In situ time-resolved optical measurements of a-Si thin films during excimer laser crystallization

An in situ time-resolved optical reflection and transmission (TRORT) monitoring system combining two He-Ne probe lasers, a digital oscilloscope and three fast photodetectors is developed to investigate the crystallization processes of Si thin films during excimer laser crystallization (ELC). The physical meaning of optical spectra obtained by TRORT measurements has been interpreted in detail. The melt duration and transient phase transformation dynamics of Si thin films can be determined and interpreted immediately. A high efficiency and non-destructive evaluation approach is proposed for determining the grain size of polycrystalline Si after ELC directly and immediacy under appropriate experimental conditions.     

Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

Procedure for retrieving the optical properties of a two-layered medium from time-resolved reflectance measurements

A procedure for retrieving the optical properties of a two-layered diffusive medium based on an exact analytical solution of the diffusion equation and on relative multidistance time-resolved reflectance measurements is presented. The method overcomes some limitations of previously developed procedures. Five parameters of the medium have been fitted: the absorption and the reduced scattering coefficients of both layers and the thickness of the first layer. The actual values of the parameters are correctly retrieved by the procedure. The inversion procedure does not require an initial guess for the unknown optical properties, but the starting value for the thickness of the first layer needs to be estimated with an error smaller than 50%.     

Random walk analysis of time-resolved transillumination measurements in optical imaging

Data relating photon migration in human tissue can be obtained from experiments in which laser light is injected into one face of a finite slab of tissue and detected at the opposite face. From the results of such experiments one would like to infer optical properties of the underlying tissue which are related to its biological properties. Random walk theory has been successfully applied to the translation of such data into the physical parameters that characterize properties of the tissue. We here outline a technique based on the theory of random walks for analyzing how well time-resolved transillumination experiments may be expected to perform for detecting hidden inclusions which have a greater absorption than surrounding tissue. We use an asymptotic evaluation of exact results to determine the degree to which detection is possible.     

Characterization of a confocal microscope for time-resolved photon counting fluorescence

A confocal microscope setup is developed for time-resolved fluorescence measurements. It is added to a traditional cuvette time-resolved setup, with a pumped Ti-Sa light source. The temporal resolution of 37 ps (FWHM) is not degraded, in comparison with the cuvette setup also described. These setups allow both decay lifetime and anisotropy relaxation time determination. Fluorescence correlation spectroscopy (FCS) is used to determine the observation point size. When associated with the calcium probe calcium green, calcium concentration in single cells can be determined in 10 ms by simultaneous acquisition of early and late fluorescence photons.     

Direct writing and in-situ material processing by a laser-micromachining projection microscope

The new laser micromachining projection microscope enables a surface to be viewed at high magnification with a built-in facility for micromachining of the same surface according to a predetermined pattern. A XeCl-excimer laser forms part of the device and provides bright viewing illumination in amplified spontaneous emission as well as a small laser spot of sufficient fluence to mark the sample surface. The basic characteristics of image enhancement in the excimer amplifier are presented. The advantage of such a technique for both pattern generation, surface cleaning and mask correction is demonstrated on Al, Cu and W specimens with a spatial resolution better than 3 m.     

Noise performance of a white-light optical signal processor

A study of the effects of spatially and partially coherent illumination on the noise performance of an optical processor is presented here. Noise arising from defects in the object plane and the Fourier plane are considered separately. Two important types of noise are analyzed in detail: (a) amplitude noise and (b) ramdom phase noise. The results illustrate that the effects of the object phase noise can be reduced when the requirements on spatial coherence are relaxed. Also, the noise at the Fourier plane can be effectively eliminated when a spatially incoherent light source is employed.     

采用随机振动法抑制白光处理系统中的颗粒噪声

本文提出在白光信息处理中,可通过使滤波孔在傅里叶面彩虹状衍射谱附近做随机振动的方法,达到抑制光栅编码片的振幅型颗粒噪声.改善其输出信噪比的目的,并给出了相应的实验结果.     

Direct observation of variations of optical properties in single quantum dots by using time-resolved near-field scanning optical microscope

We study the variations of optical properties of self-assembled In_0.5Ga_0.5As single quantum dots (QDs) in the spatial and time domains by combining a near-field scanning optical microscope with an ultrafast pulsed laser. Through the examinations of several tens of QDs, we find that the variations of photoluminescence (PL) intensity strongly depend on the condition of the initial carrier creation. The differences in quantum efficiency and those in the carrier flow rate into QDs cause the large distribution of PL intensity when the carriers are excited in the barrier layers. From the results of time-resolved PL decay measurements, we find that there are two types of QDs exhibiting quite different PL decay profiles.     

Characterization of in-situ time-resolved optical spectra during excimer-laser crystallization

A real-time in-situ time-resolved (~1 ns) optical reflectivity and transmission (TRORT) measurement system combining two He–Ne probe lasers, a digital oscilloscope, and three fast photodiodes is developed to investigate the rapid phase-change processes of Si thin films during the excimer-laser crystallization (ELC). The changes in both reflectivity and transmission of Si thin films during ELC are recorded by the TRORT measurement system. Melting and resolidification behaviors of Si thin films during ELC are interpreted. The fall time of liquid Si is reduced with increase in the excimer-laser energy density, while the rise time of liquid Si remains approximately constant at 5 ns. The first small peak in the reflectivity spectrum is proved to be not a phenomenon of explosive crystallization.     

Computer-aided processing of optical measurements in aeroballistic experiment

The idea of semiautomatic computer-aided processing of photos as applied to the problem of external ballistics, namely, determination of aerodynamic characteristics of flying objects from trajectory data, is put forward. The impetus to write this article is the development of an original software product that was used to advantage in processing data of full-scale experiments with the aim of creating a photo database of ballistic experiments performed at the Ioffe Physicotechnical Institute. This software is integrated into the Arkhimed information-retrieval system used to control this database. The advantages of the semiautomatic approach over “manual” measurements and the fully automatic version are indicated.     

Broadband near-field interference spectroscopy of metal nanoparticles using a femtosecond white-light continuum

We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.     

Quantitative amplitude and phase measurement by use of a heterodyne scanning near-field optical microscope

A coherent photon scanning tunneling microscope is presented. The setup employs heterodyne interferometry, allowing both the phase and the amplitude of the optical near field to be measured. Experimental results of measurements on a standing evanescent wave reveal the high resolution that is obtainable with such an approach. In fact we have measured the amplitude and the phase of the near field, with a resolution of 1.6 nm between sample points.     

Study of an organic nonlinear optical material for nanoscale data storage by scanning tunneling microscope

In this paper, organic thin films of a nonlinear optical (NLO) material ((E)-1-ferrocenyl-2-(l-methyl-4-pyridiniumyl) ethylene iodide) were prepared by a vacuum deposition method. The typical I–V curve of the film exhibits a favorable room-temperature bistability. Further recording experiments have been performed with a scanning tunneling microscope, which confirmed that the thin film of the donor–π–acceptor NLO material could certainly be applied for nanoscale data storage. The average size of the recorded marks is about 1.5 nm. Mechanism analyses suggest that the formation of the recording dots is due to the local change of electrical property of the thin film, and the intermolecular charge transfer induced by an electric field is proposed as the reason for the change. PACS 68.37.-d; 73.61.-r; 81.16.Rf; 68.37.Ef; 73.61.Ph     

Simultaneous microscopic measurements of thermal and spectroscopic fields of a phase change material

In this paper, simultaneous microscopic measurements of thermal and spectroscopic fields of a paraffin wax n-alkane phase change material are reported. Measurements collected using an original set-up are presented and discussed with emphasis on the ability to perform simultaneous characterization of the system when the proposed imaging process is used. Finally, this work reveals that the infrared wavelength contains two sets of important information. Furthermore, this versatile and flexible technique is well adapted to characterize many systems in which the mass and heat transfers effects are coupled.     

A curve fitting signal processing scheme for a white-light interferometric system with a synthetic source

A white-light interferometric system, which comprises two Michelson interferometers linked by a 100 μm diameter core multimode step index fibre has been investigated, where a signal processing scheme, using curve fitting, has been introduced to suppress the noise induced in the system. The theoretical resolution of such a sensor with the use of an appropriate curve fitting signal processing scheme has been discussed. The experimental results obtained show the short term repeatability (to three standard deviations) to be better than 5 nm after signal processing, under conditions where the fibre is constantly vibrated in a known and reproducible way.     

In-situ monitoring of optical near-field material processing by electron microscopes

A GaN-to-InGaN interface modification by predeposition of an ultrathin In-rich InGaN incomplete layer followed by a thin triangular InGaN well layer was employed to overcome the negative effects of polarization field on light emission efficiency of InGaN/GaN quantum wells as well as to improve the crystalline quality by avoidance of a significant strain generation and enhanced surfactant effect. Further, the interface modification induced energy band structure engineering reduces the spatial separation of electrons and holes, and thus increases the carrier recombination rate. The improvement in crystalline quality, localized potential fluctuation, and energy band engineering contribute to the significant increase of green emission of the InGaN/GaN quantum wells.     

Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.     

Linear wavelength sweep of a semiconductor laser and its consequence for dispersion measurements and interference noise in optical fibers

The wavelength shift of a semiconductor laser during triangular pulse modulation has been investigated, using both a Michelson interferometer and direct spectral measurements. This property is used to establish a method for high-resolution dispersion measurements limited to 10 ps, and not affected by material dispersion or detector rise time. A theoretical investigation shows under what conditions the method is equivalent to the standard dispersion measurement setup. The technique is applied to measure polar mode dispersion in single-mode fibers. Examples of polarization noise caused by polarization anisotropy and the variation of source wavelength are presented.     

Linear wavelength sweep of a semiconductor laser and its consequence for dispersion measurements and interference noise in optical fibers

The wavelength shift of a semiconductor laser during triangular pulse modulation has been investigated, using both a Michelson interferometer and direct spectral measurements. This property is used to establish a method for high-resolution dispersion measurements limited to 10 ps, and not affected by material dispersion or detector rise time. A theoretical investigation shows under what conditions the method is equivalent to the standard dispersion measurement setup. The technique is applied to measure polar mode dispersion in single-mode fibers. Examples of polarization noise caused by polarization anisotropy and the variation of source wavelength are presented.