基于APD单光子探测器的光子到达时间标记精度研究EI北大核心CSCD

为了准确测量X射线脉冲星导航中的光子到达时间,提出了一种X射线探测器光子到达时间精度的测试系统,该系统主要由脉冲X射线发生器、任意波形发生器、雪崩光电二极管探测器和时间标记光子计数器组成。系统测量脉冲X射线发生器的控制脉冲信号与雪崩光电二极管探测器测量的输出信号之间的时间延迟,研究时间延迟的分布情况,该分布的标准差可以反映被测探测器的光子到达时间测量精度。实验结果显示,雪崩光电二极管探测器输出信号相比控制信号的时间延迟约9.03 ns,标准差为2.23 ns,即雪崩光电二极管探测器的光子到达时间精度为2.23 ns,表明其能够实现对X射线单光子的快时间响应与高精度标记。     

单光子雪崩二极管的死时间效应分析

单光子雪崩二极管是一种采用盖革模式的单光子探测器件。一旦触发后探测器需要一段时间来重启,重启之后才能进行下一个光子事件的探测。重启所用时间(也叫死时间)的大小会影响探测器的光子计数分布。对单光子雪崩二极管的工作方式进行了数学建模,研究死时间对探测器输出响应的影响,推导出了在入射光子数服从泊松分布情况下探测器光子计数值的概率分布函数,并采用蒙特卡洛仿真进行了验证。结果表明死时间会使探测器输出的光子计数值减小,其分布会更加集中,并且死时间越大,入射光子速率越高,这种效应就越明显。     

气体雪崩过程中光电信号的时间关联

本文报道在有限正比和SQS模式下的光信号和原初雪崩的电信号时间关联的实验结果.表明伴随有限正比雪崩过程有微弱的光发射,光脉冲的半高全宽(FWHM)为4ns;自猝灭流光光脉冲相对于原初雪崩的电信号有几ns的延迟,而且这种延迟的涨落(FWHM)也有几ns.实验还给出了它们随丝电压和猝灭气体浓度的变化情况.     

四能级稳态雪崩上转换过程的计算分析

通过建立四能级雪崩上转换的速率方程,对稳态雪崩过程进行了详细的讨论和分析,阐明了雪崩上转换是个有阈值过程,并给出了各个能级布居随交叉弛豫P和跃迁速率Qi的变化。     

高时间稳定性的雪崩光电二极管单光子探测器

雪崩光电二极管单光子探测器是一种具有超高灵敏度的光电探测器件,在远距离激光测距、激光成像和量子通信等领域有非常重要的应用.然而,由于雪崩光电二极管单光子探测器的雪崩点对工作温度高度敏感,因此在外场环境下工作时容易出现增益波动,继而导致单光子探测器输出信号的延时发生漂移,严重降低了探测器的时间稳定性.本文发展了一种稳定输出延时的方法,采用嵌入式系统控制雪崩光电二极管,使其处于恒定温度,并实时补偿由环境温度引起的延时漂移,实现了雪崩光电二极管单光子探测器的高时间稳定性探测.实验中,环境温度从16 ℃变化到36 ℃,雪崩光电二极管的工作温度稳定在15 ℃,经过延时补偿,雪崩光电二极管单光子探测器输出延时漂移小于±1 ps,时间稳定度达到0.15 ps@100 s.这项工作有望为全天候野外条件和空间极端条件下的高精度单光子探测应用提供有效的解决方法.     

InGaAs单光子探测器传感检测与淬灭方式

针对InGaAs单光子雪崩光电二极管(SPAD)的光电感应特性,研究了基于门控主动式淬灭的SPAD动态偏置控制和电路实现的策略.采用门控主动淬灭控制可降低淬灭时间,有效抑制暗计数和后脉冲效应.接口感应检测电路采用标准互补金属氧化物半导体(CMOS)工艺进行制造,而SPAD则采用非标准CMOS工艺.利用铟柱互连混合封装工艺实现SPAD与感应接口电路的协同工作.在低温-30?C的条件下,实现了SPAD光触发雪崩电流信号的提取和快速淬灭.研究了感应电阻和临界检测电压对传感检测电性能的影响,并采用简单电路结构实现状态检测,实测得到的SPAD恢复时间、传输延时分别为575,563 ps,淬灭时间为1.88 ns,满足纳秒级精度传感检测应用的需要.     

近红外单光子探测器

该单光子探测器在实验中使用半导体制冷器制冷，雪崩二极管工作于盖革模式下，使用交流耦合方式提供门脉冲信号，通过延迟补偿和采样门控消除尖脉冲干扰，采用反馈门控减小后脉冲影响，优化电路参数减小暗计数.经实验测试与分析，温度在-62.5℃，门脉冲宽度为50ns，采样门控为10ns的条件下，最佳工作点的暗计数率小于4×10^-6ns^-1，量子效率约18%，噪声等效功率为2.4×10^-19W/Hz^1/2.     

与BaF2耦合的低气压多步雪崩室

研究了以纯TMAE饱和蒸汽为工作气体的紫外光敏低气压多步雪崩室的性能.在几种温度下测量了两个不同结构的丝室的气体放大倍数.在40°C时得到丝室的增益为4×10⁵,时间分辨4.1ns.和BaF₂晶体耦合,成功地观测了¹³⁷Cs γ射线(0.661MeV)的信号.     

雪崩晶体管的导通方式讨论

本文详细讨论了雪崩晶体管各种不同的导通方式,并发现由于雪崩晶体管C—E极间电压变化速率太大时,造成雪崩晶体管导通,论述了产生这种现象的原因,及此现象的存在对超快速电路的影响。     

用于LiDAR的16×1列阵CMOS单光子TOF图像传感器

设计了一款用于激光雷达的CMOS单光子飞行时间图像传感器.该传感器集成了16个结构优化的单光子雪崩二极管像素和一个双计数器结构的13-bit时间数字转换器电路.每个像素单元包括一个新型的主动淬灭-恢复电路.该设计通过优化器件的保护环来降低器件的暗噪声;带有反馈回路的主动淬灭-恢复电路用于降低死时间;时间数字转换器采用双计数器结构来避免计数器的亚稳态导致的计数错误.基于CMOS 180nm标准工艺制作了该传感器.测试结果表明:在1V的过偏压下,单光子雪崩二极管列阵的中值暗计数率为8kHz;在550nm波长光照下探测效率最高,为18%;设计的主动淬灭-恢复电路将死时间有效降低至8ns;时间数字转换器的分辨率为416ps,帮助整个系统实现厘米级距离分辨率.该传感器在0.5m距离下实现了空间分辨率为320×160的深度图像,其测距的最大非线性误差为1.9%,准确度为3.8%.     

The optical limitation effect and features of luminescence kinetics in hybrid nanosystems with CdSe/ZnS quantum dots and organic agents

The optical limitation effect in three-component systems formed in colloidal solutions of semiconductor CdSe/ZnS quantum dots with participation of fullerene C₆₀ and perylene, has been experimentally investigated. The first and second harmonics of a YAG:Nd³⁺ laser operating in the single-shot mode with a pulse duration of 7 ns and laser pulses subjected to stimulated Brillouin scattering (SBS)-stimulated Raman scattering (SRS) compression with a duration of about 20 ps near 560 nm have been used as radiation sources. It is shown that the optical limitation efficiency in the systems studied is determined by electron transfer. This is confirmed by the analysis of luminescence quenching.     

An open volume, high isolation, radio frequency surface coil system for pulsed magnetic resonance

We present an open volume, high isolation, RF system suitable for pulsed NMR and EPR spectrometers with reduced dead time. It comprises a set of three RF surface coils disposed with mutually parallel RF fields and a double-channel receiver (RX). Theoretical and experimental results obtained with a prototype operating at about 100 MHz are reported. Each surface RF coil (diameter 5.5 cm) was tuned to f₀ = 100.00 ± 0.01 MHz when isolated. Because of the mutual coupling and the geometry of the RF coils, only two resonances at f₁ = 97.94 MHz and f₂ = 101.85 MHz were observed. We show they are associated with two different RF field spatial distributions. In continuous mode (CW) operation the isolation between the TX coil and one of the RX coils (single-channel) was about −10 dB. By setting the double-channel RF assembly in subtraction mode the isolation values at f₁ or f₂ could be optimised to about −75 dB. Following a TX RF pulse (5 μs duration) an exponential decay with time constant of about 600 ns was observed. The isolation with single-channel RX coil was about −11 dB and it increased to about −47 dB with the double-channel RX in subtraction mode. Similar results were obtained with the RF pulse frequency selected to f₂ and also with shorter (500 ns) RF pulses. The above geometrical parameters and operating frequency of the RF assembly were selected as a model for potential applications in solid state NMR and in free radical EPR spectroscopy and imaging.     

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C⁺⁺) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C⁺⁺ clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (τ _r ∼ 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in τ _r of long lived species shows the possibility of energy migration along with solvent relaxation in C⁺⁺.     

Fluorescence Quenching and Time-resolved Fluorescence studies of α-Mannosidase from <Emphasis Type="Italic">Aspergillus fischeri</Emphasis> (NCIM 508)

Apart from the vital role in glycoprotein biosynthesis and degradation, α-mannosidase is currently an important therapeutic target for the development of anticancer agents. Fluorescence quenching and time-resolved fluorescence of α-mannosidase, a multitryptophan protein from Aspergillus fischeri were carried out to investigate the tryptophan environment. The tryptophans were found to be differentially exposed to the solvent and were not fully accessible to the neutral quencher indicating heterogeneity in the environment. Quenching of the fluorescence by acrylamide was collisional. Surface tryptophans were found to have predominantly positively charged amino acids around them and differentially accessible to the ionic quenchers. Denaturation led to more exposure of tryptophans to the solvent and consequently in the significant increase in quenching with all the quenchers. The native enzyme showed two different lifetimes, τ ₁ (1.51 ns) and τ ₂ (5.99 ns). The average lifetime of the native protein (τ) (3.187 ns) was not affected much after denaturation (τ) (3.219 ns), while average lifetime of the quenched protein samples was drastically reduced (1.995 ns for acrylamide and 1.537 ns for iodide). This is an attempt towards the conformational studies of α-mannosidase.     

Fluorescent Properties of a Hybrid Cadmium Sulfide-Dendrimer Nanocomposite and its Quenching with Nitromethane

A fluorescent hybrid cadmium sulphide quantum dots (QDs) dendrimer nanocomposite (DAB-CdS) synthesised in water and stable in aqueous solution is described. The dendrimer, DAB-G5 dendrimer (polypropylenimine tetrahexacontaamine) generation 5, a diaminobutene core with 64 amine terminal primary groups. The maximum of the excitation and emission spectra, Stokes’ shift and the emission full width of half maximum of this nanocomposite are, respectively: 351, 535, 204 and 212 nm. The fluorescence time decay was complex and a four component decay time model originated a good fit (χ = 1.20) with the following lifetimes: τ ₁ = 657 ps; τ ₂ = 10.0 ns; τ ₃ = 59.42 ns; and τ ₄ = 265 ns. The fluorescence intensity of the nanocomposite is markedly quenched by the presence of nitromethane with a dynamic Stern-Volmer constant of 25 M⁻¹. The quenching profiles show that about 81% of the CdS QDs are located in the external layer of the dendrimer accessible to the quencher. PARAFAC analysis of the excitation emission matrices (EEM) acquired as function of the nitromethane concentration showed a trilinear data structure with only one linearly independent component describing the quenching which allows robust estimation of the excitation and emission spectra and of the quenching profiles. This water soluble and fluorescent nanocomposite shows a set of favourable properties to its use in sensor applications.     

Saturable absorbers with concentration-dependent absorption recovery time

The ground-state bleaching of highly concentrated rhodamine 6G solutions in methanol is studied with intense picosecond light pulses. The ground-state recovery time changes with concentration from about 3.9 ns at low concentration (10^–5 mol/dm³) to about 1 ps at high concentration (0.6 mol/dm³). The shortening of the absorption recovery time is determined by the concentration dependent quenching of theS ₁-state population due to unbound dimers and by the intensity dependent longitudinal and transverse amplified spontaneous emission.     

Self quenched quantum dot avalanche photodetector for mid-infrared single photon detection

In this paper a single photon detector with operating wavelength of 6 μm is proposed and its performance characteristics are studied. Intersubband absorption of single photons in quantum dot layers leads to creation of photoelectrons which are injected to multiplication region and under above breakdown condition a large output pulse is generated. The detector is designed for self quenching operation in which an additional layer called transient carrier buffer (TCB) is used for trapping of backward avalanche generated holes at the interface of TCB and charge layer. The accumulated holes impose an additional charge in such a way that the voltage across the multiplication region drops and the output is quenched. A model is developed to analyze the performance of detector and results of simulation predict detection efficiency about 12% at T = 150 K. Also the quenching and recovering performance of detector is studied and results show that both higher temperatures and higher bias improve dynamics of detector. However higher temperatures result in higher dark count rate.     

用磁分析法观察某些铁-镍-铝合金的脱溶过程

用热磁分析方法较仔细地观测了Fe-Ni-Al三元系合金沿β+β′两相区内某一结线上的几个不同成分的合金样品。淬火所得的成分接近Fe₂NiAl的过饱和固溶体在较高温度(～850℃)分解甚快,在一分钟内脱溶基本上完成。在850℃分出的β′相还含铁约35at.％,其居里点在400℃附近。在850℃经短时间回火继以水淬的合金,在600—700℃再经适当的回火,则首次回火所生成的β′相继续迅速分解,在不超过十分钟内损失大量的铁而变成非铁磁性的;其在室温的H_c可增到500奥斯特。这和Б.Г.Лившнц所提出的“继续脱溶”的假设相符。磁性观测的结果证明:β′相的继续脱溶是“可逆的”,就是说,把曾在600—700℃回火的合金重热至850℃时β′相能在几分钟内恢复850℃的平衡成分。Fe₂NiAl过饱和固溶体在较低温度(70O℃以下)分解极慢,并且其过程是明显地“不均匀”的。因此,这种合金不能通过高温淬火继以较低温度(600—700℃)回火的处理来获得高H_c的事实可以用单畴粒子的理论加以解释。     

高密度外延电阻淬灭硅光电倍增器研究

针对表面淬灭电阻技术引起死区面积较大,以及高光子探测效率与大动态范围不能同时满足的矛盾,应用外延电阻淬灭技术,采用与雪崩光电二极管微单元相连的衬底外延层硅材料制作了淬灭电阻.研制成功的外延电阻淬灭硅光电倍增器的有源区面积为1×1mm~2,微单元尺寸为7μm,微单元密度高达21 488个/mm~2,测试结果表明:漏电流为10量级,反向击穿电压为24.5V,过偏压为2.5V时,增益达1.4×10~5,室温下暗计数率约为600kHz/mm~2,串话率低于10%,说明该器件具有良好的光子计数特性.该高密度硅光电倍增器测量的动态范围是1.8×10~4个/mm,光子探测效率为16%(@λ_(peak)=480nm),恢复时间为8.5ns,单光子分辨能力较高,并且在液氮温度环境能够探测光子,这对于拓展硅光电倍增器在极低温度条件下的应用,比如暗物质测量实验方面具有潜力.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)