Bulk Metamaterials Exhibiting Chemically Tunable Hyperbolic Responses.

Extraordinary properties of traditional hyperbolic metamaterials, not found in nature, arise from their man-made subwavelength structures causing unique light-matter interactions. However, their preparation requiring nanofabrication processes is highly challenging and merely provides nanoscale two-dimensional structures. Stabilizing their bulk forms via scalable procedures has been a sought-goal for broad applications of this technology. Herein, we report a new strategy of designing and realizing bulk metamaterials with finely tunable hyperbolic responses. We develop a facile two-step process: (1) self-assembly to obtain heterostructured nanohybrids of building blocks and (2) consolidation to convert nanohybrid powders to dense bulk pellets. Our samples have centimeter-scale dimensions typically, readily further scalable. Importantly, the thickness of building blocks and their relative concentration in bulk materials serve as a delicate means of controlling hyperbolic responses. The resulting new bulk heterostructured material system consists of the alternating h-BN and graphite/graphene nanolayers and exhibits significant modulation in both type-I and type-II hyperbolic resonance modes. It is the first example of real bulk hyperbolic metamaterials, consequently displaying the capability of tuning their responses along both in-plane and out-of-plane directions of the materials for the first time. It also distinctly interacts with unpolarized and polarized transverse magnetic and electronic beams to give unique hyperbolic responses. Our achievement can be a new platform to create various bulk metamaterials without complicated nanofabrication techniques. Our facile synthesis method using common laboratory techniques can open doors to broad-range researchers for active interdisciplinary studies for this otherwise hardly accessible technology.

Spatiotemporal plane wave expansion method for arbitrary space-time periodic photonic media.

We have derived a systematic method to calculate the photonic band structures and mode field profiles of arbitrary space-time periodic media by adopting the plane wave expansion method and extending to the space-time domain. We have applied the proposed method to a photonic crystal with time periodic permittivity, i.e., the Floquet photonic crystal, and showed that the method efficiently predicts driving-induced opening of frequency and momentum gaps and breaking of mirror symmetry in the photonic band structures. This method enables systematic investigation of various optical phenomena in space-time periodic media, such as nonreciprocal propagation of light, parametric processes, and photonic Floquet topological phases.

Electrical control of terahertz frequency conversion from time-varying surfaces.

We investigate the electrical control of frequency conversion from a time-varying interdigitated photo-conductive antenna (IPCA) and time-varying metasurface in the terahertz (THz) frequency range. Ultrafast near-infrared (NIR) optical pulses rapidly modify the conductivities of the IPCA and metasurface; however, external voltages can retard this conductivity transition. Thus, external voltages can be used to control the frequency conversion process based on the interaction between the THz waves and the time-varying surfaces. In the IPCA, both frequency up- and down-conversion processes are suppressed by external voltages. However, in the metasurface, the down-conversion is dramatically suppressed by external voltages, whereas the suppression on the up-conversion is less effective.

Husimi functions at gradient index cavities designed by conformal transformation optics.

Dielectric cavity systems, which have been studied extensively so far, have uniform refractive indices of their cavities, and Husimi functions, the most widely used phase space representation of optical modes formed in the cavities, accordingly were derived only for these homogeneous index cavities. For the case of the recently proposed gradient index dielectric cavities (called as transformation cavities) designed by optical conformal mapping, we show that the phase space structure of resonant modes can be revealed through the conventional Husimi functions by constructing a reciprocal virtual space. As examples, the Husimi plots were obtained for an anisotropic whispering gallery mode (WGM) and a short-lived mode supported in a limaçon-shaped transformation cavity. The phase space description of the corresponding modes in the reciprocal virtual space is compatible with the far-field directionality of the resonant modes in the physical space.

A terahertz metamaterial with unnaturally high refractive index.

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.

Ubiquitination-dependent degradation of p73 by the mitochondrial E3 ubiquitin ligase Hades.

p73 is a member of the p53 family of transcription factors which plays an essential role in tumor suppression. p73 is associated with the sensitivity of cancer cells to chemotherapy and the prognosis of many cancers. In this study, we showed the ubiquitination-dependent degradation of p73 by the mitochondrial E3 ubiquitin ligase Hades. First, the binding between p73 and Hades was identified by co-immunoprecipitation experiments, and it was found that the Hades RING-finger domain mediates the interaction with p73. Immunofluorescence analysis showed that p73 moves to the mitochondria and colocalizes with Hades during etoposide-induced apoptosis. By performing in vivo and in vitro ubiquitination assays, we observed that the Hades RING-finger domain promotes ubiquitination of p73. Finally, it was shown that SiRNA-mediated depletion of Hades stabilizes p73. Taken together, our results showed that Hades mediates the ubiquitination-dependent degradation of mitochondrial p73 under apoptotic conditions. These findings suggest that Hades-mediated p73 ubiquitination is a novel regulatory mechanism for the exonuclear function of p73.

Rotationally reconfigurable metamaterials based on moiré phenomenon.

Exploiting moiré interference, we make a new type of reconfigurable metamaterials and study their transmission tunability for incident electromagnetic waves. The moiré pattern is formed by overlapping two transparent layers, each of which has a periodic metallic pattern, and the cluster size of the resulting moiré pattern can be varied by changing the relative superposition angle of the two layers. In our reconfigurable metamaterials, both the size and structural shape of the unit cell can be varied simultaneously through moiré interference. We show that the transmission of electromagnetic waves can be controlled from 90% to 10% at 11 GHz by experiments and numerical simulation. The reconfigurable metamaterial proposed here can be applied in bandpass filters and tunable modulation devices.

High-Q surface-plasmon-polariton whispering-gallery microcavity.

Surface plasmon polaritons (SPPs) are electron density waves excited at the interfaces between metals and dielectric materials. Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices. A key figure of merit in this regard is the ratio of cavity quality factor, Q (related to the dissipation rate of photons confined to the cavity), to cavity mode volume, V (refs 10, 11). However, plasmonic cavity Q factors have so far been limited to values less than 100 both for visible and near-infrared wavelengths. Significantly, such values are far below the theoretically achievable Q factors for plasmonic resonant structures. Here we demonstrate a high-Q SPP whispering-gallery microcavity that is made by coating the surface of a high-Q silica microresonator with a thin layer of a noble metal. Using this structure, Q factors of 1,376 +/- 65 can be achieved in the near infrared for surface-plasmonic whispering-gallery modes at room temperature. This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method. The SPP eigenmodes, as well as the dielectric eigenmodes, are confined within the whispering-gallery microcavity and accessed evanescently using a single strand of low-loss, tapered optical waveguide. This coupling scheme provides a convenient way of selectively exciting and probing confined SPP eigenmodes. Up to 49.7 per cent of input power is coupled by phase-matching control between the microcavity SPP and the tapered fibre eigenmodes.

A printed nanobeam laser on a SiO2/Si substrate for low-threshold continuous-wave operation.

A small-footprint nanobeam photonic crystal laser made of InGaAsP material is directly integrated on a SiO2/Si substrate without using adhesive material via transfer-printing processes (i.e., dry transfer-printing). The transferred nanobeam structure with a physical volume of ~6.6 × 0.58 × 0.28 µm(3) (~10.5 (λ/n)3) shows single mode lasing near 1550 nm with continuous-wave (CW) operation at room-temperature, where effective lasing threshold power was as low as 9 µW. This CW operation was achieved mainly due to efficient heat dissipation provided by direct contact between the nanobeam and the substrate. This transfer-printed nanobeam laser could be a promising candidate for the next-generation light source with a feature of low-power consumption in ultracompact photonic integrated circuits.

Low-voltage-tunable nanobeam lasers immersed in liquid crystals.

A low-voltage-tunable one-dimensional nanobeam laser is realized by employing lithographically defined lateral electrodes. An InGaAsP nanobeam with a sub-micrometer width is transfer-printed in the middle of two electrodes using a polydimethylsiloxane stamp. Spectral tuning is achieved by controlling the molecular alignment of the surrounding liquid crystals (LCs). From µm-scale-gap structures, a total wavelength shift that exceed 6 nm is observed at a low voltage of less than 10 V. A measured spectral tuning rate of 0.87 nm/V, which is the largest value ever reported to our knowledge among LC-tuned photonic crystal lasers, was also noted.

Switching terahertz waves with gate-controlled active graphene metamaterials.

The extraordinary electronic properties of graphene provided the main thrusts for the rapid advance of graphene electronics. In photonics, the gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of photons with graphene, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications owing to its non-resonant Drude-like behaviour. Here, we demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light-matter interaction in the graphene layer can be greatly enhanced by the strong resonances of the metamaterial. Although the thickness of the embedded single-layer graphene is more than six orders of magnitude smaller than the wavelength (<λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 47% and its phase by 32.2° at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, which is indicative of persistent photonic memory effects.

Synthetic Fourier transform light scattering.

We present synthetic Fourier transform light scattering, a method for measuring extended angle-resolved light scattering (ARLS) from individual microscopic samples. By measuring the light fields scattered from the sample plane and numerically synthesizing them in Fourier space, the angle range of the ARLS patterns is extended up to twice the numerical aperture of the imaging system with unprecedented sensitivity and precision. Extended ARLS patterns of individual microscopic polystyrene beads, healthy human red blood cells (RBCs), and Plasmodium falciparum-parasitized RBCs are presented.

Non-Hermitian chiral degeneracy of gated graphene metasurfaces.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have been the focus of much attention due to their singular eigenvalue surface structure. Nevertheless, as pertaining to a non-Hermitian metasurface platform, the reduction of an eigenspace dimensionality at the EP has been investigated mostly in a passive repetitive manner. Here, we propose an electrical and spectral way of resolving chiral EPs and clarifying the consequences of chiral mode collapsing of a non-Hermitian gated graphene metasurface. More specifically, the measured non-Hermitian Jones matrix in parameter space enables the quantification of nonorthogonality of polarisation eigenstates and half-integer topological charges associated with a chiral EP. Interestingly, the output polarisation state can be made orthogonal to the coalesced polarisation eigenstate of the metasurface, revealing the missing dimension at the chiral EP. In addition, the maximal nonorthogonality at the chiral EP leads to a blocking of one of the cross-polarised transmission pathways and, consequently, the observation of enhanced asymmetric polarisation conversion. We anticipate that electrically controllable non-Hermitian metasurface platforms can serve as an interesting framework for the investigation of rich non-Hermitian polarisation dynamics around chiral EPs.

IDDI: integrated domain-domain interaction and protein interaction analysis system.

BACKGROUND: Deciphering protein-protein interaction (PPI) in domain level enriches valuable information about binding mechanism and functional role of interacting proteins. The 3D structures of complex proteins are reliable source of domain-domain interaction (DDI) but the number of proven structures is very limited. Several resources for the computationally predicted DDI have been generated but they are scattered in various places and their prediction show erratic performances. A well-organized PPI and DDI analysis system integrating these data with fair scoring system is necessary. METHOD: We integrated three structure-based DDI datasets and twenty computationally predicted DDI datasets and constructed an interaction analysis system, named IDDI, which enables to browse protein and domain interactions with their relationships. To integrate heterogeneous DDI information, a novel scoring scheme is introduced to determine the reliability of DDI by considering the prediction scores of each DDI and the confidence levels of each prediction method in the datasets, and independencies between predicted datasets. In addition, we connected this DDI information to the comprehensive PPI information and developed a unified interface for the interaction analysis exploring interaction networks at both protein and domain level. RESULT: IDDI provides 204,705 DDIs among total 7,351 Pfam domains in the current version. The result presents that total number of DDIs is increased eight times more than that of previous studies. Due to the increment of data, 50.4% of PPIs could be correlated with DDIs which is more than twice of previous resources. Newly designed scoring scheme outperformed the previous system in its accuracy too. User interface of IDDI system provides interactive investigation of proteins and domains in interactions with interconnected way. A specific example is presented to show the efficiency of the systems to acquire the comprehensive information of target protein with PPI and DDI relationships. IDDI is freely available at http://pcode.kaist.ac.kr/iddi/.

Revealing non-Hermitian band structure of photonic Floquet media.

Periodically driven systems are ubiquitously found in both classical and quantum regimes. In the field of photonics, these Floquet systems have begun to provide insight into how time periodicity can extend the concept of spatially periodic photonic crystals and metamaterials to the time domain. However, despite the necessity arising from the presence of nonreciprocal coupling between states in a photonic Floquet medium, a unified non-Hermitian band structure description remains elusive. We experimentally reveal the unique Bloch-Floquet and non-Bloch band structures of a photonic Floquet medium emulated in the microwave regime with a one-dimensional array of time-periodically driven resonators. These non-Hermitian band structures are shown to be two measurable distinct subsets of complex eigenfrequency surfaces of the photonic Floquet medium defined in complex momentum space.

Mode junction photonics with a symmetry-breaking arrangement of mode-orthogonal heterostructures.

Junction structures provide the foundation of digital electronics and spintronics today. An equivalent, a photonic junction to achieve systematic and drastic control of photon flow is currently missing, but is mandatory for serious all-optical signal processing. Here we propose a photonic junction built upon mode-orthogonal hetero-structures, as a fundamental structural unit for photonic integrated circuits. Controlling the optical potential of mode-orthogonal junctions, the flow of photons can be dynamically manipulated, to complete the correspondence to the electronic junction structures. Of the possible applications, we provide examples of a photonic junction diode and a multi-junction half-adder, with exceptional performance metrics. Highly directional (41dB), nearly unity throughput, ultra-low threshold-power, high quality signal regeneration at 200Gb/s, and all-optic logic operations are successfully derived with the self-induced, bi-level dynamic mode-conversion process across the junction.

All-liquid photonic microcavity stabilized by quantum dots.

We demonstrate two simple methods to fabricate QD-stabilized toluene microdroplets in water as whispering gallery mode microscale resonators in an all-liquid phase. The toluene microdroplets show size-dependently high Q-factors up to 5100 resulting from the stable QD-loaded microdroplets. The highly QD-stabilized toluene microdroplet resonators in the all-liquid phase would be promising for multiple all-liquid lasers.

Low-loss surface-plasmonic nanobeam cavities.

One-dimensional surface-plasmonic nanobeam cavities are proposed as a means to confine surface plasmons to a subwavelength-scale mode volume, while maintaining a relatively high Q-factor. By bonding one-dimensional photonic-crystal nanobeam structures to a low-loss metallic substrate, a clear plasmonic TM bandgap can be formed. The introduction of a single-cell defect alongside the engineering of side-air-hole shifts to this plasmonic-crystal nanobeam provides subwavelength-scale plasmonic mode localization within the plasmonic TM bandgap. This suppresses radiation and scattering loss to render a maximum Q-factor of 413 and a modal volume of 3.67x10(-3) microm3 at room temperature. The possibility of further reduction in the intrinsic loss of the cavity is investigated by lowering the operating temperature and the Q-factor of 1.34x10(4) is predicted at a temperature of 20 K for the optimistic case.

One-dimensional parabolic-beam photonic crystal laser.

We report one-dimensional (1-D) parabolic-beam photonic crystal (PhC) lasers in which the width of the PhC slab waveguide is parabolically tapered. A few high-Q resonant modes are confirmed in the vicinity of the tapered region where Gaussian-shaped photonic well is formed. These resonant modes originate from the dielectric PhC guided mode and overlap with the gain medium efficiently. It is also shown that the far-field radiation profile is closely associated with the symmetry of the structural perturbation.

Metamaterials for Enhanced Optical Responses and their Application to Active Control of Terahertz Waves.

Metamaterials, artificially constructed structures that mimic lattices in natural materials, have made numerous contributions to the development of unconventional optical devices. With an increasing demand for more diverse functionalities, terahertz (THz) metamaterials are also expanding their domain, from the realm of mere passive devices to the broader area where functionalized active THz devices are particularly required. A brief review on THz metamaterials is given with a focus on research conducted in the authors' group. The first part is centered on enhanced THz optical responses from tightly coupled meta-atom structures, such as high refractive index, enhanced optical activity, anomalous wavelength scaling, large phase retardation, and nondispersive polarization rotation. Next, electrically gated graphene metamaterials are reviewed with an emphasis on the functionalization of enhanced THz optical responses. Finally, the linear frequency conversion of THz waves in a rapidly time-variant THz metamaterial is briefly discussed in the more general context of spatiotemporal control of light.