Short-wavelength Light Sources Research Articles

Analysis of the extreme ultraviolet broadband spectral characteristics and spatiotemporal evolution of laser-produced germanium plasmas

This paper presents comprehensive spectral measurements and a theoretical analysis of laser-produced germanium (Ge) plasma within the 7.5–17 nm wavelength range, with the measurements being performed using spatiotemporally-resolved emission spectroscopy. We obtained and analyzed the extreme ultraviolet (EUV) spectra and their variations meticulously at various positions adjacent to the target surface. The 3p and 3d excitations of Ge ions spanning from Ge4+ to Ge13+ were calculated using the Cowan configuration interaction codes, with particular emphasis being placed on analysis of the spectral line distributions in the 3d-np, 3d-nf, and 3p-3d, 4s, 4d transition arrays. This analysis indicated the contributions made by these transition arrays to the broadband spectral features that were observed experimentally. By using a steady-state collisional radiation model along with a normalized Boltzmann distribution assumption, we clarified the predominant contributions of the 3d-4p and 3p-3d transitions from the Ge7+ to Ge9+ ions in close proximity to the target surface, along with those from the Ge5+ and Ge6+ ions at greater distances. Furthermore, by combining the results above with a plasma dynamics evolution analysis, we gained insights into how the plasma's transient and nonuniform properties influence the analysis of complex EUV band spectral profiles and examine their implications for spectral diagnostics. These results will be useful for plasma spectral diagnosis and for application to development of pulsed short-wavelength light sources.

Analysis of extreme ultraviolet radiation and hydrodynamics simulation at the core of laser-produced nickel plasmas

The non-uniformity and transient nature of laser-produced plasma are critical factors that affect the analysis of the extreme ultraviolet spectra of highly charged ions and the diagnosis of plasma states. This paper systematically investigates the characteristics of extreme ultraviolet radiation and the hydrodynamic evolution of laser-produced nickel plasmas from two perspectives: high-spatio-temporal-resolution extreme-ultraviolet spectroscopic measurement and radiation hydrodynamics simulation. The consistency between the four-band experimental spectra and their theoretically simulated spectra confirms the accuracy of the atomic structure parameters and plasma state parameters. We also analyze the significant contribution of the 3d-4f double-excited state radiation to the spectral profile and discuss the influence of the self-absorption caused by plasma opacity on the characteristics of extreme ultraviolet radiation. The findings are crucial for accurately understanding the characteristics of extreme ultraviolet radiation, the hydrodynamic evolution, and the application of medium- and high-Z laser-produced plasma as a pulsed short-wavelength light source.

Azimuthally extreme-ultraviolet focal splitter by modified spiral photon sieves

Extreme Ultraviolet (EUV) radiation is a short-wavelength light source that has important applications in many fields, such as optical communication, particle manipulation, and ultrahigh resolution imaging. However, the highly absorptive nature of EUV light makes it challenging to design suitable focusing optics, such as focal splitters, to properly manipulate the energetic light. Here, we propose modified spiral photon sieves to transform EUV laser light into azimuthally splitting focusing. A genetic algorithm was used to design and optimize the azimuthally focal splitters. A capillary discharge EUV laser at 46.9 nm was used to verify the effectiveness of our proposed method, and PMMA targets were used to record the focused laser spot. The profile of the recorded patterns measured by atomic force microscopy shows that the focal spots in the experiment are diffraction-limited and agreed with the theoretical analysis. The proposed technique provides a new way for manipulating EUV light and further extends the applications ranging from EUV to soft x rays.

Design and manufacturing of aspherical GaAs-SILs for a versatile optical semiconductor failure analysis system

In this study, we have developed an aspherical solid immersion lens (SIL) made of gallium arsenide (GaAs) for semiconductor failure analysis. A SIL is an optical component to dramatically increase the numerical aperture and the spatial resolution of a microscopy system by attaching it to the backside of the device-under-test. To further enhance the spatial resolution of the optical microscopy system, a shorter wavelength light source has also been introduced. In the past, we have developed a spherical SIL made of silicon (Si), which is the same material as the device under test. However, Si is not transparent for wavelengths shorter than around 1100 nm. To enable the use of a SIL at shorter wavelengths, we chose GaAs as SIL material, which is transparent at shorter wavelengths than Si and has a refractive index very close to that of Si. However, a spherical GaAs SIL features a more pronounced spherical aberration due to the difference in refractive index between the device-under-test (Si) and that of the SIL (GaAs) deteriorating the optical resolution of the microscopy system. We have therefore designed an aspherical GaAs-SIL which makes it possible to correct spherical aberration. We applied diamond tooling as a technique for manufacturing the SIL shape with a required surface irregularity below 100 nm peak-to-valley, resulting in a diffraction limited performance of the optical system for semiconductor failure analysis with a performance at around 0.18 μm cutoff.

Self-probed ptychography from semiconductor high-harmonic generation.

We demonstrate a method to image an object using a self-probing approach based on semiconductor high-harmonic generation. On the one hand, ptychography enables high-resolution imaging from the coherent light diffracted by an object. On the other hand, high-harmonic generation from crystals is emerging as a new source of extreme-ultraviolet ultrafast coherent light. We combine these two techniques by performing ptychography measurements with nanopatterned crystals serving as the object as well as the generation medium of the harmonics. We demonstrate that this strong field in situ approach can provide structural information about an object. With the future developments of crystal high harmonics as a compact short-wavelength light source, our demonstration can be an innovative approach for nanoscale imaging of photonic and electronic devices in research and industry.

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d3/2 and 4d5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Dips in high-order harmonics spectra from a subcycle-driven two-level system reflected in the negativity structure of the time-frequency Wigner function

We investigate high-order harmonics spectra radiated from a two-level model system driven by strong, ultrabroadband, half- and single-cycle pulses, which are shorter than the inverse of the transition frequency. In this driving regime, the plateau in frequency spectra typical for radiation from strongly driven systems, has noticeable modulation in amplitude due to interference between waves of same frequency and emitted at different time instants. Specifically, there are characteristic ‘dip’ structures at a set of frequencies in the radiation spectra, where the corresponding amplitudes are suppressed by several orders of magnitude. Understanding these structures is required for applications such as the generation of attosecond pulse, where the number of composing modes and their relative phases are important. Therefore, we demonstrate a systematic way to find frequencies at which the dips are formed. To further illustrate the interference mechanism, we extract the phase information with the help of time-frequency distribution functions, namely the Husimi and Wigner functions. In particular, we found that the negativity structure of the Wigner function corresponds to each dip frequency and that the information regarding the type of interference is encoded in the pattern of the negative region of the Wigner function. Since such time-frequency Wigner function can actually be measured, we envisage utilizing its negativity structure to extract the phase information between radiation components emitted at time points within a subcycle timescale. This should provide an efficient tool for understanding and designing photonic applications, including short-wavelength coherent light sources.

Neighboring Atom Collisions in Solid-State High Harmonic Generation

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

An open-source, end-to-end workflow for multidimensional photoemission spectroscopy

Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at 3rd and 4rd generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.

Attosecond light science and its application for probing quantum materials

In this paper, we review the development and application of coherent short wavelength light sources implemented using the high harmonic generation (HHG) process. The physics underlying HHG brought quantum physics into the domain of attosecond time-scales for the first time. The observation and manipulation of electron dynamics on such short time-scales—a capability not conceived—of just a few decades ago—is becoming both more-and-more sophisticated and useful as a route to achieve exquisite control over short wavelength light. New experimental techniques are enabling HHG light sources to provide new insights into fundamental quantum interactions in materials, making it possible for the first time to capture the fastest charge, spin, photon and phonon interactions and to achieve diffraction-limited imaging at short wavelengths.

Molecular pathological mechanism of blue ray induces murine photoreceptor cell 661W apoptosis

Modern life is full of artificial light in day and night. Smart phones, laptops, televisions, and high definition monitors provide unnatural human light source into human eye and cause ocular defects in modern human being. Understanding the molecular mechanism of the interplay between artificial light and ocular cells may help researcher society understanding the methods that preventing artificial light‐induced visual impairment. Here, we reported a short‐term exposure at short wavelength light source from blue and green light‐emitting diode (LED) could cause the murine photoreceptor cells 661W cells death, but not in red LED. However, the molecular mechanism remaining unclear. Using the molecular probe of caspase 3 reporter, we found the blue and green LED caused caspase 3 activation even as short as 10 minutes exposure and subsequently cell death at 16 hours later. We further confirmed 661W cells apoptosis by cleavage form caspase 3 and PARP formation. Moreover, we found the decreasing of mitochondria membrane potential and increasing of cellular relative‐oxygen species in 661W cells both indicate early cell apoptosis under short‐term exposure. Up to our knowledge, there was no transcriptome analysis which revealed immediately early gene expression in light‐induced cell death. Thus, we analyzed the 661W transcriptome at 2 hours after 10 minutes’ short wavelength light exposure and validated the candidate genes expression by RT‐Q‐PCR. Using the ingenuity pathway analysis, we found valuable up‐stream regulators, pathways, and potential mechanisms in short wavelength induced photoreceptor cell death and may provide novel therapy niches. In conclusion, our preliminary results prove that short wavelength light exposure can cause the photoreceptor cells death and the potential molecular clues. Further pathohistological analysis of key protein expression in the animal models may help us to prove the mechanism inside the eye damage from short wavelength light in future.Support or Funding InformationThis study was support by Shu Zen College of Medicine and Management with grant number (SZL10800012) and Academia Sinica

最先端短波長光源（DUV, EUV）と半導体製造への応用

最先端短波長光源（DUV, EUV）と半導体製造への応用

Orbital angular momentum from semiconductor high-order harmonics.

Light beams carrying orbital angular momentum (OAM) have led to stunning applications in various fields from quantum information to microscopy. We examine OAM from the recently observed high-harmonic generation (HHG) in semiconductor crystals. HHG from solids could be a valuable approach for integrated high-flux short-wavelength coherent light sources. First, we verify the transfer and conservation of the OAM in the strong-field regime of interaction from the generation laser to the harmonics. Secondly, we create OAM beams by etching a spiral zone structure directly at the surface of a zinc oxide crystal. Such diffractive optics act on the generated harmonics and produces focused optical vortices with sub-micrometric size.

High-harmonic generation from solids

High-harmonic generation in atomic gases has been studied for decades, and has formed the basis of attosecond science. Observation of high-order harmonics from bulk crystals was, however, reported much more recently, in 2010. This Review surveys the subsequent efforts aimed at understanding the microscopic mechanism of solid-state harmonics in terms of what it can tell us about the electronic structure of the source materials, how it can be used to probe driven ultrafast dynamics and its prospects for novel, compact short-wavelength light sources. Although most of this work has focused on bulk materials as the source, recent experiments have investigated high-harmonic generation from engineered structures, which could form flexible platforms for attosecond photonics. This Review surveys recent efforts at understanding and characterizing generation of high harmonics from solid-state materials.

Coherent Diffractive Imaging Using Table-top High Harmonic Generation

Coherent diffractive imaging (CDI) is a lensless microscopy technique in which the structure of a specimen of interest is probed using a coherent short-wavelength light source. CDI has been widely used in nanotechnology and structural biology to capture high resolution images of non-crystalline objects. In this paper, we review the theoretical and experimental aspects of coherent diffractive imaging using a focused narrow-bandwidth table-top high harmonic source. The review begins with an outline of generation and characterization of the high harmonic source. Theoretical description of coherent diffractive imaging technique is then summarized. The review concludes with our recent results in imaging using a single harmonic beam selected by employing XUV focusing mirrors. These achievements provide a promising technique for the non-crystallographic structural determination of membrane proteins using a table-top extreme ultraviolet source.

Photolithography-Based Nanopatterning Using Re-entrant Photoresist Profile.

Photolithography based on optical mask is widely used in academic research laboratories due to its low cost, simple mechanism, and ability to pattern in micron-sized features on a wafer-scale area. Because the resolution is bound by diffraction limits of the light source, nanoscale patterning using photolithography requires short-wavelength light source combined with sophisticated optical elements, adding complexity and cost. In this paper, a novel method of subwavelength patterning process using conventional i-line mercury lamp is introduced, without the use of such advanced optical tools. The method utilizes the re-entrant geometry of image reversal photoresist produced from the developing process, where a secondary mask is generated by isotropically depositing a metal layer to cover the re-entrant profile of the photoresist. Removing the photoresist by applying ultrasonic vibrations in acetone bath uniformly cracks the metal layer at the sidewalls of the re-entrant profile, exposing the substrate with a reduced feature size. The width of the initial mask pattern can be reduced by 400 nm in a controlled manner, regardless of the original width choice. As a result, the method is shown to achieve sub-100 nm scale linear patterns compatible for both subsequent deposition process and dry-etching process. Our approach is applicable to various shapes of the patterns and can be used in electronic device fabrication requiring nanoscale lithography patterning, such as the gate fabrication of AlGaN/GaN high-electron-mobility transistor.

Enhancing the spontaneous emission rate by modulating carrier distribution in GaN-based surface plasmon light-emitting diodes.

Based on the nanorod structure, we have fabricated GaN-based surface plasmon light-emitting diodes with Ag nanoparticles deposited laterally proximity to the multiple quantum wells (MQWs) region, which allows us to investigate the quantum well - surface plasmon (QW-SP) coupling effect. Our results show that the QW-SP coupling effect increases significantly when the SP resonant wavelength of Ag nanoparticles is close to the QW emission wavelength, especially by using a shorter wavelength light source, which will further enhance the spontaneous emission rate. Combined with the simulations, we find that the enhancement is due to the decreased excitation light penetration depth into the active region, which can modulate the carrier distribution and increase the proportion of SP-coupled carriers in the MQWs of LEDs. To increase the spontaneous emission rate for the electrical QW-SP coupled LEDs, we can use single QW or MQW structure to confine the carriers in the topmost QW, which will effectively increase the proportion of SP-coupled carriers. Our findings pave a way to design the ultrafast LED light source for the application of visible light communication (VLC).

Introducing Optical Cascode GaN HEMT

A novel optically activated cascode gallium nitride (GaN) high-electron-mobility transistor (HEMT) is introduced and evaluated in this paper. Furthermore, optical triggering of GaN HEMT structures by cost-effective and high-power long-wavelength light sources is proposed for the first time. In electrical domain, GaN HEMTs suffer from being normally on devices, making it more complicated to design and implement gate drivers for these devices. In optical domain, GaN devices suffer from being triggered by low-power short-wavelength light sources, making them significantly more expensive and practically unprofitable. Therefore, a new optical cascode (OC) GaN HEMT is proposed in this paper to solve the problems of both electrical and optical domains. The structure of this OC GaN HEMT includes a high-power normally on (depletion mode) GaN HEMT in combination with a low-power optical switch in cascode configuration. This OC not only realizes an overall normally off (enhancementmode) structure, but also offers activationwith cost-effective long-wavelength optical sources. It also offers a single-biased configurationwithout the need for complicated electrical gate drivers. Unlike conventional electrically activated cascode GaN HEMT structures, which may be prone to electromagnetic interference noise due to the high-frequency switching operation, the optical link in this proposed scheme is immune to external noise. The results showa high-frequency switching capability of 1MHz for theOCGaN HEMT under a bias voltage of 600 V and a current of 10 A. Less voltage and current ringing are observed in the OC compared with the electrical alternative due to the elimination of gate controlling inductance introduced in the electrical cascode configuration.

Towards the next generation of short wavelength light sources

After the laser revolution, a major breakthrough comes from the short wavelength free electron lasers that offer tunable mJ power short pulse light sources for users. Present features and new perspectives will be discussed.

Panorama of new generation of accelerator based short wavelength coherent light sources

The newly developed intense short wavelength light sources (from Extreme Ultra-Violet (EUV) to X-rays) have open the path to the exploration of matter for revealing structures and electronic processes and for following their evolution in time. After drawing the panorama of existing accelerator based short wavelength light sources, the new trends of evolution of short wavelengths FEL are described, with some illustrations with the example of the LUNEX5 (free electron Laser a New accelerator for the Exploitation of X-ray radiation of 5th generation) demonstrator project of advanced compact Free Electron Laser.