Polarized Probe Beam Research Articles

Observation of nonstationary effects in saturation spectroscopy

We have studied the nonstationary effects in saturated absorption spectroscopy of the 87Rb D2 line. Varying the size of the σ+ polarized pump laser beam, we observed saturated absorption spectra for the σ± polarized probe beam. For equal polarizations of the pump and probe beams, we found that the resonance signal for the Fg=1→Fe=2 line, and the crossover lines between Fg=1→Fe=2 and Fg=1→Fe=1 (and 0) lines increased to a greater extent than the others. This observation can be understood from the calculated time evolution of the populations of the ground-state sublevels by means of a rate equation model. We also compared experimental data for other conditions with the calculated results. We found good agreement between the calculated results and the data.

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

AbstractThe techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N2Ar with concentrations varying from 0 to 79% N2. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.

Faraday rotation in photoexcited semiconductors: A composite-exciton many-body effect

We assign Faraday rotation in a photoexcited semiconductor with bright excitons all having the same spin, to the exciton composite nature through the ``Pauli interactions,'' i.e., carrier exchanges, between the real excitons present in the sample and the virtual excitons coupled to the ${\ensuremath{\sigma}}_{\ifmmode\pm\else\textpm\fi{}}$ parts of the linearly polarized probe beam. While direct Coulomb interactions scatter bright excitons into bright excitons whatever their spins are, Pauli interactions scatter bright excitons into bright excitons if they have the same spin only. This makes Pauli interactions entirely responsible for the Faraday rotation: Indeed, the refractive index differenc between ${\ensuremath{\sigma}}_{+}$ and ${\ensuremath{\sigma}}_{\ensuremath{-}}$ photons comes from processes in which the virtual bright exciton, which is created by the unabsorbed probe photon, and the one which recombines to regenerate this probe photon, have zero or one common carrier. The new many-body theory for interacting composite excitons we have constructed provides quite appropriate tools to write this difference in terms of the physical parameters of the problem, namely, the photon detuning and the density of the excitons present in the sample. The multiarm ``Shiva'' diagrams that we have recently introduced to visualize $N$-body exchanges between composite excitons, make transparent the physics involved in the various terms. This work also shows the interesting link that exists between Faraday rotation and the exciton optical Stark effect we studied long ago.

Anisotropic photonic gratings formed in photocross-linkable polymer liquid crystals

We demonstrated anisotropic photonic gratings in photocross-linkable polymer liquid crystals irradiated with three mutually coherent beams. Both birefringence and surface relief modulation were induced by reorientation and/or migration of mesogenic molecules. Anisotropy and optical properties of photonic gratings were strongly dependent on the combination of polarization states of the three beams. Atomic force microscopy revealed that topological patterns of gratings formed according to interference patterns of the three beams and diffraction efficiencies of recorded gratings depended on polarization direction of the linearly polarized probe beam. These diffraction properties including efficiency and optical anisotropy could be explained by the theory based on Jones calculus.

Heterodyne double-channel polarimeter for mapping birefringence and thickness of flat glass panels

A new cross-polarized heterodyne optical technique is developed for two-dimensional (2D) simultaneous mapping of both birefringence and thickness variations in large flat glass panels commonly used in liquid-crystal displays (LCDs). Weak depolarization of a linearly polarized probe beam due to glass birefringence is detected by means of heterodyne mixing of the two cross-polarized and frequency shifted waves generated by Zeeman-type laser. Amplitude variations of the transmitted laser beam due to interference of the partial waves reflected from the both sides of a sample provide information about glass thickness. Measurements are being performed at the intermediate frequency of 2.3MHz, providing several orders of magnitude higher speed of data acquisition with respect to traditional polarimeters. That high speed of measurements makes it possible to perform quality assessment of LCD glass panels not only in few randomly chosen points as it was in common practice before but to obtain entire 2D maps of both birefringence and thickness variations with millimeter scale spatial resolution. The medium-scale prototype of the LCD glass inspection system is developed and tested. Design and performance of the prototype are described.

Effect of polarization of THz pulse and probe pulse on THz electro-optic detection

We studied the effect of polarization of terahertz pulse and probe beam on terahertz detection in optically isotropic detection crystals using wave coupling theory of linear electro-optic effect. The study results show that the effect is remarkable, and the detection can get its largest sensitivity when the probe beam propagates along (110) direction which is most suitable for terahertz detection. Some of the study results are in good agreement with the previously published experimental results. So our study is helpful for the design of electro-optic detectors.

Velocity map ion imaging of H atoms from the dissociation of HCO ( [formula omitted]) using Doppler-free multi-photon ionization

Velocity map ion imaging using Doppler-free 2+1 multi-photon ionization at λ = 243.1 nm has been applied to monitor the nascent H atoms from the predissociation of HCO ( A ∼ 2 A ″ ) in the (0, 0, 10 1) vibrational state. Employing two balanced, counterpropagating circularly (σ + and σ −) polarized probe beams, complete velocity map images were accumulated with every laser shot without having to scan the probe wavelength over the strongly Doppler-broadened line profile. The H–CO bond dissociation energy, CO internal energy distribution, and anisotropy parameter are presented and compared with literature data.

Vector-component isolation of an arbitrary modulating electric field in zincblende electro-optic probes

Analysis of the field-induced linear birefringence in zincblende crystals shows that one can obtain complete isolation of a single vector component of an arbitrary modulating electric field. For an optical probe beam path aligned parallel to the [110] direction and an optical probe beam polarization aligned parallel to the [110] direction, the field-induced birefringence occurs only for the component of the modulating electric field aligned parallel to the [110] direction. Measurements using a modulating electric field with known polarization and electro-optic probes machined from (110) gallium arsenide wafers demonstrate an alignment-limited isolation between orthogonal modulating electric field components of 17 dB.

Spin dynamics and lifetime of exciton polaritons in CuCl

In CuCl, biexcitons may be excited from the semiconductor ground state by two circularly polarized exciton polaritons of opposite helicity. We use this property to develop a new method in order to determine the spin-relaxation dynamics by means of non-degenerate pump–probe experiments. A pump beam first excites a spin-polarized exciton population. Then, we measure the induced absorption of a circularly polarized probe beam at the exciton biexciton transition. It depends on its polarization and the induced absorption dynamics gives information on the spin state of the exciton polariton population. When comparing results obtained in a (σ +σ +) configuration with that of a (σ +σ −) configuration, we determine the time evolution of the pseudo-spin of the exciton polaritons as well as their lifetime.

Probe spectroscopy in an operating magneto-optical trap: The role of Raman transitions between discrete and continuum atomic states

We report on cw measurements of probe beam absorption and four-wave-mixing spectra in a $^{85}$Rb magneto-optical trap taken while the trap is in operation. The trapping beams are used as pump light. We concentrate on the central feature of the spectra at small pump-probe detuning and attribute its narrow resonant structures to the superposition of Raman transitions between light-shifted sublevels of the ground atomic state and to atomic recoil processes. These two contributions have different dependencies on trap parameters and we show that the former is inhomogeneously broadened. The strong dependence of the spectra on the probe-beam polarization indicates the existence of large optical anisotropy of the cold-atom sample, which is attributed to the recoil effects. We point out that the recoil-induced resonances can be isolated from other contributions, making pump-probe spectroscopy a highly sensitive diagnostic tool for atoms in a working MOT.

Electron-spin polarization and its relaxation probed by femtosecond laser absorption

Based on the rate equations of a two_level system, the analytical solutions of r elaxation of carrier populations with spin_up and spin_down polarization are obt ained under the condition of incomplete initial spin polarization. In the small signal regime, the saturated absorption_change expression of right_ and left_cir cularly polarized probe beam is given. The parameter, initial degree of spin pol arization of electron population, appears in the expression. Therefore, the init ial degree of spin polarization of electron population, which is a very importan t key parameter in the transport of spin polarization, can be extracted by fitti ng the expression to the saturated absorption trace obtained experimentally. Fin ally, the initial degree of spin polarization of photo_injected electron populat ion and its relaxation time constant in GaAs/AlGaAs multiple quantum wells are a cquired in experiments.

Circular dichroism effect for linear molecules induced by a resonant circularly polarized pumping optical field.

In this paper we propose and discuss the laser-induced circular dichroism (LICD) effect, which is expected to occur in linear molecules pumped by a strong circular resonant light beam. The effect is to be detected via the absorption of a weak circularly polarized probe beam on another transition. Analogous to the external magnetic field in magnetic circular dichroism the resonant circular polarized pumping optical field can induce the nonzero antisymmetric rotational polarizabilities of a linear molecule, and cause the LICD effect. LICD contains three distinct contributions from M-dependent splittings of the sublevels mid R:JM due to the ac Stark effect, from the differences of Boltzmann statistical distributions among the ground state sublevels mid R:JM due to the ac Stark splittings, and from the changes of occupation probability in rotational sublevels mid R:JM due to the pumping effect. The fundamental formulas for the above three terms of LICD have been deduced by the density matrix method. As an example, the LICD for CO molecules have been calculated. The results indicate that in comparison with the rotationally resolved magnetic vibrational circular dichroism experiment, LICD may be measurable and form a basis of a different kind of CD spectroscopy.

Long-lived coherent acoustic waves generated by femtosecond light pulses

We report on photoexcitation of coherent longitudinal acoustic phonons in single-crystal cuprate thin films on epitaxialy matched substrates. The photoinduced reflectance oscillations are unusually long lived; in some samples we could easily resolve them for hundreds of periods. We studied the effect of varying a number of parameters, including the film doping level, thickness, and temperature, as well as the pump and probe beam wavelength, power, polarization, and incidence angle. We account quantitatively for the oscillation period, dispersion, phase, and decay of amplitude with time.

UNPOLAR AND POLAR HOLOGRAPHIC GRATING RECORDING BY CIRCULARLY POLARIZED LIGHT ON PHOTOANISOTROPIC AZOBENZENE LANGMUIR–BLODGETT FILMS

A comparative study of photo-induced optical anisotropy was carried out on Langmuir–Blodgett films prepared from five different amphiphilic azobenzene derivatives. The anisotropy was induced by a linearly polarized pump beam of an Ar-ion laser at λ=514 nm and monitored by a linearly polarized probe beam of a He–Ne laser at λ=633 nm . Large optical anisotropy up to 0.35 has been induced. Holographic gratings were recorded by two left circularly polarized beams of the Ar-ion laser and time evolution of the first-order diffraction efficiency of the probe beam was recorded in an automatic regime. In such a geometry, the spatial modulation of refraction index reaches 0.14. The experimental data is consistent with a model for a photo-induced collective chromophore realignment in the heterogeneous domain structure of the films studied. The influence of an electric field of the corona discharge on the grating recording process was also studied and polar diffraction gratings have been prepared by a spatially periodic photo-poling process.

Laser-induced birefringence by using a linearly polarized field in Na D 1 line and its application to laser frequency stabilization

We have observed laser-induced birefringence (LIB) in the Na D 1-line using a linearly polarized probe beam and a linearly polarized pump beam. The pure dispersion LIB spectrum could be achieved using the differential technique. The dispersion LIB spectrum could be effectively employed in the frequency stabilization of the ring dye laser. The experimentally obtained spectra has been compared to and analyzed with theoretical results which considered a velocity-selective optical pumping effect of a four-level atomic system.

Photo-induced anisotropic photoelectric response in oriented bacteriorhodopsin films

The photo-induced anisotropic properties of the chromophore protein bacteriorhodopsin (bR) are of interest for possible applications in optical image processing and spatial filtering. Here we report experimental results on the photo-induced anisotropic photoelectric response of dried oriented bR films. A polarization dependent photovoltage is measured across an ITO/bR/ITO photodetector. The anisotropy is introduced by a 632.8 nm linearly polarized pump beam and is detected with a 594.1 nm polarized probe beam. The influence of the pump light intensity on the photovoltage anisotropy has also been investigated. A model, based on the polarization dependent photoselection of the bR molecules reasonably explains the behavior of the measured data. The observed effect can be used to construct a polarization sensitive bR-based bio-photoreceiver.

State-to-state differential cross sections for spin–multiplet-changing collisions of NO(X 2Π1/2) with argon

Rotational state-resolved differential cross sections (DCS) for spin–multiplet-changing collisions of NO(X 2Π1/2→X 2Π3/2) with Ar are presented and compared to results from previous experimental and theoretical studies performed on the NO/Ar system. A crossed molecular beam apparatus coupled with velocity-mapped ion imaging was used to measure complete (θ=0°–180°) DCS for scattering of NO(X 2Π1/2,j=0.5) into NO(X 2Π3/2,j′) rotational states ranging from j′=1.5 to j′=12.5. Scattered products were detected by state-selective ionization using (1+1′) resonance-enhanced multiphoton ionization via the A 2Σ+ state. State-to-state DCS were extracted in the center-of-mass frame of reference for energy transfer at a center-of-mass collision energy of ∼530 cm−1. Studies performed using horizontally and vertically polarized excitation laser beams yielded DCS which were remarkably similar, indicating that state-to-state scattering for this system is insensitive to probe beam polarization. Experimentally determined angular scattering distributions show primarily forward scattering for low-energy rotational states (j<7.5), with side- and back-scattering increasing with product angular momentum. The scattering results are compared and contrasted to results from earlier experimental investigations and to theoretical results from quantum close-coupling calculations based on ab initio coupled cluster CCSD(T) potential energy surfaces.

Polarization spectroscopy of a conducting surface

A method of polarization spectroscopy for a conducting surface characterized by a high sensitivity to the state of the surface and its transition layer is proposed. The method uses excitation of surface electromagnetic waves (SEWs) by a linearly polarized probe beam incident on the surface under study and compensation for the phase shift between the p and s components arising upon the SEW excitation. The applicability and competitiveness of the method in both the visible and the IR range is established. The method is tested on an LEF-3M ellipsometer in the visible range by studying the oxidation process in an evaporated copper film.

Picosecond transient dichroism and birefringence spectroscopy on pheophorbide-a molecules in solution

Picosecond transient dichroism and birefringence on pheophorbide-a in solution is investigated using a pump-and-probe experiment. In this experiment the sample is pumped resonantly to the S0-S1 transition by linear polarized light and probed by a continuum polarized 45° with respect to the pump beam polarization plane. The photoselective excitation of the sample generates a transient anisotropy resulting in a rotation of the probe beam polarization plane due to light-induced dichroism and in the generation of elliptically polarized light due to light-induced birefringence. The polarization state of the probe beam after passing the sample is analysed by means of angle-resolved measurements carried out at different time delays. From these measurements (i) the light-induced changes of the absorptive index and (ii) the light-induced changes of the refractive index are obtained separately. The results are in good agreement with calculations using Kramers-Kronig transforms. Moreover, (iii) the anisotropy spectrum of the absorptive index is measured to obtain information about the orientation of the transition dipole moments of the excited-state transitions and (iv) the orientation relaxation time of the S1-state is determined from the decay time of the transient anisotropy.

Measurement and calculation of the orientation dependence of terahertz pulse detection in ZnTe

We have performed measurements and calculations of the efficiency of terahertz (THz) detection in ZnTe as functions of the angle between the THz polarization and the crystal (001) axis of the angle between the THz polarization and the probe-beam polarization. We find that, for angles of 0° and 90° between the probe and THz polarization, the THz detection process is most efficient, whereas the commonly used angle of 45° gives less than optimal results. In addition, we show how the polarization direction of a THz beam can be found if the direction of the crystal (001) axis is known. Our results are valid for ZnTe and other zinc-blende crystals such as GaP.