Tengiz Svimonishvili

Secondary electron yield measurements from materials with application to collectors of high-power microwave devices

An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies are presented for the following materials: copper, titanium, and Poco graphite. Observation of time-dependent SEY behavior in these samples suggests that surface processes play an important role during measurements. In addition, SEY at low energies and as a function of the angle of incidence of primary electrons has been measured for plasma sprayed boron carbide (PSBC). The experimental results presented here are benchmarked with existing SEY data in the literature, empirically and to first principle formulae

Three-dimensional simulation of oxide-confined vertical-cavity surface-emitting semiconductor lasers

Several vertical-cavity surface-emitting laser (VCSEL) structures are investigated by means of 3D steady-state electrical-thermal-optical numerical modeling. Electrical and thermal models are coupled via: (i) heat generation by current passing through the diode; (ii) temperature dependence of the diffusion potential of the junction; and (iii) temperature dependence of the bulk resistivity of passive material at both sides of the junction. Optical waveguide model is coupled to electrical-thermal model through position-dependent carrier recombination lifetime and temperature-dependent refractive-index. Simulation is performed for cylindrically symmetric two-sided oxide- confined intracavity-contact VCSELs. For comparison purposes, numerical data are acquired for materially identical bottom-emitting mesa laser and p-side intracavity- contact VCSEL. Nonuniformity of the main device characteristics is studied. Several different phenomena are shown to contribute to nonuniformity: (i) current crowding due to device geometry; (ii) current crowding induced by stimulated emission processes; (iii) current spreading related to oxide positioning; (iv) temperature related effects.

The Dose Effect in Secondary Electron Emission

In this paper, total incident electron dose as an inherent parameter in secondary electron emission is experimentally demonstrated. A completely automated experimental setup allows for measuring of secondary electron yield (SEY) as a function of beam energy, angle of incidence of primary electrons, electron dose, and time. SEY data are presented for copper, plasma-sprayed boron carbide, and titanium nitride samples with principal focus on dose dependence. Experiments were conducted in the low-energy range (5-1000 eV) and direct-current regime. Experimental results have been compared with formulas in literature, and good agreement was observed. Modified empirical formulas incorporating the dose effect have also been proposed.

Optoelectronic closed-loop auto-oscillator for fluorescence lifetime detection: a new fluorimetry technique with applications to chemical/biosensors

We present a new approach to excited state fluorescence lifetime-based measurements which is inexpensive and highly sensitive. The detection system consists of a closed-loop optoelectronic arrangement containing an intermediate frequency resonance amplifier, a fluorescence excitation light source (for example, a light emitting diode or a semiconductor laser), a fiber optic delay line, and a photodetector. The system exhibits self-oscillations in the vicinity of the frequency (Omega) approximately 1/(tau) (where (tau) is the excited state lifetime) which manifest themselves as modulation of the light. Changes in the excited state lifetime alter the phase delay of the loop, which in turn causes a frequency shift in the modulation signal. The frequency shift can be measured very precisely with a frequency counter. With appropriate averaging, this technique can yield sub-picosecond resolution of shifts in lifetime. This technique is suited for chemical/biological sensing applications, and can be easily duplicated for chemical/biological sensor arrays.

Overview of photonic materials and components for application in space environments

Future space systems will be based on components evolving from the development and refinement of new and existing photonic materials. Optically based sensors, inertial guidance, tracking systems, communications, diagnostics, imaging and high speed optical processing are but a few of the applications expected to widely utilize photonic materials. Knowledge of the response of these materials to space environment effects such as spacecraft charging, orbital debris, atomic oxygen, ultraviolet irradiation, temperature and ionizing radiation will be paramount to ensuring successful space applications. The intent of this paper is to address the latter two environments via a succinct comparison of the known sensitivities of selected photonic materials to the temperature and ionizing radiation conditions found in space and enhanced space radiation environments. Delineation of the known temperature and radiation induced responses in LiNbO3, AlGaN, AlGaAs, TeO2, Si:Ge, and several organic polymers are presented. Photonic materials are realizing rapid transition into applications for many proposed space components and systems including: optical interconnects, optical gyros, waveguides and spatial light modulators, light emitting diodes, lasers, optical fibers and fiber optic amplifiers. Changes to material parameters such as electrooptic coefficients, absorption coefficients, polarization, conductivity, coupling coefficients, diffraction efficiencies, and other pertinent material properties are examined for thermooptic and radiation induced effects. Conclusions and recommendations provide the reader with an understanding of the limitations or attributes of material choices for specific applications.

3D electrothermal simulation of intracavity-contacted oxide-confined VCSELs operating at room temperature and at 77 K

3D numerical simulation of intracavity-contacted oxide-confined VCSELs operating at cryogenic temperatures is reported. Spatial profiles and 3D distributions of electrical potential, current and ture are calculated. The device geometry is found to cause strong current crowding effects and nonuniformity of injected carrier distribution. Numerical experiments are performed for both spontaneous and stimulated modes of operation, and the results are compared with room-temperature simulation. Improved thermal conduction at low temperatures more than compensates for an increase in electrical resistivity, resulting in a significant reduction of active-region temperature increase at cryogenic temperatures.

Development of a new detection technique for fluorescence lifetime-based chemical/biological sensor arrays monitoring: dual closed-loop optoelectronic auto-oscillatory detection circuit

We present a new detection instrument for chemical/biological fluorescence lifetime-based sensors. The instrument comprises a primary, closed loop with a secondary loop controlling a variable phase delay within the primary loop. The primary loop consists of a fluorescence excitation light source, a fiber-optic delay line (with a gap for placement of a fluorescent sensor), an electronic phase shifter, a photo-detector, and a resonance-type RF amplifier. The secondary loop consists of a long-wavelength-pass optical filter, multimode fiber, a PMT, and an electronic phase detector (which is connected to the phase shifter of the primary loop). The system exhibits self-oscillations in the form of RF sinusoidal intensity modulation with frequency dependent on the fluorescence lifetime. Since the primary loop does not contain an optical filter, it is easier to obtain self-oscillations (compared to single loop systems). The feedback also improves the stability of the detection platform. The detection system is simple, inexpensive, and scalable for sensor array purposes. We demonstrate the use of a cost-effective, multi-channel, computer-algorithm-based frequency counter with this new system. We illustrate the detection capabilities of this detection system with the pH-sensitive, fluorescent probe carboxy seminaphthofluorescein (SNAFL-2) and an immunosensor based on fluorescence resonance energy transfer.

THZ radiation source based on two-stream instability

THz radiation straddles microwave and infrared bands (50 GHz – 10 THz), thus combining the penetrating power of lower-frequency waves and imaging capabilities of higher-energy infrared radiation. Since THz radiation is not absorbed by most dry, non-polar materials, it can be used for imaging internal structures. Besides its military uses, THz radiation is employed in such important applications as spectroscopy, industrial bio-medical imaging, and scattering. What also makes THz radiation attractive is its non-ionizing photon energy, which is less than 0.1 eV at 1 THz. Several conventional devices are used presently to generate THz radiation. For example, slow-wave devices require very small structures (mm or sub-mm) in size. This complicates fabrication and alignment and results in merely milliwatts of average output power. Conventional FELs and synchrotrons are bulky and very expensive to operate.

Investigation of Interaction of an Electron Beam and Metallic Grating in a Smith-Purcell Free Electron Laser (SPFEL)

Summary form only given. Smith-Purcell (SP) radiation is produced when an electron beam passes over a metallic periodic structure and a continuous spectrum of modes associated with the beam is scattered by the grating. Most of the scattered modes are evanescent, while some may propagate. The SP radiation wavelength is found to be proportional to the grating period, L, and inversely proportional to beam velocity, V. Therefore, the SP radiation wavelength can be varied by changing L and V. Based on recent experimental reports and theoretical calculations, a compact SP-FEL promises to be an attractive and affordable source of THz radiation. The major drawback of the traditional SP-FEL approach is that the electron beam must propagate very close to the grating surface. Furthermore, as one scales the concept to the THz regime, the metallic gratings become more lossy. We had proposed to improve the coupling of the beam to the grating by taking advantage of surface plasma waves in the grating. Here, we evaluate several models for the SP-FEL, including the effects of coupling to the surface plasma wave. Parametric scans are made to suggest optimal regimes to test the concept of a SP-FEL invoking surface plasma wave coupling.

Characterization of the Dose Effect in Secondary Electron Emission

Summary form given only. Secondary electron emission (SEE) results from bombarding materials with electrons, atoms, or ions. When studying SEE, one is usually interested in determining the secondary electron yield, defined as the number of secondary electrons produced per incident primary electron. The amount of secondary emission is well known to depend on factors such as bulk and surface properties of materials, energy of incident particles, and their angle of incidence. However, results presented here indicate large variation of yield with incident electron dose/current density in addition to the above factors. There has been little effort in literature to quantify this effect. Experiments in this thesis aim to fill this gap.