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Wire waveguides have recently been shown to be valuable for transporting pulsed terahertz radiation. This technique relies on the use of a scattering mechanism for input coupling. A radially polarized surface wave is excited when a linearly polarized terahertz pulse is focused on the gap between the wire waveguide and another metal structure. We calculate the input coupling efficiency using a simulation based on the Finite Element Method (FEM). Additional FEM results indicate that enhanced coupling efficiency can be achieved through the use of a radially symmetric photoconductive antenna. Experimental results confirm that such an antenna can generate terahertz radiation which couples to the radial waveguide mode with greatly improved efficiency.

Conventional optics depend on the gradual accumulation of spatially dependent phase shifts imparted on light propagating through a medium to modify the wavefront of an incident beam. A similar effect may be obtained by the imposition of abrupt, discrete phase changes on a propagating wavefront over a subwavelength scale using photonic metasurfaces. Highly efficient metasurfaces have applications ranging from conventional optics to high-efficiency solar energy conversion, optical communications, and more. We present here the design, computational modeling, and experimental demonstration of all-dielectric transmissive Huygens metasurfaces exhibiting anomalous refraction, defined as the controlled deflection of light at an interface as a function of subwavelength nanostructures. These metasurfaces are composed of dielectric, cylindrical elements, characterized by balanced electric and magnetic dipole resonances. For infrared wavelengths, optical efficiency of 91.3% is demonstrated computationally, and experimental efficiency of 63.6% is measured. Metasurfaces are designed and modeled in each of three experimentally realizable material systems, corresponding to incident wavelengths in the ultraviolet, visible, and infrared, all demonstrating high optical efficiency of at least 78%. A ground-up approach is presented that enables this design of highly efficient all-dielectric Huygens metasurfaces with nonzero phase gradients, in spite of difficulties due to strong interantenna coupling effects. Additionally, we computationally demonstrate a stacked metasurface device, capable of independent manipulation of four adjacent spectral bands, with midband optical efficiency as high as 55%. Taking advantage of the high sensitivity of this resonant dielectric Huygens metasurface approach, we discuss routes to the development of optical sensors and dynamically tunable metasurfaces.

Monolayer, few‐layer, and thin‐film MoS2 is synthesized using chemical vapor deposition (CVD) and thermal vapor sulfurization (TVS) methods. The complex refractive index of these samples is assessed using variable angle spectroscopic ellipsometry (VASE) measurements over a broad spectral range between 190 and 1700 nm. The ellipsometry data are sensitive to birefringence effects in the thickest thin‐film sample. These birefringence effects are investigated, and an analysis method is developed to extract the in‐plane and out‐of‐plane optical properties. The complex refractive index is then used to calculate reflectance, transmittance, and absorption of the MoS2 films using the transfer‐matrix method (TMM) and is matched with experimentally measured transmittance of the same samples. The modeled results show that the monolayer, few‐layer, and thin‐film MoS2 absorbs 7.4%, 12.6%, and 32.4% of the incident light, respectively, between 300 and 700 nm. When normalized to per unit‐thickness absorption, they absorb 12.1%, 5.9%, and 1.1% nm−1, respectively, clearly showing superior light–matter interaction in the monolayer and few‐layer films. These new complex refractive index data are further used to design optical coatings for these films to either confine absorption in a narrow bandwidth for photodetector applications or enhance broadband absorption for photovoltaic applications.

Monolayer molybdenum disulfide (MoS 2 ) is an atomically thin, direct bandgap semiconductor crystal potentially capable of miniaturizing optoelectronic devices to an atomic scale. However, the development of 2D MoS 2 -based optoelectronic devices depends upon the existence of a high optical quality and large-area monolayer MoS 2 synthesis technique. To address this need, we present a thermal vapor sulfurization (TVS) technique that uses powder MoS 2 as a sulfur vapor source. The technique reduces and stabilizes the flow of sulfur vapor, enabling monolayer wafer-scale MoS 2 growth. MoS 2 thickness is also controlled with great precision; we demonstrate the ability to synthesize MoS 2 sheets between 1 and 4 layers thick, while also showing the ability to create films with average thickness intermediate between integer layer numbers. The films exhibit wafer-scale coverage and uniformity, with electrical quality varying depending on the final thickness of the grown MoS 2 . The direct bandgap of grown monolayer MoS 2 is analyzed using internal and external photoluminescence quantum efficiency. The photoluminescence quantum efficiency is shown to be competitive with untreated exfoliated MoS 2 monolayer crystals. The ability to consistently grow wafer-scale monolayer MoS 2 with high optical quality makes this technique a valuable tool for the development of 2D optoelectronic devices such as photovoltaics, detectors, and light emitters.

A density-matrix based theory of transport and lasing in quantum-cascade lasers reveals that large disparity between luminescent linewidth and broadening of the tunneling transition changes the design guidelines to favor strong coupling between injector and upper laser level. This conclusion is supported by the experimental evidence.

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A sensing platform is presented that uses dielectric Huygens source metasurfaces to measure refractive index changes in a microfluidic channel with experimentally measured sensitivity of 323 nm/RIU, FOM of 5.4, and a response of 8.2 (820%) change in transmittance per refractive index unit (T/RIU). Changes in the refractive index of liquids flown through the channel are measured by singlewavelength transmittance measurement, requiring only a simple light source and photodetector, significantly reducing device expense in comparison to state-of-the-art refractive index sensing technologies. A techno-economic analysis predicts a device costing ~$2,400 that is capable of detecting refractive index changes on the order of 2*10 -8 . The metasurfaces utilized are low profile, scalable, and use materials and fabrication processes compatible with CMOS and other technologies This article is protected by copyright. All rights reserved.2 making them suitable for device integration. The Huygens metasurface system, characterized by spectrally overlapping electric and magnetic dipole modes, offers a high degree of customizability.Interplay between the two resonances may be controlled via metasurface geometry, leading to tunability of device sensitivity and measurement range. Ultra-high sensitivity of 350 nm/RIU with FOM of 219, corresponding to single-wavelength sensitivity of 360 T/RIU, is demonstrated computationally through use of antisymmetric resonances of a Huygens metasurface illuminated at small incidence angles.