Ruwen Peng’s research while affiliated with Nanjing University and other places

Orthogonal polarization multiplexing is typically required to avoid crosstalk between channels. However, for practical applications in optical information encryption, orthogonal polarization channels are vulnerable to decoding. Using non-orthogonal polarization channels can enhance information security. Here, we extend the conventional Jones matrix approach to achieve non-orthogonal tri-channel polarization multiplexing metasurfaces for optical encryption via optical holography. Using a supercell configuration with two pairs of α-Si meta-atoms, we experimentally demonstrate metasurface-based multiplexing under three linear polarization channels, including copolarized and non-copolarized channels. This design strategy is further extended to circular and elliptical polarization channels, which exhibits minimum crosstalk. As a proof-of-concept demonstration, we implement two optical encryption applications, i.e., image encryption and character encryption, based on non-orthogonal polarization-multiplexed metasurfaces. For image encryption, encrypted images are generated only in the correct polarization channels. While using seven different statuses of metasurface under various polarization channels, we also present a character encryption scheme. We envision that the non-orthogonal polarization multiplexing metasurface platform will open new possibilities in optical communication, optical encoding, and information security.

Metasurface polarization optics, manipulating polarization using metasurfaces composed of subwavelength anisotropic nanostructure array, has enabled a lot of innovative integrated strategies for versatile and on-demand polarization generation, modulation, and detection. Compared with conventional bulky optical elements for polarization control, metasurface polarization optics provides a feasible platform in a subwavelength scale to build ultra-compact and multifunctional polarization devices, greatly shrinking the size of the whole polarized optical system and network. Here, we review the recent progresses of metasurface polarization optics in both classical and quantum regimes, including uniform and spatially varying polarization-manipulating devices. Basic polarization optical elements such as meta-waveplate, meta-polarizer, and resonant meta-devices with polarization singularities provide compact means to generate and modulate uniform polarization beams. Spatial-varying polarization manipulation by employing the pixelation feature of metasurfaces, leading to advanced diffraction and imaging functionalities, such as vectorial holography, classic and quantum polarization imaging, quantum polarization entanglement, quantum interference, and modulation. Substituting conventional polarization optics, metasurface approaches pave the way for on-chip classic or quantum information processing, flourishing advanced applications in displaying, communication, imaging, and computing.

Smith-Purcell radiation (SPR) is a versatile platform for finely tuning nanoscale light across a broad spectral range. This study introduces a theoretical approach for shaping SPR wavefronts using aperiodic metagratings (AMGs). The AMGs consist of arrays of identical metal nano-rods (MNRs), with each MNR's spatial position precisely adjustable. This precise adjustment allows for effective modulation of the spatial phase distribution of SPR. To demonstrate the efficacy of this method, we conduct simulations to achieve diverse wavefront profiles of focusing, deflection, Bessel beams, and Airy beams. Additionally, our approach allows for integrating multiple SPR wavefront functionalities within a combo AMG. By employing the asymmetric L-shaped meta-atom design, we achieve simultaneous SPR polarization conversion and wavefront shaping. This method is promising for developing highly adaptable and multifunctional nanoscale light sources.

Conventional dielectric solid materials, both natural and artificial, lack electromagnetic self-duality and thus require additional coatings to achieve impedance matching with free space. Here, we present a class of dielectric metamaterials that are effectively self-dual and vacuum-like, thereby exhibiting full-polarization omnidirectional impedance matching as an unusual Brewster effect extended across all incident angles and polarizations. With both birefringence and reflection eliminated regardless of wavefront and polarization, such anisotropic metamaterials could establish the electromagnetic equivalence with “stretched free space” in transformation optics, as substantiated through full-wave simulations and microwave experiments. Our findings open a practical pathway for realizing unprecedented polarization-independence and omnidirectional impedance-matching characteristics in pure dielectric solids.

Dewdrops, the droplets of water naturally occurring on leaves and carapaces of insects, are a fascinating phenomenon in nature. Here, a man‐made array of dewdrops with arbitrary shapes and arrangements, which can function as an electromagnetic metasurface, is demonstrated. The realization of the dewdrop array is enabled by a surface covered by a tailored pattern of hydrophilic and hydrophobic coatings, where tiny droplets of water can aggregate and form dewdrops on the former. Interestingly, this metasurface made of dewdrops can be modulated by the condensation and evaporation process. By increasing relative humidity and decreasing temperature, the dewdrop metasurface is gradually formed with increasing amounts of water. While the reverse operation can make it completely disappear. This idea is demonstrated through two examples with different functions of dynamically controllable microwave absorption and scattering. The work shows a principle to construct functional electromagnetic devices with dewdrops, as well as a mechanism of dynamic control based on condensation and evaporation, promising unprecedented applications.

The long-standing paradox between matte appearance and transparency has deprived traditional matte materials of optical transparency. Here, we present a solution to this centuries-old optical conundrum by harnessing the potential of disordered optical metasurfaces. Through the construction of a random array of meta-atoms tailored in asymmetric backgrounds, we have created transparent matte surfaces that maintain clear transparency regardless of the strength of disordered light scattering or their matte appearances. This remarkable property originates in the achievement of highly asymmetric light diffusion, exhibiting substantial diffusion in reflection and negligible diffusion in transmission across the entire visible spectrum. By fabricating macroscopic samples of such metasurfaces through industrial lithography, we have experimentally demonstrated transparent windows camouflaged as traditional matte materials, as well as transparent displays with high clarity, full color, and one-way visibility. Our work introduces an unprecedented frontier of transparent matte materials in optics, offering unprecedented opportunities and applications.

Conventional dielectric solid materials, both natural and artificial, lack electromagnetic self-duality and fail to achieve impedance matching with free space. Here, we present a class of dielectric metamaterials that exhibit effective self-duality and also enable full-polarization omnidirectional impedance matching, thereby extending the Brewster effect across all incident angles and polarizations. The self-duality eliminates birefringence despite significant anisotropy in dispersion. Meanwhile, the full-polarization omnidirectional Brewster effect ensures near-zero reflection on interfaces with free space, regardless of incidence wavefront and polarization. These unique properties establish electromagnetic equivalence with "stretched free space" in transformation optics, as substantiated through full-wave simulations and microwave experiments. Our findings open a pathway for realizing unprecedented polarization-independence and omnidirectional impedance-matching characteristics in pure dielectric solids.

We report a giant enhancement of the third harmonic generation (THG) at 343 nm by periodically notched silicon waveguide arrays supporting guided mode resonances (GMRs) at 1030 nm. Maximum efficiency of the third harmonic generation η = 7.71 × 10⁻⁵ is achieved with a peak pump power density of 5.31 GW/cm². The enhancement factor of the THG from the GMR metasurface reaches up to 1.3 × 10⁷ compared to a flat silicon film with the same thickness. This observation demonstrates a promising approach to design high-efficiency nonlinear optical metasurfaces.

The bound states in the continuum (BICs) have attracted much attention in designing metasurface due to their high Q-factor and effectiveness in suppressing radiational loss. Here we report on the realization of the third harmonic generation (THG) at a near-ultraviolet wavelength (343 nm) via accidental BICs in a metasurface. The absolute conversion efficiency of the THG reaches 1.13 × 10⁻⁵ at a lower peak pump intensity of 0.7 GW/cm². This approach allows the generation of an unprecedentedly high nonlinear conversion efficiency with simple structures.