High-field Physics Research Articles

High-average-power 2 μm few-cycle optical parametric chirped pulse amplifier at 100 kHz repetition rate.

Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 μm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 μJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.

Picosecond pulses compression at 1053-nm center wavelength by using a gas-filled hollow-core fiber compressor**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204328, 61221064, 61078037, 11127901, and 11134010), the National Basic Research Program of China (Grant No. 2011CB808101), the Commission of Science and Technology of Shanghai, China (Grant No. 12dz1100700), the Natural Science Foundation of Shanghai, China

We theoretically study the nonlinear compression of picosecond pulses with 10-mJ of input energy at the 1053-nm center wavelength by using a one-meter-long gas-filled hollow-core fiber (HCF) compressor and considering the third-order dispersion (TOD) effect. It is found that when the input pulse is about 1 ps/10 mJ, it can be compressed down to less than 20 fs with a high transmission efficiency. The gas for optimal compression is krypton gas which is filled in a HCF with a 400-μm inner diameter. When the input pulse duration is increased to 5 ps, it can also be compressed down to less than 100 fs efficiently under proper conditions. The results show that the TOD effect has little impact on picosecond pulse compression and the HCF compressor can be applied on compressing picosecond pulses efficiently with a high compression ratio, which will benefit the research of high-field laser physics.

Unexpected richness in the high-field physics of graphite

Unexpected richness in the high-field physics of graphite

Revealing quantum path details in high-field physics

The tunnelling of an electron through a suppressed atomic potential followed by its motion in the continuum, is the fundamental mechanism underlying strong-field laser-atom/molecule interactions. Due to its quantum nature, the interaction is governed by the phase of the released electron wave-packets. Thus, detailed mapping of the electron wave-packet interferences provides essential insight into the physics underlying the interaction. A process which is providing access to the intricacies of the interaction is the generation of high order harmonics of the laser frequency. The phase-amplitude distribution of the emitted extreme-ultraviolet (EUV), carries the complete information about the harmonic generation process and vice-versa. Thus, the visualization of the EUV-spatial-amplitude-distribution, as it results from interfering electron wave-packet contributions, is of crucial importance. Restrictions to this accomplishment are due to the spatially integrating measurement approaches applied so far, that average out the phase effects in the generation process. In this work, we demonstrate a method which overcomes this obstacle. An EUV-spatial-amplitude-distribution-image is induced from the imprint on the measured spatial distribution of ions, produced through EUV-photon ionization of atoms. Interference extremes in the image carry phase information about the interfering electron wave-packets. The present approach provides detailed insight on the strong-field laser-atom interaction mechanism, while establishing the era of phase selective interaction studies. Furthermore, it paves the way for substantial enhancement of the spectral and temporal precision of measurements by in-situ controlling the phase distribution of the emitted radiation and/or spatially selecting the EUV-radiation-atom interaction products.

Quantum-optical nature of the recollision process in high-order-harmonic generation

Coherent processing of quantum information and attosecond science have had so far little in common. We here show that recent data in high harmonic emission reveal a realization of a qubit and its coherent manipulation at the attosecond time scale. By observing the interference pattern created by the spatiotemporal overlap of photons emitted by two interfering electron paths we generate a photon Hadamard gate and thus erase the electron-trajectory information. This allows the measurement of the relative phase in electron-trajectory quantum superpositions which realize the qubit, opening the possibility for more elaborate schemes of coherent information processing within high-field physics.

Self-compression to sub-3-cycle duration of mid-infrared optical pulses in dielectrics

We present the first demonstration of sub-3 cycle optical pulses at 3.1 μm central wavelength generated through self-compression in the anomalous dispersion regime in a dielectric. The pulses emerging from this compact and efficient self-compression setup could be focused to intensities exceeding 10(14) W/cm(2), a suitable range for high field physics experiments. Numerical simulations performed with a 3D nonlinear propagation code, provide theoretical insight on the processes involved and support our experimental findings.

Modelocking and femtosecond pulse generation in chip-based frequency combs

We investigate simultaneously the temporal and optical and radio-frequency spectral properties of parametric frequency combs generated in silicon-nitride microresonators and observe that the system undergoes a transition to a mode-locked state. We demonstrate the generation of sub-200-fs pulses at a repetition rate of 99 GHz. Our calculations show that pulse generation in this system is consistent with soliton modelocking. Ultimately, such parametric devices offer the potential of producing ultra-short laser pulses from the visible to mid-infrared regime at repetition rates from GHz to THz.

18- \mu J energy, 160-kHz repetition rate, 250-MW peak power mid-IR OPCPA

Progresses on the development of a high repetition rate mid-IR laser source suitable for the next generation of high-field physics experiments are reported. The presented optical parametric chirped pulse amplification (OPCPA) source currently delivers carrier-envelope phase (CEP)-stable 67-fs duration optical pulses with up to 18- \mu J output energy at 160-kHz repetition rate. The focusability of the output beam (M2~2) enables peak intensities exceeding 1014 W/cm2 and the record output energy stability-below 1% power fluctuation over 4.5 h makes this source a key enabler for the strong field physics community.

Bessel-Gauss beam enhancement cavities for high-intensity applications

We introduce Bessel-Gauss beam enhancement cavities that may circumvent the major obstacles to more efficient cavity-enhanced high-field physics such as high-harmonic generation. The basic properties of Bessel-Gauss beams are reviewed and their transformation properties through simple optical systems (consisting of spherical and conical elements) are presented. A general Bessel-Gauss cavity design strategy is outlined, and a particular geometry, the confocal Bessel-Gauss cavity, is analyzed in detail. We numerically simulate the confocal Bessel-Gauss cavity and present an example cavity with 300 MHz repetition rate supporting an effective waist of 33 μm at the focus and an intensity ratio from the focus to the cavity mirror surfaces of 1.5 × 10(4).

Numerical simulations of THz emission from the laser interaction with magnetized plasmas

One-dimensional particle-in-cell (PIC) program is used to simulate the generation of high power terahertz (THz) emission from the interaction of an ultrashort intense laser pulse with underdense magnetized plasma. The spectra of THz radiation are discussed with different laser intensity, pulse width, density scale length and external magnetic field. A high-amplitude electron plasma wave driven by a laser pulse can produce powerful THz emission through linear mode conversion under certain conditions. With incident laser intensity of 1018W/cm2, the generated emission is computed to be of the order of several MV/cm field and tens of MW level power. The corresponding energy converrsion efficiency is several ten thousandths, which is higher than the efficiency of other THz source and suitable for the studies of high-field and nonlinear physics in the THz regime.

Numerical simulations of THz emission from the laser wakefields through linear mode conversion

A nonlinear one-dimensional particle-in-cell (PIC) program is used to simulate the generation of high power terahertz (THz) emission from the interaction of an ultrashort intense laser pulse with underdense plasma under magnetized and unmagnetized cases. The magnetic field in laser-irradiated plasma has an important effect on the resonant absorption, and makes the results different. The spectra of THz radiation are compared under different cases (with or without external magnetic field), and the optimized parameters including plasma density scale length, incident angle and external magnetic field are calculated. High-amplitude electron plasma wave driven by a laser wakefield can produce powerful THz emission through linear mode conversion under certain conditions. With incident laser intensity of 1018W/cm2, the generated emission is computed to be of the order of several MV/cm field and tens of MW level power. It is suitable for the studies of high-field and nonlinear physics in the THz regime.

High field physics and extreme nonlinear optics

Despite the fact that high order harmonic generation (HHG) aims at serving as a table-top light source for imaging applications with extremely high spatial resolution, a general lack of accurate conversion efficiency measurements exists in the field. Here, we present such a measurement for a HHG setup with a semi-infinite gas cell. By combining measurements with a calibrated photodiode sensitive in the extreme ultra-violet (XUV) and spatially resolved spectral measurements of HHG spectra, we are able to determine conversion efficiencies of the HHG process as well as brilliance values for individual harmonics. The method is explained in detail and applicable to any target geometry.

Improvement of temporal contrast for ultrashort laser pulses by cross-polarized wave generation

Temporal contrast is a key factor influencing the performance of high-field physics experiments for ultraintense and ultrashort laser systems. The purpose of this paper is to improve the temporal contrast for ultraintense and ultrashort pulses in a laser system by cross-polarized wave (XPW) generation, a new method developed recently to enhance the temporal contrast effectively. Experiments were carried out to study the XPW generation efficiency and the enhancement of temporal contrast at the SILEX-I laser facility with a wavelength of 800nm. The results show that the relationships between the efficiency of cross-polarized wave generation and crystal orientation/input intensity are in good agreement with theoretical predictions. Both the prepulse and the amplified spontaneous emission (ASE) pedestal are suppressed significantly and the temporal contrast is improved by two orders of magnitude by the method. With the amplification of the temporally cleaner pulses in the future, great opportunities will be created for high-field physics experiments at the laser facility.

Comparison of femtosecond laser-induced damage on unstructured vs. nano-structured Au-targets

The combination of high-field physics with nano-plasmonics has proven to be feasible in producing high harmonics of intense laser radiation from noble gases, assisted by the field-enhancement effect in the proximity of metallic nano-antennas. However, the intensity region where harmonics can be generated without irreversible damage to these delicate structures is rather narrow. We explore the damage threshold of gold targets that exhibit regular structures on a nanoscopic scale, either explicitly resonant to the used laser frequency, or off-resonance. These are compared to values for bulk material in order to gain insight into the role of plasmonic resonances in the response of solid targets on intense laser radiation. We find that the presence of such a resonance lowers the threshold fluence (J/cm2) where global structural damage sets in by about an order of magnitude. Statistical deviations either in local pulse energy of the damage inducing laser radiation or in the exact resonance behaviour of singular structures prove to be limited. These results should serve as a guideline for future experiments working near the damage threshold of more sophisticated antenna designs.

Relativistic birefringence induced by a high-intensity laser field in a plasma

Field-induced birefringence, also known as cross-polarization wave generation, has played an important role in ultrafast nonlinear optics. In this paper we analyze birefringence induced by relativistic collective motion of electrons driven by a high-intensity laser field. An analytical expression for the phase difference between the parallel and perpendicular polarizations of a weak probe pulse with respect to the polarization of a strong pump pulse as a function of intensity, density, and wavelengths is derived. It is shown that under typical experimental conditions of high-field physics, the effect is well above detection threshold. The analysis is compared with particle-in-cell simulations, and the agreement provides good support for the theory.

High Field Characterization of Current-Voltage Dependences in Continuous/Semicontinuous High-k Ultrathin Films Through Leakage-Path Blocking

We developed a ferroelectric assisted current-voltage characterization technique for continuous/semicontinuous high-k ultrathin films under the field as high as 17 MV/cm without invoking of dielectric breakdown. The leakage current of the ultrathin films equals domain switching current with breakdown paths blocked efficiently by underneath ferroelectric thick layer. The extracted field dependences of current density for a 1– 6 nm thick Al2O3 layer deposited on top of a 300 nm thick Pb(Zr,Ti)O3 layer obey the equation of Schottky mission. This technique is helpful for investigations of high-field charge emission and quantum physics for ultrathin films in loss of the atomic-layer flatness.

Laser Ion Acceleration: Status and Perspectives for Fusion

High power short-pulse lasers presently reach peak powers of a few hundred Terawatts up to a Petawatt, and routinely reach focal intensities of 1018 - 1021 W/cm2 . These lasers are able to produce various secondary radiation, from relativistic electrons and multi-MeV/nucleon ions to high-energetic X-rays and γ-rays. In many laboratories world-wide large efforts are presently devoted to a rapid development of this novel tool of particle acceleration, targeting nuclear, fundamental and high-field physics studies as well as various applications. Based on the Radiation Pressure Acceleration mechanism, laser-accelerated ion beams can be generated with solid-state density, thus exceeding beams from conventional accelerators by about 14 orders of magnitudes. This opens the perspective of a novel reaction scheme called ’f ssion-fusion’, where in a first step f ssion is induced both in laser-accelerated f ssile projectiles from a ’production target’ and in a second ’reaction target’ again from f ssile material hit by the accelerated projectiles. Due to the unprecedented ion density, (neutron-rich) light f ssion fragments from projectile and target can fuse again, forming extremely exotic species approaching the region of the N = 126 waiting point of the r-process. Within the next 5 years a new EU-funded large-scale research infrastructure (ELI: Extreme Light Infrastructure) will be constructed, with one of its four pillars exclusively devoted to nuclear physics based on high intensity lasers (ELI-Nuclear Physics, to be built in Magurele/Bucharest). Studies of laser-induced nuclear reactions like the ’f ssion-fusion’ mechanism will be amongst the experimental f agship projects pursued there.

OPCPA boosts high-field physics

Optical parametric chirped pulse amplification is a promising approach for amplifying few-cycle laser pulses to unprecedented powers. However, the future success of the scheme depends on the availability of suitable pump sources.

Hard x-ray generation by low energy, tightly focused laser pulses

The interaction of λ2 laser pulses with plasma is studied to scale the high-field physics from high power laser systems to low-energy, high-repetition-rate lasers. For this purpose, 20mJ, 40fs Ti:sapphire laser pulses were focused in a spot of 2.2×2.3μm2 with a maximum intensity of I∼1019W∕cm2. Efficient generations of energetic electrons (∼1MeV) and hard x-rays with energies of over 100keV were obtained by irradiating the low-energy, tightly focused laser pulses on a massive Cu target.

Decrease of pulse-contrast in nonlinear chirped-pulse amplification systems due to high-frequency spectral phase ripples

It is analytically shown that weak initial spectral phase modulations cause a pulse-contrast degradation at the output of nonlinear chirped-pulse amplification systems. The Kerr-nonlinearity causes an energy-transfer from the main pulse to side-pulses during nonlinear amplification. The relative intensities of these side-pulses can be described in terms of Bessel-functions. It is shown that the intensities of the pulses are dependent on the magnitude of the accumulated nonlinear phase-shift (i.e., the B-integral), the depth and period of the initial spectral phase-modulation and the slope of the linear stretching chirp. The results are applicable to any type of laser amplifier that is based on the technique of chirped-pulse amplification. The analytical results presented in this paper are of particular importance for high peak-power laser applications requiring high pulse-contrasts, e.g. high field physics.