The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

Based on transformation optics, we introduce another set of generalized laws of reflection and refraction (differs from that of [Science 334, 333 (2011)]), through which a transformation media slab is derived as a meta-surface, producing anomalous reflection and refraction for all polarizations of incident light.

Generalized laws of reflection and refraction from transformation optics

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency, and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schrodinger equation and Schrodinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures.

Modeling techniques for quantum cascade lasers

With the rapid development of analytical techniques, it has become much easier to detect chemical and biological analytes, even at very low detection limits. In recent years, techniques based on vibrational spectroscopy, such as surface enhanced Raman spectroscopy (SERS), have been developed for non-destructive detection of pathogenic microorganisms. SERS is a highly sensitive analytical tool that can be used to characterize chemical and biological analytes interacting with SERS-active substrates. However, it has always been a challenge to obtain consistent and reproducible SERS spectroscopic results at complicated experimental conditions. Microfluidics, a tool for highly precise manipulation of small volume liquid samples, can be used to overcome the major drawbacks of SERS-based techniques. High reproducibility of SERS measurement could be obtained in continuous flow generated inside microfluidic devices. This article provides a thorough review of the principles, concepts and methods of SERS-microfluidic platforms, and the applications of such platforms in trace analysis of chemical and biological analytes.

https://iopscience.iop.org/article/10.1088/0957-4484/26/9/092001/pdf

Analytical characterization using surface-enhanced Raman scattering (SERS) and microfluidic sampling

The terahertz (THz) lens is an essential and strategic element of THz optical systems, while a conventional THz lens cannot even reach high resolution due to the diffraction limit Optical super-oscillation paves a way to generate sub-diffraction hotspots in the far field, and demonstrates the capacity for resolution improvement of microscopic imaging in the visible range However, there are few demonstrations of THz lenses for focusing hotspots or needles based on super-oscillation We propose and experimentally demonstrate a far-field sub-diffraction focusing planar lens, consisting of a sub-wavelength concentric ring structure array, for a wavelength of 1188 μm with focal length 420λ and radius 160λ Utilizing the silicon-etching process, a sub-diffraction focusing lens is fabricated The experimental results show that the planar lens can generate a sub-diffraction needle with length 197λ in the focal region along the optic axis Moreover, the smallest focal spot, with a transverse size of 1212λ, is smaller than the diffraction limit of 1476λ The proposed sub-diffraction optical needle planar lens can substitute for its traditional counterpart, and it has great potential in super-resolution tomography THz imaging systems

Realizing a terahertz far-field sub-diffraction optical needle with sub-wavelength concentric ring structure array.

The excitation wavelength for all-optical modulation of a 106 μm mid-infrared (MIR) quantum cascade laser (QCL) was varied in order to obtain maximum modulation depth Both amplitude and wavelength modulation experiments were conducted at 820 nm and 1550 nm excitation respectively, whereby the latter matches the interband transition in the QCL active region Experimental results show that for continuous-wave mode-operated QCL, the efficiency of free carrier generation is doubled under 1550 nm excitation compared with 820 nm excitation, resulting in an increase of the amplitude modulation index from 19% to 36% At the same time, the maximum wavelength shift is more than doubled from 105 nm to 280 nm Furthermore, for the first time to our knowledge, we demonstrated the optical switching of a QCL operated in pulse mode by simple variation of the excitation wavelength

Optical modulation of quantum cascade laser with optimized excitation wavelength.

Holographic Super-Resolution Metalens for Achromatic Sub-Wavelength Focusing

In recent years, graphene nanomesh (GNM), a material with high flexibility and tunable electronic properties, has attracted considerable attention from researchers due to its wide applications in the fields of nanoscience and nanotechnology. Herein, we have processed large-area, uniform arrays of rectangular graphene nanomesh (r-GNM) and circular graphene nanomesh (c-GNM) with different neck widths by electron beam lithography (EBL). The electronic properties of those high-quality GNM samples have been characterized systematically. Electrical measurements illustrated that top-gated field effect transistors with different neck widths of the GNM possessed different Ion/Ioff ratios. In particular, the devices based on r-GNM with a neck width of 30 nm were found to possess the largest Ion/Ioff ratio of ~ 100, and the band gap of the r-GNM was estimated to be 0.23 eV, which, to the best of authors’ knowledge, is the highest value for graphene ribbons or a GNM with a neck width under 30 nm. Furthermore, the terahertz response of large-area r-GNM devices based on the photoconductive effect was estimated to be 10 mA/W at room temperature. We also explored the practical application of terahertz imaging, showing that the devices can be used in a feasible setting with a response time < 20 ms; this enables accurate and fast imaging of macroscopic samples.

/pdf/the-fabrication-of-large-area-uniform-graphene-nanomeshes-16wh1bf63h.pdf

The Fabrication of Large-Area, Uniform Graphene Nanomeshes for High-Speed, Room-Temperature Direct Terahertz Detection.

Due to its unique properties of nondiffracting propagation, highly-localized intensity distribution, small beam cross-section, and self-healing, a nondiffracting beam is attractive for materials processing, microscopy, and optical research. Various methods have been investigated to generate such beams with conventional optics. However, the transverse size of those nondiffracting beams is restricted by the diffraction-limit. To overcome the diffraction limit, we use the concepts of super-oscillation and the vectorial angular spectrum method to design a phase mask mirror with a focal length of 1 m, radius of 5 mm, and numerical aperture of about 0.005 for a wavelength of 632.8 nm. The phase mask mirror was created with a phase spatial light modulator. Under the illumination of a linearly polarized Gaussian wave, a nondiffracting beam was created with sub-diffraction transverse size. The maximum transverse size of the beam is smaller than the diffraction limit of 0.5λ/NA for a propagation distance greater than 43.3 mm. A nondiffracting beam with smaller transverse size can be realized by further increasing the NA value.

Gang Chen

Papers

Realizing a terahertz far-field sub-diffraction optical needle with sub-wavelength concentric ring structure array.

Optical modulation of quantum cascade laser with optimized excitation wavelength.

Holographic Super-Resolution Metalens for Achromatic Sub-Wavelength Focusing

The Fabrication of Large-Area, Uniform Graphene Nanomeshes for High-Speed, Room-Temperature Direct Terahertz Detection.

Creating a nondiffracting beam with sub-diffraction size by a phase spatial light modulator