Michael M. Kash

ULTRASLOW GROUP VELOCITY AND ENHANCED NONLINEAR OPTICAL EFFECTS IN A COHERENTLY DRIVEN HOT ATOMIC GAS

We report the observation of small group velocities of order 90 meters per second, and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas (≈ 360 K). Media of this kind yield strong nonlinear interactions between very weak optical fields, and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.

Nonlinear optics via double dark resonances

Double dark resonances originate from a coherent perturbation of a system displaying electromagnetically induced transparency. We experimentally show and theoretically confirm that this leads to the possibility of extremely sharp resonances prevailing even in the presence of considerable Doppler broadening. A gas of {sup 87}Rb atoms is subjected to a strong drive laser and a weak probe laser and a radio frequency field, where the magnetic coupling between the Zeeman levels leads to nonlinear generation of a comb of sidebands.

Optical imaging beyond the diffraction limit via dark states

We study the possibility of creating spatial patterns having subwavelength size by using the so-called dark states formed by the interaction between atoms and optical fields. These optical fields have a specified spatial distribution. Our experiments in Rb vapor display spatial patterns that are smaller than the length determined by the diffraction limit of the optical system used in the experiment. This approach may have applications to interference lithography and might be used in coherent Raman spectroscopy to create patterns with subwavelength spatial resolution.

Observation of the saturation effect in continuous-wave coherent anti-Stokes Raman spectroscopy of liquid nitrogen

We report the first observation to our knowledge of the saturation effect in a Raman transition of a liquid, liquid N2. Using cw coherent anti-Stokes Raman spectroscopy, Raman line broadening by a factor of ≃2 is observed. This corresponds to vibrational excitation of up to 30% of the N2 molecules in the interaction volume.

Observation of picosecond UV pulses produced by coherent scattering of IR femtosecond pulses in atomic rubidium vapor

We report the observation of coherent UV light pulses by the coherent scattering of IR pulses from atomic rubidium vapor. Rubidium atoms were first excited by a 100 fs pulse from the 5S ground state to the 5D state via a two-photon transition. The atoms were then pumped by an IR pulse resonant to the 5D–12P transition. The presence of the IR pulse triggered the instantaneous emission of a UV light pulse on the 12P–5S transition. The pulse had a time duration of tens of picoseconds, which was measured by a picosecond-resolution streak camera. The temporal shape of the generated light is explained by a simplified atom–field interaction theory.

A model experiment for stand-off sensing

Two aspects of the stand-off sensing technique (Kocharovsky et al. Proc. Natl Acad. Sci. 2005, 102, 7806–7811) were demonstrated using a femtosecond laser system and an organic dye. First, three cells of a solution of Rhodamine B dye in ethanol were pumped with a single wavelength. Amplification of the light generated by two-photon absorption occurred when the pulses were overlapped temporally, in a direction opposite to that of the pump laser beam. Second, the dispersion of the dye solution permitted two pulses of different wavelength to overlap at a specified location in the solution. The overlapped pulses generated light as the result of two-photon absorption in both the forward and backward directions, relative to that of the pump laser beam.

Diamagnetic structure of lithium: n ≈ 21

Abstract We present initial results of a study by laser spectroscopy of the diamagnetic structure of Rydberg states of lithium designed to verify the absolute accuracy to which the energy and the magnetic field can be determined, and to confirm the results of numerical calculations of the energy level structure. Spectra are displayed in the energy regime of − 252 cm −1 to − 242 cm −1 , essentially exploring the n = 21 energy manifold, at magnetic fields up to 8.6 tesla.The spectroscopic resolution, ≈ 0.001 cm −1 ,represents a major improvement over previous work.

Subwavelength imaging via dark state

We theoretically study possibilities of creating spatial patterns having subwavelength length by using so-called dark states. These dark states are formed via interaction with bichromatic optical fields. Performing experiments in Rb vapor, we experimentally demonstrated spatial patterns that are smaller than the length determined by the diffraction limit of optical system used in the experiment. This approach has applications to interference lithography and can be used in coherent Raman spectroscopy to reach subwavelength spatial resolution.

Nonlinear Optics with Radio Frequency Field

Performing experiments with Rb-atoms and RF fields, we have demonstrated several nonlinear effects, such as multi-photon transitions and excitation of coherence using far-detuned field with different time-shape pulses. Our results agree with our theoretical predictions.

Dressed state "engineering" with double-dark states: experimental observation of hyper-Raman transitions

Summary form only given. Dark states are coherent superpositions of metastable states in multilevel systems that can be generated by a pair of electromagnetic fields (e.g. by probe and pump fields), but are not coupled by the fields. As a rule, any perturbations of such a state are undesirable, since they result in reduction of coherence. However, it has been recently predicted that coherent perturbation of the dark state does not necessarily lead to simple destruction of coherence. In the case when a third meta-stable state coupled by a weak radio frequency (RF) field additional sharp resonances on the spectrum displaying electromagnetically induced transparency (EIT, dashed line) have been predicted. This phenomenon is a generalization of the well-known EIT concept to higher-order hyper-Raman transitions.