Optical harmonic generation from animal tissues by the use of picosecond and femtosecond laser pulses

Second- and third-harmonic generations of femtosecond and picosecond laser pulses have been measured from chicken skin, muscle, and fat tissues. The magnitude of the harmonic signals showed a strong structural dependence with the signal from skin interface being the strongest. The polarization dependence of the signal was also measured and found to be consistent with the fact that the tissue samples were highly scattering random media. The second-harmonic- and third-harmonic-generation conversion efficiencies were found to be in the range of ~10(-7) to ~10(-10).     

Reproducing vertically recorded information-double layer media

In this paper, we present the results of an analysis, similar to the classic analysis of Wallace, of the flying height dependence of readback signals for heads on double layer media with a sinusoidal magnetization pattern. At high frequency, the readback voltage has the same functional dependence as that of a ring head and single layer media; the well known spacing loss, 55 db per wavelength flying height dependence.     

Domain observation and its compositional dependence in RE-TMmagneto-optical media

Recording domains in magnetooptical media were observed with a polarizing microscope. Regularly shaped domains were observed for transition-metal-rich compositions, whereas jagged domains were observed for rare-earth-rich compositions with compensation temperatures close to the Curie temperature. The results were interpreted by means of a force-balance model. The dependence of the domain size and shape on the external magnetic field is discussed     

Estimation of thermal conductivity of magneto-optic media

We describe a method to estimate the thermal conductivity of the substrate, the dielectric layer, and the magneto-optic (MO) layer of MO recording media. The method relies on the disappearance of the polar Kerr rotation above the Curie temperature of the MO layer. We obtain the thermal conductivities by taking into account the differences in the heat diffusion behavior under different sized focused spots. The results are reliable to better than 5% accuracy.     

Effective source term in the diffusion equation for photon transport in turbid media

The Green's function for the diffusion equation is widely used to describe photon transport in turbid media. We have performed aseries of spectroscopy experiments on a number of uniform turbid media with different optical properties (absorption coefficient in the range 0.03-0.14 cm(-1), reduced scattering coefficient in the range 5-22 cm(-1)). Our experiments have been conducted in the frequency domain, where the measured parameters are the dc intensity (I(dc)), ac amplitude (I(ac)), and phase (?) of the light intensity wave. In an infinite medium, the Green's function predicts a linear dependence of ln(rI(dc)) and ? on the source-detector separation r. Our measurements show that the intercepts of these straight lines predicted by the Green's function do not agree with the experimental results. To reproduce the experimental results, we have introduced an effective photon source whose spatial extent and source strength depend on the optical properties of the medium. This effective source term has no effect on the slopes of the straight lines predicted by the Green'sfunction at large values of r.     

Effect of tilted ellipsoid of birefringence on readout signal in magneto-optical disk data storage

In the 2× format for second-generation magneto-optical (MO) disks, there are recording tracks next to preformat pits. It has been observed that the MO signals from the tracks adjacent to the preformat pits can be affected. We present numerical modeling results on the effect of a tilted ellipsoid of substrate birefringence on the MO readout signal. Theoretical calculations show that the observed effects can be explained if a certain tilt of the ellipsoid of birefringence exists around the preformat pits.     

Raman signal enhancement in deep spectroscopy of turbid media

A new, passive method for enhancing spontaneous Raman signals for the spectroscopic investigation of turbid media is presented. The main areas to benefit are transmission Raman and spatially offset Raman spectroscopy approaches for deep probing of turbid media. The enhancement, which is typically several fold, is achieved using a multilayer dielectric optical element, such as a bandpass filter, placed within the laser beam over the sample. This element prevents loss of the photons that re-emerge from the medium at the critical point where the laser beam enters the sample, the point where major photon loss occurs. This leads to a substantial increase of the coupling of laser radiation into the sample and consequently an enhanced laser photon-medium interaction process. The method utilizes the angular dependence of dielectric optical elements on impacting photon direction with its transmission spectral profile shifting to the blue with increase in the deviation of photons away from normal incidence. This feature enables it to act as a unidirectional mirror passing a semi-collimated laser beam through unhindered from one side, and at the other side, reflecting photons emerging from the sample at random directions back into it with no restrictions to the detected Raman signal. With substantial restrictions to the spectral range, the concept can also be applied to conventional backscattering Raman spectroscopy. The use of additional reflective elements around the sample to enhance the Raman signal further is also discussed. The increased signal strength yields higher signal quality, a feature important in many applications. Potential uses include sensitive noninvasive disease diagnosis in vivo, security screening, and quality control of pharmaceutical products. The concept is also applicable in an analogous manner to other types of analytical methods such as fluorescence or near-infrared (NIR) absorption spectroscopy of turbid media or it can be used to enhance the effectiveness of the coupling of laser radiation into tissue in applications such as photodynamic therapy for cancer treatment.     

宽温度环境下光数据存储的不稳定性分析

本文分析了磁光存储系统读出不稳定性和温度的关系,导出了信号和噪声随温度变化的函数关系,指出半导体激光器和光电探测器是导致读出不稳定的主要原因,提出了在宽温度环境下改善系统读出稳定性所应采取的措施。     

Ultrasound-modulated optical tomography of absorbing objects buried in dense tissue-simulating turbid media

Continuous-wave ultrasonic modulation of scattered laser light was used to image objects buried in tissue-simulating turbid media. The buried object had an absorption coefficient greater than the background turbid medium. The ultrasonic wave that was focused into the turbid media modulated the laser light that passed through the ultrasonic field. The modulated laser light that was collected by a photomultiplier tube reflected the local mechanical and optical properties in the zone of ultrasonic modulation. Objects buried in the middle plane of 5-cm-thick dense turbid media were imaged with millimeter resolution through the scanning and detecting alterations of the ultrasound-modulated optical signal. The optical properties of the dense turbid media included an absorption coefficient of 0.1 cm(-1) and a reduced scattering coefficient of 10 cm(-1) and were comparable with those of biological tissues in the visible and near-IR ranges. The dependence of the ultrasound-modulated optical signal on the off-axis distance of the detector from the optic axis and the area of the detector was studied as well.     

Noise of low-coercivity single-layer perpendicular recording media

The noise of single layer CoCr media is investigated. A model is developed by treating the medium as a collection of columns. It appears that there is a strong correlation between magnetization direction of the columns. Neighbouring columns tend to have the same magnetization polarity, indicating exchange interaction. It is concluded that the media consist of domains. The domain width increases with the medium thickness. These domains can also result in a poor signal response at short wavelength.     

Effect of KTa1−xNbxO3 crystal orientation on the nonlinear optical response in the picoseconds degenerate-four-wave mixing

Nonlinear optical response signals on picosecond and nanosecond time scales have been studied in the mixed-crystal system KTa_1−xNb_xO₃ using transient degenerate-four-wave mixing (DFWM). The two distinct signal components, denoted as an instantaneous response (IR) and slow response (SR) in previous studies, changed intensity as the relative arriving time between two write pulses varied. The dependence of these signals on the crystal orientation was characterized. The results were consistent with a lattice relaxation model based on the laser-induced dynamic displacement of the Nb ion. In this model the signal variations result from the competition between the dynamic displacement and the intrinsic distortion associated with the spontaneous polarization along the optic axis.     

The average power density spectrum of the readback voltage from particulate media

An excellent report on the spectrum of readback voltages from magnetic media can be found in Ref. [1] (Thurlings) where the power density spectrum was developed from basic assumptions on the distributions of the particles in the media. In attempting to fit our experimental observations with the above model, it was found that the expressions suggested in [1] had to be extended in order to fit the variations in gain as a function of frequency of the written signal. The extensions include the formulation of a simple but adequate model for magnetic interaction among particles. This interaction model explains the dependence of noise on the frequency of the written signal.     

Processing of signals from media with perpendicular magnetic anisotropy

The readback signals from a ring (Karlqvist) head and recording medium with perpendicular magnetic anisotropy are analyzed using signal processing criteria. The self-consistent model developed by Beardsley and Potter is used to study the feasibility of detecting transitions at high densities. Simulated readback signals are discussed in terms of time and frequency domain characteristics. A simplified linear system model is proposed which includes the effects of media orientation. Filtering and detection methods for the "vertical" channel are discussed.     

Acoustic emission in tests on materials in hydrogenous and corrosive media

Summary Acoustic-emission studies are surveyed for cracking resistance parameter determination in corrosive media, which have been made by the authors and by others. Acoustic emission AE has been applied in research on processes occurring in hydrogen and corrosion cracking. AE signal analysis enables one to identify crack initiation and the times between steps more accurately than do traditional methods, and also enables one to predict working life. However, further research is required on methods of identifying AE signals from various sources during failure in corrosive media.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 26, No. 5, pp. 26–36, September–October, 1990.     

Structure of weiss domains in elastic ferroelectric crystals

In this work a variational principle is proposed to study the existence and structure of Weiss domains in elastic ferroelectric crystals. Weiss domains are defined as certain subregions of the crystal in each of which the polarization vector is uniform and has a constant magnitude which is equal to the saturation polarization per unit mass for the crystal. The variational principle differs from previous ones in that the variations of the domain walls are also taken into account and it is a direct generalization of the one corresponding to the rigid crystals which we have proposed earlier. In deriving the general theory the dependence on the polarization gradients are also considered and the effect of this dependence when passing from one domain to another is represented by an appropriately chosen surface energy on domain walls. The domain structure is studied under homogeneous deformation. The effect of a small deformation field on the shape of domains is illustrated in the case of a rectangular uniaxial crystal which has initially no electric field inside. It is shown that the deformation creates a small electric field in the crystal and domain walls change slightly.     

Technique for enhancing signal in conventional backscattering fluorescence and Raman spectroscopy of turbid media

We demonstrate experimentally, for the first time, the feasibility of passively enhancing fluorescence and Raman signals from diffusely scattering media in a conventional backscattering collection geometry. The method employs transmission of the collimated excitation laser beam through a "unidirectional" dielectric mirror placed directly in front of the sample. This permits laser light that escapes from the sample surface to be reflected back into the sample where it can be more usefully employed in generating Raman and fluorescence signals. This leads to improved Raman signal, higher signal-to-noise ratio, and shorter acquisition times. Feasibility studies performed on standard pharmaceutical tablets and on sheets of Teflon, using a single enhancing element, demonstrate signal enhancement factors of 6 (fluorescence) and 3 (Raman). Potential applications of this simple device include improving quality control of pharmaceutical products, disease diagnosis of biological tissue, forensics, and security screening.     

Beam modes in graded-index media and topological phases

The change in the polarization plane of light vector beams retaining their shape (beam modes) under propagation through gradient-index media is evaluated as a topological phase acquired by cyclic and noncyclic evolutions of these beams on their projective Hilbert space (momentum sphere). The polarization changes are evaluated by means of the characteristic parameters of the light beam selected.     

Ultrasonic backscatter from rat blood in aggregating media under in vitro rotational flow

Ultrasonic backscatter from flowing and static rat red blood cells (RBCs) in autologous plasma and in 360 kDa polyvinylpyrrolidone (PVP 360) solution was measured as a function of hematocrit. The flow speed was varied by a stirring magnet in a cylindrical chamber. The radio-frequency (RF) signals backscattered by RBC samples were measured over 5 min in a pulse-echo setup with a 5 MHz focused transducer. Although the intact rat blood has poor RBC aggregability, RBC aggregation of rat blood was enhanced by replacing its plasma with a higher molecular weight polymer solution. The experimental results showed that the nonlinear relationship between hematocrit and ultrasonic backscatter from rat RBCs in plasma and aggregating media is affected by flow speed, which may provide a unified insight into hematocrit dependence of RBC aggregation under flowing and static conditions.     

Effect of island geometry on the replay signal in patterned media storage

In a move to extend the storage capabilities of magnetic storage systems beyond 1 Tb/in/sup 2/, the use of patterned media has often been cited. Here, recorded domains are constrained by the geometry of the magnetic island and not the geometry of the recording head. Conventional two-dimensional readout modeling techniques, using the reciprocity integral, rely on the assumption that the across-track medium magnetization is uniform under the giant magnetoresistive replay head. However, in the case of a geometrically constrained medium this is not the case. This work investigates the effect that the island geometry has on the characteristics of the replay signal in perpendicular patterned magnetic media storage through the extension of the reciprocity integral to three dimensions. The paper describes replay pulses that offer different characteristics from those obtained by conventional two-dimensional techniques. The origins of these differences are explained by the variation in medium magnetization across the track.     

Image contrast in polarization microscopy of magneto-optical disk data-storage media through birefringent plastic substrates

Using polarized-light microscopy, we have investigated the magnetic domains of perpendicularly magnetized media under several different conditions, including direct observation of the thin-film magnetic layer and observations through the glass or plastic substrates on which the magnetic film was deposited. The results show that the image contrast is reduced with an increasing numerical aperture of the objective lens. They also indicate that the polarization rotation caused by differences between the reflectivity-transmissivity of the p and s components of polarization deteriorate the magnetic image contrast. Furthermore, by comparing the image quality using the same objective lens on samples having different substrates, we found that the images obtained through plastic substrates are worse than those obtained through glass substrates. Birefringence of the plastic substrate is shown to be responsible for the additional degradation of the image contrast.