We propose a general topology-optimization framework for metasurface inverse design that can automatically discover highly complex multilayered metastructures with increased functionalities. In particular, we present topology-optimized multilayered geometries exhibiting angular phase control, including a single-piece nanophotonic metalens with angular aberration correction, as well as an angle-convergent metalens that focuses light onto the same focal spot regardless of the angle of incidence.
Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.
Free-running Fabry-Perot lasers normally operate in a single-mode regime until the pumping current is increased beyond the single-mode instability threshold, above which they evolve into a multimode state. As a result of this instability, the single-mode operation of these lasers is typically constrained to few percents of their output power range, this being an undesired limitation in spectroscopy applications. In order to expand the span of single-mode operation, we use an optical injection seed generated by an external-cavity single-mode laser source to force the Fabry-Perot quantum cascade laser into a single-mode state in the high current range, where it would otherwise operate in a multimode regime. Utilizing this approach, we achieve single-mode emission at room temperature with a tuning range of 36 cm(-1) and stable continuous-wave output power exceeding 1W at 4.5 mu m. Far-field measurements show that a single transverse mode is emitted up to the highest optical power, indicating that the beam properties of the seeded Fabry-Perot laser remain unchanged as compared to free-running operation. Published by AIP Publishing.
Self-starting harmonic frequency combs in quantum cascade lasers exhibit skipping of several tens of longitudinal modes of the cavity, producing widely spaced frequency combs which may be used for a number of applications, such as the generation of high-spectral-purity microwave and terahertz tones. Under pure electrical injection, the spacing of such combs is fixed by fundamental laser parameters and can hardly be controlled. Here, we demonstrate that harmonic frequency combs in quantum cascade lasers can be induced by optical injection of an external seed provided by a tunable source. This scheme enables wide tunability of the harmonic comb spacing, allowing the skipping between 44 and 171 longitudinal modes in a single device. Published by AIP Publishing.
In this Letter, we experimentally report an achromatic metalens (AML) operating over a continuous bandwidth in the visible. This is accomplished via dispersion engineering of dielectric phase shifters: titanium dioxide nanopillars tiled on a dielectric spacer layer above a metallic mirror. The AML works in reflection mode with a focal length independent of wavelength from lambda = 490 to 550 nm. We also design a metalens with reverse chromatic dispersion, where the focal length increases as the wavelength increases, contrary to conventional diffractive lenses. The ability to engineer the chromatic dispersion of metalenses at will enables a wide variety of applications that were not previously possible. In particular, for the AML design, we envision applications such as imaging under LED illumination, fluorescence, and photoluminescence spectroscopy.
V-shaped nanoantennas are among the popular choices for the unit element of a metasurface, a nanostructured surface used for its ability to mold and control the wavefront of light. In general, the motivation for choosing the V-antenna as the unit element comes from its bimodal nature, where the introduction of the second mode offers extra control over the scattered wavefronts. Here, through near-field scanning optical microscopy, we study a 1D metastructure comprised of V-antennas in the context of generating asymmetric surface plasmon polariton (SPP) wavefronts. The key point is that the use of the V-antenna allows for the creation of a two-dimensional phase gradient with a single line of antennas, where the extra phase dimension offers additional control and allows for asymmetric features. Two different asymmetries are created: (1) SPP wavefronts that have different propagation directions on either side of the metastructure, and (2) SPP wavefront asymmetry through focusing: one side of the metastructure focuses SPP wavefronts, while the other side has diverging SPP wavefronts.
Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elements-SAM-toorbital angular momentum (OAM) converters-are based on the geometric phase and only permit the conversion of left-and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We designed a metasurface that converts left-and right-circular polarizations into states with independent values of OAM and designed another device that performs this operation for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.
Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiOx wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.
We examine the motion of periodically driven and optically tweezed microspheres in fluid and find a rich variety of dynamic regimes. We demonstrate, in experiment and in theory, that mean particle motion in 2D is rarely parallel to the direction of the applied force and can even exhibit elliptical orbits with non-zero orbital angular momentum. The behavior is unique in that it depends neither on the nature of the microparticles nor that of the excitation; rather, angular momentum is introduced by the particle's interaction with the anisotropic fluid and optical trap environment. Overall, we find this motion to be highly tunable and predictable.
Efficient suppression of reflection is a key requirement for perfect absorption of light. Recently, it has been shown that reflection can be effectively suppressed utilizing a single ultrathin film deposited on metals or polar materials featuring phonon resonances. The wavelength at which reflection can be fully suppressed is primarily determined by the nature of these substrates and is pinned to particular values near plasma or phonon resonances-the former typically in the ultraviolet or visible and the latter in the infrared. Here, we explicitly identify the required optical properties of films and substrates for the design of absorbing antireflection coatings based on ultrathin films. We find that completely suppressed reflection using films with thicknesses much smaller than the wavelength of light occurs within a spectral region where the real part of the refractive index of the substrate is n less than or similar to 1, which is characteristic of materials with permittivity close to zero. We experimentally verify this condition by using an ultrathin vanadium dioxide film with dynamically tunable optical properties on several epsilon-near-zero materials, including aluminum-doped zinc oxide. By tailoring the plasma frequency of the aluminum-doped zinc oxide, we are able to tune the epsilon-near-zero point, thus achieving suppressed reflection and near-perfect absorption at wavelengths that continuously span the near-infrared and long-wave midinfrared ranges.
Bessel beams are of great interest due to their unique non-diffractive properties. Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams. However, the former approach has a limited numerical aperture (NA), and the latter suffers from low efficiency, as most of the incident light is blocked by the aperture. Furthermore, an additional phase-modulating element is needed to generate higher-order Bessel beams, which in turn adds complexity and bulkiness to the system. We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means. We demonstrate meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as similar to lambda/3 (lambda = 405 nm). Importantly, these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum. These meta-axicons can enable advanced research and applications related to Bessel beams, such as laser fabrication, imaging and optical manipulation.
High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 mu m, enabling the manipulation of light in the visible and infrared range.
Immersion objectives can focus light into a spot smaller than what is achievable in free space, thereby enhancing the spatial resolution for various applications such as microscopy, spectroscopy, and lithography. Despite the availability of advanced lens polishing techniques, hand-polishing is still required to manufacture the front lens of a high-end immersion objective, which poses major constraints for lens design. This limits the shape of the front lens to spherical. Therefore, several other lenses need to be cascaded to correct for spherical aberration, resulting in significant challenges for miniaturization and adding design complexity for different immersion liquids. Here, by using metasurfaces, we demonstrate liquid immersion meta-lenses free of spherical aberration at various design wavelengths in the visible spectrum. We report water and oil immersion meta-lenses of various numerical apertures (NA) up to 1.1 and show that their measured focal spot sizes are diffraction-limited with Strehl ratios of approximately 0.9 at 532 nm. By integrating the oil immersion meta-lens (NA = 1.1) into a commercial scanning confocal microscope, we achieve an imaging spatial resolution of approximately 200 nm. These meta-lenses can be easily adapted to focus light through multilayers of different refractive indices and mass-produced using modern industrial manufacturing or nanoimprint techniques, leading to cost-effective high-end optics.
Avalanche photodetectors (APDs) are key components in optical communication systems due to their increased photocurrent gain and short response time as compared to conventional photo-detectors. A detector design where the multiplication region is implemented in a large band gap material is desired to avoid detrimental Zener tunneling leakage currents, a concern otherwise in smaller band gap materials required for absorption at 1.3/1.55 mu m. Self-assembled III-V semiconductor nanowires offer key advantages such as enhanced absorption due to optical resonance effects, strain-relaxed hetero-2 structures, and compatibility with mainstream silicon technology. Here, we present electrical and optical characteristics of single InP and InP/InAsP nanowire APD structures. Temperature-dependent breakdown characteristics of p(+)-n-n(+) InP nanowire devices were investigated first. A clear trap-induced shift in breakdown voltage was inferred from I-V measurements. An improved contact formation to the p(+)-InP segment was observed upon annealing, and its effect on breakdown characteristics was investigated. The band gap in the absorption region was subsequently varied from pure InP to InAsP to realize spatially separate absorption and multiplication APDs in heterostructure nanowires. In contrast to the homojunction APDs, no trap-induced shifts were observed for the heterostructure APDs. A gain of 12 was demonstrated for selective optical excitation of the InAsP segment. Additional electron beam-induced current measurements were carried out to investigate the effect of local excitation along the nanowire on the I-V characteristics. Simulated band profiles and electric field distributions support our interpretation of the experiments. Our results provide important insight for optimization of avalanche photodetector devices based on III-V nanowires.
Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Delta/20 is shown (Delta being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.
Recently, developments in meta-surfaces have allowed for the possibility of a fundamental shift in lens manufacturing from the century-old grinding technology to nanofabrication opening a way toward mass producible high-end meta-lenses. Inspired by early camera lenses and to overcome the aberrations of planar single-layered meta-lenses, we demonstrate a compact meta lens doublet by patterning two metasurfaces on both sides of a substrate. This meta-lens doublet has a numerical aperture of 0.44, a focal length of 342.5 mu m, and a field of view of 50 that enables diffraction-limited monochromatic imaging along the focal plane at a wavelength of 532 mu. The compact design has various imaging applications in microscopy, machine vision, and computer vision.
Recent progress in metasurface designs fueled by advanced-fabrication techniques has led to the realization of ultrathin, lightweight, and flat lenses (metalenses) with unprecedented functionalities. Owing to straightforward fabrication, generally requiring a single-step lithography, and the possibility of vertical integration, these planar lenses can potentially replace or complement their conventional refractive and diffractive counterparts, leading to further miniaturization of high-performance optical devices and systems. Here we provide a brief overview of the evolution of metalenses, with an emphasis on the visible and near-infrared spectrum, and summarize their important features: diffraction-limited focusing, high-quality imaging, and multifunctionalities. We discuss impending challenges, including aberration correction, and also examine current issues and solutions. We conclude by providing an outlook of this technology platform and identifying promising directions for future research.