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Sweat is one of the essential biofluids produced by the human body, and it contains various physiological biomarkers. These biomarkers can indicate human health conditions such as disease and illness. In particular, imbalances in the concentration of electrolytes can indicate the onset of disease. These same imbalances affect the dielectric properties of sweat. In this study, we used attenuated total reflection terahertz time domain spectroscopy to obtain the frequency-dependent dielectric properties of human sweat in a frequency range from 200 GHz to 2.5 THz. We have investigated the variation of dielectric properties of sweat collected from different regions of the human body, and we have observed that the real and imaginary part of dielectric permittivity decreases with the increase in frequency. A combination of left-hand Jonscher and Havriliak-Negami processes is used to model the results and reveal the presence of relaxation processes related to sodium and calcium ions concentrations. This information may help design novel biosensors to understand the human health condition and provide a hydration assessment., Competing Interests: The authors declare no conflicts of interest., (© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.)

The applications of terahertz (THz) waves have been increasing rapidly in different fields such as information and communication technology, homeland security and biomedical engineering. However, study on the possible health implications due to various biological effects induced by THz waves is relatively scarce. Previously, it has been reported that the human sweat ducts play a significant role in the interaction of the THz wave with human skin due to its coiled structure. This structure imposes on them the electromagnetic character of a helical antenna. To further understand these phenomena, we investigated the morphological features of human sweat ducts and the dielectric properties of their surrounding medium. Based upon these parameters, we estimated the frequency of the resonance of the human sweat duct in a normal mode of operation and our estimation showed that there is a broad resonance around 228 GHz. This result indicates that careful consideration should be given while designing electronic and photonic devices operating in the sub-terahertz frequency region in order to avoid various effects on human health due to these waves., Competing Interests: The authors declare that there are no conflicts of interest related to this article.

Intracellular structures of HeLa cells are observed using a direct electron beam excitation-assisted fluorescence (D-EXA) microscope. In this microscope, a silicon nitride membrane is used as a culture plate, which typically has a low biocompatibility between the sample and the silicon nitride surface to prevent the HeLa cells from adhering strongly to the surface. In this work, the surface of silicon nitride is modified to allow strong cell attachment, which enables high-resolution observation of intracellular structures and an increased signal-to-noise ratio. In addition, the penetration depth of the electron beam is evaluated using Monte Carlo simulations. We can conclude from the results of the observations and simulations that the surface modification technique is promising for the observation of intracellular structures using the D-EXA microscope.

We developed a high-resolution fluorescence microscope in which fluorescent materials are directly excited using a focused electron beam. Electron beam excitation enables detailed observations on the nanometer scale. Real-time live-cell observation is also possible using a thin film to separate the environment under study from the vacuum region required for electron beam propagation. In this study, we demonstrated observation of cellular components by autofluorescence excited with a focused electron beam and performed dynamic observations of intracellular granules. Since autofluorescence is associated with endogenous substances in cells, this microscope can also be used to investigate the intrinsic properties of organelles.

We report the experimental demonstration of fluorescence enhancement in fluorescent thin film using surface plasmon excitation in deep-ultraviolet (deep-UV) region. Surface plasmon resonance in deep-UV is excited on aluminum thin film in the Kretschmann-Raether geometry. Considering the oxidation thickness of aluminum, the experimentally measured incident angle dependence of reflectance show good agreement with Fresnel theory. Surface plasmon resonance was excited at the incident angle of 49 degrees for 266 nm p-polarized excitation light on the film of 18 nm-thick aluminum with 6.5 nm-thick alumina. Fluorescence of CdS quantum dots coated on this aluminum film was enhanced to 18-fold in intensity by the surface plasmon excitation.

A novel approach to calibrate a phase-shift formed in a long-period fiber grating (LPG) is firstly proposed and numerically demonstrated, which is based on the use of either intensity- or wavelength-interrogation technique to the main loss-peak of the phase-shift LPG in the spectrum. Moreover, by using a CO 2 laser with high-repetition-rate pulses emission, an equivalent phase-shift is successfully created at middle of the LPG. As an application of the proposed calibration scheme, measurement for the temperature and the refractive index of the ambient solution has been proposed and successfully demonstrated by using a phase-shifted LPG.

Cuprous oxide (Cu(2)O) films synthesis by radical oxidation with nitrogen (N(2)) plasma treatment and different RF power at low temperature (500 °C) are studied in this paper. X-ray diffraction measurements show that synthesized Cu(2)O thin films grow on c-sapphire substrate with preferred (111) orientation. With nitrogen (N(2)) plasma treatment, the optical bandgap energy is increased from 1.69 to 2.42 eV, when N(2) plasma treatment time is increased from 0 min to 40 min. Although the hole density is increased from 10(14) to 10(15) cm(-3) and the resistivity is decreased from 1879 to 780 Ω cm after N(2) plasma treatment, the performance of Cu(2)O films is poorer compared to that of Cu(2)O using RF power of 0. The fabricated ZnO/Cu(2)O solar cells based on Cu(2)O films with RF power of 0 W show a good rectifying behavior with a efficiency of 0.02%, an open-circuit voltage of 0.1 V, and a fill factor of 24%.

This paper presents a proposal of an optical configuration acceleration method applied to optically reconfigurable gate arrays (ORGAs) using a negative logic implementation. The gate array of an ORGA is reconfigured using a holographic memory. The reading time of a holographic memory depends on the number of bright bits included in a configuration context. The proposed optical configuration acceleration method can decrease the number of bright bits. As a result, the proposed optical configuration acceleration method can increase the reconfiguration frequency. In this paper, a fabricated ORGA very large scale integration that can support the optical configuration acceleration method is estimated. Consequently, this paper shows that the reconfiguration frequency of the proposed method is 1.97 times higher than those of conventional ORGA architectures with no increase of laser power.

Previously, we have proposed a lensless coherent imaging using a nonholographic and noniterative phase-retrieval method that allows the reconstruction of a complex-valued object from a single diffraction intensity measured with an aperture-array filter. The proof-of-concept experiment of this method has been demonstrated under the Fresnel diffraction approximation. In applications to microscopy, however, the measurement of the diffraction intensity with high numerical aperture beyond the Fresnel approximation is required to obtain the object information at high spatial resolution. Thus we have also presented an extension procedure to apply the method to the cases beyond the Fresnel approximation by means of computer simulations. Here the effectiveness of the procedure is demonstrated by the experiments, in which the reconstruction with about 10 times the resolution of our previous experiment has been achieved and the object information in depth direction has been retrieved.

Multi-beam confocal microscopy without any physical pinhole was demonstrated. As a key device, a custom CMOS image sensor realizing a focal-plane pinhole array effect by special pixel addressing and discarding of the unwanted photocarriers was developed. The axial resolution in the confocal mode measured by FWHM for a planar mirror was 8.9 μm, which showed that the confocality has been achieved with the proposed CMOS image sensor.

We experimentally examined the propagation of temporally square modulated optical pulses through a coupled ring resonator. Sharp transient spikes appeared as the square pulses entered the system. The main signal gradually grew up through coupled-resonator-induced transparency (CRIT), with the time constant determined by a second resonator. Transient spikes were attributed to the higher and lower spectral components of the incident pulse, to which the resonators cannot respond; hence, they were interpreted as optical precursors. The experiments, therefore, demonstrated that precursors and the main signal can be observed separately, with amplitudes comparable to that of the incident step in CRIT.

The optical performances of the all-optical switching based on Yb3+-doped fiber Bragg grating (FBG) are investigated under the case of self-phase modulation (SPM) and cross-phase modulation (XPM). For the SPM case, the optical bistability of FBG under different parameters is investigated. It shows that the width of the hysteresis loop and threshold switching power are strongly dependent on the fiber grating length, fiber grating detuning, and coupling constant. For the XPM case, the expressions of the threshold switching power in the different detuning range are given. The influence of parameters about different detuning and coupling constant to the threshold switching power and extinction ratio are also studied. In comparison with the SPM case, the switching power of FBG under XPM case can be reduced to less than 20 mW by optimizing the parameters of FBG.

Microholographic memory is an attractive storage system for its capability to hold high-density data and for its access time. Using a photochromic chromophore (diarylethene)-doped recording medium can give rise to microholographic memory's durability and contrast. In addition, it is possible to increase the microholographic memory's density by shift-multiplexed recording, since a hologram pit is constructed in a small area. The microhologram was fabricated in the diarylethene-based sample with two counterpropagating focused beams. Also, surface images and cross-sectional images scanned by a confocal microscope indicated that shift-multiplexed recording was achieved in high contrast.

A novel formation method of a long-period fiber grating (LPFG) based on a magnetic-force-induced microbend is proposed and experimentally demonstrated. The LPFG employs a permanent magnet that exerts transversal force to the fiber by attracting a steel coil spring. The transversal force causes periodic microbending to the fiber, and therefore the transmission wave attenuates at the core-to-cladding mode resonance. This device has advantages of ease of fabrication, reconfigurability, and available for any type of fiber.

We demonstrate an all-fiber Q-switched laser operation in the 2 µm region on the basis of a dynamic periodic microbend and pulsed-pump configuration. A single-mode thulium-doped silica fiber is pumped by 1.6 µm-band laser diodes, and the dynamic loss is introduced in the fiber ring resonator by the periodic microbend that is electrically controlled with a piezoelectric actuator. When the voltage-off period of the piezoelectric actuator is set at 20 µs for the pump power of 120 mW, the output pulse power is measured by 420 mW with a pulse width of 1.3 µs.

We propose a direct electron-beam excitation assisted optical microscope with a resolution of a few tens of nanometers and it can be applied for observation of dynamic movements of nanoparticles in liquid. The technique is also useful for live cell imaging under physiological conditions as well as observation of colloidal solution, microcrystal growth in solutions, etc. In the microscope, fluorescent materials are directly excited with a focused electron beam. The direct excitation with an electron beam yields high spatial resolution since the electron beam can be focused to a few tens of nanometers in the specimens. In order to demonstrate the potential of our proposed microscope, we observed the movements of fluorescent nanoparticles, which can be used for labelling specimens, in a water-based solution. We also demonstrated an observation result of living CHO cells.

We propose a method of testing transmission fiber lines and distributed amplifiers. Multipath interference (MPI) is detected as a beat spectrum between a multipath signal and a direct signal using a synthesized chirped test signal with lightwave frequencies of f(1) and f(2) periodically emitted from a distributed feedback laser diode (DFB-LD). This chirped test pulse is generated using a directly modulated DFB-LD with a drive signal calculated using a digital signal processing technique (DSP). A receiver consisting of a photodiode and an electrical spectrum analyzer (ESA) detects a baseband power spectrum peak appearing at the frequency of the test signal frequency deviation (f(1)-f(2)) as a beat spectrum of self-heterodyne detection. Multipath interference is converted from the spectrum peak power. This method improved the minimum detectable MPI to as low as -78 dB. We discuss the detailed design and performance of the proposed test method, including a DFB-LD drive signal calculation algorithm with DSP for synthesis of the chirped test signal and experiments on single-mode fibers with discrete reflections., (© 2011 Optical Society of America)

Demand is increasing daily for large data storage systems that are useful for applications in spacecraft, space satellites, and space robots, which are all exposed to radiation-rich space environment. As candidates for use in space embedded systems, holographic storage systems are promising because they can easily provided the demanded large-storage capability. Particularly, holographic storage systems, which have no rotation mechanism, are demanded because they are virtually maintenance-free. Although a holographic memory itself is an extremely robust device even in a space radiation environment, its associated lasers and drive circuit devices are vulnerable. Such vulnerabilities sometimes engendered severe problems that prevent reading of all contents of the holographic memory, which is a turn-off failure mode of a laser array. This paper therefore presents a proposal for a recovery method for the turn-off failure mode of a laser array on a holographic storage system, and describes results of an experimental demonstration., (© 2011 Optical Society of America)

In this Letter, a scaled-down silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistor (MOSFET) is characterized as a photon detector, where photogenerated individual holes are trapped below the negatively biased gate and modulate stepwise the electron current flowing in the bottom channel induced by the positive substrate bias. The output waveforms exhibit clear separation of current levels corresponding to different numbers of trapped holes. Considering this capability of single-hole counting, a small dark count of less than 0.02 s(-1) at room temperature, and low operation voltage of 1 V, SOI MOSFET could be a unique photon-number-resolving detector if the small quantum efficiency were improved., (© 2011 Optical Society of America)

Recently, we have proposed a coherent diffractive imaging using a noniterative phase retrieval method with the filter of an aperture array. The first (to our knowledge) experimental demonstration of this coherent imaging is presented here, in which a complex-valued object illuminated by a diode laser is reconstructed from the isolated diffraction intensities of the object's wave field, transmitted through an array filter of square apertures by using the phase retrieval method. This imaging method requires only a single measurement of the diffraction intensity and does not need a tight object's support constraint utilized in iterative phase retrieval algorithms or a reference wave used in holographic techniques.

We report wavelength-tunable operation of a Tm-doped silica fiber laser by using a force-induced long-period fiber grating (LPFG) formed in a fiber ring resonator. The laser output wavelength is tuned by moving the transmission passband that is generated between adjacent resonance wavelengths due to the force-induced LPFG. By changing the grating period around 900 μm, we control the laser output wavelength between 1845 and 1930 nm.

We propose a simple iterative method for calculating the dynamic behavior of ring resonators with fast and slow (cumulative) optical nonlinearities when an optical pulse with an arbitrary-shaped envelope is incident into them. In the case of a slow nonlinearity, the nonlinear phase shift and nonlinear absorption are temporally-integrated over the incident pulse. In this paper, we consider two types of single-ring resonators made out of As2Se3 chalcogenide glass with high nonlinearity and investigate the dynamic properties (especially the effect of the cumulative nonlinearity on optical bistability) using known nonlinear material parameters. It is found that the cumulative nonlinearity suppresses overshoot and ringing after switching, decreases the width of the hysteresis loop between the input and output powers, and shifts its center corresponding to the operating point. The obtained results are useful in developing chalcogenide-based bistable optical devices and the proposed approach is applicable to modeling of a variety of nonlinear optical devices.

We present a proposal of a partial reconfiguration architecture for optically reconfigurable gate arrays and present an 11,424 gate dynamic optically reconfigurable gate array VLSI chip that was fabricated on a 96.04 mm(2) chip using an 0.35 μm three-metal complementary metal oxide semiconductor process technology. The fabricated VLSI chip achieved a 2.21 μs partial reconfiguration.

We propagated transverse two-dimensional images encoded on optical pulses through a frequency window of a coupled-image-resonator-induced transparency. The optical images are stored and delayed by 10.6ns, reflecting the tunable dispersion of the coupled resonator. The k-space bandwidth of the amplitude transfer function of the system is discussed in the presence of the off-resonance Fano interference effect between the two resonators.

This paper proposes a method of superimposing acceleration and optimizing the optical reconfiguration speed while requiring no increase of laser power. Using this technique, the optical reconfiguration speed is increased by superimposing multiple configuration contexts. Simultaneously, optimization of the number of configuration contexts and reconfiguration speed is possible. A full four-context optically reconfigurable gate array system consisting of an optically reconfigurable gate array VLSI, an easily rewritable liquid crystal holographic memory, and four vertical-cavity surface-emitting lasers was constructed to demonstrate this method. This paper clarifies the method's benefits using experimental results obtained from the demonstration system.

When a very intense beam is used for illuminating an object in coherent x-ray diffraction imaging, the intensities at the center of the diffraction pattern for the object are cut off by a beam stop that is utilized to block the intense beam. Until now, only iterative phase-retrieval methods have been applied to object reconstruction from a single diffraction pattern with a deficiency of central data due to a beam stop. As an alternative method, I present a noniterative solution in which an interpolation method based on the sampling theorem for the missing data is used for object reconstruction with our previously proposed phase-retrieval method using an aperture-array filter. Computer simulations demonstrate the reconstruction of a complex-amplitude object from a single diffraction pattern with a missing data area, which is generally difficult to treat with the iterative methods because a nonnegativity constraint cannot be used for such an object.

A multi-layered nanoparticles optical disk has been developed for a jitter-free high-density data storage system. The disk has nano structures composed of 300-nm-diameter photosensitive particles and 30-nm-width non-photosensitive buffer rings around them. With the buffer rings into the nanoparticles disk, a conventional confocal microscope equipped with a low numerical aperture (NA) objective picked up a particle's shape signal to generate a synchronous signal on its own. In the three-dimensional structured disk proposed, no electronically-produced reference signal is necessary for clock data recover (CDR); no jitter occurs in data decoding.

We propose an excitation method for the localization of photons at the apex of a metal coated axicon prism. The cone angle of the prism and the metallic film thickness are designed to match the excitation conditions for surface plasmons. The plasmons propagate along the sides of the prism and converge at its apex. The resulting nanofocusing was investigated by simulating the intensity distributions around the apex of the prism using a finite-difference time-domain algorithm. For incident radial polarization, a localized and field enhanced spot is generated by the constructive interference of surface plasmons.

We propose electron beam excitation assisted optical microscope, and demonstrated its resolution higher than 50 nm. In the microscope, a light source in a few nanometers size is excited by focused electron beam in a luminescent film. The microscope makes it possible to observe dynamic behavior of living biological specimens in various surroundings, such as air or liquids. Scan speed of the nanometric light source is faster than that in conventional near-field scanning optical microscopes. The microscope enables to observe optical constants such as absorption, refractive index, polarization, and their dynamic behavior on a nanometric scale. The microscope opens new microscopy applications in nano-technology and nano-science.

We performed optical image propagation experiments in an image resonator consisting of a Fabry-Perot resonator in reflection geometry. Two-dimensional images encoded on optical pulses of 32ns were stored, and either advanced, -6.0ns, or delayed, 10.9ns, using the dispersion relation relevant to the image resonator, in the under- or over- coupling condition, respectively. The overall images are propagated through the resonator clearly, while the diffraction effects were analyzed both in real-space and in k-space.

We present an all-fiber tunable bandpass filter based on a combination of a force-induced long-period fiber grating and a fiber coil made along a double cladding fiber. The transmission wavelength can be tuned to be in a range of more than 100 nm by changing the grating period mechanically. We can control the transmission amplitude of the bandpass filter by adjusting the periodic force on the double cladding fiber. The ambient temperature causes a positive shift in the transmission wavelength. Such a device is useful for tunable laser applications and fiber-optic sensors.

A simultaneous dispersion and dispersion-slope compensator based on a doubly sampled fiber Bragg grating (FBG) with 153 channels has been demonstrated, which is obtained by simultaneously chirping the period of the seed grating and the periods of the two sampling functions. These two sampling functions are optimized with the simulated annealing algorithm and the Gerchberg-Saxton iterative method. Moreover, the spectral inclination (in reflection) inherently due to the different magnitudes of the channel dispersion has been successfully equalized.

We propose an optical buffering technique for a programmable gate array under a space radiation environment. The technique allows rapid recovery of a programmable device that is damaged by high-energy charged particles. The technique uses invalid configuration data, including error bits that have already been damaged by the particles, while the configuration data are transferred using wireless communications and are retained on electronically erasable, programmable read-only, or static random-access-memory devices.

Almost all noninterferometric phase-retrieval methods used in coherent diffractive imaging have been based on the measurement system with low numerical aperture, in which Fresnel or Fraunhofer approximation is valid to express the wave propagation between an object and a detector. In microscopy, which is a typical application of coherent diffractive imaging, the measurement of the diffraction intensity with high numerical aperture is required for object reconstruction at high spatial resolution. We here propose an extension procedure to apply the previous phase-retrieval method using an aperture-array filter [J. Opt. Soc. Am. A25, 742 (2008)] to the system with high numerical aperture, in which the first Rayleigh-Sommerfeld integral for spherical waves is utilized instead of the Fresnel integral for parabolic waves. Computer-simulated examples in the high-numerical-aperture system demonstrate object reconstruction at high lateral resolution and retrieval of information in the depth direction of an object.

A semiconductor-optical-amplifier-based fiber laser incorporating with a phase-shifted phase-only sampled fiber Bragg grating (FBG) is proposed and experimentally demonstrated. Thanks to the intrinsic property of the phase-shifted phase-only sampled FBG, the proposed approach has an unique advantage, enabling one to tune and switch all the wavelength lasing simultaneously. The obtained fiber laser has a stable output with more than 30 wavelengths lasing. The 3 dB linewidth is approximately 13 pm, the signal-to-noise ratio is larger than 50 dB, and the nonuniformity of the output is smaller than 7.4 dB.

A double sampling method enabling to have excellent channel uniformity and high in-band energy efficiency is firstly proposed for the design of an ultrahigh-channel-count fiber Bragg grating (FBG), which is based on the simultaneously utilization of an amplitude-assisted phase sampling (AAPS) function and a phase-only sampling (POS) function. As examples, two typical 10-dB FBGs with a length of 12 cm, dispersion of - 1360 ps/nm, channel spacing of 0.8 nm, and a consecutive 135- and 405- channels are numerically designed. The maximum index-modulations required are about 0.8x10(-3), and 1.3x10(-3), respectively. Compared with the proposed method, the other two kinds of double sampling schemes by utilizing either the double AAPS (i.e., AAPS+AAPS) or the double POS (i.e., POS+POS) have also been introduced for the design of the multichannel FBGs. Fabrication tolerances to the designed 135-channel FBG obtained with the AAPS plus POS method are numerically investigated.

We investigate the dependence of the nonlinear optical properties of Cu-doped and Ag-doped As(2)Se(3) glasses on the incident pulse-width at 1.064 microm using the Z-scan technique. In this work, 11.5-ns optical pulses from a Nd:YAG laser were compressed up to 1.6 ns by using stimulated Brillouin scattering in heavy fluorocarbon liquid. The measurement of the pulse-width dependence of the optical nonlinearities leads to the separation of an ultrafast Kerr nonlinearity and a slow (cumulative) nonlinearity such as a thermal nonlinearity. The measured values of the nonlinear refractive index and the nonlinear absorption coefficient of 4 at.% Cu-doped As(2)Se(3) glass (where the normal photodarkening effect is sufficiently suppressed) and 4 at.% Ag-doped As(2)Se(3) glass were proportional to the pulse width, showing that the cumulative nonlinearity is present in these two glasses. The cumulative nonlinearity of these doped glasses is larger than one of the undoped glass. The origin of such slow nonlinearity is presumably attributed to the photostructural changes inherent in chalcogenide glasses and is directly independent of one of photodarkening.

A continuous phase-only sampling scheme enabling one to create an ultrahigh-channel-count fiber Bragg grating (FBG) with excellent channel uniformity and high in-band energy efficiency is proposed that is based on the simultaneous utilization of two phase-only sampling functions. With this method, a linearly chirped 135-channel FBG with a length of 10 cm, a dispersion of 1360 ps/nm, channel spacing of 0.8 nm, and grating strength of 10 dB is theoretically demonstrated, and the maximum refractive index change required is less than 10(-3).

Recently, optically reconfigurable gate arrays (ORGAs), which consist of a gate array VLSI, a holographic memory, and a laser array, have been developed to achieve huge virtual gate counts that vastly surpass those of currently available VLSIs. By exploiting the large storage capacity of a holographic memory, VLSIs with more than 1 teragate counts will be producible. However, compared with current field programmable gate arrays, conventional ORGAs have one important shortcoming: they cannot be reprogrammed after fabrication. To reprogram ORGAs, a holographic memory must be disassembled from its ORGA package, then reprogrammed outside of the ORGA package using a holographic memory writer. It must then be implemented onto the ORGA package with high precision techniques beyond that which can be provided by manual assembly. Therefore, to improve this shortcoming, this paper proposes what is believed to be the world's first programmable ORGA architecture with no disassembly. Finally, the availability of this architecture is discussed based on the experimental results.

We propose a novel method for the implementation of a tunable multi-channel notch filter based on a thermally induced phase-shift phase-only sampled fiber Bragg grating (FBG). The proposed method is numerically and experimentally demonstrated. A 51-channel notch filter with a bandwidth (FWHM) of 0.026 nm and a tuning range of 0.6 nm has been achieved. This proposed technique offers the potential applications to the multiwavelength fiber laser and multi-channel all optical logic devices.

We study the dependence of optical nonlinearities in As(2)Se(3) glass on the incident pulse-width using the Z-scan technique. In this work, 11.5-ns optical pulses from a Nd:YAG laser operating at 1.064 mum were compressed up to 1.6 ns by using stimulated Brillouin scattering in heavy fluorocarbon liquid, where the duration of the compressed pulses depends on the pump energy. Moreover picosecond optical pulses with a 1.053-mum wavelength were utilized for the Z-scan experiments. To investigate the pulse-width dependence of the optical nonlinearities leads to the separation of an ultrafast Kerr nonlinearity and a slow (cumulative) nonlinearity such as a thermal nonlinearity because the magnitude of the cumulative nonlinearity should be proportional to the pulse duration. The experimental results clearly show that both the nonlinear refractive index and the nonlinear absorption coefficient increase linearly with pulse width. The origin of such pulse-width dependence is presumably attributed to photostructural changes inherent in chalcogenide glasses, which are induced not by nonlinear absorption such as two-photon absorption or two-step absorption, but by linear absorption in the weak-absorption region.

We study free-carrier nonlinearities in crystalline silicon at 1.064 microm using the Z-scan technique, with special emphasis on the dependence of their nonlinearities on the width of incident pulses. In the Z-scan experiment, the pulse duration was changed from 11.5 ns to 1.6 ns by the pulse compression using stimulated Brillouin scattering in a liquid. At this excitation wavelength, linear absorption is dominant for the creation of electron-hole pairs and the photoexcited carriers can modify the refractive index and absorption coefficient just as a third-order nonlinear effect. The effective nonlinear refractive index n(2eff) and nonlinear absorption coefficient beta(eff) are proportional to the pulse duration and optical intensity, i.e. the fluence when the pulse duration is shorter than the carrier recombination lifetime. We can determine the variation of refractive index per unit of photoexcited carrier density sigma(r) and the total carrier absorption cross section sigma(ab) from the dependence of n(2eff) and beta(eff) on the pulse width, respectively. In this work we had sigma(r) =-1.0 x 10(-21) cm(3) and sigma(ab)=8.4 x 10(-18) cm(2), which agree well with previous data. We also observed the decrease in the magnitude of n(2eff) and beta(eff) at high incident fluence, which is presumably attributed to band filling. This new measurement approach has an advantage of being able to separate an ultrafast Kerr nonlinearity and a cumulative nonlinearity such as the free-carrier nonlinearity treated in this paper and can be utilized to evaluate the optical nonlinearities of other materials.

An optically reconfigurable gate array (ORGA) system, which consists of an ORGA very large scale integration (VLSI), an easily rewritable liquid crystal holographic memory recording four configuration contexts, and a laser array, is proposed. Circuits on a gate array of the ORGA-VLSI can be programmed rapidly by exploiting large parallel connections between a holographic memory and a gate array VLSI; that programming can be executed even as it is being programmed. Consequently, the gate array can be switched from a certain circuit to another circuit instantaneously. We present a demonstration of the ORGA system and experimental results.

A novel approach for the reflection equalization of a phase-only sampled fiber Bragg grating (FBG) is presented, where the grating is specially designed as a simultaneous dispersion and dispersion-slope compensator with channels up to 51. The sampling-function used is given with an analytical form with a linearly-chirped sampling period and is optimized by using the simulated annealing algorithm.

We study the linear resonance properties of several types of microrings in a two-dimensional photonic crystal (PC) consisting of a square lattice with air holes in dielectric using the plane-wave expansion method and the FDTD method. Moreover we investigate the nonlinear responses, especially optical bistability when an intense optical pulse is incident into the microrings. In this paper, Ag-As-Se chalcogenide glass is assumed as nonlinear dielectric, which has a high third-order nonlinearity. Although line-defect waveguides in an air-hole-type PC are usually multimoded, we can obtain interesting unique properties such as counter rotation of intracavity fields, transmission to all output ports, and unstable nonlinear oscillations in the multimoded PC microring. We can improve the resonance characteristics by partly introducing single-mode waveguides into microrings and can obtain stable optical bistability.

Recently we have proposed a noniterative and analytic phase retrieval method using the filter of an aperture array to reconstruct a complex-valued object from a diffraction intensity pattern [Phys. Rev. Lett.98, 223901 (2007)], but this method suffers from the restriction of the far-field condition of two distances between the object and the filter and between the filter and the detector for the intensity measurement. An improved method, which extends the adaptable condition of those distances to the region of Fresnel diffraction, is proposed here. In addition, the procedure for reducing the influence of noises on the phase retrieval is presented, in which the phase information contained in multiple groups of sampling data of a single intensity distribution is utilized. The usefulness of this method is shown in computer-simulated examples of the object reconstructions, including an object with phase vortices.

The depth-resolved luminescence of whispering gallery modes (WGMs) was studied in TiO2:Eu3+ microsphere using cathode luminescence. As the penetration depth of the electron beam into the microsphere was increased through the control of the accelerating voltage, periodic structures appeared superimposed over the intrinsic transition lines of Eu3+, which were attributable to the cavity enhancement effect of the spontaneous emission by WGM resonance. The calculated radial intensity distribution of the WGM and the penetration depth explained the observed dependence of the WGM structure's visibility on the accelerating voltage.

We present the finding that photoluminescence intensity in ZnSe crystal is enhanced after exposure to a femtosecond laser beam. After the crystal was illuminated with laser light of 1.04 MW/cm(2) during 300 s, photoluminescence intensity was increased approximately 20%. The region in which photoluminescence intensity was enhanced was localized in the optical axis, because this phenomenon occurred in the two-photon excitation process. It is possible to achieve three-dimensional control of photoluminescence intensity by illumination with laser light.

We demonstrated a compact optical amplification system using a fiber taper and rare-earth-doped microsphere. Both the signal and pump beams were injected into the microsphere via the fiber taper to increase the spatial matching between two whispering gallery modes. As the pump power was increased, the resonance dip in the transmission spectra due to intrinsic attenuation in the microsphere became filled and then transited to the resonance gain peak. An amplification factor of up to 2.5 was realized with a microsphere of 63 microm in diameter.

We study the transmission characteristics of a photonic crystal waveguide with a single stub of a finite length and propose a double-stub waveguide structure for its application to optical filters. The proposed device operates as a Fabry-Perot interferometer and can be used as a broad bandstop filter and a narrow bandpass filter. These filters are inherently compact and their center frequency is given by the resonant frequency of the stub.