p-TypeE InAs/GaAs Quantum Dot, Dot-In-Well, and Low-Frequency Noise Properties of Infrared Photodetectors

Several types of p-doped Infrared detectors were studied. These include InAs/GaAs quantum dot (QDIP), and dots-in-well (DWELL) and split off band-based heterojunction detectors. In these structures, IR absorption leading to detection is based on valence-band inter-sublevel hole transitions. For a QDIP and DWELL, at 80 K, two response bands observed at 1.5 – 3 and 3 – 10 µm were identified as due to optical transitions from the heavy hole to spin–orbit split-off QD level and from the heavy-hole to heavy/light-hole level, respectively. Unlike the n-type with bias dependent spectral response, the p-type hole response displays a well-preserved spectral profile (independent of the applied bias) observed in both QDIP and DWELL detectors. At a response peak of ~ 5.2 µm, QDIP and DWELL exhibit an external quantum efficiency of 17 % and 9 % respectively. At elevated temperatures between 100 and ~120 K (for QDIP), 130 K (for DWELL), both QDIP and DWELL detectors exhibit a strong far-infrared or terahertz (THz) response up to 70 µm which show promising potential of p-type QDs for developing THz infrared photodetectors. Based on the dark current and noise power spectral density analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material and electric field are some of the optimization parameters identified to improve the photoconductive and dark current gain of detectors. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the noise analysis, QDs based structures suppressed phonon scattering and enhanced carrier life time or photoconductive gain. Furthermore, in a GaAs/AlxGa1-xAs heterostructure, for a given width of AxlGa1-xAs barrier, the barrier thickness can be varied by varying the Al mole fraction x, which is referred to as a graded barrier. Grading the barrier and optimizing the emitter thickness of GaAs/AlGaAs heterostructures enhance the absorption efficiency, the escape probability and lower the dark current; hence, enhances the responsivity and specific detectivity of detectors. The two important methods (Arrhenius plot and Temperature Dependent Internal photoemission (TDIPS)) of determining detectors threshold wavelengths or band offsets were compared. For detectors with long threshold wavelength (\u3e\u3e 9.3 μm), the Arrhenius plot used to extract activation energy leads to energy values with deviation higher than ~ 10 % from the corresponding TDIPS values and results from the temperature dependent Fermi distribution tailing effect and Fowler–Nordheim tunneling current. Therefore, TDIPS or other methods, that take the temperature effects on the band offset and Fowler–Nordheim tunneling current into account, are needed for a precise band offset characterization of a long threshold wavelength detectors

Multiplexing Biochemical Signals

In this paper we show that living cells can multiplex biochemical signals, i.e. transmit multiple signals through the same signaling pathway simultaneously, and yet respond to them very specifically. We demonstrate how two binary input signals can be encoded in the concentration of a common signaling protein, which is then decoded such that each of the two output signals provides reliable information about one corresponding input. Under biologically relevant conditions the network can reach the maximum amount of information that can be transmitted, which is 2 bits.Comment: 4 pages, 4 figure

Radial Squeezed States and Rydberg Wave Packets

We outline an analytical framework for the treatment of radial Rydberg wave packets produced by short laser pulses in the absence of external electric and magnetic fields. Wave packets of this type are localized in the radial coordinates and have p-state angular distributions. We argue that they can be described by a particular analytical class of squeezed states, called radial squeezed states. For hydrogenic Rydberg atoms, we discuss the time evolution of the corresponding hydrogenic radial squeezed states. They are found to undergo decoherence and collapse, followed by fractional and full revivals. We also present their uncertainty product and uncertainty ratio as functions of time. Our results show that hydrogenic radial squeezed states provide a suitable analytical description of hydrogenic Rydberg atoms excited by short-pulsed laser fields.Comment: published in Physical Review

Rare switching events in non-stationary systems

Physical systems with many degrees of freedom can often be understood in terms of transitions between a small number of metastable states. For time-homogeneous systems with short-term memory these transitions are fully characterized by a set of rate constants. We consider the question how to extend such a coarse-grained description to non-stationary systems and to systems with finite memory. We identify the physical regimes in which time-dependent rates are meaningful, and state microscopic expressions that can be used to measure both externally time-dependent and history-dependent rates in microscopic simulations.Comment: 14 pages, 8 figure

Model of a fluid at small and large length scales and the hydrophobic effect

We present a statistical field theory to describe large length scale effects induced by solutes in a cold and otherwise placid liquid. The theory divides space into a cubic grid of cells. The side length of each cell is of the order of the bulk correlation length of the bulk liquid. Large length scale states of the cells are specified with an Ising variable. Finer length scale effects are described with a Gaussian field, with mean and variance affected by both the large length scale field and by the constraints imposed by solutes. In the absence of solutes and corresponding constraints, integration over the Gaussian field yields an effective lattice gas Hamiltonian for the large length scale field. In the presence of solutes, the integration adds additional terms to this Hamiltonian. We identify these terms analytically. They can provoke large length scale effects, such as the formation of interfaces and depletion layers. We apply our theory to compute the reversible work to form a bubble in liquid water, as a function of the bubble radius. Comparison with molecular simulation results for the same function indicates that the theory is reasonably accurate. Importantly, simulating the large length scale field involves binary arithmetic only. It thus provides a computationally convenient scheme to incorporate explicit solvent dynamics and structure in simulation studies of large molecular assemblies

Quantum Interference: From Kaons to Neutrinos (with Quantum Beats in between)

Using the vehicle of resolving an apparent paradox, a discussion of quantum interference is presented. The understanding of a number of different physical phenomena can be unified, in this context. These range from the neutral kaon system to massive neutrinos, not to mention quantum beats, Rydberg wave packets, and neutron gravity.Comment: 12 pages, LaTeX, 3 figure

Hydrophobic interactions: an overview

We present an overview of the recent progress that has been made in understanding the origin of hydrophobic interactions. We discuss the different character of the solvation behavior of apolar solutes at small and large length scales. We emphasize that the crossover in the solvation behavior arises from a collective effect, which means that implicit solvent models should be used with care. We then discuss a recently developed explicit solvent model, in which the solvent is not described at the atomic level, but rather at the level of a density field. The model is based upon a lattice-gas model, which describes density fluctuations in the solvent at large length scales, and a Gaussian model, which describes density fluctuations at smaller length scales. By integrating out the small length scale field, a Hamiltonian is obtained, which is a function of the binary, large-length scale field only. This makes it possible to simulate much larger systems than hitherto possible as demonstrated by the application of the model to the collapse of an ideal hydrophobic polymer. The results show that the collapse is dominated by the dynamics of the solvent, in particular the formation of a vapor bubble of critical size. Implications of these findings to the understanding of pressure denaturation of proteins are discussed.Comment: 10 pages, 4 figure

A possible mechanism for cold denaturation of proteins at high pressure

We study cold denaturation of proteins at high pressures. Using multicanonical Monte Carlo simulations of a model protein in a water bath, we investigate the effect of water density fluctuations on protein stability. We find that above the pressure where water freezes to the dense ice phase ($\approx2$ kbar), the mechanism for cold denaturation with decreasing temperature is the loss of local low-density water structure. We find our results in agreement with data of bovine pancreatic ribonuclease A.Comment: 4 pages for double column and single space. 3 figures Added references Changed conten