Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air.

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level. We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy. This was achieved by applying an external DC electric field to the air plasma filament. Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold. The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Resonance absorption of the inner shell during high-order harmonic generation.

In this work, we report the observation of resonance absorption of the inner shell during the high-order harmonic generation (HHG) from xenon (Xe) and krypton (Kr). The absorption peaks show a periodic variation with the change of carrier-envelope phase of driving laser pulses and the delay of two-color laser field, which indicates the absorption peaks come from the collective multielectron effects during the HHG. With the increase of gas pressure, the depth of absorption peak will continue to increase, while due to the phase matching effect, there will be an optimal pressure for the intensity of harmonic signal. Our experimental results pave the way to uncover the physical mechanism of the collective multielectron effects involving inner-shell electrons in the HHG process.

Background-free single-beam coherent Raman spectroscopy assisted by air lasing.

We develop a background-free single-beam coherent Raman scattering technique enabling the high-sensitivity detection of greenhouse gases. In this scheme, Raman coherence prepared by a femtosecond laser is interrogated by self-generated narrowband air lasing, thus allowing single-beam measurements without complex pulse shaping. The unique temporal and spectral characteristics of air lasing are beneficial for improving the signal-to-noise ratio and spectral resolution of Raman signals. With this method, SF6 gas present at a concentration of 0.38% was detected in an SF6-air mixture. This technique provides a simple and promising route for remote detection due to the low divergence of Raman signals and the availability of high-energy pump lasers, which may broaden the potential applications of air lasing.

Controlling of the harmonic generation induced by the Berry curvature.

High-order harmonic generation in solid state has attracted a lot of attentions. The Berry curvature (BC), a geometrical property of the Bloch energy band, plays an important role for the harmonic generation in crystal. As we all know, the influence of BC on the harmonic emission has been investigated before and BC is simplified as a 1D structure. However, many other materials including MoS2 are 2D materials. In this work, we extend the investigation for BC to 2D structure and get a generalized equation, which not only gives a new method to control the harmonic emission with BC, but also gives a deeper understanding for the influence of the BC. We show the ability to control the harmonic emission related to the BC using the orthogonal two-color (OTC) laser field. By tuning the delay of OTC laser field, one can steer the trajectory of electrons and modulate the emission of harmonics. This study can provide us a deeper insight into the role of the BC which is difficult to be measured experimentally.

Free-electron lasing at 27 nanometres based on a laser wakefield accelerator.

X-ray free-electron lasers can generate intense and coherent radiation at wavelengths down to the sub-ångström region1-5, and have become indispensable tools for applications in structural biology and chemistry, among other disciplines6. Several X-ray free-electron laser facilities are in operation2-5; however, their requirement for large, high-cost, state-of-the-art radio-frequency accelerators has led to great interest in the development of compact and economical accelerators. Laser wakefield accelerators can sustain accelerating gradients more than three orders of magnitude higher than those of radio-frequency accelerators7-10, and are regarded as an attractive option for driving compact X-ray free-electron lasers11. However, the realization of such devices remains a challenge owing to the relatively poor quality of electron beams that are based on a laser wakefield accelerator. Here we present an experimental demonstration of undulator radiation amplification in the exponential-gain regime by using electron beams based on a laser wakefield accelerator. The amplified undulator radiation, which is typically centred at 27 nanometres and has a maximum photon number of around 1010 per shot, yields a maximum radiation energy of about 150 nanojoules. In the third of three undulators in the device, the maximum gain of the radiation power is approximately 100-fold, confirming a successful operation in the exponential-gain regime. Our results constitute a proof-of-principle demonstration of free-electron lasing using a laser wakefield accelerator, and pave the way towards the development of compact X-ray free-electron lasers based on this technology with broad applications.

Photoionization-assisted, high-efficiency emission of a dispersive wave in gas-filled hollow-core photonic crystal fibers.

We demonstrate that the phase-matched dispersive wave (DW) emission within the resonance band of a 25-cm-long gas-filled hollow-core photonic crystal fiber (HC-PCF) can be strongly enhanced by the photoionization effect of the pump pulse. In the experiments, we observe that as the pulse energy increases, the pump pulse gradually shifts to shorter wavelengths due to soliton-plasma interactions. When the central wavelength of the blueshifting soliton is close to the resonance band of the HC-PCF, high-efficiency energy transfer from the pump light to the DW in the visible region can be obtained. During this DW emission process, we observe that the spectral center of the DW gradually shifts to longer wavelengths leading to a slightly increased DW bandwidth, which can be well explained as the consequence of phase-matched coupling between the pump pulse and the DW. In particular, at an input pulse energy of 6 µJ, the spectral ratio of the DW at the fiber output is measured to be as high as ∼53%, corresponding to an overall conversion efficiency of ∼19%. These experimental results, well accompanied by theoretical simulations and analysis, offer a practical and effective method of generating high-efficiency tunable visible light sources and provide a few useful insights into the fields of soliton-plasma interaction and resonance-induced DW emission.

Guiding and emission of milijoule single-cycle THz pulse from laser-driven wire-like targets.

The miscellaneous applications of terahertz have called for an urgent demand of a super intense terahertz source. Here, we demonstrate the capability of femtosecond laser-driven wires as an efficient ultra-intense terahertz source using 700 mJ laser pulses. When focused onto a wire target, coherent THz generation took place in the miniaturized gyrotron-like undulator where emitted electrons move in the radial electric field spontaneously created on wire surface. The single-cycle terahertz pulse generated from the target is measured to be radially polarized with a pulse energy of a few milijoule. By further applying this scheme to a wire-tip target, we show the near field of the 500 nm radius apex could reach up to 90 GV/m. This efficient THz energy generation and intense THz electric field mark a substantial improvement toward ultra-intense terahertz sources.

Investigation of spatio-temporal stretching in a duplex grating compressor.

We employ the modified 6×6 matrix formalism to describe a pulsed Gaussian beam diffracted by a grating with arbitrary orientation. The matrix treatment is used to analyze the evolution of a pulsed beam propagating in a duplex grating compressor (DGC). For chirped pulse incidence, the inclination angle required in DGC setups will introduce several kinds of first-order spatio-temporal couplings (STCs). We found that temporal stretching due to spatial chirp is suppressed with enlarged beam diameter. Pulse-front tilt and residual frequency chirp in the compressed pulse will be eliminated simultaneously. Pulse with the transform-limited duration can be expected in ultra-intense and ultra-short pulse laser systems employing DGC.

High-performance seed pulses at 910 nm for 100 pw laser facilities by using single-stage nondegenerate four-wave mixing.

As the first step in a 100 petawatt (PW) laser facility, seed pulses with high performance are important to guarantee the quality of the output laser pulse. Here we propose a novel method based on a single-stage four-wave mixing process for the generation of seed pulses with a smooth and broadband spectrum, high energy, and high temporal contrast (TC). As high as 250 µJ pulses at approximately 910 nm central wavelength with a high TC and broader than 200 nm bandwidth are obtained in a piece of transparent medium directly after a commercial Ti:sapphire amplifier. The angular dispersion of the generated seed pulse is linear to the wavelength, which can be compensated well by using angular dispersive optics, such as a prism. The extremely simple process and setup make the output seed pulses stable and reliable for 100 PW laser facilities.

Investigation of the temporal contrast evolution in a 10-PW-level Ti:sapphire laser facility.

We have theoretically and experimentally investigated the evolution of the temporal contrast in a 10-PW-level Ti:sapphire laser in the Shanghai Superintense Ultrafast Laser Facility (SULF). The effects induced by the grism pair, spectral shaping filter, and increase in gain on the temporal contrast were investigated. First, it was found that the energy loss of clean seed pulses in the grism pair is a major factor in contrast degradation. Because of the low transmission efficiency of the grism pair (~10%), the temporal contrast is degraded by one order of magnitude. Second, the spectral shaping filter in the regenerative amplifier degrades the temporal contrast by increasing the intracavity loss. Finally, as the amplified spontaneous emission pedestal experiences gain more than the main pulse in Ti:sapphire amplifiers, particularly during saturated amplification, the temporal contrast will further deteriorate as the gain increases in multi-stage Ti:sapphire amplifiers. In addition, the effect on the temporal contrast induced by the extraction during pumping technique in large-aperture Ti:sapphire amplifiers has been considered. According to the investigations described above, the design of the SULF can be further improved. It is predicted that a temporal contrast of over 10-11 can be achieved at a peak power of 10 PW following the improvements. The investigations conducted in this study can provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers.

Femtosecond laser filament-assisted AgI-type pyrotechnic nucleant-induced water condensation in cloud chamber.

AgI-type pyrotechnics are widely used in the field of weather modification, as a kind of artificial ice nuclei. However, their precipitation yield remains an intensively studied area. In this paper, we present a study of AgI-type pyrotechnic nucleant-induced water condensation promoted by femtosecond laser filaments in a cloud chamber. It is found that when 50-ml sample was irradiated by the laser filaments, the particles condensed on the glass slide are more soluble and slightly larger (5-15 µm). The irradiation of the laser filament on the nucleant rarely induces the generation of particles of sizes larger than 1 µm; however, it increases the decay time of particles from 13 to 18 min by the creation of numerous small particles. The amount of snow on the cold bottom plate increases by 4.2-13.1% in 2 h, compared to that without the irradiation of the laser filament. These results are associated with the production of high-concentration HNO3 by the laser filament. The concentration of HNO3 in the melt water increases by more than ten times when the sample was irradiated by the laser filaments.

339 J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility.

We report on the laser pulse output of 339 J centered at 800 nm from a chirped-pulse amplification (CPA) Ti:sapphire laser system at the Shanghai Superintense Ultrafast Laser Facility. The experimental results demonstrated that the parasitic lasing as well as the transverse amplified spontaneous emission of the homemade 235-mm-diameter Ti:sapphire final amplifier were suppressed successfully via the temporal dual-pulse pumped scheme and the index-matching liquid cladding technique. The maximum pump-to-signal conversion efficiency of 32.1% was measured for the final amplifier. With a compressor transmission efficiency of 64% and a compressed pulse duration of 21 fs obtained for the sample light at a lower energy level, this laser system could potentially generate a compressed laser pulse with a peak power of 10.3 PW. The experimental results represent significant progress with respect to the CPA laser.

Laser guided ionic wind.

We report on a method to experimentally generate ionic wind by coupling an external large electric field with an intense femtosecond laser induced air plasma channel. The measured ionic wind velocity could be as strong as >4 m/s. It could be optimized by increasing the strength of the applied electric field and the volume of the laser induced plasma channel. The experimental observation was qualitatively confirmed by a numerical simulation of spatial distribution of the electric field. The ionic wind can be generated outside a high-voltage geometry, even at remote distances.

Free-space Ν2+ lasers generated in strong laser fields: the role of molecular vibration.

We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

2.6 mJ/100 Hz CEP-stable near-single-cycle 4 µm laser based on OPCPA and hollow-core fiber compression.

A carrier-envelope-phase-stable near-single-cycle mid-infrared laser based on optical parametric chirped pulse amplification and hollow-core fiber compression is demonstrated. A 4 µm laser pulse with 11.8 mJ energy is delivered from a KTA-based optical parametric chirped pulse amplification (OPCPA) with 100 Hz repetition rate, and compressed to 105 fs by a two-grating compressor with efficiency over 50%. Subsequently, the pulse spectrum is broadened by employing a krypton gas-filled hollow-core fiber. Then, the pulse duration is further compressed to 21.5 fs through a CaF2 bulk material with energy of 2.6 mJ and energy stability of 0.9% RMS, which is about 1.6 cycles for a 4 µm laser pulse. The carrier envelope phase of the near-single-cycle 4 µm laser pulse is passively stabilized with 370 mrad.

Infrared-laser-induced ultrafast modulation on the spectrum of an extreme-ultraviolet attosecond pulse.

We present a theoretical investigation of the isolated extreme-ultraviolet (XUV) attosecond pulse propagating in the ionizing gas induced by a synchronized strong infrared (IR) laser, with the numerical solution of the nonadiabatic one-dimensional propagation model. Upon scanning the relative delay between the XUV and the IR pulse, it is found that the delay-dependent XUV transmission spectrogram exhibits the unique pattern that is controllable by the chirp of the XUV pulse. Furthermore, we demonstrate that this kind of spectrum modulation can be attributed to the term of the light energy loss involved in the propagation equation. The characteristics of the spectrum modulation dependent on the XUV chirp might provide an all-optical way for the reconstruction of the XUV spectral phase.

Terawatt-scale optical half-cycle attosecond pulses.

Extreme-ultravoilet (XUV) attosecond pulses with durations of a few tens of attosecond have been successfully applied for exploring ultrafast electron dynamics at the atomic scale. But their weak intensities limit the further application in demonstrating nonlinear responses of inner-shell electrons. Optical attosecond pulses will provide sufficient photon flux to initiate strong-field processes. Here we proposed a novel method to generate an ultra-intense isolated optical attosecond pulse through relativistic multi-cycle laser pulse interacting with a designed gas-foil target. The underdense gas target sharpens the multi-cycle laser pulse, producing a dense layer of relativistic electrons with a thickness of a few hundred nanometers. When the dense electron layer passes through an oblique foil, it emits single ultra-intense half-cycle attosecond pulse in the visible and ultraviolet spectral range. The emitted pulse has a peak intensity exceeding 1018 W/cm2 and full-width-half-maximum duration of 200 as. The peak power of this attosecond light source reaches 2 terawatt. The proposed method relaxes the single-cycle requirement on the driving pulse for isolated attosecond pulse generation and significantly boosts the peak power, thus it may open up the route to new experiments tracking the nonlinear response of inner-shell electrons as well as nonlinear attosecond phenomena investigation.

High-contrast front end based on cascaded XPWG and femtosecond OPA for 10-PW-level Ti:sapphire laser.

By combining cross-polarized wave generation and femtosecond optical parametric amplification, a high-contrast front end featuring ultrahigh contrast, a broadband spectrum, an excellent beam profile, and good stability is built for a 10-PW-level Ti:sapphire laser in the Shanghai Superintense Ultrafast Laser Facility (SULF-10PW laser). The front end can deliver a cleaned pulse with a 110 µJ energy at 1 kHz, and the bandwidth of the cleaned pulse exceeds 60 nm (FWHM), which can support a 17 fs compressed pulse duration. The measured output energy fluctuation in one hour is <1.8% in rms value. The measurement-limited contrast is 10-10 at 3 ps before the main pulse. Utilizing the high-contrast front end, single-shot contrast at 10-10 level has been demonstrated in the SULF-10PW laser at a 24 fs pulse duration.

Temporal evolution of condensation and precipitation induced by a 22-TW laser.

Water condensation and precipitation induced by 22-TW 800-nm laser pulses at 1 Hz in an open cloud chamber were investigated in a time-resolved manner. Two parts of precipitation in two independent periods of time were observed directly following each laser shot. One part started around the filament zone at t < 500 µs and ended at t ≅ 1.5 ms after the arrival of the femtosecond laser pulse. The other following the laser-induced energetic air motion (turbulence), started at t ≅ 20 ms and ended at t ≅ 120 ms. Meanwhile, the phase transitions of large-size condensation droplets with diameters of 400-500 µm from liquid to solid (ice) in a cold area (T < -30 °C) were captured at t ≅ 20 ms.

Intense broadband mid-infrared pulses of 280 MV/cm for supercontinuum generation in gaseous medium.

We produce extremely bright mid-infrared (mid-IR) pulses with a tunable wavelength of 7 µm to 15 µm through difference frequency generation. Optimization of beam quality and beam focusing results in an intense mid-IR field spatiotemporally confined in the lambda-cubic volume. A near planar wavefront is achieved through manipulating the wavefront curvature of the pumping pulse in the frequency downconversion process. Coherent mid-IR pulses are produced with the peak field of 280 MV/cm at 10 µm, and its intensity exceeds 100 TW/cm2, estimated from measured pulse energy, and spatial and temporal pulse profiles. Interaction of such an intense mid-IR field with Xe and Kr gas forms plasma and generates a supercontinuum in the visible range.