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Conference on Lasers and Electro-Optics

Quantum interference in a fs-dephasing medium induced by an intense hyper-continuum

H. Nishioka, H. Koutaka, Kenji Ueda

Published Jan 1, 2001• 0 citations
Publisher

Abstract

Summary form only given. We have demonstrated absorption enhancement and emission suppression by the coherence control in the fs-dephasing medium. The ultra-broadband differential phase excitation controls the coupling between multiple energy levels. The pumping scheme is possibly applied to further broadband materials for the control of general material properties unlimited within the optical characteristics. Especially, coherence control by the UV and VUV broadband excitations coupling to the same continuum drastically changes ionization and reaction properties of material. Applications for reaction control, material processing, or optical control of short wavelength system, that are essentially connecting with their short-live and ultra-broadband nature may be remarkable future of this scheme.

DOI
10.1109/cleo.2001.948081

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Summary form only given. We have demonstrated absorption enhancement and emission suppression by the coherence control in the fs-dephasing medium. The ultra-broadband differential phase excitation controls the coupling between multiple energy levels. The pumping scheme is possibly applied to further broadband materials for the control of general material properties unlimited within the optical characteristics. Especially, coherence control by the UV and VUV broadband excitations coupling to the same continuum drastically changes ionization and reaction properties of material. Applications for reaction control, material processing, or optical control of short wavelength system, that are essentially connecting with their short-live and ultra-broadband nature may be remarkable future of this scheme.
We show experimentally that spectral phase manipulation of ultrashort extreme-uv light pulses can induce and control coherent transient excitation of the He(1s3p)\mathrm{He}(1s3p)He(1s3p) excited state by the nonresonant components of the broadband extreme-ultraviolet light. The spectral phase manipulation of the 15th harmonic of an intense 805nm805\phantom{\rule{0.3em}{0ex}}\mathrm{nm}805nm, 80fs80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}80fs pulse is achieved by propagation of the euv light through a variable optical density of He gas. The acquired spectral phase due to the dispersive interaction of the off-resonance components in the euv pulse with the He(1s3p)\mathrm{He}(1s3p)He(1s3p) resonance enhances and modifies the transient excitation. The temporal evolution of the coherently prepared transient He(1s3p)\mathrm{He}(1s3p)He(1s3p) amplitude is probed by ionization to the continuum with a 400nm400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}400nm, 80fs80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}80fs pulse.
The resonant absorption, emission, and scattering of light are the fundamental optical processes that have been used both to probe matter and to manipulate light itself. In the last decade there has been essential progress in coherent control of both linear and nonlinear optical responses based on resonant excitation of atomic coherence in multilevel quantum systems. Some interesting and useful phenomena, resulting from coherent control of absorption and the group index, such as electromagnetically induced transparency, lasing without inversion, and ultra-slow group velocity of light have been widely studied. This work is focused on coherent control of refractive index and resonant fluorescence in multilevel medium. \n \nWe suggest two promising schemes for resonant enhancement of the refractive index with eliminated absorption and propose their implementation in transition element doped crystals with excited state absorption and in a cell of Rb atoms at natural abundance. We show how to use one of these schemes for spatial variation of the refractive index via its periodic resonant increase/decrease, remarkably keeping at the same time zero absorption/gain. It opens the way to production of transparent photonic structures (such as distributed Bragg reflectors, holey fibers, or photonic crystals) in a homogeneous resonant atomic media such as dielectrics with homogeneously distributed impurities, atomic, or molecular gases. These optically produced photonic structures could easily be controlled (including switching on/off, changing amplitude and period of modulation) and would be highly selective in frequency, naturally limited by the width of the optical resonance. \n \nWe also derive the optical fluorescence spectra of a three-level medium driven by two coherent fields at the adjacent transitions in a general case when all three transitions are allowed. We show that coherent driving can efficiently control the distribution of intensities between the fluorescent channels. In particular, the total intensity of fluorescence at the transition which is not driven by the optical fields may essentially exceed the fluorescence intensity at the driven transitions under the condition of two-photon resonance. This counter-intuitive effect is due to depletion of the intermediate state via atomic interference.
Abstract Coherent control uses interference between quantum pathways to enhance or suppress transitions to a target quantum state. In stimulated Raman excitation, such control can be achieved through interference between two indistinguishable pathways connecting the same initial and final states, with the relative phase between the pathways modulating the population transfer to the final state. Extending this strategy to the extreme ultraviolet (XUV) and X-ray regimes would enable control of ultrafast electronic dynamics and site-selective core excitations. However, Raman coupling at short wavelengths competes with photoionization and intermediate-state decay, whereas intense fields with stable and adjustable relative phases remain difficult to produce. Here we use two-colour pulses from a seeded coherent free-electron laser to excite helium from its ground state to a bound metastable state via stimulated XUV Raman excitation. We implement phase-coherent control by creating two interfering Raman pathways through the ionization continuum and modulate the transferred population by varying their relative phases. Blocking one pathway removes the modulation, confirming its interferometric origin, and time-dependent Schrödinger equation calculations reproduce the measured phase dependence. Our results establish phase-controlled stimulated Raman excitation in the XUV regime, providing a strategy for the coherent control of atomic and molecular processes at short wavelengths.
A theory of quantum control of short-wavelength sum-frequency generation, which employs the continuum states, is developed. The proposed scheme employs all-resonant coupling and trade-off optimization of the accompanying constructive and destructive quantum interference effects in the lower-order and higher-order polarizations controlled by the overlap of two autoionizinglike laser-induced continuum structures. The scheme does not rely on adiabatic passage, coherent population trapping or maximum atomic coherence as a means to facilitate maximum output. The opportunities for manipulating transparency of the medium and refractive index for the fundamental and generated radiations, as well as nonlinear polarization in the multiple-resonant medium, are shown. This opens the feasibility of creating frequency-tunable narrowband filters, polarization rotators, and dispersive elements for vacuum ultraviolet radiation. The features specific for quantum interference in Doppler-broadened media are investigated. The feasibility of almost complete conversion of long-wavelength fundamental radiation into generated short-wavelength radiation, and of a dramatic decrease in the intensity of required fundamental radiations, is shown.