Explore the vast range of titles available.

Active metasurfaces promise reconfigurable optics with drastically improved compactness, ruggedness, manufacturability and functionality compared to their traditional bulk counterparts. Optical phase-change materials (PCMs) offer an appealing material solution for active metasurface devices with their large index contrast and non-volatile switching characteristics. Here we report a large-scale, electrically reconfigurable non-volatile metasurface platform based on optical PCMs. The optical PCM alloy used in the devices, Ge2Sb2Se4Te (GSST), uniquely combines giant non-volatile index modulation capability, broadband low optical loss and a large reversible switching volume, enabling notably enhanced light–matter interactions within the active optical PCM medium. Capitalizing on these favourable attributes, we demonstrated quasi-continuously tuneable active metasurfaces with record half-octave spectral tuning range and large optical contrast of over 400%. We further prototyped a polarization-insensitive phase-gradient metasurface to realize dynamic optical beam steering.An electrically reconfigurable optical metasurface using a Ge2Sb2Se4Te phase change material shows half an octave spectral tuning and promising performances for optical beam steering applications.

The development of compact and fieldable mid-infrared (mid-IR) spectroscopy devices represents a critical challenge for distributed sensing with applications from gas leak detection to environmental monitoring. Recent work has focused on mid-IR photonic integrated circuit (PIC) sensing platforms and waveguide-integrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus and tellurene. However, material bandgaps and reliance on SiO 2 substrates limit operation to wavelengths λ  ≲ 4 μm. Here we overcome these challenges with a chalcogenide glass-on-CaF 2 PIC architecture incorporating split-gate photothermoelectric graphene photodetectors. Our design extends operation to λ  = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz 1/2 , no fall-off in photoresponse up to f  = 1 MHz, and a predicted 3-dB bandwidth of f 3dB  > 1 GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications. Mid-infrared photonic integrated circuits (PICs) are important for sensing and optical communications, but their operational wavelengths are usually limited below 4  μ m. Here, the authors report the realization of photothermoelectric graphene photodetectors incorporated in a chalcogenide glass-on-CaF2 PIC operating at 5.2  μ m, showing promising results for gas sensing applications.

The flow of photoexcited electrons in a type-II heterostructure can be imaged with energy, spatial and temporal resolution. Technological progress since the late twentieth century has centred on semiconductor devices, such as transistors, diodes and solar cells 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied electric fields 3 , 9 or by the excitation of photocarriers 2 , 4 , 5 , 8 . Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. Here, by combining femtosecond pump–probe techniques with spectroscopic photoemission electron microscopy 10 , 11 , 12 , 13 , we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observed the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure—a fundamental phenomenon in semiconductor devices such as solar cells. Quantitative analysis and theoretical modelling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.

Patterns composed of micron‐sized surface structures can abruptly change the properties of an optical wave front, including its phase, amplitude, polarization, and dispersion. Optical components featuring surface micro‐gratings not only possess optical functionalities that can rival or exceed those of traditional bulky components but can also significantly improve the compactness of optical systems. Herein, the design and fabrication process of As2S3 glass‐based micro‐gratings as well as their resulting optical functionality in the infrared regime are reported. The novel two‐step photochemical process consists of spatially controlled direct laser writing and subsequent selective solution etching. The process yields surface micro‐gratings that possess arbitrarily tunable geometries and design patterns on large‐area flat and curved optical surfaces or substrates, offering a potential new avenue for developing aberration‐corrected infrared‐imaging systems. A novel photochemical method, consisting of direct laser writing and post selective etching, is utilized to fabricate As2S3 surface micro‐gratings that possess arbitrarily tunable geometries on large‐area flat and curved platforms. A combined simulation–experimental approach to the design and realization of optically functional infrared diffractive optical elements offers a potential new avenue for developing aberration‐corrected infrared‐imaging systems.

Recently, ultrafast lasers exhibiting high peak powers and extremely short pulse durations have created a new paradigm in materials processing. The precision and minimal thermal damage provided by ultrafast lasers in the machining of metals and dielectrics also suggests a novel application in obtaining precise cross-sections of fragile, combustible paint layers in artwork and cultural heritage property. Cross-sections of paint and other decorative layers on artwork provide critical information into its history and authenticity. However, the current methodology which uses a scalpel to obtain a cross-section can cause further damage, including crumbling, delamination, and paint compression. Here, we demonstrate the ability to make controlled cross-sections of paint layers with a femtosecond pulsed laser, with minimal damage to the surrounding artwork. The femtosecond laser cutting overcomes challenges such as fragile paint disintegrating under scalpel pressure, or oxidation by the continuous-wave (CW) laser. Variations in laser power and translational speed of the laser while cutting exhibit different benefits for cross-section sampling. The use of femtosecond lasers in studying artwork also presents new possibilities in analyzing, sampling, and cleaning of artwork with minimal destructive effects.

Reconfigurable photonic systems featuring minimal power consumption are crucial for integrated optical devices in real‐world technology. Current active devices available in foundries, however, use volatile methods to modulate light, requiring a constant supply of power and significant form factors. Essential aspects to overcome these issues are the development of nonvolatile optical reconfiguration techniques which are compatible with on‐chip integration with different photonic platforms and do not disrupt their optical performances. Herein, a solution is demonstrated using an optoelectronic framework for nonvolatile tunable photonics that uses undoped‐graphene microheaters to thermally and reversibly switch the optical phase‐change material Ge2Sb2Se4Te1 (GSST). An in situ Raman spectroscopy method is utilized to demonstrate, in real‐time, reversible switching between four different levels of crystallinity. Moreover, a 3D computational model is developed to precisely interpret the switching characteristics, and to quantify the impact of current saturation on power dissipation, thermal diffusion, and switching speed. This model is used to inform the design of nonvolatile active photonic devices; namely, broadband Si3N4 integrated photonic circuits with small form‐factor modulators and reconfigurable metasurfaces displaying 2π phase coverage through neural‐network‐designed GSST meta‐atoms. This framework will enable scalable, low‐loss nonvolatile applications across a diverse range of photonics platforms. Single‐layer graphene microheaters are used to electro‐thermally switch the optical phase‐change material Ge2Sb2Se4Te1, with powers down to 8.6 mW. Nonvolatile tunable integrated photonic modulators and reconfigurable 2π phase‐modulation metasurfaces are proposed using 3D simulations and experimental data. This framework enables scalable nonvolatile photonic devices with microheaters that are compatible to all photonic platforms and do not disrupt their optical performances.

Two-dimensional (2D) materials are of tremendous interest to integrated photonics, given their singular optical characteristics spanning light emission, modulation, saturable absorption and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. Here, we present a new route for 2D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material that can be directly deposited and patterned on a wide variety of 2D materials and can simultaneously function as the light-guiding medium, a gate dielectric and a passivation layer for 2D materials. Besides achieving improved fabrication yield and throughput compared with the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light–matter interactions in the 2D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared waveguide-integrated photodetectors and modulators. Exploiting the peculiar properties of graphene, a series of high-performance glass-on-graphene devices, such as polarizers, thermo-optic switches and mid-infrared waveguide-integrated photodetectors and modulators are realized.

Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS 2 ), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects the range of key opto-electronic, structural and morphological properties of monolayer MoS 2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO 2 substrates. Our demonstration provides a way of integrating MoS 2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.

Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.

In the version of this Article originally published online, the following statement for the equally contributing authors was missing: “Hongtao Lin, Yi Song, Yizhong Huang and Derek Kita contributed equally to this work.” This has now been corrected in all versions of the Article.