Search

SUMMARY We have studied self-absorption effects of a laser-produced plasma (LPP) source at 6.x nm as a function of laser pulse duration and viewing angle. The spectral profiles are shown to have a strong dependence on the viewing angle. Absorption effects are less pronounced when a 150-ps pulse duration is used due to reduced opacity resulting from plasma expansion. Conversion efficiencies and ion energies are also measured as a function of laser pulse duration and laser power density. A maximum conversion efficiency of 0.4% within a 0.6% bandwidth was measured, and lower ion kinetic energy was observed. It is concluded that in order to reduce self-absorption effects and ion yields, and increase conversion efficiencies, a short pulse duration should be used.

Summary form only given. When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization predicted by S. C. Wilks et. al. [1]. The theory requires not only the plasma creation in time much shorter than an oscillation period of the electromagnetic wave but also plasma length much longer than a wavelength of it. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency up-conversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:S laser in ZnSe crystal.

Over the past 10 years Extreme Ultraviolet Lithography (EUVL) sources based on 13.5nm emission from Sn plasmas have been investigated. Sn plasmas produce strong resonant emission due to 4d-4f, 4p-4d and 4d-5p transitions around 13.5 nm, which overlap in adjacent ion stages to yield an intense unresolved transition array (UTA). The UTA consists of hundreds of thousands of near-degenerate resonance lines, therefore high intense emission can be obtained from Sn plasmas at around 13.5 nm. A high power EUV source is an important requirement for EUVL. Shorter wavelength sources can not only be used for EUVL, but also material science, and biological imaging near the water window. According to a previous work, it was shown that the UTA peak wavelength depends on atomic number and will therefore extend down towards the water window region1. This may be of great interest if a high power source can be achieved using UTA emission.

We demonstrate the extreme ultraviolet (EUV) and soft x-ray sources in the 2 to 7 nm spectral region related to the beyond EUV (BEUV) question at 6.x nm and the water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays (UTAs), extending below the carbon K edge (4.37 nm). An outline of a microscope design for single-shot live cell imaging is proposed based on high-Z plasma UTA source, coupled to multilayer mirror optics.

In this work we present results on the influence of laser pulse duration and irradiating power density on the conversion efficiency (CE) and the ion energy of gadolinium (Gd) laser produced plasmas. Three lasers were used with 10 ns, 150 ps and 140 fs pulse durations. By varying the lasers output energies, experiments could be carried out for a power density range of 10 11 - 10 15 W/cm 2 . A maximum CE of 0.4% was achieved within a 0.6% bandwidth in 2π steradians using the picosecond laser. A faraday cup was used to calculate ion yield and time of flight measurements of each laser. The picosecond laser also showed a reduction in the ion time of flight measurements compared with the nanosecond pulse.

We demonstrate a table-top strong band emission water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4-n = 4 transitions are overlaid with n = 4-n = 5 emission and shift to shorter wavelength with increasing atomic number. Under spectral analysis a guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a bismuth plasma source, coupled with multilayer mirror optics.

We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2%BW), which is 1.6 times larger than the 0.33% (0.6%BW) CE produced from a solid density target. Enhancement of the EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.

We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency up-conversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:S laser in ZnSe crystal.

Presented at a poster session at Advances in X-Ray/EUV Optics and Components VI, Monday 22 August 2011, San Diego, California, USA

When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization. The theory requires the plasma creation in time much shorter than an oscillation period of the electromagnetic wave. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultrashort laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave. We observed frequency up-conversion from 0.2 THz to 0.7–1.5 THz by the irradiance of the laser in ZnSe crystal. The up-conversion to the broad frequencies suggests that the flash ionization occurs in non-uniform plasmas produced in the deeper region of the crystal.

We have demonstrated that the terahertz radiation can be generate by the burst current produced by a laser created ionization front, which is induced an optical-field-induced ionization in air with a pulsed electric field. The peak frequency of the radiation spectrum depends on the rise time of the OFI. The central frequencies of the radiation are observed to be 0.2 and 0.04 THz at the pulse durations of 50 and 300 fs (FWHM), respectively. The power of the linearly polarized THz radiation is linearly increased with the square of the electrostatic field strength applied to the capacitors.

Unresolved transition array (UTA) is scalable to shorter wavelengths, and we demonstrate a table-top broadband emission "water window" soft x-ray source based on laser-produced plasmas. Resonance emission from multiply charged ions merges to produce intense UTAs in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). An outline of a microscope design for single-shot live cell imaging is proposed based on a bismuth (Bi) plasma UTA source, coupled to multilayer mirror optics.

Development of laser-produced plasma “water window” sources poses a major challenge in x-ray research and most effort has focused on line sources for use with zone plate optics. Here, a comparison of carbon and nitrogen line emission with that from both 3d – 4f and 4d – 4f unresolved transition arrays shows that, at power densities available from “table-top” solid-state lasers, 3d – 4f emission from zirconium plasmas is most intense, and calculations show that in an imaging system based on multilayer mirrors, for reflectance bandwidths >1% has superior performance than either line or broader-band sources. For bandwidths

Recent increases in reflectance at 6.x nm of La/B4C multilayer mirrors has led to the adoption of this wavelength for the development of future sources for lithography. The emission from laser produced plasmas of Gd and Tb emit strongly in this wavelength region. The transitions responsible and the optimum plasma conditions for optimum emission have been identified both theoretically and experimentally.

We have demonstrated frequency up-conversion in the terahertz (THz) region by flash ionization, which occurs due a terahertz wave interacting with a plasma created by an ultrashort laser pulse. The upshift frequency is controlled by the plasma density. We performed a proof-of-principle experiment with a plasma creation time scale much shorter than the period of the electromagnetic wave and a plasma length longer than its wavelength. We upshifted the frequency from 0.35 to 3.5 THz by irradiating a ZnSe crystal with a Ti:sapphire laser pulse.

Tungsten will be used as a wall material in ITER and therefore will be present as an intrinsic plasma impurity with the resulting emission having the potential to be used as a plasma diagnostic. We have recorded spectra of tungsten laser produced plasmas in the 1‐7 nm region using Nd:YAG lasers operating at a range of power densities. We have analysed these spectra, giving special attention to the unresolved transition arrays in the 3 nm region that appear at the highest laser power densities. We compare our results to those from previous work and also use new atomic structure calculations to identify a number of new features. (Some figures may appear in colour only in the online journal)

Recent work on multilayer mirror development for beyond extreme ultraviolet lithography indicates that their optimum reflectivity occurs at either 6.63 nm or 6.66 nm which may be too short a wavelength for Gd-based plasma sources. Calculations performed for Tb12+ to Tb28+ ions show that if the mirror reflectivity is fixed at one of these values, Tb may be a better source, though Gd is capable of providing greater intensity if the full reflection curve of the mirrors is exploited. Theoretical simulation shows that the Tb emission peaks close to 6.51 nm at an optimum electron temperature close to 120 eV.

We have demonstrated the effect of viewing angle on the extreme ultraviolet (EUV) emission spectra of gadolinium (Gd) near 6.7 nm. The spectra are shown to have a strong dependence on viewing angle when produced with a laser pulse duration of 10 ns, which may be attributed to absorption by low ion stages of Gd and an angular variation in the ion distribution. Absorption effects are less pronounced at a 150-ps pulse duration due to reduced opacity resulting from plasma expansion. Thus for evaluating source intensity it is necessary to allow for variation with both viewing angle and target orientation. Science Foundation Ireland

We have demonstrated an efficient extreme ultraviolet (EUV) source at 6.7 nm by irradiating Gd targets with 0.8 and 1.06 μm laser pulses of 140 fs to 10 ns duration. Maximum conversion efficiency of 0.4% was observed within a 0.6% bandwidth. A Faraday cup observed ion yield and time of flight signals for ions from plasmas generated by each laser. Ion kinetic energy was lower for shorter pulse durations, which yielded higher electron temperatures required for efficient EUV emission, due to higher laser intensity. Picosecond laser pulses were found to be the best suited to 6.7 nm EUV source generation. Science Foundation Ireland

We demonstrate a table-top broadband emission water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4−4 transitions are overlaid with n = 4−5 emission and shift to shorter wavelength with increasing atomic number. From spectral analysis, a guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a bismuth plasma source, coupled with multilayer mirror optics. Higher Education Authority

We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2% BW), which is 1.6 times larger than the 0.33% (0.6% BW) CE produced from a solid density target. Enhancement of the EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.

Self-compression of 20-mJ laser pulses has been demonstrated in a free-space argon-filled cell. A 130-fs pulse was compressed to less than 60 fs (full width at half maximum) with an output energy of 17 mj at an argon gas pressure of 25 kPa, corresponding to an input peak power of 3.65 times the self-focusing critical power through a single filament in a 10-mJ energy region.