Flat Optics Research Articles

Programmable Wavefront Control in the Visible Spectrum Using Low-Loss Chalcogenide Phase-Change Metasurfaces.

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change materials (PCMs), which provide fast, low energy, and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid-infrared and near-infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible optical modulation. In this work, a programmable all-dielectric Huygens' metasurface made of antimony sulfide (Sb2 S3 ) PCM is experimentally demonstrated, a low loss and high-index material in the visible spectral range with a large contrast (≈0.5) between its amorphous and crystalline states. ≈2π phase modulation is shown with high associated transmittance and it is used to create programmable beam-steering devices. These novel chalcogenide PCM metasurfaces have the potential to emerge as a platform for next-generation spatial light modulators and to impact application areas such as programmable and adaptive flat optics, light detection and ranging (LiDAR), and many more.

Chiral liquid crystal based holographic reflective lens for spectral detection.

Flat optics based on chiral liquid crystal (CLC) can be achieved using holographic polarization recording with the help of a photoalignment technique to vary the orientation of the optical axis in a thin CLC layer. A variety of reflective diffractive optical components with high efficiency and polarization selectivity can be realized employing this technique. In this work we discuss the use of CLC diffractive lenses in a spectrometer. The functionalities of two mirrors and a linear grating used in a traditional spectrometer are combined into a single holographic CLC component. Circularly polarized light entering through the slit can be reflected and projected onto a linear detector by the CLC component, with over 90% efficiency. This excellent optical functionality can be achieved with a micrometer thin CLC layer, offering the opportunity for device integration.

Graphene metasurfaces for terahertz wavefront shaping and light emission [Invited

Graphene is a promising materials platform for metasurface flat optics at terahertz wavelengths, with the important advantage of active tunability. Here we review recent work aimed at the development of tunable graphene metasurfaces for THz wavefront shaping (including beam-steering metamirrors and metalenses) and light emission. Various design strategies for the constituent meta-units are presented, ranging from metallic phase-shifting elements combined with a nearby graphene sheet for active tuning to graphene plasmonic resonators providing the required phase control or radiation mechanism. The key challenge in the development of these devices, related to the limited radiative coupling of graphene plasmonic excitations, is discussed in detail together with recently proposed solutions. The resulting metasurface technology can be expected to have a far-reaching impact on a wide range of device applications for THz imaging, sensing, and future wireless communications.

Control of Orbital Angular Momentum Regimen by Modulated Metasurface Leaky-Wave Antennas

In this paper, we present the design procedure of modulated metasurface leaky-wave antennas (MMLWAs) to generate coaxial superposition of vortex beams with several orbital angular momentum (OAM) states. Based on the Flat Optics (FO) technique and aperture field estimation (AFE) method, an analytical framework is proposed to facilitate the implementation of MMLWAs generating multiple topological charges in the OAM regimen. Furthermore, using the spectral analysis which has been derived from the proposed model, we have shown that the symmetry of aperture shape can affect the purity of the mode. Also the perfectly symmetric circular shape is introduced as an ideal choice for high-purity vortex generation. This single aperture antenna with embedded monopole feed can be an appropriate alternative to more complex vortex beam generators such as spiral phase plates and circular antenna arrays.

Static Beam Steering by Applying Metasurfaces on Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting laser (PCSEL) holds promise for laser beam with high efficacy while retain the small divergence angle, being now all the rage as laser source in modern opto-electronic technologies to replace edge emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). Arbitrary beam shaping and deflection of VCSELs with ultra-thin flat optics, aka metasurfaces, has recently demonstrated, making possibility to realize a compact system in many technologies. However, the large divergence angle and limited output power at single fundamental mode of the VCSEL limits its application. Here, we report the static steering of laser beam with high efficiency and near 1° divergence angle by shining the laser beam of the PCSEL through the metasurface array placed on top. The small divergence angle is contributed from PCSEL. On the other hand, the metasurface array offers the higher efficiency beam deflection, without considering additional collimating or focusing. This provides highly flexible platform and is capable to manipulate the laser emission through the design of the metasurfaces and the PCSELs. Moreover, the beam proprieties can be further modified for specific application through the metasurfaces. The integration of metasurface with PCSEL provides the compact and collimated laser source, promising more accurate generation of versatile laser sources on demand for automotive ultra-compact light detection and ranging (LiDAR), depth sensing, optical communication, etc.

Metasurface optofluidics for dynamic control of light fields.

The ability to manipulate light and liquids on integrated optofluidics chips has spurred a myriad of important developments in biology, medicine, chemistry and display technologies. Here we show how the convergence of optofluidics and metasurface optics can lead to conceptually new platforms for the dynamic control of light fields. We first demonstrate metasurface building blocks that display an extreme sensitivity in their scattering properties to their dielectric environment. These blocks are then used to create metasurface-based flat optics inside microfluidic channels where liquids with different refractive indices can be directed to manipulate their optical behaviour. We demonstrate the intensity and spectral tuning of metasurface colour pixels as well as on-demand optical elements. We finally demonstrate automated control in an integrated meta-optofluidic platform to open up new display functions. Combined with large-scale microfluidic integration, our dynamic-metasurface flat-optics platform could open up the possibility of dynamic display, imaging, holography and sensing applications.

Versatile optical beam routers based on inversely designed supercell metagratings

Metagratings manipulating the wavefront with a subwavelength volume have been widely explored in recent years for their potential applications in optical beam routers, such as laser beam splitters and combiners. Current metagratings are hampered from practical beam rerouting due to inhibitive computational load of engineering design and expensive nanofabrication cost. In this work, we apply the emerging concept of supercell metagratings, which arranges random binary gratings of a unit cell in a one-dimensional periodicity, to simplify such design and fabrication processes. A high-performance inverse design framework for the supercell gratings is proposed. In such a framework, we use coupled wave algorithm as a fast-converging electromagnetic solver for the forward calculation and particle swarm optimization algorithm for the backward optimization. By manipulating the building blocks of a unit cell, specific diffraction orders of supercell gratings can be intentionally strengthened or eliminated. We finally realize high-efficiency, broadband and multi-functionality optical beam splitters and combiners both in visible and near-infrared regimes. We envision that the proposed method offers an avenue towards the mass production realization of flat optics.

Inverse design in flat optics

Due to the sub-wavelength resolution, compact size, and ease of integration, flat optics has emerged as a pivotal player for applications in diffractive metalens, structured-light generation, holograms, nonlocal metasurfaces, etc. In the design of flat optics devices, the traditional forward design method demonstrates insurmountable limitations while inverse design has developed into an indispensable approach to focus designers only on high-level optical functionalities while handing over the task of finding conforming structures to machines. Here, we outline a cross-section of main advances of inverse design in flat optics: moving from basic theories and algorithms to further developments and diversified applications.

Ultrafast Terahertz Self-Induced Absorption and Phase Modulation on a Graphene-Based Thin Film Absorber

Interaction of intense terahertz (THz) waves with graphene based modulation devices holds great potential for the optoelectronic applications of the future. Here, we present a thin film graphene-based THz perfect absorbing device whose absorption and phase characteristics can be modulated through THz self-actions in the subps time scale. The device consists of a single-layer graphene placed on an ionic liquid substrate, back-plated by a metallic back-reflector, with the graphene doping level mediated through electrostatic gating. We experimentally record an absorption modulation of more than 3 orders of magnitude from the initial perfect absorption state, when the device is illuminated with THz field strengths in the range of 102 to 654 kV/cm. Furthermore, an absolute phase modulation of 130° is recorded. Detailed theoretical analysis indicates that the origin of the THz nonlinear response is the THz-induced heating of the graphene’s carriers that leads to a reduction of its conductivity and, consequently, to reduced absorption of the THz radiation. Our analysis also maps the temporal dynamics of the THz-induced temperature elevation of the graphene’s carriers, illustrating the ultrafast, subps nature of the overall process. These results can find applications in future dynamically controlled flat optics and spatiotemporal shaping of intense THz electric fields.

Compound Meta-Optics for Complete and Loss-Less Field Control.

Optical metasurfaces offer a compact platform for manipulation of the amplitude, phase, and polarization state of light. Independent control over these properties, however, is hindered by the symmetric transmission matrix associated with single-layer metasurfaces. Here, we utilize multilayer birefringent meta-optics to realize high-efficiency, independent control over the amplitude, phase, and polarization state of light. High-efficiency control is enabled by redistributing the wavefront between cascaded metasurfaces, while end-to-end inverse design is used to realize independent complex-valued functions for orthogonal polarization states. Based on this platform, we demonstrate spatial mode division multiplexing, optical mode conversion, and universal vectorial holograms, all with diffraction efficiencies over 80%. This meta-optic platform expands the design space of flat optics and could lead to advances in optical communications, quantum entanglement, and information encryption.

Flat-optics generation of broadband photon pairs with tunable polarization entanglement.

The concept of "flat optics" is quickly conquering different fields of photonics, but its implementation in quantum optics is still in its infancy. In particular, polarization entanglement, strongly required in quantum photonics, is so far not realized on "flat" platforms. Meanwhile, relaxed phase matching of "flat" nonlinear optical sources enables enormous freedom in tailoring their polarization properties. Here we use this freedom to generate photon pairs with tunable polarization entanglement via spontaneous parametric downconversion (SPDC) in a 400-nm GaP film. By changing the pump polarization, we tune the polarization state of photon pairs from maximally entangled to almost disentangled, which is impossible in a single bulk SPDC source. Polarization entanglement, together with the broadband frequency spectrum, results in an ultranarrow (12 fs) Hong-Ou-Mandel effect and promises extensions to hyperentanglement.

Diffractive Nonlocal Metasurfaces

AbstractMetasurfaces are ushering in an era of multifunctional control over optical wavefronts realized with ultrathin planarized devices. Recent advances have been enabling unprecedented control over the frequency response of these surfaces, suggesting that the future of flat optics may tailor both spectral and spatial degrees of freedom in highly multispectral and multifunctional devices. Diffractive nonlocal metasurfaces are opening new opportunities in this direction: they leverage symmetry‐protected scattering from quasi‐bound states in the continuum and, by spatially manipulating controlled geometric perturbations, they support ultrasharp optical responses with wavefront‐manipulating features. Encoded in nonlocal (i.e., spatially extended) resonant modes, the resulting response is observed exclusively within the bandwidth of the resulting Fano resonance, affording ideal features for a wide range of applications. In this perspective, this novel class of metasurfaces are discussed in the broader context of flat optics, highlighting their peculiar operation in contrast to relevant predecessors, and highlighting the opportunities for future advancement and applications. In particular, it is emphasized that nonlocality and selectivity are inherently related, but that spectral and spatial selectivity can be independently tuned in suitably tailored metasurfaces. In turn, this freedom allows the design and implementation of both wavefront‐shaping and wavefront‐selective devices. The novel optical responses, combined with the compatibility with rational design, herald new prospects for active, nonlinear and quantum metasurfaces, ultrathin devices for augmented reality, and compact tailored optical sources.

Incoherent Optoelectronic Differentiation Based on Optimized Multilayer Films

AbstractFourier‐based optical computing operations, such as spatial differentiation, have recently been realized in compact form factors using flat optics. Experimental demonstrations, however, have been limited to coherent light requiring laser illumination and leading to speckle noise and unwanted interference fringes. Here, the use of an optimized multilayer film, combined with dual color image subtraction, is demonstrated to realize differentiation with unpolarized incoherent light. Global optimization is achieved by employing neural networks combined with the reconciled level set method to optimize the optical transfer function of a multilayer film at wavelengths of 532 and 633 nm. Spatial differentiation is then achieved by subtracting the normalized incoherent images at these two wavelengths. The optimized multilayer film is experimentally demonstrated to achieve incoherent differentiation with a resolution of 6.2 m. The use of a multilayer film allows for scalable lithography‐free fabrication and results in a system that could open the door to high‐speed processing for a wide variety of incoherent, and coherent, imaging systems.

Electrical modulation of high-Q guided-mode resonances using graphene

Subwavelength dielectric resonators have been demonstrated to provide an authentic platform to excite high-Q resonances in flat optics. Here, we present a strategy to modulate the high-Q resonances electrically using graphene. In the proposed strategy, the high-Q resonance is invoked through the use of a lateral guided-mode resonance that is spectrally aligned with a vertical thin film interference, while graphene is employed to engineer the overall absorption rate. This configuration allows for near unity modulations in both the transmission and the reflection simultaneously. Theoretical analysis shows that the excitation of the high-Q guided-mode resonance is dictated by the electrically tunable interband absorption in graphene while the radiative coupling channel remains intact. In particular, it is shown that the electrical modulation of the high-Q guided-mode resonance facilitates remarkably efficient light modulation without being limited by the graphene quality.

Artificial Intelligence in Meta-optics.

Recent years have witnessed promising artificial intelligence (AI) applications in many disciplines, including optics, engineering, medicine, economics, and education. In particular, the synergy of AI and meta-optics has greatly benefited both fields. Meta-optics are advanced flat optics with novel functions and light-manipulation abilities. The optical properties can be engineered with a unique design to meet various optical demands. This review offers comprehensive coverage of meta-optics and artificial intelligence in synergy. After providing an overview of AI and meta-optics, we categorize and discuss the recent developments integrated by these two topics, namely AI for meta-optics and meta-optics for AI. The former describes how to apply AI to the research of meta-optics for design, simulation, optical information analysis, and application. The latter reports the development of the optical Al system and computation via meta-optics. This review will also provide an in-depth discussion of the challenges of this interdisciplinary field and indicate future directions. We expect that this review will inspire researchers in these fields and benefit the next generation of intelligent optical device design.

Temporal and spatial tuning of optical constants in praseodymium doped ceria by electrochemical means

Abstract Temporal and spatial tuning of the refractive index of optical thin films is desired for flat optics applications. The redistribution of mobile ions in mixed ionic-electronic conductors (MIEC) has been demonstrated to serve as a viable means for achieving optical tuning down to the nanoscale. Here we studied the dynamic range of the optical tuning achievable in the refractive index, in the MIEC oxide – Pr x Ce1−x O2−δ (PCO), for x = 0.1, 0.2 and 0.4, at 500 °C, by in-situ spectrophotometry. Significant increases in the modulation of both the imaginary and real optical constants in the visible and the adjacent spectra were obtained for increased doping levels. Device employing an electrochemical titration method was implemented to modulate the oxygen concentration, and thereby the optical transmission of PCO. Incorporation of a patterned top electrode allowed for the demonstration of spatial control of PCO thin film properties by in-situ video imaging of the optical switching process. The electrochemically induced optical state is shown to remain non-volatile upon quenching the device to room temperature under applied bias.

Active Ferromagnetic Metasurface with Topologically Protected Spin Texture for Spectral Filters

AbstractElectromagnetic metasurfaces modulate a material's response to electromagnetic waves by specifically arranged elements with dimensions below the wavelength. They have opened new fields of research, including flat optics and nanophotonics on a chip. Ferromagnetic metasurfaces could become the building blocks for manipulation of both microwaves and spin waves (magnons). So far, the functionality of magnonic devices has been limited by high intrinsic damping of the materials employed, suppressing long‐distance spin‐wave propagation. Here ferromagnetic metasurfaces are reported, which are created from periodic arrays of either 15 nm thick Co20Fe60B20, Ni80Fe20 or Co nanodisks on ferrimagnetic yttrium iron garnet (YIG) hosting topologically protected vortex states. This device, a reconfigurable spectral filter, operates in the microwave regime near 0.9 GHz and manipulates long‐distance spin‐wave transmission in thin YIG. An efficiency of 98.5% is demonstrated, with the metasurface covering only 15% of the microwave antenna. This first demonstration of a ferromagnetic metasurface opens unprecedented possibilities for on‐chip control of microwaves in low‐damping ferrimagnetic insulators.

Review of Metasurfaces and Metadevices: Advantages of Different Materials and Fabrications.

Flat optics, metasurfaces, metalenses, and related materials promise novel on-demand light modulation within ultrathin layers at wavelength scale, enabling a plethora of next-generation optical devices, also known as metadevices. Metadevices designed with different materials have been proposed and demonstrated for different applications, and the mass production of metadevices is necessary for metadevices to enter the consumer electronics market. However, metadevice manufacturing processes are mainly based on electron beam lithography, which exhibits low productivity and high costs for mass production. Therefore, processes compatible with standard complementary metal–oxide–semiconductor manufacturing techniques that feature high productivity, such as i-line stepper and nanoimprint lithography, have received considerable attention. This paper provides a review of current metasurfaces and metadevices with a focus on materials and manufacturing processes. We also provide an analysis of the relationship between the aspect ratio and efficiency of different materials.

Gigahertz free-space electro-optic modulators based on Mie resonances

Electro-optic modulators are essential for sensing, metrology and telecommunications. Most target fiber applications. Instead, metasurface-based architectures that modulate free-space light at gigahertz (GHz) speeds can boost flat optics technology by microwave electronics for active optics, diffractive computing or optoelectronic control. Current realizations are bulky or have low modulation efficiencies. Here, we demonstrate a hybrid silicon-organic metasurface platform that leverages Mie resonances for efficient electro-optic modulation at GHz speeds. We exploit quasi bound states in the continuum (BIC) that provide narrow linewidth (Q = 550 at {lambda }_{{{{{{{{rm{res}}}}}}}}}=1594 nm), light confinement to the non-linear material, tunability by design and voltage and GHz-speed electrodes. Key to the achieved modulation of frac{{{Delta }}T}{{T}_{max }}=67 % are molecules with r33 = 100 pm/V and optical field optimization for low-loss. We demonstrate DC tuning of the resonant frequency of quasi-BIC by {{Delta }}{lambda }_{{{{{{{{rm{res}}}}}}}}}= 11 nm, surpassing its linewidth, and modulation up to 5 GHz (fEO,−3dB = 3 GHz). Guided mode resonances tune by {{Delta }}{lambda }_{{{{{{{{rm{res}}}}}}}}}= 20 nm. Our hybrid platform may incorporate free-space nanostructures of any geometry or material, by application of the active layer post-fabrication.

Flat magic window

Magic windows (or mirrors) consist of optical devices with a surface deformation or thickness distribution devised in such a way to form a desired image. The associated image intensity distribution has been shown in previous works to be related to the Laplacian of the height of the surface relief. Exploiting the Laplacian theory to calculate the needed phase pattern, we experimentally realize such devices with flat optics employing optical polarization-wavefront coupling, which represent a new paradigm for light manipulation. The desired pattern and experimental specifications for designing the flat optics was implemented with a reconfigurable spatial light modulator, which acted as the magic mirror. The flat plate, an optical polarization-wavefront coupler, is then fabricated by spatially structuring nematic liquid crystals. The plate is used to demonstrate the concept of a polarization-switchable magic window, where, depending on the input circular polarization handedness, one can display either the desired image or the image resulting from the negative of the window’s phase.