Tight Focusing Geometry Research Articles

A narrow bandwidth extreme ultra-violet light source for time- and angle-resolved photoemission spectroscopy.

Here, we present a high repetition rate, narrow bandwidth, extreme ultraviolet photon source for time- and angle-resolved photoemission spectroscopy. The narrow bandwidth pulses meV for photon energies eV are generated through high harmonic generation using ultra-violet drive pulses with relatively long pulse lengths (461 fs). The high harmonic generation setup employs an annular drive beam in tight focusing geometry at a repetition rate of 250 kHz. Photon energy selection is provided by a series of selectable multilayer bandpass mirrors and thin film filters, thus avoiding any time broadening introduced by single grating monochromators. A two stage optical-parametric amplifier provides < 100 fs tunable pump pulses from 0.65 μm to 9 μm. The narrow bandwidth performance of the light source is demonstrated through angle-resolved photoemission measurements on a series of quantum materials, including high-temperature superconductor Bi-2212, WSe2, and graphene.

Predominant contribution of direct laser acceleration to high-energy electron spectra in a low-density self-modulated laser wakefield accelerator

The two-temperature relativistic electron spectrum from a low-density ($3\times10^{17}$~cm$^{-3}$) self-modulated laser wakefield accelerator (SM-LWFA) is observed to transition between temperatures of $19\pm0.65$ and $46\pm2.45$ MeV at an electron energy of about 100 MeV. When the electrons are dispersed orthogonally to the laser polarization, their spectrum above 60 MeV shows a forking structure characteristic of direct laser acceleration (DLA). Both the two-temperature distribution and the forking structure are reproduced in a quasi-3D \textsc{Osiris} simulation of the interaction of the 1-ps, moderate-amplitude ($a_{0}=2.7$) laser pulse with the low-density plasma. Particle tracking shows that while the SM-LWFA mechanism dominates below 40 MeV, the highest-energy ($>60$ MeV) electrons gain most of their energy through DLA. By separating the simulated electric fields into modes, the DLA-dominated electrons are shown to lose significant energy to the longitudinal laser field from the tight focusing geometry, resulting in a more accurate measure of net DLA energy gain than previously possible.

High efficiency second harmonic generation of nanojoule-level femtosecond pulses in the visible based on BiBO

We demonstrate high efficiency second harmonic generation (SHG) of near infrared femtosecond pulses using a $\text{BiB}_{3}\text{O}_{6}$ crystal in a single-pass tight focusing geometry setup. A frequency doubling efficiency of $63\%$ is achieved, which is, to the best of our knowledge, the highest value ever reported in the femtosecond regime for such low energy (nJ-level) pumping pulses. Theoretical analyses of the pumping scheme focusing waist and the SHG efficiency are performed, by numerically solving the three wave mixing coupled equations in the plane-wave scenario and by running simulations with a commercial full 3D code. Simulations show a good agreement with the experimental data regarding both the efficiency and the pulse spectral profile. The simulated SHG pulse temporal profile presents the characteristic features of the group velocity mismatch broadening in a ‘thick’ crystal.

Highly extended high density filaments in tight focusing geometry in water: from femtoseconds to microseconds

We report a new regime of filamentation in water in tight focusing geometry, very similar to the so-called superfilamentation seen in air. In this regime there is no observable conical emission and multiple small-scale filaments, but instead a single continuous plasma channel is formed. To achieve this specific regime the principal requirement is the usage of tight focusing and supercritical power of laser radiation. Together they guarantee extremely high intensity in the microvolume in water (∼1014 W cm−2) and clamp the energy in the ultra-thin (approximately several microns) channel with a uniform plasma density distribution in it. Each point of the ‘superfilament’ becomes a center of spherical shock wave generation. The overlapped shock waves transform into one cylindrical shock wave. At low energies, a single spherical shock wave is generated from the laser beam waist, and its radius tends toward saturation as energy increases. At higher energies, a long stable contrast cylindrical shock wave is generated, whose length increases logarithmically with laser pulse energy. The linear absorption decreases the incoming energy delivered to the focal spot, which dramatically complicates the filament formation, especially in the case of loose focusing. Aberrations added to the optical scheme lead to multiple dotted plasma sources for shock wave formation, spaced along the axis of pulse propagation. Increasing the laser energy launches the filaments at each of the dots, whose overlapping leads to enhancing the length of the whole filament and therefore the shock impact on the material.

Dynamics of multiple bubbles, excited by a femtosecond filament in water

Using shadow photography we observed the evolution of multiple cavitation bubbles, excited by a femtosecond laser pulse in water, up to the microsecond time scale. In the tight focusing geometry a single filament is formed. The filament becomes the center of the cavitation region formation. When aberrations were added to the optical scheme, aberration hot spots along the filament axis are formed. At high energies (more than 40 μJ) the filaments in the aberration hot spots are fired. Thereby a complex pattern of the cavitation bubbles is created. The bubbles can be isolated from each other or can form an exotic ‘drop-shaped’ cavitation region, whose evolution at the end of its ‘life’, before the final collapse, contains the jet emission. The dynamics of the cavitation pattern were investigated from the pulse energy and the focusing. An increase of the numerical aperture of the focusing optics leads to an increase of the cavitation area length. A strong interaction between the bubbles was also found. This leads to a significant change in the bubbles’ evolution, which is not yet in accordance with the Rayleigh model.

Spectral investigation of higher-order Kerr effects in a tight-focusing geometry

The role of the Higher-Order Kerr Effects (HOKE) in intensity clamping is experimentally investigated. We fail to observe any evidence of HOKE-based intensity clamping in a tight geometrical focusing experiment. We introduce a polarization-based technique that can distinguish between spectral components from the leading and trailing edges of the pulse. The results of this time-resolved measurement support the ionization theory of intensity clamping.

Efficient Raman amplifier in the forward and backward directions pumped by picosecond laser pulses

In this letter, we report on stimulated Raman amplification in YVO4 pumped by MHz repetition-rate picosecond pulses. We demonstrate that in the case of a tight focusing geometry the efficiency of this amplification is independent of the direction of the pump pulse propagation with respect to that of the seed pulse. The polarization properties of Raman amplification are studied in detail.

Optimization and phase matching of fiber-laser-driven high-order harmonic generation at high repetition rate

High-repetition-rate sources are very attractive for high-order harmonic generation (HHG). However, due to their pulse characteristics (low energy, long duration), those systems require a tight focusing geometry to achieve the necessary intensity to generate harmonics. In this Letter, we investigate theoretically and experimentally the optimization of HHG in this geometry, to maximize the extreme UV (XUV) photon flux and improve the conversion efficiency. We analyze the influence of atomic gas media (Ar, Kr, or Xe), gas pressure, and interaction geometries (a gas jet and a finite and a semi-infinite gas cell). Numerical simulations allow us to define optimal conditions for HHG in this tight focusing regime and to observe the signature of on-axis phase matching. These conditions are implemented experimentally using a high-repetition-rate Yb-doped fiber laser system. We achieve optimization of emission with a recorded XUV photon flux of 4.5×10(12) photons/s generated in Xe at 100 kHz repetition rate.

High-order harmonic generation at 4 MHz as a light source for time-of-flight photoemission spectroscopy

We demonstrate high-order harmonic generation (HHG) at 4 MHz driven by a long-cavity femtosecond laser oscillator. The laser output is used directly in a tight focusing geometry, where the harmonics are generated from a gas jet with high backing pressure. The harmonic light source is applied to time-of-flight photoemission spectroscopy, and the characteristic electronic structure of Cu(111) is measured. Our results suggest a straightforward design of high-order harmonic generation at megahertz repetition rate and pave the way for applications in electron spectroscopy and microscopy.

High-order harmonic generation with μJ laser pulses at high repetition rates

We investigate the generation of high-order harmonics using laser pulse energies in the few-μJ range at high repetition rates. We analyse how the conversion efficiency is influenced by the tight focusing geometry required for the generation of high-order harmonics under these conditions. A generalized phase-matching model allows us to discuss macroscopic phase effects independent of focal length. We present experimental results using the example of a 100 kHz laser system to generate harmonics up to the 27th order in Ar with a photon flux up to 3 × 109 photons s−1 into one harmonic order. High-repetition-rate femtosecond or even attosecond light sources open new possibilities for a broad range of applications such as time-resolved photoelectron spectroscopy and microscopy in the extreme ultraviolet regime.

Polarization-dependent ablation of silicon using tightly focused femtosecond laser vortex pulses

We demonstrate experimentally that, in a tight focusing geometry, circularly polarized femtosecond laser vortex pulses ablate material differently depending on the handedness of light. This effect offers an additional degree of freedom to control the shape and size of laser-machined structures on a subwavelength scale.

Broadband supercontinuum generation in air using tightly focused femtosecond laser pulses

Supercontinuum generation in air using tightly focused femtosecond laser pulses was investigated experimentally. Broadband white-light emission covering the whole visible spectral region was generated. Spectral broadening extended only to the blue side of the fundamental frequency due to the phase modulation induced by the strong ionization of air. Numerical simulation was also performed to confirm the spectral broadening mechanism. A constant UV cutoff wavelength close to 400 nm was observed in the supercontinuum spectrum. This phenomenon indicated that intensity clamping still plays a role in tight focusing geometry.

Spatially coherent, phase matched, high-order harmonic EUV beams at 50 kHz

By focusing a high repetition rate (50 kHz), compact, femtosecond laser system with low pulse energy (25 muJ) using a tight-focusing geometry, we demonstrate fully phase matched high-order harmonic generation for the first time at very high repetition rates, resulting in EUV light with full spatial coherence. The result is a practical, single-box, coherent source useful for applications in metrology, ultrafast spectroscopy, imaging and microscopy. The soft x-ray flux can be improved further by increasing the laser pulse energy and/or repetition rate.

The influence of self-focusing and filamentation on refractive index modifications in fused silica using intense femtosecond pulses

The interaction of focused femtosecond infrared laser pulses at 1 kHz repetition rate with bulk fused silica is thoroughly investigated. The interplay between self-focusing and filamentation of the laser pulses is analyzed for a broad range of focusing conditions. It is shown that even in the case of very tight focusing, filamentation is observed as evidenced by the scanning electron microscope (SEM) pictures. Preliminary results show that using such a tight focusing geometry and at input powers above the critical power for self-focusing in silica, waveguide structures with elliptical cores are inscribed within the glass by moving the sample perpendicular to the laser beam propagation direction.

High-performance, compact, prismless, low-threshold 30-MHz Ti:Al2O3 laser.

We describe the design and operation of a compact femtosecond Ti:Al2O3 laser based on a novel multipass cavity (MPC) design. The laser is all solid state, has prismless dispersion compensation with double-chirped mirrors, and uses a tight-focusing geometry to facilitate efficient low-threshold operation. We increase output pulse energies by extending the resonator length with a compact, scalable MPC, which preserves the characteristics of the Gaussian beam for the short cavity. Although the effective cavity length is approximately 5 m, an extremely compact laser that measures only 30 cm x 45 cm is achieved. With only 1.5 W of pump power, the laser generates 23-fs pulses at a repetition rate of 31.25 MHz and with 88 mW of average output power, corresponding to 2.8 nJ of pulse energy.

High-order harmonic generation at a repetition rate of 100 kHz

We report high-order harmonic generation (HHG) in rare gases using a femtosecond laser system with a very high repetition rate (100 kHz) and low pulse energy $(7\ensuremath{\mu}\mathrm{J}).$ To our knowledge, this is the highest repetition rate reported to date for HHG. The tight focusing geometry required to reach sufficiently high intensities implies low efficiency of the process. Harmonics up to the 45th order are nevertheless generated and detected. We show evidence of clear separation and selection of quantum trajectories by moving the gas jet with respect to the focus, in agreement with the theoretical predictions of the semiclassical model of HHG.

Extreme-ultraviolet high-order harmonic pulses in the microjoule range

We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation. The generation of the high-order harmonics of intense laser pulses in gases @1# has recently opened new perspectives for probing matter in the extreme-ultraviolet ~XUV! pulses on an unprecedented time scale. The ultrashort harmonic pulse duration is used in pump-probe experiments in atomic @2,3# and molecular @ 4‐6 # spectroscopy, as well as in solidstate physics @7,8#. Combined to the high intrinsic beam coherence, it has allowed ultrafast diagnosis of laser-produced plasmas through XUV interferometry @9,10#. However, the harmonic beam energy is still relatively low and many applications would become possible if the number of generated photons were increased: ultrafast XUV holography, diagnosis of dense bright plasmas, or even study of nonlinear processes in the XUV, limited so far to low harmonic orders @11#. Recently a number of studies have demonstrated high conversion efficiencies, using ultrashort laser pulses focused in hollow core fibers @12‐14# or cells @15,16#. However, these efficiencies were obtained at a very low laser energy ~less than 1 mJ in most cases! that imposed a relatively tight focusing geometry in order to reach saturation intensities of the generating rare gases. This resulted in a low harmonic energy, in the nanojoule range. The fact that much larger energies are now available on ultrashort laser systems raises a number of questions: using higher laser energies and looser focusing, is it possible to achieve similar efficiencies, and thus to generate microjoule harmonic pulses? In particular, how will phase matching be affected by these unusual generating conditions? In this Rapid Communication, we report a thorough study of harmonic generation at a high pumping energy using a long focal length lens. Using a pumping energy of 27 mJ and a f 52 m lens, we study the influence of the beam aperture, the medium length, and the atomic density on the harmonic yield produced in a pulsed gas jet. We identify different saturation regimes, thanks to the excellent quantitative agreement obtained with detailed three-dimensional ~3D! simulations. In particular, we clearly observe the interplay between phase matching, absorption, and defocusing, the main limiting factors of the macroscopic emission. Absolute photon number measurements in the optimal conditions give conversion efficiencies as high as those reported using ultrashort (,20 fs) laser pulses, but now with ten times more energy. Using a f 55 m lens, we show that even higher conversion efficiencies can be obtained, resulting in harmonic energies in excess of 1 mJ. The experiments were performed on the LUCA laser facility with an amplified Ti:sapphire system delivering 60 fs pulses at 800 nm, with an energy of up to 100 mJ at 20 Hz. In our experiment, a pumping energy of 27 mJ was focused with either a f 52 m or 5 m lens in a pulsed gas jet. The nozzle was formed by a slit of dimensions 300 mm3 3m m producing a jet at pressure 10‐100 Torr characterized by Mach-Zehnder interferometry. By rotating the jet relative to the laser axis, we can change the length of the generating medium, while keeping the same peak density. Harmonics produced in the jet are analyzed by an XUV spectrometer without entrance slit @17#, and detected with a calibrated XUV photodiode blinded for diffused IR light with two 100 nm Al filters. The absolute spectrometer response~as well as the filter transmission! was measured using the harmonic radiation as a source further monochromatized with another spectrometer, like a synchrotron beam line. Since the total aperture of the laser beam was 40 mm, the

Third-harmonic generation at atmospheric pressure in methane by use of intense femtosecond pulses in the tight-focusing limit

An experimental study of third-harmonic generation in methane with a 100-fs, 820-nm, Ti:sapphire laser in a tight focusing geometry is presented. The harmonic intensity and bandwidth were measured in a range of intensities extending from below to far above the first ionization threshold and at pressures as high as 10 atm (1 atm=760 Torr). The harmonic signal follows a power-law dependence on the laser intensity with an exponent of ∼7 and saturates at an intensity Is∼4×1014 W/cm2. The conversion efficiency was found to increase with the pressure for laser intensities smaller than the saturation intensity Is and to decrease with the pressure at larger intensities. At laser intensities larger than the saturation intensity a substantial modification in the third-harmonic bandwidth and structure was observed.

Blue-lines pumped Kerr-lens mode-locked Cr:LiSGaF laser

We report a blue-lines pumped femtosecond Kerr-lens mode-locked Cr:LiSGaF laser based on a tight-focusing geometry. With a combination of 488, 476 and 458 nm lines as a pump source, 58 fs pulses centered at 845 nm were generated. The average output power is 30 mW, and time–band product is 0.331, assuming a sec h 2 pulse shape. Neither acousto-modulator nor physical aperture or slit were used in this cavity.

Operation of a Kerr-Lens Mode-Locked Cr:LiSGaF Laser with a Tight-Focusing Cavity Geometry

A femtosecond Kerr-lens mode-locked Cr:LiSGaF laser based on a compact cavity geometry is demonstrated. With a combination of 488, 476, and 458 nm lines as a pump source, 58 fs pulses centered at 845 nm were generated. The average output power is 30 mW, and time-band product is 0.331, assuming a sech2 pulse shape. Neither acousto-modulator nor physical aperture or slit were used in this cavity.