Nonlinear Metasurfaces Research Articles

Recent Progress on Optical Frequency Conversion in Nonlinear Metasurfaces and Nanophotonics

Recent Progress on Optical Frequency Conversion in Nonlinear Metasurfaces and Nanophotonics

Electrically tunable nonlinear polaritonic metasurface

Nonlinear polaritonic metasurfaces created by the coupling of intersubband nonlinearities in semiconductor heterostructures with optical modes in nanoresonators have recently demonstrated efficient frequency mixings at very low pumping intensities of the order of a few tens of kilowatts per square centimetre. In these subwavelength structures, the efficiency, spectral bandwidth and local nonlinear phase of wave mixing do not depend on phase matching but only on the nonlinear response of the constituent meta-atoms. We exploit this property to demonstrate an electrically tunable nonlinear metasurface that combines a plasmonic nanocavity and a quantum-engineered semiconductor heterostructure, in which the magnitude and phase of the local nonlinear responses are controlled by a bias voltage through the quantum-confined Stark effect. We demonstrate spectral tuning, dynamic intensity modulation and dynamic beam manipulation for second-harmonic generation. Our work suggests a route for electrically reconfigurable flat nonlinear optical elements with versatile functionalities.

Overcoming Intensity Saturation in Nonlinear Multiple-Quantum-Well Metasurfaces for High-Efficiency Frequency Upconversion.

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light-matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixingsystems.

Impact of nonlocal response in plasmonic metasurfaces on four-wave mixing

We report the impact of nonlocal response in metallic nanostructures on four-wave mixing (FWM) process in nonlinear plasmonic metasurfaces which consist of Au nanostrips coupled with Au film spaced by an ultrathin dielectric film. When the dielectric film is linear and FWM only from the Au nanostructures, the FWM efficiency of around two orders of magnitude enhancement is obtained when the nonlocal effect of Au, not the traditionally local Drude model of Au, is considered. However, when the dielectric film is nonlinear and FWM from the Au nanostructures is negligible, the almost half FWM response from the nonlinear metasurface under the nonlocal, not the local model, of Au is confirmed. These results are both ascribed to the different local electric field distributions near the surface of Au nanostructures and in the gap at the local and nonlocal response of Au. The results have an important significance to design ultra-compacted integrated nonlinear optical devices or to explain the experimental measurements of nonlinear response involving plasmonic nanostructures of ultra-small gaps.

Rayleigh wave propagation in nonlinear metasurfaces

We investigate the propagation of Rayleigh waves in a half-space coupled to a nonlinear metasurface. The metasurface consists of an array of nonlinear oscillators attached to the free surface of a homogeneous substrate. We describe, analytically and numerically, the effects of nonlinear interaction force and energy loss on the dispersion of Rayleigh waves. We develop closed-form expressions to predict the dispersive characteristics of nonlinear Rayleigh waves by adopting a leading-order effective medium description. In particular, we demonstrate how hardening nonlinearity reduces and eventually eliminates the linear filtering bandwidth of the metasurface. Softening nonlinearity, in contrast, induces lower and broader spectral gaps for weak to moderate strengths of nonlinearity, and narrows and eventually closes the gaps at high strengths of nonlinearity. We also observe the emergence of a spatial gap (in wavenumber) in the in-phase branch of the dispersion curves for softening nonlinearity. Finally, we investigate the interplay between nonlinearity and energy loss and discuss their combined effects on the dispersive properties of the metasurface. Our analytical results, supported by finite element simulations, demonstrate the mechanisms for achieving tunable dispersion characteristics in nonlinear metasurfaces.

Nonlinear wavefront engineering with metasurface decorated quartz crystal

Abstract In linear optical processes, compact and effective wavefront shaping techniques have been developed with the artificially engineered materials and devices in the past decades. Recently, wavefront shaping of light at newly generated frequencies was also demonstrated using nonlinear photonic crystals and metasurfaces. However, the nonlinear wave-shaping devices with both high nonlinear optical efficiency and high wave shaping efficiency are difficult to realize. To circumvent this constraint, we propose the idea of metasurface decorated optical crystal to take the best aspects of both traditional nonlinear crystals and photonic metasurfaces. In the proof-of-concept experiment, we show that a silicon nitride metasurface decorated quartz crystal can be used for the wavefront shaping of the second harmonic waves generated in quartz. With this crystal-metasurface hybrid platform, the nonlinear vortex beam generation and nonlinear holography were successfully demonstrated. The proposed methodology may have important applications in nonlinear structured light generation, super-resolution imaging, and optical information processing, etc.

Full Complex‐Amplitude Modulation of Second Harmonic Generation with Nonlinear Metasurfaces

AbstractMetasurfaces have shown unprecedented capabilities and flexibilities for optical wave manipulation, which provide a powerful platform for the integration and minimization of multifunctional optical devices. However, the realization of the multidimensional manipulation of harmonic waves generated by nonlinear metasurfaces is still a challenge due to the lack of a theoretical guidance. Here, an efficient design strategy of nonlinear metasurfaces based on the hydrodynamic model of the free electron dynamics is demonstrated to realize the full complex‐amplitude modulation of the second harmonic generation (SHG). As a proof concept, three multifunctional nonlinear metasurfaces are designed, in which both the amplitude and the phase of the SHG waves are efficiently and independently manipulated. With numerical and experimental validations, the proposed nonlinear metasurfaces can realize the spin‐selective SHG optical vortices with independent topological charges. The design strategy of nonlinear metasurfaces shall boost the applications of nonlinear metasurfaces in optical information, optical multifunctional integration, and so on.

Reconfigurable nonlinear response of dielectric and semiconductor metasurfaces

Abstract Optically resonant dielectric and semiconductor metasurfaces are an emerging and promising area of nanophotonics and light–matter interaction at the nanoscale. Recently, active tuning of the linear response and nonlinear effects of these components has received an increasing amount of interest. However, so far these research directions have remained separated with only few sporadic works that study their combination beginning to appear in the literature. The evolution of nonlinear metasurfaces based on dielectric and semiconductor materials toward reconfigurable and dynamic components could potentially answer the demand of integrated on-chip components that realize essential functionalities such as frequency conversion, active switching, optical isolation, and all-optical routing. This review provides an overview of recent investigations in this field, reviews the main physical phenomena enabling the dynamic control of the nonlinear response and compares the temporal dynamics of the diverse approaches that have been explored so far. Finally, future directions of dynamic nonlinear metasurfaces are outlined.

Photonic memory with nonlinear plasmonic nanotubes

Photonic memory devices are capable of giving different outputs for the same present input, judging from the input’s past values and, in this way, “remembering” what happened the previous times. Nonlinear cylindrical metasurfaces, when fed centrally by line sources of various angular momenta, are found to possess exactly this property, which is called multistability; in particular, they can develop multiple responses in proportion to what the history of excitation intensities is. Interestingly, some of the potential radiation patterns of the considered nanotubes, despite the homogeneous and isotropic nature of the structure, are azimuthally rotated, even for symmetric excitations. Such a feature of several alternative stable solutions with different amplitudes and angular tilts may provide a promising route for designing reconfigurable optical setups with versatile memory and storage utilities.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Integrated Terahertz Generator-Manipulators Using Epsilon-near-Zero-Hybrid Nonlinear Metasurfaces.

In terahertz (THz) technologies, generation and manipulation of THz waves are two key processes usually implemented by different device modules. Integrating THz generation and manipulation into a single compact device will advance the applications of THz technologies in various fields. Here, we demonstrate a hybrid nonlinear plasmonic metasurface incorporating an epsilon-near-zero (ENZ) indium tin oxide (ITO) layer to seamlessly combine efficient generation and manipulation of THz waves across a wide frequency band. The coupling between the plasmonic resonance of the metasurface and the ENZ mode of the ITO thin film enhances the THz conversion efficiency by more than 4 orders of magnitude. Meanwhile, such a hybrid device is capable of shaping the polarization and wavefront of the emitted THz beam via the engineered nonlinear Pancharatnam-Berry (PB) phases of the plasmonic meta-atoms. The presented hybrid nonlinear metasurface opens a new avenue toward miniaturized integrated THz devices and systems with advanced functionalities.

Enhanced generation of nondegenerate photon pairs in nonlinear metasurfaces

We reveal a novel regime of photon-pair generation driven by the interplay of multiple bound states in the continuum resonances in nonlinear metasurfaces. This non-degenerate photon-pair generation is derived from the hyperbolic topology of the transverse phase-matching and can enable orders-of-magnitude enhancement of the photon rate and spectral brightness, as compared to the degenerate regime. We show that the entanglement of the photon-pairs can be tuned by varying the pump polarization, which can underpin future advances and applications of ultra-compact quantum light sources.

On the Use of Nonlinear Metasurfaces for Circumventing Fundamental Limits of Mantle Cloaking for Antennas

The aim of this communication is to investigate and demonstrate the possibility to overcome the fundamental limitations of mantle cloaking for antennas by exploiting nonlinear metasurfaces. First, we recap and give additional physical insights about the fundamental bounds that apply to the electric properties of an antenna that is made invisible at its own resonance frequency. Then, an innovative strategy is proposed to circumvent these limits through the introduction of nonlinear elements that are able to dynamically transform the geometry of the cloaking metasurface depending on the power level of the impinging field. Different nonlinear designs are discussed, and their effectiveness is assessed through the relevant benchmark example of a half-wavelength dipole antenna able to efficiently transmit high-power signals while being invisible to low-power ones. In addition, the capability of such nonlinear mantle cloaks to enable the shaping of the radiation pattern of an antenna array depending on the power level of the received/transmitted signal is demonstrated. These innovative cloaking devices may find applications in different radiating systems enabling unprecedented functionalities.

High-Efficiency Spatial-Wave Frequency Multiplication Using Strongly Nonlinear Metasurface.

In the past decades, metasurfaces have opened up a promising venue for manipulating lights and electromagnetic (EM) waves. In the field of nonlinearity, second‐harmonic generation (SHG) is a research focus due to its diverse applications. There have been many researches for realizing SHG in optical regime using nonlinear characteristics of optical materials, but its efficiency is low. In microwave frequencies, SHGs are basically studied in the guided‐wave systems. Here, high‐efficiency SHGs of spatial waves are presented in the microwave frequency using nonlinear metasurface loaded with active chips at the subwavelength scale. The nonlinear meta‐atom is composed of receiving antenna, transmitting antenna, and active circuit of frequency multiplier, which can realize strongly nonlinear response and link the EM signals from the receiving to transmitting antennas. Correspondingly, to achieve the function of spatial‐wave frequency multiplication, the working frequency of the transmitting antenna in the meta‐atom should be twice as that of the receiving antenna, and hence the active chip is well matched to obtain the signal transforming with high efficiency. Good performance of the spatial‐wave frequency multiplication is demonstrated in the proof‐of‐concept experiments with the best transform efficiency of 85.11% under normal incidence, validating the proposed method.

Nonlinear Wavy Metasurfaces with Topological Defects for Manipulating Orbital Angular Momentum States

Nonlinear Wavy Metasurfaces with Topological Defects for Manipulating Orbital Angular Momentum States

Quasi-Bragg plasmon modes for highly efficient plasmon-enhanced second-harmonic generation at near-ultraviolet frequencies.

Boosting nonlinear frequency conversions with plasmonic nanostructures at near-ultraviolet (UV) frequencies remains a great challenge in nano-optics. Here we experimentally design and fabricate a plasmon-enhanced second-harmonic generation (PESHG) platform suitable for near-UV frequencies by integrating aluminum materials with grating configurations involved in structural heterogeneity. The SHG emission on the proposed platform can be amplified by up to three orders of magnitude with respect to unpatterned systems. Furthermore, the mechanism governing this amplification is identified as the occurrence of quasi-Bragg plasmon modes near second-harmonic wavelengths, such that a well-defined coherent interplay can be attained within the hot spot region and facilitate the efficient out-coupling of local second-harmonic lights to the far-field. Our work sheds light into the understanding of the role of grating-coupled surface plasmon resonances played in PESHG processes, and should pave an avenue toward UV nanosource and nonlinear metasurface applications.

Nonlinear metasurface combining telecom-range intersubband transitions in GaN/AlN quantum wells with resonant plasmonic antenna arrays

We realize and investigate a nonlinear metasurface taking advantage of intersubband transitions in ultranarrow GaN/AlN multi-quantum well heterostructures. Owing to huge band offsets, the structures offer resonant transitions in the telecom window around 1.55 µm. These heterostructures are functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second-harmonic generation at normal incidence which is completely absent for an antenna array without the multi-quantum well structure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits the redirection of the second-harmonic light to free-space radiation, which is emitted perpendicular to the surface.

Infrared upconversion imaging in nonlinear metasurfaces

Infrared imaging is a crucial technique in a multitude of applications, including night vision, autonomous vehicles navigation, optical tomography, and food quality control. Conventional infrared imaging technologies, however, require the use of materials like narrow-band gap semiconductors which are sensitive to thermal noise and often require cryogenic cooling. Here, we demonstrate a compact all-optical alternative to perform infrared imaging in a metasurface composed of GaAs semiconductor nanoantennas, using a nonlinear wave-mixing process. We experimentally show the up-conversion of short-wave infrared wavelengths via the coherent parametric process of sum-frequency generation. In this process, an infrared image of a target is mixed inside the metasurface with a strong pump beam, translating the image from infrared to the visible in a nanoscale ultra-thin imaging device. Our results open up new opportunities for the development of compact infrared imaging devices with applications in infrared vision and life sciences.

Resonant Nonlinear Synthetic Metasurface with Combined Phase and Amplitude Modulations

AbstractNonlinear metasurfaces provide a promising platform for integrated nonlinear photonic devices owing to their unprecedented capability of nonlinear wavefront manipulation. However, the previously reported second‐harmonic (SH) metasurfaces are mainly based on the nonlinear Pancharatnam–Berry phase modulation. While this method is quite convenient and robust, it shows limitations of modulation efficiency and the difficulty in extending to multidimensional combined modulations. Here, the resonant nonlinear synthetic metasurface is proposed and experimentally demonstrated for independent phase and amplitude modulations of the SH beam. A high SH modulation efficiency of 75% (with a theoretical limit of 90%) is achieved in the polarization‐dependent SH metalens. Moreover, SH holographic imaging with phase and amplitude (at 2 and 4 levels) combined modulation is realized experimentally. Compared with the pure‐phase modulation, the signal‐to‐noise ratios are increased by 2 and 3 times when 2‐ and 4‐level amplitude controls are introduced. This is not only an important step in the improvement and innovation of holographic display technology, but also paves a distinct avenue toward multifunctional, higher efficiency, and ultracompact nonlinear optical devices.

Nonlinear THz‐Nano Metasurfaces: Nonlinear THz‐Nano Metasurfaces (Adv. Funct. Mater. 24/2021)

In article number 2100463, Ranjan Singh, Weili Zhang, Xiaojun Wu, and co-workers successfully fabricate novel nonlinear THz metasurfaces with nanometer capacitive gaps and large size and experimentally realized with precise frequency tuning. The gigantic resonant electric field enhancement in nanogaps induces carrier multiplication in silicon substrates, resulting in conductivity increase in the capacitive gap regions of the metasurface, serving as the source for the nonlinear response.