High-conversion-efficiency and broadband tunable femtosecond noncollinear optical parametric amplifier

We build a compact high-conversion-efficiency and broadband tunable noncollinear optical parametric amplifier (OPA) in the infra-red (IR) pumped by a femtosecond Ti:sapphire CPA laser. The OPA consists of an internal seed of white-light continuum generator (WLG) and two noncollinear optical parametric amplifiers. The tunable wavelength range is from 1.2 mu m to 2.4 mu m for both signal and idle pulses. The total OPA efficiency in the last OPA stage reaches about 40% in a wider tunable spectral range (from 1.3 mu m to 1.7 mu m for signal pulse, from 1.5 mu m to 2.0 mu m for idle pulse respectively).

X-shaped third harmonic generated by ultrashort infrared pulse filamentation in air

The authors report the measurement of the angularly resolved spectrum of the third harmonic generated in a femtosecond filament in air and its evolution with increasing pump power. Pumped by a focused infrared ultrashort pulse with a carrier wavelength of 1270 nm, a pulse duration of similar to 20 fs, and pulse energy up to 487 mu J, the generated third harmonic is composed of an on-axis emission and a conical ring emission. When the pump power is sufficiently high, angularly resolved spectra with significant X-like feature could be observed, indicating the formation of nonlinear X wave at third harmonic. (c) 2008 American Institute of Physics.

Optical gain at 1550 nm from colloidal solution of Er3+-Yb3+ codoped NaYF4 nanocubes

We demonstrated optical amplification at 1550 nm with a carbon tetrachloride solution of Er3+-Yb3+ codoped NaYF4 nanocubes synthesized with solvo-thermal route. Upon excitation with a 980 nm laser diode, the nanocube solution exhibited strong near-infrared emission by the I-4(13/2) -> I-4(15/2) transition of Er3+ ions due to energy transfer from Yb3+ ions. We obtained the highest optical gain coefficient at 1550 nm of 0.58 cm(-1) for the solution with the pumping power of 200 mW. This colloidal solution might be a promising candidate as a liquid medium for optical amplifier and laser at the optical communication wavelength. (C) 2009 Optical Society of America

Filamentation control in the temperature gradient argon gas

A novel technique of controlling the evolution of the filamentation was experimentally demonstrated in an argon gas-filled tube. The entrance of the filament was heated by a furnace and the other end was cooled with air, which resulted in the temperature gradient distribution along the tube. The experimental results show that multiple filaments are merged into a single filament and then no filament by only increasing the temperature at the entrance of the filament. Also, the filament can appear and disappear after increasing the local temperature and input pulse energy in turn. This technique offers another degree of freedom to control the filamentation and opens a new way for multi-mJ level monocycle pulse generation through filamentation in the noble gas.

Bright-field and fluorescence chip-scale microscopy for biological imaging

Optical microscopy is an essential tool in biological science and one of the gold standards for medical examinations. Miniaturization of microscopes can be a crucial stepping stone towards realizing compact, cost-effective and portable platforms for biomedical research and healthcare. This thesis reports on implementations of bright-field and fluorescence chip-scale microscopes for a variety of biological imaging applications. The term “chip-scale microscopy” refers to lensless imaging techniques realized in the form of mass-producible semiconductor devices, which transforms the fundamental design of optical microscopes.

Our strategy for chip-scale microscopy involves utilization of low-cost Complementary metal Oxide Semiconductor (CMOS) image sensors, computational image processing and micro-fabricated structural components. First, the sub-pixel resolving optofluidic microscope (SROFM), will be presented, which combines microfluidics and pixel super-resolution image reconstruction to perform high-throughput imaging of fluidic samples, such as blood cells. We discuss design parameters and construction of the device, as well as the resulting images and the resolution of the device, which was 0.66 µm at the highest acuity. The potential applications of SROFM for clinical diagnosis of malaria in the resource-limited settings is discussed.

Next, the implementations of ePetri, a self-imaging Petri dish platform with microscopy resolution, are presented. Here, we simply place the sample of interest on the surface of the image sensor and capture the direct shadow images under the illumination. By taking advantage of the inherent motion of the microorganisms, we achieve high resolution (~1 µm) imaging and long term culture of motile microorganisms over ultra large field-of-view (5.7 mm × 4.4 mm) in a specialized ePetri platform. We apply the pixel super-resolution reconstruction to a set of low-resolution shadow images of the microorganisms as they move across the sensing area of an image sensor chip and render an improved resolution image. We perform longitudinal study of Euglena gracilis cultured in an ePetri platform and image based analysis on the motion and morphology of the cells. The ePetri device for imaging non-motile cells are also demonstrated, by using the sweeping illumination of a light emitting diode (LED) matrix for pixel super-resolution reconstruction of sub-pixel shifted shadow images. Using this prototype device, we demonstrate the detection of waterborne parasites for the effective diagnosis of enteric parasite infection in resource-limited settings.

Then, we demonstrate the adaptation of a smartphone’s camera to function as a compact lensless microscope, which uses ambient illumination as its light source and does not require the incorporation of a dedicated light source. The method is also based on the image reconstruction with sweeping illumination technique, where the sequence of images are captured while the user is manually tilting the device around any ambient light source, such as the sun or a lamp. Image acquisition and reconstruction is performed on the device using a custom-built android application, constructing a stand-alone imaging device for field applications. We discuss the construction of the device using a commercial smartphone and demonstrate the imaging capabilities of our system.

Finally, we report on the implementation of fluorescence chip-scale microscope, based on a silo-filter structure fabricated on the pixel array of a CMOS image sensor. The extruded pixel design with metal walls between neighboring pixels successfully guides fluorescence emission through the thick absorptive filter to the photodiode layer of a pixel. Our silo-filter CMOS image sensor prototype achieves 13-µm resolution for fluorescence imaging over a wide field-of-view (4.8 mm × 4.4 mm). Here, we demonstrate bright-field and fluorescence longitudinal imaging of living cells in a compact, low-cost configuration.

Infrared luminescence properties of bismuth-doped barium silicate glasses

Infrared (IR) luminescence covering 1.1 to similar to 1.6 mu m wavelength region was observed from bismuth-doped barium silicate glasses, excited by a laser diode at 808 nm wavelength region, at room temperature. The peak of the IR luminescence appears at 1325 nm. A full width half-maximum (FWHM) and the lifetime of the fluorescence is more than 200 nm and 400 mu s, respectively. The fluorescence intensity increases with Al2O3 content, but decreases with BaO content. We suggest that the IR luminescence should be ascribed to the low valence state of bismuth Bi2+ or Bi+, and Al3+ ions play an indirect dispersing role for the infrared luminescent centers.

Multiphoton-excited upconversion luminescence of Nd : YVO4

Fluorescence spectra of Nd: YVO4 under excitation of a continuous wave (CW) diode laser and a femtosecond laser at 800nm were investigated. It was found that Nd: YVO4 shows different upconversion and downconversion luminescencent behaviors when excited by the diode laser and the femtosecond laser. The dependence of the upconversion luminescence intensity on the pump power of the femtosecond laser was discussed. The populations of the upper energy levels for upconversion and downconversion luminescence were calculated based on the Bloch equations. The calculations agree well with the experimental results. (c) 2007 Optical Society of America.

Design strategies for ultra-high efficiency photovoltaics

While concentrator photovoltaic cells have shown significant improvements in efficiency in the past ten years, once these cells are integrated into concentrating optics, connected to a power conditioning system and deployed in the field, the overall module efficiency drops to only 34 to 36%. This efficiency is impressive compared to conventional flat plate modules, but it is far short of the theoretical limits for solar energy conversion. Designing a system capable of achieving ultra high efficiency of 50% or greater cannot be achieved by refinement and iteration of current design approaches.

This thesis takes a systems approach to designing a photovoltaic system capable of 50% efficient performance using conventional diode-based solar cells. The effort began with an exploration of the limiting efficiency of spectrum splitting ensembles with 2 to 20 sub cells in different electrical configurations. Incorporating realistic non-ideal performance with the computationally simple detailed balance approach resulted in practical limits that are useful to identify specific cell performance requirements. This effort quantified the relative benefit of additional cells and concentration for system efficiency, which will help in designing practical optical systems.

Efforts to improve the quality of the solar cells themselves focused on the development of tunable lattice constant epitaxial templates. Initially intended to enable lattice matched multijunction solar cells, these templates would enable increased flexibility in band gap selection for spectrum splitting ensembles and enhanced radiative quality relative to metamorphic growth. The III-V material family is commonly used for multijunction solar cells both for its high radiative quality and for the ease of integrating multiple band gaps into one monolithic growth. The band gap flexibility is limited by the lattice constant of available growth templates. The virtual substrate consists of a thin III-V film with the desired lattice constant. The film is grown strained on an available wafer substrate, but the thickness is below the dislocation nucleation threshold. By removing the film from the growth substrate, allowing the strain to relax elastically, and bonding it to a supportive handle, a template with the desired lattice constant is formed. Experimental efforts towards this structure and initial proof of concept are presented.

Cells with high radiative quality present the opportunity to recover a large amount of their radiative losses if they are incorporated in an ensemble that couples emission from one cell to another. This effect is well known, but has been explored previously in the context of sub cells that independently operate at their maximum power point. This analysis explicitly accounts for the system interaction and identifies ways to enhance overall performance by operating some cells in an ensemble at voltages that reduce the power converted in the individual cell. Series connected multijunctions, which by their nature facilitate strong optical coupling between sub-cells, are reoptimized with substantial performance benefit.

Photovoltaic efficiency is usually measured relative to a standard incident spectrum to allow comparison between systems. Deployed in the field systems may differ in energy production due to sensitivity to changes in the spectrum. The series connection constraint in particular causes system efficiency to decrease as the incident spectrum deviates from the standard spectral composition. This thesis performs a case study comparing performance of systems over a year at a particular location to identify the energy production penalty caused by series connection relative to independent electrical connection.

Measurement of the neutron (^3He) spin structure function at low Q^2 : a connection between the Bjorken and Drell-Hearn-Gerasimov sum rules

The spin dependent cross sections, σT_1/2 and σT_3/2 , and asymmetries, A_∥ and A_⊥ for ³He have been measured at the Jefferson Lab's Hall A facility. The inclusive scattering process ³He(e,e)X was performed for initial beam energies ranging from 0.86 to 5.1 GeV, at a scattering angle of 15.5°. Data includes measurements from the quasielastic peak, resonance region, and the deep inelastic regime. An approximation for the extended Gerasimov-Drell-Hearn integral is presented at a 4-momentum transfer Q² of 0.2-1.0 GeV².

Also presented are results on the performance of the polarized ³He target. Polarization of ³He was achieved by the process of spin-exchange collisions with optically pumped rubidium vapor. The ³He polarization was monitored using the NMR technique of adiabatic fast passage (AFP). The average target polarization was approximately 35% and was determined to have a systematic uncertainty of roughly ±4% relative.

New optoelectronic devices using GaAs-GaAlAs epitaxy

Three subjects related to epitaxial GaAs-GaAlAs optoelectronic devices are discussed in this thesis. They are:

1. Embedded Epitaxy

This is a technique of selective multilayer growth of GaAs- Ga_1-xAl_xAs single crystal structures through stripe openings in masking layers on GaAs substrates. This technique results in prismatic layers of GaAs and Ga_1-xAl_xAs "embedded" in each other and leads to controllable uniform structures terminated by crystal faces. The dependence of the growth habit on the orientation of the stripe openings has been studied. Room temperature embedded double heterostructure lasers have been fabricated using this technique. Threshold current densities as low as 1.5 KA/cm² have been achieved.

2. Barrier Controlled PNPN Laser Diode

It is found that the I-V characteristics of a PNPN device can be controlled by using potential barriers in the base regions. Based on this principle, GaAs-GaAlAs heterostructure PNPN laser diodes have been fabricated. GaAlAs potential barriers in the bases control not only the electrical but also the optical properties of the device. PNPN lasers with low threshold currents and high breakover voltage have been achieved. Numerical calculations of this barrier controlled structure are presented in the ranges where the total current is below the holding point and near the lasing threshold.

3. Injection Lasers on Semi-Insulating Substrates

GaAs-GaAlAs heterostructure lasers fabricated on semi-insulating substrates have been studied. Two different laser structures achieved are: (1) Crowding effect lasers, (2) Lateral injection lasers. Experimental results and the working principles underlying the operation of these lasers are presented. The gain induced guiding mechanism is used to explain the lasers' far field radiation patterns. It is found that Zn diffusion in Ga_1-xAl_xAs depends on the Al content x, and that GaAs can be used as the diffusion mask for Zn diffusion in Ga_1-xAl_xAs. Lasers having very low threshold currents and operating in a stable single mode have been achieved. Because these lasers are fabricated on semi-insulating substrates, it is possible to integrate them with other electronic devices on the same substrate. An integrated device, which consists of a crowding effect laser and a Gunn oscillator on a common semi-insulating GaAs substrate, has been achieved.

Aperiodic structures in optics and integrated optics and the transverse Bragg reflector laser

The first part of this work describes the uses of aperiodic structures in optics and integrated optics. In particular, devices are designed, fabricated, tested and analyzed which make use of a chirped grating corrugation on the surface of a dielectric waveguide. These structures can be used as input-output couplers, multiplexers and demultiplexers, and broad band filters.

Next, a theoretical analysis is made of the effects of a random statistical variation in the thicknesses of layers in a dielectric mirror on its reflectivity properties. Unlike the intentional aperiodicity introduced in the chirped gratings, the aperiodicity in the Bragg reflector mirrors is unintentional and is present to some extent in all devices made. The analysis involved in studying these problems relies heavily on the coupled mode formalism. The results are compared with computer experiments, as well as tests of actual mirrors.

The second part of this work describes a novel method for confining light in the transverse direction in an injection laser. These so-called transverse Bragg reflector lasers confine light normal to the junction plane in the active region, through reflection from an adjacent layered medium. Thus, in principle, it is possible to guide light in a dielectric layer whose index is lower than that of the surrounding material. The design, theory and testing of these diode lasers are discussed.

Study of the effect of alcohol on single human red blood cells using near-infrared laser tweezers Raman spectroscopy

The effect of alcohol solution on single human red blood Cells (RBCs) was investigated using near-infrared laser tweezers Raman spectroscopy (LTRS). In our system, a low-power diode laser at 785 nm was applied for the trapping of a living cell and the excitation of its Raman spectrum. Such a design could simultaneously reduce the photo-damage to the cell and suppress the interference from the fluorescence on the Raman signal. The denaturation process of single RBCs in 20% alcohol solution was investigated by detecting the time evolution of the Raman spectra at the single-cell level. The vitality of RBCs was characterized by the Raman band at 752 cm(-1), which corresponds to the porphyrin breathing mode. We found that the intensity of this band decreased by 34.1% over a period of 25 min after the administration of alcohol. In a further study of the dependence of denaturation on alcohol concentration, we discovered that the decrease in the intensity of the 752 cm(-1) band became more rapid and more prominent as the alcohol concentration increased. The present LTRS technique may have several potential applications in cell biology and medicine, including probing dynamic cellular processes at the single cell level and diagnosing cell disorders in real time. Copyright (c) 2005 John Wiley T Sons, Ltd.

Double injection: high frequency noise and temperature dependence

Noise measurements from 140°K to 350°K ambient temperature and between 10kHz and 22MHz performed on a double injection silicon diode as a function of operating point indicate that the high frequency noise depends linearly on the ambient temperature T and on the differential conductance g measured at the same frequency. The noise is represented quantitatively by〈i^2〉 = α•4kTgΔf. A new interpretation demands Nyquist noise with α ≡ 1 in these devices at high frequencies. This is in accord with an equivalent circuit derived for the double injection process. The effects of diode geometry on the static I-V characteristic as well as on the ac properties are illustrated. Investigation of the temperature dependence of double injection yields measurements of the temperature variation of the common high-level lifetime τ(τ ∝ T^2), the hole conductivity mobility µ_p (µ_p ∝ T^(-2.18)) and the electron conductivity mobility µ_n(µ_n ∝ T^(-1.75)).

Application of sample-and-hold circuits in the laser frequency-shifting

A new method of frequency-shifting for a diode laser is realized. Using a sample-and-hold circuit, the error signal can be held by the circuit during frequency shifting. It can avoid the restraint of locking or even lock-losing caused by the servo circuit when we input a step-up voltage into piezoelectric transition (PZT) to achieve laser frequency-shifting.

Avaliação da eficácia da terapia fotodinâmica como adjunto ao tratamento periodontal de pacientes com periodontite agressiva

A periodontite agressiva é um processo inflamatório de origem bacteriana mediado pelo sistema imunológico do hospedeiro e é provavelmente a forma mais grave de doença periodontal, apresentando destruição das estruturas tanto de proteção quanto de suporte dentário, num período relativamente rápido, normalmente levando a perda prematura dos elementos dentários e, em alguns casos, terapia antimicrobiana adjunta é necessária em adição à terapia mecânica. O objetivo do presente ensaio clínico randomizado controlado foi avaliar o efeito clínico da terapia fotodinâmica como adjunto ao tratamento periodontal não cirúrgico no tratamento da periodontite agressiva Os seguintes parâmetros clínicos foram avaliados: índice de placa visível, sangramento à sondagem, profundidade de bolsa à sondagem, nível de inserção clínica relativo, envolvimento de furca e mobilidade. Foram selecionados dez pacientes com periodontite agressiva, os quais foram examinados no dia zero e após três meses.O desenho do estudo consistiu em um modelo de boca dividida, onde um hemiarco foi tratado com raspagem e alisamento radicular e terapia fotodinâmica (laser diodo) e o outro apenas com raspagem e alisamento radicular. Três meses após o término do tratamento, os grupos terapêuticos apresentaram resultados semelhantes para todos os parâmetros clínicos avaliados: ambas as terapias tiveram sucesso, como redução de profundidade de bolsa, ganho de nível de inserção clínica relativo, redução de índice de placa visível, redução de sangramento à sondagem, diminuição de envolvimento de furca e diminuição de mobilidade, porém sem diferenças estatisticamente significantes entre elas. Dentro das limitações do presente estudo, os resultados sugerem que a terapia fotodinâmica adjuntamente ao tratamento periodontal não cirúrgico mecânico foi tão eficaz quanto o tratamento periodontal não cirúrgico mecânico sozinho.