Phase Conjugation Experiments Research Articles

TIME REVERSAL EXPERIMENTS IN THE MICROWAVE RANGE: DESCRIPTION OF THE RADAR AND RESULTS

We present a new RADAR system able to perform Phase Conjugation experiments over the ultrawideband (2{4)GHz. The system is equipped with a transmit/receive linear array made of eight antennas connected to a 2-port Vector Network Analyzer through eight independent couples of digitally-controlled RF attenuators and phase shifters. Thus, each channel can selectively transmit or receive and can as well attenuate and phase shift the RF signal. For each frequency, either the Phase Conjugation or the Decomposition of the Time Reversal Operator (DORT) is applied to the received signal and the appropriate amplitude and phase law is coded into the prototype; the focusing wave is then experimentally re-emitted by the array. The quality of the achieved backpropagation is evaluated both in frequency and time domain: In this sense we can speak of Time Reversal. The excellent agreement between measured and theoretical results validates the potential of our system.

An in vivo study of turbidity suppression by optical phase conjugation (TSOPC) on rabbit ear

We present a holography-based in vivo optical phase conjugation experiment performed on a living rabbit ear. The motion of live tissues caused the phase conjugate signal to decay with a consistent decay time of less than two seconds. We monitor the signal decay time variation after the ear is excised to postulate different mechanisms that cause the signal decay. The experimental findings address the minimum speed limit of a broad range of optical time reversal experiments for in vivo applications on tissues.

Polarization dependence of four-wave mixing in a degenerate two-level system

Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate two-level atomic transition is theoretically and experimentally examined. Using the model presented by A. Lezama et al [Phys. Rev. A 61, 013801 (2000)] the NDFWM spectra corresponding to different pump and probe polarization cases are calculated and discussed. The calculated spectra are compared to the observation of NDFWM within the $6S_{1/2}(F=4)\to 6P_{3/2}(F=5)$ transition of cesium in a phase conjugation experiment using magneto optically cooled atoms

Spatial resolution of time reversal arrays in shallow water

A series of time reversal (phase conjugation) experiments was performed in shallow water including a range-dependent slope environment. Time reversal arrays implemented with center frequencies of 445 and 3500 Hz achieved sharp focal regions up to ranges of 30 and 13 km, respectively, in 110–130 m shallow water. Analysis with an image method indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions, i.e., waveguide geometries and attenuation. The measured focal size has implications for the maximum achievable resolution of matched field processing, which is a computational implementation of the phase conjugation process. [Work supported by ONR.]

Characterization of near-field optical probes

Radiation and collection characteristics of four different near-field optical-fiber probes, namely, three uncoated probes and an aluminum-coated small-aperture probe, are investigated and compared. Their radiation properties are characterized by observation of light-induced topography changes in a photosensitive film illuminated with the probes, and it is confirmed that the radiated optical field is unambiguously confined only for the coated probe. Near-field optical imaging of a standing evanescent-wave pattern is used to compare the detection characteristics of the probes, and it is concluded that, for the imaging of optical-field intensity distributions containing predominantly evanescent-wave components, a sharp uncoated tip is the probe of choice. Complementary results obtained with optical phase-conjugation experiments with the uncoated probes are discussed in relation to the probe characterization.

A long-range and variable focus phase-conjugation experiment in shallow water

A second phase-conjugation experiment was conducted in the Mediterranean Sea in May 1997 extending the results of the earlier time-reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. New results reported here include (1) extending the range of focus from the earlier result of 6 km out to 30 km, (2) verifying a new technique to refocus at ranges other than that of the probe source [Song et al., J. Acoust. Soc. Am. 103, 3234–3240 (1998)], and (3) demonstrating that probe-source pulses up to 1 week old can be refocused successfully.

Photorefractive Properties of Co-Doped Potassium Sodium Strontium Barium Niobate Crystals

The transmissivity and the results of two-beam coupling and self-pumped phase conjugation (SPPC) experiments on Co-doped potassium sodium strontium barium niobate (KNSBN) single crystals are reported. The Co ions doped into KNSBN can form a deep energy level near 2.25 eV. The two-beam coupling gain coefficients of the Co-doped KNSBN are nearly 20 cm-1 at 2 teta = 370 for the transmission photorefractive index grating and larger than 29.2 cm-1 for the reflecting photorefractive index grating. A striking single-beam asymmetric transmission was observed. The maximum SPPC reflectivity is larger than 70% at teta = 49.70 for an intensity W = 77 mW, which is the largest value measured in the family of KNSBN crystals.

Resonant Enhancement of Parametric Processes via Radiative Interference and Induced Coherence

Nonlinear optical parametric processes are studied in a resonantly driven multilevel system displaying quantum interference effects. It is shown that in such systems a new regime of nonlinear amplification is possible, in which a pair of correlated Stokes and anti-Stokes fields can be generated from infinitesimally small initial values. An atomic coherence grating emerges during this process of efficient nonlinear amplification. The present analysis explains the results of recent optical phase conjugation experiments involving atomic phase coherence.

Phase conjugation under two-photon absorption conditions in polydiacetylene crystals

Phase-conjugation experiments have been performed in a backward degenerate four-wave mixing geometry using 1064 nm picosecond light pulses in thick p-toluene sulphonate polydiacetylene crystals. Two-photon absorption is identified and characterized. The relevant propagation equations are resolved analytically in different experimental geometries. Nonlinear absorption cancels out all the phase-conjugate signal at light fluences. A time-dependent analysis of the effect in a pump and nonlinear-probe experiment reveals an excited-state-induced nonlinearity.

Self-pumped phase conjugation in a ring cavity at 1.06 μm in cw and nanosecond regimes using photorefractive BaTiO3:Rh

We present self-pumped phase conjugation experiments in a ring cavity at 1.06 μm in a BaTiO3:Rh crystal both in the cw and in the nanosecond regime. The measured reflectivities are respectively 50% and 66%. At 4 W cm−2 of cw illumination, it only takes 90 s to reach the steady state. A preliminary experiment of correction of the focusing effect of a lens is reported.

Growth and photorefractive properties of Mn-doped (KNa)0.1(Sr0.6Ba0.4)0.9Nb2O6 crystals

(KNa)0.1(Sr0.6Ba0.4)0.9Nb2O6, or KNSBN, crystals have been grown with two different Mn concentrations: 0.02 wt % and 0.04 wt %. Two-wave mixing and self-pumped phase conjugation experiments were carried out on these crystals at 632.8 nm wavelength. The measured two-wave coupling gains for the 0.02 wt % and 0.04 wt % crystals are larger than 7 cm−1 and 9 cm−1, respectively, and the two-wave coupling time response is faster for the 0.4 wt % crystal. In addition, the self-pumped phase conjugation reflectivity of the 0.04 wt % crystal is measured to be as high as 70%. It is shown that the Mn dopant can increase effectively the absorption coefficient and enhance the photorefractive properties of KNSBN.

Degenerate four-wave mixing in the copolymer of phenylacetylene and norbornene

The results of optical phase conjugation experiments in a degenerate four-wave mixing geometry are reported at 532 nm in a new block copolymer of phenylacetylene and norbornene. The copolymer sample with an equal number of repeating units for the two homopolymers prepared by sequential addition of the monomers gives a third-order susceptibility value which is twice that of the homopolymer of phenylacetylene.

Copper-ion point defects in the photorefractive material (KxNa1-x)0.4(SryBa1-y)0.8Nb2O6.

We show that Cu ions doped in (${\mathrm{K}}_{\mathit{x}}$${\mathrm{Na}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${)}_{0.4}$(${\mathrm{Sr}}_{\mathit{y}}$${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${)}_{0.8}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ (KNSBN) form two energy levels in the band gap, which increase the effective charge-combination-center concentration ${\mathit{N}}_{\mathrm{eff}}$. The two-wave coupling gain and the response speed of KNSBN crystals are increased while the photoconductivity and the effects of a dc field on the photorefractive properties are reduced. Self-pumped phase conjugation (SPPC) experiments have revealed that the effects of Cu ions on the photorefractive properties have a wavelength dependence. The SPPC response time at the ${\mathit{e}}^{\mathrm{\ensuremath{-}}1}$ level can be shortened to 200 ms.

Probe transmission in a one-dimensional optical molasses: Theory for circularly-cross-polarized cooling beams.

We present a detailed theoretical investigation of the transmission spectrum of a probe beam interacting with atoms in a one-dimensional optical molasses obtained with linearly cross-polarized counterpropagating pump beams. The study is performed for a ${\mathit{J}}_{\mathit{g}}$=1/2\ensuremath{\rightarrow}${\mathit{J}}_{\mathit{e}}$=3/2 atomic transition in the limit where the Hamiltonian part of the atom-field coupling is predominant over the relaxation part. We analyze the stimulated Raman transitions occurring between different vibrational levels of the atoms in the periodic potential created by the light shifts, and we show a dramatic lengthening of the damping time of coherences between such levels due to the Lamb-Dicke effect. Very narrow Rayleigh resonances with a shape sensitive to the probe polarization appear for a probe frequency close to the pump frequency. We interpret these resonances in terms of scattering of the pump waves on density and magnetization gratings, and show that they provide important information about the dynamics and localization of atoms at the bottom of the potential wells. Such information should also be accessed by phase-conjugation experiments. Finally, indications on the treatment of other atomic transitions are given.

Probe transmission in one-dimensional optical molasses: Theory for linearly cross-polarized cooling beams.

We present a detailed theoretical investigation of the transmission spectrum of a probe beam interacting with atoms in a one-dimensional optical molasses obtained with linearly cross-polarized counterpropagating pump beams. The study is performed for a ${\mathit{J}}_{\mathit{g}}$=1/2\ensuremath{\rightarrow}${\mathit{J}}_{\mathit{e}}$=3/2 atomic transition in the limit where the Hamiltonian part of the atom-field coupling is predominant over the relaxation part. We analyze the stimulated Raman transitions occurring between different vibrational levels of the atoms in the periodic potential created by the light shifts, and we show a dramatic lengthening of the damping time of coherences between such levels due to the Lamb-Dicke effect. Very narrow Rayleigh resonances with a shape sensitive to the probe polarization appear for a probe frequency close to the pump frequency. We interpret these resonances in terms of scattering of the pump waves on density and magnetization gratings, and show that they provide important information about the dynamics and localization of atoms at the bottom of the potential wells. Such information should also be accessed by phase-conjugation experiments. Finally, indications on the treatment of other atomic transitions are given.

Picosecond light-induced noncentrosymmetry in a dye solution.

By means of a phase-conjugation experiment by nondegenerate six-wave mixing, we demonstrate a transient ${\mathrm{\ensuremath{\chi}}}^{(2)}$ grating induced by a nonzero ${\mathit{E}}^{3}$ time average in a solution of noncentrosymmetric dye molecules. The origin of the induced phase-matched second-harmonic generation is found in an orientational hole-burning effect. Its lifetime, limited by molecule rotation, is 210\ifmmode\pm\else\textpm\fi{}10 psec and the ${\mathrm{\ensuremath{\chi}}}^{(2)}$ magnitude reaches 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ pm/V for 1 GW ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ pumps at 1064 nm.

Longitudinal plasma resonances excited by optical mixing and detected by phase conjugation

The first phase conjugation experiment in a plasma, using wavelengths in the microwave range (X band) is reported. A longitudinal resonance (plasmon or phonon) is excited by optical mixing of two pump waves in a low density discharge plasma. The resonance is detected by a three wave phase conjugation process described by the coupled mode equations accounting for pump depletion, either in transmission or in reflexion, as opposed to the usual reflection conjugation by four wave mixing. We also stress the optimal coherent scattering event achieved by the phase conjugation process.

Resolution of the nuclear and electronic contributions to the optical nonlinearity in polysilanes

We present a method of resolving the nuclear and electronic contributions to the third-order susceptibility χ3 in a phase conjugation experiment. The technique is applied to octylmethylpolysilane yielding χ3 (electronic)=(1.8±0.5)×10−12 esu and χ3(nuclear)=(1.1±0.4)×10−12 esu at 532 nm.

Optical phase conjugation for postgraduates

The use of the very efficient nonlinear medium of dye molecules held in glue films is described for optical phase conjugation (OPC) experiments using an argon ion laser. The medium is attractive because it is inexpensive, easy to prepare and a large signal-to-noise ratio can be obtained. It is suitable for introducing OPC in postgraduate laboratory courses.

Conical emissions and phase conjugation in atomic sodium vapor

Observations of a new type of conical emission in atomic sodium provide the motivation for a series of phase conjugation experiments. These experiments show large variations in the reflectivity as a function of probe angle, which are not explained by existing theory. Analysis which includes self-induced refractive index changes leads to numerical calculations, which agree well with the observations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>