effect of the applied DC bias on these EFI effects was investigated. The results show that the electro-optic signal from Pockels effect in silicon linearly increases with the applied DC voltage and the modulating voltage, and the signal of optical rectification is linearly enhanced by the DC bias as well, but the polarization characteristic of optical rectification does not vary. The enhancement of these EFI effects is mainly owed to the strengthening of the built-in field and the extension of the space charge region in the silicon sample. The Kerr effect of silicon was also detected and contrasted against the EFI Pockels effect, and it is verified that the EFI Pockels effect is much stronger than the Kerr effect in the silicon sample. These EFI effects are significant for the development of silicon photonics or silicon optoelectronics.     

High Speed Signal Wavelength Conversion Using Stimulated Raman Effect in Ultrasmall Silicon-on-Insulator Optical Waveguides

We propose the high speed signal wavelength conversion based on stimulated Raman effect on silicon waveguides. Simulation results of non-return-to-zero （NRZ） pseudorandom bit sequence （2^7-1 code） at 500-Gb/s rate of conversion in an ultrasmall silicon-on-insulator （SOI） optical waveguide are presented by co-propagating pump optical field. The most attractive issue is that the inverted converted signal can be obtained at the same wavelength as that of primary signal. In addition, the conversion performances, including extinction ratio （ER） and average peak power of conversion signal, depend strongly on the launching pump intensity.     

Quick-response film photodetector of high-power laser radiation based on the optical rectification effect

The efficiency of conversion of high-power laser radiation to an electric signal based on the optical rectification effect in nanographite films is studied experimentally. The amplitude of the signal is found to significantly depend on the size of the film, as well as on the length and arrangement of measuring electrodes. The maximal sensitivity of the photodetector (above 500 mV/MW at a wavelength of 1064 nm) consisting of the film with electrodes and operating without an external power supply and add-on components is shown to be achieved when the size of the film is comparable to the laser beam diameter. The sensitivity of the photodetector is studied under the condition when a nanosecond beam from a pulsed laser scans the surface of the film in two mutually perpendicular directions. The local sensitivity increases near the free ends of the photodetector. It is shown that the nanographite detector and a similar photodetector made of a polished silicon wafer have radically different parameters.     

Optical harmonic generation in amorphous silicon nitride microcavities

Second harmonic (SH) and third harmonic (TH) generation in amorphous silicon nitride microcavity are experimentally investigated. The transmitted SH and TH signals are measured in the 0.9-1.4 μm spectral range, showing enhanced nonlinear conversion efficiency corresponding to resonant wavelength and optical band edges. The efficiencies of the SH and TH generation processes are found to be enhanced by about two and one orders of magnitude, respectively, in comparison with the case of reference amorphous silicon nitride sample. The SH spectra can be reasonably interpreted as due to surface/interface harmonic generation, while the TH signal is related to bulk isotropic third-order polarization. The results obtained for the TH signal are discussed in terms of the linear optical properties of amorphous silicon nitride thin films.     

带孔硅纳米薄膜热整流及声子散射特性研究

本文基于非平衡的分子动力学模拟方法计算了带有三角形孔的硅纳米薄膜的界面热阻特性,结果表明300-1100 K范围内随着热流方向的改变,在含有三角形孔的硅纳米薄膜中存在热整流效应,热整流系数达28％.同时借助于声子波包动力学模拟方法,获得了不同频率下的纵波声子在三角形孔处的散射特性,结果表明纵波声子在散射过程中产生了横波声子,并且从三角形底部向顶部入射的声子能量透射系数比反向时平均低22％.不对称结构引起的声子透射率的差异是引起热整流效应的主要因素.     

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn_(1-x)Cd_xSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.     

双波泵浦非对称量子阱的光学整流效应

为了实现基于光整流方式的室温下宽调谐高效率太赫兹源,设计了一种适于双波长CO2激光器共振子带跃迁泵浦的双阱嵌套形非对称量子阱结构,结构组分为Al0.5Ga0.5As/Ga As/Al0.2Ga0.8As,采用密度矩阵及迭代方法计算了其二阶非线性光整流系数χo(2)表达式,在导带为抛物线形和非抛物线形两种条件下对χo(2)进行对比研究。计算结果表明,其偶极跃迁矩阵元随量子阱总阱宽的增大而逐渐减小。当固定量子阱总阱宽及其中一束泵浦光波长不变时,χo(2)随着另一束泵浦光波长的增加,呈现出先增大后减小的变化趋势。当深阱为7 nm、总阱宽为23 nm、两束泵浦光相等为10.64μm时,χo(2)达到最大值5.925×10-6m/V;随着总阱宽的增大,χo(2)曲线呈现"红移"现象,其原因为量子限制效应导致了不同阱宽条件下的量子阱能级值差不同,从而造成满足泵浦光光子能量与能级差共振条件的变化。导带为抛物线形和非抛物线形两种条件下的χo(2)的最大值对应泵浦光波长基本相同,χo(2)数值上的差异主要由跃迁矩阵元的不同导致。     

Empirical study of nonlinearity tensor dominating THz generation in barium borate crystal through optical rectification

The optical rectification is an important optical method to generate the THz wave. However, there is a lack of knowledge about the properties of the nonlinearity tensor which governs the optical rectification process. In this work, we demonstrate that some key information of the nonlinearity tensor of barium borate (BBO) crystal could be revealed by 2-dimensional time-domain spectroscopic measurements. The experimental results indicate that d ₂₂ of the nonlinearity tensor coefficients of BBO crystal plays negligible role during the THz generation by optical rectification. At the same time, a proportional relationship among three other nonzero nonlinearity tensor coefficients, i.e., d ₃₃, d ₃₁, and d ₁₅, could be obtained empirically. It is worth noting that our method is also applicable to similar nonlinearity study in THz region of other types of crystals.     

Design of two-dimensional zero reference codes with a genetic algorithm

In mask-alignment systems a reference signal is needed to align the mask with the silicon wafers. The optical reference signal is the autocorrelation of two two-dimensional (2D) codes with binary transmittance. For a long time, one-dimensional codes have been used in grating-measurement systems to obtain a reference signal. The design of this type of code has needed a great computational effort, which limits the size of the code to about 100 elements. Recently, we have applied genetic algorithms to design codes with arbitrary length. We propose the application of these algorithms to design 2D codes to generate 2D optical signals used in mask-alignment systems.     

Testing the reliability of ESR dating of optically exposed buried quartz sediments

Optical dating of quartz by optically stimulated luminescence has a time range that is generally less than about 500 ka, due to relatively rapid saturation of the available luminescence defects in quartz. We test here a new method, electron spin resonance (ESR) optical dating of quartz, in which radiation-sensitive defects at aluminum and titanium atoms on silicon sites give rise to signals which can only be detected near liquid nitrogen temperature and which have a much higher capacity to absorb radiation dose before saturating than optical luminescence-detected signals. Our results show this method yields agreement with independent age control out to about 2.5 million years, extending here the dating range of optically exposed quartz in sediments in along-shore sediments (aeolian and waterlain) by a factor of about 5. Three sites in along-shore lacustrine and marine aeolian environments yielded very good agreement with independent age control. Details of single saturating exponential fitting in relation to agreement with expected burial doses and annealing of Al and Ti signals provide additional data to consider the best approaches to the dating method. Furthermore, we propose a new criterion for ESR optical dating: both the Al signal and Ti signal ages must agree to insure accurate burial ages. Moreover, when Al signal ages are lower than Ti signal ages, then the Al signal may be taken as the minimum burial age.     

Non-linear optical diagnostic of a-Si:H thin films deposited by RF-glow discharge

We have evidenced the high sensitivity of infra red-induced second harmonic generation (IR-ISHG) to the structural changes that occurred in amorphous hydrogenated silicon films (a-Si:H) prepared by Rf-glow discharge technique at different substrate temperatures and doping types. In every case, a maximal signal of the IR-induced SHG is achieved at a temperature of about 110 K and pump–probe delaying time about 22–38 ps. This one indicates a marked effect of doped subsystems in the observed non-linear optical effects. A tremendous effect of doping is established from a drastic change of the IR-induced SHG behavior presenting an anomaly at about 400 MW/cm² for a pumping power with wavelength 3.7 μm. A minimum of the SHG is observed in that case for standard non-doped films. Note here that the doping type does not markedly affect the behavior of the second-order non-linear optical susceptibility. The thermo-annealing leads to a slight decrease of the effective second-order susceptibilities. More drastic changes are observed with doped samples for the pump-probe delaying time from about 39 till 24 ps.     

Optical rectification in CdS

Optical rectification non-linear process in CdS single crystals at the Nd-YA1G and CO₂ frequencies is reported. V_OR signal measurements versus θ angle, between radiation E-field and crystal c-axis, give non-zero terms values of the optical rectification tensor. The frequency dispersion effect of this non-linear process is discussed.     

Compact all-optical switch for WDM networks based on Raman effect in silicon nanowavegide

We propose an all-optical switching scheme based on Raman gain in a silicon nanowaveguide suitable for multichannel optical communication. Raman gain is used for amplification of a control pulse with a higher wavelength, which depletes the tuned channel signal. Separation between control and signal pulses should be equal to the Raman shift in silicon. By employing a 3 mm channel nanowaveguide, we demonstrate a channel attenuation of about 12 dB, while the suppression ratios for the first and second neighboring channels are about 1.6 dB and 1 dB, respectively. This scheme can be used as an all-optical switch in dense wavelength division multiplexing networks. Moreover, we demonstrate that the depleted channel can be retrieved by a control pulse with lower wavelength in which the pulse amplifies the channel in contrast to the prior situation.     

Three-dimensional rectification of a gradient force in a strong nonmonochromatic field

A new three-dimensional scheme for rectifying a gradient force is proposed and analyzed. The scheme is based on the use of a strong, partially coherent optical field involving a component with a fluctuating phase. It is shown that the rectification of a gradient force acting on atoms with a nondegenerate ground state is a second-order effect with respect to field strength in this scheme, whereas an analogous effect is third-order in coherent bichromatic fields. Conditions for three-dimensional confinement of atoms are obtained by using the velocity dependence of the rectified radiative force. For a large class of atoms, such as even-even isotopes of ytterbium and alkaline-earth elements, these conditions can be implemented at a relatively high effective temperature (of the particle ensemble) of about 10 K. This finding can be used to widen substantially the range of energies of atoms amenable to effective three-dimensional optical manipulation.     

Influence of the Depolarization Effect on Optical Rectification in Electric-Field-Biased Quantum Well

The effect of the depolarization field on the optical rectification due to resonant intersubband transition in electric-field-biased quantum well is studied. Calculations are performed using a compact density-matrix approach. The obtained results show that the depolarization effect shifts the peak of the optical rectification coefficient, and the shift increases with increasing of the bias field.     

Polaron effects on the optical rectification in a two-dimensional quantum pseudodot system

Using the compact density-matrix approach and the iterative method, the optical rectification in a two-dimensional quantum pseudodot system under the influence of a static magnetic field is theoretically investigated. The effects of an external magnetic field and the geometrical size of the pseudodot system on the optical rectification are presented. What’s more, we find that the polaron effect has an important influence on the optical rectification.     

基于硅波导的喇曼光放大器

在硅波导上添加反向偏压的PIN结构,当波导产生受激喇曼散射时,可以将波导中双光子吸收(TPA)产生的光生自由载流子扫出波导,降低了波导的非线性损失,极大地提高了硅波导中泵浦光对信号光的喇曼增益。为了应用已经非常成熟的硅工艺,并且应用硅波导使器件小型化,根据法布里-帕罗(F-P)腔和行波放大器理论,在硅波导两端的解理面蒸镀增透膜,应用这种波导的喇曼效应设计了一种光放大器,即基于硅波导的喇曼光放大器。建立了计算放大器增益的方程,给出了不同波导长度和输入功率情况下的放大器增益,得出适当增加波导长度和泵浦光功率可以得到较高喇曼增益的结论。基于硅的光放大器有较高的饱和功率且没有泵浦源的限制,通过调整泵浦激光的波长可以放大不同波长的信号光。     

薄膜硅/晶体硅异质结电池中本征硅薄膜钝化层的性质及光发射谱研究

本文采用甚高频等离子体化学气相沉积技术 (VHF-PECVD) 制备薄膜硅/晶体硅异质结电池中的本征硅薄膜钝化层, 光发射谱 (OES) 测量技术研究了硅薄膜沉积过程中等离子体发光谱随时间的变化. 结果表明: 在实验优化条件下等离子体发光谱很快达到稳定 (大约25 s), 并且SiH^*/H_α^* 的比值随时间变化较小, 避免了生长过程中硅薄膜结构的不均匀性, 这主要是SiH₄没有完全耗尽避免了SiH₄的反向扩散. 进一步研究了沉积参数对稳态发光谱和硅薄膜性质的影响, 结果表明: 随着硅烷浓度增加, H_α^*峰强度减小, SiH^*峰强度增加, 薄膜从微晶转变成非晶, 非晶硅薄膜钝化效果好; 随着沉积气压增大, H_α^*和 SiH^*峰强度先增加后减小, 高气压下H_α^*和 SiH^*峰强度下降主要是反应前驱物的聚合形成高聚合物, 不利于形成高质量的硅薄膜, 因此钝化效果下降; 随着反应功率密度增加, H_α^*和 SiH^*峰强度增大, 当功率密度为150 mW/cm² 趋于饱和, 硅薄膜的致密度和钝化效果也开始下降, 50 mW/cm²的低功率密度下硅薄膜钝化效果差可能是由于原子H 浓度低, 不能完全钝化单晶硅表面的悬挂键. 关键词： 薄膜硅 异质结 光发射谱 钝化     

Generation of a terahertz supercontinuum by the self-scattering of a femtosecond pulse in the optical-rectification regime

The possibility of generating a terahertz spectral supercontinuum in the regime of the optical rectification of a femtosecond optical pulse is predicted. The transient and asymptotic stages of generation are distinguished. The supercontinuum consists of the soliton and nonsoliton components of a terahertz signal at the asymptotic stage. It is shown that the formation of supercontinuums is accompanied by a redshift of the median of the opticalpulse spectrum.     

All-Optical Wavelength Conversion Based on Four-Wave Mixing in Dispersion-Engineered Silicon Nanowaveguides

We demonstrate experimentally all-optical wavelength conversion based on four-wave mixing in dispersion-engineered silicon nanowaveguides with a picosecond pulse pump. We find that the conversion efficiency is significantly limited by nonlinear losses induced by the two-photon absorption and freecarrier absorption. Using a picosecond pulse pump centered at 1,550 nm, we show that the input continuous-wave signals can efficiently be converted into a broadband idler pulse in silicon waveguides with various dimensions. Conversion efficiencies versus signal wavelengths are different for silicon waveguides with different dimensions due to the variation in the phase mismatch; we obtain a conversion efficiency of – 32 dB in silicon nanowaveguides with a length of 5.8 mm. Such on-chip optical wavelength converters can find important potential applications in highly-integrated optical circuits for all-optical ultrafast signal processing.