Temperature measurement by degenerate four-wave mixing with strong absorption of the excitation beams

We have made simultaneous temperature measurements by degenerate four-wave mixing (DFWM) and absorption spectroscopy of OH in a CH(4)-air, lifted-diffusion flame. After we corrected the DFWM data for laser beam absorption of as much as 60%, the DFWM-based temperatures were in good agreement with temperatures derived strictly from the absorption data, as well as a one-dimensional reacting flow simulation.     

Resonant degenerate four-wave mixing in I(2): effect of buffer gas pressure

The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.     

Two-photon absorption and degenerate four-wave mixing studies of sulfide semiconductor nanoparticles in polymeric solutions

The two-photon absorption coefficients (beta) and the third-order nonlinear susceptibilities (chi(3)) of several semiconductor nanoparticles (CdS, Cd(x)Ag(1-x)S, and core-shell CdS/Ag2S) that are confined and stabilized by random and block ionomers have been measured by nonlinear transmission and degenerate four-wave mixing techniques using 21 picosecond laser pulses at near-infrared spectral region. The imaginary part of the third-order nonlinear susceptibility that is related to the two-photon absorption coefficient was then calculated. The absorptive nonlinearity of the nanoparticles (2 approximately 9 nm) was found to be dependent on the particle size, composition and wavelength, i.e., larger CdS particles exhibit higher two-photon absorption coefficients and the presence of Ag improves two-photon absorption of CdS nanoparticles. The obtained two-photon absorption coefficients of nanoparticles corrected for their volume fraction in solution are significantly greater that those of corresponding bulk semiconductors.     

Coherence-based imaging through turbid media by use of degenerate four-wave mixing in thin liquid-crystal films and photorefractives

We describe a coherence-based imaging technique based on degenerate four-wave mixing in two materials. First, a 45 degrees -cut BaTiO(3) crystal was used as a self-pumped phase conjugator to obtain depth-resolved images through a 4 mean-free-path (mfp) scattering media. In addition, an HITC dye-doped K15 liquid-crystal layer was used to provide single-shot image acquisition through a 2 mfp scattering media. This technique can be used to provide instantaneous (single-pulse), depth-resolved, two-dimensional images of the internal structure of scattering materials. Potential applications of this technique include subsurface imaging of biomedical tissue and nondestructive evaluation of composite materials.     

Trace-concentration detection of cobalt in a liquid flow cell by degenerate four-wave mixing using low-power off-resonant laser excitation.

Optical phase conjugation by degenerate four-wave mixing (D4WM) in an absorbing metal-ion solution using a low-power argon-ion laser as the excitation source is demonstrated. This nonlinear laser technique can be used as a sensitive analytical spectroscopic method for trace-concentration measurement of metal ions in a small-volume continuously flowing analyte cell. Several important characteristics are discussed, including the effects of solvent properties, excitation wave-length, laser intensity, and analyte absorptivity on signal intensity. Detection of 0.26 ng (4.4 pmol) of cobalt inside the laser probe volume of 0.14 microL is reported using an excitation wavelength that is 136 nm away from the maximum absorption wavelength of the analyte solution. The minimum absorbance measured in our D4WM experiment is 2.0 X 10(-5) without complex formation for cobalt. The D4WM detection sensitivity, in terms of the concentration-absorptivity product, is 4.05 X 10(-4) cm-1 for cobalt(II) in ethanol. Our preliminary detection sensitivity compares favorably with other laser-based spectrometric methods. This nonlinear laser technique is applicable to both fluorescing and nonfluorescing analytes.     

Imaging in turbid media by photon density waves: spatial resolution and scaling relations

Spatial resolution of transillumination imaging through highly scattering media normalized to sample thickness d depends on only the normalized wavelength ?/d of photon density waves and normalized penetration depth delta/d. This was concluded theoretically and verified experimentally by the derivation of edge-spread functions from measured time-resolved transmittance and its Fourier transform by the use of dilute milk at various concentrations as scatterer in cuvettes of d = 2 cm and d = 4 cm. In the frequency domain and the time domain, spatial resolution was found experimentally to be given by approximately 0.3 d obtained at ? approximately d or when only the first arriving 1% of all photons detected were taken into account.     

Solvent influence on the nonlinear optical properties of molecular systems in the presence of degenerate and non-degenerate four-wave mixing

The nonlinear optical properties are studied theoretically of a molecular system immersed in a thermal bath and interacting with a four-wave mixing signal. A methodology based on cumulant expansions to obtain the average in the Fourier components associated with the coherences and populations, calculated by the optical stochastic Bloch equations, is employed. The system properties are analyzed as a function of the frequency detuning of the high-intensity pump beam and parameterized by the frequency detuning of the probe beam, both with respect to the Bohr frequency of the system. The longitudinal and transversal relaxation times and the bath effects, modeled as white noise, are also included as parameters in the calculation. Different cases, with degenerate and non-degenerate optical frequencies are examined and compared, resulting in differences related to the maximum in the population pulsation effects.     

Imaging resolution and properties analysis of super resolution microscopy with parallel detection under different noise,detector and image restoration conditions

Parallel detection, which can use the additional information of a pinhole plane image taken at every excitation scan position, could be an efficient method to enhance the resolution of a confocal laser scanning microscope. In this paper, we discuss images obtained under different conditions and using different image restoration methods with parallel detection to quantitatively compare the imaging quality. The conditions include different noise levels and different detector array settings. The image restoration methods include linear deconvolution and pixel reassignment with Richard–Lucy deconvolution and with maximum-likelihood estimation deconvolution. The results show that the linear deconvolution share properties such as high-efficiency and the best performance under all different conditions, and is therefore expected to be of use for future biomedical routine research.     

Analysis of spatial resolution of an experiment on verification of the equivalence principle for a neutron by the diffraction method

Investigations of the spatial resolution of a setup for verification of the equivalence of inertial and gravitational masses of a neutron by the method of diffraction in a perfect crystal have been performed. Tests were conducted at Bragg angles of 74°–82°. A decrease in spatial resolution at Bragg angles >78° is detected.     

Discrete diffraction transform for propagation, reconstruction, and design of wavefield distributions

A discrete diffraction transform (DDT) is a novel discrete wavefield propagation model that is aliasing free for a pixelwise invariant object distribution. For this class of distribution, the model is precise and has no typical discretization effects because it corresponds to accurate calculation of the diffraction integral. A spatial light modulator (SLM) is a good example of a system where a pixelwise invariant distribution appears. Frequency domain regularized inverse algorithms are developed for reconstruction of the object wavefield distribution from the distribution given in the sensor plane. The efficiency of developed frequency domain algorithms is demonstrated by simulation.     

High resolution, two-dimensional image mapping of ZnO nanowires by confocal microphotoluminescence and microraman spectroscopy

High-quality ZnO nanowires were synthesized using both Au catalysts and ZnO seeds by chemical vapor depositionon basal plane sapphire substrates. The nanowires were hexagonal and aligned with their c-axis closely perpendicular to the sapphire substrate surface. The structural characteristics of the nanowiresgrown using the different catalysts/seeds were compared using scanning electron microscopyand X-ray diffraction. Their optical properties were assessed using microphoto-luminescence and confocal microRaman spectroscopy and compared. The nanowires exhibited a strong near band-edge related UV luminescence emission along with a defect related visible emission. The dependence of the luminescence as a function of incident excitation power and depth along the axis of the nanowires was studied. The wurtzite structure of the ZnO was confirmed from the Raman measurements. Two-dimensional mappings of the microphotoluminescence emission at different wavelengths and microRaman scattering from the nanowire samples were carried out using a confocal laser scanning microscope. This enabled the ability to correlate the near band-edge UV and visible emissions over the mapped area.     

Evaluation by Monte Carlo simulations of the power limits and bit-error rate degradation in wavelength-division multiplexing networks caused by four-wave mixing

Fiber nonlinearities can degrade the performance of a wavelength-division multiplexing optical network. For high input power, a low chromatic dispersion coefficient, or low channel spacing, the most severe penalties are due to four-wave mixing (FWM). To compute the bit-error rate that is due to FWM noise, one must evaluate accurately the probability-density functions (pdf) of both the space and the mark states. An accurate evaluation of the pdf of the FWM noise in the space state is given, for the first time to the authors' knowledge, by use of Monte Carlo simulations. Additionally, it is shown that the pdf in the mark state is not symmetric as had been assumed in previous studies. Diagrams are presented that permit estimation of the pdf, given the number of channels in the system. The accuracy of the previous models is also investigated, and finally the results of this study are used to estimate the power limits of a wavelength-division multiplexing system.     

Time-resolved diffraction and interference: Young's interference with photons of different energy as revealed by time resolution

We present time-resolved diffraction and two-slit interference experiments using a streak camera as a detector for femtosecond pulses of photons. These experiments show how the diffraction pattern is built by adding frames of a few photons to each frame. It is estimated that after 300 photons the diffraction pattern emerges. With time resolution we can check the speed of light and put an upper limit of 2 ps at our resolution to the time for wave function collapse in the quantum measurement process. We then produce interference experiments with photons of different energies impinging on the slits, i.e. we know which photon impinges on each slit. We show that for poor time resolution, no interference is observed, but for high time resolution, we have interference that is revealed as beats of 100 GHz frequency. The condition for interference is that the two pulses should overlap spatially at the detector, even if the pulses have different energies but are generated from the same pulse of the laser. The interference seems to be in agreement with classical theory at first sight. However, closer study and analysis of the data show deviations in the visibility of the interference fringes and of their phase. These experiments are discussed in connection with quantum mechanics and it may be concluded that the time resolution provides new data for understanding the longstanding and continuing arguments on wave-particle duality initiated by Newton, Young, Fresnel, Planck and others. A thought experiment is presented in the appendix to try to distinguish the photons at the detector by making it sensitive to colour.     

Synthesis,z-scan and degenerate four wave mixing characterization of certain functionalized photosensitive polyesters containing ortho-hydroxyazo chromophores

The preparation and NLO characterization of photosensitive polyesters containing azoaromatic residues in the molecular backbone, functionalized with orthohydroxy chromophores is presented. Samples were studied for its UV–vis absorption, FT-IR and intensity dependent nonlinear absorption properties. Nonlinear characterization was carried out with z-scan using frequency doubled, Q-switched Nd:YAG laser operating at 532 nm. The closed aperture z-scan spectra reveal the self defocusing effects of the samples with negative nonlinearity coefficient (n₂) showing values as high as −1.28 × 10⁻¹⁰ (esu) for certain samples and the corresponding third order susceptibility coefficient of the order of 29.9 × 10⁻¹² (esu). Degenerate four wave mixing technique was employed to substantiate the findings. The numerical fits show that the molecules exhibit reverse saturable absorption. A study of beam fluence dependence of nonlinear absorption coefficient (β_eff) has been presented. All phenomena indicate that molecules are reverse saturable absorbers whose optical limiting property gets enhanced with increasing conjugation length.     

Optoacoustic tomography: time-gated measurement of pressure distributions and image reconstruction

Optoacoustic imaging is a potential novel medical imaging technology to image structures in turbid media to depths of several millimeters with a resolution of some tens of micrometers. Thereby short laser pulses generate thermoelastic pressure waves inside a tissue, which are detected on the surface with a wideband ultrasonic transducer. Image reconstruction has the goal of calculating the distribution of the absorbing structures in the tissue. We present a method in which the acoustic field distribution is captured as a two-dimensional snapshot at the sample surface, using an optical-reflectance-based detection principle with a detection resolution of 20 mum. A new image reconstruction is accomplished by backprojection of the detected two-dimensional pressure distributions into the sample volume by use of the delay between the laser pulse and the time the snapshot was taken. Two-dimensional pressure-wave distribution and image reconstruction are demonstrated by simulations and experiments, in which small objects are irradiated with laser pulses of 6-ns duration. The method opens the possibility to irradiate the sample hidden in a light-scattering medium directly through the detector plane, thus enabling front-surface detection of the optoacoustic signals, which is especially important if structures close to the tissue surface have to be imaged. Reconstructed tomography images with a depth resolution of 20 mum and a lateral resolution of 200 mum are presented.     

Image transmission through a turbulent medium using a point reflector and four-wave mixing. 2: Characteristics of the system

The characteristics of the one-way image transmission system presented in Part 1 are investigated in detail [Appl. Opt. 22, 2192 (1983)]. First, a general expression of the expectation of the transmitted image is derived for turbulence that may be typical in image transmission in the horizontal direction. Then, with the help of numerical examples, the image quality is discussed in terms of the point spread function for both thin layer and uniformly distributed turbulence. It is shown that the image transmission system is effective especially where turbulence exists relatively close to the transmission plane.     

Particle image velocimetry analysis using an optically addressed spatial light modulator: effects of nonlinear transfer function

Optical processors for generating a two-dimensional squared autocorrelation function have been presented for postprocessing particle image velocimetry photographs of fluid flows. The incoherent-tocoherent conversion can be performed by an optically addressed spatial light modulator. The transfer function of these devices is far from linear and will influence the performance of the optical processor.

Two different transfer functions, characterizing the two main types of commercial optically addressed spatial light modulators as an analog and a binary transfer function, have been simulated digitally.

Results of numerical simulations on the influence of introducing these nonlinear transfer functions to the correlation function for particle image velocimetry analysis are presented.

     

Imaging through scattering media by parametric amplification of images: study of the resolution and the signal-to-noise ratio

An imaging scheme through scattering media in which parametric image amplification is used is presented. An image of a resolution chart through a solution of latex microspheres with an attenuation of 22 mean free paths is obtained with a resolution of 20 mum. The evolution of the signal-to-noise ratio with respect to the medium attenuation is studied and compared with a rough modeling of the imaging process.