Gate-tunable plasmons in mixed-dimensional van der Waals heterostructures

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Machine Learning for Optical Scanning Probe Nanoscopy

The ability to perform nanometer-scale optical imaging and spectroscopy is key to deciphering the low-energy effects in quantum materials, as well as vibrational fingerprints in planetary and extraterrestrial particles, catalytic substances, and aqueous biological samples. These tasks can be accomplished by the scattering-type scanning near-field optical microscopy (s-SNOM) technique that has recently spread to many research fields and enabled notable discoveries. Herein, it is shown that the s-SNOM, together with scanning probe research in general, can benefit in many ways from artificial-intelligence (AI) and machine-learning (ML) algorithms. Augmented with AI- and ML-enhanced data acquisition and analysis, scanning probe optical nanoscopy is poised to become more efficient, accurate, and intelligent.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Transverse Magnetic Surface Plasmons in Graphene Nanoribbon Qubits: The Influence of a VO2 Substrate

We study the influence of the phase-change material VO₂ on transverse magnetic (TM) surface plasmon (SP) modes in metallic arm-chair graphene nanoribbon (AGNR) qubits in the Lindhard approximation. We assess the effects of temperature as a dynamic knob for the transition from the insulating to the metallic phase on the TM SP modes in single-band (SB) and two-band (TB) transitions. We show that a VO₂ substrate leads to TM SP modes in both SB and TB transitions. In addition, we observe that the SP modes have a lower frequency than those for a substrate of constant permittivity. In addition, we study the influence of the substrate-induced band gap Δ' on SP modes in TB transitions for the insulating and metallic phases of VO₂.

High performance 1D-2D CuO/MoS2 photodetectors enhanced by femtosecond laser-induced contact engineering

The integration of 2D materials with other dimensional materials opens up rich possibilities for both fundamental physics and exotic nanodevices. However, current mixed-dimensional heterostructures often suffer from interfacial contact issues and environment-induced degradation, which severely limits their performance in electronics/optoelectronics. Herein, we demonstrate a novel BN-encapsulated CuO/MoS₂ 2D-1D van der Waals heterostructure photodetector with an ultrahigh photoresponsivity which is 10-fold higher than its previous 2D-1D counterparts. The interfacial contact state and photodetection capabilities of 2D-1D heterojunctions are significantly improved via femtosecond laser irradiation induced MoS₂ wrapping and contamination removal. These h-BN protected devices show highly sensitive, gate-tunable and robust photoelectronic properties. By controlling the gate and bias voltages, the device can achieve a photoresponsivity as high as 2500 A W^-1 in the forward bias mode, or achieve a high detectivity of 6.5 × 10¹¹ Jones and a typical rise time of 2.5 ms at reverse bias. Moreover, h-BN encapsulation effectively protects the mixed-dimensional photodetector from electrical depletion by gas molecules such as O₂ and H₂O during fs laser treatment or the operation process, thus greatly improving the stability and service life in harsh environments. This work provides a new way for the further development of high performance, low cost, and robust mixed-dimensional heterostructure photodetectors by femtosecond laser contact engineering.

Lightwave nano-converging enhancement by an arrayed optical antenna based on metallic nano-cone-tips for CMOS imaging detection

A kind of gold-coated glass nano-cone-tips (GGNCTs) is developed as an arrayed optical antenna for highly receiving and converging incident lightwaves. A local light field enhancement factor (LFEF) of ~ 2 × 10⁴ and maximum light absorption of ~ 98% can be achieved. The near-field lightwave measurements at the wavelength of 633 nm show that the surface net charges over a single GGNCT make a typical dipole oscillation and the energy transmits along the wave vector orientation, thus leading to a strong local light field enhancement. An effective detection method by near-field coupling an arrayed GGNCT and complementary metal-oxide-semiconductor (CMOS) sensor for highly efficient imaging detection is proposed. The lightwave detection at several wavelengths, including typical 473 nm, 532 nm, 671 nm, and 980 nm, shows a notable characteristic that a better capability of the net charge distribution adjusting and localized aggregating can be obtained at the absorption peak of the GGNCT developed and a stronger signal detection achieved. The research lays a foundation for further developing a light detector with an ideal optoelectronic sensitivity and broad spectral suitability, which is based on integrating GGNCTs as an arrayed optical antenna with common sensors.

Direct Visualization of Ultrastrong Coupling between Luttinger-Liquid Plasmons and Phonon Polaritons

Ultrastrong coupling of light and matter creates new opportunities to modify chemical reactions or develop novel nanoscale devices. One-dimensional Luttinger-liquid plasmons in metallic carbon nanotubes are long-lived excitations with extreme electromagnetic field confinement. They are promising candidates to realize strong or even ultrastrong coupling at infrared frequencies. We applied near-field polariton interferometry to examine the interaction between propagating Luttinger-liquid plasmons in individual carbon nanotubes and surface phonon polaritons of silica and hexagonal boron nitride. We extracted the dispersion relation of the hybrid Luttinger-liquid plasmon-phonon polaritons (LPPhPs) and explained the observed phenomena by the coupled harmonic oscillator model. The dispersion shows pronounced mode splitting, and the obtained value for the normalized coupling strength shows we reached the ultrastrong coupling regime with both native silica and hBN phonons. Our findings predict future applications to exploit the extraordinary properties of carbon nanotube plasmons, ranging from nanoscale plasmonic circuits to ultrasensitive molecular sensing.