A. V. Mitrofanov

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.

Highly birefringent silicate glass photonic-crystal fiber with polarization-controlled frequency-shifted output: A promising fiber light source for nonlinear Raman microspectroscopy.

A highly birefringent silicate glass photonic-crystal fiber (PCF) is employed for polarization-controlled nonlinear-optical frequency conversion of femtosecond Cr: forsterite laser pulses with a central wavelength of 1.24 mum to the 530--720-nm wavelength range through soliton dispersion-wave emission. The fiber exhibits a modal birefringence of 1.2.10(-3) at the wavelength of 1.24 mum due to a strong form anisotropy of its core, allowing polarization switching of the central wavelength of its blue-shifted output by 75 nm. Polarization properties and the beam quality of the blue-shifted PCF output are shown to be ideally suited for polarization-sensitive nonlinear Raman microspectroscopy.

Post-filament self-trapping of ultrashort laser pulses.

Laser filamentation is understood to be self-channeling of intense ultrashort laser pulses achieved when the self-focusing because of the Kerr nonlinearity is balanced by ionization-induced defocusing. Here, we show that, right behind the ionized region of a laser filament, ultrashort laser pulses can couple into a much longer light channel, where a stable self-guiding spatial mode is sustained by the saturable self-focusing nonlinearity. In the limiting regime of negligibly low ionization, this post-filamentation beam dynamics converges to a large-scale beam self-trapping scenario known since the pioneering work on saturable self-focusing nonlinearities.

Angle-resolved multioctave supercontinua from mid-infrared laser filaments.

Angle-resolved spectral analysis of a multioctave high-energy supercontinuum output of mid-infrared laser filaments is shown to provide a powerful tool for understanding intricate physical scenarios behind laser-induced filamentation in the mid-infrared. The ellipticity of the mid-infrared driver beam breaks the axial symmetry of filamentation dynamics, offering a probe for a truly (3+1)-dimensional spatiotemporal evolution of mid-IR pulses in the filamentation regime. With optical harmonics up to the 15th order contributing to supercontinuum generation in such filaments alongside Kerr-type and ionization-induced nonlinearities, the output supercontinuum spectra span over five octaves from the mid-ultraviolet deep into the mid-infrared. Full (3+1)-dimensional field evolution analysis is needed for an adequate understanding of this regime of laser filamentation. Supercomputer simulations implementing such analysis articulate the critical importance of angle-resolved measurements for both descriptive and predictive power of filamentation modeling. Strong enhancement of ionization-induced blueshift is shown to offer new approaches in filamentation-assisted pulse compression, enabling the generation of high-power few- and single-cycle pulses in the mid-infrared.

New Horizons of Optics of the Midinfrared Spectral Range

Optical physics and laser technologies are rapidly moving in the direction of exploring the midinfrared spectral range. New methods of mid-IR ultrashort pulse generation allow forming very short flashes of electromagnetic radiation with record high peak power for this range. The first experiments conducted with such systems make possible implementing new regimes of laser–matter interaction and shed light on unusual properties of the nonlinear-optical response of materials in the mid-IR spectral range.

Spectral-temporal properties and nonlinear-optical transformation of supercontinuum radiation with an energy over 1 μJ generated by a large-mode-area photonic-crystal fiber

Nonlinear-optical transformation of chirped femtosecond Cr:forsterite-laser pulses in a large-mode-area photonic-crystal fiber gives rise to the generation of a supercontinuum in the near infrared spectral range with an energy exceeding 1 μJ. A broadband light source with a wavelength tunability range from 550 to 1800 nm is implemented through second-harmonic-and sum-frequency-generation transformation of the supercontinuum photonic-crystal fiber output.

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.

Ultrafast Photonics with Microstructures Fibers

Microstructure fibers – new optical fibers with tailored dispersion and nonlinearity – provide a constantly growing platform for the development of advanced fiber-format devices and components for ultrafast photonics . Unique options offered by microstructure fiber technology, such as dispersion management through fiber structure engineering and enhancement of optical nonlinearity due to a strong field confinement in a small-size fiber core, are pushing the frontiers of ultrafast photonics , allowing the creation of efficient sources of supercontinuum radiation, novel compact fiber lasers, as well as frequency converters, pulse compressors, and fiber components for microscopy and bioimaging.

Waveform shaping of stretched-pulse fiber laser output with a hollow photonic-crystal fiber

A hollow-core photonic-crystal fiber (PCF) is integrated with a stretched-pulse ytterbium fiber oscillator to enable ultrafast optical waveform shaping, demonstrated through the pulse compression of single-pulse fiber-laser output and tunable ultrashort-pulse pair generation in the regime where the fiber laser delivers a double-pulse output. A hollow PCF with a properly designed dispersion profile is shown to allow the stretched 6.2-ps Yb fiber laser output to be compressed to a pulse width of less than 160 fs, which is only 6% longer than the pulse width supported by the entire 18-nm bandwidth of the fiber laser output.

Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber

In this work, we demonstrate microjoule supercontinuum generation using an LMA PCF with a mode area of 380 mum2 pumped by an amplified stretched-pulse output of a mode-locked Cr: forsterite laser with a peak power of a few megawatts.