Optical Tweezing Terahertz Probing for a Single Metal Nanoparticle.

Recently, extensive research has been reported on the detection of metal nanoparticles using terahertz waves, due to their potential for efficient and nondestructive detection of chemical and biological samples without labeling. Resonant terahertz nanoantennas can be used to detect a small amount of molecules whose vibrational modes are in the terahertz frequency range with high sensitivity. However, the positioning of target molecules is critical to obtaining a reasonable signal because the field distribution is inhomogeneous over the antenna structure. Here, we combine an optical tweezing technique and terahertz spectroscopy based on nanoplasmonics, resulting in extensive controllable tweezing and sensitive detection at the same time. We observed optical tweezing of a gold nanoparticle and detected it with terahertz waves by using a single bowtie nanoantenna. Furthermore, the calculations confirm that molecular fingerprinting is possible by using our technique. This study will be a prestep of biomolecular detection using gold nanoparticles in terahertz spectroscopy.

Ultrastrong Coupling Enhancement with Squeezed Mode Volume in Terahertz Nanoslots.

Metallic nanogaps have emerged as a versatile platform for realizing ultrastrong coupling in terahertz frequencies. Increasing the coupling strength generally involved reducing the gap width to minimize the mode volume, which presents challenges in fabrication and efficient material coupling. Here, we propose employing terahertz nanoslots, which can efficiently squeeze the mode volume in an extra dimension alongside the gap width. Our experiments using 500 nm wide nanoslots integrated with an organic-inorganic hybrid perovskite demonstrate ultrastrong phonon-photon coupling with a record-high Rabi splitting of 48% of the original resonance (Ω = 0.48ω0), despite having a gap width 5 times larger than previously reported structures with Ω = 0.45ω0. Mechanisms underlying this effective light--matter coupling are investigated with simulations using coupled mode theory. Moreover, bulk polariton analyses reveal that our results account for 68% of the theoretical maximum Rabi splitting, with the potential to reach 82% through further optimization of the nanoslots.

Identification of Chemical and Structural Characteristics of Acrylic Paint Layer Using Terahertz Metasurfaces.

The precise investigation and monitoring of the internal structural change within complex layered systems are crucial, as the emergence of undesirable defects or formation of secondary internal structures significantly exerts a profound influence on the overall properties of the system. We demonstrate an advanced sensing platform utilizing terahertz metasurfaces, allowing chemical detection and precise identification within an acrylic paint layer with a noticeable sensitivity, reaching down to several hundreds of nanometers, in nondestructive and noncontact manners. The identification of solid and mixed paint samples was achieved by analyzing their optical properties, including the refractive index and absorption coefficient. Notably, the presence of internal pore defects within the mixed acrylic paint led to geometric distortions, affecting the state of the overall system. Intriguingly, even in cases where acrylic paint exhibited identical colors perceptible under visible light, distinct discrimination and identification of chemical compositions were successfully proposed.

Direct comparison with terahertz metamaterials and surface-enhanced Raman scattering in a molecular-specific sensing performance.

Signal enhancement of spectroscopies including terahertz time-domain spectroscopy (THz-TDS) and surface-enhanced Raman scattering (SERS) is a critical issue for effective molecular detection and identification. In this study, the sensing performance between THz-TDS and SERS individually accompanied by the proper plasmonic subwavelength structures was compared. For the precisely quantitative study on the optical properties of rhodamine 6G (R6G) dyes, SERS incorporates with the non-linearly enhanced Raman emissions at the molecular characteristic peaks while THz-TDS refers to the transmittance change and the shift of the spectral resonance. The local molecular density-dependent trade-off relationship between limit-of-detection and quenching was observed from both measurements. The specificity for two samples, R6G and methylene blue, is determined by the discriminations in spectral features such as the intensity ratio of assigned peaks in SERS and transmittance difference in THz-TDS. The comprehension of field enhancement by the specific nanostructures was supported by the finite-element method-based numerical computations. As a result, both spectroscopic techniques with the well-tailored nanostructures show great potential for highly sensitive, reproducible, label-free, and cost-effective diagnosis tools in the biomedical fields.

Single-layer metamaterial bolometer for sensitive detection of low-power terahertz waves at room temperature.

This study demonstrates a metamaterial bolometer that can detect terahertz (THz) waves by measuring variations in electrical resistance. A metamaterial pattern for enhanced THz waves absorption and a composite material with a high temperature coefficient of resistance (TCR) are incorporated into a single layer of the bolometer chip to realize a compact and highly sensitive device. To detect the temperature change caused by the absorption of the THz waves, a polydimethylsiloxane mixed with carbon black microparticles is used. The thermosensitive composite has TCR ranging from 1.88%/K to 3.11%/K at room temperature (22.2-23.8°C). In addition, a microscale metamaterial without a backside reflector is designed to enable the measurement of the resistance and to enhance the sensitivity of the bolometer. The proposed configuration effectively improves thermal response of the chip as well as the absorption of the THz waves. It was confirmed that the irradiated THz waves can be detected via the increment in the electrical resistance. The resistance change caused by the absorption of the THz waves is detectable in spite of the changes in resistance originating from the background thermal noise. The proposed metamaterial bolometer could be applied to detect chemical or biological molecules that have fingerprints in the THz band by measuring the variation of the resistance without using the complex and bulky THz time-domain spectroscopy system.

Strongly Localized ohmic Absorption of Terahertz Radiation in Nanoslot Antennas.

Ohmic absorption of light is an indication of a light-matter interaction within metals, where many interesting phenomena and application potentials can be found. To realize the ohmic absorption of light at long wavelengths, where metals are highly reflective, one can use a metamaterial absorber design to concentrate the electromagnetic field within a thin metal film. This concept has enabled thinning of perfect absorbers from a quarter-wave thickness to several tens of nanometers, greatly improving the utility and efficiency of light-metal interactions. Further improvements on the performance are expected if the absorption can be additionally focused laterally, which is a possibility not yet explored. In this study, we report that nanoslot antennas can be a unique ohmic absorber of the low-frequency radiations, where it can incorporate 70% of incident light to ohmic absorption, focused laterally onto 1% of the unit cell area. The inductive field that drives both field enhancement and ohmic absorption is localized within a skin depth distance from the slots with amplitude being as large as 30% of the incident field. Mode-matching calculations and terahertz spectroscopy measurements confirm the inductive and localized nature of the absorption. The strong confinement of the inductive field and of the resulting ohmic absorption is expected to open a new venue in nanocalorimetry, optical nonlinearities of metals, and bolometer applications.

Highly Sensitive and Selective Detection of Steroid Hormones Using Terahertz Molecule-Specific Sensors.

Discrimination and quantification of trace amounts of steroid hormones in biological specimens are needed to elucidate their changing expression because their biological functions are responsible for the development and prevention of endocrine disorders. Although mass-spectrometry-based assays are most commonly recommended, development of a new type of highly sensitive and selective detection methods in clinical practices is needed. Here, we introduce a label-free type of terahertz molecule sensor capable of sensing and identifying progesterone and 17α-OH-progesterone selectively. Nanoslot-array-based sensing chips were used as launching pads for absorption cross-section enhancement of molecules at a reliable terahertz frequency. With use of nanoslots with resonances at 1.17 THz corresponding to intrinsic THz absorption resonance mode for progesterone and at 1.51 THz for 17α-OH-progesterone, respectively, each steroid shows prominent transmittance change in terms of its amount. In particular, the sensing performance has been much improved by controlling evaporation speed, in turn resulting in an efficient, homogeneous distribution of the molecules onto a sensing hot spot.

Terahertz optical characteristics of two types of metamaterials for molecule sensing.

We investigate spectral responses of two different terahertz (THz) metamaterials of double split ring resonator (DSRR) and the nano slot resonator (NSR) for molecule sensing in low concentration. Two different resonant frequencies of DSRR can be controlled by polarization angle of incident THz beam. For comparison of THz optical characteristics, two NSRs were made as showing the same resonant frequencies as DSRR's multimode. The monosaccharide molecules of glucose and galactose were detected by these two types of metamaterials matching the resonant frequencies in various concentration. NSR shows higher sensitivity in very low concentration range rather than DSRR, although the behavior was easily saturated in terms of concentration. In contrast, DSRR can cover more broad concentration range with clear linearity especially under high quality factor mode in polarization of 67.5 degree due to the Fano resonance. THz field enhancement distributions were calculated to investigate sensing performance of both sensing chips in qualitative and quantitative manner.

Terahertz Nanoprobing of Semiconductor Surface Dynamics.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (∼1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz nanoprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors.

Discrimination of Avian Influenza Virus Subtypes using Host-Cell Infection Fingerprinting by a Sulfinate-based Fluorescence Superoxide Probe.

The current gold-standard diagnosis method for avian influenza (AI) is an embryonic egg-based hemagglutination assay followed by immunoblotting or PCR sequencing to confirm subtypes. It requires, however, specialized facilities to handle egg inoculation and incubation, and the subtyping methods relied on costly reagents. Now, the first differential sensing approach to distinguish AI subtypes is demonstrated using series of cell lines and a fluorescent sensor. Susceptibility of AI virus differs depending on genetic backgrounds of host cells. Cells were examined from different organ origins, and the infection patterns against a panel of cells were utilized for AI virus subtyping. To quantify AI infection, a highly cell-permeable fluorescent superoxide sensor was designed to visualize infection. This differential sensing strategy successfully proved discriminations of AI subtypes and demonstrated as a useful primary screening platform to monitor a large number of samples.

Terahertz transmission control using polarization-independent metamaterials.

We present terahertz (THz) transmission control by several uniquely designed patterns of nano-slot antenna array. Collinearly aligned slot antenna arrays have been usually applied to THz filters with frequency band tunability by their geometry. Normally the amplitude in transmission (reflection) in the collinear alignment case can be varied via rotating the azimuthal angle with a sinusoidal trend, which can limit their utilization and performance only at fixed angle between the alignment of the resonant antennas and incident beam polarization. To pursue a variety of metamaterial uses, here, we present polarization-independent THz filters using variously aligned antenna array (asterisk, chlorophyll, and honeycomb patterns) in such counter-intuitive aspects. Besides, unprecedented multi resonance behaviors were observed in chlorophyll and honeycomb patterns, which can be explained with interferences by adjacent structures. The measured spectra were analyzed by harmonic oscillator model with simplified coupling between slots and their adjacent.

Ultrasensitive terahertz sensing of gold nanoparticles inside nano slot antennas.

We introduce a robust control method of terahertz (THz) transmission by tuning filling factors of Au nanoparticles (AuNPs) inside nano slot antennas. AuNPs in sub-100 nm diameters were spread over the nano slot antennas, followed by sweeping them into the slots. AuNPs can be efficiently localized and inserted into nano slots where the THz fields are greatly enhanced, by a "squeegee" made of the polydimethylsiloxane (PDMS). The sweeping of the AuNPs results in further dramatic reduction of THz transmission by suppressing the fundamental resonance mode of the nano slot, as compared to a typical random dropping case. It definitely works for an accurate THz transmission control, as well as the removal of unwanted ions that occasionally confuse signal accuracy from the target signals. Our approach provides a complete reinterpretation of sample deposition for further steady demands in developing ultrasensitive terahertz (THz) molecule sensors.

Broadband supercontinuum generation using a hollow optical fiber filled with copper-ion-modified DNA.

We experimentally demonstrated supercontinuum generation through a hollow core photonic bandgap fiber (HC-PBGF) filled with DNA nanocrystals modified by copper ions in a solution. Both double-crossover nano DNA structure and copper-ion-modified structure provided a sufficiently high optical nonlinearity within a short length of hollow optical fiber. Adding a higher concentration of copper ion into the DNA nanocrystals, the bandwidth of supercontinuum output was monotonically increased. Finally, we achieved the bandwidth expansion of about 1000 nm to be sufficient for broadband multi-spectrum applications.

Ultrafast phase transition via catastrophic phonon collapse driven by plasmonic hot-electron injection.

Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications technologies by modulating signals in integrated nanocircuits at terahertz speeds. In quantum phase-changing materials (PCMs), microscopic charge, lattice, and orbital degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known about the ultrafast dynamics of nanostructured PCMs when interfaced to another class of materials as in this case to active plasmonic elements. Here, we demonstrate how a mesh of gold nanoparticles, acting as a plasmonic photocathode, induces an ultrafast phase transition in nanostructured vanadium dioxide (VO2) when illuminated by a spectrally resonant femtosecond laser pulse. Hot electrons created by optical excitation of the surface-plasmon resonance in the gold nanomesh are injected ballistically across the Au/VO2 interface to induce a subpicosecond phase transformation in VO2. Density functional calculations show that a critical density of injected electrons leads to a catastrophic collapse of the 6 THz phonon mode, which has been linked in different experiments to VO2 phase transition. The demonstration of subpicosecond phase transformations that are triggered by optically induced electron injection opens the possibility of designing hybrid nanostructures with unique nonequilibrium properties as a critical step for all-optical nanophotonic devices with optimizable switching thresholds.

Advancements of Intense Terahertz Field Focusing on Metallic Nanoarchitectures for Monitoring Hidden Interatomic Gas-Matter Interactions.

With the advancements of nanotechnology, innovative photonic designs coupled with functional materials provide a unique way to acquire, share, and respond effectively to information. It is found that the simple deposition of a 30 nm-thick palladium nanofilm on a terahertz (THz) metasurface chip with a 14 nm-wide effective nanogap of asymmetric materials and geometries allows the tracking of both interatomic and interfacial gas-matter interactions, including gas adsorption, hydrogenation (or dehydrogenation), metal phase changes, and unique water-forming reactions. Combinatorial analyses by simulation and experimental measurements demonstrate the distinct nanostructures, which leads to significant light-matter interactions and corresponding THz absorption in a real-time, highly repeatable, and reliable manner. The complex lattice dynamics and intrinsic properties of metals influenced by hydrogen gas exposure are also thoroughly examined using systematically controlled ternary gas mixture devices that mimic normal temperature and pressure. Furthermore, the novel degrees of freedom are utilized to analyze various physical phenomena, and thus, analytical methods that enable the tracking of unknown hidden stages of water-forming reactions resulting in water growth are introduced. A single exposure of the wave spectrum emphasizes the robustness of the proposed THz nanoscopic probe, bridging the gap between fundamental laboratory research and industry.

Synchronous Diagnosis of Respiratory Viruses Variants via Receptonics Based on Modeling Receptor-Ligand Dynamics.

The transmission and pathogenesis of highly contagious fatal respiratory viruses are increasing, and the need for an on-site diagnostic platform has arisen as an issue worldwide. Furthermore, as the spread of respiratory viruses continues, different variants have become the dominant circulating strains. To prevent virus transmission, the development of highly sensitive and accurate on-site diagnostic assays is urgently needed. Herein, a facile diagnostic device is presented for multi-detection based on the results of detailed receptor-ligand dynamics simulations for the screening of various viral strains. The novel bioreceptor-treated electronics (receptonics) device consists of a multichannel graphene transistor and cell-entry receptors conjugated to N-heterocyclic carbene (NHC). An ultrasensitive multi-detection performance is achieved without the need for sample pretreatment, which will enable rapid diagnosis and prevent the spread of pathogens. This platform can be applied for the diagnosis of variants of concern in clinical respiratory virus samples and primate models. This multi-screening platform can be used to enhance surveillance and discriminate emerging virus variants before they become a severe threat to public health.

Sampling Rate Decay in Hindsight Experience Replay for Robot Control.

Training agents via deep reinforcement learning with sparse rewards for robotic control tasks in vast state space are a big challenge, due to the rareness of successful experience. To solve this problem, recent breakthrough methods, the hindsight experience replay (HER) and aggressive rewards to counter bias in HER (ARCHER), use unsuccessful experiences and consider them as successful experiences achieving different goals, for example, hindsight experiences. According to these methods, hindsight experience is used at a fixed sampling rate during training. However, this usage of hindsight experience introduces bias, due to a distinct optimal policy, and does not allow the hindsight experience to take variable importance at different stages of training. In this article, we investigate the impact of a variable sampling rate, representing the variable rate of hindsight experience, on training performance and propose a sampling rate decay strategy that decreases the number of hindsight experiences as training proceeds. The proposed method is validated with three robotic control tasks included in the OpenAI Gym suite. The experimental results demonstrate that the proposed method achieves improved training performance and increased convergence speed over the HER and ARCHER with two of the three tasks and comparable training performance and convergence speed with the other one.

Detection and discrimination of SARS-CoV-2 spike protein-derived peptides using THz metamaterials.

The development of effective assay techniques for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has recently received research attention due to its rapid worldwide spread and considerable risk to human health. The receptor-binding domain (RBD) of the spike (S) protein in SARS-CoV-2, a key component for viral entry that has a unique sequence compared to other structural proteins, has been considered an important diagnostic target. In this respect, low-frequency vibrational modes have the advantage of providing information about compositional and structural dependencies at the peptide level. In this study, the sensitive and selective detection of peptides derived from the RBD in SARS-CoV-2 and SARS-CoV was investigated using metamaterial-based sensing chips with a terahertz time-domain spectroscopy (THz-TDS) system. Based on their RBD sequences, two pairs of peptides with 20 residues each were prepared. The sensitivity, specificity, and reproducibility of the proposed system were examined via quantitative analysis using THz metamaterials at three resonance frequencies, and it was found that the species could be discriminated based on their sequences. The THz signals were analyzed with regard to the major amino acid components of the peptides, and the molecular distributions were also investigated based on the hydropathy and net charge of the peptides.

Effective terahertz shielding properties of extreme graphene-silver nanowire surfaces investigated by nanoprobing.

In the terahertz (THz) electromagnetic wave regime, which has recently received great attention in the fields of communication and security, shielding of THz waves is a significant issue. Therefore, carbon-based nanostructures or polymer-carbon nanocomposites have been widely explored. Herein, significantly enhanced THz shielding efficiency is reported for silver nanowires coated with reduced graphene oxide (rGO) and nanoscale THz metamaterials, as compared to the cases without nanoscale metamaterials. Using a nanoslot-patterned metamaterial with strong resonances at certain frequencies, THz transmission in intensity is enhanced up to three orders of magnitude. Enhanced transmission by nanopatterns substantially increases the shielding performance to the external THz waves, even for ultrathin films (several tens of nanometers) produced by a simple spray of rGO (a few nm of flakes) on a complex random nanowire network. Excellent shielding performance is presented and the shielding mechanism is investigated by the nanoprobing configuration at the same time.

Active terahertz nanoantennas based on VO2 phase transition.

Unusual performances of metamaterials such as negative index of refraction, memory effect, and cloaking originate from the resonance features of the metallic composite atom(1-6). Indeed, control of metamaterial properties by changing dielectric environments of thin films below the metallic resonators has been demonstrated(7-11). However, the dynamic control ranges are still limited to less than a factor of 10,(7-11) with the applicable bandwidth defined by the sharp resonance features. Here, we present ultra-broad-band metamaterial thin film with colossal dynamic control range, fulfilling present day research demands. Hybridized with thin VO(2) (vanadium dioxide) (12-18) films, nanoresonator supercell arrays designed for one decade of spectral width in terahertz frequency region show an unprecedented extinction ratio of over 10000 when the underlying thin film experiences a phase transition. Our nanoresonator approach realizes the full potential of the thin film technology for long wavelength applications.