Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30?nm Au layer thickness, 500?nm grating constant) upon fs laser irradiation (785?nm, 150?fs, 1?kHz,???1?mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15°?fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2?fJ in the 0.3?C0.38?THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).     

Corrugated neat thin-film conjugated polymer distributed-feedback lasers

Wave-guided thin-film distributed-feedback (DFB) polymer lasers are fabricated by spin coating a PPV-derived semiconducting polymer, thianthrene-DOO-PPV, onto oxidised silicon wafers with corrugated second-order periodic gratings. The gratings are written by reactive ion beam etching. Laser action is achieved by transverse pumping with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The DFB-laser surface emission and edge emission are analysed. Outside the grating region the polymer film is used for comparative wave-guided travelling wave laser (amplified spontaneous emission (ASE)) studies. The pump pulse threshold energy density for wave-guided DFB-laser action (4–9 μJ cm^-2) is found to be approximately a factor of two lower than the threshold for wave-guided travelling wave laser action. The spectral width of the DFB laser (down to Δλ_DFB≈0.07 nm) is considerably narrower than that of the travelling wave laser (Δλ_TWL≈14 nm). The DFB-laser emission is highly linearly polarised transverse to the grating axis (TE mode). Only at high pump pulse energy densities does an additional weak TM mode build up. The surface-emitted DFB-laser radiation has a low divergence along the grating direction. For both the DFB lasers and the travelling wave lasers, gain saturation occurs at high excitation energy densities. Received: 7 January 2002 / Revised version: 15 February 2002 / Published online: 14 March 2002     

THz radiation generation via the interaction of two‐colour ultra‐short laser pulses with SO2 and NH3 gases

In this work, using a two‐dimensional particle‐in‐cell Monte Carlo collision computation method, terahertz (THz) radiation generation via the interaction of two‐colour, ultra‐short, high‐power laser pulses with the polyatomic molecular gases sulphur dioxide (SO₂) and ammonia (NH₃) is examined. The influence of SO₂ and NH₃ pressures and two‐colour laser pulse parameters, i.e., pulse shape, pulse duration, and beam waist, on the THz radiation generation is studied. It is shown that the THz signal generation from SO₂ and NH₃ increases with the background gas pressure. It is seen that the THz emission intensity for both gases at higher laser pulse durations is higher. Moreover, for these polyatomic gases, the plasma current density increases with increase in the laser pulse beam waist. A more powerful THz radiation intensity with a larger time to peak of the plasma current density is observed for SO₂ compared to NH₃. In addition, many THz signals with small intensities are observed for both polyatomic gases. It is seen that for both SO₂ and NH₃ the generated THz spectral intensity is higher at higher gas pressures.     

Efficient and widely step-tunable terahertz generation with a dual-wavelength CO<Subscript>2</Subscript> laser

Coherent terahertz radiation in a widely step-tunable range of 72.3–2706 μm (0.11–4.15 THz) has been generated in GaAs crystal by difference-frequency generation using one CO₂ laser with dual-wavelength output. The peak power of THz pulse reaches 35 W at the wavelength of 236.3 μm, which corresponds to a pulse energy of 2.1 μJ. An average power of 10 μW has been achieved when working repetitively. This efficient terahertz radiation source is more compact and widely tunable than other THz sources pumped by CO₂ laser.     

Simulation of THz emission from laser interaction with 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 plasma. The spectra of THz radiation are discussed under different laser intensity, pulse width, incident angle and density scale length. 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 10¹⁸ W/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 conversion efficiency is several ten thousandths, which is higher then the efficiency of other THz source and suitable for the studies of THz nonlinear physics.     

飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

Experimental study of the acceleration of protons emitted from thin foils irradiated by ultrahigh-contrast laser pulses

Experimental results are presented for proton acceleration from the back of a target irradiated by laser pulses with intensities up to 2 × 10¹⁹ W/cm² generated by the SOKOL-P facility. The proton acceleration efficiency increases with decreasing of the target thickness. However, thin targets are destroyed by the amplified spontaneous emission (ASE) prepulse before the main pulse arrival. An additional optical switch based on a Pockels cell has been used in the amplification section to carry out the experiments with ultrathin foils. As a result, the energy contrast with respect to the ASE prepulse has been increased up to 4 × 10⁶. Owing to high contrast, the experiments on studying proton acceleration from foils with thicknesses less than 100 nm have been carried out.     

THz Radiation Generation via the Interaction of Ultra‐short Laser Pulses with the Molecular Hydrogen Plasma

In this paper, a two dimensional Particle In Cell‐Monte Carlo Collision simulation scheme is used to examine the THz generation via the interaction of high intensity ultra‐short laser pulses with an underdense molecular hydrogen plasma slab. The influences of plasma density, laser pulse duration and its intensity on the induced plasma current density and the subsequent effects on the generated THz signal characteristics are studied. It is observed that the induced current density in the plasma medium and THz spectral intensity are increased at the higher laser pulse intensities, laser pulse durations and plasma densities. Moreover, the generated THz electric field amplitude is reduced at the higher laser pulse durations. A wider frequency range for the generated THz signal is shown at the lower laser pulse durations and higher plasma densities. Additionally, it is found that the induced current density in hydrogen plasma medium is the dominant factor influencing the generation of THz pulse radiation. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High repetition rate plasma mirror for temporal contrast enhancement of terawatt femtosecond laser pulses by three orders of magnitude

We present a plasma mirror configuration that improves the temporal pulse contrast of femtosecond terawatt laser pulses by a factor of thousand using a single antireflection coated glass target. The device provides ultra-high contrast for experiments with a maximum repetition rate of 10 Hz. A third-order cross-correlator has been used to measure the temporal pulse contrast for several different plasma mirror targets. It is shown that the ASE can be suppressed to a level of 10⁻¹¹. A comparison between a triggered and an untriggered plasma mirror reveals differences in the intensity distribution of the focused beam. The triggered plasma mirror produces a slightly larger focus due to the expansion of the triggered plasma mirror at −3 ps before the main pulse. We propose a cost-effective AR-coated and a blank glass target to reduce the costs of the consumable target material. High-harmonic radiation on solid surfaces has been generated with different plasma mirror targets to demonstrate the high laser contrast.     

Polymeric distributed-feedback resonator with sub-micrometer fibers fabricated by two-photon induced photopolymerization

The authors observed amplified spontaneous emission (ASE) action with an ultra-low threshold under optical excitation from an active polymeric distributed-feedback (DFB) resonator which consists of sub-micrometer fiber grating. A dendrimer was used to modify the laser dye and increased its concentration up to 4.95 wt. % in the photocured resin. The DFB resonator was fabricated by using two-photon induced polymerization (TPIP) technique. The ASE at 556 nm was observed with a threshold of 0.30 μJ/pulse due to fourth-order diffraction feedback of the optical gain. This DFB could be expected to open a way for the fabrication and application of microscale polymeric mirrorless laser. PACS 42.70.-a; 81.40.Tv; 82.35.Ej; 82.50.Pt; 42.60.Da     

A GaAs/AlAs superlattice autocorrelator for picosecond THz radiation pulses

We report on a GaAs/AlAs, wide-miniband, superlattice autocorrelator for picosecond THz radiation pulses (operated at room temperature); the autocorrelator is based on the THz radiation-induced reduction of current through the superlattice. THz radiation (frequency 7.2 THz) from the FELIX (free-electron laser for infrared experiments) was coupled into the superlattice with an antenna system. We measured the current reduction for two time-delayed pulses and found that the signal decreased when the time delay was smaller than the pulse duration. With this superlattice autocorrelator we were able to resolve laser pulses that had a duration of a few picoseconds.     

Optimization of highly efficient terahertz generation in lithium niobate driven by Ti:sapphire laser pulses with 30 fs pulse duration

We systematically study the optimization of highly efficient terahertz(THz) generation in lithium niobate(LN)crystal pumped by 800 nm laser pulses with 30 fs pulse duration. At room temperature, we obtain a record optical-to-THz energy conversion efficiency of 0.43% by chirping the pump laser pulses. Our method provides a new technique for producing millijoule THz radiation in LN via optical rectification driven by joule-level Ti:sapphire laser systems, which deliver sub-50-fs pulse durations.     

Growth of thin Fe(0 0 1) films for terahertz emission experiments

The electrical and magnetic properties of thin iron (Fe) films have sparked significant scientific interest. Our interest, however, is in the fundamental interactions between light and matter. We have discovered a novel application for thin Fe films. These films are sources of terahertz (THz) radiation when stimulated by an incident laser pulse. After intense femtosecond pulse excitation by a Ti:sapphire laser, these films emit picosecond, broadband THz frequencies. The terahertz emission provides a direct measure of the induced ultrafast change in magnetization within the Fe film. The THz generation experiments and the growth of appropriate thin Fe films for these experiments are discussed. Several criteria are used to select the substrate and film growth conditions, including that the substrate must permit the epitaxial growth of a continuous, monocrystalline or single crystal film, yet must also be transparent to the emitted THz radiation. An Fe(0 0 1) film grown on the (0 0 1) surface of a magnesium oxide (MgO) substrate makes an ideal sample. The Fe films are grown by physical vapor deposition (PVD) in an ultrahigh vacuum (UHV) system. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are used to characterize the Fe(0 0 1) films. Two substrate surface preparation methods are investigated. Fe(0 0 1) films grown on MgO(0 0 1) substrates that are used as-received and films grown on MgO(0 0 1) substrates that have been UV/ozone-cleaned ex vacuo and annealed in vacuo produce the same results in the THz generation experiments. Either substrate preparation method permits the growth of samples suitable for the THz emission experiments.     

Eye-safe radiation source based on an optical parametric oscillator

We have studied an optical parametric oscillator (OPO) with an unstable telescopic cavity, placed inside the cavity of an actively Q-switched multimode Nd³⁺:KGW pump laser. We used a KTP crystal as the nonlinear medium for the OPO. We have compared the emission characteristics of OPOs with unstable telescopic and planar cavities. We have established that compared with the planar cavity, the unstable cavity reduces the OPO beam divergence and improves the spatial distribution of the radiation energy in the far wave zone. Based on our investigations, we have designed a compact eye-safe (λ = 1.578 μm) laser source with natural cooling, emitting (for electrical pumping energy 7.3 J) pulses with pulse energy 22 mJ and pulse duration 6 nsec. The FWHM beam divergence for the source is no greater than 3.5 mrad. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 254–259, March–April, 2006.     

Double chirped-pulse-amplification laser: a way to clean pulses temporally

We demonstrate a double chirped-pulse-amplification (CPA) Ti: sapphire laser system that includes two CPA stages with intermediate nonlinear temporal pulse filtering. The method makes it possible to reduce substantially the background of amplified spontaneous emission (ASE), including prepulses and postpulses. An ASE temporal contrast of 10(10) was demonstrated at 20 mJ of output energy and 50-fs pulse duration. The demonstrated scheme is applicable to petawatt power-level laser systems.     

Investigation of collisionally induced stimulated scattering in sodium vapor with temporal and spectral resolution

In this work we present fully time-resolved spectra of Amplified Spontaneous Emission (ASE) and Stimulated Raman Scattering (SRS) in sodium vapor. We observe that the ASE signal precedes the SRS emission, and the peak of the amplified Stokes pulse is delayed with respect to the peak of the laser pulse. The time evolution of the line widths of the ASE and Raman signals is also shown. We find that the line widths of the stimulated fluorescence component (ASE) and the stimulated Raman radiation change with time. Both spectra start as small narrow fluctuating lines, then the line widths reach maximum values and thereafter decrease with time.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

Passive Q-switching of erbium glass laser by magnesium aluminosilicate sitall with cobalt ions

We have studied the radiation output parameters for an erbium glass laser, lasing at a wavelength of 1.54 μm, with passive Q-switching by means of a cobalt-containing magnesium aluminosilicate sitall compared with a saturable absorber based on a magnesium aluminum spinel crystal with cobalt ions. We have shown that the output characteristics of the laser emission when using sitall are not inferior to the analogous characteristics of a laser based on a spinel crystal, and are practically independent of the temperature of the saturable absorber in the range 0°C–80°C. The duration (energy) of the output pulses was 70 nsec (∼4 mJ), the energy dispersion of the radiation pulse relative to the average value was no greater than 3%, the beam divergence was 2.8 mrad, the laser beam quality factor was M² = 1.2. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 126–131, January–February, 2007.     

Nonequilibrium laser-produced plasma of volume-structured media and inertial-confined-fusion applications

We review the results of experimental and theoretical studies of the properties of a nonequilibrium plasma produced from volume-structured media, containing micro- and nano-size internal elements, under laser-pulse irradiation. We consider two types of materials, i.e., regularly and stochastically structured materials. The first type is either a set of flat layers or cylindrical and spherical shells of micrometer thickness, and the second type is either foams of light elements or light foams containing clusters of heavy elements with dimensions in the range of 10–100 nm. We study the properties of high-temperature laser-produced plasmas of such materials and applications directed to developing the design of inertial confinement fusion (ICF) targets and creating powerful sources of thermonuclear neutron and soft X-ray emission initiated by the laser pulse. The foam materials can be used as absorbers capable of providing homogeneity of laser-energy absorption by the target. A neutron yield up to 1014−1015 DT neutrons per shot can be achieved by heating regularly structured materials using a laser pulse in the regime of the consequent thermal explosions of solid elements containing isotopes of hydrogen. Laser-radiation conversion into soft X-ray emission with the efficiency controlled in a wide range may be realized in laser-produced plasmas of porous media doped with clusters of heavy elements. In particular, such a material can be used as an absorber–converter of laser radiation in inertial confinement fusion targets. Under direct irradiation of an ICF target by a laser pulse, such a converter can provide transformation of 20–30% of the absorbed laser energy into the energy of X-ray radiation transferred to thermonuclear capsules.     

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<t_p<3 ps) at the wavelength =810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured. PACS 06.60.Jn; 42.50.Md; 78.47.-p; 81.16.-c; 82.53.-k