E. A. Stepanov

Mid-infrared laser filaments in the atmosphere

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 μm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

Solid-State Source of Subcycle Pulses in the Midinfrared.

We have demonstrated an approach for the generation of 20-fs-pulses at 6.8 μm. Subcycle pulse widths have been achieved due to self-compression dynamics and ultrabroadband phase matching for FWM near the zero-GVD wavelength of GaAs.

Frequency-tunable sub-two-cycle 60-MW-peak-power free-space waveforms in the mid-infrared

A physical scenario whereby freely propagating mid-infrared pulses can be compressed to pulse widths close to the field cycle is identified. Generation of tunable few-cycle pulses in the wavelength range from 4.2 to 6.8 μm is demonstrated at a 1-kHz repetition rate through self-focusing-assisted spectral broadening in a normally dispersive, highly nonlinear semiconductor material, followed by pulse compression in the regime of anomalous dispersion, where the dispersion-induced phase shift is finely tuned by adjusting the overall thickness of anomalously dispersive components. Sub-two-cycle pulses with a peak power up to 60 MW are generated in the range of central wavelengths tunable from 5.9 to 6.3 μm.

Mapping the electron band structure by intraband high-harmonic generation in solids

High-order harmonic generation (HHG) in atomic gases is one of the key phenomena in strong-field laser-matter interactions, playing a central role in optical physics and rapidly growing attosecond technologies. When applied to solid materials [1-3], approaches of strong-field physics are subject to natural limitations, related to absorption and much lower laser damage thresholds of solids. As a reward, such solid-state extensions promise breakthroughs toward petahertz solid-state optoelectronics, open avenues toward attosecond science on the platform of solid-state materials, and suggest new all-optical methods for crystallographic analysis.

Pulse-width considerations for nonlinear Raman brain imaging: whither the optimum?

We propose simple, yet efficient strategies of pulse-width optimization applicable for nonlinear Raman brain imaging. With the spectral bandwidth of laser pulses accurately matched against the bandwidth of molecular vibrations, the coherent Raman signal is shown to be radically enhanced, enabling higher sensitivities and higher frame rates in nonlinear Raman brain imaging. As a specific example, we show that subpicosecond pulses offer a powerful tool for the detection of brain tumors using stimulated Raman microscopy, as they provide a strong signal without compromising the molecular specificity.

Nonlinear spectroscopy with few-cycle pulses in mid-infrared: Mapping the electron band structure by intraband high-harmonic generation in solids

High-order harmonic generation (HHG) in atomic gases is one of the key phenomena in strong-field laser-matter interactions, playing a central role in optical physics and rapidly growing attosecond technologies. When applied to solid materials [1-3], approaches of strong-field physics are subject to natural limitations, related to absorption and much lower laser damage thresholds of solids. As a reward, such solid-state extensions promise breakthroughs toward petahertz solid-state optoelectronics, open avenues toward attosecond science on the platform of solid-state materials, and suggest new all-optical methods for crystallographic analysis.

Mapping anomalous dispersion of air with ultrashort mid-infrared pulses

We present experimental studies of long-distance transmission of ultrashort mid-infrared laser pulses through atmospheric air, probing air dispersion in the 3.6–4.2-μm wavelength range. Atmospheric air is still highly transparent to electromagnetic radiation in this spectral region, making it interesting for long-distance signal transmission. However, unlike most of the high-transmission regions in gas media, the group-velocity dispersion, as we show in this work, is anomalous at these wavelengths due to the nearby asymmetric-stretch rovibrational band of atmospheric carbon dioxide. The spectrograms of ultrashort mid-infrared laser pulses transmitted over a distance of 60 m in our experiments provide a map of air dispersion in this wavelength range, revealing clear signatures of anomalous dispersion, with anomalous group delays as long as 1.8 ps detected across the bandwidth covered by 80-fs laser pulses.

Nonlinear optics in the mid-infrared: new morning

Recent breakthroughs in ultrafast photonics in the mid-IR help understand complex interactions of high-intensity mid-IR field waveforms with matter, offer new approaches for x-ray generation, enable mid-IR laser filamentation in the atmosphere, facilitate lasing in filaments, give rise to unique regimes of laser-matter interactions, and reveal unexpected properties of materials in the mid-IR range.

Multimodal nonlinear Raman microspectroscopy with ultrashort chirped laser pulses

We demonstrate the physical principles of multimodal nonlinear optical microspectroscopy, integrating methods of coherent and stimulated Raman scattering of ultrashort chirped laser pulses in a single optical scheme. Nonlinear phase distortions of ultrashort laser pulses are accurately compensated within a broad spectral range in this scheme to enable a high-spectral-resolution laser microspectroscopy that can reliably resolve groups of fingerprint molecular vibrations with close frequencies, thus facilitating an analysis of complex multicomponent systems.