Gas-filled Hollow Waveguide Research Articles

Generation of Intense Low-Divergence Isolated Soft-X-Ray Attosecond Pulses in a Gas-Filled Waveguide Using Three-Color Synthesized Laser Pulses

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

Resonant dispersive wave emission in hollow capillary fibers filled with pressure gradients.

Resonant dispersive wave (RDW) emission in gas-filled hollow waveguides is a powerful technique for the generation of bright few-femtosecond laser pulses from the vacuum ultraviolet to the near infrared. Here, we investigate deep-ultraviolet RDW emission in a hollow capillary fiber filled with a longitudinal gas pressure gradient. We obtain broadly similar emission to the constant-pressure case by applying a surprisingly simple scaling rule for the gas pressure and study the energy-dependent dispersive wave spectrum in detail using simulations. We further find that in addition to enabling dispersion-free delivery to experimental targets, a decreasing gradient also reduces the pulse stretching within the waveguide itself, and that transform-limited pulses with 3 fs duration can be generated by using short waveguides. Our results illuminate the fundamental dynamics underlying this frequency conversion technique and will aid in fully exploiting it for applications in ultrafast science and beyond.

A tabletop experimental system for investigating ultrafast atomic dynamics based on femtosecond extreme ultraviolet photons

Femtosecond extreme ultraviolet (XUV) light pulses play an important role in investigating the ultrafast dynamics of atoms and molecules, and are complementary to the conventional large facilities like synchrotron radiation and free electron laser. We build a table-top femtosecond extreme ultraviolet light source based on the high-order harmonic generation (HHG) process of gaseous medium in a strong laser field. We implement HHG by focusing an intense IR laser into a 5 cm long gas-filled hollow waveguide, instead of the conventional tightly focusing geometry with gas jet. Inside the waveguide, the laser peak intensity is nearly constant and the gas pressure is well-controlled, making it possible to maintain the phase matching condition over an extended distance. And a fully coherent high harmonic beam builds up along the waveguide, leading to a dramatically higher HHG efficiency. Monochromatic XUV light pulses are obtained by spectral selection of the HHG through employing the conical diffraction method of grating. With this geometry used, the pulse broadening caused by wave front tilting during the diffraction can be strongly suppressed, especially for the case of grazing incidence. And the femtosecond temporal character of the light pulse can be preserved while keeping a high reflectivity. The temporal broadening of the XUV light pulse in our setup is estimated to be within 100 femtosecond. By using different noble gases, photons with energy values ranging from 20 eV to 90 eV are produced. For the 27<sup>th</sup>-order harmonic centered at 41.9 eV, the flux is measured to be 1 × 10<sup>10</sup> photons per second, with an energy spread of 0.4 eV. In order to investigate the ultrafast dynamic behaviors of gaseous atoms and molecules with an HHG-based XUV source, we develop a reaction microscope with ultrahigh vacuum of about 10<sup>–11</sup> mbar. The combination of HHG-based XUV with the newly developed reaction microscope provides a unique tool for studying the XUV photon and atom/molecule interaction. A series of experiments has been successfully carried out on the platform and the system shows good performance.

Observation of saturated absorption spectrum of 12C2H2 at 1536 nm in a 300 μm hollow glass waveguide

A system for saturated absorption spectroscopy is firstly built (to our knowledge) based on a gas-filled hollow glass waveguide with a core diameter of 300 μm. The performance of the system is demonstrated by observing the saturated absorption of the P(17) transition of the (ν1 +ν3) combination band of acetylene around 1536 nm using an erbium-doped fiber amplifier boosted external cavity diode laser. The saturated absorption spectrum is observed and the observed linewidth of the saturation dip varies from 10 to 20 MHz depending on the gas pressure and the optical launching condition into the GW. Its derivative spectrum is obtained using wavelength modulation spectroscopy and the third derivative signal is used as the error signal to lock the external cavity diode laser to the transition center. A frequency stability of ~60 kHz at 1 s integration time was achieved. Our study shows it is possible to build a mid-IR frequency reference using a large core gas-filled hollow GW.

Optimal generation of spatially coherent soft X-ray isolated attosecond pulses in a gas-filled waveguide using two-color synthesized laser pulses.

We numerically demonstrate the generation of intense, low-divergence soft X-ray isolated attosecond pulses in a gas-filled hollow waveguide using synthesized few-cycle two-color laser waveforms. The waveform is a superposition of a fundamental and its second harmonic optimized such that highest harmonic yields are emitted from each atom. We then optimize the gas pressure and the length and radius of the waveguide such that bright coherent high-order harmonics with angular divergence smaller than 1 mrad are generated, for photon energy from the extreme ultraviolet to soft X-rays. By selecting a proper spectral range enhanced isolated attosecond pulses are generated. We study how dynamic phase matching caused by the interplay among waveguide mode, neutral atomic dispersion, and plasma effect is achieved at the optimal macroscopic conditions, by performing time-frequency analysis and by analyzing the evolution of the driving laser’s electric field during the propagation. Our results, when combined with the on-going push of high-repetition-rate lasers (sub- to few MHz’s) may eventually lead to the generation of high-flux, low-divergence soft X-ray tabletop isolated attosecond pulses for applications.

Generation of Bright, Spatially Coherent Soft X-Ray High Harmonics in a Hollow Waveguide Using Two-Color Synthesized Laser Pulses.

We investigate the efficient generation of low-divergence high-order harmonics driven by waveform-optimized laser pulses in a gas-filled hollow waveguide. The drive waveform is obtained by synthesizing two-color laser pulses, optimized such that highest harmonic yields are emitted from each atom. Optimization of the gas pressure and waveguide configuration has enabled us to produce bright and spatially coherent harmonics extending from the extreme ultraviolet to soft x rays. Our study on the interplay among waveguide mode, atomic dispersion, and plasma effect uncovers how dynamic phase matching is accomplished and how an optimized waveform is maintained when optimal waveguide parameters (radius and length) and gas pressure are identified. Our analysis should help laboratory development in the generation of high-flux bright coherent soft x rays as tabletop light sources for applications.

Compression of TW class laser pulses in a planar hollow waveguide for applications in strong-field physics

We demonstrate pulse post-compression of a TW class chirped pulse amplification laser employing a gas-filled planar hollow waveguide. A waveguide throughput of 80% is achieved for 50 mJ input pulse energy. Good focusability is found and after compression with chirped mirrors a pulse duration of sub-15 fs is measured in the beam center. Whereas a total energy efficiency of ≈70% should be achievable, our post-compressor currently delivers 20 mJ output pulse energy (≈40% efficiency), mostly limited by apertures of chirped mirrors and vacuum windows. The viability of the planar hollow waveguide compression scheme for applications in strong-field physics is demonstrated by generating high-order harmonics in a pulsed Ar gas cell.

Models for supercontinuum generation beyond the slowly-varying-envelope approximation

We show numerically that both the modified Korteweg--de Vries and the sine-Gordon models are conducive to the generation of supercontinua with spectral bandwidths of several octaves, when an intense short pulse is launched as the initial condition. These models beyond the slowly-varying-envelope approximation could play an important role in modeling supercontinuum generation in gas-filled hollow waveguides.

Efficient generation of ultra-intense few-cycle radially polarized laser pulses.

We report on efficient generation of millijoule-level, kilohertz-repetition-rate few-cycle laser pulses with radial polarization by combining a gas-filled hollow-waveguide compression technique with a suitable polarization mode converter. Peak power levels >85 GW are routinely achieved, capable of reaching relativistic intensities >10(19) W/cm2 with carrier-envelope-phase control, by employing readily accessible ultrafast high-energy laser technology.

Ionization-assisted guided-wave pulse compression to extreme peak powers and single-cycle pulse widths in the mid-infrared

Ionization-assisted spectral broadening of high-energy 10.6 μm laser pulses in a gas-filled hollow waveguide is shown to yield single-cycle pulses with multiterawatt peak powers in the mid-IR. While the highest quality of pulse compression is achieved in the regime of weak ionization, careful management of complex ionization-assisted spectral broadening of guided-wave fields is the key to compressing the output of advanced high-power mid-IR laser sources to single-cycle pulse widths.

Frequency-Tunable Multigigawatt Sub-Half-Cycle Light Pulses from Coupled-State Dynamics of Optical Solitons and Impulsively Driven Molecular Vibrations

Coupling ultrashort optical field waveforms to ultrafast molecular vibrations in an impulsively excited Raman medium is shown to enable the generation of frequency-tunable sub-half-cycle multigigawatt light pulses. In a gas-filled hollow waveguide, this coupled-state dynamics is strongly assisted by soliton effects, which help to suppress temporal stretching of subcycle optical pulses, providing efficient Raman-type impulsive excitation of ultrafast molecular vibrations over large propagation paths.

Pulse compression with planar hollow waveguides: a pathway towards relativistic intensity with table-top lasers

We study in detail the compression of high-energy ultrashort laser pulses to the few-cycle regime in gas-filled planar hollow waveguides. In this scheme, the laser beam is guided in only one transverse dimension, whereas the other dimension is free to adjust, allowing scalability to high pulse energies. We report on various practical aspects of the planar hollow waveguide compression scheme and characterize the dependence of the performance of the method on several experimental parameters: (i) we evaluate different materials for the construction of planar waveguides; (ii) we investigate the dependence of the pulse duration on gas type and pressure; (iii) we measure the spatial intensity and phase; (iv) we characterize the pulse duration along the transverse beam direction; and (v) we investigate the focusability. An output pulse energy of 10.6 mJ at a duration of 10.1 fs (FWHM) in the beam center after compression is demonstrated. A careful estimation reveals that the radiation should be focusable to a relativistic intensity exceeding 1019 W cm−2 in the few-cycle regime. The experimental results are supported by numerical modeling of nonlinear pulse propagation inside planar hollow waveguides. We discuss energy up-scalability exceeding the 100 mJ level.

Compression of ultrashort laser pulses in planar hollow waveguides: a stability Analysis

We investigate compression of ultrashort laser pulses by nonlinear propagation in gas-filled planar hollow waveguides, using (3+1)- dimensional numerical simulations. In this geometry, the laser beam is guided with a fixed size in one transverse dimension, generating significant spectral broadening, while it propagates freely in the other, allowing for energy up-scalability. In this respect the concept outperforms compression techniques based on hollow core fibers or filamentation. Small-scale self-focusing is a crucial consideration, which introduces mode deterioration and finally break-up in multiple filaments. The simulation results, which match well with initial experiments, provide important guidelines for scaling the few-cycle pulse generation to higher energies. Pulse compression down to few-cycle duration with energies up to 100 mJ levels should be possible.

High-energy ultrashort laser pulse compression in hollow planar waveguides

We demonstrate compression of high-energy ultrashort laser pulses by nonlinear propagation inside gas-filled planar hollow waveguides. We adjust the input beam size along the nonguided dimension of the planar waveguide to restrain the intensity below photoionization, while the relatively long range guided propagation yields significant self-phase modulation and spectral broadening. We compare the compression in different noble gases and obtain 13.6 fs duration with output pulse energy of 8.1 mJ in argon and 11.5 fs duration with 7.6 mJ energy in krypton. The broadened spectra at the output of the waveguide are uniform over more than 70% of the total pulse energy. Shorter duration could be obtained at the expense of the introduction of spatial structure in the beam (and eventual formation of filaments) resulting from small-scale self-focusing in the nonguided direction.

Design of fully spatially coherent extreme-ultraviolet light sources

We demonstrate experimentally that, in order to generate fully spatially coherent extreme-ultraviolet (EUV) beams using high-harmonic generation, it is necessary to guide the driving laser beam over long interaction lengths in gas-filled hollow waveguides. Numerical simulations show that, in propagating the laser through a long plasma-filled guide, the laser beam forms a stable eigenmode with uniform spatial phase, even at very high levels of ionization. This results in a compact, highly spatially coherent, EUV source useful for applications in EUV metrology, microscopy, interferometry, and holography.

Coherent soft x-ray generation in the water window with quasi-phase matching.

We demonstrate enhanced generation of coherent light in the "water window" region of the soft x-ray spectrum at 4.4 nanometers, using quasi-phase-matched frequency conversion of ultrafast laser pulses. By periodically modulating the diameter of a gas-filled hollow waveguide, the phase mismatch normally present between the laser light and the generated soft x-ray light can be partially compensated. This makes it possible to use neon gas as the nonlinear medium to coherently convert light up to the water window, illustrating that techniques of nonlinear optics can be applied effectively in the soft x-ray region of the spectrum. These results advance the prospects for compact coherent soft x-ray sources for applications in biomicroscopy and in chemical spectroscopy.

Pulse contrast enhancement of high-energy pulses by use of a gas-filled hollow waveguide

Using nonlinear ellipse rotation in a gas-filled hollow waveguide, we have increased the pulse contrast of a microjoule femtosecond laser pulse by several orders of magnitude. This scheme offers a number of advantages over competing techniques, including a high degree of tunability that allows for a broad range of input pulse parameters, higher throughput, greater stability, and an output pulse with high spatial quality that is compressible to a quarter of the original temporal width.

High-order harmonic and attosecond pulse generations: Bulk media versus hollow waveguides

Generation and propagation of high-order harmonics by intense, ultrashort laser pulses in static gas cells and gas-filled hollow waveguides are numerically investigated. The calculations are performed utilizing a self-consistent solution of the time-dependent Schr\odinger and wave equations. It is shown that, when the 30-fs laser pulse is focused into a gas cell the emission spectrum consists of discrete and well-resolved harmonics. These high-order harmonics can be used for generation of a train of subfemtosecond pulses. When the 5-fs laser pulse propagates along a gas-filled hollow waveguide the emission spectrum exhibits a quasicontinous structure that permits the generation of a single subfemtosecond pulse. The effects of laser intensity and position of the interacting medium relative to the laser focus on the emission spectrum and pulse profile are investigated. The differences between results obtained by our model and by the model that uses the strong-field approximation for single-atom calculations are discussed.

Self-and cross-phase modulation accompanying third-harmonic generation in a hollow waveguide

The influence of self-and cross-phase modulation on third-harmonic generation in a hollow waveguide is investigated. Analytic solutions of the coupled equations for the slowly varying amplitudes of the pump pulse and the third harmonic in a gas-filled hollow waveguide are obtained with consideration of self-and cross-phase modulation and first-order dispersion effects. The possibility of controlling the nonlinear phase trajectory of the third harmonic by cross-phase modulation is demonstrated.

Theory of ultrabroadband optical pulse generation by induced phase modulation in a gas-filled hollow waveguide

Generation of an ultrabroadband optical pulse with a fluent frequency dependency of the phase is important for creating a monocyclelike optical pulse and for shaping multiwavelength optical pulses. A previously proposed method that uses induced phase modulation between a femtosecond fundamental wave ω1 and its second-harmonic wave ω2=2ω1 in a fused-silica fiber is applied to a capillary fiber filled with noble gas. Analytic results of chirps without dispersion but with loss in the fiber are shown, and the optimum conditions relating to a delay time between two pulses and to input peak powers are found for fully covering the spectrum between ω1 and ω2. Furthermore, numerical calculations, including dispersion effects of the fundamental and the second-harmonic waves from a Ti:sapphire laser-amplifier system with experimentally realizable parameters, are presented. These calculations show that it is possible to generate an ultrabroadband optical pulse whose spectrum ranges from 300 to 900 THz (330 to 1000 nm) with quasi-linear chirp by this method.