Opposite Circular Polarization Research Articles

Two-color interference effect involving three-photon atomic excitation and four-wave mixing in crossed laser beams

Through multiphoton ionization measurements, the polarization effects in destructive quantum interference under three-photon resonant excitation have been studied. Recent observations [V. Peet, Phys. Rev. A 74, 033406 (2006)] have indicated that contrary to the well-known pattern of a total suppression of resonance excitation, the destructive interference becomes incomplete if three-photon transition is driven by crossed beams with orthogonal polarization planes. These observations have been tested for a more general case of two-color excitation and very similar polarization-dependent anomalies in the interference character have been registered. It has been shown that the destructive interference is modified and the resonance excitation does occur if two crossed laser beams have opposite circular polarizations. The pressure-induced evolution of the uncanceled ionization peaks has the ratio of blue shift to width close to 0.5 exactly as it is known for resonance ionization peaks registered under excitation by counterpropagating laser beams.

Photo-induced birefringence in semiconductors compared with optical fibers

We analyze the photon-induced birefringence in semiconductors based on pump-probe setups, within the semiconductor Bloch equations formalism and the Luttinger–Kohn model for the band structure. When the pump and probe pulses are well separated in time, the anisotropic momentum space filling of the photo-excited electrons is the only mechanism causing the induced birefringence. The birefringence ratio is then Br=34 for pump and probe having perpendicular vs. parallel linear polarizations. This ratio is Br=16 for opposite vs. identical circular polarization. When the pump and probe pulses overlap in time, these birefringence ratios become Br=13 for linear polarizations and Br=19 in case of circular polarizations. These predictions differ markedly from those for optical fibers.

Excitation of vortices in semiconductor microcavities

We predict that the transverse electric-magnetic polarization splitting of exciton polaritons allows a simple technique for the generation of polariton vortices of winding number 2 in semiconductor microcavities. The vortices can be excited by circularly polarized light having no orbital angular momentum and observed as phase vortices in the opposite circular polarization or directly as linear polarization vortices. The prediction is explained by a simplified analytical linear model and shown fully with a numerical model based on the Gross-Pitaevskii equations, which includes the nonlinear effects of polariton-polariton interactions.

Cavity-quantum-electrodynamics entangled photon source based on two truncated Rabi oscillations

We discuss a cavity-QED scheme to deterministically generate entangled photons pairs by using a three-level atom successively coupled to two single longitudinal mode high-Q cavities presenting polarization degeneracy. The first cavity is prepared in a well defined Fock state with two photons with opposite circular polarizations while the second cavity remains in the vacuum state. A half-of-a-resonant Rabi oscillation in each cavity transfers one photon from the first to the second cavity, leaving the photons entangled in their polarization degree of freedom. The feasibility of this implementation and some practical considerations are discussed for both, microwave and optical regimes. In particular, Monte Carlo wave function simulations have been performed with state-of-the-art parameter values to evaluate the success probability of the cavity-QED source in producing entangled photon pairs as well as its entanglement capability.

A Nematic Liquid Crystal as an Amplifying Replica of a Holographic Polarization Grating

ABSTRACT Holographic gratings of different periods were recorded in the photopolymer by two laser beams with opposite circular polarization and a nematic liquid crystal was brought in contact with them. The anchoring conditions for the nematic were modulated in a similar way and an induced, field controlled replica manifested a characteristic asymmetric diffraction for the beams of opposite circular polarizations with dramatically enhanced diffraction efficiency. In fact the situation is more complicated than expected and our modelling of the director distribution for a finite anchoring strength explains the experimental data by considering an additional modulated structure at the polymer-nematic interface.

A NEMATIC REPLICA OF A HOLOGRAPHIC POLARIZATION GRATING: MODELING OF THE IDEAL AND A CORRUGATED SURFACE

When a nematic liquid crystal layer is brought in contact with a photosensitive polymer, on which a holographic grating is recorded by two laser beams with opposite circular polarization, due to a spatial modulation of the direction of the local optical axis, the photopolymer modulates the anchoring conditions for NLC in a similar way. As a result, a similar grating is induced in NLC with the enhanced diffraction efficiency and specific diffraction properties controlled by electric field.6 Here we show that, even in the case of a typical finite value of the anchoring energy, the numerical modeling confirms the characteristic asymmetric diffraction of the circularly polarized reconstructing beam and exchange by energy between the zero and first-order diffracted beams. Some other properties, such as reduced diffraction efficiency, appearance of higher diffraction orders and too-large optical phase retardation of a nematic cell at the areas occupied by gratings, can also be modeled by assuming some corrugation of the surface.

Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer

We create planar-periodic alignment in nematic liquid crystal (LC) cell by using a command layer of azo-dye molecules directly deposited on the cell substrates and exposed with two interfering laser beams of opposite circular polarizations. Permanent high efficiency polarization gratings are thus created. The diffraction efficiency of those gratings is controlled by a uniform electric field applied across the cell. The electro-optical properties of such polarization gratings are studied. Obtained gratings can be used for electrically controlled discrimination and detection of polarized components of light.

Polarization-modulated magnetic soft-x-ray transmission microscopy

An adjustable aperture element has been integrated into the full-field soft-x-ray microscope at the Advanced Light Source to select either the right or left elliptically polarized x rays emitted at an inclined angle from a bending magnet. Magnetic contrast recorded at the Fe L3 edge in a 59-nm-thin Gd25Fe75 layer can be modulated and scales with the degree of circular polarization in agreement with theoretical calculations. Nonmagnetic background contributions can be reduced and magnetic contrast is enhanced by comparing two images taken with opposite circular polarization. The fast modulation speed of this technique allows for lock-in recording schemes in high-resolution magnetic soft-x-ray microscopy.

Electric field controlled polarization grating based on a hybrid structure “photosensitive polymer-liquid crystal”

An investigation was carried out on a hybrid structure consisting of a photosensitive polymer in contact with a nematic liquid crystal (NLC) layer. Before assembling the structure, holographic gratings of different periods were recorded in the photopolymer by two laser beams with opposite circular polarization. Such gratings are known for their unique asymmetric diffraction in case of elliptically polarized light and can be used in optical polarization devices. Due to a spatial modulation of the direction of the local optical axis, the photopolymer modulates the anchoring conditions for NLC in a similar way. As a result, a new grating is induced in NLC with the enhanced diffraction efficiency (in a particular experiment it was four orders of magnitude higher than that of the photopolymer). The NLC grating also shows the characteristic asymmetric diffraction for the reconstructing beams of opposite circular polarizations. The diffraction efficiency is easily controlled by an external electric field.

Cavity QED quantum phase gates for a single longitudinal mode of the intracavity field

A single three-level atom driven by a longitudinal mode of a high-Q cavity is used to implement two-qubit quantum phase gates for the intracavity field. The two qubits are associated to the zero-and one-photon Fock states of each of the two opposite circular polarization states of the field. The three-level atom yields the conditional phase gate provided the two polarization states and the atom interact in a $V$-type configuration and the two photon resonance condition is fulfilled. Microwave and optical implementations are discussed with gate fidelities being evaluated against several decoherence mechanisms such as atomic velocity fluctuations or the presence of a weak magnetic field. The use of coherent states for both polarization states is investigated to assess the entanglement capability of the proposed quantum gates.

Measurement of Zeeman pattern with VLBI

Magnetic field is believed to play an important role in the evolution of compact HII regions where massive stars are formed. In fact, it exists very few methods to measure it. One of the best method used, is based on the Zeeman effect: under the effect of the magnetic field, a line can be split into two separated lines of opposite circular polarization. The separation in frequency between the lines is directly proportional to the intensity of the magnetic field. One of the difficulty of the method, is to made a correct identification of the Zeeman pairs. The very high resolution achievable by VLBI allows us to demonstrate whether the pairs are spatially coincident (as expected in true Zeeman pairs) or not. The OH emission traces the densest pockets of the neutral gas environment. In addition, because of its non zero electronic angular momentum, the OH radical is a natural magnetometer allowing us to determine both the intensity and the direction of the magnetic field in the outer layers of the compact HII regions. In this poster I reported on the possibilities of VLBI observations of OH masers in the first excited state (J = 5/2) lying at 6 GHz.

Energy renormalization and binding energy of the biexciton

We deduce the energy renormalization of the biexciton resonance in ZnSe quantum wells from polarization and intensity dependent pump-probe spectroscopy in the coherent regime. We confirm that the biexciton renormalization is exactly what one would expect from a compound particle of two excitons in opposite circular polarization states. Since the biexciton binding is measured with respect to the exciton resonance one has to consider the renormalization of the latter resonance as well. We present an exact method to determine the real biexciton binding energy using the low-excitation extrapolation of the exciton-to-biexciton transition energy.

One-dimensional bichromatic standing-wave cooling of cesium atoms

We report the results of an experimental study on the interaction of cooled cesium atoms with the optical field of two standing waves having different wavelengths (852 and 894 nm) and opposite circular polarizations. The spatial modulation of the superposition of the two optical potentials and the polarization properties of this configuration are expected to produce cooling of the atoms and a spatial modulation of their density with the periodicity of the beat of the two wavelengths. We performed temperature measurements of the cesium sample and observed the density distribution of the atoms for several configurations of the standing wave by means of time-of-flight absorption imaging and fluorescence imaging techniques. Experimentally we could not observe a pronounced density modulation on the length scale of the superperiod. Reasons for this are revealed by a one-dimensional numerical simulation including the complexity of the full Zeeman structure of the cesium atoms. That simulation reproduces the experimental results for the temperatures and spatial confinement.

Polarization holographic techniques: a method to produce diffractive devices in polymer dispersed liquid crystals

We report a study of holographic gratings recorded by exposing a homogeneous mixture of prepolymer and liquid crystal to polarization light patterns. The method is based on the alignment control of the liquid crystal domains during the photo-induced polymerization process. In contrast, the usual methods to write diffraction gratings in polymer dispersed liquid crystals are based on an intensity holographic technique that controls the polymerization process. The polarization patterns here are made using different geometries: in particular we report results from exposure to polarization patterns realized by the superposition of two waves having orthogonal linear polarizations, or two waves having opposite circular polarizations. Analyses of the recorded gratings are reported.

Two-color stabilization of atomic hydrogen in circularly polarized laser fields

Dynamic stabilization of atomic hydrogen against ionization in high-frequency single- and two-color, circularly polarized laser pulses is observed by numerically solving the three-dimensional, time-dependent Schr\"odinger equation. The single-color case is revisited and numerically determined ionization rates are compared with both, exact and approximate high-frequency Floquet rates. The position of the peaks in the photoelectron spectra can be explained with the help of dressed initial states. In two-color laser fields of opposite circular polarization the stabilized probability density may be shaped in various ways. For laser frequencies $\omega_1$ and $\omega_2=n\omega_1$, $n=2,3,...$ and sufficiently large excursion amplitudes $n+1$ distinct probability density peaks are observed. This may be viewed as the generalization of the well-known ``dichotomy'' in linearly polarized laser fields, i.e, as ``trichotomy,'' ``quatrochotomy,'' ``pentachotomy'' etc. All those observed structures and their ``hula-hoop''-like dynamics can be understood with the help of high-frequency Floquet theory and the two-color Kramers-Henneberger transformation. The shaping of the probability density in the stabilization regime can be realized without additional loss in the survival probability, as compared to the corresponding single-color results.

Time-Resolved Photoluminescence Cross-Correlation Measurements on InAs Quantum Dots

We have studied recombination and relaxation dynamics in InAs quantum dots by means of photoluminescence cross-correlation techniques with sub-picosecond time-resolution. We have determined the relative radiative efficiencies for carrier recombination from different energy levels of the quantum dots and show that the radiative efficiency decreases with increasing transition energy, indicating that carrier access to non-radiative recombination centres increases for higher-energy states inside the dots. Cross-correlation measurements with both excitation beams of the same circular polarization and with the beams having opposite circular polarization are shown to give direct insight into the spin relaxation dynamics in the quantum dots and the wetting layer.

Storage ring free electron laser: operation with two undulators having opposite circular polarizations

In Storage Ring Free Electron Lasers two undulators, having opposite helical polarizations and arranged as an Optical-Klystron, may be exploited to obtain linearly polarized radiation, without additional problems associated with mirror degradation, due to the higher on axis harmonic emission. In this paper we explore the dynamical behavior of this device and discuss possible configurations allowing such a possibility.

OH masers and magnetic fields in the bipolar outflow source W75N

MERLIN polarization measurements of 1665-,1667- and 1720-MHz OH masers in the bipolar outflow source W75N are presented. The OH masers are distributed over a region 5 arcsec across (10 000 au at 2 kpc), which is elongated in a north-west‐ south-east (NW ‐SE) direction. The distribution is interpreted in terms of a rotating disc orthogonal to the bipolar outflow that is seen in CO and H2. A compact cluster of OH masers with a large velocity spread appears to mark the source of the outflow. Most of the OH masers detected have strong circular polarization up to 100 per cent. Seven Zeeman groups are identified, including the first example of a complete Zeeman triplet (two s-components and one p-component). The field strength measured from Zeeman pairs of opposite circular polarization ranges from 2 8 toa 8 mG, with a field reversal on opposite sides of the disc. This is interpreted in terms of a toroidal component of the magnetic field in the disc. Linear polarization is also detected in many of the OH masers with some 1665-MHz masers showing up to 100 per cent linear polarization. The polarization position angles are preferentially along the direction of the bipolar outflow or perpendicular to it. The polarization data lend support to the twisted magnetic field model originally proposed by Uchida & Shibata.

Cavity-mediated dark-state cooling without spontaneous emission

We extend the concept of dark state cooling to enclose the fields in a high-$Q$ resonator. Choosing a sufficiently large detuning between the atomic transition and the driving fields, spontaneous emission during the cooling process is largely suppressed and the energy dissipation occurs dominantly via cavity decay. For a ring cavity with two degenerate counterpropagating waves of opposite circular polarization, the final dark state is identical as in free space. However, we find a strong cavity mediated force with a large capture range as a precooling mechanism. In this scheme spontaneous emission is only important for the final jump to the dark state in the dynamics and the reabsorption problem is strongly reduced. In principle the scheme could also be applied to molecules with a suitable transition.

Dynamical behaviour of free electron laser devices operating with two undulators having opposite circular polarizations

Optical-Klystron free electron lasers operating with undulators having opposite circular polarizations are characterized by a spontaneous emission spectrum which does not exhibit the characteristic interference pattern. The use of the Madey theorem may allow the conclusion that, for such configuration, the dispersive section does not provide any gain enhancement. In this paper we analyse the problem from a dynamical point of view and clarify how the optical field evolve, what is the role of the bunching and how the dynamics of the system can be followed up to the saturation.