Broadband terahertz generation by optical rectification of ultrashort multiterawatt laser pulses near the beam breakup threshold.

We identify the physical factors that limit the terahertz (THz) yield of an optical rectification (OR) of ultrashort multiterawatt laser pulses in large-area quadratically nonlinear crystals. We show that the THz yield tends to slow its growth as a function of the laser driver energy, saturate, and eventually decrease as the laser beam picks up a spatiotemporal phase due to the intensity-dependent refraction of the OR crystal. We demonstrate that, with a careful management of the driver intensity aimed at keeping the nonlinear length larger than the coherence length, OR-based broadband THz generation in large-area lithium niobate (LN) crystals is energy-scalable, enabling an OR of multiterawatt laser pulses, yielding ∼10µJ/cm2 of THz output energy per unit crystal area. With a 27-fs, 10-TW, 800-nm Ti:sapphire laser output used as a driver for OR in large-area LN crystals, this approach is shown to provide a THz output with a pulse energy above 10 µJ and a bandwidth extending well beyond 6 THz, supporting single-cycle THz waveforms.

High-harmonic-driven inverse Raman scattering.

Spectral analysis of high-order harmonics generated by ultrashort mid-infrared pulses in molecular nitrogen reveals well-resolved signatures of inverse Raman scattering, showing up near the frequencies of prominent vibrational transitions of nitrogen molecules. When tuned on a resonance with the v'=0→v''=0 pathway within the B3Πg→C3Πu second positive system of molecular nitrogen, the eleventh harmonic of a 3.9 µm, 80 fs driver is shown to acquire a distinctive antisymmetric spectral profile with red-shifted bright and blue-shifted dark features as indicators of stimulated Raman gain and loss. This high-harmonic setting extends the inverse Raman effect to a vast class of strong-field light-matter interaction scenarios.

Coherently enhanced microwave pulses from midinfrared-driven laser plasmas.

Ultrafast ionization of a gas medium driven by ultrashort midinfrared laser pulses provides a source of bright ultrabroadband radiation whose spectrum spans across the entire microwave band, reaching for the sub-gigahertz range. We combine multiple, mutually complementary detection techniques to provide an accurate polarization-resolved characterization of this broadband output as a function of the gas pressure. At low gas pressures, the lowest-frequency part of this output is found to exhibit a drastic enhancement as this field builds up its coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. This behavior of the intensity, coherence, and polarization of the microwave output is shown to be consistent with Cherenkov-type radiation by ponderomotively driven plasma currents.

Chirp-controlled high-harmonic and attosecond-pulse generation via coherent-wake plasma emission driven by mid-infrared laser pulses.

Coherent-wake plasma emission induced by ultrashort mid-infrared laser pulses on a solid target is shown to give rise to high-brightness, high-order harmonic radiation, offering a promising source of attosecond pulses and a probe for ultrafast subrelativistic plasma dynamics. With 80-fs, 0.2-TW pulses of 3.9-µm radiation used as a driver, optical harmonics up to the 34th order are detected, with their spectra stretching from the mid-infrared region to the extreme ultraviolet region. The harmonic spectrum is found to be highly sensitive to the chirp of the driver. Particle-in-cell analysis of this effect suggests, in agreement with the generic scenario of coherent-wake emission, that optical harmonics are radiated as trains of extremely short, attosecond ultraviolet pulses with a pulse-to-pulse interval varying over the pulse train. A positive chirp of the driver pulse can partially compensate for this variation in the interpulse separation, allowing harmonics of the highest orders to be generated in the plasma emission spectrum.

Chirp-controlled filamentation and formation of light bullets in the mid-IR.

Formation of light bullets-tightly localized in space and time light packets, retaining their spatiotemporal shape during propagation-is, for the first time, experimentally observed and investigated in a new regime of mid-infrared filamentation in ambient air. It is suggested that the light bullets generated in ambient air by multi-mJ, positively chirped 3.9-µm pulses originate from a dynamic interplay between the anomalous dispersion in the vicinity of CO2 resonance and positive chirp, both intrinsic, carried by the driver pulse, and accumulated, originating from nonlinear propagation in air. By adjusting the initial chirp of the driving pulses, one can control the spatial beam profile, energy losses, and spectral-temporal dynamics of filamenting pulses and deliver sub-3-cycle mid-IR pulses in high-quality beam on a remote target.

High-order harmonic analysis of anisotropic petahertz photocurrents in solids.

Polarization maps of high-order harmonics are shown to enable a full vectorial characterization of petahertz electron currents generated in a crystalline solid by an ultrashort laser driver. As a powerful resource of this methodology, analysis of energy-momentum dispersion landscapes, defined by the electron band structure, can help identify, as our analysis shows, special directions within the Brillouin zone that can provide a preferable basis for polarization-sensitive high-harmonic mapping of anisotropic petahertz photocurrents in solids.

High-order harmonic generation from a solid-surface plasma by relativistic-intensity sub-100-fs mid-infrared pulses.

High-order harmonic generation (HHG) in plasmas induced by ultrashort, relativistic-intensity laser pulses on solid surfaces can provide an efficient source of attosecond pulses and opens routes toward new regimes of laser-matter interactions, x-ray generation, laser particle acceleration, and relativistic nonlinear optics. However, field intensities in the range of Irel∼1019 W/cm2 are typically needed to achieve the relativistic regime of HHG in experiments with near-infrared laser pulses. Here, we show that, in the mid-infrared range, due to the λ-2 scaling of Irel with the driver wavelength λ, relativistic HHG can be observed at much lower levels of laser field intensities. High-peak-power 80-fs, 3.9-µm pulses are focused in our experiments on a solid surface to provide field intensities in the range of 1017 W/cm2. Remarkably, this level of field intensities, considered as low by the standards of relativistic optics in the near infrared, is shown to be sufficient for generation of high-order harmonics with signature properties of relativistic HHG-beam directionality, spectra with extended plateaus, and a high HHG yield sustained for both p- and s-polarized driver fields.

Filamentation of mid-IR pulses in ambient air in the vicinity of molecular resonances.

Properties of filaments ignited by multi-millijoule, 90 fs mid-infrared pulses centered at 3.9 µm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25 m) to loose (f=7 m) focusing, we observe a shift from plasma-assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the nonlinear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 µm. The dynamic CO2 absorption in the case of 3.9 µm filaments with their low plasma content is the main mechanism of energy losses and, either alone or together with other nonlinear processes, contributes to the arrest of intensity.

Reconnectable fiberscopes for chronic in vivo deep-brain imaging.

Reconnectable bundles consisting of thousands of optical fibers are shown to enable high-quality image transmission, offering a platform for the creation of implantable fiberscopes for minimally invasive in vivo brain imaging. Experiments on various lines of transgenic mice verify the performance of this fiberscope as a powerful tool for chronic in vivo neuroimaging using genetically encoded calcium indicators, neuronal activity markers as well as axon growth regulators and brain-specific protein drivers in deep regions of live brain.

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.

Fiber-optic electron-spin-resonance thermometry of single laser-activated neurons.

Optically detected electron spin resonance in fiber-coupled nitrogen-vacancy (NV) centers of diamond is used to demonstrate a fiber-optic quantum thermometry of individual thermogenetically activated neurons. Laser-induced temperature variations read out from single neurons with the NV-diamond fiber sensor are shown to strongly correlate with the fluorescence of calcium-ion sensors, serving as online indicators of the inward Ca2+ current across the cell membrane of neurons expressing transient receptor potential (TRP) cation channels. Local laser heating above the TRP-channel activation threshold is shown to reproducibly evoke robust action potentials, visualized by calcium-ion-sensor-aided fluorescence imaging and detected as prominent characteristic waveforms in the time-resolved response of fluorescence Ca2+ sensors.

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.

Fiber-optic control and thermometry of single-cell thermosensation logic.

Thermal activation of transient receptor potential (TRP) cation channels is one of the most striking examples of temperature-controlled processes in cell biology. As the evidence indicating the fundamental role of such processes in thermosensation builds at a fast pace, adequately accurate tools that would allow heat receptor logic behind thermosensation to be examined on a single-cell level are in great demand. Here, we demonstrate a specifically designed fiber-optic probe that enables thermal activation with simultaneous online thermometry of individual cells expressing genetically encoded TRP channels. This probe integrates a fiber-optic tract for the delivery of laser light with a two-wire microwave transmission line. A diamond microcrystal fixed on the fiber tip is heated by laser radiation transmitted through the fiber, providing a local heating of a cell culture, enabling a well-controlled TRP-assisted thermal activation of cells. Online local temperature measurements are performed by using the temperature-dependent frequency shift of optically detected magnetic resonance, induced by coupling the microwave field, delivered by the microwave transmission line, to nitrogen--vacancy centers in the diamond microcrystal. Activation of TRP channels is verified by using genetically encoded fluorescence indicators, visualizing an increase in the calcium flow through activated TRP channels.

Mid-infrared-to-mid-ultraviolet supercontinuum enhanced by third-to-fifteenth odd harmonics.

A high-energy supercontinuum spanning 4.7 octaves, from 250 to 6500 nm, is generated using a 0.3-TW, 3.9-µm output of a mid-infrared optical parametric chirped-pulse amplifier as a driver inducing a laser filament in the air. The high-frequency wing of the supercontinuum spectrum is enhanced by odd-order optical harmonics of the mid-infrared driver. Optical harmonics up to the 15th order are observed in supercontinuum spectra as overlapping, yet well-resolved peaks broadened, as verified by numerical modeling, due to spatially nonuniform ionization-induced blue shift.

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.

Fiber-optic magnetic-field imaging.

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.

Fiber-optic magnetometry with randomly oriented spins.

We demonstrate fiber-optic magnetometry using a random ensemble of nitrogen-vacancy (NV) centers in nanodiamond coupled to a tapered optical fiber, which provides a waveguide delivery of optical fields for the initialization, polarization, and readout of the electron spin in NV centers.

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.

Electron spin manipulation and readout through an optical fiber.

The electron spin of nitrogen--vacancy (NV) centers in diamond offers a solid-state quantum bit and enables high-precision magnetic-field sensing on the nanoscale. Implementation of these approaches in a fiber format would offer unique opportunities for a broad range of technologies ranging from quantum information to neuroscience and bioimaging. Here, we demonstrate an ultracompact fiber-optic probe where a diamond microcrystal with a well-defined orientation of spin quantization NV axes is attached to the fiber tip, allowing the electron spins of NV centers to be manipulated, polarized, and read out through a fiber-optic waveguide integrated with a two-wire microwave transmission line. The microwave field transmitted through this line is used to manipulate the orientation of electron spins in NV centers through the electron-spin resonance tuned by an external magnetic field. The electron spin is then optically initialized and read out, with the initializing laser radiation and the photoluminescence spin-readout return from NV centers delivered by the same optical fiber.

Pulse-width-tunable 0.7 W mode-locked Cr: forsterite laser.

A mode-locked chromium forsterite laser with output power in excess of 0.7 W, a central wavelength of 1.25 µm, a pulse repetition rate of 29 MHz, and an output pulse-width-tunable from 40 to 200 fs is demonstrated. The dynamics behind the buildup of ultrashort light pulses in this laser is shown to involve spectral and temporal breathing due to the interplay of gain, Kerr nonlinearity, and dispersion effects. The pulse-width-tunable 1.25 µm output delivered by the developed laser source suggests a powerful tool for nonlinear-optical bio-imaging and offers an advantageous front end for extreme-power laser technologies.