Nonlinearity measurements of silicon photodetectors

Nonlinearities of the responsivity of various types of siliconphotodetectors have been studied. These detectors are based onphotodiodes with two sizes of the active area (10 x 10 mm(2) and 18 x 18 mm(2)). The detectorconfigurations investigated include single photodiodes, two reflectiontrap detectors, and a transmission trap detector. For all devices, the measured nonlinearity was less than 2 x 10(-4) forphotocurrents up to 200 muA. The diameter of themeasurement beam was found to have an effect on thenonlinearity. The measured nonlinearity of the trap detectorsdepends on the polarization state of the incident beam. Theresponsivity of the photodetectors consisting of the large-areaphotodiodes reached saturation at higher photocurrent values comparedwith the devices based on the photodiodes with smaller activearea.     

Quantitative measurements of amino acids by terahertz time-domain transmission spectroscopy

The quantitative analysis of amino acids by terahertz (THz) time-domain absorption spectroscopy is demonstrated. The optical densities of the amino acids were found to be linearly proportional to the concentration. The molar absorption coefficients of L-glutamic acid (L-Glu), L-glutamic acid sodium salt (Na-L-Glu), L-glutamic acid hydrochloric salt (HCl-L-Glu), L-cysteine (L-Cys), and L-histidine (L-His) were calculated by averaging the THz spectra of the amino acids at several different concentrations in approximately the 0.2-1.0 mol L(-1) range. The concentrations of L-Glu, L-Cys, and L-His mixed samples were successfully calculated with errors of less than 11% and 20% when their concentrations were higher than 0.45 and 0.22 mol L(-1), respectively, by using the obtained molar absorption coefficient.     

Intrinsic response time of graphene photodetectors

Graphene-based photodetectors are promising new devices for high-speed optoelectronic applications. However, despite recent efforts it is not clear what determines the ultimate speed limit of these devices. Here, we present measurements of the intrinsic response time of metal-graphene-metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses. We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers. This time translates into a bandwidth of ～262 GHz. Moreover, we investigate the dependence of the response time on gate voltage and illumination laser power.     

Analysis of error sources in time-domain dielectric spectroscopy

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 61–63, August, 1992.     

Dynamic response of rigid strip-foundations by a time-domain boundary element method

The dynamic response of rigid strip-foundations placed on or embedded in a homogeneous, isotropic, linear elastic half-space under conditions of plane strain to either external forces or obliquely incident seismic waves of arbitrary time variation is numerically obtained. The above mixed boundary-value problems are treated by the time domain boundary element method which is used in a step-by-step timewise fashion to provide the foundation response to a rectangular impulse. Numerical examples are presented in detail to demonstrate the use and importance of the proposed method. The method appears to be more advantageous than frequency domain techniques, because it provides the transient foundation response in a natural and direct way and can form the basis for extension to the non-linear case.     

Improving the accuracy of time intervals measurements by pulse counting

A method of improving the accuracy of time interval measurements by pulse counting is described. It is shown that the proposed method can double the measurement accuracy.Translated from Izmeritel'naya Tekhnika, No. 12, pp. 29–32, December, 1993.     

Polarization response of two-dimensional metallic photonic crystals studied by terahertz time-domain spectroscopy

The polarization characteristics of a terahertz (THz) wave transmitted through two-dimensional (2-D) metallic photonic crystals (MPCs) are investigated. The 2-D MPCs studied in this paper are metal slabs perforated periodically with circular holes. We measured the polarization characteristics of the THz wave using a THz time-domain spectroscopic system with wire grid polarizers in the time and frequency domains. The linearly polarized incident THz wave changes its polarization direction and becomes elliptic after it transmits through the sample. This phenomenon is highly sensitive to the incident angle. It is shown that the frequency range at which the polarization rotation occurs is related to the lattice constant of a photonic crystal, indicating the importance of photonic band modes of the 2-D MPC in the mechanism of the phenomenon.     

Optical noise induced by Gaussian sources in stokes parameter measurements

A novel formalism for determining the source-induced noise in Stokes parameter measurements is derived for sources with Gaussian statistics. The formalism is based on a concise expression for the autocovariance functions of the Stokes parameters in terms of the second-order correlation properties of the optical field. At the output of an optical system, source-induced noise can result not only from the intensity fluctuations of the source but also from phase or polarization fluctuations. To describe the effect of the system, another formalism for the propagation of the second-order correlation properties of the optical field is derived. We apply the formalisms to analyze source-induced noise at the output of a birefringent medium, and in coherence-multiplexing networks.     

Wide-energy-range,high-resolution measurements of neutron pulse shapes of polyethylene moderators

We report measurements of the emission time distributions (pulse shapes) as functions of energy for neutrons emerging from two heavily irradiated, ambient-temperature polyethylene moderators in IPNS. A time-focused crystal spectrometer arrangement provided resolution such that instrumental broadening was insignificant; cooling the Ge monochromator to 10 K provided adequate reflectivity in the high energy region. Measurements covered the range 2.5 < E < 1000 meV.We introduce a novel set of functions which fit the pulse shapes over the entire range of energies with four wavelength-independent parameters.     

时域响应中传感器附加质量影响消除

本文提出了一种时域内的传感器附加质量影响消除方法。利用时域实测自由响应数据,通过特征系统实现算法进行模态参数识别,获得传感器附加质量影响情况下的模态参数。以实测模态参数为基准,采用信赖域和非线性最小二乘算法对考虑传感器附加质量的结构初始有限元模型参数进行识别与修正,并以修正后的有限元模型为基础预测传感器附加质量消除所引起的模态参数改变。基于振型叠加法原理建立时域内传感器附加质量影响消除的识别方程组,并结合有限元预测的模态参数改变反演传感器附加质量影响消除情况下的时域响应。通过对一个实验室两端夹支梁模型实测时域响应中传感器附加质量影响消除进行研究,对所提出的方法进行验证。     

Transient response of a semi-permeable crack between dissimilar anisotropic piezoelectric layers by time-domain BEM

This paper studied the transient response of a semi-permeable crack between two dissimilar anisotropic piezoelectric layers by a time-domain BEM with a sub-domain technique and an iterative process for the non-linear crack-face boundary conditions. The present time-domain BEM uses a quadrature formula for the temporal discretization to approximate the convolution integrals and a collocation method for the spatial discretization. A universal matrix-form displacement extrapolation formula and its explicit formula are used to determine the dynamic intensity factors. Several examples are presented and discussed to show the effects of the electrical crack-face boundary condition on dynamic intensity factors.     

Analysing the pattern of pulse waves in arterial networks: a time-domain study

The mechanisms underlying the shape of pulse waves in the systemic arterial network are studied using the time-domain, one-dimensional (1-D) equations of blood flow in compliant vessels. The pulse waveform at an arbitrary location in the network is initially separated into a peripheral component that depends on the cardiac output, total compliance and total peripheral resistance of the network, and a conduit component governed by reflections at the junctions of the large conduit arteries and at the aortic valve. The dynamics of the conduit component are then analysed using a new algorithm that describes all the waves generated in the linear 1-D model network by a single wavefront starting at the root. This algorithm allows one to systematically follow all the waves arriving at the measuring site and identify all the reflection sites that these waves have visited. Application of this method to the pulse waves simulated using a 1-D model of the largest 55 systemic arteries in the human demonstrates that peripheral components make a larger contribution to aortic pressure waveforms than do the conduit components. Conduit components are closely related to the outflow from the left ventricle in early systole. Later in the cardiac cycle, they are the result of reflections at the arterial junctions and aortic valve. The number of reflected waves increases approximately as 3 ^m , with m being the number of reflection sites encountered. The pressure changes associated with these waves can be positive or negative but their absolute values tend to decrease exponentially. As a result, wave activity is minimal during late diastole, when the peripheral components of pressure and the flow are dominant, and aortic pressures tend to a space-independent value determined by the cardiac output, total compliance and total peripheral resistance. The results also suggest that pulse-wave propagation is the mechanism by which the arterial system reaches the mean pressure dictated by the cardiac output and total resistance that is required to perfuse the microcirculation. The total compliance determines the rate at which this pressure is restored when the system has departed from its equilibrium state of steady oscillation. This study provides valuable information on identifying and measuring the parameters and pathways of the arterial network that have the largest effect on the simulated pulse waveforms.     

A method for improved standardization of in vivo calcaneal time-domain speed-of-sound measurements

Although calcaneal speed of sound (SOS) is an effective predictor of osteoporotic fracture risk, clinical SOS measurements exhibit a high degree of inter-system variability. Calcaneal SOS is usually computed from time-of-flight measurements of broadband ultrasound pulses that propagate through the foot. In order to minimize the effects of multi-path interference, many investigators measure time-of-flight from markers near the leading edge of the pulse. The calcaneus is a highly attenuating, highly inhomogeneous bone that distorts propagating ultrasound pulses via frequency-dependent attenuation, reverberation, dispersion, multiple scattering, and refraction. This pulse distortion can produce errors in leading-edge transit-time marker-based SOS measurements. In this paper, an equation to predict dependence of time-domain SOS measurements on system parameters (center frequency and bandwidth), transit-time marker location, and bone properties (attenuation coefficient and thickness) is validated with through-transmission measurements in a bone-mimicking phantom and in 73 women in vivo, using a clinical bone sonometer. In order to test the utility of the formula for suppressing system dependence of SOS measurements, a wideband laboratory data acquisition system was used to make a second set of through-transmission measurements on the phantom. The compensation formula reduced system-dependent leading-edge transit-time marker-based SOS measurements in the phantom from 41 m/s to 5 m/s and reduced average transit-time marker-related SOS variability in 73 women from 40 m/s to 10 m/s. The compensation formula can be used to improve standardization in bone sonometry.     

Sensitivity characterization of a time-domain fluorescence imager: eXplore Optix

A key issue in the practical application of fluorescence imaging is the presence of a background signal detected during data acquisition when no target fluorescent material is present. Regardless of the technology employed, background signals cannot be completely eliminated, which limits the detection sensitivity of fluorescence imaging systems, especially for in vivo applications. We present a methodology to characterize the sensitivity of fluorescence imaging devices by taking the background effect into account through the fluorescent signal-to-background ratio (SBR). In an initial application of the methodology, tissuelike liquid phantoms with Cy5.5 fluorescent inclusions were investigated experimentally over a wide range of varying parameters, such as tissue absorption coefficient, scattering coefficient, fluorophore concentration, and inclusion location. By defining detectable and quantifiable SBR thresholds, empirical relations are established, and the sensitivity performance of Advanced Research Technologies's eXplore Optix using Cy5.5 is characterized.