Laser Plasma Light Source Research Articles

Laboratory system for optical coherence tomography (OCT) using a laser plasma source of soft x-rays and extreme ultraviolet and focusing ellipsoidal optics.

Optical coherence tomography (OCT) with the use of soft x-rays (SXR) and extreme ultraviolet (EUV) has been recently demonstrated [Fuchs et al. Sci. Rep.6, 20658 (2016)10.1038/srep20658; Fuchs et al. Optica4, 903 (2017)10.1364/OPTICA.4.000903]. This new imaging technique, named XCT, makes it possible to obtain cross-sectional and tomographic images of objects with nanometer spatial resolution. The article presents a newly developed laboratory system for XCT using a compact laser plasma light source operating in the SXR and EUV spectral ranges. The source is based on a gas puff target containing Kr gas or a Kr/Xe gas mixture irradiated with nanosecond laser pulses from an Nd:YAG laser. The use of the gas puff target enables efficient emission of SXR and EUV radiation without generating target debris associated with laser ablation when using a solid target. The system is equipped with an ellipsoidal mirror to collect radiation from the source and focus on the imaged object. The XCT measurements are made by processing the spectrum of the radiation reflected from the object recorded with a transmission grating spectrometer equipped with an identical focusing mirror and a CCD camera. The paper presents the characterization and optimization of the new XCT system and its application to the measurements of layered nanostructures.

Development of a cryogenic ion mobility spectrometer for an isomer-selected photoexcitation and reaction studies

SynopsisA new development aiming experimental studies of isomer-selected gas-phase reactions and photoexcitation processes is being conducted. The experiment is based on isomer separation by an ion mobility spectrometry and ion accumulation in a temperature-variable radiofrequency ion trap. The laser plasma light source at Rikkyo University is currently under an upgrade for providing ultraviolet light at the wavelength region from 90 to 200 nm. We report on the details of the experimental setup and current status of the development.

A table-top EUV focusing optical system with high energy density using a modified Schwarzschild objective and a laser-plasma light source.

For investigating extreme ultraviolet (EUV) damage on optics, a table-top EUV focusing optical system was developed in the laboratory. Based on a modified Schwarzschild objective with a large numerical aperture and a laser-plasma light source, this optical system can generate a focusing spot with the maximum energy density of 2.27 J/cm2 at the focal plane of the objective at the wavelength of 13.5 nm. The structures and the characterized properties of this optical system are presented in this paper. For testing the performance of this setup, single-shot EUV damage tests were carried out on an optical substrate of CaF2 and a gold thin film. The damage thresholds and morphologies of these samples were given and discussed with former research studies.

Single-Shot near Edge X-ray Fine Structure (NEXAFS) Spectroscopy Using a Laboratory Laser-Plasma Light Source.

We present a proof of principle experiment on single-shot near edge soft X-ray fine structure (NEXAFS) spectroscopy with the use of a laboratory laser-plasma light source. The source is based on a plasma created as a result of the interaction of a nanosecond laser pulse with a double stream gas puff target. The laser-plasma source was optimized for efficient soft X-ray (SXR) emission from the krypton/helium target in the wavelength range from 2 nm to 5 nm. This emission was used to acquire simultaneously emission and absorption spectra of soft X-ray light from the source and from the investigated sample using a grazing incidence grating spectrometer. NEXAFS measurements in a transmission mode revealed the spectral features near the carbon K-α absorption edge of thin polyethylene terephthalate (PET) film and L-ascorbic acid in a single-shot. From these features, the composition of the PET sample was successfully obtained. The NEXAFS spectrum of the L-ascorbic acid obtained in a single-shot exposure was also compared to the spectrum obtained a multi-shot exposure and to numerical simulations showing good agreement. In the paper, the detailed information about the source, the spectroscopy system, the absorption spectra measurements and the results of the studies are presented and discussed.

Optical coherence tomography (OCT) with 2\u2009nm axial resolution using a compact laser plasma soft X-ray source

We present optical coherence tomography (OCT) with 2 nm axial resolution using broadband soft X-ray radiation (SXR) from a compact laser plasma light source. The laser plasma was formed by the interaction of nanosecond laser pulses with a gaseous target in a double stream gas puff target approach. The source was optimized for efficient SXR emission from the krypton/helium gas puff target in the 2 to 5 nm spectral range, encompassing the entire “water-window” spectral range from 2.3 nm to 4.4 nm wavelength. The coherence parameters of the SXR radiation allowed for the OCT measurements of a bulk multilayer structure with 10 nm period and 40% bottom layer thickness to period ratio, with an axial resolution of about 2 nm and detect multilayer interfaces up to a depth of about 100 nm. The experimental data are in agreement with OCT simulations performed on ideal multilayer structure. In the paper, detailed information about the source, its optimization, the optical system, OCT measurements and the results are presented and discussed.

Compact system for near edge X-ray fine structure (NEXAFS) spectroscopy using a laser-plasma light source.

We present a compact laboratory system for near edge soft X-ray fine structure (NEXAFS) spectroscopy that was developed using a laser-plasma light source. The source is based on a double stream gas puff target. The plasma is formed by the interaction of a laser beam with the double stream gas puff target approach. The laser plasma source was optimized for efficient soft X-ray emission from a krypton/helium target in the range of 1.5 to 5 nm wavelength. This emission is used to acquire simultaneously the emission and absorption spectra of soft X-ray light from the source and from the investigated sample using a grazing incidence spectrometer. The measurements in the transmission mode reveal the features near the carbon K-α absorption edge of thin PET film. From those features, the composition of the sample was successfully obtained. The data are in agreement with synchrotron measurements. In the paper, the detailed information about the source, its optimization, the system, spectral measurements and the results are presented and discussed.

Spectral irradiance of singly and doubly ionized zinc in low-intensity laser-plasma ultraviolet light sources

The absolute spectral irradiances of laser-plasmas produced from planar zinc targets are determined over a wavelength region of 150 to 250 nm. Strong spectral radiation is generated using 60 ns full-width-at-half-maximum, 1.0 μm wavelength laser pulses with incident laser intensities as low as ∼5 × 108 W cm−2. A typical radiation conversion efficiency of ∼2%/2πsr is measured. Numerical calculations using a comprehensive radiation-hydrodynamics model reveal the strong experimental spectra to originate mainly from 3d94s4p-3d94s2, 3d94s4d-3d94s4p, and 3d94p-3d94s, 3d94d-3d94p unresolved-transition arrays in singly and doubly ionized zinc, respectively.

A desktop extreme ultraviolet microscope based on a compact laser-plasma light source

A compact, desktop size microscope, based on laser-plasma source and equipped with reflective condenser and diffractive Fresnel zone plate objective, operating in the extreme ultraviolet (EUV) region at the wavelength of 13.8 nm, was developed. The microscope is capable of capturing magnified images of objects with 95-nm full-pitch spatial resolution (48 nm 25–75% KE) and exposure time as low as a few seconds, combining reasonable acquisition conditions with stand-alone desktop footprint. Such EUV microscope can be regarded as a complementary imaging tool to already existing, well-established ones. Details about the microscope, characterization, resolution estimation and real sample images are presented and discussed.

Highly vibrationally excited O2 molecules in low-pressure inductively-coupled plasmas detected by high sensitivity ultra-broad-band optical absorption spectroscopy

Inductively-coupled plasmas in pure O2 (at pressures of 5–80 mTorr and radiofrequency power up to 500 W) were studied by optical absorption spectroscopy over the spectral range 200–450 nm, showing the presence of highly vibrationally excited O2 molecules (up to vʺ = 18) by Schumann–Runge band absorption. Analysis of the relative band intensities indicates a vibrational temperature up to 10,000 K, but these hot molecules only represent a fraction of the total O2 density. By analysing the (11-0) band at higher spectral resolution the O2 rotational temperature was also determined, and was found to increase with both pressure and power, reaching 900 K at 80 mTorr 500 W. These measurements were achieved using a new high-sensitivity ultra-broad-band absorption spectroscopy setup, based on a laser-plasma light source, achromatic optics and an aberration-corrected spectrograph. This setup allows the measurement of weak broadband absorbances due to a baseline variability lower than 2 × 10−5 across a spectral range of 250 nm.

レーザープラズマ光源を用いた希ガス混合凝縮層からの光励起脱離の観測

レーザープラズマ光源を用いた希ガス混合凝縮層からの光励起脱離の観測

Plume dynamics of a laser produced plasma from a dot target

To generate a plasma light source with a small spot size by reducing the expansion of the plasma plume, we studied the plume expansion dynamics of a plasma generated from a dot target that consisted of a photoresist with a hole coated on a pure Al plate. A time-resolved ultraviolet (UV) image of the plasma plume and a UV spectrometer were used to study the expansion of the plasma plume produced from a dot target and from an Al plate target. The measurements of the plume size from the emission of Al II ions showed that due to the strong collision between the core Al plasma and the plasma generate from the outer part of the dot, the expansion speed in the lateral direction was reduced to 70 % that of the plasma generated from the pure Al plate. However, for the case of the emission from neutral Al atoms, the plume sizes of the two targets were almost the same because of the weak collision between the Al atoms and the surrounding plasmas. Thus, these collisional effects in the plasma produced from a dot target can reduce the size of the core plasma ion and can be used to generate a small-spot-size laser plasma light source.

Integrated experimental setup for angle resolved photoemission spectroscopy of transuranic materials

We have developed the Angle Resolved Photoemission Spectroscopy (ARPES) system for transuranic materials. The ARPES transuranic system is an endstation upgrade to the Laser Plasma Light Source (LPLS) at Los Alamos National Laboratory. The LPLS is a tunable light source for photoemission with a photon energy range covering the vacuum ultraviolet (VUV) and soft x-ray regions (27-140 eV). The LPLS was designed and developed for transuranic materials. Transuranic photoemission is currently not permitted at the public synchrotrons worldwide in the VUV energy range due to sample encapsulation requirements. With the addition of the ARPES capability to the LPLS system there is an excellent opportunity to explore new details centered on the electronic structure of actinide and transuranic materials.

Vacuum-ultraviolet resonant photoabsorption imaging of laser produced plasmas

We present results from a vacuum-ultraviolet (VUV) “photoabsorption imaging” technique based on the measurement of the time and space resolved absorption of a quasimonochromatic VUV beam from a laser plasma light source. The use of VUV radiation as a probe beam permits direct access to resonance lines of (singly and more highly charged) ions and also to the resonant and nonresonant continua of atoms and ions. In this experiment we have confined ourselves to measurements using the 3p–3d resonances of Ca, Ca+, and Ca2+ as markers of the temporal and spatial distribution of ground state atoms and ions in an expanding laser plasma plume. We show how time resolved column density maps may be extracted from such images. In addition we have extracted plasma plume velocities from the data, which compare well with an analytical laser ablation model.

Electronic structure of α- and δ-Pu from photoelectron spectroscopy

We report photoemission results on \ensuremath{\alpha}- and \ensuremath{\delta}-Pu using a laser plasma light source (LPLS) as well as He light as the exciting radiation. The LPLS is a pseudocontinuum tunable light source with intensities rivaling some second-generation synchrotrons. Both phases of Pu display a narrow, temperature-independent, $5f$-related feature at the Fermi energy, which is narrower in \ensuremath{\delta}-Pu than in \ensuremath{\alpha}-Pu, suggestive of possible heavy-fermion-like behavior. In both \ensuremath{\alpha}-Pu and \ensuremath{\delta}-Pu the photon-energy dependence of this feature suggests some $6d$ admixture, albeit somewhat smaller in \ensuremath{\delta}-Pu. In this respect it differs qualitatively from Ce and U heavy fermions.

A tunable bench top light source for photoelectron spectroscopy: first results for alpha and delta Pu

We report the first ever utilization of a laser plasma light source for photoelectron spectroscopy. Using a continuous mercury stream as the target, the light source is essentially debris free with intensities rivaling some second generation syncrotrons. Its primary use is likely to be in connection with hazardous materials which are undesirable at synchrotrons, such as Pu. Toward this end we report the first photoemission data on α- and δ-Pu. Both phases display a 5f-related feature at E F, which is much sharper in δ-Pu than in α-Pu, suggestive of heavy fermion behavior. In both phases, however, the hν-dependence of the intensity of this feature can only be understood if there exists substantial 6d-admixture.

Resonance photoelectron spectroscopy using a tunable laser plasma light source

We report the first resonance photoemission experiments on plutonium metal, with surface preparation accomplished by means of laser ablation. The photoemission experiments were conducted using the newly commissioned tunable light source for transuranic research at Los Alamos. The light source is a tunable, laser plasma system capable of producing photons in the energy range 30–150 eV, with a photon flux of order 109 photons s−1 at 0.1% bandwidth. First results on δ-phase plutonium reveal the 5d–5f resonance enhancement to be maximum at a photon energy of 113 eV. Lower energy photoemission results show a narrow peak very near the Fermi level, with a linewidth of order 100 meV. This peak is verified by resonance photoemission to be of 5f character but also having strong 6d–7s contributions. The combination of high-energy resolution, tunable energies for orbital character determination and surface sensitivity of the UPS energy range make the laser plasma light source ideal for surface and interface studies. © 1998 John Wiley & Sons, Ltd.

X-UV Absorption Spectroscopy with Laser-Produced Plasmas: A Review

Exteme-ultraviolet (X-UV) absorption spectroscopy has gained impetus in recent years, spurred on by a growing interest in the physics of core excited atomic, molecular and solid species. Progress in this field has been helped in no small measure by developments in X-UV light sources. One such source is the laser-produced plasma which, by suitable choice of target material, can be used to produce a clean, line free continuum virtually throughout the X-UV and V-UV spectral regions from 40-2000 Å. We briefly address here the issues surrounding the origin and characteristics of laser-plasma continuum light sources. In addition we review progress to date on the application of these sources to the study of the X-UV photoabsorption spectra of atoms, ions and molecules.

Temperature dependence of the electronic structure of oxides: MgO, MgAl2O4 and Al2O3

We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Laser-plasma sourced, temperature dependent, VUV spectrophotometer using dispersive analysis

We have developed a vacuum ultraviolet spectrophotometer with wide energy and temperature range coverage, utilizing a laser-plasma light source (LPLS), CO2-laser sample heating and time-resolved dispersive analysis. Reflection and transmission spectra can be taken from 1.7 to 40eV (31-700nm) on samples at 15-1800K with a time resolution of 20-400ns. These capabilities permit the study of the temperature dependence of the electronic structure, encompassing the effects of thermal lattice expansion and electron-phonon interaction, and changes in the electronic structure associated with equilibrium and metastable phase transitions and stress relaxation.The LPLS utilizes a samarium laser-plasma created by a Q-switched Nd:YAG laser (500mJ/pulse) to produce high brightness, stable, continuum radiation. The spectrophotometer is of a single beam design using calibrated iridium reference mirrors. White light is imaged off the sample in to the entrance slit of a l-m polychromator. The resolution is 0.1 to 0.3nm. The dispersed light is incident on a focal plane phosphor, fiber-optic-coupled to an image-intensified reticon detector. For spectroscopy between 300 and 1800K, the samples are heated in situ with a 150 Watt CO2 laser. The signal to noise ratio in the VUV, for samples at 1800 K, is excellent. From 300 K to 15 K samples are cooled using a He cryostat.

Optical reflectivity measurements using a laser plasma light source

Light produced by a laser plasma light source (LPLS) is used to perform optical reflectivity measurements on single crystals from 5 to 40 eV in a single experiment. The intense continuum generated by the rare-earth plasma allows a significantly higher resolution above 15 eV and extends the measurements to higher energies than those attainable with other laboratory based light sources. This is the first application of a LPLS to vacuum ultraviolet spectroscopy of solids and we demonstrate this capability on two insulating materials, α-Al2O3 and MgAl2O4.