Cell samples flowing along a microfluidic tube are scanned with an optical coherence tomography (OCT) system and their correlation times in M-mode scans are calibrated. In particular, the variations of correlation time with waiting time after 5 and 10 % ethanol are applied to the cell samples are compared for understanding the evolution of cell morphology in the cell death pathways of apoptosis and necrosis, respectively. Also, Au nanorings (NRIs) are taken up by cells for increasing the scattering strength in OCT scanning and hence increasing the signal-to-noise ratio. It is found that when cells are incubated with 5 % ethanol, the correlation time keeps decreasing with waiting time and then increases at 7 hours. On the other hand, when cells are incubated with 10 % ethanol, the correlation time keeps decreasing with waiting time all the way up to 7 hours. This difference may imply that the correlation time from OCT scanning may be determined not only by the size of cell fragment, but also by the smoothness of the cell fragment in a scale of several hundred nm during the apoptosis and necrosis processes. In particular, the results imply that the surface smoothness of the apoptotic bodies formed at the final stage of an apoptosis process is higher than that of the cell fragments formed at the final stage of a necrosis process. This OCT scanning technique has the potential application to the determination of cell death condition with the function similar to cell flow cytometry.
A cell spheroid can be as large as 1 mm in diameter, containing more than 300,000 cells. It can be used for simulating a cancer tumor in cell treatment study. In this study, the cell uptake behaviors in cell spheroids are investigated. In particular, the penetration depths of Au nanoring, Au nanorod, and photosensitizer (AlPcS) in cell spheroids are evaluated based on a newly proposed technique of using confocal fluorescent microscopy. By using a geometric computation process, we can estimate the width of a fluorescent belt region, which corresponds to the uptake depth. Based on such evaluations, we can understand whether the quiescent viable cells in the middle layer of a cell spheroid. Also, by using a cell culture insert, we can estimate the uptake Au nanoparticle number per cell through mass spectrometry measurement. It is found that after 48-hour incubation time, the Au nanoparticles and photosensitizer all have the penetration depths in the range of 110-140 m. Such uptake penetration depths control the cell damage depths under laser illuminations. For observing the uptake depths, we use different fluorescent dyes to incubate the cell spheroids. It is found that the penetration depth of a dye also relies on its molecular weight. The smaller uptake penetration depths of Au nanoparticles into a cell spheroid can be attributed to the settlement of Au nanoparticles in the incubation solutions such that the upper portion of a cell spheroid has no access to the Au nanoparticles.
Photosensitizer can be taken up directly by cells or through the linkage with gold nanoparticles (NPs). When gold NPs linked with a photosensitizer are taken up by cells, both photodynamic (PDT) and photothermal effects for damaging cells can be generated under the illumination of a light source of an appropriate wavelength. The gold NPs and photosensitizer can escape from cells through an exocytosis process such that the efficiency of cell damage may decay with time. In this study, we investigate the dependencies of cell damage efficiency, gold nanoring (NRI) and photosensitizer amounts stayed in cells on the delay time. In particular, we differentiate the two possible escape pathways of the photosensitizer, AlPcS, i.e., AlPcS escape alone after its linkage with Au NRI is broken and AlPcS escape together with the linked Au NRI. Also, we intend to understand the dependence of the aforementioned AlPcS escape pathway on the location of Au NRIs inside a cell after they are taken up. It is found that generally, the PDT-induced cell damage efficiency decays more slowly when AlPcS is taken up together with linked Au NRI, compared with the case that AlPcS is taken up alone by cells. This trend indicates that the photosensitizer is more stable when it is linked with an Au NP. This result is confirmed by the observations of Au NRI and photosensitizer escapes through mass spectrometry, flow cytometry, and confocal fluorescent microscopy.
In AlGaN, the dominating emission polarization depends on the Al content. Generally speaking, a higher Al content leads to a stronger TM-polarized emission. Normally, the dominating emission polarization of an AlGaN layer changes from the TE polarization into the TM polarization when the emission wavelength is shorter than 300 nm. Because a TM-polarized photon propagate along the lateral dimension of a c-axis grown LED sample, its light extraction efficiency is lower, when compared with a TE-polarized photon. In this study, the material characterization techniques of transmission electron microscopy observation, reciprocal space mapping and omega-2theta scan in X-ray diffraction measurement, and geometric phase analysis are used for first identifying the existence of the high-Al layers (HALs) on both sides of a quantum well (QW) in three 3-period AlGaN QW structures of different deep-UV emission wavelengths. Then, optical analyses, including transmission and photoluminescence (PL) measurements, particularly the PL measurements under an applied stress along the sample c-axis, are undertaken for understanding the effects of such HALs on the band structures and hence the polarized emission behaviors of the samples. Simulation studies are also performed for providing the favorable comparisons with the experimental data. Basically, the HALs produce an extra compressive strain in the c-plane for lowering the heavy-hole (HH) band edge (lower than the edge of the split-off band) such that the TE-polarized emission through the electron transition between the conduction and HH band becomes dominating. In this situation, the light extraction efficiency of such a deep-UV light-emitting diode can be enhanced.
p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.04 Ohm-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. A model for estimating the current penetration behavior in a p-GaN/u-GaN alternating-layer structure when its I-V characteristics is to be measured is proposed. In this model, an exponential decay with a characteristic penetration depth is assumed for a layered structure of low effective conductivity. In a high-conductivity structure, this penetration depth is regarded as infinity such that the depth-dependent current distribution is uniform. By growing p-type structures with an upper portion of a layered structure of unknown effective conductivity and a lower portion of high conductivity, which can be a layered or a uniform structure, of different individual thicknesses, we can have a sequence of sheet conductance data for best-fitting with the proposed model to simultaneously obtain the characteristic penetration depth and effective conductivity of the upper layered structure. Simulation studies are performed to provide the results supporting the proposed model. From the simulation results, it is found that the key factor hindering the current penetration is the low conductivity and finite thickness of a sub-layer around the middle of a u-GaN layer, which is not covered by the hole diffusion range from the neighboring p-GaN layers.
By increasing the Mg-doping level and hence the hole concentration in the p-AlGaN electron-blocking layer, the polarization field in this layer can be screened for reducing the potential barrier of hole and hence enhancing the hole tunneling efficiency such that the overall LED emission efficiency is increased. The increase of Mg-doping level is implemented based on an Mg pre-flow growth technique, in which Mg source is supplied into the metalorganic chemical vapor deposition chamber for several minutes before the growth of p-AlGaN or p-GaN. Based on a simulation study, we observe that the energy difference between the valence band-edge and the quasi-Fermi level of hole in the EBL is reduced by increasing the Mg-doping level in this layer such that the total hole density in the quantum wells is increased for enhancing the LED emission efficiency. Based on this technique, the high performance of an LED with the total p-type thickness as small as 38 nm is demonstrated. The surface plasmon coupling effects, including the enhancement of internal quantum efficiency, increase of output intensity, reduction of efficiency droop, and increase of modulation bandwidth, among the thin p-type LED samples of different p-type thicknesses are compared. These advantageous effects are stronger as the p-type layer becomes thinner. With a circular mesa size of 10 micron in radius, we achieve the record-high modulation bandwidth of 625.6 MHz among c-plane GaN-based LEDs.
The use of a high-resolution optical coherence tomography (OCT) system with the operation wavelength around 800 nm to scan SCC4 cancer cells under different laser illumination conditions is demonstrated. The cancer cells are incubated with Au nanorings (NRIs), which are linked with photosensitizer, AlPcS, for them to be up-taken by the cells. Two Au NRI samples of different geometries for inducing localized surface plasmon (LSP) resonance around 1310 and 1064 nm are used. Four different lasers are utilized for illuminating the cells under OCT scanning, including 1310-nm continuous (cw) laser, 1064-nm cw laser, 1064-nm femtosecond (fs) laser, and 660-nm cw laser. The 1310- and 1064-nm cw lasers mainly produce the photothermal effect through the LSP resonance of Au NRIs for damaging the observed cells. Besides the photothermal effect, the 1064-nm fs laser can produce strong two-photon absorption through the assistance of the LSP resonance of Au NRI for exciting AlPcS to effectively generate singlet oxygen and damage the observed cells. The 660-nm laser can excite AlPcS through single-photon absorption for generating singlet oxygen and damaging the observed cells. With the photothermal effect, the observed cells can be killed through the process of necrosis. Through the generation of singlet oxygen, the cell membrane can be preserved and the interior substances are solidified to become a hard body of strong scattering. In this situation, the cells are killed through the apoptosis process. Illuminated by the 660-nm cw laser, a process of interior substance escape is observed through high-speed OCT scanning.
In combining the photothermal and photodynamic effects for killing cancer cells through the localized surface plasmon resonance (LSP) of photosensitizer-linked Au nanorings (NRIs), which are up-taken by the cells, the cells can be killed via different processes, including necrosis and apoptosis. In particular, the dominating effect, either photothermal or photodynamic effect, for cancer cell killing leading to either necrosis or apoptosis process is an important issue to be understood for improving the therapy efficiency. In this paper, we demonstrate the study results in differentiating the necrosis and apoptosis processes of cell death under different laser illumination conditions. With the LSP resonance wavelength of the Au NRIs around 1064 nm, the illumination of a 1064-nm cw laser can mainly produce the photothermal effect. The illumination of a 1064-nm fs laser can lead to LSP resonance-assisted two-photon absorption of the photosensitizer (AlPcS) for generating singlet oxygen and hence the photodynamic effect, besides the photothermal effect. Also, the illumination of a 660-nm cw laser can result in single-photon absorption of the photosensitizer for generating singlet oxygen and the photodynamic effect. By comparing the necrosis and apoptosis distributions in dead cells between the cases of different laser illumination conditions, we can differentiate the cancer cell killing processes between the photothermal effect, photodynamic effect, and the mixed effect.
Cancer cell killing efficiencies based on the photothermal effect caused by the surface plasmon resonance of metal nanoparticles (NPs) and the photodynamic effect caused by the singlet oxygen generation of a photosensitizer rely on the cell uptake efficiency of metal NP and photosensitizer. Perforation and heating can increase cell membrane permeability and hence can increase the cell uptake efficiency of NPs and drugs. In this paper, we demonstrate the variations of the cell damage efficiency under the illuminations of different lasers, which can produce mainly photothermal effect, mainly photodynamic effect, and mixed effect, when a pre-perforation and a pre-heating processes are applied. Au nanorings (NRIs) with their localized surface plasmon resonance wavelength around 1064 nm are used. The perforation process is undertaken by illuminating the cell samples by a femtosecond laser at 1064 nm with the power density lower than the cell damage threshold intensity. The heating process is implemented by illuminating cells with a low power continuous laser at 1064 nm. It is found that with the pre-perforation and pre-heating processes, the photodynamic effect is enhanced because the internalized Au NRI number and hence the internalized photosensitizer (AlPcS) molecule number are increased. However, the photothermal effect can be reduced because the adsorbed Au NRIs on cell membrane are effectively internalized during the pre-perforation and pre-heating processes. The photothermal effect is more effective when Au NRIs are adsorbed on cell membrane.
In MOCVD growth, two key factors for growing a p-type structure, when the modulation growth or delta-doping technique is used, include Mg memory and diffusion. With high-temperature growth (>900 degree C), doped Mg can diffuse into the under-layer. Also, due to the high-pressure growth and growth chamber coating in MOCVD, plenty Mg atoms exist in the growth chamber for a duration after Mg supply is ended. In this situation, Mg doping continues in the following designated un-doped layers. In this paper, we demonstrate the study results of Mg preflow, memory, and diffusion. The results show that pre-flow of Mg into the growth chamber can lead to a significantly higher Mg doping concentration in growing a p-GaN layer. In other words, a duration for Mg buildup is required for high Mg incorporation. Based on SIMS study, we find that with the pre-flow growth, a high- and a low-doping p-GaN layer are formed. The doping concentration difference between the two layers is about 10 times. The thickness of the high- (low-) doping layer is about 40 (65) nm. The growth of the high-doping layer starts 10-15 min after Mg supply starts (Mg buildup time). The diffusion length of Mg into the AlGaN layer beneath (Mg content reduced to <5%) is about 10 nm. The memory time of Mg in the growth chamber is about 60 min, after which the Mg doping concentration is reduced to <1%.
KEYWORDS: Solid state lighting, Current controlled current source, Light emitting diodes, Modulation, Surface plasmons, Light sources and illumination, Data transmission, Quantum wells, Silver, Nanoparticles
Besides lighting, LEDs can be used for indoor data transmission. Therefore, a large modulation bandwidth becomes an important target in the development of visible LED. In this regard, enhancing the radiative recombination rate of carriers in the quantum wells of an LED is a useful method since the modulation bandwidth of an LED is related to the carrier decay rate besides the device RC time constant To increase the carrier decay rate in an LED without sacrificing its output power, the technique of surface plasmon (SP) coupling in an LED is useful. In this paper, the increases of modulation bandwidth by reducing mesa size, decreasing active layer thickness, and inducing SP coupling in blue- and green-emitting LEDs are illustrated. The results are demonstrated by comparing three different LED surface structures, including bare p-type surface, GaZnO current spreading layer, and Ag nanoparticles (NPs) for inducing SP coupling. In a single-quantum-well, blue-emitting LED with a circular mesa of 10 microns in radius, SP coupling results in a modulation bandwidth of 528.8 MHz, which is believed to be the record-high level. A smaller RC time constant can lead to a higher modulation bandwidth. However, when the RC time constant is smaller than ~0.2 ns, its effect on modulation bandwidth saturates. The dependencies of modulation bandwidth on injected current density and carrier decay time confirm that the modulation bandwidth is essentially inversely proportional to a time constant, which is inversely proportional to the square-root of carrier decay rate and injected current density.
The controlling mechanism for determining the growth direction of a Ga-doped ZnO (GaZnO) nanoneedle (NN) by using an Ag nanoparticle (NP) as vapor-liquid-solid (VLS) growth catalyst is disclosed. It is found that the local Ag (111) orientation of the catalytic Ag portion in an Ag NP determines the ZnO (002) orientation of the grown GaZnO and hence the NN growth direction. The ZnO (002) plane of the grown GaZnO is always parallel with the Ag (111) planes of the Ag portions involved in VLS growth in either the top or bottom Ag NP of an NN. When GaN is used as NN growth template, at a sufficiently high temperature (350-450 degrees C), a small Ag NP can become a quasi-single crystal with its Ag (111) plane consistent with the GaN (002) plane and hence results in the growth of a vertical GaZnO NN. However, tilted NNs can be grown from a larger Ag NP or a cluster of Ag NP on GaN due to its non-uniform Ag (111) orientation distribution. At the early stage of GaZnO growth, GaZnO precipitation can be observed between Ag layers within an Ag NP, indicating the growth of a semiconductor on Ag. On other templates, like Si, sapphire, or silicon diode, single-crystal Ag NP cannot be formed such that GaZnO NNs of random orientations are grown.
In a core-shell quantum-well (QW) nanorod (NR) structure, because of the non-uniform constituent atom supply for QW growth at different heights on a sidewall, the QW thickness and indium content vary with NR height. Multi-section NRs can be grown by controlling the supply duration of Ga source for decreasing the size of catalytic Ga droplet and hence tapering the NR cross-sectional size. The sidewall QWs of such a multi-section NR can emit light of a broad spectrum due to the larger variation ranges of QW thickness and indium content between sections of different cross-sectional sizes. In this paper, besides the growth processes of the aforementioned NR structures are reported, two models are built for simulating the sidewall QW growth and the tapering process of NR. Based on one of the models, the theories show the consistent results of increasing QW thickness and indium content in increasing NR height with the experimental observations. Based on another model, Ga adatoms diffuse on the slant facets from the Ga droplet on the NR top to deposit GaN on the slant facets for forming a gradient layer. In this situation, the angle of the slant facet increases from ~43 to ~62 degrees during the tapering process. The results are consistent with what observed in experiment. In this paper, besides NRs grown from patterned circular holes, the growth results of NRs from patterned elliptical holes are illustrated.
In this paper, we demonstrate simulation results of top-emission organic light-emitting devices (TOLEDs) with red, green and blue (RGB) colors. We take the RGB spectral peaks at 460, 520, and 600 nm with full width at half maximum of 100 nm. Device structures are thick silver (Ag) anode /hole-transport layer 60 nm/ emitting layer (EML) /semi-transparent Ag cathode 20 nm/ dielectric layer. The dielectric material capped upon the cathode is zinc selenium with a refractive index of 2.6 for providing high output intensity and narrow FWHM. When monitoring the peak wavelengths of RGB device and varying the EML and dielectric thicknesses, we found the optimized value of the EML are 71, 47 and 31 nm for the red, green and blue devices, respectively. The optimized dielectric thicknesses are 80, 70 and 50 nm with periods of 117, 98 and 90 nm, respectively, for the RGB devices. Due to the limitations of the experiments, the EML thicknesses can be different and the dielectric thickness must be the same of the RGB devices. For optimizing the BGB devices simultaneously, the thickness of dielectric layer of the OLED is 667 nm. The RGB peak intensities are 0.96, 0.99 and 0.83, normalized to their optimized value. Typically, in a TOLED, green device exhibits higher efficiency than red and blue ones. That means the intensity of the green TOLED can be lower. When the dielectric layer thickness is 314 nm, the normalized RGB peak intensities are 0.99, 0.26 and 0.97.
A polarization-sensitive optical coherence tomography (PSOCT) system using a femtosecond-laser as the broadband light source is implemented with the axial resolution of 5 µm in free space. Through the design of path-length difference between the two polarization inputs and the modulation of one of the polarization inputs, the PSOCT images of various input and output polarization combinations can be distinguished and simultaneously collected. The PSOCT system is then used for in vitro scanning of the myocardium tissues of normal and infarcted rat hearts. The destruction of the birefringence nature of the fiber muscle in the infarcted heart can be clearly observed.
In this paper, we demonstrate the numerical simulations of the organic light-emitting device (OLED) based on a rigorous and efficient numerical method. The input parameters of such a program include the layer thickness and complex refractive index of each layer, the locations and density of the oscillating dipoles, and the emission photoluminescence spectrum. In evaluating the device performances, the output spectrum, the intensity distribution, and the viewing-angle characteristics of an OLED are concerned. Since the numerical difficulty arising from the large thickness of the glass layer in the OLED is carefully overcome, the program can simulate the optical performances with different glass substrate thickness ranged from less than one to hundreds of μm. When the glass substrate becomes thinner, multi-peak spectrum of an OLED is observed due to the strong interference effect between the two sides of the glass substrate. When the thickness of the glass substrate is reduced to less than 1 μm, the device is identical to a top-emission OLED with a two-microcavity structure. The simulation results are consistent with the Fabry-Perot cavity equation, which can be used as a guideline for optimizing the optical characteristics of an OLED from the normal direction. We have also demonstrated the procedures to maximize the total flux from an OLED which is more important than the luminance from the normal direction for the lighting application. Since our development of numerical algorithms is based on the general electromagnetic theory, the proposed model is, in principle, applicable to an OLED consisting of any number of layers.
For a photonic crystal, the larger the photonic band gap (PBG), the greater the bandwidth for manipulating the optical wave propagation. Therefore, to enlarge the photonic band gaps would be an important research topic. In this paper, we adopt the simulated annealing (SA) algorithm to synthesize two-dimensional photonic crystals for large band gaps. The SA is an iterative procedure and its probabilistic nature provides an opportunity to escape from the local minima and to reach the global minimum in a nonlinear optimization problem. To speed up the numerical calculations, we also use the fast plane wave expansion method for calculating the band diagram of a photonic crystal at each iteration step of the SA procedure. Numerical simulations are carried out to demonstrate the feasibility and effectiveness of our synthesis algorithms.
In this paper, we demonstrate simulation results of a top-emission organic light-emitting device (TOLED) with a passivation layer and a dielectric layer. Passivation layer is usually composed of silicon dioxide (SiO2) and/or silicon nitride (Si3N4) to protect organic layers from oxygen and moisture. Dielectric layer is a high refractive index thin film for enhancing the external quantum efficiency. The TOLED device has a microcavity structure which comprised of an opaque and high reflective anode and a thin semitransparent cathode. When varying dielectric layer thickness, the output intensity changes and the spectrum peak shifts. The peaks oscillate as a function of the dielectric thickness and the period is around hundreds of nanometers depending on the refractive index of the dielectric layer. When adding the passivation layer, which is on the order of micrometers, more than single peak are observed. With a simple model, we found that the frequency difference between two peaks corresponds to the free spectral range of the fabry-perot cavity formed by passivation layer. When a passivation layers is added on the TOLED, the microcavity effect results in the presence of multi-peaks. It limits the view angle and decreases the color purity.
In this paper, we report simulation results of a top-emission organic light-emitting device (TOLED) with a passivation layer composed of silicon dioxide (SiO2) and silicon nitride (Si3N4) to protect organic layers from oxygen and moisture. Usually, the thickness of such a layer is about several micrometers. The electrode material of the device used for simulation is silver (Ag). The anode is thick and opaque while the cathode is thin and semi-transparent. The structure is capped with three pair of SiO2 and Si3N4 as the passivation layer. When SiO2 thickness is less than 2μm, the output intensity changes and the spectrum peak shifts. The peaks oscillate as a function of the SiO2 thickness and the period is about 170nm. When the SiO2 thickness is over 2μm, more than single peak are observed. With a simple model, we found that the frequency difference between two peaks corresponds to the free spectral range of the fabry-perot cavity formed by passivation layer. When a passivation layers is added on the TOLED, the microcavity effect results in the presence of multi-peaks. It limits the view angle and decreases the color purity.
We first demonstrated the effectiveness of imaging in a tissue phantom with isotropic scattering by using polarization discrimination combined with the time gating method. In this situation with lean pork as targets and diluted milk as tissue phantom, the reduced scattering coefficient mapping manifests clear images. However, such an imaging technique became less effective in filamentous tissues, such as chicken breast tissues, because filamentous tissue had a deterministically anisotropic property. It led to coherent coupling between the two linear polarization components. In this situation, we employed the time-gated degree of polarization (DOP) imaging technique that based on the Stokes formalism. The results showed that the DOP measurement was quite effective in high-quality imaging of objects in filamentous tissues. The improvement of this method was attributed to the unchanged polarization part under the coupling processes ofvarious polarization components.
The band structures of dispersive photonic crystals are numerically investigated. Based on the fmite-difference time-domain (FDTD) method with some auxiliary variables, time-stepping formulas are derived to compute the evolution of electromagnetic fields. The photomc crystals made of dispersive materials can be described either by the Lorentzian model or by the Drude model. In this paper, we describe how to use the time domain technique to calculate the dispersion relation of photonic crystals and then introduce the recursive convolution scheme to treat the photonic crystals with frequency-dependent materials. Band structures of photomc crystals without dispersion are calculated with the conventional FDTD time-stepping formulas and those with Lorentzian type dispersion and Drude type dispersion are also calculated with the recursive convolution scheme.
Periodical perturbations along an optical fiber can cause power coupling between the core mode and cladding modes for the applications of spectral filtering and derivative operations. Such perturbations can be generated through periodical loading on fiber. By applying loading onto fiber with two face-to-face, identical periodical groove structures, it was found that the long-period grating effects were dependent on the relative phase of the two periodical corrugations. Particularly, when the relative phase was zero (crest-to-crest), spectral filtering effects disappeared completely. The comparisons of such results between the cases of jacketed and un-jacketed fibers led to the conclusion that geometric deformation, instead of direct pressure-induced effect, was the dominating mechanism for generating spectral filtering functions in the double-sided loading configuration. The same conclusion can be applied to the single-sided loading device.
This paper presents our study of time-resolved propagation of polarized light in scattering media. Monte Carlo simulated time-resolved Stokes vectors of transmitted light were compared with the experimental results. A satisfying match has been obtained.
To explore the polarization-dependent optical properties of skeletal muscle tissues, we calibrated the extinction coefficients of the polarization components parallel with and perpendicular to chicken breast tissue filaments and the cross-polarized intensity coupling coefficients between the two polarization components, based on the measured quasi- coherent photons. The calibrated values of these coefficients were quite consistent with previously reported. The extinction coefficient in the polarization along tissue filaments was significantly higher than that of the other polarization. Also, the cross-polarized coupling coefficient of the coupling from the polarization of tissue filaments into the other was stronger than that of inverse coupling.
Flexural waves on fiber and fiber Bragg gratings generated with applied ultrasounds and loading were used for switching the reflection wavelength of fiber Bragg grating and for controlling the transmission spectra of fiber. By producing lateral vibration of a fiber Bragg grating, which has been etched in cladding, it was found that the reflection wavelength could be switched from the Bragg wavelength to other wavelengths, which corresponded to the conditions of strong coupling between core mode and cladding modes. By adjusting the applied voltage for acoustic waves, the relative strength of different reflection wavelengths could be well adjusted. Theoretical analysis of phase-matching condition indicated that the diameter of etched cladding played a key role in controlling the switched reflection wavelength. Flexural waves were also generated on fiber by applying loading with one or two periodically corrugated plates. By adjusting the orientation of the periodical corrugation, loading level, and the relative position of the two corrugated plates, depressions of transmission spectrum at various wavelengths could be well controlled. The spectral depressions were caused by the coupling between core mode and various cladding modes. Such depressions were used for gain equalization of erbium-doped fiber amplifiers. The equalization operation was more efficient with curved fiber under loading. Control of fiber curvature for various forms of spectral depression will be discussed.
Gain saturation and the induced refractive index variation in semiconductor optical amplifiers (SOAs) have been widely used for many optoelectronics operations, including frequency conversion, phase conjugation, switching, modulation, and laser mode locking. In this paper, we report the experimental and numerical results of using gain saturation in SOAs for all-optical switching and novel laser mode locking configurations. For all-optical switching, we fabricated an all-SOA nonlinear optical loop mirror and implemented efficient power-dependent switching in both cw and pulse modes. Also, numerical modeling was also conducted to show consistent trends with experimental data. For novel laser mode-locking configurations, we demonstrated numerically efficient operation of mode-locked semiconductor laser with multi-mode interference SOA structures. With the nonlinear coupling process, it was found that efficient pulse compression could be achieved, implying that stable mode locking is feasible. Also, with a ring cavity colliding-pulse mode locking can be implemented.
Photons are seriously scattered when entering turbid medium; this the images of objects hidden in turbid medium can not be obtained by just collecting the transmitted photons. Early-arriving photons, which are also called ballistic or snake protons, are much less scattered when passing through turbid medium, and contains more image information than the late-arriving ones. Therefore, objects embedded in turbid medium can be imaged by gathering the ballistic and snake photons. In the present research we try to recover images of objects in turbid medium by simultaneously time-gate and polarization-gate to obtain the snake photons. An Argon-pumped Ti-Sapphire laser with 100fs pulses was employed as a light source. A streak camera with a 2ps temporal resolution was used to extract the ballistic and snake photons. Two pieces of lean swine meat, measured 4mmX3mm and 5xxX4mm, respectively, were placed in a 10cmX10cmX3cm acrylic tank, which was full of diluted milk. A pair of polarizer and an analyzer was used to extract the light that keeps polarization unchanged. The combination of time gating and polarization gating resulted in good images of objects hidden in turbid medium.
Imaging of inhomogeneities in a turbid medium with transmitted optical signal represents a crucial technique for optical diagnosis of abnormal cells in human tissues, particularly important for early detection of female breast cancer. In this paper, we present the results of our simulation and experimental studies. In the simulation study, we propose a new technique using transient diffuse photon density waves for imaging reconstruction. In the experimental study, we demonstrate the assistance of ultrasounds to optical imaging through a turbid medium. For the imaging, we basically utilize transmitted ballistic and snake photons.
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