Excess noise in InAs/GaSb type-II superlattice pin-photodiodes

We characterize the low-frequency white noise behavior of a large set of InAs/GaSb superlattice infrared pin-photodiodes for the mid-wavelength infrared regime at 3–5 μm. For diodes with an increased dark current in comparison to the dark current of generation–recombination limited bulk material, the standard shot-noise model fails to describe the noise experimentally observed in the white part of the spectrum. Instead, we find that McIntyre’s noise model for avalanche multiplication processes is compliant with our data. We suggest that within high electric field domains localized around macroscopic defects, avalanche multiplication processes leading to increased dark current and excess noise.     

Dephasing of a qubit due to quantum and classical noise

The qubit (or a system of two quantum dots) has become a standard paradigm for studying quantum information processes. Our focus is decoherence due to interaction of the qubit with its environment, leading to noise. We consider quantum noise generated by a dissipative quantum bath. A detailed comparative study with the results for a classical noise source such as generated by a telegraph process, enables us to set limits on the applicability of this process vis à vis its quantum counterpart, as well as lend handle on the parameters that can be tuned for analysing decoherence. Both Ohmic and non-Ohmic dissipations are treated and appropriate limits are analysed for facilitating comparison with the telegraph process.     

Theoretical performance of novel multipliers at millimeter and submillimeter wavelengths

A theoretical comparison of various low and high order multipliers for 200 GHz and 1 THz has been carried out. Novel diodes including single barrier varactors, barrier-intrinsic-n⁺ diodes and high electron mobility varactors are shown to have excellent theoretical performance, comparable or better than the conventional Schottky varactors for single and double diode frequency multipliers at millimeter and submillimeter wavelengths, whereas quantum well diodes, since they suffer from high resistive losses, are shown to be less attractive. In comparison to the conventional Schottky varactor, these new diodes have some potential advantages in their characteristics such as nonlinearity or a special symmetry. For future optimization some general comments on these advantages as well as other factors affecting multiplication is given.     

Survival,Extinction and Approximation of Discrete-time Branching Random Walks

We consider a general discrete-time branching random walk on a countable set X. We relate local, strong local and global survival with suitable inequalities involving the first-moment matrix M of the process. In particular we prove that, while the local behavior is characterized by M, the global behavior cannot be completely described in terms of properties involving M alone. Moreover we show that locally surviving branching random walks can be approximated by sequences of spatially confined and stochastically dominated branching random walks which eventually survive locally if the (possibly finite) state space is large enough. An analogous result can be achieved by approximating a branching random walk by a sequence of multitype contact processes and allowing a sufficiently large number of particles per site. We compare these results with the ones obtained in the continuous-time case and we give some examples and counterexamples.     

Identifiability of Age-Dependent Branching Processes from Extinction Probabilities and Number Distributions

Consider an age-dependent, single-species branching process defined by a progeny number distribution, and a lifetime distribution associated with each independent particle. In this paper, we focus on the associated inverse problem where one wishes to formally solve for the progeny number distribution or the lifetime distribution that defines the Bellman-Harris branching process. We derive results for the existence and uniqueness (the identifiability) of these two distributions given one of two types of information: the extinction time probability of the entire process (extinction time distribution), or the distribution of the total number of particles at one fixed time. We demonstrate that perfect knowledge of the distribution of extinction times allows us to formally determine either the progeny number distribution or the lifetime distribution. Furthermore, we show that these constructions are unique. We then consider “data” consisting of a perfectly known total number distribution given at one specific time. For a process with known progeny number distribution and exponentially distributed lifetimes, we show that the rate parameter is identifiable. For general lifetime distributions, we also show that the progeny distribution is globally unique. Our results are presented through four theorems, each describing the constructions in the four distinct cases.     

Enhancing optomechanical force sensing via precooling and quantum noise cancellation

We theoretically investigate optomechanical force sensing via precooling and quantum noise cancellation in two coupled cavity optomechanical systems.We show that force sensing based on the reduction of noise can be used to dramatically enhance the force sensing and that the precooling process can eifectively improve the quantum noise cancellation.Specifically,we examine the effect of optomechanical cooling and noise reduction on the spectral density of the noise of the force measurement;these processes can significantly enhance the performance of optomechanical force sensing,and setting up the system in the resolved sideband regime can lead to an optimization of the cooling processes in a hybrid system.Such a scheme serves as a promising platform for quantum back-action-evading measurements of the motion and a framework for an optomechanical force sensor.     

Burst and 1/f noise in light-emitting diodes with quantum dots

We study the low-frequency electric noise characteristics of light-emitting diodes with InAs quantum dots in a GaInAs layer. Burst noise having the character of random telegraph signal (RTS) is found against the 1/f noise background in the noise voltage of some specimens. A procedure based on the standard theory of signal detection against the noise background is proposed for a separate study of these noise components. It is found that Hooge’s empirical relation applied to p-n diodes for the first time by Kleinpenning is also applicable to 1/f noise in quantum-dot diodes. The current dependences of statistical characteristics of the 1/f and RTS noise components are compared to show that the physical origins of RTS noise and 1/f noise in the studied specimens are different. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 5, pp. 437–447, May 2006.     

Towards quantitative chemical imaging with ToF-SIMS

Methods for processing ToF-SIMS spectra and images have advanced significantly in recent years to include multivariate analysis methods. Multivariate methods can reduce noise, identify and quantify chemical components, and segment images into discrete chemical phases. To date, these methods have focused on the analysis of single images; however, quantitative or semi-quantitative methods for comparison of multiple images collected across multiple samples have lagged in development. This study evaluates simple noise reduction and scaling methods to facilitate semi-quantitative comparison of images collected across several samples with varying acquisition conditions. Down-binning, Poisson-scaling, and nearest-neighbor smoothing methods improved signal-to-noise in image datasets, with nearest-neighbor smoothing providing the greatest improvement. Image scaling methods including pixel-by-pixel (PbP) normalization and scalar multiplication were found to improve the relative quantification of images. While PbP normalization methods performed well for relatively flat samples, such methods were not suitable for samples with significant topography. Scaling methods using scalar multiplication of individual secondary ion images and histogram analyses facilitated semi-quantitative comparison of these samples.     

The importance of the privilege for appearance of inverse-power solutions in Ito equations

It is shown that the true cause of inverse-power distributions in the Ito equation is some kind of privilege which is hidden in the course of evolution of the system. Connections between Ito equations with additive noise or/and multiplicative noise with additive processes, multiplicative processes, multiplication of probabilities and return-to-the-origin problem are found. On the basis of two toy models, the appearance of particular functions for deterministic and stochastic forces in the Ito equation is explained. The paper stands as the next contribution confirming the hypothesis that the adequate privilege is the cause for the origin of inverse-power distributions in many phenomena.     

Exclusive Semi-leptonic Decays b→s(d)ν\bar{ν} in the Standard Model with Fourth Generation

We consider the effects of the fourth generation quarks on the rareBdecay processes described by the b→ s(d)ν \bar{ν} processes at the quark level. We find that the values of the branching ratios for some of these rare decay processes are much larger than the standard model (SM) predictions.
These processes can be used to constrain the relevant free parameters.     

The Mott diode as a heterodyne receiver element

This paper considers a novel doping profile for Schottky barrier mixer diodes called the Mott barrier. The structure consists of a metal-semiconductor junction in which the semiconductor's epitaxial layer is very lightly doped and thin enough so that it remains depleted even under substantial forward bias. It has been proposed that Mott barrier diodes will generate less noise and have lower series resistance-junction capacitance products than standard Schottky diodes, thus increasing the sensitivity and cut-off frequency of heterodyne receivers. In this paper, the band structure and electron transport properties of the Mott diode are evaluated. This analysis shows that the Mott diode actually will have a large series resistance-junction capacitance product and excessive hot electron noise, making it a poor candidate for high-frequency applications. Experimental results are presented which substantiate these conclusions.     

Noise in optical fibre backscatter measurement

We consider in this paper the origin and effect of those sources of noise present in optical fibre backscatter systems which employ large-area semiconductor laser diodes and avalanche photodetectors. Source noise due to the filamentary lasing action of the photodiodes is shown to dominate the near-end performance. The clear superiority of two-point processing over boxcar integration in the presence of source noise is demonstrated.     

Random response time of thin avalanche photodiodes

The avalanche built-up time using random response time model for avalanche photodiode (APD) is presented. A random response time model considers the randomness of times at which the primary and secondary carriers exit the multiplication region. The dead-space effect is included in our model to demonstrate its effect on response time of APDs especially for the thin devices. Our results show that feedback impact ionisation process and dead-space prolong the response time in APDs. The time response of homojunction InP p⁺-i-n⁺ diodes with the multiplication region of 0.281, 0.582 and 1.243 m are calculated.     

High Doping Density Schottky Diodes in the 3MM Wavelength Cryogenic Heterodyne Receiver

The paper describes a 3mm cryogenic mixer receiver using high doping density (“room-temperature”) Schottky diodes. The measured equivalent noise temperature T_eq of the diodes is 109 K at 20 K, which is much higher than the T_eq of the low doping density (“cryogenic”) diodes. In spite of this, the double-sideband (DSB) noise temperature of the cryogenic receiver developed is 55 K at 110 GHz, owing to the low conversion loss of the mixer and ultra-low noise of the PHEMT IF amplifier. This is the lowest noise temperature ever reported for a Schottky diode mixer receiver. The results obtained are useful for the development of submm receivers in which high doping density Schottky diodes are used.     

Accurate time delay determination for femtosecond X-ray diffraction experiments

Intensity correlations and noise reduction are observed and characterized in the broadband supercontinuum generated by spatio-temporal solitons propagating in air, i.e., in filamentation of ultrashort laser pulses. Large correlations and reduction of the laser noise are observed already at the first steps of the filamentation process, while further propagation results in cascaded χ⁽³⁾ broadening processes and yield complex correlation maps. The spectral range yielding an optimal laser noise reduction of 3.6 dB is found to cover 10 nm around the fundamental wavelength. PACS 42.65.Jx; 42.65.Tg; 42.65.Ky; 42.50.Lc     

Random Walk of Second Class Particles in Product Shock Measures

We consider shock measures in a class of conserving stochastic particle systems on ℤ. These shock measures have a product structure with a step-like density profile and include a second class particle at the shock position. We show for the asymmetric simple exclusion process, for the exponential bricklayers’ process, and for a generalized zero range process, that under certain conditions these shocks, and therefore the second class particles, perform a simple random walk. Some previous results, including random walks of product shock measures and stationary shock measures seen from a second class particle, are direct consequences of our more general theorem. Multiple shocks can also be handled easily in this framework. Similar shock structure is also found in a nonconserving model, the branching coalescing random walk, where the role of the second class particle is played by the rightmost (or leftmost) particle.     

Sources of 1/F noise in gallium arsenide IMPATT diodes

The quasistatic approximation is used to analyze 1/F noise in IMPATT diodes in the static and dynamic (self-oscillating) modes. Sources of 1/F noise are defined in accordance with the fluctuator model: allowance is made for fluctuations of the charge of traps and fluctuations of the electron drift velocity caused by their scattering by traps and metastable neutral centers. It is shown that the fluctuations of the voltage across the diode and the fluctuations of the oscillation frequency are mainly determined by the fluctuations of the trap charge, while the fluctuations of the oscillation amplitude are determined by scattering by neutral centers. A method is developed to determine the intensity of noise sources using the results of measurements of the fluctuations in the static and dynamic modes of IMPATT diodes and a method of checking the model as a whole is checked. Experimental results are presented and these show satisfactory agreement with the calculations. Zh. Tekh. Fiz. 67, 65–70 (August 1997)     

The effect of determined chaos on light collection in optical systems

In terms of the dynamic approach, the collection of light in optical systems and the influence of determined chaos on the photometry and fluctuations of regular and chaotic collection are considered. The photometric relationships generalizing the formula of the integrating sphere as applied to chaotic collection are obtained. A universal law for noise in the regular light collection is predicted and found to be in good agreement with the available experimental data. The relationships studied can find use in the elaboration of a new-design detectors, light guides, light-emitting diodes, etc., for the enhancement of their efficiency and the reduction of noise.