Low-threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate.

We report on high average power, low threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate with amplified as well as unamplified pulses from a Yb:KGW oscillator. An octave-spanning supercontinuum was produced in undoped potassium gadolinium tungstate (KGW), which demonstrated robust, damage-free long-term performance with a total average pump power of 6.4 W. The supercontinuum generation was unambiguously attested by the distinctive features of the phenomenon: beam filamentation visualized via filament-induced luminescence; conical emission and its characteristic angular distribution captured by angle-resolved spectral measurements; and pulse splitting that produced the sub-pulses with well-behaved phases, as retrieved from the measurements employing a second harmonic frequency-resolved optical gating technique.

KGW and YVO4: two excellent nonlinear materials for high repetition rate infrared supercontinuum generation.

We present an experimental investigation of supercontinuum generation in potassium gadolinium tungstate (KGW) and yttrium vanadate (YVO4) crystals pumped with 210 fs, 1030 nm pulses from an amplified Yb:KGW laser operating at 2 MHz repetition rate. We demonstrate that compared to commonly used sapphire and YAG, these materials possess considerably lower supercontinuum generation thresholds, produce remarkable red-shifted spectral broadenings (up to 1700 nm in YVO4 and up to 1900 nm in KGW) and exhibit less bulk heating due to energy deposition during filamentation process. Moreover, durable damage-free performance was observed without any translation of the sample, suggesting that KGW and YVO4 are excellent nonlinear materials for high repetition rate supercontinuum generation in the near and short-wave infrared spectral range.

Broadband conical third harmonic generation in femtosecond filament-modified fused silica.

High repetition rate femtosecond filaments in transparent solids produce conical third harmonic generation due to filament-induced material reorganization in the form of periodic volume nanogratings. Here we report on conical third harmonic generation that accompanies supercontinuum generation in fused silica using broadly tunable femtosecond pulses. The measurement of third harmonic cone angles with driving wavelengths in the 1-3-µm range fully supports the noncollinear phase-matching scenario that involves a reciprocal lattice vector of the filament-inscribed nanograting. The nanograting provides an octave-spanning phase-matching bandwidth, as attested by the measurements of the angle-resolved spectra of broadband conical third harmonic emission.

Low spatial frequency laser-induced periodic surface structures in fused silica inscribed by widely tunable femtosecond laser pulses.

The formation and evolution of laser-induced periodic surface structures in fused silica under irradiation of widely tunable (in the 1-3 [Formula: see text]m range) linearly polarized femtosecond (200 fs) pulses was studied experimentally. The structures were inscribed in high fluence regime (exceeding the surface ablation threshold for a single pulse) and characterized by using scanning electron microscopy and two dimensional Fourier transform. The results revealed rapid (after irradiation with a few successive pulses) formation of periodic laser-induced periodic surface structures aligned parallel to laser polarization, whose period increases with increasing the inscription wavelength, obeying the [Formula: see text] law. With further increase of number of pulses, the generated structures gradually reorganize into laser polarization-independent low spatial frequency annular structures associated with formation of the damage crater, which fully established after irradiation with a few tens of successive laser pulses. This particular evolution scenario was observed over the entire wavelength tuning range of incident pulses.

Conical third harmonic generation from volume nanogratings induced by filamentation of femtosecond pulses in transparent bulk materials.

We report on observations of conical third harmonic emission that emerges during supercontinuum generation produced by self-focusing and filamentation of high (20-200 kHz) repetition rate 180 fs, 1035 nm pulses from an amplified Yb:KGW laser in various nonlinear crystals and glasses: YAG, sapphire, YLF, LiF, CaF2, MgF2, LiSAF, fused silica and BK-7 glass. We show that conical third harmonic generation is a phase-matched four-wave mixing process, where noncollinear phase matching is achieved by means of reciprocal lattice vector, inversely proportional to the period of nanograting, which is inscribed by femtosecond filament in the volume of nonlinear material. The existence of a particular period required to phase match conical third harmonic generation was indirectly verified by investigations of periodicity features of high and low spatial frequency laser-induced periodic surface structures, in which matter is reorganized in a similar fashion.

High repetition rate green-pumped supercontinuum generation in calcium fluoride.

We compare supercontinuum generation in [Formula: see text] crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing ([Formula: see text]) of the pump beam into a long (25 mm) [Formula: see text] sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing ([Formula: see text]) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.

Supercontinuum generation and optical damage of sapphire and YAG at high repetition rates.

We have experimentally investigated supercontinuum (SC) generation and the evolution of optical damage in sapphire and YAG crystals with 180 fs, 1035 nm pulses from an amplified Yb:KGW laser with variable repetition rates, both in tight and loose focusing conditions. In this Letter, we demonstrate that the extinction of the SC spectrum always correlates with an occurrence of conical third harmonic generation, which readily serves as an indication of the onset of in-bulk optical damage. Damage-related structural changes of the nonlinear material are also justified by an increased intensity and large red shift of crystal luminescence spectrum corresponding to the F center emission. The SC spectrum in sapphire starts shrinking on the time scale between seconds and minutes by varying the focusing condition from tight to loose at the laser repetition rate of 200 kHz, whereas the YAG crystal produces stable performance for several hours at least.

Optical Time Reversal from Time-Dependent Epsilon-Near-Zero Media.

Materials with a spatially uniform but temporally varying optical response have applications ranging from magnetic field-free optical isolators to fundamental studies of quantum field theories. However, these effects typically become relevant only for time variations oscillating at optical frequencies, thus presenting a significant hurdle that severely limits the realization of such conditions. Here we present a thin-film material with a permittivity that pulsates (uniformly in space) at optical frequencies and realizes a time-reversing medium of the form originally proposed by Pendry [Science 322, 71 (2008)SCIEAS0036-807510.1126/science.1162087]. We use an optically pumped, 500 nm thick film of epsilon-near-zero (ENZ) material based on Al-doped zinc oxide. An incident probe beam is both negatively refracted and time reversed through a reflected phase-conjugated beam. As a result of the high nonlinearity and the refractive index that is close to zero, the ENZ film leads to time reversed beams (simultaneous negative refraction and phase conjugation) with near-unit efficiency and greater-than-unit internal conversion efficiency. The ENZ platform therefore presents the time-reversal features required, e.g., for efficient subwavelength imaging, all-optical isolators and fundamental quantum field theory studies.

Filamentation-free self-compression of mid-infrared pulses in birefringent crystals with second-order cascading-enhanced self-focusing nonlinearity.

We experimentally demonstrate virtually lossless, filamentation-free and energy-scalable more than three-fold self-compression of mid-infrared laser pulses at 2.1 µm in a birefringent medium (ß-BBO crystal), which stems from favorable interplay between the second-order cascading-enhanced self-phase modulation and anomalous group velocity dispersion. By choosing an appropriate input beam diameter and intensity, the self-compression down to sub-30 fs pulse widths with gigawatt peak power is achieved without the onset of beam filamentation and associated nonlinear losses due to the multiphoton absorption, yielding the energy throughput greater than 86%.

Second-order cascading-assisted filamentation and controllable supercontinuum generation in birefringent crystals.

We experimentally investigate filamentation and supercontinuum generation in a birefringent medium (BBO crystal), in the self-focusing regime where intrinsic cubic nonlinearity is either enhanced or reduced by the second-order cascading due to phase-mismatched second harmonic generation. We demonstrate that the supercontinuum spectral extent is efficiently controlled by varying the phase mismatch parameter. In the range of negative phase mismatch, we achieve full control of the blue-shifted spectral broadening, which is very robust and independent on the input pulse energy. In the range of positive phase mismatch, both the blue-shifted and the red-shifted spectral broadenings are controlled simultaneously, however showing a certain dependence on the input pulse energy. The results are interpreted in terms of complex interplay between the self-phase-matched second harmonic generation, which is a process inherent to narrow ultrashort pulsed laser beams and concurrent self-steepening processes which arise from cubic and cascaded-quadratic nonlinearities.

Ultrabroadband supercontinuum and third-harmonic generation in bulk solids with two optical-cycle carrier-envelope phase-stable pulses at 2 µm.

We report on the generation of ultrabroadband supercontinuum (SC) by filamentation of two optical-cycle, carrier-envelope phase-stable pulses at 2 µm in fused silica, sapphire, CaF2 and YAG. The SC spectra extend from 450 nm to more than 2500 nm, and their particular shapes depend on dispersive properties of the materials. Prior to spectral super-broadening, we observe third-harmonic generation, which occurs in the condition of large phase and group velocity mismatch and consists of free and driven components. A double-peaked third-harmonic structure coexists with the SC pulse as demonstrated by the numerical simulations and verified experimentally. The SC pulses have stable carrier envelope phase with short-term rms fluctuations of ∼ 300 mrad, as simultaneously measured in YAG crystal by f-2f and f-3f interferometry, where the latter makes use of intrinsic third-harmonic generation.

Generation of carrier-envelope phase-stable two optical-cycle pulses at 2 µm from a noncollinear beta-barium borate optical parametric amplifier.

We report on the generation of two optical-cycle, carrier-envelope phase-stable pulses with energy of 15 µJ at central wavelength of 2 µm. Pulses of 15 fs (2.3 optical cycles) duration are obtained by difference-frequency generation, which at the same time provides passive carrier-envelope phase stabilization, and noncollinear optical parametric amplification in beta-barium borate crystal, which is shown to provide broad phase-matching bandwidth if seeded by pulses in the 1.6-2.6 µm wavelength range. Pulse compression is achieved by means of a simple propagation through the optical setup and by precisely controlling the initial chirp of the pulses to be frequency downconverted.

Noctilucent clouds: modern ground-based photographic observations by a digital camera network.

Noctilucent, or "night-shining," clouds (NLCs) are a spectacular optical nighttime phenomenon that is very often neglected in the context of atmospheric optics. This paper gives a brief overview of current understanding of NLCs by providing a simple physical picture of their formation, relevant observational characteristics, and scientific challenges of NLC research. Modern ground-based photographic NLC observations, carried out in the framework of automated digital camera networks around the globe, are outlined. In particular, the obtained results refer to studies of single quasi-stationary waves in the NLC field. These waves exhibit specific propagation properties--high localization, robustness, and long lifetime--that are the essential requisites of solitary waves.

Accurate retrieval of pulse-splitting dynamics of a femtosecond filament in water by time-resolved shadowgraphy.

By means of a time-resolved, shadowgraphic method we observed directly the development of the pulse-splitting dynamics of a femtosecond laser pulse propagating in the filamentary regime in water. For the first time to our knowledge, the relative splitting velocity was measured. We compare the experimental data with numerical simulations. A possible scenario for the splitting event and evolution of the fragments is discussed.

Time-resolved refractive index and absorption mapping of light-plasma filaments in water.

By means of a quantitative shadowgraphic method, we performed a space-time characterization of the refractive index variation and transient absorption induced by a light-plasma filament generated by a 120 fs laser pulse in water. The formation and evolution of the plasma channel in the proximity of the nonlinear focus were observed with a 23 fs time resolution.

Stimulated Raman X waves in ultrashort optical pulse filamentation.

We demonstrate that ultrashort pulse filamentation in liquids with strong Raman gain leads to the spontaneous formation of nonlinear X waves at a Raman-shifted wavelength. We measured as much as 75% energy conversion efficiency into a Raman X wave in ethanol starting from 1 ps pulses due to the group velocity matching between the pump and Raman X pulses. Large Raman gain of a weak seed signal was observed in water, associated with a strong spatiotemporal transformation of the seed into an X wave.

Competition between phase-matching and stationarity in Kerr-driven optical pulse filamentation.

Experiments show that the spatiotemporal spectral broadening of an intense pump pulse in a Kerr medium in the presence of strong higher-order dispersion does not lead to symmetric profiles, and hence cannot be interpreted as standard modulational instability of a plane and monochromatic nonlinear eigenmode. The highly asymmetric features of the generated (K perpendicular,Omega) spectrum are due to odd-order dispersion terms and are interpreted in terms of spontaneous formation of stationary conical waves.

Conical emission, pulse splitting, and X-wave parametric amplification in nonlinear dynamics of ultrashort light pulses.

The precise observation of the angle-frequency spectrum of light filaments in water reveals a scenario incompatible with current models of conical emission (CE). Its description in terms of linear X-wave modes leads us to understand filamentation dynamics requiring a phase- and group-matched, Kerr-driven four-wave-mixing process that involves two highly localized pumps and two X waves. CE and temporal splitting arise naturally as two manifestations of this process.

High localization, focal depth and contrast by means of nonlinear Bessel beams.

We show an experimental and computational comparison between the resolution power, the contrast and the focal depth of a nonlinearly propagated diffraction-free beam and of other beams (a linear and a nonlinearly propagated Gaussian pulse): launching a nondiffractive Bessel pulse in a solution of Coumarine 120 in methanol creates a high contrast, 40 mm long, 10 microm width fluorescence channel excited by 3-photon absorption process. This fluorescence channel exhibits the same contrast and resolution of a tightly focused Gaussian pulse, but reaches a focal depth that outclasses by orders of magnitude that reached by an equivalent Gaussian pulse.

From X- to O-shaped spatiotemporal spectra of light filaments in water.

We show that the angle-wavelength spectra of light filaments excited by ultrashort pulses experience a transition from X- to O-like structures when their carrier wavelengths are switched from normal to anomalous dispersion. Calculations confirm that the O-shaped conical emission follows the elliptic geometry of the nonlinear Schrödinger equation with anomalous dispersion.