Polarization-sensitive Detection Research Articles

An external electrooptic sampling technique based on the Fabry-Perot effect

Electrooptic sampling has proved itself as the best technique for the measurement of high-speed electronic signals. This is due to its high bandwidth and noninvasiveness. Standard sampling geometries rely on a rotation of polarization within an electrooptic material and subsequent polarization sensitive detection. In this paper, we present results using a simple technique which relies on Fabry-Perot resonances inside the electrooptic material to produce an amplitude modulation. This simplifies the experiment considerably.

Structure and dynamics from polarized parent/product photofragmentation spectroscopy

Energy-level selected nitrosyl chloride (ClNO) was rotationally polarized (aligned/oriented) and photodissociated at 355 nm. Polarization sensitive detection of the NO(2Π) product was used to measure the magnitudes and relative phases of the transition amplitudes (T-matrix elements) for the coherent excitation of continuum wavefunctions representing the dissociating molecule on the excited potential energy surface. Rotational state-populations were also measured and a smaller number of ∣NΩ) levels were found to be occupied than in non-state-selected, jet-cooled experiments. The theoretical formalism necessary for the extraction of the T-matrix elements from fully polarized experiments is also presented. Finally, hexapole focusing curves demonstrate excellent ∣JK) energy-level selection for the parent ClNO and the NO(2Π) photofragment angular distribution demonstrates for the first time dipole orientation and rotational alignment, i.e., ∣M∣ selection, of a state-selected asymmetric top by an electrostatic hexapole.

Application of a passive polarimetric infrared sensor in improved detection and classification of targets

Automatic detection and recognition of targets by means of passive IR sensors suffer from limitations due to lack of sufficient contrast between the targets and their background, and among the facets of a target. In this paper the results of a suite of polarization-sensitive automatic target detection and recognition algorithms on sets of simulated and real polarimetric IR imagery are presented. A custom designed Polarimetric IR (PIR) imaging sensor is used for collecting real polarimetric target data-three of the four Stokes parameters under a variety of conditions. Then a set of novel algorithms are designed and tested that uses the target and background Stokes parameters for detection, segmentation and classification of targets. The empirical performance results are obtained in terms of the probabilities of detection, false alarm rate, segmentation accuracy, and recognition probabilities as functions of number of pixels on target, aspect and depression angles and under several background conditions (clutter densities) on the polarimetric and non-polarimetrirc data. These results show that a noticeable improvement over the non-polarimetric ATR can be achieved.

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

Emission of Submillimeter Electromagnetic Waves by Coherent Phonons.

We report on the first observation of the emission of electromagnetic radiation from coherent lattice vibrations in a semiconductor. Coherent optical phonons are excited by ultrashort laser pulses in single-crystal tellurium. THz frequency radiation emitted by the Dember-field-driven phonons is detected by time-resolved THz-emission spectroscopy. The measurements are complemented by optical pump-probe experiments with a polarization-sensitive detection scheme utilizing the symmetry of the Raman tensor.

Near-field magneto-optical microscopy

A scanning tunneling optical microscope (STOM) operating in total reflexion condition with variable incident angle and polarization sensitive detection of the evanescent mode has been developed to study near-field magneto-optical properties of thin magnetic films. The sample is deposited on the external face of a prism and illuminated in total reflexion conditions with linearly polarized laser light. The evanescent mode close to the surface is detected with a tip-ending monomode optical fiber connected at its other end to a photomultiplier tube equipped with a light-polarization analyzer. The polarization sensitivity of the whole system, which was found to depend on the tip condition, was characterized on the bare prism with s- and p-polarized excitations. The magneto-optical effect in the evanescent mode is measured through a lock-in amplifier by modulating the external magnetic field produced by a coil surrounding the tip. With this set-up we have mainly studied a dielectric garnet film exhibiting perpendicular magnetization. The images, obtained on this sample by measuring the magneto-optical effect under very low amplitude of the external magnetic field modulation, show up submicronic details due to magnetic domain wall motion.

ChemInform Abstract: Dynamics of Third‐Order Nonlinearity of Canthaxanthin Carotenoid by the Optically Heterodyned Phase‐Tuned Femtosecond Optical Kerr Gate.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Dynamics of third-order nonlinearity of canthaxanthin carotenoid by the optically heterodyned phase-tuned femtosecond optical Kerr gate

It is shown that polarization sensitive (optically heterodyned) detection combined with π/2 optical phase biasing between the optical Kerr gate signal and the local oscillator signal provides an effective method for the investigation of dynamics of third-order nonlinear optical processes. The method permits selective enhancement of either the real or the imaginary component of the nonlinear optical response of the medium under study. Both the magnitudes and the signs of the real and the imaginary components of the complex third-order optical susceptibility, contributing to the Kerr gate phenomenon, can thus be determined independently. The method is used to investigate the third-order nonlinear optical response of solutions of canthaxanthin (4,4′-dioxo-β-carotene) in tetrahydrofuran. With the use of ultrafast 60 fs laser pulses at 620 nm, the instantaneous, coherent part of the response is separated from an incoherent, time-delayed response. The real and the imaginary components of the instantaneous complex second hyperpolarizability γ are derived. The analysis of the delayed part of the signal suggests that it originates in an effective χ(5) nonlinear process due to the change of linear susceptibility after population of a two-photon excited state. However, an unusual feature observed is that the two-photon pumped excited state does not produce a significant change of linear susceptibility, but it is a lower state, most probably the 2 1Ag state, subsequently populated by nonradiative relaxation which produces a dominant change in linear susceptibility responsible for the incoherent nonlinear optical response. From a theoretical fit of the temporal behavior of the delayed nonlinear response, the rates of nonradiative population and subsequent decay of the 2 1Ag state are derived. These rates are in qualitative agreement with published spectroscopic results for this class of compounds.

Techniques for measuring small changes in the intensity of a pulsed laser

Several techniques are described that compensate for the pulse-to-pulse laser fluctuations that occur when small absorption or gain measurements are taken with pulsed lasers. The best approach tested used two matched diodes that were mounted in a bridge circuit with their bias batteries. One diode was illuminated by a single beam, and a second was illuminated by a reference beam. The bridge circuit output forms the difference in the two laser intensities. Using pulsed lasers with 8-ns widths, we observed that the error in subtraction was 0.01% of the pulse area. This was only 1.5 times the shot noise of the pulses. The technique is examined for application to Raman-induced Kerr-effect spectroscopy, in which difficulties arise because of the use of polarization-sensitive detection.

A Faraday Laser Magnetic Resonance spectrometer for spectroscopy of molecular radical ions

We describe a mid-infrared Laser Magnetic Resonance (LMR) spectrometer which is based on a cryomagnet and a sealed-off CO-laser. Faraday rotation combined with a multireflection cell is used for polarization sensitive detection of molecular radical ions. Experimental details and first spectroscopic examples are given.