Valleytronics: Opportunities, Challenges, and Paths Forward.

A lack of inversion symmetry coupled with the presence of time-reversal symmetry endows 2D transition metal dichalcogenides with individually addressable valleys in momentum space at the K and K' points in the first Brillouin zone. This valley addressability opens up the possibility of using the momentum state of electrons, holes, or excitons as a completely new paradigm in information processing. The opportunities and challenges associated with manipulation of the valley degree of freedom for practical quantum and classical information processing applications were analyzed during the 2017 Workshop on Valleytronic Materials, Architectures, and Devices; this Review presents the major findings of the workshop.

Optical nondestructive dynamic measurements of wafer-scale encapsulated nanofluidic channels.

Nanofluidic channels are of great interest for DNA sequencing, chromatography, and drug delivery. However, metrology of embedded or sealed nanochannels and measurement of their fill-state have remained extremely challenging. Existing techniques have been restricted to optical microscopy, which suffers from insufficient resolution, or scanning electron microscopy, which cannot measure sealed or embedded channels without cleaving the sample. Here, we demonstrate a novel method for accurately extracting nanochannel cross-sectional dimensions and monitoring fluid filling, utilizing spectroscopic ellipsometric scatterometry, combined with rigorous electromagnetic simulations. Our technique is capable of measuring channel dimensions with better than 5-nm accuracy and assessing channel filling within seconds. The developed technique is, thus, well suited for both process monitoring of channel fabrication as well as for studying complex phenomena of fluid flow through nanochannel structures.

Fluidic microoptics with adjustable focusing and beam steering for single cell optogenetics.

Electrically controlled micron-scale liquid lenses have been designed, fabricated and demonstrated, that provide both adjustable focusing and beam steering, with the goal of applying them to optogenetic in vivo mapping of brain activity with single cell resolution. The liquid lens is formed by the interface between two immiscible liquids which are contained in a conically tapered lens cavity etched into a fused silica substrate. Interdigitated electrodes have been patterned along the sidewall of the taper to control the liquid lens curvature and tilt. Microlenses with apertures ranging in size from 30 to 80 µm were fabricated and tunable focusing ranging from 0.25 to 3 mm and beam steering of ± 1 degree have been demonstrated.

Electrically switchable diffractive waveplates with metasurface aligned liquid crystals.

Diffractive waveplates and equivalent metasurfaces provide a promising path for applications in thin film beam steering, tunable lenses, and polarization filters. However, fixed metasurfaces alone are unable to be tuned electronically. By combining metasurfaces with tunable liquid crystals, we experimentally demonstrate a single layer device capable of electrically switching a diffractive waveplate design at a measured peak diffraction efficiency of 35%, and a minimum switching voltage of 10V. Furthermore, the nano-scale metasurface aligned liquid crystals are largely independent of variations in wavelength and temperature. We also present a computational analysis of the efficiency limits of liquid crystal based diffractive waveplates, and compare this analysis to experimental measurements.

Engineered liquid crystal anchoring energies with nanopatterned surfaces.

The anchoring energy of liquid crystals was shown to be tunable by surface nanopatterning of periodic lines and spaces. Both the pitch and height were varied using hydrogen silsesquioxane negative tone electron beam resist, providing for flexibility in magnitude and spatial distribution of the anchoring energy. Using twisted nematic liquid crystal cells, it was shown that this energy is tunable over an order of magnitude. These results agree with a literature model which predicts the anchoring energy of sinusoidal grooves.

Aluminum plasmonics: optimization of plasmonic properties using liquid-prism-coupled ellipsometry.

We have established a method to quantify and optimize the plasmonic behavior of aluminum thin films by coupling spectroscopic ellipsometry into surface plasmon polaritons using a liquid prism cell in a modified Otto configuration. This procedure was applied to Al thin films deposited by four different methods, as well as to single crystal Al substrates, to determine the broadband optical constants and calculate plasmonic figures of merit. The best performance was achieved with Al films that have been sputter-deposited at high temperatures of 350°C, followed by chemical mechanical polishing. This combination of temperature and post-processing produced aluminum films with both large grain size and low surface roughness. Comparing these figures of merit with literature values of gold, silver, and copper shows that at blue and ultraviolet wavelengths, optimized aluminum has the highest figure of merit of all materials studied. We further employ the Ashcroft and Sturm theory of optical conductivity to extract the electron scattering times for the Drude and effective interband transitions, interband transition energies, and the optical mass of electrons.

22-nm immersion interference lithography.

Immersion interference lithography was used to pattern gratings with 22-nm half pitch. This ultrahigh resolution was made possible by using 157-nm light, a sapphire coupling prism with index 2.09, and a 30-nm-thick immersion fluid with index 1.82. The thickness was controlled precisely by spin-casting the fluid rather than through mechanical means. The photoresist was a diluted version of a 193-nm material, which had a 157-nm index of 1.74. An analysis of the trade-off between fluid index, absorption coefficient, gap size and throughput indicated that, among the currently available materials, employing a high-index but absorbing fluid is preferable to using a highly transparent but low-index immersion media.

A variable-transmittance apodizing filter at 157 nm.

A variable-transmittance apodizing filter has been designed and demonstrated at 157 nm. The Gaussian transmission function is created by flowing oxygen gas, which is absorptive below 185 nm, between the two spherical surfaces of meniscus lenses. By varying the oxygen partial pressure, the degree of apodization can be controlled.