Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

We have characterized the fundamental photoluminescence (PL) properties of individual, isolated indium phosphide (InP) nanowires to define their potential for optoelectronics. Polarization-sensitive measurements reveal a striking anisotropy in the PL intensity recorded parallel and perpendicular to the long axis of a nanowire. The order-of-magnitude polarization anisotropy was quantitatively explained in terms of the large dielectric contrast between these free-standing nanowires and surrounding environment, as opposed to quantum confinement effects. This intrinsic anisotropy was used to create polarization-sensitive nanoscale photodetectors that may prove useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and high-resolution detectors.

https://web.stanford.edu/group/cui_group/papers/Yi%20Cui-NW%20light%20emission.pdf

Highly polarized photoluminescence and photodetection from single indium phosphide nanowires.

We review progress in quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO3. Using the electric-field poling process, we can reliably fabricate 0.5-mm-thick crystals with uniform domain structures over a 15-mm length. Periodically poled material retains the low-loss and bulk power handling properties of single-domain LiNbO3, and quasi phase matching permits noncritical phase matching with d33, the highest-valued nonlinear coefficient. Optical parametric oscillators pumped by 1.064-μm pulsed Nd:YAG lasers have been operated over the wavelength range 1.4–4 μm with tuning by temperature or by quasi-phase-matched period. We have shown an oscillation threshold as low as 0.012 mJ with a Q-switched pump laser and pumping at greater than ten times threshold without damage. We have also demonstrated a cw doubly resonant oscillator near 1.96 μm pumped directly with a commercial cw diode laser at 978 nm.

/pdf/quasi-phase-matched-optical-parametric-oscillators-in-bulk-3gnsflghce.pdf

Quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO 3

Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Analysis and applications of optical diffraction by gratings

Optical Waveguide Theory

The selected research activities on integrated optics components and devices using periodic structures are reviewed, with emphasis on the authors' works employing the electron-beam writing technique. The periodic structures include static gratings and dynamic ones produced through acoustooptic (AO) and electrooptic (EO) effects. They provide a variety of passive functions and effective means for guided-wave control. The review is made from the integration point of view, including the most recent results. First, the theoretical fundamentals are outlined and the electron-beam writing techniques, including the writing system, are discussed. Next, passive components (grating deflectors, filters, lenses, couplers, etc.) and elements for guided-wave controlling and detecting (AO and EO grating elements and photodetectors) are described. Then, integrated optic devices, i.e., wavelength demultiplexers, RF spectrum analyzers, optical disk pickup, etc., are presented. Finally, the possibility of future applications is discussed.

Integrated optics components and devices using periodic structures

Theoretical analysis of second-harmonic generation (SHG) phase matched with gratings in a channel waveguide is presented from the viewpoint of device design. The analysis includes cases where efficiency is high and residual phase mismatch is involved. Nonlinear couples mode equations, with general expressions for effective SHG coefficients and coupling coefficients, are derived, solved, and discussed for uniform and chirped gratings. It was found that very high efficiency with broadened phase matching bandwidth can be simultaneously obtained with chirped gratings. The results are summarized in graphic data with normalized parameters, and guidelines and criterion for optimum design are given. >

Theoretical analysis of waveguide second-harmonic generation phase matched with uniform and chirped gratings

A monolithic-integrated optical-disk pickup device capable of detecting readout and focus/tracking error signals is proposed, designed, and fabricated. The integrated-optic disk pickup device (IODPU) is constructed by integrating a focusing grating coupler, a grating beam splitter, and photodiodes in a film waveguide on a Si substrate. The function of detecting readout and error signals is demonstrated by butt-coupling a laser diode to the fabricated IODPU.

An integrated-optic disk pickup device

Waveguide nonlinear-optic devices

The characteristics of the ferroelectric-domain-inverted gratings fabricated by applying voltage at room temperature in Z-cut LiNbO/sub 3/ crystal were examined for waveguide quasiphase-matched second-harmonic generation (QPM-SHG) devices. Conditions suitable for reproducible fabrication of uniform grating structure were clarified. Devices with uniform and fan-out grating for green and blue light generation were fabricated, and the performances were examined. A normalized SHG efficiency as high as 150%/W was obtained in blue light generation device with uniform grating of 3 mm interaction length.

Toshiaki Suhara

Papers

Integrated optics components and devices using periodic structures

Theoretical analysis of waveguide second-harmonic generation phase matched with uniform and chirped gratings

An integrated-optic disk pickup device

Waveguide nonlinear-optic devices

High-efficiency LiNbO/sub 3/ waveguide second-harmonic generation devices with ferroelectric-domain-inverted gratings fabricated by applying voltage