Session N43: Quantum Optics and Strong Field Physics

Large Double-EIT and Mutual Phase Shifts in Rubidium

We propose a scheme to achieve large double-EIT and mutual phase shifts for two slow, co-propagating pulses of light through a Rubidium gas, with the additional advantages of enabling equal group velocities for the two pulses and avoiding cancellations of nonlinearities at resonance.   

Electromagnetically induced transparency and precision measurement of atomic transitions in a laser-cooled sample of cesium atoms

Electromagnetically induced transparency (EIT) has observed in a cascade system of laser-cooled Cs atoms and the atomic energy levels have been measured to an accuracy of 0.0003 $cm^{-1}$. In our experiment, Cs atoms are loaded into the magneto-optical trap (MOT) from a background vapor that has a pressure of 10$^{- 9}$ torr. The number of Cs atoms is estimated using a CCD camera to be 10$^7$ occupying a roughly spherical volume having a radius of 2 mm and the temperature of the atom cloud is measured using a time of flight technique to be about 100 $\mu$K. A diode laser excites the Cs atoms from $|6\,^2S_{1/2}, F=4\rangle$ state to $|6\,^2P_{3/2}, F=5\rangle$ state, then a dye laser couples the $6\,P_{3/2}$ state to the higher excited, $|9\,^2D_{3/2}\rangle$, $|9\,^2D_{5/2}\rangle$, $|10\,^2D_{3/2} \rangle$, $|10\,^2D_{5/2}\rangle$, and $|11\,^2S_{1/2}\rangle$ atates. The signal is monitored by detecting the trap loss while changing the pump or coupling laser frequency. The effect on the EIT signals by changing the pump laser detuning and the coupling laser power is also discussed. The line positions are measured by comparing the saturation absorption spectrum of molecular iodine (accuracy $<$ 3 MHz).   

Femtosecond Coherent Raman Spectroscopy in Biomolecules

We present a scheme for identification of biomolecules in real time by using Coherent Anti-Stokes Raman Spectroscopy (CARS) with femtosecond pulses. The nonlinear interaction of the pulses with the molecules is calculated analytically as well as numerically and comparison is made with the experimental results. We show that the commonly used rotating wave approximation needs to be waved for our fast dephasing molecules. Propagation effects are simulated, in addition to the nonlinear interaction, and it is shown how one needs to balance the electronic resonance enhancement and the losses due to absorption, in order to maximize the signal at the output of the optically dense medium. In the transient regime, we present and analyze the generation of polarization beats. These beats are generated due to the very broad bandwidth of the femtosecond pulses, which allows for the coupling of more that one vibrational modes at a time.   

Dephasing of excitons in single ZnSe quantum wells using 30 fs pulses

The dephasing of excitons in homogenously broadened 10nm wide ZnMgSe/ZnSe single quantum wells is investigated using ultra short ($<$30fs) light pulses in two beam and three beam four wave mixing (FWM) configuration. The observed FWM traces show marked quantum beats due to excitation of higher exciton transitions within the 80 meV spectrally broad pulses. For pulse delay times shorter than 500 fs, non-Markovian signal decay is observed. For longer delay times the exciton dephasing becomes single exponential indicating the transition from quantum kinetics to classical Boltzmann kinetics. Intensity and polarization dependent FWM measurements give further information on the physical nature and scattering processes that take place in both time regimes. This work is supported by the National Science Foundation (DMR-0305076).   

Dynamics of high amplitude coherent phonons in photoexcited bismuth

We report on studies of high amplitude coherent phonons in photoexcited bismuth. All optical experiments, in which two pump pulses are used to coherently control the amplitude $A_{1g}$ phonon at a fixed carrier density, allow us to separate the effects of carrier dynamics from anharmonicity. The results show that the the time dependent frequency of the phonon is dominated by electronic softening of the interatomic potential. Seperate first-principals theoretical calculations confirm these results for photoexcited carrier densities up to approximately 2\% of the valence electrons. We comment on the possibility of x-ray diffraction and diffuse scattering as a means of measuring the detailed dynamics.   

Optimization and Rate-Equation Model for Second-Harmonic Generation in Mid-Infrared Quantum Cascade Lasers

We present the rate-equation simulation of a mid-infrared quantum cascade laser with optimized second-harmonic generation. The simulation is performed by optimization of the structure design followed by self-consistently solving the rate equations for the carriers in the various levels. The optimized structure was obtained by supersymmetric quantum mechanics with both position-depended mass and band nonparabolicity included. In the rate-equation model, the second harmonic generation process is described by sequentially two single photon absorption and one second-harmonic photon generation. Nonunity pumping efficiency is accounted by all relevant electron-electron and electron-LO phonon scatterings between injector/collector and active region are included. The modal gain, linear power and second-harmonic power can then be calculated based on the steady-state populations in the active region. Results show that the optimized structure has higher modal gain and linear to nonlinear conversion efficiency.   

Observation of a coherent exciton-LO phonon resonance in a ZnSe single quantum well

A new coherent signal has been observed while performing two- beam degenerate four-wave mixing (FWM) experiments on a 3nm ZnMgSSe/ZnSe single quantum well (SQW) using 30fs laser pulses. In this SQW structure the exciton binding energy exceeds the LO-phonon energy (31.6 meV). The observed spectral feature is blue shifted with respect to the heavy-hole bound exciton transition by $\sim $32 meV and indicates the formation of a coherent exciton-LO phonon resonance with a dephasing time of $\sim $500 fs. This tentative assignment is further supported by photoluminescence excitation (PLE) and by reflection measurements. This work is supported by the National Science Foundation (DMR-0305076).   

Vibrational Lifetimes and Frequency-Gap Law of Hydrogen Bending Modes in Semiconductors

Vibrational lifetimes of hydrogen and deuterium related bending modes in semiconductors are measured by transient bleaching spectroscopy and high-resolution infrared absorption spectroscopy. We find that the vibrational lifetimes follow a universal frequency-gap law, i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multi-phonon process. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems.   

High field gradient for electron acceleration and ultra-short electron pulse formation

We explore a possibility of strongly inelastic scattering and large energy exchange between the tightly focused laser and electron beams in the ``transverse'' geometry, whereby the beams propagate normally to each other. In the most basic configuration of the laser standing wave we demonstrate that multi-MeV electron acceleration per pass is attainable, if the field has large spatial gradient along the direction of electron motion. The electron motion in this case is relativistic, and the energy gain or loss can be retained by an electron with very high efficiency. The proposed scheme may provide enormous $\sim 0.1$ TeV/cm acceleration gradient. We also show that the transverse electron and laser beam configuration can cause strong temporal electron focusing resulting in formation of ultra-short electron bunches. Such a system has a potential to operate as a full-switch laser gate for electrons, a new base element of a free-electron laser and laser electron accelerators.   

Relativistic effect of ponderomotive force direction reversal in a standing laser wave

In our research, we theoretically discovered a relativistic effect of the direction reversal of the field-gradient (ponderomotive) force (PF) in a standing laser wave. This reversal makes the high-field areas attractive for electrons, in contrast to the regular PF, and it represents the only effect known so far that pins down a distinct borderline between relativistic and nonrelativistic motion. We demonstrated that the collinear configuration, in which the laser wave is linearly polarized with electric field, $\vec{E}$ parallel to the initial electron momentum, $\vec{p}_0$, is the optimal configuration for the relativistic reversal. In that case, the transverse PF reverses its direction when the incident momentum is $p_0 = mc$. The reversal effect vanishes in the cases of circular and linear with $\vec{E} \perp \vec{p} _0$ polarizations. We have discovered, however, that the counter-rotating circularly polarized standing waves develop attraction and repulsion areas along the axis of laser, in the laser field whose intensity is homogeneous in that axis, i.e. has no field gradient.   

Spatial and pulse shape dependence of K$\alpha$ source from high contrast fs laser plasmas in regime of Relativistic Engineering

Interaction of intense Ti: Sapphire laser with Cu foil targets has been studied by measuring hard X-ray generation. Hard x-ray spectroscopy and K$\alpha $ X-ray conversion efficiency (\textit{$\eta $}$_{K})$ from Cu plasma have been studied as a function of laser intensity via pulse duration scan (\textit{60 fs $\sim $ 600 fs}), laser pulse energy scan (\textit{60 mJ $\sim $ 600 mJ}) and target displacement scan from best focus. For intensity \textit{I $>$ 1x10}$^{17}$\textit{ W/cm}$^{2}$, the Cu \textit{$\eta $}$_{K}$ keep on increasing to reach a maximum value of $1x10^{-4}$ at an intensity $I = 1x10^{18}$\textit{ W/cm}$^{2}$. The focusing was varied widely to give a range of intensities from $10^{15}$\textit{ W/cm}$^{2}$\textit{ $\sim $ 10}$^{18}$\textit{ W/cm}$^{2}$. Two individual emission peaks are obtained, one is at best focal spot and the other is at larger target offset corresponding to \textit{$\sim $ 10}$^{15}$\textit{ W/cm}$^{2}$. Each peak is corresponding to different energy absorption mechanism. In addition, when we introduce slightly detuning of compressor gratings at the best focal condition, it shows \textit{$\eta $}$_{K}$ generated by negatively skewed \textit{100 fs} pulse width laser irradiation reach $5x10^{-4}$ and almost $7$ times greater than the case of positively skewed pulse. Vacuum Heating is greatly stimulated in this case and preciously control of pre-plasma is the key factor in tuning control of X-ray emission in relativistic \textit{fs} regime.   

Guiding of 100 TW Relativistic Laser Pulses by 10 mm Plasma Channels

First experiments for laser-gas plasma interaction about electron acceleration have been performed with 30 fs, 100 TW relativistic Ti:Sapphier laser pulse interaction with long slit (\textit{1.2 x 10 mm}$^{2})$ gas plasmas. The world record \textit{10 mm} length plasma channel formed that was longer than 20 times the Rayleigh length. Plasma density was the key factor for this long channel stimulation under \textit{100 TW} laser pulse irradiation that was much higher than critical power for relativistic self-focusing. For the first time, channel characteristics such as laser bending, hosing and cavity formation were demonstrated experimentally. In case of long channel guiding, accelerated electron bunch was tightly collimated with low emmitance \textit{$<$ 0.8 $\pi $ mm mrad} and quasi-monoenergetic electron bunch (\textit{$\sim $70 MeV}) was obtained as well. Accelerated electron charge current with electron energy \textit{$>$ 1 MeV} was \textit{$\sim $ 10 nC/shot} which was highest value in laser accelerator, to our knowledge, and ascribed to the contribution of long plasma channel.