A high-efficiency programmable modulator for extreme ultraviolet light with nanometre feature size based on an electronic phase transition

The manipulation of light in the extreme ultraviolet (EUV) and X-ray spectral ranges remains a significant challenge, hindering real-world applications that require complex beam patterns. While optical manipulation techniques are well-established for infrared, visible, and ultraviolet light, these methods face considerable difficulties when applied to higher-energy photons. Current EUV and X-ray diffraction gratings, produced through methods like lithographic etching of thin-film transmission gratings or the design of multilayer blazed gratings, suffer from limitations such as narrow working spectral ranges and complex fabrication processes requiring precise control over etching. Temporal modulation of X-ray beams often relies on mechanically moving optical elements, which results in limited speed, precision, and potential geometrical constraints. Recent advancements such as miniature microelectromechanical systems (MEMS) offer improved modulation frequencies (e.g., 350 MHz), but these face challenges in matching light source repetition rates and may limit photon throughput due to small beam footprint requirements. Phase-change materials, like Ge₂Sb₂Te₄, offer a promising alternative for reflective spatial light modulators (SLMs) but are limited by their relatively large feature size due to heat propagation effects inherent in their thermal switching mechanism. This research addresses these limitations by presenting a novel approach to EUV/soft X-ray modulation that combines ultrafast switching with sub-90-nm feature size.

Existing methods for EUV and X-ray manipulation have inherent limitations. Lithographic etching, while capable of producing diffraction gratings, results in narrow working spectral ranges and complex manufacturing processes. Multilayer blazed gratings offer an alternative, but these too are technologically challenging to produce with high precision. Temporal modulation using mechanical movement of optical elements is slow, imprecise, and often limited by the physical constraints of the optical setup. MEMS-based modulators, while faster, have to be carefully matched to the light source repetition rate and often have limitations in total photon throughput. Reflective SLMs using phase-change materials like Ge₂Sb₂Te₄ demonstrate high stability and fast writing speeds but suffer from a large feature size due to thermal effects. The authors highlight the need for an approach that combines ultrafast switching with a sub-90 nm feature size, addressing the limitations of current technologies.

The researchers utilized a transient grating experimental configuration to demonstrate their novel modulator. The setup involved using two crossed coherent EUV beams (write beams) to imprint a metastable periodic structure onto a 1T-TaS₂ sample. The 1T-TaS₂ material was chosen due to its controllable lifetime of electronic states. This imprinted structure acts as a diffraction grating, deflecting short-wavelength photons. Experiments were conducted at both room temperature (nearly commensurate CDW state) and 100 K (commensurate CDW state). At room temperature, a typical transient grating response was observed on the picosecond timescale, driven by a near-resonant pump (31.1 eV) and probed with non-resonant photons (93.3 eV). The signal exhibited an exponential background with superimposed oscillations attributed to periodic lattice temperature modulation. At 100 K, after the initial excitation by crossed pump beams, a strong scattering signal persisted even after the pump beams were blocked, indicating a persistent, imprinted periodic structure. This structure remained stable for at least 750 seconds before being erased by a single EUV beam (a heating pulse). The process was repeatable, indicating no surface degradation. The long lifetime is attributed to a photoinduced phase transition to a metastable hidden electronic order. The diffraction efficiency of the imprinted structure was significantly higher at low temperature than at room temperature. The fluence in the bright fringes of the crossed pump beams was much higher than in previous optical experiments, resulting in a high energy density absorbed by the sample, but the density of absorbed photons was comparable to that of the previously published results. The erasing of the structure by a single pump beam is explained through two possible scenarios: exceeding the switching threshold or direct current heating. Experimental data supports the latter, where the longer erasing time constant (48 ± 3 s) suggests that the direct current heating is the dominant process.

The study demonstrates a novel EUV and soft X-ray spatial light modulator where the periodic structure can be written and erased on demand using short laser pulses. The high diffraction efficiency stems from the significant out-of-plane lattice contraction associated with the photoinduced phase transition in the 1T-TaS2 material. The stability of this imprinted structure directly correlates with the long lifetime of the photoinduced hidden charge-ordered states. The authors achieved a diffraction efficiency at least an order of magnitude higher than the transient grating at 300K. Modeling shows that the efficiency is influenced by the amplitude of the grating and the wavelength of the light. The diffraction efficiency is modeled using η = R|J₁(α)|², where R is the reflectivity, α is the peak-to-peak phase excursion, and J₁ is the first-order Bessel function. By changing the experimental geometry (incident angle), it is theoretically possible to enhance the efficiency by several orders of magnitude. The modelling indicates that by tuning experimental parameters, it should be possible to reach efficiencies exceeding 1%, approaching those of state-of-the-art multilayer etched gratings. The ability to control the structure suggests that the device can also be used as an intensity and spectral monitor, providing flexibility in EUV and soft X-ray optical layouts. The observed effect hinges on the material's ability to exhibit a long-lived, metastable electronic state after being excited by an EUV beam. The use of bulk exfoliated crystals, while effective in the proof-of-concept, presents a scalability challenge. However, the authors suggest that advancements in chemical vapor deposition and molecular beam epitaxy may pave the way for industrial-scale production of high-quality thin-film devices. The experimental results suggest that the feature size could potentially be further reduced.

The results directly address the need for efficient and programmable modulators for EUV and soft X-ray light. The demonstrated modulator overcomes the limitations of existing technologies by combining high diffraction efficiency with fast switching speed and a sub-90 nm feature size. The unexpectedly high efficiency is attributed to the large out-of-plane lattice contraction associated with the photoinduced phase transition in 1T-TaS2. The stability and programmability of the modulator are crucial advantages for numerous applications. The ability to tune the device's parameters and the potential for scalability offer significant implications for various fields, such as EUV lithography, X-ray microscopy, and spectroscopy. The proposed hybrid electro-optical device, combining laser imprinting and current injection, warrants further investigation for enhancing the control and versatility of the modulator.

This research successfully demonstrates a programmable EUV and soft X-ray spatial light modulator based on photoinduced phase transitions in 1T-TaS2. The high diffraction efficiency, ultrafast switching speed, and sub-90 nm feature size represent a significant advancement in EUV/soft X-ray optics. Future research should focus on optimizing the device design, exploring different materials and wavelengths, and developing scalable manufacturing techniques for large-area devices. The potential applications of this technology are broad, spanning various fields where precise control over EUV and X-ray beams is essential.

The current study primarily utilizes bulk exfoliated crystals of 1T-TaS2, which hinders scalability for industrial applications. Although promising pathways like chemical vapor deposition and molecular beam epitaxy are mentioned, further research is needed to optimize the growth of high-quality thin films suitable for device fabrication. The detailed nature of the photoinduced metastable state requires further investigation, and the precise mechanism of erasing needs more thorough characterization. The model used for efficiency calculations employs a simplified representation of the grating, and more sophisticated models might be needed to capture all aspects of the diffraction process.