High-harmonic generation in metallic titanium nitride

Toward Complete All-Optical Intensity Modulation of High-Harmonic Generation from Solids

Optical modulation of high-harmonics generation in solids enables the detection of material properties, such as the band structure, and promising new applications, such as super-resolution imaging in semiconductors. Various recent studies have shown optical modulation of high-harmonics generation in solids, in particular, suppression of high-harmonics generation has been observed by synchronized or delayed multipulse sequences. Here we provide an overview of the underlying mechanisms attributed to this suppression and provide a perspective on the challenges and opportunities regarding these mechanisms. All-optical control of high-harmonic generation allows for femtosecond, and in the future possibly subfemtosecond, switching, which has numerous possible applications: These range from super-resolution microscopy to nanoscale controlled chemistry and highly tunable nonlinear light sources.

Ultrafast Electron-Electron Scattering in Metallic Phase of 2H-NbSe_{2} Probed by High Harmonic Generation

Electron-electron scattering on the order of a few to tens of femtoseconds plays a crucial role in the ultrafast electron dynamics of conventional metals. When mid-infrared light is used for driving and the period of light field is comparable to the scattering time in metals, unique light-driven states and nonlinear optical responses associated with the scattering process are expected to occur. Here, we use high-harmonics spectroscopy to investigate the effect of electron-electron scattering on the electron dynamics in thin film 2H-NbSe_{2} driven by a mid-infrared field. We observed odd-order high harmonics up to 9th order as well as a broadband emission from hot electrons in the energy range from 1.5 to 4.0 eV. The electron-electron scattering time in 2H-NbSe_{2} was estimated from the broadband emission to be almost the same as the period of the mid-infrared light field. A comparison between experimental results and a numerical calculation reveals that competition and cooperation between the driving and scattering enhances the nonperturbative behavior of high harmonics in metals, causing a highly nonequilibrium electronic state corresponding to several thousand Kelvin.

Hollow core optical fiber enabled by epsilon-near-zero material

Hollow core optical fibers of numerous guiding mechanisms have been studied in the past decades for their advantages on guiding light in air core. This work demonstrates a new hollow core optical fiber based on a different guiding mechanism, which confines light with a cladding made of epsilon-near-zero (ENZ) material through total internal reflection. We show that the addition of a layer of ENZ material coating (e.g. indium tin oxide layer) significantly reduces the loss of the waveguide compared to the structure without the ENZ layer. We also show that the propagation loss of the ENZ hollow core fiber can be further improved by integrating ENZ materials with lower loss. This study presents a novel type of hollow core fiber, and can find advanced in-fiber photonic applications such as laser surgery/spectroscopy, novel gas-filled/discharge laser, in-fiber molecular/gas sensing, and low-latency optical fiber communication.

Solid-state high harmonic spectroscopy for all-optical band structure probing of high-pressure quantum states

High pressure has triggered various novel states/properties in condensed matter, as the most representative and dramatic example being near-room-temperature superconductivity in highly pressured hydrides (~200 GPa). However, the mechanism of superconductivity is not confirmed, due to the lacking of effective approach to probe the electronic band structure under such high pressures. Here, we theoretically propose that the band structure and electron-phonon coupling (EPC) of high-pressure quantum states can be probed by solid-state high harmonic generation (sHHG). This strategy is investigated in high-pressure Im-3m H3S by the state-of-the-art first-principles time-dependent density-functional theory simulations, where the sHHG is revealed to be strongly dependent on the electronic structures and EPC. The dispersion of multiple bands near the Fermi level is effectively retrieved along different momentum directions. Our study provides unique insights into the potential all-optical route for band structure and EPC probing of high-pressure quantum states, which is expected to be helpful for the experimental exploration of high-pressure superconductivity in the future.

Photonic time crystals: a materials perspective [Invited]

Recent advances in ultrafast, large-modulation photonic materials have opened the door to many new areas of research. One specific example is the exciting prospect of photonic time crystals. In this perspective, we outline the most recent material advances that are promising candidates for photonic time crystals. We discuss their merit in terms of modulation speed and depth. We also investigate the challenges yet to be faced and provide our estimation on possible roads to success.

High-harmonic generation in polycrystalline CdTe nano-films via macroscopic investigations

Bright high harmonics generation (HHG) in CMOS-compatible nano-films can provide new opportunities for integrated coherent ultra-violet sources and attosecond photonic devices. Up to now, most HHG studies have been limited to single crystals. Polycrystalline materials, which consist of many grains separated by grain boundaries and normally have random crystallographic orientations, have rarely been explored for HHG. Understanding and predicting the HHG properties in polycrystalline nano-films are important owing to its merits of low cost and diversified properties, but challenging due to their complicated electronic structures. Here, we for the first time experimentally discover the correspondence between HHG in polycrystalline matters and macroscopic material parameters, to the best of our knowledge. Pumped by a mid-infrared femtosecond laser centered at 7.1 µm wavelength, bright and long-term stable harmonics extending to 25^th orders (284 nm) are demonstrated in polycrystalline cadmium telluride (CdTe) nano-films. It is found that the HHG strengths in the transmission and the reflection behave differently as a function of the material thickness in the range from 6 nm to 4 µm, which is highly correlated to the measured macroscopic conductivity. The experimental findings agree well with the recent theoretical prediction [Phys. Rev. B103(15), 155426 (2021)10.1103/PhysRevB.103.155426]. This work provides a simple gauge to study and predict HHG in complicated polycrystalline and amorphous nano-systems, and paves the way for novel strong-field nanophotonics based on polycrystalline nano-films.

Formula Graph Self-Attention Network for Representation-Domain Independent Materials Discovery

The success of machine learning (ML) in materials property prediction depends heavily on how the materials are represented for learning. Two dominant families of material descriptors exist, one that encodes crystal structure in the representation and the other that only uses stoichiometric information with the hope of discovering new materials. Graph neural networks (GNNs) in particular have excelled in predicting material properties within chemical accuracy. However, current GNNs are limited to only one of the above two avenues owing to the little overlap between respective material representations. Here, a new concept of formula graph which unifies stoichiometry-only and structure-based material descriptors is introduced. A self-attention integrated GNN that assimilates a formula graph is further developed and it is found that the proposed architecture produces material embeddings transferable between the two domains. The proposed model can outperform some previously reported structure-agnostic models and their structure-based counterparts while exhibiting better sample efficiency and faster convergence. Finally, the model is applied in a challenging exemplar to predict the complex dielectric function of materials and nominate new substances that potentially exhibit epsilon-near-zero phenomena.