Metasurface Biosensors Enabling Single-Molecule Sensing of Cell-Free DNA.

In this study, we have revealed that highly fluorescence (FL)-enhancing all-dielectric metasurface biosensors are capable of detecting single-target DNA, which is cell-free DNA (cfDNA) specific to the human practice effect. The ultimately high-precision detection was achieved in a scheme combining the metasurface biosensors with a short-time nucleic acid amplification technique, that is, a reduced-cycle polymerase chain reaction (PCR). In this combined scheme, we obtained a series of FL signals at a single-molecule concentration, reflecting the Poisson distribution, and moreover elucidated that the FL signals exhibit the single-molecule cfDNA detection with more than 84% statistical confidence in an automated FL detection system and with 99.9% statistical confidence in confocal FL microscopy. As a result, we have found a simple and practical test to discriminate the target of 1 copy/test from zero using metasurface biosensors, which has not been realized by other elaborate techniques such as digital PCR.

High-performance metasurface polarizers with extinction ratios exceeding 12000.

High-performance ultrathin polarizers have been experimentally demonstrated employing stacked complementary (SC) metasurfaces, which were produced using nanoimprint lithography. It is experimentally determined that the metasurface polarizers composed of Ag and Au have large extinction ratios exceeding 17000 and 12000, respectively, in spite of the subwavelength thickness. It is also shown that the ultrathin polarizers of the SC structures are optimized at telecommunication wavelengths.

Selective Plasmonic Enhancement of Electric- and Magnetic-Dipole Radiations of Er Ions.

Lanthanoid series are unique in atomic elements. One reason is because they have 4f electronic states forbidding electric-dipole (ED) transitions in vacuum and another reason is because they are very useful in current-day optical technologies such as lasers and fiber-based telecommunications. Trivalent Er ions are well-known as a key atomic element supporting 1.5 µm band optical technologies and also as complex photoluminescence (PL) band deeply mixing ED and magnetic-dipole (MD) transitions. Here we show large and selective enhancement of ED and MD radiations up to 83- and 26-fold for a reference bulk state, respectively, in experiments employing plasmonic nanocavity arrays. We achieved the marked PL enhancement by use of an optimal design for electromagnetic (EM) local density of states (LDOS) and by Er-ion doping in deep subwavelength precision. We moreover clarify the quantitative contribution of ED and MD radiations to the PL band, and the magnetic Purcell effect in the PL-decay temporal measurement. This study experimentally demonstrates a new scheme of EM-LDOS engineering in plasmon-enhanced photonics, which will be a key technique to develop loss-compensated and active plasmonic devices.

Heteroplasmon hybridization in stacked complementary plasmo-photonic crystals.

We constructed plasmo-photonic crystals in which efficient light-trapping, plasmonic resonances couple with photonic guided resonances of large density of states and high-quality factor. We have numerically and experimentally shown that heteroplasmon hybrid modes emerge in stacked complementary (SC) plasmo-photonic crystals. The resonant electromagnetic-field distributions evidence that the two hybrid modes originate from two different heteroplasmons, exhibiting a large energy splitting of 300 meV. We further revealed a series of plasmo-photonic modes in the SC crystals.

Ultraviolet-nanoimprinted packaged metasurface thermal emitters for infrared CO2 sensing.

Packaged dual-band metasurface thermal emitters integrated with a resistive membrane heater were manufactured by ultraviolet (UV) nanoimprint lithography followed by monolayer lift-off based on a soluble UV resist, which is mass-producible and cost-effective. The emitters were applied to infrared CO2 sensing. In this planar Au/Al2O3/Au metasurface emitter, orthogonal rectangular Au patches are arrayed alternately and exhibit nearly perfect blackbody emission at 4.26 and 3.95 µm necessary for CO2 monitoring at the electric power reduced by 31%. The results demonstrate that metasurface infrared thermal emitters are almost ready for commercialization.

Robust Detection of Cancer Markers in Human Serums Using All-Dielectric Metasurface Biosensors.

One of the most significant characteristics, which biosensors are supposed to satisfy, is robustness against abundant molecules coexisting with target biomolecules. In clinical diagnoses and biosensing, blood, plasma, and serum are used daily as samples. In this study, we conducted a series of experiments to examine the robustness of all-dielectric metasurface biosensors, which comprise pairs of a highly fluorescence-enhancing silicon nanopellet array and a transparent microfluidic chip. The metasurface biosensors were shown to have high performance in detecting various targets from nucleic acids to proteins, such as antigens and antibodies. The present results show almost four-order wide dynamic ranges from 0.16 ng/mL to 1 µg/mL for prostate-specific antigen (PSA) and from 2 pg/mL to 25 ng/mL for carcinoembryonic antigen (CEA). The ranges include clinical criteria for PSA, 4 ng/mL and CEA, 5 ng/mL. To date, a systematic demonstration of robustness has not been reported regarding the metasurface biosensors. In detecting cancer markers of PSA and CEA in human serums, we demonstrate that the metasurface biosensors are robust enough in a wide target concentrations, including the clinical diagnosis criteria.

A Design Strategy for Surface Nanostructures to Realize Sensitive Refractive-Index Optical Sensors.

Refractive-index optical sensors have been extensively studied. Originally, they were surface plasmon resonance sensors using only a flat gold film. Currently, to develop practically useful label-free optical sensors, numerous proposals for refractive index sensors have been made using various nanostructures composed of metals and dielectrics. In this study, we explored a rational design strategy for sensors using surface nanostructures comprising metals or dielectrics. Optical responses, such as reflection and transmission, and resonant electromagnetic fields were computed using a numerical method of rigorous coupled-wave analysis combined with a scattering-matrix algorithm. As a result, good performance that almost reached the physical limit was achieved using a plasmonic surface lattice structure. Furthermore, to precisely trace the refractive-index change, a scheme using two physical quantities, resonant wavelength and reflection amplitude, was found to be valid for a 2D silicon metasurface.

Photonic metamaterials: a new class of materials for manipulating light waves.

A decade of research on metamaterials (MMs) has yielded great progress in artificial electromagnetic materials in a wide frequency range from microwave to optical frequencies. This review outlines the achievements in photonic MMs that can efficiently manipulate light waves from near-ultraviolet to near-infrared in subwavelength dimensions. One of the key concepts of MMs is effective refractive index, realizing values that have not been obtained in ordinary solid materials. In addition to the high and low refractive indices, negative refractive indices have been reported in some photonic MMs. In anisotropic photonic MMs of high-contrast refractive indices, the polarization and phase of plane light waves were efficiently transformed in a well-designed manner, enabling remarkable miniaturization of linear optical devices such as polarizers, wave plates and circular dichroic devices. Another feature of photonic MMs is the possibility of unusual light propagation, paving the way for a new subfield of transfer optics. MM lenses having super-resolution and cloaking effects were introduced by exploiting novel light-propagating modes. Here, we present a new approach to describing photonic MMs definitely by resolving the electromagnetic eigenmodes. Two representative photonic MMs are addressed: the so-called fishnet MM slabs, which are known to have effective negative refractive index, and a three-dimensional MM based on a multilayer of a metal and an insulator. In these photonic MMs, we elucidate the underlying eigenmodes that induce unusual light propagations. Based on the progress of photonic MMs, the future potential and direction are discussed.

Rapid Detection of Attomolar SARS-CoV-2 Nucleic Acids in All-Dielectric Metasurface Biosensors.

Worldwide infection due to SARS-CoV-2 revealed that short-time and extremely high-sensitivity detection of nucleic acids is a crucial technique for human beings. Polymerase chain reactions have been mainly used for the SARS-CoV-2 detection over the years. However, an advancement in quantification of the detection and shortening runtime is important for present and future use. Here, we report a rapid detection scheme that is a combination of nucleic acid amplification and a highly efficient fluorescence biosensor, that is, a metasurface biosensor composed of a pair of an all-dielectric metasurface and a microfluidic transparent chip. In the present scheme, we show a series of proof-of-concept experimental results that the metasurface biosensors detected amplicons originating from attomolar SARS-CoV-2 nucleic acids and that the amplification was implemented within 1 h. Furthermore, this detection capability substantially satisfies an official requirement of 100 RNA copies/140 µL, which is a criterion for the reliable infection tests.

Two-Way Detection of COVID-19 Spike Protein and Antibody Using All-Dielectric Metasurface Fluorescence Sensors.

COVID-19 (or SARS-CoV-2) has deeply affected human beings worldwide for over two years, and its flexible mutations indicate the unlikeliness of its termination in a short time. Therefore, it is important to develop a quantitative platform for direct COVID-19 detection and human status monitoring. Such a platform should be simpler than nucleic acid amplification techniques such as polymerase chain reaction, and more reliable than the disposable test kits that are based on immunochromatography. To fulfill these requirements, we conducted proof-of-concept experiments for the quantitative detection of spike glycoprotein peptides and antibodies in one platform, i.e., all-dielectric metasurface fluorescence (FL) sensors. The high capability to enhance FL intensity enabled us to quantitatively measure the glycoproteins and antibodies more efficiently compared with the previous methods reported to date. Furthermore, the intrinsic limit of detection in the metasurface FL sensors was examined via confocal microscopy and found to be less than 0.64 pg/mL for glycoprotein peptides. Moreover, the sensors had a dynamic range more than five orders that of the target concentrations, indicating extremely high sensitivity. These two-way functions of the metasurface FL sensors can be helpful in reducing daily loads in clinics and in providing quantitative test values for proper diagnosis and cures.

Smart Shape-Memory Polymeric String for the Contraction of Blood Vessels in Fetal Surgery of Sacrococcygeal Teratoma.

Shape-memory polymers (SMPs) are promising materials in numerous emerging biomedical applications owing to their unique shape-memory characteristics. However, simultaneous realization of high strength, toughness, stretchability while maintaining high shape fixity (Rf ) and shape recovery ratio (Rr ) remains a challenge that hinders their practical applications. Herein, a novel shape-memory polymeric string (SMP string) that is ultra-stretchable (up to 1570%), strong (up to 345 MPa), tough (up to 237.9 MJ m-3 ), and highly recoverable (Rf averagely above 99.5%, Rr averagely above 99.1%) through a facile approach fabricated solely by tetra-branched poly(ε-caprolactone) (PCL) is reported. Notably, the shape-memory contraction force (up to 7.97 N) of this SMP string is customizable with the manipulation of their energy storage capacity by adjusting the string thickness and stretchability. In addition, this SMP string displays a controllable shape-memory response time and demonstrates excellent shape-memory-induced contraction effect against both rigid silicone tubes and porcine carotids. This novel SMP string is envisioned to be applied in the contraction of blood vessels and resolves the difficulties in the restriction of blood flow in minimally invasive surgeries such as fetoscopic surgery of sacrococcygeal teratoma (SCT).

In-plane plasmonic modes of negative group velocity in perforated waveguides.

Eigenmodes in a typical perforated metal-insulator-metal waveguide structure have been analyzed. It is shown that the lowest eigenmode originates from a plasmonic waveguide mode between metallic layers and has net negative group velocity on the lower branch under TM polarization. The negative group velocity is directly derived from the dispersion curve in the plane of in-plane wave number and photon energy, and is responsible for the negative refraction effect reported in fishnet metamaterials.

Highly sensitive wide-range target fluorescence biosensors of high-emittance metasurfaces.

We demonstrate highly sensitive fluorescence (FL) biosensors made of plasmon-photon-hybrid high-emittance metasurfaces, which are hybrid structures composed of perforated silicon waveguides and stacked complementary (SC) gold nanostructures. The SC metasurfaces are applicable to a wide range of targets from antibodies to nucleic acids. As a test bed, a representative antibody of immunoglobulin G is immobilized on the metasurfaces through microfluidic paths and then is directly detected in a scaled manner even at a very low concentration of 5 pg mL-1, i.e., 34 fM. Moreover, a cancer marker of p53 antibody is indirectly detected on the SC metasurfaces at a low concentration of 50 pg mL-1, which is significantly lower than the medical diagnosis criterion of a few ng mL-1. Furthermore, single-strand DNAs that are oligonucleotides and complementary to SARS-CoV-2 RNA are detected with 1 h immobilization time in the range of fmol mL-1 in a scaled manner. These experimental results indicate that the present FL metasurface sensors function efficiently as biosensors for a wide range of biomarkers.

High-Sensitivity High-Throughput Detection of Nucleic Acid Targets on Metasurface Fluorescence Biosensors.

Worldwide infection disease due to SARS-CoV-2 is tremendously affecting our daily lives. High-throughput detection methods for nucleic acids are emergently desired. Here, we show high-sensitivity and high-throughput metasurface fluorescence biosensors that are applicable for nucleic acid targets. The all-dielectric metasurface biosensors comprise silicon-on-insulator nanorod array and have prominent electromagnetic resonances enhancing fluorescence emission. For proof-of-concept experiment on the metasurface biosensors, we have conducted fluorescence detection of single-strand oligoDNAs, which model the partial sequences of SARS-CoV-2 RNA indicated by national infection institutes, and succeeded in the high-throughput detection at low concentrations on the order of 100 amol/mL without any amplification technique. As a direct detection method, the metasurface fluorescence biosensors exhibit high performance.

Subwavelength electromagnetic dynamics in stacked complementary plasmonic crystal slabs.

Resonant electromagnetic fields in stacked complementary plasmonic crystal slabs (sc-PlCSs) are numerically explored in subwavelength dimensions. It is found that the local plasmon resonances in the sc-PlCSs are composite states of locally enhanced electric and magnetic fields. Two sc-PlCSs are analyzed in this paper and it is shown that each sc-PlCS realizes a resonant electromagnetic state suggested by one of Maxwell equations. It is moreover clarified that the local plasmons open efficient paths of Poynting flux, those result in high-contrast polarized transmission.

Second harmonic generation in periodically polarity-inverted zinc oxide.

We report on the second harmonic generation (SHG) in 2D periodically polarity-inverted (PPI) ZnO heterostructures. The grating structures with nanometer-scale periodicity are fabricated on (0001) Al(2)O(3) substrates by using the in situ polarity inversion method. The achievements of SHG with grating in fabricated PPI ZnO structures are demonstrated under consideration of quasi phase matching conditions. In general, grating formation using the this periodical array of differnet polar surface can be extended to the other heteroepitaxial systems with polarity characteristics.

Subwavelength orthogonal polarization rotator.

A concrete design for an orthogonal polarization rotator at optical frequencies is presented. The subwavelength rotator is formed from a pair of ultrathin metamaterial plates of transparency and remarkable anisotropy. By traveling through the rotator, the incident light of linear polarization undergoes the azimuth-angle rotation by about 90 degrees for a wide range of incident azimuth angles from 0 degrees to 90 degrees . The orthogonal rotator proposed here works with the 1/4 thickness of the incident wavelength and keeps the transmittance as high as 50%, showing a high efficiency as a subwavelength optical element.

All-Dielectric Metasurface Fluorescence Biosensors for High-Sensitivity Antibody/Antigen Detection.

Protein biomolecules are used as markers for various diseases and infections and are targets in biosensing research and development. One of the highest-sensitivity biosensing techniques is considered to be fluorescence (FL) sensing. However, to our knowledge, no study has shown that all-dielectric metasurfaces can contribute to highly sensitive FL sensing. Here, we introduce an efficient type of FL-sensing platforms and show that all-dielectric metasurface FL biosensors are able to directly detect a representative antibody, immunoglobulin G, at very small concentrations on the order of pg/mL or tens of femtomolars. Furthermore, it is shown that they work efficiently as an indirect detection platform for standard cancer marker antigens, such as carcinoembryonic antigens, at concentrations well below the medical diagnosis criterion at 5 ng/mL. Importantly, the metasurface biosensors simultaneously suppress inhomogeneous FL responses, exhibit high reproducibility, and retain sensitivity, even in human serum. These results indicate that the present metasurface FL biosensors provide a high-sensitivity practical platform, suggesting that they are a better option than the commercially standard of enzyme-linked immunosorbent assays.

Non-Empirical Large-Scale Search for Optical Metasurfaces.

Metasurfaces are artificially designed, on-top, thin structures on bulk substrates, realizing various functions in recent years. Most metasurfaces have been conceived of for attaining optical functions, based on elaborate human knowledge-based designs for complex structures. Here, we introduce a method for a non-empirical, large-scale structural search to find optical metasurfaces, which enable us to access intended functions without depending on human knowledge and experience. This method is different from the optimization and modification reported so far. To illustrate the outputs in the non-empirical search, we show unpredictable, optically high-performance, all-dielectric metasurfaces found in the machine search. As an extension of the finding of a higher order diffractive structure, we furthermore show a light-focusing metadevice, which is diffraction-limited and has the unique feature that the focal length is almost invariant even when the distance from the incident spot to the metadevice largely varies.

Optical rectification effect in 1D metallic photonic crystal slabs with asymmetric unit cell.

Photo-induced voltage due to photon drag effect is investigated for metallic photonic crystal slabs (PCS) with symmetric and asymmetric unit cells. In the symmetric structure, the signal is antisymmetric as a function of the incident angle, while in the asymmetric structure it is asymmetric. When the laser beam is normally incident to the sample, the photovoltage is observed only for PCS with asymmetric unit cells and its laser wavelength dependence is readily described in terms of uneven diffraction. The phenomenon can be referred to as optical rectification due to photonic scale asymmetry.