Oscar M. Stafsudd

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Ferroelectric materials (FEMs) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Absorption and Fluorescence Spectra of Several Praseodymium‐Doped Crystals and the Change of Covalence in the Chemical Bonds of the Praseodymium Ion

The absorption and fluorescence spectra of praseodymium‐doped lanthanum trifluoride, lanthanum ethylsulfate, and yttrium aluminum garnet single crystals are reported. The center of gravity of each J level is shifted to the red when the nearest neighbors of the praseodymium ion change in the order of fluorine, water, chlorine, bromine, and oxygen. This corresponds to an increase of covalent character by 0.7, 0.9, 1.1, and 2.0% for Pr–H2O, Pr–Cl, Pr–Br, and Pr–O bonds when compared with Pr–F.

Prospects for Silicon Mid-IR Raman Lasers

This paper presents the case for the silicon Raman laser as a potential source for the technologically important midwave infrared (MWIR) region of the optical spectrum. The mid-IR application space is summarized, and the current practice based on the optical parametric oscillators and solid state Raman lasers is discussed. Relevant properties of silicon are compared with popular Raman crystals, and linear and nonlinear transmission measurements of silicon in the mid-IR are presented. It is shown that the absence of the nonlinear losses, which severely limit the performance of the recently demonstrated silicon lasers in the near IR, combined with unsurpassed crystal quality, high thermal conductivity and excellent optical damage threshold render silicon a very attractive Raman medium, even when compared to the very best Raman crystals. In addition, silicon photonic technology, offering integrated low-loss waveguides and microcavities, offers additional advantages over todays bulk crystal Raman laser technology. Using photonic crystal structures or microring resonators, the integrated cascaded microcavities can be employed to realize higher order Stokes emission, and hence to extend the wavelength coverage of the existing pump lasers. Exploiting these facts, the proposed technology can extend the utility of silicon photonics beyond data communication and into equally important applications in biochemical sensing and laser medicine

Type conversion, contacts, and surface effects in electroplated CdTe films

Efficient electroplated CdS/CdTe solar cells can be fabricated by heat treating and type‐converting the n‐CdTe films deposited on CdS layers. In this paper, various mechanisms which may give rise to the conversion of electroplated CdTe films from n to p type are investigated. It is concluded that Cd‐vacancy generation is the main mechanism of type conversion. Possible effects of oxygen on this mechanism are also discussed. Evaporated Au contacts to electroplated p‐CdTe films were studied. It was found that the Au contacts depleted the excess Te present on the surface of Br2‐methanol etched p‐CdTe films. Oxygen was found to affect the electrical characteristics of such contacts.

Invited review article: practical guide for pyroelectric measurements.

The characterization of pyroelectric materials is a necessary stage in the design of a large variety of pyroelectric-based devices ranging from intrusion alarms to IR cameras. The sample configurations and measurement techniques currently in use vary widely and require careful attention in order to avoid artifacts. In this review, we provide a practical guide to the measurement of the pyroelectric coefficient, paying particular attention to the new instrumental possibilities (fast sinusoidally modulated light sources, low impedance broad band current meters, and fast averaging oscilloscopes) that have become available during the last decade. Techniques applicable to bulk specimens, substrate-supported films, and self-supported films are described in detail. The most commonly used procedures are classified according to the type of thermal excitation: continuous ramping, heat pulse, and continuous oscillation. In the appendices, we describe the practical realization of these measurement schemes and provide mathematical descriptions for the extraction of the pyroelectric coefficient from the measured data.

Quantification and modeling of the dynamic changes in the absorption coefficient of water at /spl lambda/ = 2.94 /spl mu/m

A stable, high-energy Q-switched Er:YAG laser operating at 2.94 /spl mu/m with a nearly diffraction-limited spatial beam profile was used to quantify the dynamic changes in the absorption coefficient of liquid water as a function of incident fluence. The data from transmission measurements across water layers of known thicknesses shows that the effective absorption coefficient of water decreases by almost an order of magnitude for fluence levels less than 2 J/cm/sup 2/. From the measured transmission data, we have developed a phenomenological, finite-difference absorption model (the dynamic saturable absorption model) that can, at least to a first-order approximation, accurately predict the dynamic and effective absorption coefficient of water at the wavelength /spl lambda/ = 2.94 /spl mu/m. The model developed in the present study should prove useful in efforts to understand the underlying mechanisms of laser-tissue interaction in applications such as skin resurfacing and corneal sculpting, wherein Er:YAG lasers are used to target water as the dominant chromophore.

Electronic effects of ion mobility in semiconductors: Semionic behaviour of CuInSe2

Abstract Application of strong electric fields at ambient temperatures to crystals of CuInSe 2 and (Cu,Ag)InSe 2 semiconductors can create non-equilibrium doping profiles, stable after removal of the electric field, as illustrated by formation of μm-sized single and multiple diode structures in initially homogeneous and uniformly doped single crystals. Phototransistor action and amplification was obtained, clear evidence for the presence of junctions, resulting from non-equilibrium doping profiles. Electron beam-induced current measurements, combined with current-voltage, capacitance-voltage and time-dependent current measurements, as well as numerical simulations were performed to characterize and understand the phenomena. Control experiments exclude formation of extended defects or the occurrence of contact diffusion. Localized Joule heating that occurs around the contact cannot by itself account for the observed phenomena. Because these chalcopyrites contain relatively mobile components [Cu,Ag], which are also dopants, we interpret these observations as a result of internal dopant redistribution, aided by localized temperature increases and directed by the electric field.

Luminescence of Cu+-β“-alumina

Abstract A series of monovalent copper containing β -alumina single crystals have been prepared by standard ion exchange techniques. The crystals exhibit strong luminescence which we attribute to the 3d 9 4s → 3d 10 interconfigurational transition. The peak wavelength of this transition can be varied by 135 nm by the presence of other cations in the β “-alumina conduction plane, allowing the luminescence to be varied throughout the visible spectrum. Single-pass gain measurements of several of these crystals show gains of the order of 0.1/cm, suggesting the potential use of these materials for solid-state tunable lasers.

Current transport mechanisms of electrochemically deposited CdS/CdTe heterojunction

Abstract The dark current transport mechanism in electrochemically deposited n-CdS/p-CdTe thin film heterojunctions is investigated. The forward current measured in the temperature range between 200° and 305°K can be expressed as Jf = J0(T) exp (AV) and the reverse current can be expressed as J r = −CV exp −CV[−λ(V d −V) −( 1 2 ) ] . The current mechanisms are consistent with a multi step recombination-tunneling model.