In this paper, we present results from our theoretical and experimental exploration of tailoring the absorption spectrum of a type of metamaterial absorber through manipulating the symmetricity and uniformity of the metallic submicron particle array on the top layer. The absorber under study is a metal-insulator-metal (MIM) trilayer structure made up of a top layer of engineered metallic submicron particles, a middle insulator spacer layer and an opaque ground metal reflector layer. We first studied the structure with a top layer consisting of a uniform array of raindrop-shaped gold (Au) submicron disks. We designed the raindrop shape with a reflectional symmetry on the 45° line. We compared the spectrum generated with that of a similar structure but the top layer which is filled with uniformly arranged circular submicron discs. It has been well reported that an array of circular particles each with both reflectional and rotational symmetries usually generates a spectrum with one absorption spike. By changing the circular shape to raindrop shape, the MIM absorber has been predicted to generate two absorption peaks with significantly broadened absorption bandwidth. Subsequently, we found that even wider spectra could be achieved if the top layer is built with a periodic arrangement of the unit cells containing differently sized raindrop-shaped disks. This leads to a wider bandwidth of higher than 50% absorbance ranging from 2.80 μm to 3.90 μm.
We focused on the effects of group-velocity dispersion (GVD) on the coherent pulse progression in mid-infrared (MIR) quantum-cascade lasers (QCLs). Comparison of GVD effects on the two kinds of typical QCL cavities, i.e., FP and ring cavities, brings insight into the interaction between the GVD and the spatial hole burning (SHB) effect which is only supported by FP cavities but not ring cavities. The theoretical model is built based on the Maxwell-Bloch formulism accounting for two-way propagations of electric field and polarization as well as the couplings among the electric field, the polarization, and the population inversion. The pulse evolution in time-spatial domains is simulated by the finite difference method with prior nondimensionalization, which is necessary for a convergent solution. Results predict that the SHB could broaden the QCL gain bandwidth and induce additional side modes closely around the central lasing mode with an intensity more pronounced than that of GVD associated side modes. Moreover, owing to the SHB, the lasing instability caused by GVD is weaker in a FP cavity than a ring cavity.
We focused on the effects of group-velocity dispersion (GVD) on the coherent pulse progression in midinfrared quantum-cascade lasers (QCLs). We carried out the study on Fabry–Perot (FP) cavities. Comparison of GVD effects on the two kinds of typical QCL cavities, i.e., FP and ring cavities, brings insight into the interaction between the GVD and spatial hole burning (SHB) effect, which is only supported by FP cavities but not ring cavities. The theoretical model is built based on the Maxwell–Bloch formulism accounting for two-way propagations of electric field and polarization as well as the couplings among the electric field, polarization, and population inversion. The pulse evolution in time–spatial domains is simulated by the finite-difference method with prior nondimensionalization, which is necessary for a convergent solution. Results predict that the SHB could broaden the QCL gain bandwidth and induce additional side modes closely around the central lasing mode with an intensity more pronounced than that of GVD associated side modes. Moreover, owing to the SHB, the lasing instability caused by GVD is weaker in an FP cavity than a ring cavity.
The effect of group-velocity dispersion (GVD) and self-phase modulation (SPM) on the coherent pulse progression in mid-infrared (MIR) quantum-cascade lasers (QCLs) is investigated. The background saturable absorber (SA) effect is included in the study. In this case, the lasing pulses can be built up from the instable continuous wave (CW) operation condition related to both SA effect and GVD under the influence of a small initial disturbance. The theoretical model is built based on the Maxwell-Bloch formulism accounting for the couplings among the electric field, the polarization, and the population inversion. The pulse evolution in time-spatial domains is simulated by the finite difference method with prior nondimensionalization, which is necessary for convergent solution. We first studied the QCLs with ring cavity to illustrate the interplay between GVD and SPM effects. It is found that the anomalous GVD, which receives less attention in the study of QCL dynamics, can significantly narrow the spectrum splitting between side modes. The SPM can broaden the linewidth of the spectral modes. Their combined effects can lead the possibility of forming solitons. We also extended our study to the same QCL medium with a Fabry-Perot (FP) cavity. The additional effect of spatial-hole burning (SHB) is identified in results obtained.
As the effect of group-velocity dispersion (GVD) on the stability of midinfrared quantum-cascade lasers (QCLs) has not been explored so far, both amplitude and phase instabilities of QCLs were analyzed in the presence of a background saturable absorber in the lasing medium. The investigation was carried out through the linearization of the Maxwell–Bloch formalism for the lasing medium by including second-order dispersion of group velocity and intensity-dependent loss accounting for the saturable absorber. The effect of the GVD on laser instability was found to be strongly related to the strength of the saturable absorber and pumping. The effect of the GVD is more significant at lower pumping levels and with weaker saturable absorbers.
Theoretical analysis of lasing instability in quantum-cascade lasers was carried out. A mid-infrared quantum-cascade laser structure with triple resonance levels was chosen as the prototype of the lasing medium with high Kerr nonlinearity from optical transitions. The self-phase modulation effect induced by Kerr nonlinearity was found to possibly change the unstable frequency domain significantly, i.e., not only broaden the unstable domain but also drive it from single-mode to multi-mode. The instability mechanisms of quantum-cascade lasers include one like the coherent multi-mode Rishen-Nummedal-Graham-Haken instability and the spatial-hole-burning-induced single-mode instability.
We study mid-infrared quantum-cascade lasers (QCL's) with a pair of triply harmonic resonant levels. Potential
applications of such designs are discussed based on the resonant third-order nonlinear susceptibility χ(3) at the third-harmonic
(TH) frequency, χ(3)(3ω; ω, ω, ω), and that at fundamental mode (FM) frequency χ(3)( ω; ω, ω,-ω). Mode
overlap and the phase mismatch effect are considered in the TH power evaluation. In addition to single-photon
processes, resonant two-photon processes are included in the evaluation of χ(3)( ω; ω, ω,-ω), which results in the
enhancement of the positive nonlinear (Kerr) refractive index, and thus induce stronger self-phase modulation (SPM). It
is concluded that QCL's with multiple-resonance design are not only favorable for tunable light emission in the near- to
mid-infrared region, but are also promising candidates for spectrum broadening by SPM.
In the visualization of three-dimensional (3D) images, specific isosurfaces are usually extracted from 3D images and used to represent (approximate) boundary surfaces of certain structures within 3D images. In order to well approximate the boundary surfaces of these structures, it is important to determine a good isosurface for each boundary surface. An isosurface is said to be a good isosurface of a boundary surface if it can approximate the boundary surface with the smallest error under certain error measuring criteria. The mathematical model describing the approximation problem of a boundary surface by isosurfaces is constructed and studied. The method used to deduce good isosurfaces for the boundary surfaces within 3D discrete images is presented. The proposed method is illustrated by examples with different real 3D biomedical images.
Due to intensity inhomogeneities, partial volume effects, as well as organ shape variations, automatic segmentation of abdominal organs has always been a high challenging task. To conquer these difficulties, we employ a pre-labeled atlas (VIP-Man) to supplement anatomical knowledge to the segmentation process. First, an atlas-subject registration is carried out to establish the proper correspondence between the atlas and the subject. The registration consists of two steps. In the global registration step, a similarity transformation is found to eliminate the stature differences. In the organ registration step, organs of interest are registered respectively to achieve better alignments. Second, we utilize the fuzzy connectedness framework to segment abdominal organs of interest from the subject image. Under the guidance of the registered atlas, the seeds and intensity parameters of organs are specified in an auto-adaptive way. Further more, the anatomical knowledge contained in the atlas is blended into the frame work, to make the segmentation result more reliable. To remove possible jags on boundary, a level set smooth method which implements fuzzy connectedness as external speed forces, is utilized on the segmentation result. Our purpose is to accomplish the segmentation task like how anatomy experts do. So far, this approach has been applied to segment organs, including liver, spleen and kidneys, in the female MRI T1 data set from the VHP. All organs of interest are identified correctly, and delineated with considerable precision.
FDG ([18F] 2-fluoro-2-deoxy-D-glucose) is the typical tracer used in clinical PET (positron emission tomography) studies. The FDG-PET is an important imaging tool for early diagnosis and treatment of malignant tumor and functional disease. The main purpose of this work is to propose a method that represents FDG metabolism in human body through the simulation and visualization of 18F distribution process dynamically based on the segmented VHP (Visible Human Project) image dataset. First, the plasma time-activity curve (PTAC) and the tissues time-activity curves (TTAC) are obtained from the previous studies and the literatures. According to the obtained PTAC and TTACs, a set of corresponding values are assigned to the segmented VHP image, Thus a set of dynamic images are derived to show the 18F distribution in the concerned tissues for the predetermined sampling schedule. Finally, the simulated FDG distribution images are visualized in 3D and 2D formats, respectively, incorporated with principal interaction functions. As compared with original PET image, our visualization result presents higher resolution because of the high resolution of VHP image data, and show the distribution process of 18F dynamically. The results of our work can be used in education and related research as well as a tool for the PET operator to design their PET experiment program.
In this paper, we presents a newly developed near-infrared optical tissue imaging system with single channel detection based on the principles of frequency-domain spectroscopy, which uses diffusive photons to detect the breast cancer. The patient’s breast is slightly compressed between two parallel glass plates, which are located between the source fiber and the detector fiber. The laser beam travels in the source fiber to the breast, and the transmitted light is detected by a photomultiplier tube and then demodulated. The ac amplitude of the signal is sampled to the computer by an A/D board. The source fiber and the detector fiber are driven by stepper motors and move synchronously in two dimensions, which enable the fibers to scan the entire breast. The scanning process is automatically controlled by computer. And the optical mammograms are displayed on the computer screen after the scanning process. In comparison with our former instrument that uses multichannel and scans only in one dimension to shorten the time of scanning, the new prototype has only one transmitter and one detector. This structure not only reduces the costs of the apparatus but also leads to a
much more simplified system. Unfortunately, it makes the scanning time much longer. However, a new sampling mode is developed for the system to sample the data continuously, which compensates the disadvantage of the single-channel structure and reduces the scanning time. The results of intralipid experiments and pre-clinical experiments prove the potential of this approach to distinguish between tumors and healthy tissues.
Characterization of human tissue using near-IR (NIR) light is becoming increasingly popular. The light signal transmitted from the tissue contains information concerning inhomogeneities in tissue, such as size, position, and pathological states (benign or malignant). We discuss the most probable diffuse path (MPDP) related to frequency-domain diffuse photon density waves (DPDWs) propagating inside turbid media. We find that for a medium of finite size, the existence of boundaries between tissue and nonscattering media would have considerable impact on the path shape. It is also demonstrated that such paths can be used to obtain higher accuracy in localizing absorbers embedded in a homogeneous background. Based on the proposed MPDP, a new method for 3-D localization of heterogeneities in turbid media is proposed, which is validated by experiments using Intralipid and pork fat. The experiments are performed with an NIR breast cancer detection system designed and assembled in our lab, using 780-nm NIR light. In Intralipid, when the size of a single absorber is less than 1 cm, the localization error is about 2 mm. The results from pork fat are also acceptable.
Early detection of the breast cancer is one of the keys to decrease the mortality of this disease. Now more and more attention is paid on development of non-invasive mammography. To study the feasibility of breast imaging with near infrared light, a frequency-domain optical scanner of breast cancer was designed and developed in our lab, based on the theory of interaction of Near Infrared (NIR) light and soft tissues. In this paper, the imaging prototype as well as the imaging processing system based on Matlab is described. The results of phantom experiments using the prototype of this scanner show a resolution in millimeter scale. The initial clinical trials show the feasibility of our prototype in clinical study. The prototype can scan a breast in 4 minutes. Images of the same breast obtained both with the prototype and with x-ray mammography are presented.
KEYWORDS: Diffusion, Scattering, Finite element methods, Monte Carlo methods, Photon transport, Absorption, Sensors, Light scattering, Chemical elements, Tissues
In medical visualization, multiple isosurfaces are usually extracted from medical volume image and used to represent (approximate) the boundary surfaces of different structures in the image. In this paper, we will discuss the approximating problem of the boundary surface (contained within volume image) by isosurface. It is quite common that a medical volume image can contain multiple interesting structures; we present a novel approach for the selection of multiple isosurfaces to approximate the boundary surfaces of these multiple structures. With this approach, the discrete sampling points of the gray values of the boundary surfaces within volume image are computed first. Then by identifying appropriate clusters from the discrete sampling points and computing the mean of each cluster, we can determine the corresponding isosurfaces for approximating these multiple boundary surfaces.
Detection and extraction of the boundary surfaces of interested regions within the medical volume image are important research topics. In this paper, we will introduce several methods to detect, extract and approximate the boundary surfaces within volume image.
Optical spectroscopy for tumor demarcation was investigated in this study. Laser-induced fluorescence and Raman spectra were measured from normal and tumorous human blood serum in an attempt to discover some values useful in discrimination between normal and tumorous cases. Three Raman peaks were consistently observed from normal blood serum emission using 488.0nm and 514.5nm excitation of an Ar-ion laser, whereas no peak or only slight Raman peaks were detected from tumorous cases. In addition, red shift of fluorescence peak and decrease of fluorescence intensity were observed after samples radiated by laser. According to one thousand twenty-two samplesí» spectra, three parameters (alpha) , (beta) and (delta) (lambda) are introduced to distinguish normal, benign and malignant from one another. The application of such parameters in clinical diagnosis was researched. Four hundred and fifty-nine samples were investigated before knowing the result of clinical diagnosis in order to estimate the availability of our criteria.
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