Zi Jing Wong

An ultrathin invisibility skin cloak for visible light

Wrap-around invisibility cloak An invisibility cloak can be used to conceal an object from view by guiding light around it. Most cloaks developed so far have bulky structures that are difficult to scale up for hiding large objects. To design a thin invisibility cloak that can be wrapped around an object such as a sheet or skin, Ni et al. designed a two-dimensional metamaterial surface. Such flexible, highly reflective materials could be manufactured at large scale to hide large objects. Science, this issue p. 1310 A metamaterial surface can be designed to operate as an invisibility skin cloak. Metamaterial-based optical cloaks have thus far used volumetric distribution of the material properties to gradually bend light and thereby obscure the cloaked region. Hence, they are bulky and hard to scale up and, more critically, typical carpet cloaks introduce unnecessary phase shifts in the reflected light, making the cloaks detectable. Here, we demonstrate experimentally an ultrathin invisibility skin cloak wrapped over an object. This skin cloak conceals a three-dimensional arbitrarily shaped object by complete restoration of the phase of the reflected light at 730-nanometer wavelength. The skin cloak comprises a metasurface with distributed phase shifts rerouting light and rendering the object invisible. In contrast to bulky cloaks with volumetric index variation, our device is only 80 nanometer (about one-ninth of the wavelength) thick and potentially scalable for hiding macroscopic objects.

Observation of piezoelectricity in free-standing monolayer MoS2

Piezoelectricity allows precise and robust conversion between electricity and mechanical force, and arises from the broken inversion symmetry in the atomic structure. Reducing the dimensionality of bulk materials has been suggested to enhance piezoelectricity. However, when the thickness of a material approaches a single molecular layer, the large surface energy can cause piezoelectric structures to be thermodynamically unstable. Transition-metal dichalcogenides can retain their atomic structures down to the single-layer limit without lattice reconstruction, even under ambient conditions. Recent calculations have predicted the existence of piezoelectricity in these two-dimensional crystals due to their broken inversion symmetry. Here, we report experimental evidence of piezoelectricity in a free-standing single layer of molybdenum disulphide (MoS₂) and a measured piezoelectric coefficient of e₁₁ = 2.9 × 10(-10) C m(-1). The measurement of the intrinsic piezoelectricity in such free-standing crystals is free from substrate effects such as doping and parasitic charges. We observed a finite and zero piezoelectric response in MoS₂ in odd and even number of layers, respectively, in sharp contrast to bulk piezoelectric materials. This oscillation is due to the breaking and recovery of the inversion symmetry of the two-dimensional crystal. Through the angular dependence of electromechanical coupling, we determined the two-dimensional crystal orientation. The piezoelectricity discovered in this single molecular membrane promises new applications in low-power logic switches for computing and ultrasensitive biological sensors scaled down to a single atomic unit cell.

Phase Mismatch–Free Nonlinear Propagation in Optical Zero-Index Materials

Nonlinear Optics Made Easier Nonlinear optical materials can change their optical properties in the presence of light. The nonlinearity results from the constructive addition of interacting photons, but the amount of nonlinear light produced is crucially dependent on meeting strict phase-matching conditions of the interacting photon fields. Suchowski et al. (p. 1223; see the Perspective by Kauranen) now show that metamaterials can be designed with optical properties that relax the phase-matching requirements. At a specific wavelength where the metamaterial exhibits zero refractive index, the photons are found to interact nonlinearly with the phasematching done automatically. Metamaterials relax the requirement for phase matching in nonlinear optics. Phase matching is a critical requirement for coherent nonlinear optical processes such as frequency conversion and parametric amplification. Phase mismatch prevents microscopic nonlinear sources from combining constructively, resulting in destructive interference and thus very low efficiency. We report the experimental demonstration of phase mismatch–free nonlinear generation in a zero-index optical metamaterial. In contrast to phase mismatch compensation techniques required in conventional nonlinear media, the zero index eliminates the need for phase matching, allowing efficient nonlinear generation in both forward and backward directions. We demonstrate phase mismatch–free nonlinear generation using intrapulse four-wave mixing, where we observed a forward-to-backward nonlinear emission ratio close to unity. The removal of phase matching in nonlinear optical metamaterials may lead to applications such as multidirectional frequency conversion and entangled photon generation.

Magnetic hyperbolic optical metamaterials

Strongly anisotropic media where the principal components of electric permittivity or magnetic permeability tensors have opposite signs are termed as hyperbolic media. Such media support propagating electromagnetic waves with extremely large wave vectors exhibiting unique optical properties. However, in all artificial and natural optical materials studied to date, the hyperbolic dispersion originates solely from the electric response. This restricts material functionality to one polarization of light and inhibits free-space impedance matching. Such restrictions can be overcome in media having components of opposite signs for both electric and magnetic tensors. Here we present the experimental demonstration of the magnetic hyperbolic dispersion in three-dimensional metamaterials. We measure metamaterial isofrequency contours and reveal the topological phase transition between the elliptic and hyperbolic dispersion. In the hyperbolic regime, we demonstrate the strong enhancement of thermal emission, which becomes directional, coherent and polarized. Our findings show the possibilities for realizing efficient impedance-matched hyperbolic media for unpolarized light.

Axial Plane Optical Microscopy

We present axial plane optical microscopy (APOM) that can, in contrast to conventional microscopy, directly image a samples cross-section parallel to the optical axis of an objective lens without scanning. APOM combined with conventional microscopy simultaneously provides two orthogonal images of a 3D sample. More importantly, APOM uses only a single lens near the sample to achieve selective-plane illumination microscopy, as we demonstrated by three-dimensional (3D) imaging of fluorescent pollens and brain slices. This technique allows fast, high-contrast, and convenient 3D imaging of structures that are hundreds of microns beneath the surfaces of large biological tissues.

Single-photon test of hyper-complex quantum theories using a metamaterial

In standard quantum mechanics, complex numbers are used to describe the wavefunction. Although this has so far proven sufficient to predict experimental results, there is no theoretical reason to choose them over real numbers or generalizations of complex numbers, that is, hyper-complex numbers. Experiments performed to date have proven that real numbers are insufficient, but the need for hyper-complex numbers remains an open question. Here we experimentally probe hyper-complex quantum theories, studying one of their deviations from complex quantum theory: the non-commutativity of phases. We do so by passing single photons through a Sagnac interferometer containing both a metamaterial with a negative refractive index, and a positive phase shifter. To accomplish this we engineered a fishnet metamaterial to have a negative refractive index at 780 nm. We show that the metamaterial phase commutes with other phases with high precision, allowing us to place limits on a particular prediction of hyper-complex quantum theories.

Three-dimensional metasurface carpet cloak

We experimentally demonstrate a three-dimensional ultra-thin metasurface carpet cloak can cover on an arbitrary-shaped object and make it undetectable by the visible light owing to the phase control capability of the metasurface.

Parity-time symmetric optical cavities(Conference Presentation)

Optical cavities are imperative in micro/nanophotonics for their ability to provide resonance feedback and radiation enhancement. In recent years, the interplay between gain and loss using parity-time (PT) symmetry has opened up a new degree of freedom for cavity mode and emission control. I will first discuss a PT micro-ring cavity with the unique features of thresholdless PT symmetry breaking and single-mode lasing. Next, I will reveal a novel PT optical cavity which can support lasing and coherent perfect absorption modes within a single device and enable strong modulation from coherent amplification to coherent absorption.

Parity-time optical metamaterial devices

The interplay between gain and loss in optical metamaterials can lead to novel device functionalities. Here we demonstrate a single-mode laser with unique cavity mode manipulation capability based on thresholdless parity-time symmetry breaking.

Monolayer tungsten disulfide laser

When transition metal dichalcogenide (TMDC) crystals are thinned to monolayers, they undergo an indirect to direct bandgap transition, enabling rich electroluminescent and photoluminescent behaviors. Coupling and integration of TMDC monolayers with photonic crystal and distributed Bragg reflector microcavities have recently been reported with Purcell enhancement of spontaneous emission and strong light-matter interaction. However, realization of laser - a fundamental building block of optoelectronic system - remains a bottleneck, mainly due to the limited overall materials gain volume and difficulty to design efficient optical confinement structure. In fact, the quantum confinement on these layered d-electron materials leads to layer-dependent evolution of electronic structure with step like density of states for monolayers comparing to their bulk counterparts, Making TMDC monolayers a unique optical gain medium for superior lasing characteristics. Here, we report the first realization of monolayer tungsten disulfide (WS2) laser embedded in a microdisk resonator. To reduce the 2D material lasing threshold, we utilized a whispering gallery mode resonator with a high quality factor. The Si3N4/WS2/HSQ sandwich configuration provides a strong feedback and mode overlap. An excitonic laser emission has bee observed in the visible wavelength. Our work marks a major step towards monolayer-based on-chip active optoelectronics and integrated 2D photonic platforms for new optical communication and computing applications.