Microexplosions in bulk sapphire driven by simultaneously spatially and temporally focused femtosecond laser beams.

We report experimental results on the formation of microvoids in bulk sapphire produced through the single-shot illumination of the sample by near-infrared, simultaneously spatially and temporally focused (SSTF) laser beams. Compared with the cases of tightly focused femtosecond Gaussian and flattop beams, the SSTFs produce internal microvoids with significantly larger volumes and without cracks between the interaction sites, which may be beneficial for applications in volumetric data storage and for the studies of exotic, super-dense elemental phases contained in the densified shells surrounding the microvoids.

In-line Spectral Interferometry in Shortwave-Infrared Laser Filaments in Air.

We investigate the nonlinear propagation of intense, two-cycle, carrier-envelope phase (CEP) stable laser pulses at 1.7 µm center wavelength in air. We observe CEP-dependent spectral interference in the visible part of the forward-propagating white light generated on propagation. The effect is robust against large fluctuations of the input pulse energy. This robustness is enabled by rigid clamping of both the peak optical field and the phase of the propagating waveform, which has been revealed by numerical simulations. The CEP locking can enhance the yield of the CEP-dependent strong-field processes in gaseous media with long-wavelength drivers, while the observed spectral interference enables single-shot, stand-off CEP metrology in the atmosphere.

Robust Remote Sensing of Trace-Level Heavy-Metal Contaminants in Water Using Laser Filaments.

Water is the major natural resource that enables life on our planet. Rapid detection of water pollution that occurs due to both human activity and natural cataclysms is imperative for environmental protection. Analytical chemistry-based techniques are generally not suitable for rapid monitoring because they involve collection of water samples and analysis in a laboratory. Laser-based approaches such as laser-induced breakdown spectroscopy (LIBS) may offer a powerful alternative, yet conventional LIBS relies on the use of tightly focused laser beams, requiring a stable air-water interface in a controlled environment. Reported here is a proof-of-principle, quantitative, simultaneous measurement of several representative heavy-metal contaminants in water, at ppm-level concentrations, using ultraintense femtosecond laser pulses propagating in air in the filamentation regime. This approach is straightforwardly extendable to kilometer-scale standoff distances, under adverse atmospheric conditions and is insensitive to the movements of the water surface due to the topography and water waves.

Measurements of fluence profiles in femtosecond laser filaments in air.

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our approach is based on the dependence of single-shot laser ablation threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser filaments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the external beam focusing. The limiting value of the peak intensity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f-number above ∼500), is about 55 TW/cm2.

Generation of multi-millijoule red-shifted pulses for seeding stimulated Raman backscattering amplifiers.

The efficient generation of redshifted pulses from chirped femtosecond joule level Bessel beam pulses in gases is studied. The redshift spans from a few 100 cm⁻¹ to several 1000 cm⁻¹ corresponding to a shift of 50-500 nm for Nd:glass laser systems. The generated pulses have an almost perfect Gaussian beam profile insensitive of the pump beam profile, and are much shorter than the pump pulses. The highest measured energy is as high as 30 mJ, which is significantly higher than possible with solid state nonlinear frequency shifters.

Laser interaction with materials: introduction.

Laser-materials interaction is the fascinating nexus where laser physics, optical physics, and materials science intersect. Applications include microdeposition via laser-induced forward transfer of thin films, clean materials processing with femtosecond beams, creating color filters with nanoparticles, generating very high density storage sites on subpicosecond time scales, structuring solar cell surfaces for higher efficiency, making nanostructures that would be impossible by other means, and creating in-volume waveguiding structures using femtosecond laser filaments.

Generation of multiterawatt vortex laser beams.

We report the fabrication of large-area phase masks on thin fused-silica substrates that are suitable for shaping multiterawatt femtosecond laser beams. We apply these phase masks for the generation of intense femtosecond optical vortices. We further quantify distortions of the vortex beam patterns that result from several common types of mask defects.

Low-threshold bidirectional air lasing.

Air lasing refers to the remote optical pumping of the constituents of ambient air that results in a directional laserlike emission from the pumped region. Intense current investigations of this concept are motivated by the potential applications in remote atmospheric sensing. Different approaches to air lasing are being investigated, but, so far, only the approach based on dissociation and resonant two-photon pumping of air molecules by deep-UV laser pulses has produced measurable lasing energies in real air and in the backward direction, which is of the most relevance for applications. However, the emission had a high pumping threshold, in hundreds of GW/cm^{2}. We demonstrate that the threshold can be virtually eliminated through predissociation of air molecules with an additional nanosecond laser. We use a single tunable pump laser system to generate backward-propagating lasing in both oxygen and nitrogen in air, with energies of up to 1 µJ per pulse.

Third and fifth harmonic generation by tightly focused femtosecond pulses at 2.2 µm wavelength in air.

We report experiments on the generation of third and fifth harmonics of millijoule-level, tightly focused, femtosecond laser pulses at 2.2 µm wavelength in air. The measured ratio of yields of the third and fifth harmonics in our setup is found equal to 2 · 10(-4). This result contradicts the recent suggestion that the Kerr effect in air saturates and changes sign in ultra-intense optical fields.

Self-focusing of femtosecond diffraction-resistant vortex beams in water.

We report experiments on self-focusing of femtosecond diffraction-resistant vortex beams in water. These beams are higher-order Bessel beams with weak azimuthal modulation of the transverse intensity patterns. The modulation overrides the self-focusing dynamics and results in the formation of regular bottlelike filament distributions. The peak-power thresholds for filamentation, at a particular distance, are relatively accurately estimated by the adaptation of the Marburger formula derived earlier for Gaussian beams. The nonlinear conversion of the incident conical waves into the localized spatial wave packets propagating near the beam axis is observed.

Experimental tests of the new paradigm for laser filamentation in gases.

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multiphoton ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments. Our results consistently support the established paradigm.

Standoff spectroscopy via remote generation of a backward-propagating laser beam.

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.

Supercontinuum generation with femtosecond self-healing Airy pulses.

We report experiments and numerical simulations on supercontinuum generation with femtosecond Airy pulses in a highly nonlinear optical fiber. The ability of the Airy waveform to regenerate its dominant intensity peak results in the generation of distinct spectral features. Airy pulses and other self-healing temporal waveforms may be useful for the generation of spectra with desired properties.

Filamentation of femtosecond laser Airy beams in water.

We report experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water. The supercontinuum radiation generated along the curved beam path is angularly resolved in the far field. Spectral maps of this radiation reveal the changing character of the laser-pulse evolution on propagation.

Curved plasma channel generation using ultraintense Airy beams.

Plasma channel generation (or filamentation) using ultraintense laser pulses in dielectric media has a wide spectrum of applications, ranging from remote sensing to terahertz generation to lightning control. So far, laser filamentation has been triggered with the use of ultrafast pulses with axially symmetric spatial beam profiles, thereby generating straight filaments. We report the experimental observation of curved plasma channels generated in air using femtosecond Airy beams. In this unusual propagation regime, the tightly confined main intensity feature of the axially nonsymmetric laser beam propagates along a bent trajectory, leaving a curved plasma channel behind. Secondary channels bifurcate from the primary bent channel at several locations along the beam path. The broadband radiation emanating from different longitudinal sections of the curved filament propagates along angularly resolved trajectories.

Extended filamentation with temporally chirped femtosecond Bessel-Gauss beams in air.

We report experimental results on ultrafast filamentation with temporally chirped femtosecond Bessel-Gauss beams. We find that by chirping the pulses, the longitudinal range of the generated plasma channels can be extended relative to filaments generated by fully compressed, transform-limited femtosecond pulses. We find a clear correlation between the extent of filamentation and the intensity of the on-axis emission by the femtosecond Bessel-Gauss beam. The on-axis emission is negligible for fully compressed pulses, but it can become quite substantial (up to 10% of the input pulse energy) when chirped pulses are used. Under certain conditions, the on-axis emission becomes sufficient for generating its own plasma channel thus resulting in extended filamentation. This effect may offer means of remote control over filament formation with femtosecond Bessel-Gauss beams.We identify a four-wave mixing process, enhancement of which is likely to result in a maximum of the on-axis emission, and derive a simple expression for estimating the duration of the chirped pulse that is required for such enhancement. Our estimations are in good agreement with the experiment.

Generation of extended plasma channels in air using femtosecond Bessel beams.

Extending the longitudinal range of plasma channels created by ultrashort laser pulses in atmosphere is important in practical applications of laser-induced plasma such as remote spectroscopy and lightning control. Weakly focused femtosecond Gaussian beams that are commonly used for generating plasma channels offer only a limited control of filamentation. Increasing the pulse energy in this case typically results in creation of multiple filaments and does not appreciably extend the longitudinal range of filamentation. Bessel beams with their extended linear foci intuitively appear to be better suited for generation of long plasma channels. We report experimental results on creating extended filaments in air using femtosecond Bessel beams. By probing the linear plasma density along the filament, we show that apertured Bessel beams produce stable single plasma channels that span the entire extent of the linear focus of the beam. We further show that by temporally chirping the pulse, the plasma channel can be longitudinally shifted beyond the linear-focus zone, an important effect that may potentially offer additional means of controlling filament formation.

Optimized multiemitter beams for free-space optical communications through turbulent atmosphere.

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.

All-fiber passively mode-locked laser oscillator at 1.5 microm with watts-level average output power and high repetition rate.

We report on a passively mode-locked all-fiber laser oscillator at 1.5 microm based on heavily doped phosphate-glass active fiber. An active fiber only 20 cm long is sufficient to produce as much as 2.4 W of average output power directly from the oscillator. The width of the mode-locked pulses varies from 8 ps at the lowest output power in the mode-locked state to 44 ps at the highest power. Our picosecond laser oscillator features a high repetition rate of 95 MHz and high peak pulse power of approximately 540 W. The oscillator combines the convenience of all-fiber construction with power performance that was previously achievable only with mode-locked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.

Single-frequency laser oscillator with watts-level output power at 1.5 microm by use of a twisted-mode technique.

We report an all-fiber laser oscillator producing as much as 1.9 W of single-frequency direct output at 1.5 microm. Spatial gain hole burning in the active fiber has been eliminated by use of a twisted-mode cavity approach. The two short pieces of a polarization-maintaining fiber that were spliced to the ends of the active fiber served as ultracompact quarter-wave plates. To our knowledge, the use of such a wave plate to manipulate the polarization state of light inside a fiber laser cavity is reported here for the first time. The laser output is linearly polarized and delivered through a polarization-maintaining fiber pigtail. We believe that the output power of our laser is the highest among all single-frequency fiber laser oscillators reported to date.