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

https://iopscience.iop.org/article/10.1088/1361-6463/ab46c4/pdf

Optimization-free approach for broadband achromatic metalens of high-numerical-aperture with high-index dielectric metasurface

Independent manipulation of phase and polarization of optical fields is of great interest in various applications, including vector-field generation, optical tweezers, and nanolithography. The integration of phase and polarization manipulation on a single optical device may greatly simplify optical systems and eases optical alignment. In this Letter, a family of reflective cross-shaped quarter-wave birefringent metasurfaces is proposed to achieve full control of polarization and phase of reflected waves. Based on the proposed metasurfaces, two meta-mirrors are designed with integrated functions of polarization conversion and sub-diffraction focusing. Numerical investigations also reveal the achromatic focusing performance of the two meta-mirrors. The proposed metasurfaces with independent manipulation of polarization and phase provide flexible building blocks for constructing complicated vector optical fields.

Broadband quarter-wave birefringent meta-mirrors for generating sub-diffraction vector fields

We present a self-consistent approach to simulate the output characteristics of a quantum cascade laser (QCL), such as the current-light (I-L) and current-voltage (I-V) curves. Unlike the conventional 1(1/2) period model, our new QCL model includes the spontaneous emission and the stimulated emission, which allow the numerical study of the laser behavior above its threshold. The corresponding numerical method is proposed to solve the full laser rate equations in a self-consistent way. For a given QCL, the I-L and I-V curves are calculated, showing a good agreement with the typical QCL output characteristics. Besides the output properties, this new model also provides a way to study the photon and electron dynamics in the QCL cavity above the laser threshold. An example of the QCL photon and electron response to the interband ultrafast optical excitation is given at the end of this paper. All simulation results show the validity of the new model.

Self-Consistent Approach for Quantum Cascade Laser Characteristic Simulation

Three-dimensional hollow spots have attracted significant interest for a wide variety of optical applications including optical trapping, simulated emission depletion microscopy, and optical tweezers. In this paper, an integrated superoscillatory metalens is proposed based on all-dielectric metasurfaces for independent control of azimuthally and radially polarized waves for broadband operation. The reported metalens integrates the functions of two independent lenses on single substrate, whose phase profiles are overlapped on the same plane and are optimized for independent modulation of azimuthal and radial components in vector cylindrically polarized waves. The independent manipulation of the two orthogonal polarizations allows for significantly reducing the transverse size of the 3D hollow spot. Numerical simulation shows the formation of a 3D hollow spot with inner sizes of 0.33 λ and 1.32 λ in the transverse and axial directions respectively. More importantly, the broadband performance of the proposed superoscillatory lens was also demonstrated with a bandwidth of 40 nm (from 612.8 nm to 652.8 nm). Such a superoscillatory lens is promising for applications of optical trapping, optical tweezers, and super-resolution microscopy.

https://iopscience.iop.org/article/10.1088/1361-6463/ab3210/pdf

Broadband integrated metalens for creating super-oscillation 3D hollow spot by independent control of azimuthally and radially polarized waves

Polarization is a significant factor in a great variety of optical phenomena, playing an important role in determining the focusing properties of lenses, in the resolution of optical systems, and in the performance during laser processing. Knowing the polarization distribution in focused light is critical to understanding and designing relevant optical devices and systems. However, it remains challenging to characterize the vectorial polarization distribution in optical fields. We develop a polarization-conversion-based optical microscope for directly acquiring the distribution of three orthogonal polarizations in focused light and theoretically prove and experimentally demonstrate its validity by characterizing super-resolution focused light with different incident polarizations.

Gang Chen

Papers

Optimization-free approach for broadband achromatic metalens of high-numerical-aperture with high-index dielectric metasurface

Broadband quarter-wave birefringent meta-mirrors for generating sub-diffraction vector fields

Self-Consistent Approach for Quantum Cascade Laser Characteristic Simulation

Broadband integrated metalens for creating super-oscillation 3D hollow spot by independent control of azimuthally and radially polarized waves

Polarization-conversion microscopy for imaging the vectorial polarization distribution in focused light