Andrey Gandman

Enhancement of intermediate-field two-photon absorption by rationally shaped femtosecond pulses

We extend the powerful frequency-domain analysis of femtosecond two-photon absorption to the intermediate-field regime of considerable absorption yields, where additionally to the weak-field nonresonant two-photon transitions also four-photon transitions play a role. Consequently, we rationally find that the absorption is enhanced over the transform-limited pulse by any shaped pulse having a spectral phase that is antisymmetric around one-half of the transition frequency and a spectrum that is asymmetric around it (red or blue detuned according to the system). The enhancement increases as the field strength increases. The theoretical results for Na are verified experimentally.

Pulse-bandwidth dependence of coherent phase control of resonance-mediated ( 2 + 1 ) three-photon absorption

We study in detail coherent phase control of femtosecond resonance-mediated

Frequency-domain coherent control of femtosecond two-photon absorption: intermediate-field versus weak-field regime

(2+1)

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays

three-photon absorption and its dependence on the spectral bandwidth of the excitation pulse. The regime is the weak-field regime of third perturbative order. The corresponding interference mechanism involves a group of three-photon excitation pathways that are on resonance with the intermediate state and a group of three-photon excitation pathways that are near resonant with it. The model system of the study is atomic sodium (Na), for which experimental and numerical-theoretical results are obtained. Prominent among the results is our finding that with simple proper pulse shaping an increase in the excitation bandwidth leads to a corresponding increase in the enhancement of the three-photon absorption over the absorption induced by the (unshaped) transform-limited pulse. For example, here, a 40 nm bandwidth leads to an order-of-magnitude enhancement over the transform-limited absorption.

Intermediate-field two-photon absorption enhancement by shaped femtosecond pulses: Tolerance to phase deviation from perfect antisymmetry

Coherent control of femtosecond two-photon absorption in the intermediate-field regime is analysed in detail in the powerful frequency domain using an extended fourth-order perturbative description. The corresponding absorption is coherently induced by the weak-field non-resonant two-photon transitions as well as by four-photon transitions involving three absorbed photons and one emitted photon. The interferences between these two groups of transitions lead to a difference between the intermediate-field and weak-field absorption dynamics. The corresponding interference nature (constructive or destructive) strongly depends on the detuning direction of the pulse spectrum from one-half of the two-photon transition frequency. The model system of the study is atomic sodium, for which both experimental and theoretical results are obtained. The detailed understanding obtained here serves as a basis for coherent control with rationally-shaped femtosecond pulses in a regime of sizeable absorption yields.

NIR femtosecond phase control of resonance-mediated generation of coherent UV radiation

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 μm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different line shape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points to the interferences origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in a nanoconfined environment.

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas

We study in detail the coherent interference mechanism leading to the intermediate-field two-photon absorption enhancement recently found for shaped femtosecond pulses with spectral phases that are antisymmetric around one-half of the transition frequency. We particularly investigate the tolerance of the phenomenon to the phase deviation from perfect antisymmetry. We theoretically and experimentally find that this tolerance increases as the field strength increases. For the present Na excitation, the enhancement occurs even when

Non-poissonian formation of multiple excitons in photoexcited CdTe colloidal quantum qots by femtosecond nonresonant two-photon absorption

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Near-infrared resonance-mediated chirp control of a coherently generated broadband deep-ultraviolet spectrum

30% of the phase pattern is not antisymmetric. Our findings are of particular importance for multichannel coherent control scenarios.

Coherent phase control of resonance-mediated ( 2 + 1 ) three-photon absorption

Shaped near-infrared (NIR) femtosecond pulses are used for the first time to control the generation of coherent deep-ultraviolet (UV) radiation in an atomic resonance-mediated (2+1) three-photon excitation. The broadband excitation coherently involves pathways that are on resonance with the intermediate resonance state as well as pathways that are near resonance with it. Experimental and theoretical results are presented for phase controlling the total emitted UV yield in atomic sodium. Depending on the NIR spectrum of the excitation pulse, the coherent UV emission is either predominantly due to a single excited real state that is accessed resonantly or due to a manifold of virtual states. The former leads to a narrowband UV emission, while the latter leads to a broadband UV radiation. Basic phase control is exercised in both cases, with excellent agreement between experiments and calculations. The tunability is over an order-of-magnitude UV-yield range.