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

In the past two decades, there is an increasing interest in developing new infrared photodetectors based on novel
nanostructures, such as quantum well infrared photodetector (QWIP) and quantum dot infrared photodetector (QDIP).
However, the commonly used electrical read-out approach limits the resolution of QWIP/QDIP infrared imaging to
around 1 mega pixel. In this paper, we reported our theoretical study on an all-optical readout based on quantum dot
phase modulation, which provides a new way for the intersubband infrared detection by measuring the phase change in
the transmitted interband near infrared (NIR) and allows a high-resolution middle infrared (MIR) or far infrared (FIR)
imaging. Utilizing the long life time in the quantum dots, the intersubband infrared resonant light is used to control the
interband NIR resonant light phase. An infrared image can be converted into a visible or near infrared image, which can
be easily captured with a high resolution CCD camera. It provides a new way to obtain a high resolution infrared image.

Infrared imaging based on quantum dot optical phase modulation

A scheme of a novel hybrid integrated micro spectrometer is proposed, which greatly reduces the number of optical units used in the system and can easily realize the integration of the optical unit and the detecting array. At the same time, the effective area of the grating is increased. One model of such device is fabricated. It is about 60mm×40mm×40mm and the volume can be further reduced. In experiments, the spectrum signal of Hg lamp and tungsten lamp is obtained. The result shows that a resolution of 7nm is achieved when a single mode fiber is used as the light input device.

Novel micro fiber spectrometer

The invention provides a far-field sub-diffraction limited focusing lens based on a medium-metal bar-type structure array. The far-field sub-diffraction limited focusing lens comprises a base, a medium bar-type structure unit and a metal bar-type structure unit, wherein the medium bar-type structure with multi-value phase regulation capability and the metal bar-type structure with two-value vibration amplitude regulation capability are adopted, the phase of emergent lights is determined by the thickness of the medium bar-type structure, and the vibration amplitude of the emergent lights is determined by the thickness of the metal bar-type structure; by virtue of changing the thickness of medium bars and metal bars, multi-value phase regulation and two-value vibration amplitude are realized; by virtue of a spatial planar array formed by metal bar structure units and metal bar structure units, lens transmission function vibration amplitude-phase spatial distribution needed for far-field sub-diffraction limited focusing is realized, so that a far-field sub-diffraction limited focusing function capable of breaking a diffraction limit is realized, far-field sub-diffraction limited focusing spots are reduced, the focusing efficiency is improved, sidelobes are inhibited and the field range is enlarged.

Far-field sub-diffraction limited focusing lens based on medium-metal bar-type structure array

Nondiffracting light beams receive substantial attention in various areas due to their properties of diffractionless propagation and achievable subwavelength beam size. Extending the propagation distance and compressing the beam size of nondiffracting beams are crucial to their practical applications. However, it remains challenging to achieve nondiffracting light beams with a subwavelength subdiffracting transverse size and a long propagation distance with controllable intensity profiles in both transverse and longitudinal directions. Here, we propose an optimization method to generate an ultralong-superoscillation nondiffracting light beam with full control of the beam intensity profile. A 45\ensuremath{\lambda}-long-superoscillation nondiffracting light beam, the transverse sizes of which are within the range of 0.37\ensuremath{\lambda}--0.4\ensuremath{\lambda} along the propagation axis, is experimentally demonstrated by a metalens based on continuous phase modulation using geometrical phase metasurfaces. Our method provides an attractive way to controllably generate ultralong-superoscillation nondiffracting beams on a subwavelength scale and might find fascinating applications in optical manipulation, materials processing, data storage, microscopy, and spectroscopy. 

Generating Ultralong-Superoscillation Nondiffracting Beams with Full Control of the Intensity Profile

The far-field resolution of optical imaging systems is restricted by the Abbe diffraction limit, a direct result of the wave nature of light. One successful technological approach to circumventing this limit is to reduce the effective size of a point-spread-function. In the past decades, great endeavors have been made to engineer an effective point-spread-function by exploiting different mechanisms, including optical nonlinearities and structured light illumination. However, these methods are hard to be applied to objects in a far distance. Here, we propose a new way to achieve super-resolution in a far field by utilizing angular magnification. We present the first proof-of-concept demonstration of such an idea and demonstrate a new class of lenses with angular magnification for far-field super-resolution imaging. Both theoretical and experimental results demonstrate a more than two-fold enhancement beyond the angular-resolution limit in the far-field imaging. The proposed approach can be applied to super-resolution imaging of objects in far distance. It has promising potential applications in super-resolution telescopes and remote sensing.

Gang Chen

Papers

Infrared imaging based on quantum dot optical phase modulation

Novel micro fiber spectrometer

Far-field sub-diffraction limited focusing lens based on medium-metal bar-type structure array

Generating Ultralong-Superoscillation Nondiffracting Beams with Full Control of the Intensity Profile

Super-Resolution Imaging via Angular Magnification