Abstract

Transition metal dichalcogenides (TMDs) are layered van der Waals materials that exhibit
exceptional physical, electronic, and optical properties in both bulk and two-dimensional
(2D) forms1, 2
. The rapid advancements in terahertz (THz) science and technology have
driven extensive research into novel materials and material structures to gain a deeper
understanding of fundamental processes occurring at THz energy scales3, 4. In this context,
ultrafast THz time-domain spectroscopy has proven to be an invaluable tool for studying THz
emission mechanisms in these materials. In this work, we explore the enhancement of THz
radiation generation in bilayer MoS2 upon femtosecond laser excitation, with a particular
focus on the impact of niobium (Nb) doping. We identify that different microscopic
mechanisms contribute to the THz photocurrent generation, with their relative significance
depending on excitation above the electronic bandgap of the bilayer. The THz emission
measurements were conducted in a reflection geometry using a 480 nm excitation wavelength
with a pump fluence of 150 µJ/cm². The complete details about the experimental setup can be
found elsewhere5, 6
. The recorded time-domain waveforms and the corresponding Fourier
transform spectrum of the emitted THz signals from both pristine and Nb-doped bilayer
MoS2 are shown in Fig. 1(a) and (b), respectively. Notably, even at a relatively low Nbdoping concentration of approximately 0.05%, we observe a multifold enhancement in THz
emission from the Nb-doped bilayer MoS2 compared to its pristine counterpart. The
enhancement in THz signal is attributed to the breaking of inversion symmetry and a
reduction in the screening of the surface depletion field induced by Nb doping. Furthermore,
along with the increased THz generation efficiency, we detect a reversal in the THz pulse
polarity in Nb-doped layers, which correlates with the reversed surface depletion field, due to
the p-type behavior of the doped MoS2
5
. This work provides a potential pathway for
optimizing TMD-based THz emitters

Abstract

Metadevices are artificial periodic micro/nanostructures used to manipulate the propagation of electromagnetic waves. By properly engineering the geometrical dimensions and material compositions of metadevices subwavelength periodic unit cells, the permittivity and permeability of metadevices can be manually controlled. Thus, metadevices are widely studied in energy harvesting, medical imaging, cloaking devices, negative refraction index, and so on. The operating wavelength range of metadevices can be spanned in the entire electromagnetic spectrum by physically scaling the geometry from visible (Vis), infrared (IR), terahertz (THz), microwave (MW) specrea. There have been many metadevices demonstrated to have powerful performances. However, these rigid devices cannot be actively tuned once fabricated. The controllable metadevices help to improve flexibility by using an external stimulus. To satisfy this requirement, there are many techniques proposed for tuning mechanisms, including thermal annealing, laser pumping, phase-transition materials, two-dimensional (2D) material, liquid crystals, and nano-/micro-electro-mechanical systems (N/MEMS) technology. Among these, N/MEMS technology is a promising technique for actively tunable metadevices. These tunable metadevices exhibit assuredly unprecedented capabilities in the manipulation of incident electromagnetic wave, e.g., frequency, amplitude, phase, and polarization. They allow for the implementations of many traditional optical devices. Moreover, N/MEMS tuning methods can provide an ideal platform for metadevices, which is not limited by the nonlinear characteristic of natural materials and exhibits a large tuning range of resonant frequency. Recently, the actively tunable N/MEMS metadevices developed by Dr. Lin’s research group provide the possibility to manually manipulate electromagnetic waves from Vis light to THz wave. Here, the fruitful results in Dr. Lin’s research group for the actively tunable N/MEMS metadevices and their potential applications will be presented and summarized.

On the 3^rd and the 4^th of March, 2025, the Lunar Laser Ranging Observatories (LLROs) in Grasse, France and Wettzell, Germany and later the APOLLO  LLRO obtained hundreds of successful range measurements to our Next Generation Lunar Reflector (NGLR-1) on the Moon in Mare Crisium. The launch was on a SpaceX’s Faclon9 rocket at 1:11am on the 15^th of January from the Kennedy Space Center. NGLR-1 was carried to the Moon by Firefly Aerospace’s Blue Ghost lander. The Blue Ghost successfully deployed NGLR-1 on the Moon on the 2nd of March at which time the LLROs began Lunar Laser Ranging (LLR) operations to search for NGLR-1.

The objective of the NGLR-1 project and later the deployments of NGLR-2 and NGLR-3 is to greatly improve the precision and accuracy of the LLR measurements. The preliminary data obtained during the first week address the ranging precision, the relative local accuracy, the relative extended accuracy, and the accuracy. These objectives and early results obtained during the initial test phase will be discussed.

Simulations of a 6-year mission addressing the expected improvement in the accuracy of the scientific results were performed with Jim Williams of JPL*. This analysis indicates that the scientific results , beginning when all three NGLRs are deployed, may be expected to provide an order of magnitude improvement in the accuracy of the various scientific results as compared to the current results obtained with the Apollo Retroreflector Arrays (ARAs) and the Lunokhod retroreflector arrays. The analysis of the latter has already discovered the liquid core of the Moon, obtained the best estimate of any deviations of the Weak Equivalence Principle, provided the best evaluation of the temporal and spatial changes in Big G, and provided many of the best tests of General Relativity.

A brief review of the operational issues and design history of NGLR-1^# will be compared to our Apollo 11 Retroreflector  Array.

   Abstract

Optical device systems designed to steer light in specific directions have been extensively researched and developed for a variety of applications. Although non-mechanical beam steering systems using liquid crystals have been introduced for their compactness, there is still a demand for devices that provide wider viewing angles. To implement patterned Indium tin oxide (ITO) with a smaller pitch, a beam deflector (BD) with a pixel pitch of 2 µm, consisting of a 1.5 µm electrode and a 0.5 µm spacing, allowing a maximum angle of 7.643° is achieved at a wavelength of 532 nm. However, the total size of the active area is reduced due to the limited number of channels, but this can be improved by introducing a via-hole process, which enables a periodic tiling process and expands the active area by repeating the unit bank. When a total of 720 electrodes are grouped into a unit bank and repeated 10 times, the active area, which is only 1.44 mm with 2 µm pitch patterns, can be expanded to 14.4 mm. When a specific voltage is applied to channel #1, this voltage is simultaneously transferred to each channel #1 of the 10 unit banks, as all corresponding channels are interconnected through the via-hole process.

Prof. Kanghee Won is an assistant professor at the department of Information Display at Kyung Hee University. He completed his PhD studies in Electrical Engineering at the University of Cambridge. He worked as senior researcher at the Samsung Advanced Institute of Technology (2011-2023). His research topic is mainly include: Next-generation displays (holographic, AR/VR systems), optical and Nanophotonic devices (beam deflector. LiDAR, MetaLens), and electro-optical materials (liquid crystals).