Session J28 - Quantum Optics and Strong Field Physics.
ORAL session, Tuesday midday, March 23
516E, Palais des Congres

[J28.001] Resonant Photonic Bandgap (RPBG) Structures and Self-Induced Transparency (SIT) Solitons

The most basic RPBG structure consists of Bragg-spaced planes of two-level atoms, i.e. planes spaced with half the emission wavelength in the medium. Radiative coupling of the planes of atoms changes the response from absorption in individual atoms to superradiant decay in the collective structure. The nonlinear response of RPBG structures shows formation of SIT solitons, both moving and zero-velocity. Moving SIT-solitons tunnel through the RPBG, while zero-velocity solitons store the optical pulse as a coherent polarization state within the structure. Both linear and initial nonlinear measurements will be presented on a RPBG structure constructed from excitons in periodic InGaAs/GaAs semiconductor quantum wells. Linear measurements show suppression of the single quantum well heavy-hole exciton absorption, while resonant pump-probe nonlinear measurements show adiabatic following of the material response to the pump pulse. Additionally, numerical simulations will be presented demonstrating optical pulse storage in RPBG structures consisting of periodic planes of two-level atoms.

[J28.002] Persistent Perfect Entanglement in Atomic Systems

It has been shown recently [1] that a pure entangled state of two-level atoms can be obtained in an optical resonator through the exchange by cavity photons. Unfortunately, the lifetime of such an entangled state, caused by the radiative decay time for the dipole transitions is very short.

The situation can be improved through the use of three-level atoms with lambda-type transition [2]. In this case, the cavity field pumps transition between the lowest (ground) and highest (excited) states. Then, the decay of the excited state can populate the intermediate state. This is just the Raman-type process with emission of Stokes photon in atomic system. Because of the selection rules by the parity conservation, the dipole decay from the intermediate state to the ground state is forbidden. If the Stokes photons created by the transitions from the excited state to the ground state are discarded (through the use of cavity leakage of absorption), the final state of atomic system is stabile or at least durable.

In the case of 2n three-level atoms, this can lead to the N-qubit perfect entangled state, where N=2j+1 and j is an odd ``spin'' corresponding to the SU(2) algebra in the Hilbert space

$H=(C\^2)\^\\backslash otimes N\$

In fact, these are the SU(2) phase states of odd ``spin'' have been discussed in [3] in the context of two-level atoms. The possibility to create and observe these states with present experimental technique is discussed.

[1] A. Beige, S. Bose, D. Braun, S.F. Huelga, P.L. Knight, M.B. Plenio, and V. Verdal. J. Mod. Optics 47, 2583 (2000).

[2] M.A. Can, A.A. Klyachko, and A.S. Shumovsky. Appl. Phys. Lett. 81, 5072 (2002).

[3] M.A. Can, A.A. Klyachko, and A.S. Shumovsky. Phys. Rev. A 66, 022111 (2002).

[J28.003] Polarization properties of speckle from scattering media

We have shown the importance of the relationship between photon travel time statistics and the second-order frequency-dependent speckle correlation function in imaging. Recently, we discovered that the third-order intensity statistics provides information regarding the Fourier phase, which allows the reconstruction of the temporal response for scattering media. Here we extend the second-order correlation treatment to include co- and cross-polarized light. A tunable external-cavity laser diode provides frequency scanning and a small spot is imaged with a CCD camera. We measured a series of intensity speckle patterns as a function of scan frequency. From this data we determined the degree of polarization and second-order intensity frequency correlation results for co- and cross-polarized light as a function of sample thickness. The second-order correlations showed distinct differences, depending on polarization, for less strongly scattering samples.

[J28.004] Space-time control of ultrafast nano-optics

Electric near-field distributions are at the center of many experimental techniques such as scanning tunneling microscopy (STM) or near-field two-photon fluorescence microscopy. Ultrahigh spatial resolution is provided by making use of the optical field enhancement in the vicinity of a sharp tip. We simulate the field distribution near an STM tip/sample geometry by solving Maxwell's equations (taking into account the vectorial nature of the electric field by means of the boundary element method), under the irradiation with polarization-shaped femtosecond laser pulses. This allows for the first time shaping all three mutually orthogonal polarization components of a femtosecond light pulse at a given point in space in a complex fashion. Using an evolutionary algorithm, this technique has not only the potential to enhance nonlinear signals in the above-mentioned near-field techniques (by applying the optimal light pulse), but it also offers the possibility for "three-dimensional" quantum control on surface-adsorbed molecules, accessing their 3D wavefunction properties with light fields optimized in all three polarization directions.

[J28.005] Coherent manipulation of hydrogenic states in GaAs with polarized radiation

Donor bound electrons in GaAs have orbital states analogous to those of free Hydrogen, suggesting the possibility of combining the precise state manipulation of atomic optics with the scalability of semiconductor systems for applications in quantum information processing. We report on the use of polarized radiation to selectively excite coherent transitions between orbital states. Polarized short pulses of THz radiation are generated by ''slicing'' long pulses from the UCSB Free Electron Laser into ps widths. This radiation illuminates an unintentionally doped 15 micron layer of GaAs with an electron density of 2.8 x 10^14 cm-3. Cooling to 2K initializes the population in the ground (1s) state and excitations to higher orbital states are detected by photoconductivity. Rabi oscillations between orbital states are visible when radiation is circularly polarized to obey the selection rules for the 1s to 2p+ transition. With linearly polarized light, a reduced effective field strength is observed. With oppositely handed circularly polarized radiation, the coupling is sufficiently weak that no oscillations are observed. This work was supported by DARPA QuIST Grant No. MDA972-01-1-0027

[J28.006] Slow Light Using Excitonic Population Pulsation in Semiconductor Quantum Wells

We theoretically model and successfully explain the experimental data [1] on slow light using the excitonic population pulsation in semiconductor quantum wells. In a two-level system, beating between a resonant pump and a signal can cause population pulsation. The pulsation can induce a rapid change in the refractive index within a spectral width determined by the radiative recombination, which results in a reduced group velocity. In our model, the spin-dependent polarization selection rules for the optical transition and the excitation-induced dephasing are taken into account. Our theory explains very well the experimental absorbance and refractive index spectra and their polarization dependence. A theoretical value for the slowdown factor of 31,300 is obtained and agrees well with the experimental value. [1] P. C. Ku, P. Palinginis, T. Li, F. G. Sedgwick, S. W. Chang, H. Wang, C. J. Chang-Hasnain and S. L. Chuang, (submitted).

[J28.007] Giant above-threshold absorption and cascade ionization of diatomic molecules in ultra-short pulses of strong lasers

Recent experiments demonstrate that the dynamics of laser - molecule interaction in strong pulses containing only few optical cycles is clearly different from that in the multi-cycle pulses.

In the present work the molecular non-linear excitation and multi-electron ionization are studied for the few-cycle and multi-cycle pulses of resonant and off-resonant frequency. It is shown that in some cases the molecule can undergo cascade ionization and giant above-threshold absorption. In the cascade ionization the molecule emits one electron with the same probability as an electron pair. This phenomenon arises due to one-photon resonance with an intermediate electronic state. In the giant above-threshold absorption the molecule absorbs a big amount of laser energy far above the second ionization threshold but can still exist like a whole molecule. This phenomenon is invoked by rapid energy transmission from the laser pulse to the molecule. The detailed analysis of physics of both phenomena is presented.

[J28.008] Fragmentation of polyatomic ions assisted by nonadiabatic charge localization

The processes of molecular excitation, ionization, and fragmentation caused by strong non-resonant laser field may involve charge build-up in selected locations within a polyatomic molecule. Using kinetic energy distributions of produced H+ ions, we probe the electron-nuclear dynamics of the Coulomb-explosion dissociation of anthracene subjected to 60 fs, 800 nm laser pulses of intensity between 0.4 and 4.0×10^14 W·cm^-2. The counter-intuitive evolution of the proton energy distribution with growing intensity suggests substantial nonadiabatic charge localization within the molecule. We propose strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the duration of the charge localization significantly exceeds the laser period and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of H+ kinetic energy on the laser intensity. The dissociative ionization of polyatomic molecules assisted by long-lived charge localization is a new type of strong-field electron-nuclear dynamics, essential for understanding the pathways of molecular/ionic fragmentation and for effective control of the fragmentation process.

[J28.009] Spin-orbit coupling and Berry phase with ultracold atoms in 2D optical lattices

We show how spin-orbit coupling and Berry phase can appear in two-dimensional optical lattices by coupling atoms' internal degrees of freedom to radiation. The Rashba Hamiltonian, a standard description of spin-orbit coupling for two-dimensional electrons, is obtained for the atoms under certain circumstances. We discuss the possibility of observing associated phenomena, such as the anomalous Hall and spin Hall effects, with cold atoms in optical lattices.

[J28.010] Electron spin polarization of ions produced by two-photon pulsed laser ionization of Sr atoms

Spin polarization of ions and electrons plays an important role in the fields of surface physics, atomic and molecular physics, and high-energy physics, in particular, for the study of spin related characteristics. When gas phase atoms are ionized by multi-photon ionization process, the degrees of spin polarization of photoions and photoelectrons are the same if all of the produced photoions are in the s state [1]. We have achieved the degree of electron-spin polarization of Sr^+ ions as large as 64% by ionizing Sr atoms through one-photon resonant two-photon pulsed laser ionization via 5s5p ^3P_1 (M_J=+1) state [2,3]. In other words, we have produced 64% electron-spin polarized photoions and photoelectrons simultaneously. The degree of electron-spin polarization is measured at three different wavelengths (308nm, 266nm, 248nm) for the ionization laser.

[1] T. Nakajima et. al. J. Chem. Phys., 117, 2112 (2002). [2] T. Nakajima et. al., Appl. Phys. Lett. 83, 2103 (2003) [3] N. Yonekura et. al., J. Chem. Phys. in press.

[J28.011] Spectroscopy of the 9s and 8p levels of atomic francium

We use two-photon resonant excitation and time-correlated single-photon counting techniques on a sample of ^210Fr atoms. The atoms are produced at the Stony Brook superconducting LINAC , and confined and cooled in a magneto-optical trap.

We excite atoms from the 7s ground level through the 7P_1/2 level to the 9s level with lasers at 817 nm and 744 nm. We observe the decay of the 9s level to the 7P_3/2 level at 851 nm from which we measure the lifetime of the 9s level. The decay of the 9s level also populates the 8p levels. We detect the decay of the 8P_3/2 and 8P_1/2 levels to the 7s level at 423 nm and 433 nm, respectively, from which we determine their lifetimes. With the use of a scanning Fabry-Perot interferometer, we measure the hyperfine splitting of the 9s level.

The lifetime and hyperfine splitting measurements are important tests for atomic structure calculations and future parity non-conservation experiments in francium.

Work supported by NSF. E.G. acknowledges support from CONACYT.

[J28.012] Xenon Clusters in Intense VUV Laser Fields

Only recently, in an experiment using the new free-electron laser at DESY (Hamburg, Germany), intense VUV-cluster interactions were observed [1]. Xenon clusters were found to absorb a very large number of VUV photons, many more than had been anticipated. A theoretical description of the experiment is presented [2], which accounts for the main observations. The key aspects analyzed in this theory include the formation of a nanoplasma, and the subsequent photon absorption during electron-ion collisions, a process known as inverse bremsstrahlung. Inclusion of electron-ion interactions beyond the level of standard hydrogenic models proves to be crucial for understanding the experiment. [1] H. Wabnitz et al., Nature 420, 482 (2002). [2] R. Santra and C. H. Greene, Phys. Rev. Lett. 91, 233401 (2003).

[J28.013] Phase Control over Decaying Molecular States in Laser Pulses

It is well - known that the quantum interference between two pathways, created by the fundamental laser wave and its third harmonic, provides one scenario for the laser control over molecular processes. Here we consider the effect of various factors previously neglected on the extent of control. Included are the effects of radiative decay times, the finite frequency bandwidth of the pulsed laser, and the frequency mismatch between the fundamental and the third harmonic. We show that the non-linear time-dependent interference of quantum transitions induced by the two electromagnetic waves plays a crucial role in the extent of control. We obtain that the triple wave frequency mismatch, even if small, creates additional time-dependent phase shift which reduces the control. The computational analysis confirms that the more stable molecular states are better controlled and are less sensitive to the frequency mismatch. We further demonstrate that shortening the laser pulse duration from nanoseconds to picoseconds or changing the peak intensity of fundamental pulse between 10^10 W/cm^2 and 10^12 W/cm^2 can strongly improve phase control.

Part J of program listing