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We design and experimentally demonstrate an optical switch based on the interference of plasmonic modes in whispering gallery mode (WGM) antennas. Simultaneous excitation of even and odd WGM modes, enabled by a small symmetry breaking via non-normal illumination, allows switching the plasmonic near field between opposite sides of the antenna, depending on the excitation wavelength used in a wavelength range of 60 nm centered around 790 nm. This proposed switching mechanism is experimentally demonstrated by combining photoemission electron microscopy (PEEM) with a tunable wavelength femtosecond laser source in the visible and infrared.

Quantum computing has the power to break current cryptographic systems, disrupting online banking, shopping, data storage and communications. However, quantum mechanics can also be used to make these systems stronger and more resilient. In this paper we describe the transmissibility of a quantum money scheme, which was proposed by Dmitry Gavinsky and implemented by the authors, and discuss some of its benefits and limitations.

After almost three decades of research, commercial sources for extreme ultraviolet lithography (EUVL) at 13.5 nm are now being manufactured and EUVL is expected to be introduced finally in 2018 or 2019. The source is based on laser produced plasmas (LPPs) from tin micro-droplets using double pulse irradiation in a pre-pulse/main pulse configuration. The currently accepted configuration uses a CO 2 laser main pulse to minimize self-absorption in the transitions responsible for the emission at 13.5 nm. The conversion efficiency (CE) of laser to in-band EUV energy in commercial systems is currently around 4% and values up to 6% have recently been observed. However the possibility of attaining significantly higher figures has been predicted by plasma modelling and research is still ongoing to increase the CE. The present paper describes the physical processes that occur in Sn LPPs and possible routes to attaining higher CE values.

Transformation of light carrying spin angular momentum (SAM) to optical field vortices carrying orbital angular momentum (OAM) has been of wide interest in recent years. The interactions between two optical fields, each carrying one of those degrees of freedom, and furthermore, the transfer of the resulting angular momentum product to matter are seldom discussed. Here, we measure the interaction between 3D light carrying axial SAM and 2D plasmon-polariton vortices carrying high-order transverse OAM. The interaction is mediated by two-photon absorption within a gold surface, imprinting the resulting angular-momentum mixing into matter by excitation of electrons that are photo-emitted into vacuum. Interestingly, the spatial distribution of the emitted electrons carries the signature of a subtraction of the spin from the orbit angular momenta. We show experimentally and theoretically that the absorptive nature of this interaction leads to both single and double photon-plasmon angular momentum mixing processes by one- and two- photon interactions. Our results demonstrate high order angular momenta light-matter interactions, provide a glimpse into specific electronic excitation routes, and may be applied in future electronic sources and coherent control.

Time-resolved imaging of the propagation and interference of isolated ultrashort surface plasmon polariton wave packets is demonstrated using two photon photoemission microscopy. The group- and phase velocity of individual wave packets are determined experimentally. Using two counter-propagating surface plasmon polariton pulses, the transient formation of a standing surface plasmon polariton wave is imaged in time and space. We demonstrate that using a normal incidence geometry in time-resolved photoemission microscopy provides great advantages for in-situ imaging of surface plasmon polaritons in arbitrary plasmonic structures. A simple 1D wave-simulation is used to confirm the experimental results.

It is well known that the lanthanide and actinide elements are formed by the filling of $4f$ and $5f$ subshells which occurs after the filling of $5d$ and $6d$ subshells, respectively, has begun. With increasing ionization one expects the energy levels to eventually regroup to their hydrogenic ordering, i.e., in terms of principal quantum number. In the lanthanides, the $4f$ electron binding energy overtakes that of $5p$ near the 6th or 7th ion stage and $5s$ near the 14th or 15th ion stage, leading to dramatic rearrangements of ground-state configurations. In this paper we report on the results of a study to explore the effects of increasing ionization on the ground-state configurations of actinide ions as a result of $5f$ and $6p$ or $6s$ level crossings. It is seen that the effects generally occur later and are more strongly influenced by spin-orbit splitting than in the lanthanides. The near degeneracies of $5f$ and $6l$ energies in these stages lead to configuration interaction (CI) amongst configurations with variable numbers of $5f$ and $6p$ electrons. The effects of CI on the level complexity are explored for ions along the Rn I sequence and are found to lead to the formation of ``compound states'' as predicted for the lanthanides. The extreme ultraviolet and soft x-ray spectra of medium and highly charged lanthanides are dominated by emission from unresolved transition arrays (UTAs) of the type $\mathrm{\ensuremath{\Delta}}n=0$, $4{p}^{6}4{d}^{N+1}\ensuremath{-}4{p}^{5}4{d}^{N+2}+4{p}^{6}4{d}^{N}4f$, which, in general, overlap in adjacent ion stages of a particular element. Here, the corresponding $\mathrm{\ensuremath{\Delta}}n=0$, $5{p}^{6}5{d}^{N+1}\ensuremath{-}5{p}^{5}5{d}^{N+2}+5{p}^{6}5{d}^{N}5f$ UTAs have been studied theoretically with the aid of Hartree-Fock with configuration interaction calculations. As well as predicting the wavelengths and spectral details of the anticipated features, the calculations show that the effects of configuration interaction are quite different for the two different families of $\mathrm{\ensuremath{\Delta}}n=0$ transitions and, once more, spin-orbit interactions play a major role.

Configuration interaction (CI) effects can greatly influence the way in which extreme ultraviolet (EUV) and soft X-ray (SXR) spectra of heavier ions are dominated by emission from unresolved transition arrays (UTAs), the most intense of which originate from Δn = 0, 4p64dN+1−4p54dN+2+4p64dN4f1 transitions. Changing the principle quantum number n, from 4 to 5, changes the origin of the UTA from Δn = 0, 4p64dN+1−4p54dN+2+4p64dN4f1 to Δn = 0, 5p65dN+1−5p55dN+2+5p65dN5f1 transitions. This causes unexpected and significant changes in the impact of configuration interaction from that observed in the heavily studied n = 4 – n = 4 arrays. In this study, the properties of n = 5 – n = 5 arrays have been investigated theoretically with the aid of Hartree-Fock with configuration interaction (HFCI) calculations. In addition to predicting the wavelengths and spectral details of the anticipated features, the calculations show that the effects of configuration interaction are quite different for the two different families of Δn = 0 transitions, a conclusion which is reinforced by comparison with experimental results.

Photoemission electron microscopy (PEEM) is an attractive and advantageous technique in the field of plasmonics. Whilst surface plasmons are excited at the metal dielectric interface by light, it is the near-field photoelectron distribution that is imaged, with

The design of noble-metal plasmonic devices and nanocircuitry requires a fundamental understanding and control of the interference of plasmonic modes. Here we report the first visualization of the propagation and interference of guided modes in a showcase plasmonic nanocircuit using normal-incidence nonlinear two-photon photoemission electron microscopy (PEEM). We demonstrate that in contrast to the commonly used grazing-incidence illumination scheme, normal-incidence PEEM provides a direct image of the structure’s near-field intensity distribution due to the absence of beating patterns and despite the transverse character of the plasmonic modes. Based on a simple heuristic numerical model for the photoemission yield, we are able to model all experimental findings if global plane wave illumination and coupling to multiple input/output ports, and the resulting interference effects are accounted for.

We experimentally reveal the sub-optical cycle dynamic formation, revolution and decay of plasmonic vortices using time-resolved two-photon photoemission electron microscopy with

The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light.

We reconstruct the optical response of nanostructures of increasing complexity by fitting interferometric time-resolved photoemission electron microscopy (PEEM) data from an ultrashort (21 fs) laser excitation source with different harmonic oscillator-based models. Due to its high spatial resolution of similar to 40 nm, PEEM is a true near-field imaging system and enables in normal incidence mode a mapping of plasmon polaritons and an intuitive interpretation of the plasmonic behaviour. Using an actively stabilized Mach-Zehnder interferometer, we record two-pulse correlation signals with 50 as time resolution that contain information about the temporal plasmon polariton evolution. Spectral amplitude and phase of excited plasmon polaritons are extracted from the recorded phase-resolved interferometric two-pulse correlation traces. We show that the optical response of a plasmon polariton generated at a gold nanoparticle can be reconstructed from the interferometric two-pulse correlation signal using a single harmonic oscillator model. In contrast, for a corrugated silver surface, a system with increased plasmonic complexity, in general an unambiguous reconstruction of the local optical response based on coupled and uncoupled harmonic oscillators, fails. Whereas for certain local responses different models can be discriminated, this is impossible for other positions. Multidimensional spectroscopy offers a possibility to overcome this limitation.

The continuation of Moore's law for semiconductor fabrication envisages the introduction of extreme ultraviolet lithography (EUVL) based on a source wavelength of 13.5 nm for high-volume manufacturing within the next few years. While exposure tools have already been developed and the feasibility of the technology well demonstrated, the key source requirement in terms of power output remains to be achieved. Currently, sources based on laser-produced plasmas from tin droplet targets appear to be the most promising and are being deployed in manufacturing tools. Progress in CO2 laser design aimed at increasing conversion efficiency to close to 5% should make possible the attainment of greater than 200 W of in-band optical power. Recently, research has commenced on the development of sources operating at a wavelength near 6.7 nm for beyond 13.5 nm lithography and gadolinium has been identified as the fuel of choice. The results of these experiments are described and show many similarities to the behavior of ti...

It is known that no one statistical test by itself can give conclusive evidence for the presence or absence of quantum chaos within a given system. For this reason a range of detailed tests, namely the nearest neighbour spacing distribution, covariance of adjacent spacings, spectral rigidity, correlation-hole method and inverse participation ratio have been applied to the quasienergies and quasieigenstates of a periodically kicked particle in a 1-D infinite potential well. The results are compared with the predictions of random matrix theory for various kick strengths in order to search for signatures of quantum chaos within this system.

The classical-quantum correspondence of a periodically kicked particle in a 1-D infinite potential well is investigated. Stroboscopic state space portraits are presented for various kick strengths and a classical diffusion study reveals anomalous behaviour and the presence of both regular islands of stability and accelerator modes. Quantum diffusion is subsequently studied and the quantum diffusion coefficient is found to mimic the classical diffusion coefficient by rescaling k . Wigner and Husimi distribution functions are derived and comparisons are made between the classical stroboscopic state space portraits and these quantum quasi-probability distribution functions.

Using the configuration interaction Hartree–Fock method, the even and odd Hamiltonian matrices of Sm IX are studied in the ls and jj coupling schemes. Sm IX is isoelectronic with Xe but because of 4f orbital contraction, and the near degeneracy of 4f and 5p binding energies, the lowest configuration is a mixture of states with differing numbers of 4f and 5p electrons. Diagonalization of the Hamiltonian results in a set of eigenvalues and eignvectors which are characterized using various statistical tests of random matrix theory.

4f collapse in rare earth ions produces an enormous level density due to the occurrence of near-degeneracies of 4f/5p and 4f/5s binding energies. As a result, there is a complete breakdown of the single-particle approximation and the level and line spectra satisfy many of the distributions associated with random matrix theory. Similar behaviour has been previously reported in spectra associated with wavefunction contraction effects or atoms containing open 4f subshells; here the effects of combining both phenomena are explored. In particular it is shown that the adjacent level separations satisfy a Wigner distribution, while transitions between them obey the Porter-Thomas distribution. Detailed Hartree-Fock calculations show that there is a strong correlation between line strength and transition energy. This correlation is pursued and parametrized for the particular case of 4f→5d, 5p→5d and 5s→5p transitions in Sm IX within the unresolved transition array model.

Extreme ultraviolet radiation emitted from highly-charged dysprosium ions was measured at the National Institute of Standards and Technology. The ions were created, trapped, and excited in an electron beam ion trap (EBIT), and the spectra were recorded with a flat-field grazing-incidence spectrometer in the wavelength range 3 nm to 17 nm. Tuning the electron beam energies between 1.2 keV and 2.0 keV resulted in a selection of Rb-like Dy29+ to Ni-like Dy38+ ions. Identification of strong n = 4-n = 4 transitions was achieved by collisional-radiative modeling of the EBIT plasma. A total of 64 spectral lines were recorded, including 54 new identifications.

A large variety of software has been developed for numerical simulations of plasma radiation emission and transport and the design of optics. Many of these codes are not specifically for short wavelengths but they can be helpful in the design of X-ray and extreme ultraviolet (XUV) sources. The astrophysics community has written codes to study the spectra and evolution of stars and gas clouds while civilian and military applicants of atomic processes were aware very early of the need for atomic, plasma and hydrodynamic software to simulate nuclear fission and fusion and their effects. Of more relevance to low cost EUV sources, the large synchrotron community has also generated efficient tools for the simulation of radiation from electrons and the design of short wavelength optics. The present intention is not to present all the available software, since resources such as Computer Physics Communications and Plasma Gate give much more comprehensive, if never complete, reviews. Instead, some of the more popular programs will be introduced, as they are proven and tested assistance is available from experienced users. The software may be downloaded from the web sites listed in the references; registration is sometimes required.

Extreme ultraviolet and soft X-ray sources have been identified, using the FAC relativistic code, as emission arising from 4d–4f and 4p–4d transitions in palladium-like to rubidium-like ions of lanthanum through actinium. The spectra of these strong emitters are seen to separate and move to shorter wavelength as the nuclear charge increases. The emission was comparable from each source and was characterized using the unresolved transition array model. In future applications such as microscopy, spectroscopy and lithography, these radiation sources may be generated by employing the laser-produced plasma technique.

Measurements of extreme ultraviolet radiation from highly charged gadolinium ions were made at the National Institute of Standards and Technology. The ions were produced and confined in an electron-beam ion trap (EBIT), and the spectra were recorded with a flat-field grazing-incidence spectrometer in the wavelength range 3--17 nm. Ionization stages from Rb-like to Cu-like gadolinium were selected by tuning the electron-beam energies between 0.97 and 1.7 keV. Strong intrashell $n=4$--$n=4$ transitions were identified by performing detailed collisional-radiative modeling of the EBIT plasma. A total of 73 spectral features were recorded, including 59 new identifications.

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 summarize detailed calculations of dielectronic recombination rate coefficients for Sn4+ - Sn13+ using three approaches of differing degrees of complexity. These are: configuration-mixed Breit-Pauli using the AUTOSTRUCTURE code, bundled-nl using the Burgess-Bethe General Program (BBGP) and configuration-average (CA) using the DRACULA code. We describe the implications for the finite-density modelling of tin plasma sources for microlithography and tin markers for magnetic fusion plasmas as well as for heavy species in general such as Tungsten ions which are of great importance for magnetic fusion plasmas.

We have measured extreme ultraviolet (EUV) spectra in the 20 nm region from tungsten ions in plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science. The spectra are recorded by a grazing incidence spectrometer following an injection of a tungsten pellet. A quasicontinuum feature has been observed in the 20 nm range under low temperature conditions below 1.0 keV, where large contributions of charged states lower than W27+ are expected. In order to give an insight into this spectral feature, we have carried out atomic structure calculations for W7+-W27+ using Cowan's code. The calculated distributions of wavelengths and gA values suggest that the emission largely arises from n = 5-5 transitions in stages lower than W27+.

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission. We also have investigated the dependence of the spectral behavior and conversion efficiencies of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6 × 10 12 W/cm 2 at an operating wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.

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

Theoretical extreme ultraviolet emission spectra of gadolinium and terbium ions calculated with the Cowan suite of codes and the flexible atomic code (FAC) relativistic code are presented. 4d–4f and 4p–4d transitions give rise to unresolved transition arrays in a range of ions. The effects of configuration interaction are investigated for transitions between singly excited configurations. Optimization of emission at 6.775 nm and 6.515 nm is achieved for Gd and Tb ions, respectively, by consideration of plasma effects. The resulting synthetic spectra are compared with experimental spectra recorded using the laser produced plasma technique. Science Foundation Ireland

Theoretical ground-state configurations of lanthanide ions calculated with the Cowan suite of codes are presented. Theoretical 4d - 4f and 4p - 4d spectra of Pd-like to Rb-like lanthanide ions calculated using the FAC relativistic code are also shown. The effects of configuration interaction are investigated and results compare favorably with experiments where, for increasing nuclear charge, strong emission peaks are observed to move towards shorter wavelength. The application of these strong emitters as extreme ultraviolet radiation sources, a topic of emerging interest, is discussed. Science Foundation Ireland

The primary requirement for the development of tools for extreme ultraviolet lithography (EUVL) has been the identification and optimization of suitable sources. These sources must be capable of producing hundreds of watts of extreme ultraviolet (EUV) radiation within a wavelength bandwidth of 2% centred on 13.5 nm, based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ~70% at this wavelength. Since, with the exception of large scale facilities, such as free electron lasers, such radiation is only emitted from plasmas containing moderately to highly charged ions, the source development prompted a large volume of studies of laser produced and discharge plasmas in order to identify which ions were the strongest emitters at this wavelength and the plasma conditions under which their emission was optimized. It quickly emerged that transitions of the type 4p64dn − 4p54dn+1 + 4dn−14f in the spectra of Sn IX to SnXIV were the best candidates and work is still ongoing to establish the plasma conditions under which their emission at 13.5 nm is maximized. In addition, development of other sources at 6.X nm, where X ~ 0.7, has been identified as the wavelength of choice for so-called Beyond EUVL (BEUVL), based on the availability of La/B based MLMs, with theoretical reflectance approaching 80% at this wavelength. Laser produced plasmas of Gd and Tb have been identified as potential source elements, as n = 4 − n = 4 transitions in their ions emit strongly near this wavelength. However to date, the highest conversion efficiency (CE) obtained, for laser to BEUV energy emitted within the 0.6% wavelength bandwidth of the available mirrors is only 0.8%, compared with values of 5% for the 2% bandwidth relevant for the Mo/Si mirrors at 13.5 nm. This suggests a need to identify other potential sources or the selection of other wavelengths for BEUVL. This review deals with the atomic physics of the highly-charged ions relevant to EUV emission at these wavelengths. It considers the developments that have contributed to the realization of the 5% CE at 13.5 nm which underpins the production of high-volume lithography tools, and those that will be required to realize BEUV lithography.

Lithography tools are being built and shipped to semiconductor manufacturers for high volume manufacturing using extreme ultraviolet lithography (EUVL) at a wavelength of 13.5 nm. This wavelength is based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ~70% at this wavelength. Moreover, the primary lithography tool manufacturer, ASML, has identified 6.x nm, where x~7, as the wavelength of choice for so-called Beyond EUVL, based on the availability of La/B4C MLMs, with theoretical reflectance approaching 80% at this wavelength. The optimum sources have been identified as laser produced plasmas of Gd and Tb, as n = 4–n = 4 transitions in their ions emit strongly near this wavelength. However, to date, the highest conversion efficiency obtained, for laser to EUV energy emitted within the 0.6% wavelength bandwidth of the mirror is only 0.8%, pointing to the need to identify other potential sources or consider the selection of other wavelengths. At the same time, sources for other applications are being developed. Conventional sources for soft x-ray microscopy use H-like line emission from liquid nitrogen or carbon containing liquid jets which can be focused using zone plates. Recently the possibility of using MLMs with n = 4−n = 4 emission from a highly charged Bi plasma was proposed and subsequently the possibility of using Δn = 1 transitions in 3rd row transition elements was identified. All of these studies seek to identify spectral features that coincide with the reflectance characteristics of available MLMs, determine the conditions under which they are optimized and establish the maximum conversion efficiencies obtainable. Thus, there is a need for systematic studies of laser produced plasmas of a wide range of elements as some of the challenges are similar for all of these sources and some recent results will be presented.

Current research on sources for extreme ultraviolet lithography (EUVL) has converged on the use of discharge or laser produced plasmas containing xenon, tin, or lithium with tin showing by far the most promise. Because of their density, radiation transport from these plasmas is a major issue and accurate photoabsorption cross sections are required for the development of the plasma models needed to optimize conditions for source operation. The relative EUV photoionization cross sections of Sn I through Sn IV have been measured and from a comparison with the results of many body calculations, the cross section has been estimated to be close to 11 Mb in each species at 13.5 nm (91.8 eV), the wavelength of choice for EUVL.

We report here some observations and preliminary findings from a study focussed on the vacuum-UV (λ, 40-60 nm) radiation emitted during the interaction of 150 ps laser pulses (100-400 mJ) with copper pre-plasmas formed by an electro-optically synchronised (0.1 - 0.8 J, 8 ns) long pulse laser. We have observed significant gains in VUV flux that scale with inter-laser delay. We also report preliminary observations on total X-ray emission from the interaction of a superintense 80 fs, 200 mJ laser pulse at the UK ASTRA laser facility with a similar pre-plasma at irradiances approaching 1019 W/cm2

Extreme ultraviolet (EUV) spectra from gold laser produced plasmas were recorded in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 150 ps and 10 ns, respectively, operating at a range of power densities. The maximum focused peak power density was 9.5 × 1013 W cm−2 for the former and 5.3 × 1012 W cm−2 for the latter. Two intense quasicontinuous intensity bands resulting from n = 4–n = 4 and n = 4–n = 5 unresolved transition arrays dominate the 4–5.5 and 1.5–3.6 nm regions of both spectra. Comparison with atomic structure calculations performed with the Cowan suite of atomic structure codes as well as consideration of previous experimental and theoretical results aided identification of the most prominent features in the spectra. For the ns spectrum, the highest ion stage that could be identified from the n = 4–n = 5 arrays was Au28+ while for the ps plasma the presence of significantly higher stages was deduced and lines due to 4d104f–4d94f2 transitions in Ag-like Au32+ give rise to the strongest observed features within the n = 4–n = 4 array while in the n = 4–n = 5 array it was possible to identify a number of previously unidentified spectral features as resulting from 4f–5g transitions in the spectra of Au XX to Au XXXIII.

Measurements of extreme ultraviolet radiation from gadolinium, dysprosium and tungsten ions with an open n = 4 shell were performed at the National Institute of Standards and Technology. The ions were produced and confined in an electron beam ion trap, and the spectra were recorded with a flat-field grazing-incidence spectrometer in the wavelength range 3.5–17.5 nm. These data are useful for the development of future lithography sources and for diagnostics of hot plasmas in fusion devices.

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.

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)

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 measured extreme ultraviolet (EUV) spectra from tungsten ions around 20 nm region in plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The spectra after injecting a tungsten pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer. Quasicontinuum spectral feature arising from unresolved transition array (UTA) was observed around 20 nm region in plasmas with temperatures below 1.0 keV. This structure is reasonably considered to be the same one as those observed in another tokamak device or laser produced plasmas under low temperature conditions. Atomic structure calculations have been performed for tungsten ions with open 5p, 5s and 4f subshells (W7+–W27+) to interpret this commonly observed feature around 20 nm. Wavelengths and gA values for these transitions were calculated, and their mean wavelengths and extent were compared with the observations, which suggests that the emission largely arises from n = 5–5 transitions in stages lower than W27+. Science Foundation Ireland

Potential extreme ultraviolet and soft x-ray radiation sources have been identified, using the flexible atomic code (FAC), as emission peaks arising from the 4d–4f and 4p–4d transitions in Pd-like to Rb-like ions of hafnium through actinium. The effects of configuration interaction are investigated and for increasing nuclear charge, these strong emitters are seen to separate and move to shorter wavelengths. Each source is characterized using the unresolved transition array model. They are proposed to complement the currently used nitrogen and argon sources in the 'water window', and as possible successors to tin in next-generation lithography. Science Foundation Ireland

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm (in-band: 6.7 nm +/-1%) suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission.

Near-field imaging and spectroscopy of ring resonators is performed with photoemission electron microscopy and a tuneable femtosecond laser source. Phase- and time-resolved near-field imaging is achieved with an actively stabilized interferometer.

Employing unique metasurface-based plasmonic generators and an ultrafast probing technique, we experimentally demonstrate plasmonic vortex compounds carrying arbitrary orbital angular momentum. These can be used for functional near-field shaping and sculpting of optical potentials.

Dynamics of plasmonic vortices are observed using time-resolved two-photon PEEM (40 nanometers, 100 attoseconds resolution). The measured orbital angular momentum surprisingly differs from theory due to the observation from the rotating frame of circular polarization.