Subpicosecond Laser Research Articles

Broadband terahertz wave emission from liquid metal

Metals have been studied as terahertz sources for decades. Recent research has shown the potential of metals in generating extremely high THz pulse energy excited by intense laser pulses. To avoid the metal surface debris caused by laser pulses, here, we report the results of the broadband terahertz wave emission from a flowing liquid metal line excited by subpicosecond laser pulses. The THz signal emitted from the liquid gallium line shows the stronger field with the broader bandwidth compared to the signal from water under the identical optical excitation conditions. Our preliminary study suggests that the liquid metals have the potential to serve as efficient and powerful THz sources for the intense lasers with a high repetition rate.

Theoretical and experimental progress of high-order harmonics and attosecond pulse generation based on solid overdense plasma

<p indent="0mm">Since the laser was invented, harmonic generation and wavelength conversion have been important research topics in nonlinear optics regime. In 1987, McPherson A et al. used sub-picosecond KrF laser <sc>(248 nm</sc> wavelength) to interact with noble gas to obtain high-order harmonics for the first time. In 2001, the first single attosecond <sc>(10^–18 s)</sc> pulse with a pulse width of 650 as was measured experimentally by Krausz et al. by using the broadband high-order harmonic generation. Since then, high-order harmonics and attosecond pulses have been widely used in atomic and molecular physics, materials science and other fields. However, due to the low efficiency of attosecond pulses generated by the gas media, the energy of attosecond pulses is limited, which limits the detection methods of attosecond time dynamics research (currently mainly IR+XUV pumping/detection) and its applications in many fields. How to obtain high-brightness, high-energy attosecond pulses has always been the pursuit in this field since the generation of the first attosecond pulse. With the development of femtosecond laser technology, the output peak power of the femtosecond laser system has been rapidly increased in the past two decades. At present, the laser system can output the pulse with a peak power of up to 10 PW (1 PW<sc>=10¹⁵ W).</sc> Its intensity can be much higher than <sc>10¹⁸ W/cm²</sc> and its interaction with the overdense plasma can efficiently generate high-order harmonics at the level of hundreds of eV or even keV, which brings new opportunities for the development and application of high-order harmonics and attosecond pulses. Generally speaking, the interaction of high peak power femtosecond laser pulses with solid-density plasmas has unique advantages in high-luminance, high-energy high-order harmonics and attosecond pulse generation. In the ELI-ALPS plan, solid plasma high-order harmonics and attosecond pulse generation are very important parts, especially to promote their high brightness and their high photon energy. In terms of the attosecond pulse energy, the goal of their first phase is that the attosecond pulse energy reaches the μJ level, which will be advanced to the mJ level in the next phase, and the photon energy of the attosecond pulse will reach a few keV. This review gives a brief introduction to its physical mechanisms and their current experimental research progress. Three mechanisms (coherent wake emission (CWE), relativistic oscillating mirror (ROM), coherent synchrotron emission (CSE)) will be discussed. They are working under different laser intensity. CWE can emit high efficiency harmonics at the laser intensity below <sc>10¹⁸ W/cm².</sc> But when the peak laser intensity is very high, the research of the CSE mechanism shows the generation of high-luminance high-order harmonics under super-relativistic light intensity, which can be used to produce extremely high field-strength pulse of attoseconds, even tens of zeptoseconds. If the laser parameters and the plasma condition can be well controlled, the laser pulse may even generate higher harmonic radiation with field strength much higher than the pump laser field strength, which may be a possible way to study the 39th question (“What is the most powerful laser researchers can build? ”) among the 125 most challenging scientific questions published by <italic>Science</italic> on its 125th anniversary.

Formation dynamics of exciton-polariton vortices created by nonresonant annular pumping

We study the spontaneous formation of exciton polariton vortices in an all-optical non-resonantly excited annular trap, which is formed by ring-shaped subpicosecond laser pulses. Since the light emitted by these vortices carries orbital angular momentum (OAM) corresponding to their topological charge, we apply a dedicated OAM spectroscopy technique to detect the OAM of the measurement signal with picosecond time resolution. This allows us to identify the formation of OAM modes and investigate the dynamics of the vortex formation process. We also study the power dependence of this process and how the ring diameter influences the formation of OAM modes.

Investigation of an interlaced laser beam scanning method for ultrashort pulse laser micromachining applications

Abstract This article investigates picosecond and sub-picosecond laser micromachining of Borofloat®33 glass and provides clear evidence that a simple modification of the laser beam scanning strategy can lead to significant improvement of machining efficiency and hence process throughput. Besides studying the impact of the fundamental laser machining parameters, such as laser fluence, pulse overlap, pulse repetition frequency (PRF), pulse duration and laser spot diameter, on the machined depth, surface roughness and material removal rate (MRR), it also compares the machining results for two different laser beam scanning strategies, called here sequential” method (SM) and “interlaced” method (IM). By changing the scanning strategy from SM to IM, the MRR can be significantly increased because IM allows high-quality machining of the glass at higher PRF values. The experimental results show that this simple, cost-free modification allows the MRR value to be increased by more than 4 times, i.e. from 0.12 mm3/s to 0.53 mm3/s. Moreover, by using a Phantom V2512 high-speed camera, the picosecond laser micromachining process using both SM and IM was filmed. The videos show that SM leads to the accumulation of glass particles within the laser-machined area, whereas in IM the glass material is removed layer by layer which leads to the generation of “cleaner” and deeper areas. The mechanisms associated with these machining improvements are discussed.

Few-layer silicene nanosheets as saturable absorber for subpicosecond pulse generation in all-fiber laser

Two-dimensional silicene nanosheets is an emerging material in many fields. However, there is a lack of research on nonlinear optical applications of silicene nanosheets so far. Here, we demonstrate a subpicosecond soliton mode-locked erbium-doped fiber laser using microfiber-based silicene nanosheets as a saturable absorber for the first time, to the best of our knowledge. Few-layer silicene nanosheets, which were prepared by liquid-phase exfoliation method, have a modulation depth is 20% and the saturation intensity is 5.78 MW/cm2. Stable soliton pulses were generated at 1531.48 nm with a 3-dB bandwidth of 2.84 nm, a pulse duration of 937 fs, and a time-bandwidth product of 0.34. Our results indicate that few-layer silicene nanosheets could be used as an excellent photonic device for short-pulse laser, and promote the applications of silicene-based nonlinear photonics.

Conjoint influence of quantum interference and Freeman resonance on substructures of the photoelectron spectra in above-threshold ionization

Based on the numerical solution of the full-dimensional time-dependent Schr\odinger equation, we study the above-threshold ionization of atomic hydrogen by subpicosecond laser pulses at wavelengths ranging from 300 nm to 800 nm, in which regime substructures of the photoelectron energy spectra resulting from quantum interference are entangled with those induced by the Freeman resonance. By analyzing the time-dependent populations and ac Stark energy shifts of the relevant atomic levels, and observing the pulse intensity dependence and the pulse length evolution of the spectra, we clearly identify the origins of the substructures exhibited in the spectra.

Properties of heavy ion beams produced by a PW sub-picosecond laser

The paper reports the results of numerical studies on the generation of heavy ions from a sub-micrometre gold target irradiated by a high-energy (60 J) high-intensity (∼ 2 × 1020 W/cm2) sub-ps laser pulse. The properties of the heavy ion beam produced from a 0.5-μm gold target with and without hydrogen contaminants are investigated using a multi-dimensional (2D3 V) particle-in-cell code. It was found that in the case of the pure (without the contaminants) target, the ion beam contains over 20 ion species with a charge state > 30 and a total number of high-energy (> 20 MeV) ions reaching values well above 1012. The ion energy spectrum is broad, with maximum energies > 1 GeV and mean energies > 100 MeV . At a distance ∼ 1 mm from the target, the intensity of the ion beam is ∼ 1015 W/cm2, the ion fluence reaches ∼ 1016 ions/cm2, and the ion pulse duration is ∼ 100 ps. The contaminants significantly reduce the ion beam parameters and both the mean ion energy and the intensity and fluence of the ion beam generated from the contaminated gold target are almost an order of magnitude lower than those produced from the pure target. Heavy ion beams with the parameters obtained in the case of the pure gold target are barely achievable in conventional RF-driven accelerators, so they can open the door to new areas of research, in particular in high energy-density physics.

Highly efficient stimulated Raman scattering of sub-picosecond laser pulses in BaWO4 for 10.6 µm difference frequency generation.

Transient stimulated Raman scattering (SRS) of 0.3 ps 515 nm laser pulses in ${\rm BaWO_4}$BaWO4 crystal was experimentally demonstrated with efficiency up to ${\sim}{20}\% $∼20% for the Stokes component with a wavenumber of ${\sim}{925}\;{{\rm cm}^{ - 1}}$∼925cm-1 in a simple single-pass geometry. This anomalous high efficiency was obtained due to the laser pulse self-phase modulation resulting in spectral broadening and seeding the SRS. The applicability of seed pulse production for a high-pressure sub-picosecond ${{\rm CO}_2}$CO2 laser amplifier via difference frequency generation in ${{\rm LiGaS}_2}$LiGaS2 crystal was numerically verified.

Preference of subpicosecond laser pulses for terahertz wave generation from liquids

Terahertz (THz) wave generation from laser-induced air plasma generally requires a short temporal laser pulse. In contrast, it was observed that THz radiation from ionized liquid water prefers a longer pulse, wherein the mechanism remains unclear. We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water, which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse. The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied. Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.

Self-focusing and pulse sharpening of sub-picosecond laser pulse in n-InSb

The self-focusing and pulse sharpening of an intense short pulse laser in n-type indium antimonide, a semiconductor with non-parabolic energy band, is studied. The nonlinearity arises through the dependence of electron effective mass on quiver velocity and gives effective permittivity a saturating function of intensity. A laser beam with Gaussian transverse intensity profile undergoes periodic self-focusing. The faster self-focusing of the peak of the pulse causes significant sharpening of the pulse in course of propagation. For 10.6μm CO2 laser of picosecond pulse length and intensity ∼1 – 2 GW/cm2, significant self-focusing and pulse length shortening occurs over a length of 0.6 mm.

Generation of Laser-Induced Periodic Surface Structures on Different Glasses by a Picosecond-Pulsed Laser

The high cost of sub-picosecond pulsed lasers currently used to produce laser-induced periodic surface structures (LIPSS), also called ripples, on borosilicate and soda-lime glass limits the technique’s industrial and scientific utility. To increase accessibility, we describe here a LIPSS-fabrication process using a less expensive 15 ps-pulsed laser system equipped with a galvanometer scanner system. By carefully considering interpulse effects, we combined novel scanning strategies with optimized laser and scanning parameters to generate the nanostructures over a macroscopic area. The resulting LIPSS’s spatial periods display both metal-like and dielectric-like behaviour. Using different scanning strategies allows for low processing times and highly homogenized surfaces.

Broadband terahertz radiation from metal targets irradiated by a short pulse laser

The generation of low-frequency radiation from sub-picosecond laser pulses incident on metal targets is investigated. The laser field drives time-varying currents in a thin sub-surface layer of the metal, which emits broadband radiation that peaks at terahertz frequencies. We present a one-dimensional electrostatic model for copper appropriate for the interaction of laser pulses at normal incidence combined with a radiation model for an infinitely thin disk. The latter uses as input a single parameter, the temporal dependence of the integrated current density on axis, which is derived from the electrostatic model. The salient characteristics of the emitted radiation, such as power, energy, and spectra, are calculated for laser pulses with various intensities and pulse durations. The radiated energy per unit solid angle peaks at a small angle off the target normal and tapers off at larger angles. Analytical scaling of radiated energy with incident laser energy, in the low frequency limit, is obtained in the form εrad∼εlaser3/2. For accurate results, it is imperative to use the full expression for the heat capacity of electrons, in both the degenerate and ideal gas limits. Failure to do so may result in inaccuracies for the computed radiated energy, as large as one order of magnitude. A comparison of calculated and measured radiation energy in the 8–12 GHz frequency range indicates a similar trend with laser energy and comparable magnitude (∼1 fJ).

Complex-conjugate Pole-residue Pair-Based FDTD Method for Assessing Ultrafast Transient Plasmonic Near Field

The study of the optical properties of plasmonic nanostructures in the stationary regime has greatly benefited from the development of numerical methods, among which Finite Difference Time Domain (FDTD) is popular. In contrast, the use of these numerical tools for assessing the transient plasmonic optical response triggered by ultrashort laser pulses is hampered by the difficulty to address small variations of the material optical properties with reasonable computational time. Yet, many of the developments based on this ultrashort response rely on the dynamics of the near-field topography around the nanostructures. In this article, we present a way to bridge this gap with the complex-conjugate pole-residue pair (CCPRP) approach. A CCPRP-based FDTD simulator has been developed. First, a simple methodology to check the end-to-end accuracy of the FDTD simulation is provided. Then, in conjunction with a three-temperature model, the approach enables us to calculate the ultrafast transient near field inside and around a gold nanoparticle (AuNP) upon absorption of a subpicosecond laser pulse. The transient variation of the field intensity inside and around the AuNP is compared with the one determined by the Mie theory. The dependence of the transient field intensity on the distance away from the nanoparticle surface and on the delay time after laser pulse absorption is finally analyzed.

Intense sub-terahertz radiation from wide-bandgap semiconductor based large-aperture photoconductive antennas pumped by UV lasers

The characteristics of terahertz (THz) radiation generated from large-aperture photoconductive antennas (LAPCAs) were investigated. The antennas were fabricated using different wide-bandgap semiconductor crystals (ZnSe, GaN, 6H–SiC, 4H–SiC and β–Ga2O3) as the substrate. We used an amplified sub-picosecond KrF excimer laser for illumination of the LAPCAs. THz emission scaling was studied as a function of the bias field and the pump laser energy. It was found that the radiated THz energy scales quadratically as a function of the bias field and sub-linearly as a function of the optical fluence for most of the substrates. Further, we demonstrate that SiC, and especially 4H–SiC LAPCAs offer the best THz generation performances. In order to generate intense THz radiation, we fabricated both 6H- and 4H–SiC LAPCAs with an interdigitated structure. From the field autocorrelation trace, it was found that the spectra lie in the sub-THz regime, extending up to 400 GHz, with a peak frequency at 50 GHz, making the bridge between the microwaves band and the THz band. The maximum generated THz energy was 11 μJ, which is to date the highest THz energy measured from LAPCA sources, with a corresponding peak electric field of 115 kV cm−1 and a corresponding ponderomotive potential of 60 eV. Nonlinear THz experiments were performed using these energetic THz pulses, and open aperture Z-scan experiments in an n-doped InGaAs layer revealed a transmission enhancement of 1.7. It was also shown that in order to have efficient THz generation, the energy contrast of the laser must be kept high.

Generation of spectrally unmodulated and broad pulses through non-linear propagation with elliptical polarization

A fiber-based spectral broadening technique is reported which improves the temporal quality of ultrashort pulses by generating a wide, smooth spectrum from a sub-picosecond laser. The experimental results are confirmed by numerical simulations. It is demonstrated that the interplay between third order nonlinear effects such as cross-phase modulation and nonlinear elliptical polarization rotation have an important role in birefringent fibers and this is an appealing alternative to highly modulated spectrum caused by self-phase modulation. The theoretical model predicts the efficient generation of smooth, wide spectra with compensable phase and shows that proper polarization control is also crucial. An additional model is proposed which allows the determination of the proper set of parameters for the highest efficiency.

High-resolution measurements of Cl15+ line shifts in hot, solid-density plasmas

The shifts of the $1s3p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}\ensuremath{\rightarrow}1{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ He-$\ensuremath{\beta}$ transition of ${\mathrm{Cl}}^{15+}$ were measured in hot ($\ensuremath{\ge}600$ eV), dense (1--2 g/${\mathrm{cm}}^{3}$) plasmas generated by a (66--151)-J short-pulse laser beam at the Orion laser facility. The subpicosecond laser beam irradiated optically thin, KCl microdot targets buried in layers of plastic. The measured red shifts ranged from $4.8\ifmmode\pm\else\textpm\fi{}1.1$ eV for unshocked to $5.9\ifmmode\pm\else\textpm\fi{}1.2$ eV for shocked targets. These values are significantly smaller than recent predictions from a self-consistent-field ion-sphere model.

X-ray Lasers in Cluster Flows and in Nanostructured Targets

The paper presents a brief review of recent works concerning the modeling of X-ray lasers in cluster flows and in nanostructured targets. Calculations of the atomic characteristics are based on relativistic perturbation theory with a model potential of zero approximation. Two new results are discussed: (1) it is shown that a subpicosecond X-ray laser with λ = 41.8 nm formed in a xenon cluster flow can serve as an alternative to a free-electron laser and (2) in heavy Ni-like ions (Z ≥ 60), the ionization of ions and recombination of electrons are balanced at electronic temperatures ≥1500 eV; thus, the state of a Ni-like ion is quasi-steady-state. The inversions of many transition levels of an X-ray laser are also quasi-steady-state. The possibility of experimental observation of X-ray lasers based on 3p54d104p [J = 0] – 3p63d94p [J = 1] intrashell transitions in Gd36+ with wavelengths in the water window region is discussed.

Study of laser produced plasma in a longitudinal magnetic field

Laser produced plasma embedded in a longitudinal magnetic field was studied using a 1 MA pulsed power generator coupled with a 50 TW laser. Half turn coil loads with an internal diameter of 2.5–3.5 mm generate a 50–70 T axial magnetic field near the load. A subpicosecond laser pulse with an intensity of 1018–1019 W/cm2 irradiates a thin Si foil target in the magnetic field of the coil load. A laser produced plasma plume collimates within the longitudinal field to a narrow jet 0.2–0.3 mm in diameter with a length of 3–4 mm and an electron plasma density of (0.2–1) × 1020 cm−3 on the jet axis. The jet propagates with a velocity of 160–200 km/s in general agreement with magnetohydrodynamic simulations. X-ray spectral measurements show an increase in the plasma electron density resulting from the magnetic confinement of the jet.

Divergence and direction control of laser-driven energetic proton beam using a disk-solenoid target

A scheme for controlling the divergence and direction of an energetic proton beam driven by intense laser pulse is proposed. Simulations show that a precisely directed and collimated proton bunch can be produced by a sub-picosecond laser pulse interacting with a target consisting of a thin solid-density disk foil with a solenoid coil attached to its back at the desired angle. It is found that two partially overlapping sheath fields are induced. As a result, the accelerated protons are directed parallel to the axis of the solenoid, and their spread angle is also reduced by the overlapping sheath fields. The proton properties can thus be controlled by manipulating the solenoid parameters. Such highly directional and collimated energetic protons are useful in the high-energy-density as well as medical sciences.

Direct instantaneous 2-D imaging for photoacoustic waves by ultrashort single pulse interferometry

We perform ultrafast optical imaging by means of single pulse interferometry (SPI) using sub-picosecond ultrashort laser pulses. Laser-induced photoacoustic waves propagating at the sonic speed are captured in 2-D instantly without time-consuming area scanning or averaging-out of electronic noise. A common digital camera offering merely a millisecond-scale exposure is employed in synchronization with pulse picking so as to receive only a single pulse per camera frame. High fringe visibility is obtained with low background noise by blocking asynchronous spontaneous emission (ASE) mixed in source pulses by second harmonic filtering. Our proof-of-concept experimental scheme permits a 0.25 ps effective exposure time with almost ideal image contrast by optical coherence pulse gating. The 2-D ultrafast imaging capability is evaluated through a pump-probe measurement of a 60.8 MHz laser-induced photoacoustic wave in liquid.