Metasurfaces are planar structures that locally modify the polarization, phase and amplitude of light in reflection or transmission, thus enabling lithographically patterned flat optical components with functionalities controlled by design. Transmissive metasurfaces are especially important, as most optical systems used in practice operate in transmission. Several types of transmissive metasurface have been realized, but with either low transmission efficiencies or limited control over polarization and phase. Here, we show a metasurface platform based on high-contrast dielectric elliptical nanoposts that provides complete control of polarization and phase with subwavelength spatial resolution and an experimentally measured efficiency ranging from 72% to 97%, depending on the exact design. Such complete control enables the realization of most free-space transmissive optical elements such as lenses, phase plates, wave plates, polarizers, beamsplitters, as well as polarization-switchable phase holograms and arbitrary vector beam generators using the same metamaterial platform.

/pdf/dielectric-metasurfaces-for-complete-control-of-phase-and-3pa0oy7av0.pdf

Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission

Flat optical devices thinner than a wavelength promise to replace conventional free-space components for wavefront and polarization control. Transmissive flat lenses are particularly interesting for applications in imaging and on-chip optoelectronic integration. Several designs based on plasmonic metasurfaces, high-contrast transmitarrays and gratings have been recently implemented but have not provided a performance comparable to conventional curved lenses. Here we report polarization-insensitive, micron-thick, high-contrast transmitarray micro-lenses with focal spots as small as 0.57 λ. The measured focusing efficiency is up to 82%. A rigorous method for ultrathin lens design, and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. The micro-lenses, composed of silicon nano-posts on glass, are fabricated in one lithographic step that could be performed with high-throughput photo or nanoimprint lithography, thus enabling widespread adoption.

Subwavelength-thick Lenses with High Numerical Apertures and Large Efficiency Based on High Contrast Transmitarrays

This article reviews recent progress leading to the realization of planar optical components made of a single layer of phase shifting nanostructures. After introducing the principles of planar optics and discussing earlier works on subwavelength diffractive optics, we introduce a classification of metasurfaces based on their different phase mechanisms and profiles and a comparison between plasmonic and dielectric metasurfaces. We place particular emphasis on the recent developments on electric and magnetic field control of light with dielectric nanostructures and highlight the physical mechanisms and designs required for efficient all-dielectric metasurfaces. Practical devices of general interest such as metalenses, beam deflectors, holograms, and polarizing interfaces are discussed, including high-performance metalenses at visible wavelengths. Successful strategies to achieve achromatic response at selected wavelengths and near unity transmission/reflection efficiency are discussed. Dielectric metasurfaces and dispersion management at interfaces open up technology opportunities for applications including wavefront control, lightweight imaging systems, displays, electronic consumer products, and conformable and wearable optics.

Recent advances in planar optics: from plasmonic to dielectric metasurfaces

Multi-wavelength light is directed to an optic including a substrate and achromatic metasurface optical components deposited on a surface of the substrate. The achromatic metasurface optical components comprise a pattern of dielectric resonators. The dielectric resonators have nonperiodic gap distances between adjacent dielectric resonators; and each dielectric resonator has a width, w, that is distinct from the width of other dielectric resonators. A plurality of wavelengths of interest selected from the wavelengths of the multi-wavelength light are deflected with the achromatic metasurface optical components at a shared angle or to or from a focal point at a shared focal length.

/pdf/achromatic-metasurface-optical-components-by-dispersive-47jfe0x7a1.pdf

Achromatic Metasurface Optical Components by Dispersive Phase Compensation

Developing light-harvesting materials with tunable emission colours has always been at the forefront of colour display technologies. The variation in materials composition, phase and structure can provide a useful tool for producing a wide range of emission colours, but controlling the colour gamut in a material with a fixed composition remains a daunting challenge. Here, we demonstrate a convenient, versatile approach to dynamically fine-tuning emission in the full colour range from a new class of core-shell upconversion nanocrystals by adjusting the pulse width of infrared laser beams. Our mechanistic investigations suggest that the unprecedented colour tunability from these nanocrystals is governed by a non-steady-state upconversion process. These findings provide keen insights into controlling energy transfer in out-of-equilibrium optical processes, while offering the possibility for the construction of true three-dimensional, full-colour display systems with high spatial resolution and locally addressable colour gamut.

Temporal full-colour tuning through non-steady-state upconversion

Multiview three-dimensional (3D) displays can project the correct perspectives of a 3D image in many spatial directions simultaneously They provide a 3D stereoscopic experience to many viewers at the same time with full motion parallax and do not require special glasses or eye tracking None of the leading multiview 3D solutions is particularly well suited to mobile devices (watches, mobile phones or tablets), which require the combination of a thin, portable form factor, a high spatial resolution and a wide full-parallax view zone (for short viewing distance from potentially steep angles) Here we introduce a multi-directional diffractive backlight technology that permits the rendering of high-resolution, full-parallax 3D images in a very wide view zone (up to 180 degrees in principle) at an observation distance of up to a metre The key to our design is a guided-wave illumination technique based on light-emitting diodes that produces wide-angle multiview images in colour from a thin planar transparent lightguide Pixels associated with different views or colours are spatially multiplexed and can be independently addressed and modulated at video rate using an external shutter plane To illustrate the capabilities of this technology, we use simple ink masks or a high-resolution commercial liquid-crystal display unit to demonstrate passive and active (30 frames per second) modulation of a 64-view backlight, producing 3D images with a spatial resolution of 88 pixels per inch and full-motion parallax in an unprecedented view zone of 90 degrees We also present several transparent hand-held prototypes showing animated sequences of up to six different 200-view images at a resolution of 127 pixels per inch

A multi-directional backlight for a wide-angle, glasses-free three-dimensional display

Dielectric high-contrast sub-wavelength grating (SWG) structures have received much attention in recent years, offering a new paradigm for the integration of optical systems. Their nanoscale resonant properties can result in a complex and unintuitive far-field behavior that, if carefully crafted, allows the full control of the optical phase front from a thin sub-wavelength planar layer. To date, experimental demonstrations of these new devices have only been realized with polarized light in a reflective mode, greatly limiting their use for practical systems. In this letter, we demonstrate a highly efficient, sub-wavelength thick, transmissive grating lens configuration using symmetrical resonant posts to achieve polarization-independent operation. Our transmissive SWG lenses are easily fabricated using low-cost scalable semiconductor process technology. To illustrate their performance, we demonstrate the generation of high-order orbital angular momentum beams and their use in an optical mode-isolator application that achieves a suppression ratio of over 25 dB.

Sub-Wavelength Grating Lenses With a Twist

We utilize cross-phase modulation to observe all-optical switching in microring resonators fabricated with hydrogenated amorphous silicon (a-Si:H). Using 2.7-ps pulses from a mode-locked fiber laser in the telecom C-band, we observe optical switching of a cw telecom-band probe with full-width at half-maximum switching times of 14.8 ps, using approximately 720 fJ of energy deposited in the microring. In comparison with telecom-band optical switching in undoped crystalline silicon microrings, a-Si:H exhibits substantially higher switching speeds due to reduced impact of free-carrier processes.

Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators

We present a low-cost light-weight wristband Vital as the basic element for emergency response network, which senses and reports hazardous surroundings for people who need immediate assistance such as children and seniors. Based on a multi-sensor Arduino microsystem and a low-power Bluetooth 4.1 module [1], the Vital band samples data from multiple sensors and reports to a base station, such as the guardian's cell phone or the emergency response network, for an emergency situation. With a 3D-printed case and a lithium-ion coin battery, the Vital case is weighed 13 grams and sized 35 × 30 × 11 mm3. With 10 ms short sampling pulses and adaptive sampling rates, the estimated battery lifetime can be extended to more than 100 hours for activity tracking.

Wristband Vital: A wearable multi-sensor microsystem for real-time assistance via low-power Bluetooth link

An optical coupling system includes an optical signal source to provide an optical signal from an aperture. The system also includes a substantially planar high-contrast grating (HCG) lens to convert an optical mode of the optical signal to provide a converted optical signal having a mode-isolating intensity profile. The system further includes an optical element to receive the converted optical signal. The optical signal source and the substantially planar HCG lens can be arranged to substantially mitigate coupling of a reflected optical signal associated with the converted optical signal that is reflected from the optical element to the aperture of the optical signal source based on a reflected mode-isolating intensity profile.

Sonny Vo

Papers

A multi-directional backlight for a wide-angle, glasses-free three-dimensional display

Sub-Wavelength Grating Lenses With a Twist

Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators

Wristband Vital: A wearable multi-sensor microsystem for real-time assistance via low-power Bluetooth link

Mode conversion for optical isolation