Gold-black manufacture, microstructure, and optical characterization.

A previous contribution formulates a first-principle dipole antenna theory for predicting the polarization-sensitive directional spectral absorptance of gold-black in the near infrared. The current contribution chronicles a successful effort to validate that theory. After a brief review of gold-black history, we describe in some detail the design and construction of a vapor-deposition cell for laying down gold-black coatings on a mirrorlike gold substrate. The microstructure of 4- and 8-µm-thick coatings is revealed using scanning electron microscopy. An automated bench-level reflectometer has been used to measure the in-plane bidirectional reflectivity of the gold-black coatings in the visible (532 nm) and near-infrared (800 nm) for p and s polarization. Measurements are reported over incident zenith angles ranging between 10 and 50 deg. Results obtained using the apparatus are consistent with the dipole antenna theory in this range of incident zenith angles.

Graphene-Ta2O5 heterostructure enabled high performance, deep-ultraviolet to mid-infrared photodetection.

Ultrafast, high sensitive, low cost photodetectors operating at room temperature sensitive from the deep-ultraviolet to mid-infrared region remain a significant challenge in optoelectronics. Achievements in traditional semiconductors using cryogenic operation and complicated growth processes prevent the cost-effective and practical application of broadband detectors. Alternative methods towards high-performance photodetectors, hybrid graphene-semiconductor colloidal quantum dots have been intensively explored. However, the operation of these photodetectors has been limited by the spectral bandwidth and response time. Here, we have demonstrated hybrid photodetectors operating from the deep-ultraviolet to the mid-infrared region with high sensitivity and ultrafast response by coupling graphene with a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 thin film. Photons with energy higher than the energy of the defect centers release holes from neutral acceptors. The holes are transferred into graphene, leaving behind ionized acceptors. Due to the advantage of two-dimensional heterostructure including homogeneous thickness, extending in a two-dimensional plane, large contact area between the N-Ta2O5 thin film and graphene, and the high mobility of carriers in graphene, holes are transferred rapidly to graphene and recirculated during the long lifetime of ionized acceptors. The photodetectors achieve a high photo-responsivity (up to 3.0 × 106 A W-1), ultrafast rise time (faster than 20 ns), and a specific detectivity (up to ∼2.2 × 1012 Jones). The work provides a method for achieving high-performance optoelectronics operating in the deep-ultraviolet to mid-infrared region.

Monte Carlo ray-trace diffraction based on the Huygens-Fresnel principle.

The goal of this effort is to establish the conditions and limits under which the Huygens-Fresnel principle accurately describes diffraction in the Monte Carlo ray-trace (MCRT) environment. This goal is achieved by systematic intercomparison of dedicated experimental, theoretical, and numerical results. We evaluate the success of the Huygens-Fresnel principle by predicting and carefully measuring the diffraction fringes produced by both single slit and circular apertures. We then compare the results from the analytical and numerical approaches with each other and with dedicated experimental results. We conclude that use of the MCRT method to accurately describe diffraction requires that careful attention be paid to the interplay among the number of aperture points, the number of rays traced per aperture point, and the number of bins on the screen. This conclusion is supported by standard statistical analysis, including the adjusted coefficient of determination, Radj2, the rms deviation, and the reduced chi-square statistic, χv2.

Photoluminescence quantum efficiency of Er optical centers in GaN epilayers.

We report the quantum efficiency of photoluminescence processes of Er optical centers as well as the thermal quenching mechanism in GaN epilayers prepared by metal-organic chemical vapor deposition. High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN under resonant excitation excitations were performed. Data provide a picture of the thermal quenching processes and activation energy levels. By comparing the photoluminescence from Er ions in the epilayer with a reference sample of Er-doped SiO2, we find that the fraction of Er ions that emits photon at 1.54 µm upon a resonant optical excitation is approximately 68%. This result presents a significant step in the realization of GaN:Er epilayers as an optical gain medium at 1.54 µm.

High-precision gigahertz-to-terahertz spectroscopy of aqueous salt solutions as a probe of the femtosecond-to-picosecond dynamics of liquid water.

Because it is sensitive to fluctuations occurring over femtoseconds to picoseconds, gigahertz-to-terahertz dielectric relaxation spectroscopy can provide a valuable window into water's most rapid intermolecular motions. In response, we have built a vector network analyzer dielectric spectrometer capable of measuring absorbance and index of refraction in this frequency regime with unprecedented precision. Using this to determine the complex dielectric response of water and aqueous salt solutions from 5.9 GHz to 1.12 THz (which we provide in the supplementary material), we have obtained strong new constraints on theories of water's collective dynamics. For example, while the salt-dependencies we observe for water's two slower relaxations (8 and 1 ps) are easily reconciled with suggestions that they arise due to rotations of fully and partially hydrogen bonded molecules, respectively, the salt-dependence of the fastest relaxation (180 fs) appears difficult to reconcile with its prior assignment to liberations of single hydrogen bonds.

New terahertz dielectric spectroscopy for the study of aqueous solutions.

We present the development of a high precision, tunable far-infrared (terahertz) frequency-domain dielectric spectrometer for studying the dynamics of biomolecules in aqueous solutions in the gigahertz-to-terahertz frequency. As an important benchmark system, we report on the measurements of the absorption and refractive index for liquid water in the frequency range from 5 GHz to 1.12 THz (0.17-37.36 cm(-1) or 0.268-60 mm). The system provides a coherent radiation source with power up to 20 mW in the gigahertz-to-terahertz region. The dynamic range of our instrument reaches 10(12) and the system achieves a spectral resolution of less than 100 Hz. The temperature of samples can be controlled precisely with error bars of ±0.02 °C from 0 °C to 90 °C. Given these attributes, our spectrometer provides unique capabilities for the accurate measurement of even very strongly absorbing materials such as aqueous solutions.

Inducing an incipient terahertz finite plasmonic crystal in coupled two dimensional plasmonic cavities.

We measured a change in the current transport of an antenna-coupled, multigate, GaAs/AlGaAs field-effect transistor when terahertz electromagnetic waves irradiated the transistor and attribute the change to bolometric heating of the electrons in the two dimensional electron channel. The observed terahertz absorption spectrum indicates coherence between plasmons excited under adjacent biased device gates. The experimental results agree quantitatively with a theoretical model we developed that is based on a generalized plasmonic transmission line formalism and describes an evolution of the plasmonic spectrum with increasing electron density modulation from homogeneous to the crystal limit. These results demonstrate an electronically induced and dynamically tunable plasmonic band structure.

Dielectric spectroscopy of proteins as a quantitative experimental test of computational models of their low-frequency harmonic motions.

Decades of molecular dynamics and normal mode calculations suggest that the largest-scale collective vibrational modes of proteins span the picosecond to nanosecond time scale. Experimental investigation of these harmonic, low-amplitude motions, however, has proven challenging. In response, we have developed a vector network analyzer-based spectrometer that supports the accurate measurement of both the absorbance and refractive index of solvated biomolecules over the corresponding gigahertz to terahertz frequency regime, thus providing experimental information regarding their largest-scale, lowest frequency harmonic motions. We have used this spectrometer to measure the complex dielectric response of lysozyme solutions over the range 65 to 700 GHz and an effective medium model to separate the dielectric response of the solvated protein from that of its buffer. In doing so, we find that each lysozyme is surrounded by a tightly bound layer of 165 ± 15 water molecules that, in terms of their picosecond dynamics, behave as if they are an integral part of the protein. We also find that existing computational descriptions of the protein's dynamics compare poorly with the results of our experiment. Specifically, published normal mode and molecular dynamics simulations do not explain the measured dielectric response unless we introduce a cutoff frequency of 250 GHz below which the density of vibrational modes drops to zero. This cutoff is physically plausible, given the known size of the protein and the known speed of sound in proteins, raising questions as to why it is not apparent in computational models of the protein's motions.

Coherent control of Rydberg states in silicon.

Laser cooling and electromagnetic traps have led to a revolution in atomic physics, yielding dramatic discoveries ranging from Bose-Einstein condensation to the quantum control of single atoms. Of particular interest, because they can be used in the quantum control of one atom by another, are excited Rydberg states, where wavefunctions are expanded from their ground-state extents of less than 0.1 nm to several nanometres and even beyond; this allows atoms far enough apart to be non-interacting in their ground states to strongly interact in their excited states. For eventual application of such states, a solid-state implementation is very desirable. Here we demonstrate the coherent control of impurity wavefunctions in the most ubiquitous donor in a semiconductor, namely phosphorus-doped silicon. In our experiments, we use a free-electron laser to stimulate and observe photon echoes, the orbital analogue of the Hahn spin echo, and Rabi oscillations familiar from magnetic resonance spectroscopy. As well as extending atomic physicists' explorations of quantum phenomena to the solid state, our work adds coherent terahertz radiation, as a particularly precise regulator of orbitals in solids, to the list of controls, such as pressure and chemical composition, already familiar to materials scientists.

Continuous voltage tunability of intersubband relaxation times in coupled SiGe quantum well structures using ultrafast spectroscopy.

We demonstrate continuous voltage control of the nonradiative transition lifetime in semiconductor heterostructures. The results were obtained by picosecond time-resolved experiments on biased SiGe valence band quantum well structures using a free electron laser. By varying the applied voltage, the intersubband hole relaxation times for quantum well structures were varied by a factor of 2 as the wave functions and their overlaps were tuned. The range of magnitudes for the lifetime indicates a possible route to silicon-based quantum cascade lasers.

THz operation of asymmetric-nanochannel devices.

The THz spectrum lies between microwaves and the mid-infrared, a region that remains largely unexplored mainly due to the bottleneck issue of lacking compact, solid state, emitters and detectors. Here, we report on a novel asymmetric-nanochannel device, known as the self-switching device, which can operate at frequencies up to 2.5 THz for temperature up to 150 K. This is, to our knowledge, not only the simplest diode but also the quickest acting electronic nanodevice reported to date. The radiation was generated by the free electron laser FELIX (Netherlands). The dependences of the device efficiency as a function of the electric bias, radiation intensity, radiation frequency and temperature are reported.

Donor-state-enabling Er-related luminescence in silicon: direct identification and resonant excitation.

We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.

Isotope dependence of the lifetime of the vibration of oxygen in silicon.

By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate tau of the asymmetric-stretch vibration (nu3=1136 cm-1) of oxygen (Oi) in silicon changes by a factor of approximately 2.5. These data establish that nu3 decays by creating one nu1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (nu3-nu1) is less than the maximum frequency num of the host lattice, as for 28Si-16O-28Si in natural silicon, then it is emitted as one lattice mode, and tau depends on the density of one-phonon states at nu3-nu1. If (nu3-nu1)>num, as for 16O in single-isotope 30Si silicon, two lattice modes are created in addition to nu1, increasing tau. Prediction of tau for a particular defect clearly requires a detailed knowledge of that defect.

Microscopic structure of Er-related optically active centers in crystalline silicon.

A successful observation and analysis of the Zeeman effect on the lambda approximately 1.54 microm photoluminescence band in Er-doped crystalline MBE-grown silicon are presented. The symmetry of the dominant optically active centers is conclusively established as orthorhombic I(C(2v)) with g axially approximately 18.39 and g radially approximately 0. In this way the long standing puzzle as regards the paramagnetism of optically active Er-related centers in silicon is settled. Preferential generation of a single type of an optically active Er-related center confirmed in this study is essential for photonic applications of Si:Er.

Cloning and sequence analysis of a cDNA encoding a novel truncated form of the chicken TrkB receptor.

A cDNA clone encoding a novel truncated form of the chicken TrkB receptor has been isolated and sequenced. Compared to two previously reported forms from mouse and rat, this clone contains an 141-bp insert (47 amino acids) in the cytoplasmic region.