Orietation-controlled synthesis and Raman study of 2D SnTe.

Tin telluride (SnTe), as a narrow bandgap semiconductor material, has great potential for developing photodetectors with wide spectra and ultra-fast response. At the same time, it is also an important topological crystal insulator material, with different topological surface states on several common surfaces. Here, we introduce different Sn sources and control the growth of regular SnTe nanosheets along the (100) and (111) planes through the atmospheric pressure chemical vapor deposition method. It has been proven through various characterizations that the synthesized SnTe is a high-quality single crystal. In addition, the angular resolved Raman spectra of SnTe nanosheets grown on different crystal planes are first demonstrated. The experimental results showed that square SnTe nanosheets grown along the (100) plane exhibit in-plane anisotropy. At the same time, we use micro-nanofabrication technology to manufacture SnTe-based field effect transistors and photodetectors to explore their electrical and optoelectronic properties. It has been confirmed that transistors based on grown SnTe nanosheets exhibit p-type semiconductor characteristics and have a high response to infrared light. This work provides a new approach for the controllable synthesis of SnTe and adds new content to the research of SnTe-based infrared detectors.

Ultralow-Loss Phonon Polaritons in the Isotope-Enriched α-MoO3.

α-MoO3, a natural van der Waals (vdWs) material, has received wide attention in nano-optics for supporting highly confined anisotropic phonon polaritons (PhPs) from the mid-infrared to the terahertz region, which opens a new route for manipulating light at the nanoscale. However, its optical loss hinders light manipulation with high efficiency. This work demonstrates that the isotope-enriched Mo element enables ultralow-loss PhPs in the α-MoO3. Raman spectra reveal that the isotope-enriched Mo element in the α-MoO3 allows different optical phonon frequencies by efficiently altering the Reststrahlen band's dispersion. The Mo isotope-enriched α-MoO3 significantly reduces the PhPs' optical loss due to efficient optical coherence, which enhances the propagation length revealed by infrared nanoimaging. These findings suggest that the isotope-enriched α-MoO3 is a new feasible 2D material with an ultralow optical loss for possible high-performance integrated photonics and quantum optics devices.

Strong Light-Matter Interactions between Gap Plasmons and Two-Dimensional Excitons under Ambient Conditions in a Deterministic Way.

Strong exciton-plasmon interactions between layered two-dimensional (2D) semiconductors and gap plasmons show a great potential to implement cavity quantum electrodynamics under ambient conditions. However, achieving a robust plasmon-exciton coupling with nanocavities is still very challenging, because the layer area is usually small in the conventional approaches. Here, we report on a robust strong exciton-plasmon coupling between the gap mode of a bowtie and the excitons in MoS2 layers with gold-assisted mechanical exfoliation and nondestructive wet transfer techniques for a large-area layer. Due to the ultrasmall mode volume and strong in-plane field, the estimated effective exciton number contributing to the coupling is largely reduced. With a corrected exciton transition dipole moment, the exciton numbers are extracted as being 40 for the case of a single layer and 48 for eight layers. Our work paves the way to realize strong coupling with 2D materials with a small number of excitons at room temperature.

Single-Vesicle Infrared Nanoscopy for Noninvasive Tumor Malignancy Diagnosis.

Protein heterogeneity in molecular expression and structures determines tumorigenesis and is the diagnostic and therapeutic cancer biomarker. Small extracellular vesicles (sEVs) are cell-released nanoscaled membrane-bound vesicles transferring bioactive molecules for intercellular communication and playing essential roles in tumor progression and metastasis. Therefore, protein heterogeneity in tumor-derived sEVs indicates the degree of malignant transformation, providing a noninvasive biomarker for cancer diagnosis and malignancy evaluation. We employ near-field infrared (nano-FTIR) spectroscopy to investigate malignancy-related protein heterogeneity in a single sEV and demonstrate the discriminability of sEV protein heterogeneity to evaluate tumor malignancy and metastasis. We found that the amide I/II adsorption ratio of the sEVs increases with tumor malignancy, the proportion of α-helix + random coil (α-helix and random coil) in sEV proteins decreases with tumor malignancy, and the proportion of ß-sheet + ß-turn (ß-sheet and ß-turn) increases with tumor malignancy. These nano-FTIR spectral signatures of the sEVs from the primary tumor tissue of breast cancer patients show high sensitivity and specificity in evaluating tumor metastasis. This study shows the advantages of nano-FTIR in single sEV characterization and demonstrates the significance of sEV protein heterogeneity in cancer diagnosis. It provides a noninvasive solution to elucidate cancer development and facilitates the exploitation of potential cancer biomarkers.

The small-molecule drug homoharringtonine targets HSF1 to suppress pancreatic cancer progression.

Heat shock factor 1 (HSF1), an essential transcription factor for stress response, is exploited by various tumors to facilitate their initiation, progression, invasion, and migration. Amplification of HSF1 is widely regarded as an indicator in predicting cancer severity, the likelihood of treatment failure and reduced patient survival. Notably, HSF1 is markedly amplified in 40% of pancreatic cancer (PC), which typically have limited treatment options. HSF1 has been proven to be a promising therapeutic target for multiple cancers. However, a direct small molecule HSF1 inhibitor with sufficient bioactivity and reliable safety has not been developed clinically. In this study, we successfully established a high-throughput screening system utilizing luciferase reporter assay specifically designed for HSF1, which leads to the discovery of a potent small molecule inhibitor targeting HSF1. Homoharringtonine (HHT) selectively inhibited PC cell viability with high HSF1 expression and induced a markedly stronger tumor regression effect in the subcutaneous xenograft model than the comparator drug KRIBB11, known for its direct action on HSF1. Moreover, HHT shows promise in countering the resistance encountered with HSP90 inhibitors, which have been observed to increase heat shock response intensity in clinical trials. Mechanistically, HHT directly bound to HSF1, suppressing its expression and thereby inhibiting transcription of HSF1 target genes. In conclusion, our work presents a preclinical discovery and validation for HHT as a HSF1 inhibitor for PC treatment.

Manipulating hyperbolic transient plasmons in a layered semiconductor.

Anisotropic materials with oppositely signed dielectric tensors support hyperbolic polaritons, displaying enhanced electromagnetic localization and directional energy flow. However, the most reported hyperbolic phonon polaritons are difficult to apply for active electro-optical modulations and optoelectronic devices. Here, we report a dynamic topological plasmonic dispersion transition in black phosphorus via photo-induced carrier injection, i.e., transforming the iso-frequency contour from a pristine ellipsoid to a non-equilibrium hyperboloid. Our work also demonstrates the peculiar transient plasmonic properties of the studied layered semiconductor, such as the ultrafast transition, low propagation losses, efficient optical emission from the black phosphorus's edges, and the characterization of different transient plasmon modes. Our results may be relevant for the development of future optoelectronic applications.

Broadband plasmonic indium arsenide photonic antennas.

An on-chip integrated mid-infrared Fabry-Perot (F-P) polariton resonator exhibits excellent biosensing, thermal emission, and quantum laser utility potential. However, the narrow optical response range and absence of optoelectronic tunability have hindered the development of a F-P phonon polariton resonator. The discovery of surface plasmons in semiconductor nanowires provides a novel route to F-P polariton resonator devices with a broadband optical response and multi-field tunability. Due to their high electron mobility and crystalline quality, InAs twinning superlattice (TSL) nanowires have become a promising candidate in plasmonic electronics. We systemically studied the F-P plasmonic resonance of individual InAs TSL nanowires with a scattering-type near-field optical microscope. Using a metallic AFM tip to excite surface plasmons, we can observe odd-order and even-order modes of F-P polariton resonance, breaking the symmetric selection rules. Through nano Fourier transform infrared spectroscopy, we found that InAs nanowires' F-P polariton resonances appear in a broadband frequency range (650-1100 cm-1) and calculated that the corresponding Q factor is 5-10. This semiconductor F-P polariton resonator with inherent electrical tunability will be essential in integrated nanophotonic circuits.

Optically Tunable Transient Plasmons in InSb Nanowires.

Optical carrier incubation can effectively alter the electron transport properties of semiconductors; thus, optical switching of the plasmonic response of the semiconductor enables the ultrafast manipulation of the light at the nanoscale. Semiconductor nanostructures are promising platforms in on-chip high-speed plasmonic devices, owing to their high photoinduced electron injection efficiency at sub-picosecond and compatibility with contemporary semiconductor technologies. The pure single crystalline InSb nanowires are promising plasmonic materials in the mid-infrared region due to their high electron mobility and small electron effective mass. Here, the pump-probe nanoscopy is utilized to investigate the pump fluence dependency and the dynamics of the non-equilibrium plasmons in the InSb nanowires. The InSb plasmon is successfully switched by injecting the photoinduced electrons and the practical tuning of the plasmon frequency to one octave is shown by increasing the pump fluence from 0 to 90 µJ cm-2 . The density of the photoinduced electrons in InSb nanowires is 18.8 × 1018  cm-3 with pump fluence as low as 90 µJ cm-2 . The high electron mobility of the InSb supports the low-loss plasmon with a damping rate of ≈200 cm-1 . The InSb nanowires' excellent plasmonic properties ensure that they are a promising platform for upcoming high-speed mid-infrared plasmonic materials for informatic devices.

Tunable heterostructural prism for planar polaritonic switch.

The study of phonon polaritons in van der Waals materials at the nanoscale has gained significant attention in recent years due to its potential applications in nanophotonics. The unique properties of these materials, such as their ability to support sub-diffraction imaging, sensing, and hyperlenses, have made them a promising avenue for the development of new techniques in the field. Despite these advancements, there still exists a challenge in achieving dynamically reversible manipulation of phonon polaritons in these materials due to their insulating properties. In this study, we present experimental results on the reversible manipulation of anisotropic phonon polaritons in α-MoO3 on top of a VO2 film, a phase-change material known for its dramatic changes in dielectric properties between its insulating and metallic states. Our findings demonstrate that the engineered VO2 film enables a switch in the propagation of polaritons in the mid-infrared region by modifying the dielectric properties of the film through temperature changes. Our results represent a promising approach to effectively control the flow of light energy at the nanoscale and offer the potential for the design and fabrication of integrated, flat sub-diffraction polaritonic devices. This study adds to the growing body of work in the field of nanophotonics and highlights the importance of considering phase-change materials for the development of new techniques in this field.

Gate-tunable negative refraction of mid-infrared polaritons.

Negative refraction provides a platform to manipulate mid-infrared and terahertz radiation for molecular sensing and thermal emission applications. However, its implementation based on metamaterials and plasmonic media presents challenges with optical losses, limited spatial confinement, and lack of active tunability in this spectral range. We demonstrate gate-tunable negative refraction at mid-infrared frequencies using hybrid topological polaritons in van der Waals heterostructures. Specifically, we visualize wide-angle negatively refracted polaritons in α-MoO3 films partially decorated with graphene, undergoing reversible planar nanoscale focusing. Our atomically thick heterostructures weaken scattering losses at the interface while enabling an actively tunable transition of normal to negative refraction through electrical gating. We propose polaritonic negative refraction as a promising platform for infrared applications such as electrically tunable super-resolution imaging, nanoscale thermal manipulation, enhanced molecular sensing, and on-chip optical circuitry.

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3.

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO3 . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 µm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO3 permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO3 are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Doping-driven topological polaritons in graphene/α-MoO3 heterostructures.

Control over charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties of materials. This approach can also be used to induce topological transitions in the optical response of photonic systems. Here we report a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and α-phase molybdenum trioxide. By chemically changing the doping level of graphene, we observed that the topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, when the substrate was changed, the dispersion contour became dominated by flat profiles at the topological transition, thus supporting tunable diffractionless polariton propagation and providing local control over the optical contour topology. We achieved subwavelength focusing of polaritons down to 4.8% of the free-space light wavelength by using a 1.5-µm-wide silica substrate as an in-plane lens. Our findings could lead to on-chip applications in nanoimaging, optical sensing and manipulation of energy transfer at the nanoscale.

Manipulating phonon polaritons in low loss 11B enriched hexagonal boron nitride with polarization control.

Hexagonal boron nitride (hBN) supports two types of hyperbolic phonon polaritons (HPPs), whose properties of strong electromagnetic field confinement and low propagation loss have been proposed for various applications in nanophotonics. Conventionally, real-space imaging of HPPs by scattering-type scanning near-field optical microscopy (s-SNOM) with vertical polarization excitation contains both tip and edge launched polariton modes, which leads to hybrid interference fringes. In this work, we symmetrically study the tip and edge excited HPPs in both boron nitride with the natural distribution of boron isotopes (natural hBN) and 11B isotope-enriched boron nitride (99.2% 11B hBN). The intrinsic HPPs excited in 99.2% 11B hBN exhibit a lower damping rate and longer propagation length than that in natural hBN. We experimentally realize a tuning from tip-dominated to edge-dominated excited HPPs by rotating the polarization of incident light. The near-field electric field intensity (NEFI) of edge-excited HPPs Eedge and the angle ß (between the hBN edge and the projective direction of the incident electric field on the hBN plane) present a sine function relationship as Eedge∝|sin ß| under an s-polarized incident light. The NEFI of edge-excited HPPs in 99.2% 11B hBN shows a 10% enhancement compared to natural hBN under the same measurement conditions. Our findings demonstrate an effective approach to reducing phonon polariton damping and manipulating phonon polariton excitation in hBN, which are beneficial for developing HPPs-based nanophotonic applications.

Observation and Ultrafast Dynamics of Inter-Sub-Band Transition in InAs Twinning Superlattice Nanowires.

A variety of infrared applications rely on semiconductor superlattices, including, notably, the realization of high-power, compact quantum cascade lasers. Requirements for atomically smooth interface and limited lattice matching options set high technical standards for fabricating applicable heterostructure devices. The semiconductor twinning superlattice (TSL) forms in a single compound with periodically spaced twin boundaries and sharp interface junctions and can be grown with convenient synthesis methods. Therefore, employing semiconductor TSL may facilitate the development of optoelectronic applications related to superlattice structures. Here, it is shown that InAs TSL nanowires generate inter-sub-band transition channels due to the band projection and the Bragg-like electron reflection. The findings reveal the physical mechanisms of inter-sub-band transitions in TSL structure and suggest that TSL structures are promising candidates for mid-infrared optoelectronic applications.

Tin diselenide van der Waals materials as new candidates for mid-infrared waveguide chips.

Mid-infrared is a spectral region of molecular vibration and rotation modes and thus, it has been widely used in chem/bio analysis. On-chip MIR waveguides combining attenuated total reflection spectroscopy provide an efficient way to minimize equipment size and benefit chemical trace analysis. But, inevitable surface roughness-induced scattering is harmful for waveguide mode propagation in traditional sensors. Two-dimensional materials are natural thin slabs with atomic-scale smooth surfaces and thus could be excellent for building weak surface scattering waveguides. Here, we used near-field microscopy to investigate a waveguide mode of 1T tin diselenide slabs at nanoscale resolution in 5.13-6.57 µm and manipulate the mode strengths and wavelengths by controlling the slab thickness. This work extends two-dimensional materials as building blocks for integrated MIR chips.