Supercontinuum Radiation Research Articles

Nonlinear spectral features of the relativistic interaction between electron and ultrashort laser pulse

Features of the radiation spectra are investigated with the quantum electrodynamic theory for the nonlinear scattering interaction between relativistic electron and few-cycle linearly polarized laser pulse. The angle between the moving direction of the electron and that of the laser pulse affects the scattered radiation characteristics. When the electron has a head-on collision with the laser pulse having a carrier envelope phase as zero, the angular distribution of the scattered radiation spectrum is symmetrical with respect to the laser pulse propagation direction. Such a symmetry disappears when the electron collides with the laser obliquely and the direction of most of the radiation energy shifts at an acute angle towards the electron direction. The CEP has significant effects on the spectral angular distribution. The CEP influence is determined by the exertion of the electromagnetic field on the motion of the electron, whose trajectory overlap in phase space leads to the interference between scattered radiation in different intervals of the interaction process. The supercontinuum radiation could be produced under certain CEPs. An external electrostatic field applied as the environment for the laser-electron interaction is another important factor affecting the scattered radiation features by modulating the harmonic components and their angular distribution. This study may help the analysis of the spectral data from relativistic laser-solid target experiments.

Intermolecular energy transfer-enhanced super-broadband stimulated Raman scattering in cyclohexane-benzene mixtures.

Supercontinuum radiation has found numerous applications in diverse fields encompassing spectroscopy, pulse compression, and tunable laser sources. Anomalous enhanced stimulated Raman scattering (SRS) of cyclohexane-benzene mixtures was obtained in this study. SRS of the pure solvent, the multi-order Stokes of the strongest fundamental vibration modes, and energy transfer in intra-molecular modes were observed. SRS of the mixture revealed that the cross-pumping effect was generated between the C-H stretching (v2) mode of cyclohexane and the C=C ring skeleton (v1) mode of benzene, thereby producing the intermolecular secondary stimulated Raman emission and the appearance of two super-broadband radiations at 664.36-673.9nm and 704.62-729.22nm. The results suggest that the energy transfer of intermolecular vibrational modes, where the strongest vibrational mode excites other vibrational modes, is a simple approach for generating supercontinuum coherent radiation.

Multicolor and supercontinuum radiation generation in terahertz and mid-infrared ranges due to the gas ionization by two-color chirped laser pulses

A method for generating multicolor phase-stable pulses tunable in a wide range from terahertz (THz) to mid-infrared (mid-IR) is proposed. It is based on gas ionization by a laser pulse containing a linearly chirped fundamental field and several double-frequency components with different time delays. Our closed-form analytical formulas and numerical calculations show that the generated current density contains unchirped components at several detuning frequencies determined by the difference between the instantaneous frequencies of the additional laser components and twice the instantaneous frequency of the fundamental field, as well as at all triple combination detuning frequencies. The generated frequencies can be changed by varying the time delay or the chirp value. The generated pulse durations are determined by the gas ionization duration, which is much shorter than the laser pulse duration and can be varied in a wide range by changing the laser pulse duration or intensity. Under a sufficiently short ionization duration, the peak widths at the generated frequencies increase greatly, forming a supercontinuum extending from THz to the mid-IR range.

Experimental Determination of Aerosol Particle Sizes Using Supercontinuum Radiation and Estimation of the Directional Pattern of Radiation from the Filamentation Region of Femtosecond Pulses

Experimental Determination of Aerosol Particle Sizes Using Supercontinuum Radiation and Estimation of the Directional Pattern of Radiation from the Filamentation Region of Femtosecond Pulses

Intense vector supercontinuum radiation from femtosecond filamentation.

Intense vector supercontinuum (SC) radiation with spatial polarization is obtained by using 800nm femtosecond vector laser beams in the air. The SC generated by azimuthally, radially, cylindrically polarized beams, and higher-order vector beams are investigated, respectively. The results show that the SC generated by vector beams is greatly enhanced compared to that by a Gaussian beam. The energy density of SC radiation reaches the order of 1µJ/nm in a bandwidth of 258 nm from 559 nm to 817 nm and 0.1 µJ/nm from 500 nm to 559 nm. Furthermore, by checking the polarization distribution of SC in different wavelengths from visible to near-infrared bands, we find that the SC maintains nearly the same polarization distribution as pump pulses. This work provides an effective and convenient way to generate powerful SC vector beams which may facilitate potential applications including optical communication, micro/nano-fabrication, and super-resolution microscopy.

Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources.

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M2 factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

Experimental Determination of Aerosol Particle Sizes Using Supercontinuum Radiation and Estimation of the Directivity Pattern of Radiation From the Filamentation Region of Femtosecond Pulses

Experimental Determination of Aerosol Particle Sizes Using Supercontinuum Radiation and Estimation of the Directivity Pattern of Radiation From the Filamentation Region of Femtosecond Pulses

Electron trajectory backanalysis for spectral profile in two-color terahertz generation

The gas-phase medium ionized by two-color femtosecond field produces supercontinuum radiation ranging from the terahertz to midinfrared band. Under the strong-field approximation, the electron trajectory backanalysis is exploited to interpret the spectral profile of terahertz radiation. Meanwhile, the electron trajectory method can correlate terahertz spectral profiles and photoelectron momentum distributions (PEMDs). The coherent superposition of electron trajectories released from the consecutive cycles is found to induce the high-frequency cutoff and sharpen the spectral bandwidth of supercontinuum radiation, tightly coinciding with the intercycle interference fringes in PEMDs. The trajectory analysis exhibits that single electron interference plays an important role in the terahertz generation process. Our method provides an intuitive interpretation in terms of electron trajectory perspective and sheds light on the microscopic mechanisms of terahertz generation.

Total internal reflection of the mid-IR near single-cycle light bullet in LiF.

Mid-IR light bullets formation and self-reconstruction after total internal reflection in LiF is investigated experimentally and through numerical simulations. The experiment showed that spatial-temporal deformations of the wave packet in inhomogeneous wave on a reflecting interface can be neglected for this process and a near single-cycle light bullet can be formed or recovered after a distance determined by its energy loss for ionization and supercontinuum radiation. The estimation of the reconstruction distance by an empirical formula for self-focusing distance on the base of the peak power and the radius of the LB numerical simulation are in a good agreement with experimental data received.

Third-order harmonic generation in a bi-chromatic elliptical laser field.

The low-order harmonic generation induced by a strong laser field produces a bright, ultrashort, supercontinuum radiation ranging from the terahertz to ultraviolet band. By controlling the phase-delay and ellipticity of the bi-chromatic laser fields, the third harmonic generation is experimentally and theoretically investigated for elucidating the mechanism of the low-order harmonics. The third harmonic generation is found to be strongly suppressed in the counter-rotating bi-chromatic laser field due to the selection rule for harmonic emissions. The continuum-continuum transition in the strong field approximation is extended to explain the third harmonic generation as a function of the phase delay and ellipticity of the bi-chromatic laser fields. Compared with the semi-classical photocurrent model, the continuum-continuum transition on the basis of quantum-mechanical treatment achieves better agreement with the experimental observations. Our work indicates that the overlapping in continuum states via different quantum paths of a single electron plays a role in low-order harmonics generation under elliptical bi-chromatic laser fields.

Coherent Control of High Harmonic Generation Driven by Metal Nanotip PhotoemissionSupported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0307700 and 2016YFA0401100), the National Natural Science Foundation of China (Grant Nos. 11774215, 11674209, 91950101, 11947243, 11334009, 11425414, and 11947080), Sino-German Mobility Programme (Grant No. M-0031), Department of Education of Guangdong Province (Grant No. 2018KCXTD011), High Level

Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.

Random quasi-phase-matching in polycrystalline media and its effects on pulse coherence properties.

Polycrystalline materials can mediate efficient frequency up-conversion for mid-infrared light. Motivated by the need to understand the properties of the harmonic and supercontinuum radiation from such media, we utilize realistic numerical simulations to reveal its complex temporal and spatial structure. We show that the generated radiation propagates in the form of long-duration pulse trains that can be difficult to compress and that optical filamentation in high-energy pulses gives rise to fine-structured beam profiles. We identify trends concerning pulse energy, sample length, and the microstructure of the material that can inform optimization for different applications.

A multi-terawatt two-color beam for high-power field-controlled nonlinear optics.

Two-color laser beams are instrumental in light-field control and enhancement of high-order harmonic, spectral supercontinuum, and terahertz radiation generated in gases, plasmas, and solids. We demonstrate a multi-terawatt two-color beam produced using a relativistic plasma mirror, with 110 mJ at 800 nm and 30 mJ at 400 nm. Both color components have high spatial quality and can be simultaneously focused, provided that the plasma mirror lies within a Rayleigh range of the driving fundamental beam. Favorable scaling of second-harmonic generation by plasma mirrors at relativistic intensities suggests them as an excellent tool for multi-color waveform synthesis beyond the petawatt level.

Coherent mid-infrared supercontinuum generation in tapered suspended-core As39Se61 fibers pumped by a few-optical-cycle Cr:ZnSe laser.

We report on efficient supercontinuum generation in tapered suspended-core $ {{\rm As}_{39}}{{\rm Se}_{61}} $As39Se61 fibers pumped by a femtosecond mode-locked Cr:ZnSe laser. The supercontinuum spectrum spans the mid-infrared spectral region from 1.4 to 4.2 µm, and its spectral coherence is proved by heterodyning with a single-frequency narrow-linewidth Er-fiber laser at 1.55 µm, measuring a beat note with 27-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz. The intensity stability of the supercontinuum radiation is also characterized by relative intensity noise measurements.

Dual-band infrared optical coherence tomography using a single supercontinuum source.

Recent developments and commercial availability of low-noise and bright infrared (IR) supercontinuum sources initiated intensive applied research in the last few years. Covering a significant part of near- and mid-infrared spectral ranges, supercontinuum radiation opened up unique possibilities and alternatives for the well-established imaging technique of optical coherence tomography (OCT). In this contribution, we demonstrate the development, performance, and maturity of a cost-efficient dual-band Fourier-domain IR OCT system (2 µm and 4 µm central wavelengths). The proposed OCT setup is elegantly employing a single supercontinuum source and a pyroelectric linear array. We discuss adapted application-oriented approaches to signal acquisition and post-processing when thermal detectors are applied in interferometers. In the experimental part, the efficiency of the dual-band detection is evaluated. Practical results and direct comparisons of the OCT system operating within the employed sub-bands are exhibited and discussed. Furthermore, we introduce the 2 µm OCT sub-system as an affordable alternative for art diagnosis; therefore, high resolution and sensitive measurements of the painting mock-ups are presented. Finally, potentials of the dual-band detection are demonstrated for lithography-based manufactured industrial ceramics.

Ultraviolet-to-millimeter-band supercontinua driven by ultrashort mid-infrared laser pulses

Combined optical nonlinearity of bound and free electrons in a fast-ionizing medium driven by ultrashort, mid-infrared (mid-IR) pulses gives rise to a vast variety of ultrafast nonlinear-optical scenarios, producing bright, broadband radiation in spectral ranges as different as ultraviolet (UV) and terahertz (THz). Given its enormous bandwidth, a quantitative experimental analysis of this type of nonlinear response is anything but simple. Here, we confront this challenge by ultrabroadband spectral measurements performed across the spectral range from the UV to the millimeter-wave (MMW) band jointly with beam profile analysis in the THz-to-MMW band and direct time-domain field waveform characterization. As one of the most striking results, the nonlinear response of a fast-ionizing gas driven by a two-color field, consisting of a high-peak-power sub-100-fs mid-IR pulse and its second harmonic, is shown to provide a source of a bright multiband supercontinuum (SC) radiation, whose spectrum spans over about 14 octaves, stretching from below 300 nm all the way beyond 4.3 mm. The MMW-to-THz part of this SC is emitted, as direct measurements show, in the form of half-cycle field waveforms that can be focused to yield a field strength of ≈ 5 M V / c m . At least 1.5% of the MMW–THz supercontinuum energy is emitted in the MMW range, giving rise to MMW field strengths up to 100 kV/cm in the beam waist region.

Генерация суперконтинуума в режиме филаментации в водяной капле с наночастицами серебра при низкой температуре

The supercontinuum generation in water droplets with nanoparticles of citrate silver in the temperature range of 2–22 °C, as well as in the ice droplets frozen to −15.0 °C, has been studied. It was found that the intensity of the supercontinuum emission under the excitation by a train of femtosecond laser pulses exponentially decays along the droplet diameter and it increases linearly with increasing NP concentration. The emission spectrum of supercontinuum in water droplet with NPs and the generation of localized plasmons with fluorescence at the 430 nm wavelength was studied. The movement of a heat wave along the diameter of a small frozen drop with a speed of 190 mm / s accompanying exponentially decaying supercontinuum radiation was recorded. The modeling of heat transfer processes in the frozen droplet during the formation of a heat wave has been carried out.

Compact all-fiber source of coherent linearly polarized octave-spanning supercontinuum based on normal dispersion silica fiber

We report the generation of coherent octave-spanning supercontinuum in an all-fiber system, without any free-space optical components. The setup uses the femtosecond fiber laser as a pump and an all-normal dispersion microstructured fiber as a medium for supercontinuum generation. The generated spectrum is characterized both experimentally and numerically and shows a broad bandwidth (1.1−2.2 μm), a high signal to noise ratio reaching 100 at maximum, a high coherence (closing to 1), linear polarization and average output power up to 57 mW. The source is characterized by exceptional simplicity and does not require any alignment (the nonlinear fiber is spliced to the pump) which finally opens the path to outside-lab applications of supercontinuum radiation.

Ultrafast Broadband Diagnostics of the Filling of the s Band at the Two-Photon Femtosecond Laser Excitation of a Gold Film

The transmission of near infrared femtosecond laser pulses (wavelength of 800 nm) and supercontinuum radiation (300–750 nm) generated by them through a 50-nm gold film immersed in water has been experimentally studied at the power of focused laser radiation in the range of 1–10 GW. An increase in the excitation intensity is accompanied by the blueshift of the edge of the extinction band of the film, which is attributed to the filling of electronic states of the s band with a low density of states caused by two-photon interband transitions from d bands with a high density of states.

Ultrafast Broadband Nonlinear Spectroscopy of a Colloidal Solution of Gold Nanoparticles

The generation of supercontinuum radiation at the filamentation of femtosecond laser pulses (800 nm) with a supercritical gigawatt peak power in pure water and colloidal solutions of plasmon gold nanoparticles (perturbative regime) has been studied experimentally. After correction to the extinction of the fluid behind a filament, the supercontinuum radiation yields from the active filamentation zone in water (broadband radiation source) and in colloidal solution (broadband radiation source modified by various in situ interactions with nanoparticles) have been compared and analyzed for various peak radiation powers. As a result, the proposed method of ultrafast broadband nonlinear spectroscopy has allowed the observation of effects of the saturated two-photon interband absorption of gold nanoparticles in the spectral range of supercontinuum generation and the enhancement of this generation in the range of plasmon resonance of nanoparticles.