Decoherence Effects of Terahertz Generation in Solids under Two-Color Femtosecond Laser Fields

We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects.Quantum-mechanical simulations are based on the length gauge semiconductor Bloch equations describing the optical excitation and decoherence with phenomenological dephasing and depopulation times.Contributions of interband and intraband mechanisms are identified in time domain,and the latter has dominated THz generation in solid-state sys...     

Limitations of interface sharpness in a-Si:H/a-SiC:H multilayers

The goal of this work is to study the sharpness of interfaces in amorphous silicon based compositional multilayers (superlattices) by a number of different techniques and to discuss their limitations. From monitoring plasma transients during glow discharge deposition of a-Si:H/a-SiC:H multilayers, a lower limit of 3 Å for interface sharpness is estimated. Transmission electron microscopy (TEM) images yield an upper limit of 5–10 Å. These images directly show the increase in undulation from the substrate towards the film surface. From the comparison of simulated X-ray diffraction (XRD) spectra with measurements the interface sharpness is found to be between 5 and 8 Å. Using a series of multilayers with increasing number of interfaces, structural characteristics of the interfacial region can be extracted. For example, infrared absorption spectroscopy (FTIR) and elastic recoil detection (ERD) lead to an estimate of 1.1 × 10¹⁴ cm^-2 additional hydrogen atoms per single interface.     

非晶/微晶两相硅薄膜电池的计算机模拟

在对不同晶相比硅薄膜的实验研究的基础上，利用有效介质理论估算了这种两相材料的光吸 收系数、迁移率寿命乘积及带隙宽度等参量，计算机模拟了不同结晶比硅薄膜电池的伏安特 性及光谱响应；结果为随着本征层微晶成分的增多，电池的开路电压逐渐减小，短路电流逐 渐增大，本征层的最佳厚度逐渐增大，填充因子有降低的趋势，电池的效率随晶相比的增大 而减小. 电池的光谱响应曲线表明，随晶相比的增大电池的长波响应明显提高. 根据这些模 拟结果，分析讨论了在考虑Lambertian背反射的情况下，非晶/微晶叠层电池的底电池采用 晶相比为40％—50％的两相硅薄膜材料做本征层是最佳选择. 关键词： 两相硅薄膜 太阳能电池 计算机模拟     

a-Si:H/c-Si 异质结太阳电池 J-V 曲线的 S-Shape 现象

通过异质结界面分析与 AMPS 模拟计算研究了 a-Si:H/c-Si 异质结太阳电池在低温工作、a-Si:H 层低掺杂、高价带补偿以及高界面态时光态 J-V 曲线出现 S-Shape 现象的物理过程,总结了 S-Shape 现象的物理原因.分析结果表明,当空穴输运受到界面势垒的限制时,空穴在 c-Si 界面附近聚集,能带重新分配,c-Si 耗尽区的电场减小,更多的电子从 c-Si 准中性区反转至 c-Si 界面及耗尽区与空穴复合,复合速率显著增大,光电流的损失显著增大,光态 J-V< 关键词： 模拟 异质结太阳电池 a-Si:H/c-Si 异质结     

Determination of electron-diffusion length from photocurrent characteristics of the structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd

In this study the electron diffusion length L _n is determined from the relative spectral response of the photocurrent characteristics of the p/i/n sandwich structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd. The techniques used for the preparation of the a-Sic:H and a-Si:H amorphous films were glow-discharge and rf magnetron sputtering, respectively. The thickness of the p-type, intrinsic and n-type layer were 400 Å, 7000 Å and 600 Å, respectively. The response of the short-circuit current density J _sc was measured versus the photon energy hv at both constant light intensity and constant temperature. The electron diffusion length was found to be 0.31 m by means of the method of Agarwala and Tewary. Although, in the case of single crystals many diffusion length measurements have been made, there are only few papers for amorphous silicon this films [1]. As it is well-known, the diffusion length of the charge carriers is the most important parameter from the point of view of solar cell applications [2]. In order to obtain a high efficiency in a solar cell all carriers created under illumination in the intrinsic layer should reach the electrodes [3]. In the case that the thickness of the intrinsic layer is much larger than the diffusion length, not all carriers can reach the electrodes and, accordingly, a low efficiency results [4]. On the other hand, carriers which reach the electrodes without thermalizing do not contribute to the photocurrent and finally the efficiency of the solar cell is negatively affected. In order to avoid such an effect to a large extent, the thickness of the amorphous layers in a p/i/n solar cell must be conveniently chosen compared to the diffusion length of the carriers.Here it is aimed to determine the electron diffusion length. In order to achieve this goal, the photocurrent characteristics of an ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd structure was measured versus the photon energy at constant light intensity and constant temperature. In order to determine the electron diffusion length, the method of Agarwala and Tewary [5] was utilized.     

On a presence of SimHn clusters in a-Si:H/c-Si structures

Dominant aim of the paper was to verify the existence of the Si_mH_n clusters in a-Si:H layers. Thin layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) on both glass and crystalline silicon substrates. Their IR and structural properties were investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction at grazing incidence angle (XRDGI). We have found that the layer probably consists of larger structurally ordered parts corresponding to Si_mH_n clusters and separated groups of (Si-H_x)_N. The ordered parts could be identified as some of Si_mH_n clusters ranging from (10, 16) to (84, 64) represented by corresponding vibration frequencies in three following IR regions: 600-750, 830-900 and 2080-2180 cm⁻¹. XRDGI measurement indicates that diffraction maximum at around 2Θ = 28° can be attributed to an existing Si_mH_n cluster.     

Resonance Selective Excitation of Terahertz Plasma Waves in a Semiconductor Quantum Well Using Two-Color Laser Radiation

A new method of coherent resonance excitation of plasma instability in a 2D quantum well driven by twocolor laser radiation due to photoinduced generation of electron–hole pairs is proposed. It is shown that under the resonance condition, where the frequency and wavenumber of the beat wave obey the 2D plasmon dispersion relation, a spacecharge wave is excited efficiently. A selfconsistent treatment of the problem is presented and the smallsignal response of the 2D system in the THz frequency region is found. This new method provides the selective resonance excitation of plasmons, tuning of their frequency, and efficient control of plasma wave parameters crucial for THz semiconductor technology.     

Control of epitaxial growth at a-Si:H/c-Si heterointerface by the working pressure in PECVD

The epitaxial-Si(epi-Si) growth on the crystalline Si(c-Si) wafer could be tailored by the working pressure in plasmaenhanced chemical vapor deposition(PECVD).It has been systematically confirmed that the epitaxial growth at the hydrogenated amorphous silicon(a-Si:H)/c-Si interface is suppressed at high pressure(hp) and occurs at low pressure(1p).The hp a-Si:H,as a purely amorphous layer,is incorporated in the 1p-epi-Si/c-Si interface.We find that:(i) the epitaxial growth can also occur at a-Si:H coated c-Si wafer as long as this amorphous layer is thin enough;(ii) with the increase of the inserted hp layer thickness,lp epi-Si at the interface is suppressed,and the fraction of a-Si:H in the thin films increases and that of c-Si decreases,corresponding to the increasing minority carrier lifetime of the sample.Not only the epitaxial results,but also the quality of the thin films at hp also surpasses that at lp,leading to the longer minority carrier lifetime of the hp sample than the lp one although they have the same amorphous phase.     

Extreme Ultraviolet Radiation Source Based on a Discharge Sustained by a Radiation Pulse from a Terahertz Free-Electron Laser

Journal of Experimental and Theoretical Physics - We consider the possibility of sustaining a discharge in xenon efficiently emitting light in a wavelength range of 11.2 nm ± 1% by a...     

Influence of the semiconductor/metal rear contact on the performance of n(a-Si:H)/i(a-Si:H)/p(c-Si) heterojunction solar cells

The aim of this work is to analyze on the results of using of Al/Ag layer as a rear contact to improve the performance of heterojunction silicon solar cells. An analytical method is presented to extract the physical parameters of the equivalent circuit. These parameters are extracted to simulate the I(V) characteristic of heterojunction silicon solar cells, with Al and Al/Ag rear-metal contact. A good agreement between our analytical method and experimental measurement of electrical characteristics is obtained which show clearly how the Al/Ag rear contact can improve the characteristics of silicon solar cells. The influence of the rear-metal contact on the performance of the c-Si(p)-based bifacial HIT solar cell, i.e., the ZnO/Al/a-Si:H(n)/a-Si:H(i)/c-Si(p)/metal solar cell, is investigated in detail by computer simulation using the AFORS-HET software. Accordingly, the design optimization of the bifacial HIT solar cells on c-Si(p) substrates is provided. These simulation show an optimal conversion efficiency of 23% when the rear-metal contact is perfectly ohmic.     

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet

JETP Letters - Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (I&nbsp;~&nbsp;1014 W/cm2) femtosecond mid-infrared radiation of a laser...     

Real-Time Monitoring of the Energy Deposition under the Tight Focusing of Femtosecond Laser Radiation into a Bulk Transparent Dielectric Based on Third Harmonic Signal

It has been found that the third harmonic generated under the tight focusing of a near-infrared (1.24-μm) femtosecond laser pulse indicates the energy deposition into the bulk transparent dielectric and can be used as a feedback system in the process of microstructuring. A third harmonic signal is sensitive to a change in both the size of a laser-induced microplasma and the free-electron density, which allows the detection of the microplasma with submicron longitudinal sizes. It has been shown that the method based on the third harmonic is universal and can be applied in both single-pulse and two-color microstructuring.     

Influence of the broken symmetry of defect state distribution at the a-Si:H/c-Si interface on the performance of hetero-junction solar cells

Taking into account the fact that the distribution of defect states at the interface does not have strictly symmetrical shape, we present a simulation study of a-Si:H(n)/c-Si(p) and a-Si:H(p)/c-Si(n) structures with regard to the defect states at the interface, band offsets and doping concentration of the emitter. The presented results suggest for a-Si:H(n)/c-Si(p) solar cells a strong influence of the introduced broken symmetry between acceptor and donor defect states on the open-circuit voltage, whereas the a-Si:H(p)/c-Si(n) structure benefits from inherent favorable band alignment and remains unaffected.     

Generation of Coherent Extreme Ultraviolet Radiation in an Air Gas Cell with a High Power Femtosecond Laser System

Optics and Spectroscopy - High harmonic generation (HHG) in air medium is experimentally produced by using a state-of-the-art high power laser system. The harmonic orders up to 35th harmonic are...     

钙钛矿/硅叠层太阳电池中平面a-Si:H/c-Si异质结底电池的钝化优化及性能提高

最近,旋涂法制备的钙钛矿/平面硅异质结高效叠层太阳电池引起人们广泛关注,主要原因是相比于绒面硅衬底制备的钙钛矿/硅叠层太阳电池,其制备工艺简单、制备成本低且效率高.对于平面a-Si:H/c-Si异质结电池, a-Si:H/c-Si界面的良好钝化是获得高转换效率的关键,进而决定了钙钛矿/硅异质结叠层太阳电池的性能.本文主要从硅片表面处理、a-Si:H钝化层和P型发射极等方面展开研究,通过对硅片表面的氢氟酸(HF)浸泡时间和氢等离子体预处理气体流量、a-Si:H钝化层沉积参数、钝化层与P型发射极(I/P)界面富氢等离子体处理的综合调控,获得了相应的优化工艺参数.对比研究了p-a-Si:H和p-nc-Si:H两种缓冲层材料对I/P界面的影响,其中高电导、宽带隙的p-nc-Si:H缓冲层既能够降低I/P界面的缺陷态,又可以增强P型发射层的暗电导率,提高了前表面场效应钝化效果.通过上述优化,制备出最佳的P-type emitter layer/aSi:H(i)/c-Si/a-Si:H(i)/N-type layer (inip)结构样品的少子寿命与implied-Voc分别达到2855μs和709 mV,表现出良好的钝化效果.应用于平面a-Si:H/c-Si异质结太阳电池,转换效率达到18.76%,其中开路电压达到681.5 mV,相对于未优化的电池提升了34.3 mV.将上述平面a-Si:H/c-Si异质结太阳电池作为底电池,对应的钙钛矿/硅异质结叠层太阳电池的开路电压达到1780 mV,转换效率达到21.24%,证明了上述工艺优化能够有效地改善叠层太阳电池中的硅异质结底电池的钝化及电池性能.     

Study on Real-Time Measurement of Femtosecond Laser Pulse Width Based on Noncollinear Second-Harmonic Generation Effect

Journal of Russian Laser Research - We propose an autocorrelator based on noncollinear second-harmonic generation effect and a pulse-width measurement program written by LabVIEW software providing...     

Generation of Stable Near－infrared and Violet Femtosecond Pulses from a Self－mode－locked Ti：Sapphire Laser

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Effects of Terahertz Radiation on Living Cells: a Review

Optics and Spectroscopy - This review discusses the current state of research concerning the effects of terahertz (THz) radiation on living cells in the context of biosafety of THz radiation for...     

Generation of Femtosecond Laser Pulse at 1.43 GHz from an Optical Parametric Oscillator Based on LBO Crystal

A femtosecond LBO optical parametric oscillator(OPO) with widely adjustable repetition rate by fractionally decrement of the cavity length is demonstrated. The repetition rate of 755 MHz to 1.43 GHz at an interval of 75.5 MHz is realized, which is 10 to 19 times that of the pump laser. The properties of output signal at 750 nm at different repetition rates are studied. The power of signal decreases with increasing the repetition rate. The maximum power of 194 mW at the repetition rate of 755 MHz and the minimum power of 22 mW at repetition rate of 1.43 GHz for the signal at 750 nm are obtained for the pump power of 3 W.