Effect of Strong Pump Depletion on Two-beam Coupling in Photorefractive BaTiO3 at 442 nm

Abstract

Photorefractive BaTiO₃ crystal illuminated at 442 nm gives a large initial scattering level, which leads to strong pump depletion into beam fanning. Studies on the time development of beam fanning show a temporal shift of the fanned beam maximum towards the c-axis direction. The effect on the signal beam amplification of the strong pump depletion into beam fanning has been investigated, revealing different temporal development of the amplified signal for various cross-angles. Our observations on beam fanning in the presence and in the absence of a signal reveal that there is appreciable coupling between the fanned beam and the signal.     

Reduction of Quantum Entropy by Reversible Extraction of Classical Information

Abstract

We inquire under what conditions some of the information in a quantum signal source, namely a set of pure states ψ_a emitted with probabilities p _a, can be extracted in classical form by a measurement leaving the quantum system with less entropy than it had before, but retaining the ability to regenerate the source state exactly from the classical measurement result and the after-measurement state of the quantum system. We show that this can be done if and only if the source states ψ_a fall into two or more mutually orthogonal subsets.     

Angular dependence of the signal in nonlinear polarization spectroscopy under conditions of noncollinear excitation by partially polarized radiation

The light-induced anisotropy tensor of initially isotropic media is calculated for the case of partially nominally polarized radiation. An analytic expression is obtained for the first time for the signal in a noncollinear nonlinear polarization spectroscopy system with a polarization-incoherent pump wave. Pis’ma Zh. Tekh. Fiz. 25, 79–82 (December 26, 1999)     

Collapse-revival Phenomenon in the Process of k-photon Down Conversion with Superpositions of Number States

Abstract

We calculate numerically the time evolution of the mean photon number in the process of k-photon down conversion process with quantized pump. The pump mode was supposed to be initially in a superposition of number states and the down converted mode in a number state. We analysed in some detail the influence of the initial field statistics of the pump mode as well as the presence of non-vacuum number states in the down converted mode on the appearance of collapses and revivals.     

Relativistic doppler effect in quantum communication

Abstract

When an electromagnetic signal propagates in vacuo, a polarization detector cannot be rigorously perpendicular to the wave-vector because of diffraction effects. The vacuum behaves as a noisy channel, even if the detectors are perfect. The ‘noise’ can, however, be reduced and nearly cancelled by a relative motion of the observer towards the source. The standard definition of a reduced density matrix fails for photon polarization, because the transversality condition behaves like a superselection rule. We can, however, define an effective reduced density matrix which corresponds to a restricted class of positive operator-valued measures. There are no pure photon qubits and no exactly orthogonal qubit states.     

Atomic interference in the micromaser and statistical properties of the micromaser field

Abstract

In this paper we report on atomic interferometry in the micromaser. The atomic inversion is recorded while scanning the cavity frequency across the atomic resonance. For high pump rates, interference patterns are observed on the low-frequency wing of the maser line. The interferences are due to the non-adiabatic mixing of dressed states of the atoms at the entrance and exit holes of the maser cavity, leading to a Ramsey-type two-field interaction. Furthermore, statistical properties of the maser field are investigated via a measurement of the statistics of the pump atoms that leave the maser cavity. Theoretical expressions for the time dependence of the Fano–Mandel parameter Q _A(t) are compared with the experimental data. We demonstrate, that metastability of the maser field and atomic interference strongly influence the approach to a steady-state value of Q _A(t).     

Correlation effects of chaotic and phase-diffusion fields on polarization beats in a V-type three-level system

Abstract

The correlation effects of fourth-order on polarization beats in a V-type three-level system (PBVTS) are investigated using chaotic and phase-diffusion models. It has been found that the temporal behaviour of the beat signal depends on the stochastic properties of the lasers and transverse relaxation rate of the transitions, and the modulation terms of the beat signal depend on the second-order coherence function, which is determined by the laser line shape. Since different stochastic models of the laser field affect only the fourth-order coherence function, they have little influence on the general temporal modulation behaviour of the beat signal. The different roles of the phase fluctuation and amplitude fluctuation have been pointed out in the time domain. The fourth-order coherence function theory of chaotic field is of vital importance in PBVTS. The cases where the pump beams have either narrow band or broadband linewidth are considered and it has been found that for both cases a Doppler-free precision in the measurement of the energy-level splitting between two states can be achieved. Finally, the difference between the PBVTS and DeBeer's ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.     

Packed Powder as Superleak for Spin Pump Experiments in Superfluid 3He A1

Experimental exploration of highly spin-polarized states of liquid ³He by applying external magnetic field is limited by the availability of static magnetic field. In the “ferromagnetic” superfluid A₁ phase of liquid ³He there is an alternate method for boosting spin-polarization by the process of spin pumping without requiring such high magnetic field. The spin pumping in the A₁ phase takes advantage of a superleak (SL) acting simultaneously as a filter for both entropy and spin. The spin pump technique that uses the SL-spin filter and a mechanical actuator enables us to directly boost polarization of ³He. The amount of enhancement of spin polarization has been limited so far. We are now developing a new type of SL filter made of packed aluminum oxide powder (referred as PAP-SL), in order to achieve greater enhancement of spin polarization. Several kinds of the PAP-SL filter were constructed by pressing aluminum oxide powders into a cylinder holder. The packed structures were carefully characterized by a flow-rate-measurement, X-ray tomography, and mercury intrusion porosimetry. The preliminary result shows that the PAP-SL works as SL filter for the superfluid ³He.     

High-Resolution Stimulated Raman Gain Spectroscopy of Parahydrogen Crystals

High-resolution stimulated Raman gain spectroscopy of the Q₁(0) transition of para-H₂ crystals with varying ortho-H₂ concentrations is reported. A crystal containing 0.06% of ortho-H₂ showed a single sharp Raman transition with the width of less than 7 MHz. The Raman signal became broader with increasing concentration of ortho-H₂. At the concentration of 2%, the spectrum splits into a doublet. The components of the doublet have different dependence on the relative polarization of the pump and probe laser radiations. Physical origin of the splitting and the polarization dependence is yet to be understood.PACS numbers: 33.20 Fb, 42.65 Dr, 67.80 −s     

Two-photon Resonance Dichroism in Caesium Vapours

Abstract

Caesium vapours are shown to become dichroic when the conditions for two-photon resonance excitation are created for the 6S_1/2–7S_1/2 transition. The observed phenomenon may be explained by the dependence of two-photon resonance absorption on the beam polarization in accordance with the selection rules for transitions between the magnetic sub-levels of 6S_1/2 and 7S_1/2 states.     

Femtosecond Pump-Probe Polarization Dependent Investigation of Relaxation Dynamics in YBa2Cu4O8

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YBa₂Cu₄O₈ as a function of temperature and polarization. We compare our results with the data obtained on YBa₂Cu₃O_7?δ and show that similar two-component relaxation is present in both cases. A strong polarization anisotropy of the picosecond response is observed below T _c and interpreted with the aid of a simple model which considers the anisotropy of the probe transition matrix elements.     

Expanded quantum cryptographic entangling probe

A generalized quantum circuit and design are given for an optimal entangling probe to be used in attacking the BB84 protocol of quantum key distribution and yielding maximum information to the probe. Probe photon polarization states become optimally entangled with the BB84 signal states on their way between the legitimate transmitter and receiver. The present design generalizes an earlier one by Brandt [J. Mod. Optics 52 2177 (2005)] to include a complete range of error rates that can be induced by the probe.     

High-power,high-repetition-rate picosecond optical parametric oscillators for the near-to mid-infrared

Abstract

We report high-repetition-rate, singly-resonant, picosecond optical parametric oscillators based on the nonlinear crystals LiB₃O₅ and KTiOAsO₄ which are synchronously pumped by a self-mode-locked Ti:sapphire laser operating at 81 MHz. These devices allow tunable pulse generation from 1·116-3·160 μm to be achieved. The LiB₃O₅ system produces average nearinfrared output powers of 325 mW and is continuous tuning over the wavelength range 1·16-2·26 μm. For 1·8 ps input pump pulses, transform-limited signal pulses with durations of 1-1·2 ps and idler pulses with durations of 2-2·2 ps have been generated over 1·2-2·2 μm, without requirement for dispersion compensation. The KTiOAsO₄ system produces average near-infrared output powers of 403 mW, with the signal tuning over 1·116-1·281 μm and idler tuning over 2·260-3·160 μm. Without dispersion compensation, signal (idler) pulses with durations between 1·01-1·03 (1·61-2·91) ps have been obtained for 1·2 ps input pump pulses.     

Dynamic mode evolution and phase transition of twisted light in nonlinear process

Abstract

We study the dynamic evolution of light carrying orbital angular momentum (OAM) in sum frequency generation (SFG). The dynamic evolution of the SFG intensity profiles is studied for the case where the two pump beams are in a well-defined OAM value or a superposition of OAM modes. When two pump beams carry OAM superposition modes, we observe two kinds of spatial patterns and evolution behaviours of SFG signal, and one pattern of them has characteristic intensity distribution which is unchanged during propagation. In addition, SFG signal obtained by two pump beams with opposite OAM evolves into a beam in a quasi-Gaussian mode. These observations show that the pump phases coherently transfer to the SFG signal in the nonlinear frequency conversion process.     

Prism-coupled excitation of multiple Tamm waves

Abstract

A Tamm wave is guided by the interface of a homogeneous dielectric material and a periodically nonhomogeneous dielectric material, both isotropic. We found theoretically that multiple Tamm waves of different linear polarization states can be guided by such an interface incorporated in a prism-coupled configuration. The Tamm waves have different linear polarization states, phase speeds, and spatial profiles, but the same frequency. The narrow absorptance peaks associated with the excitation of Tamm waves in the prism-coupled configuration offer the application as chemical sensors with very high sensitivity.     

Pump- and Probe-polarization Analyses of Ultrafast Carrier Dynamics in Organic Superconductors

We investigated photo-excited carrier relaxation dynamics in the strongly correlated organic superconductors κ-(BEDT-TTF)₂Cu(NCS)₂ and κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, using different polarizations of pump and probe pulses. Below the glasslike transition temperature (T _g) anisotropic responses for probe polarization were observed in both compounds. Decomposing the data into anisotropic and isotropic components, we found the anisotropic component shows no pump polarization dependence, meaning that dissipative excitation process was dominant for the anisotropic carrier relaxation. This behavior indicates that the appearance of anisotropic responses can be associated with spatial symmetry breaking due to structural change of BEDT-TTF molecules.     

Switchable and dual-wavelength ultrafast fibre lasers with an MoTe2-based saturable absorber

ABSTRACT

Based on monolayer MoTe₂ film, we have proposed a mode-locked fibre laser delivering single-wavelength and dual-wavelength solitons at the telecommunication band. The saturable absorber (SA) is synthesized by coating monolayer MoTe₂ film on the pinhole of fibre pigtail. The experimental results show that our soliton lasers can work at the single- and dual-wavelength states, respectively, by appropriately adjusting the polarization controller. The proposed laser can work stably at the mode-locking state for several hours, indicating that monolayer MoTe₂ film is an excellent SA material for ultrafast optics.     

Spatial and temporal dependence of single-pass parametric gain

Abstract

The use of focused Gaussian beams and short optical pulses to pump single-pass optical parametric amplifiers is examined. A simple model is introduced to mimic the experimental conditions used, and the expected intensity profile of the output signal for degenerate parametric amplification in LBO and MgO:LiNbO₃ is calculated. Experimental and theoretical results are compared for LBO. Second harmonic autocorrelation measurements were performed in order to determine the pulse duration of the output signal from the LBO parametric amplifier. These measurements showed that the amplified pulse was slightly shorter, and the deamplified pulse slightly longer, than the input signal—a result of the variation in the parametric gain over the 3 ps signal pulse. The implications of this spatial and temporal variation of the gain in squeezing are discussed.     

Tungsten tri-oxide (WO3) film absorber for generating Q-switched pulses in erbium laser

ABSTRACT

This paper reports a new type of passive saturable absorber (SA) made of transition metal oxide (TMO) embedded in polyvinyl alcohol (PVA). The Tungsten trioxide (WO₃)-PVA SA is placed in an erbium-doped fibre laser cavity to produce Q-switched pulses operating at 1562.82?nm. The pulse laser starts to manifest at the threshold pump power of 40?mW and continues to exist until the maximum pump power of 195?mW. Within that pump power range, its pulse energy, repetition rate and pulse width vary from 98 to 142.85?nJ, 29.86 to 56.7?kHz and 5.032 to 1.85?µs, respectively. The pulse train is stable with a signal to noise ratio of 70?dB. This is the first demonstration of a Q-switched laser using such a SA.