Coherent Extreme Ultraviolet Radiation Research Articles

Why plasma harmonics?

We discuss the emergence of interest in the high-order harmonic generation (HHG) of ultrashort pulses propagated through laser-produced plasmas. It is shown that, during the last few years, substantial amendments of plasma HHG allowed in some cases the characteristics of gas HHG to be surpassed. The attractiveness of a new approach in coherent extreme ultraviolet radiation generation is demonstrated, which can also be used as a tool for laser-ablation-induced HHG spectroscopy of a giant class of solids. We present general ideas and prospects for this relatively new field of nonlinear optics.

Extreme ultraviolet high-harmonic spectroscopy of solids.

Extreme ultraviolet (EUV) high-harmonic radiation emerging from laser-driven atoms, molecules or plasmas underlies powerful attosecond spectroscopy techniques and provides insight into fundamental structural and dynamic properties of matter. The advancement of these spectroscopy techniques to study strong-field electron dynamics in condensed matter calls for the generation and manipulation of EUV radiation in bulk solids, but this capability has remained beyond the reach of optical sciences. Recent experiments and theoretical predictions paved the way to strong-field physics in solids by demonstrating the generation and optical control of deep ultraviolet radiation in bulk semiconductors, driven by femtosecond mid-infrared fields or the coherent up-conversion of terahertz fields to multi-octave spectra in the mid-infrared and optical frequencies. Here we demonstrate that thin films of SiO2 exposed to intense, few-cycle to sub-cycle pulses give rise to wideband coherent EUV radiation extending in energy to about 40 electronvolts. Our study indicates the association of the emitted EUV radiation with intraband currents of multi-petahertz frequency, induced in the lowest conduction band of SiO2. To demonstrate the applicability of high-harmonic spectroscopy to solids, we exploit the EUV spectra to gain access to fine details of the energy dispersion profile of the conduction band that are as yet inaccessible by photoemission spectroscopy in wide-bandgap dielectrics. In addition, we use the EUV spectra to trace the attosecond control of the intraband electron motion induced by synthesized optical transients. Our work advances lightwave electronics in condensed matter into the realm of multi-petahertz frequencies and their attosecond control, and marks the advent of solid-state EUV photonics.

Perturbative optical parametric amplification in the extreme ultraviolet.

The response of a medium illuminated with a light field can be expanded in many orders in perturbative non-linear optics. Here we use two multiple-cycle high-intensity laser pulses at 800 and 1,400 nm to generate extreme ultraviolet radiation where the multi-photon processes can be treated as the driving force of a perturbative optical parametric amplification. When a very high-intensity pulse (>7.10(14) W cm(-2)) at 800 nm is applied in addition to a high-intensity pulse at 1,400 nm, we are able to enhance the flux of the coherent extreme ultraviolet radiation in the photon energy range around 80 eV by more than an order of magnitude compared with the generation with a single-wavelength pulse. This opens the way to extend the powerful techniques of perturbative non-linear optics to the case of a high-intensity-driving field in multiple-photon processes.

Laser control for harmonic selective enhancement

We theoretically investigate harmonic-selective enhancement by controlling the time-delayed two-color laser field. By properly controlling the delay times of the two pulses, selective enhancement of the harmonics with photon energies between 75 and 125 eV can be obtained, resulting in wavelength-tunable and bandwidth-controllable coherent extreme ultraviolet radiation. In addition, the other laser parameters’ effects on the harmonic-selective-enhancement have also been investigated.

Cancer cell classification with coherent diffraction imaging using an extreme ultraviolet radiation source.

In cancer treatment, it is highly desirable to classify single cancer cells in real time. The standard method is polymerase chain reaction requiring a substantial amount of resources and time. Here, we present an innovative approach for rapidly classifying different cell types: we measure the diffraction pattern of a single cell illuminated with coherent extreme ultraviolet (XUV) laser-generated radiation. These patterns allow distinguishing different breast cancer cell types in a subsequent step. Moreover, the morphology of the object can be retrieved from the diffraction pattern with submicron resolution. In a proof-of-principle experiment, we prepared single MCF7 and SKBR3 breast cancer cells on gold-coated silica slides. The output of a laser-driven XUV light source is focused onto a single unstained and unlabeled cancer cell. With the resulting diffraction pattern, we could clearly identify the different cell types. With an improved setup, it will not only be feasible to classify circulating tumor cells with a high throughput, but also to identify smaller objects such as bacteria or even viruses.

Advantages of plasma harmonic generation using high pulse repetition rate lasers: Review of recent studies

The studies of coherence properties of the harmonics generating in laser-produced plasmas, the analysis of the optical nonlinearities of deoxyribonucleic acid components, the resonance enhancement of harmonic in the cases of excitation of indium plasma by multi- and few-cycle pulses, and the application of nanoparticle-based emitters of harmonics using high-pulse repetition rate lasers are reviewed. The analysis of various aspects of plasma harmonic generation at the conditions of optimal excitation of the ablated targets irradiating by 1 kHz lasers is presented. The growth of plasma harmonic conversion efficiency, single harmonic emission, nonlinear spectroscopy of complex organic components, as well as high coherency of harmonic radiation show the advantages of using plasma harmonic technique for optimization of the sources of coherent extreme ultraviolet radiation and for the material science studies. These studies allowed a significant growth of the average power of harmonics compared with the case of 10 Hz lasers.

Harmonic generation from partially ionized plasma [Invited

The latest achievements in high-order harmonic generation of ultrashort pulses in plasma plumes are reviewed. These studies demonstrate the attractiveness of this advanced approach of coherent extreme ultraviolet radiation generation. We analyze early studies in this field and review the application of quasi-phase matching, extended plasma plumes, cluster-containing ablation, etc., for improvement of the harmonic generation from partially ionized plasmas.

High repetition rate plasma mirror device for attosecond science

This report describes an active solid target positioning device for driving plasma mirrors with high repetition rate ultra-high intensity lasers. The position of the solid target surface with respect to the laser focus is optically monitored and mechanically controlled on the nm scale to ensure reproducible interaction conditions for each shot at arbitrary repetition rate. We demonstrate the target capabilities by driving high-order harmonic generation from plasma mirrors produced on glass targets with a near-relativistic intensity few-cycle pulse laser system operating at 1kHz. During experiments, residual target surface motion can be actively stabilized down to 47 nm (root mean square), which ensures sub-300-as relative temporal stability of the plasma mirror as a secondary source of coherent attosecond extreme ultraviolet radiation in pump-probe experiments.

Generation of highly coherent extreme ultraviolet source and its application in diffraction imaging

We report the generation of small bandwidth highly coherent extreme ultraviolet radiation and the application of this source in coherent diffractive transmission microscope. Using a focussed narrow-bandwidth high harmonic generation (HHG) source with wavelength around 30nm we achieve a resolution of ∼45nm with a sample size down to 3μm×3μm in a very short exposure time of <8s. The experimental scheme is able for imaging sub-10nm scale objects with an inexpensive table-top, commercially available femtosecond laser system.

Photoionization of resonantly driven atomic states by an extreme ultraviolet-free-electron laser: intensity dependence and renormalization of Rabi frequencies

We analyze theoretically the high intensity photoionization dynamics of a system with two atomic states resonantly coupled by coherent extreme ultraviolet laser radiation that also gives rise to the ionization. The ground state occupation of such a system is shown to exhibit damped Rabi oscillations. The corresponding ionization, which is responsible for the damping, scales almost linearly with the field intensity when the pulse length exceeds the Rabi period. For shorter pulses a quadratic scaling is found. The Rabi frequency is shifted compared to its value for an isolated two-level system. The shift increases with excitation intensity and can acquire a high percentage of the unrenormalized frequency at high intensities. Analytical results obtained within a simplified solvable model demonstrate that the damping and the shift both result from the coupling of the discrete states to the ionization continuum and are therefore closely related. Numerical simulations for a two-electron system reveal at high intensities the importance of off-resonant ionization channels.

RECENT PROGRESS AND PERSPECTIVES OF PLASMA HARMONICS

We review recent achievements of high-order harmonic generation in the plasma plumes produced during laser ablation of various materials. These studies have demonstrated the attractiveness of a new approach in coherent extreme ultraviolet radiation generation. We also present the general ideas of potential goals of plasma harmonic studies and perspectives of the development of this relatively new field of nonlinear optics.

Scalability limits of Talbot lithography with plasma-based extreme ultraviolet sources

Lithography has been faced with a challenge to bring resolution down to the 10-nm level. One of the promising approaches for such ultra-high-resolution printing is self-imaging Talbot lithography with extreme ultraviolet (EUV) radiation. However, as the size of structures on the mask approaches the wavelength of the radiation, diffraction influence needs to be evaluated precisely to estimate the achievable resolution and quality of the patterns. Here, the results of finite-difference time-domain simulations of the diffraction on EUV transmission masks in dependence to the period (pitch) of the mask are presented with the aim to determine the resolution that can be realistically achieved with the EUV Talbot lithography. The modeled experimental setup is utilizing partially coherent EUV radiation with the wavelength of 10.9 nm from Xe/Ar discharge plasma EUV source and Ni/Nb-based amplitude transmission mask. The results demonstrate that the method can be used to produce patterns with resolution down to 7.5-nm half-pitch with 2∶1 mask demagnification utilizing achromatic Talbot effect and transverse electric (TE)-polarized light.

Probing of laser-irradiated solid targets using coherent extreme ultra-violet radiation

The diagnostic potential of extreme ultraviolet (EUV) coherent probing within a laser produced plasma is investigated. A fluid code is used to model the interaction of a 35 fs, 2 × 1014 Wcm−2 800 nm laser pulse with an 800 nm thick aluminium target. A post processor is used to calculate the refractive index and transmission to 45 eV radiation of the target. The effects of EUV radial phase variations at the rear of the target on the intensity distribution at a detector 1.5 m from the target are studied. An irradiated aluminium target is found to have little effect on the transmission of 45 eV radiation, however, there are significant phase retardation differences of the probing beam in the radial direction. These phase variations affect the subsequent propagation of the radiation, suggesting that a simple diagnostic that measures the far-field footprint of the coherent EUV radiation passing through an irradiated target is sensitive to radial variations of the target heating. Sample calculated footprint variations associated with a drop in laser absorption to an irradiance of 1014 Wcm−2 at a radius from the focal centre of 50 μm are shown.

Diffraction-assisted extreme ultraviolet proximity lithography for fabrication of nanophotonic arrays

In this article, the possibilities and limitations of proximity lithography with extreme ultraviolet (EUV) radiation are explored theoretically and experimentally. Utilizing partially coherent EUV radiation with a wavelength of 10.88 nm from a Xe/Ar discharge plasma EUV source, proximity patterning of various nanoantenna arrays has been performed. The experimental results are compared with the results of numerical scalar diffraction simulations, and it is shown that proximity printing in the Fresnel diffraction mode can enable production of high-resolution features even with lower resolution masks, successfully demonstrating sub-30 nm edge resolution in the resist. The potential of the method is explored by simulation of the patterning through circular and triangular apertures as well as through bowtie antenna patterns, with the results suggesting that precise control of the proximity gap and the exposure dose together with simulation-supported mask design optimizations may allow for a wide variety of high-resolution structures to be printed through relatively simple transmission masks. The method is especially suited for high-performance manufacturing of submicrometer sized nanophotonic arrays.

Strong Field-Induced Frequency Conversion of Laser Radiation in Plasma Plumes: Recent Achievements

New findings in plasma harmonics studies using strong laser fields are reviewed. We discuss recent achievements in the growth of the efficiency of coherent extreme ultraviolet (XUV) radiation sources based on frequency conversion of the ultrashort pulses in the laser-produced plasmas, which allowed for the spectral and structural studies of matter through the high-order harmonic generation (HHG) spectroscopy. These studies showed that plasma HHG can open new opportunities in many unexpected areas of laser-matter interaction. Besides being considered as an alternative method for generation of coherent XUV radiation, it can be used as a powerful tool for various spectroscopic and analytical applications.

Light-induced states in attosecond transient absorption spectra of laser-dressed helium

Laser-dressed absorption in atomic helium is studied, both theoretically and experimentally, by transient absorption spectroscopy using isolated 400-as pulses centered at 22 eV and 12-fs near-infrared (NIR) pulses with 780-nm central wavelength. Multiple features in the helium singly excited bound-state spectrum are observed only when the NIR and attosecond pulses are overlapped in time. Theoretical analysis indicates that these light-induced structures (LISs) are the intermediate states in resonant, second-order processes that transfer population to multiple dipole forbidden states. The use of broadband, coherent extreme ultraviolet (XUV) radiation allows observation of these LISs without specifically tuning to a two-photon resonance, as would be required with narrowband XUV light. The strength and position of the LISs depend strongly on the NIR intensity and the pump-probe delay.

Phase-matched generation of highly coherent radiation in water window region

Highly coherent extreme ultraviolet radiation around the water window region (~4.4 nm) is generated in a semi-infinitive helium gas cell using infrared pulses of wavelength 1300 nm, energy 2.5 mJ, duration 40 fs, and repetition rate 1 kHz. The pressure-squared dependence of the intensity and the almost-perfect Gaussian profile and low divergence of the high harmonic source indicate a phase-matched generation process. The spatial coherence of the source is studied using Young's double-slit measurements.

Relativistic frequency upshift to the extreme ultraviolet regime using self-induced oscillatory flying mirrors

Coherent short-wavelength radiation from laser–plasma interactions is of increasing interest in disciplines including ultrafast biomolecular imaging and attosecond physics. Using solid targets instead of atomic gases could enable the generation of coherent extreme ultraviolet radiation with higher energy and more energetic photons. Here we present the generation of extreme ultraviolet radiation through coherent high-harmonic generation from self-induced oscillatory flying mirrors—a new-generation mechanism established in a long underdense plasma on a solid target. Using a 30-fs, 100-TW Ti:sapphire laser, we obtain wavelengths as short as 4.9 nm for an optimized level of amplified spontaneous emission. Particle-in-cell simulations show that oscillatory flying electron nanosheets form in a long underdense plasma, and suggest that the high-harmonic generation is caused by reflection of the laser pulse from electron nanosheets. We expect this extreme ultraviolet radiation to be valuable in realizing a compact X-ray instrument for research in biomolecular imaging and attosecond physics.

Coherent continuum extreme ultraviolet radiation in the sub-100-nJ range generated by a high-power many-cycle laser field

High-energy coherent continuum radiation is generated by the interaction of rare gases with a high-power many-cycle laser field, utilizing the interferometric polarization gating technique. A narrow temporal gate is formed in the laser pulse within which the extreme ultraviolet (XUV) emission is restricted. An analytical expression for the gate function is derived. A super-XUV continuum down to 15 nm, broad enough to support synthesis of single pulses of 260 as duration and a few tens of nanojoule energy, has been measured. These results directly challenge the perspectives of single-attosecond pulse XUV-pump-XUV-probe applications.

Ion-Pair Dissociation Dynamics of Cl2 in the Range 13.26−13.73 eV Studied by Using XUV Laser and the Velocity Map Imaging Method

The ion-pair dissociation dynamics of Cl2 in the range 13.26-13.73 eV have been studied employing coherent extreme ultraviolet radiation (XUV) and the velocity map imaging method. The Cl- (1S(0)) ion-pair yield spectrum has been measured, and 80 velocity map images of Cl- (1S(0)) have been recorded for the strong peaks in the spectrum. From the images, two dissociation channels corresponding to Cl- (1S(0)) + Cl+ (3P(j)) and Cl- (1S(0)) + Cl+ (1D(2)) have been found. The branching ratios among the fragments Cl+ (3P(0,1)), Cl+ (3P(2)), and Cl+ (1D(2)) and their corresponding anisotropy parameters beta have been determined. It is concluded that the ion-pair dissociation occurs via the predissociation of Rydberg states and its dynamics strongly depend on the excitation energies. The perpendicular and parallel transitions correspond to the excitation to two major Rydberg series converging to ion-core Cl2+ (A2Pi(u)), ...sigma(g)2pi(u)3pi(g)4[A2Pi(u)]ns sigma(g), 1Pi(u) (n = 6,7), and ...sigma(g)2pi(u)3pi(g)4[A2Pi(u)]nd pi(g), 1Sigma(u)+ (n = 4,5), respectively. For the production of fragments Cl+ (1D(2)), it is proposed that the perpendicular transitions are from the excitation to Rydberg states [A2Pi(u)]ns sigma(g), 1Pi(u) that is predissociated by ion-pair state 2 1Pi(u), while the parallel transitions are from the excitation to [A2Pi(u)]nd pi(g), 1Sigma(u)+ that interacts with ion-pair states 0(u)+ (1D(2)), 0(u)+ (3P(2)), and 0(u)+ (3P(0)).