Resonant cavity structures integrated with phase change material germanium antimony telluride (GST) function as angular insensitive mid-infrared optical filters. The one-dimensional (1D) sub-wavelength grating structures consisting of GST resonators embedded in metallic (Ag) film act as Fabry-Perot resonators with observable transmission resonances, that can be tuned by thermal excitation when GST switches from amorphous to crystalline state, while maintaining angular insensitivity up to 60 degrees. Modeling and experimental results of an interdigitated design of the 1D resonant structure for reconfigurable applications by electrical switching of the transmission spectra of the optical filter will be presented.
We present the design of a transmission filter in mid-infrared and its experimental verification using phase change material GST (Germanium Antimony Telluride). Progressively increasing the annealing temperature of GST controls the crystalline structure, allowing the refractive index to increase significantly in a steady rate. One-dimensional metal dielectric sub-wavelength grating device, such that the dielectric segments are proportioned to decouple any angular-dependent resonance. Incorporating GST between metal gratings allows transmission wavelengths to be actively tuned as annealing increases refractive index. Amorphous and crystalline GST devices show transmission resonances up to 60 degrees indicate that angular-independence is preserved during material excitation.
Given that breast cancer is the second leading cause of cancer-related deaths among women in the United States, it is necessary to continue improving the sensitivity and specificity of breast imaging systems that diagnose breast lesions. Photoacoustic (PA) imaging can provide functional information during in vivo studies and can augment the structural information provided by ultrasound (US) imaging. A full-ring, all-reflective, illumination system for photoacoustic tomography (PAT) coupled to a full-ring US receiver is developed and tested. The US/PA tomography system utilizes a cone mirror and conical reflectors to optimize light delivery for PAT imaging and has the potential to image objects that are placed within the ring US transducer. The conical reflector used in this system distributes the laser energy over a circular cross-sectional area, thereby reducing the overall fluence. This, in turn, allows the operator to increase the laser energy achieving better cross-sectional penetration depth. A proof-of-concept design utilizing a single cone mirror and a parabolic reflector is used for imaging cylindrical phantoms with light-absorbing objects. For the given phantoms, it has been shown that there was no restriction in imaging a given targeted cross-sectional area irrespective of vertical depth, demonstrating the potential of mirror-based, ring-illuminated PAT system. In addition, the all-reflective ring illumination method shows a uniform PA signal across the scanned cross-sectional area.
Among various types of cancer, breast cancer is considered to be the most common that affects thousands of women all over the world. Several imaging tools are being used for breast cancer detection and diagnosis. Mammography and B-mode ultrasound (US) are the primary screening tools for breast lesions. However, mammography is limited with low sensitivity especially in women with dense breasts, who appear to be at higher risk of breast cancer. Additionally, the B-mode US suffers from low specificity in the differential diagnosis of breast lesions. Therefore, it is clinically significant to develop screening techniques that could eliminate previous limitations. Photoacoustic (PA) has been showing potential for early stage detection and staging breast cancer due to its unique abilities to acquire functional and molecular information of the breast lesions. We have developed an optimized US and PA tomography system, which uses custom designed all reflective based optics to create an omnidirectional ring-shaped beam to illuminate a cross-section of the breast tissue and acquire thegenerated acoustic waves by using a full-ring US transducer. The developed PA tomography (PAT) system can potentially make a more uniform illumination of the breast tissue and more importantly enhance the imaging depth. In this study, development of the full-ring illumination and the results of our initial feasibility US/PA tests are presented.
Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 μm. The bulk plasma frequency ωp and high-frequency limit to the permittivity ε∞ were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time τ and effective mass m . This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity.
Conventional optical spectrometers that are based on bulk optical components tend to be relatively large and expensive
compared to the other components used in systems designed for detecting chemical/biological agents. Microspectrometers
based on focusing waveguide gratings incorporate both spectral dispersion and focusing functions into a
single component that can be fabricated hundreds at a time at the wafer level using nano-imprint lithography techniques.
These types of spectrometers are ideal for integration into micro-fluidic systems because the signals can be directly
coupled into the planar waveguide. We present preliminary data from a prototype system and explore potential
applications for these devices.
Waveguide grating couplers (WGC) are used for input and output coupling in many planar waveguide
based sensors. By using a chirped grating, guided light can be decoupled and focused to a desired location. The
location and spot size of focused beam depends upon the size and chirp of the grating as well as the wavelength of
the light. The locus of the focused beams for different wavelengths lies along a curve. In many applications a planar
detector array is used to capture spectral data. Wherever the planar detector array does not intersect this focal curve,
images of a point source will be defocused. We develop a theoretical model to calculate the image location and
geometric spot size for a given set of grating parameters.
In order to experimentally verify the model, chirped waveguide grating couplers were fabricated on
HfO2/Quartz planar waveguides using e-beam lithography. The spot size, intensity, and location of diffracted beam
was measured at several wavelengths and compared with the theoretical results.
Optical multiplexers/demultiplexers developed for the telecommunication industry, at the level of the basic principles,
perform essentially the same functions as general-use optical spectrometers. The spectrometer design inspired by the
telecom devices would represent an extremely compact device compatible with the manufacturing procedures in
integrated optoelectronics and micro-optics. In this paper we summarize our recent results on development of a
miniature optical spectrometer. The spectrometer uses a diffractive optical element integrated with a planar optical
waveguide. It is designed to provide spectral resolution of at least 2nm in the entire visible spectral range from 400nm-
700nm, and simultaneously resolve spectra from up to 35 independent optical inputs. The optical part of the
spectrometer fits volume below 10mm3. The spectrometer is designed for on-chip diagnostic systems, in particular for
fluorescence detection of hazardous materials. A device prototype with diffractive optical element fabricated using
electron beam lithography is manufactured and tested.
Two-dimensional periodic waveguides have been made by depositing colloidal crystals on top of a planar waveguide. We present a fabrication technique and discuss applications of 2D periodic waveguides in optical coupling, filtering, and wavelength demultiplexing.
A few major schemes for all-optical switching/modulation is considered and the comparison of the values of nonlinear index change required for the device operation is presented. Each design is assumed to be optimized to meet the requirement for insertion losses of 1 dB (about 80% transmission or reflectance depending on the geometry of the device) and the switching contrast of 10 dB. A number of numerical results of transmission and reflectance in those devices are proposed in this paper. The advantage of the design of optimized integrated optical switchers/modulators with losses include extremely low switching refractive index change (delta) n. In addition, we suggest one new optimized design based on Bragg grating.
Tunable distributed Bragg reflectors with comb-like reflection spectrum have been shown to allow semiconductor lasers tuning over about 100 nm wavelength range. Recently we proposed a concept of BSG as a multiwavelength reflector and looked into a number of key design issues for widely tunable lasers. In this paper we analyze the technical aspects of designing and implementing the BSG. We show that the calculated performance of the binary grating is at least as good as that of the existing approaches. However, BSG offers additional design freedoms in positions and amplitudes of reflectance peaks, which in turn allows extension of the tuning range of the laser or relaxing the tuning current requirements. The principles of widely tunable lasers have been adapted to a practical design which should significantly simplify the control requirements for the tuning. We show that a BSG reflector for 1.55 micrometers tunable laser can be successfully implemented with the smallest feature size as large as 0.12 micrometers , which is well within the capacity of electron beam lithography. A simulated tuning curve for a BSG laser predicts a high side- mode suppression ratio (> 35 dB).
We present an algorithm and results of calculation of the binding energy and the oscillator strength of an exciton in a GaAs/InGaAs/GaAs single strained quantum well taking into account self consistency effects and phase space filling. We assume that the well is placed in a depletion region and the well filling by free carriers is governed by a surface voltage. The wave functions of the charge carriers motion in the direction transverse to the well plane were calculated via self-consistent solution of Schreodinger and Poisson equations. The exciton binding energy, wave functions of in-plane motion and oscillator strength were found by a variational approach. The variational function of the exciton was chosen in the form that explicitly includes the phase space filling. It is found that the dependence of the binding energy and oscillator strength of the exciton versus the surface voltage is not monotonic. This is explained by a competition of the self consistency effects and the phase space filling.
Photoreflectance spectra of InGaAs/GaAs single quantum well structures are studied at pump power densities from 0.5 to 300 mW/cm2. The spectra measured at different pump powers were found to differ not only in amplitude but also in spectral position. Moreover, a linear correlation between the amplitude and the spectral position of the modulation spectra takes place. We explain this dependence by means of the conventional scheme of modulation spectra modeling, taking into account the second order expansion of the quantum well refraction and absorption indices with respect to the excitonic resonance frequency. Plotting spectral positions of modulation spectra features versus modulation spectra amplitudes we can find the spectral position limit when the amplitude tends to zero. This limit is an excitonic resonance frequency in the absence of perturbation induced by pumping light. We have found that the observed spectral displacement of the modulation spectrum with respect to the nonperturbated excitonic frequency is equal to a half of the excitonic frequency shift induced by external modulation. Using the pumping-induced excitonic frequency shift, derivatives (delta) R/(delta) n and (delta) R/(delta) k, and modulation spectrum (Delta) R/R itself, it is easy to find the absolute value of the excitonic absorption (the excitonic oscillator strength) in a quantum well.
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