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Although the nonradiative decay of surface plasmons was once thought to be only a parasitic process within the plasmonic and metamaterial communities, hot carriers generated from nonradiative plasmon decay offer new opportunities for harnessing absorption loss. Hot carriers can be harnessed for applications ranging from chemical catalysis, photothermal heating, photovoltaics, and photodetection. Here, we present a review on the recent developments concerning photodetection based on hot electrons. The basic principles and recent progress on hot electron photodetectors are summarized. The challenges and potential future directions are also discussed.

Rapid developments in machine vision technology have impacted a variety of applications, such as medical devices and autonomous driving systems. These achievements, however, typically necessitate digital neural networks with the downside of heavy computational requirements and consequent high energy consumption. As a result, real-time decision-making is hindered when computational resources are not readily accessible. Here we report a meta-imager designed to work together with a digital back end to offload computationally expensive convolution operations into high-speed, low-power optics. In this architecture, metasurfaces enable both angle and polarization multiplexing to create multiple information channels that perform positively and negatively valued convolution operations in a single shot. We use our meta-imager for object classification, achieving 98.6% accuracy in handwritten digits and 88.8% accuracy in fashion images. Owing to its compactness, high speed and low power consumption, our approach could find a wide range of applications in artificial intelligence and machine vision applications. A metasurface-based approach is used to implement computationally expensive digital convolution operations in high-speed, low-power optics for improving the latency and power consumption of machine vision systems.

Metamaterials offer unprecedented flexibility for manipulating the optical properties of matter, including the ability to access negative index 1 , 2 , 3 , 4 , ultrahigh index 5 and chiral optical properties 6 , 7 , 8 . Recently, metamaterials with near-zero refractive index have attracted much attention 9 , 10 , 11 , 12 , 13 . Light inside such materials experiences no spatial phase change and extremely large phase velocity, properties that can be applied for realizing directional emission 14 , 15 , 16 , tunnelling waveguides 17 , large-area single-mode devices 18 and electromagnetic cloaks 19 . However, at optical frequencies, the previously demonstrated zero- or negative-refractive-index metamaterials have required the use of metallic inclusions, leading to large ohmic loss, a serious impediment to device applications 20 , 21 . Here, we experimentally demonstrate an impedance-matched zero-index metamaterial at optical frequencies based on purely dielectric constituents. Formed from stacked silicon-rod unit cells, the metamaterial has a nearly isotropic low-index response for transverse-magnetic polarized light, leading to angular selectivity of transmission and directive emission from quantum dots placed within the material. Previously demonstrated zero- or negative-refractive-index metamaterials at optical frequencies suffer from large ohmic losses because of the need to use metals. Metamaterials formed by stacked silicon rod unit cells allow the realization of all-dielectric impedance-matched zero-index metamaterials operating at optical frequencies, potentially benefiting the development of angular-selective optical devices.

Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO 2 ). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO 2 phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ  = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ 3 in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm 2 or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale. Here the authors develop a subwavelength nonreciprocal optical component harnessing the effect is thermal phase transition of VO2 boosted by the Mie resonant response of the dielectric meta-surface.

Tunnel junctions fabricated over a large area provide a platform for efficient generation and utilization of hot carriers.

Image processing has become a critical technology in a variety of science and engineering disciplines. Although most image processing is performed digitally, optical analog processing has the advantages of being low-power and high-speed, but it requires a large volume. Here, we demonstrate flat optics for direct image differentiation, allowing us to significantly shrink the required optical system size. We first demonstrate how the differentiator can be combined with traditional imaging systems such as a commercial optical microscope and camera sensor for edge detection with a numerical aperture up to 0.32. We next demonstrate how the entire processing system can be realized as a monolithic compound flat optic by integrating the differentiator with a metalens. The compound nanophotonic system manifests the advantage of thin form factor as well as the ability to implement complex transfer functions, and could open new opportunities in applications such as biological imaging and computer vision.Vertical integration of a metalens to realize compound nanophotonic systems for optical analog image processing is realized, significantly reducing the size and complexity of conventional optical systems.

Optical metasurfaces, composed of subwavelength scattering elements, demonstrate remarkable control over the transmitted amplitude, phase, and polarization of light. However, manipulating the amplitude upon transmission, without the use of surface waves, requires loss if a single metasurface is used. Herein, high‐efficiency independent manipulation of the amplitude and phase of a beam is described using two lossless phase‐only metasurfaces separated by a distance. With this configuration, optical components such as combined beam‐forming and splitting devices are experimentally demonstrated, as well as those for forming complex‐valued, 3D holograms. The compound meta‐optic platform provides a promising approach for achieving high‐performance holographic displays and compact optical components, while exhibiting a high overall efficiency. High‐efficiency independent manipulation of an optical beam amplitude and phase profile is experimentally shown using two phase‐only metasurfaces separated by a distance, forming a meta‐optic. A combined beam‐forming and splitting function and displaying 3D holograms are demonstrated. The meta‐optic platform provides a promising approach for applications requiring detailed field control, while exhibiting a high overall efficiency.

Metasurface analogues of electromagnetically induced transparency (EIT) have been a focus of the nanophotonics field in recent years, due to their ability to produce high-quality factor (Q-factor) resonances. Such resonances are expected to be useful for applications such as low-loss slow-light devices and highly sensitive optical sensors. However, ohmic losses limit the achievable Q-factors in conventional plasmonic EIT metasurfaces to values <~10, significantly hampering device performance. Here we experimentally demonstrate a classical analogue of EIT using all-dielectric silicon-based metasurfaces. Due to extremely low absorption loss and coherent interaction of neighbouring meta-atoms, a Q-factor of 483 is observed, leading to a refractive index sensor with a figure-of-merit of 103. Furthermore, we show that the dielectric metasurfaces can be engineered to confine the optical field in either the silicon resonator or the environment, allowing one to tailor light–matter interaction at the nanoscale. Electromagnetically induced transparency—an effect in atomic physics caused by interference between transitions—has found analogues in other areas, like nanophotonics. Yang et al . exploit this effect in an all-dielectric metasurface to produce high-Q-factor resonances ideal for refractive index sensing.

Previous demonstrations of cloaking, where objects are rendered invisible at certain frequencies, have been limited to the microwave regime. Moving us a significant step closer to invisibility in a region that can been seen by humans, a cloaking device has now been demonstrated for a broad range of frequencies in the near-infrared. Invisibility devices have captured the human imagination for many years. Recent theories have proposed schemes for cloaking devices using transformation optics and conformal mapping 1 , 2 , 3 , 4 . Metamaterials 5 , 6 , with spatially tailored properties, have provided the necessary medium by enabling precise control over the flow of electromagnetic waves. Using metamaterials, the first microwave cloaking has been achieved 7 but the realization of cloaking at optical frequencies, a key step towards achieving actual invisibility, has remained elusive. Here, we report the first experimental demonstration of optical cloaking. The optical ‘carpet’ cloak is designed using quasi-conformal mapping to conceal an object that is placed under a curved reflecting surface by imitating the reflection of a flat surface. The cloak consists only of isotropic dielectric materials, which enables broadband and low-loss invisibility at a wavelength range of 1,400–1,800 nm.

Circularly polarized light is utilized in various optical techniques and devices. However, using conventional optical systems to generate, analyse and detect circularly polarized light involves multiple optical elements, making it challenging to realize miniature and integrated devices. While a number of ultracompact optical elements for manipulating circularly polarized light have recently been demonstrated, the development of an efficient and highly selective circularly polarized light photodetector remains challenging. Here we report on an ultracompact circularly polarized light detector that combines large engineered chirality, realized using chiral plasmonic metamaterials, with hot electron injection. We demonstrate the detector’s ability to distinguish between left and right hand circularly polarized light without the use of additional optical elements. Implementation of this photodetector could lead to enhanced security in fibre and free-space communication, as well as emission, imaging and sensing applications for circularly polarized light using a highly integrated photonic platform. Analysis and detection of circularly polarized light involves the use of multiple optical elements. Here, the authors demonstrate an ultracompact circularly polarized light detector using chiral plasmonic metamaterials with hot electron injection, realizing its implementation on an integrated photonic platform.