Coherent optical coupling to surface acoustic wave devices.

Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>105 measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems.

Plasmonic Terahertz Devices and Sensors Based on Carbon Electronics.

Tunable terahertz (THz) photonic devices are imperative in a wide range of applications ranging from THz signal modulation to molecular sensing. One of the currently prevailing methods is based on arrays of metallic or dielectric resonators integrated with functional materials in response to an external stimulus, in which for the purpose of sensing the external stimuli may introduce inadvertent undesirable effects into the target samples to be measured. Here we developed an alternative approach by postprocessing nanothickness macro-assembled graphene (nMAG) films with widely tunable THz conductivity, enabling versatile solid-state THz devices and sensors, showing multifunctional nMAG-based applications. The THz conductivities of free-standing nMAGs showed a broad range from 1.2 × 103 S/m in reduced graphene oxide before annealing to 4.0 × 106 S/m in a nMAG film annealed at 2800 °C. We fabricated nMAG/dielectric/metal and nMAG/dielectric/nMAG THz Salisbury absorbers with broad reflectance ranging from 0% to 80%. The highly conductive nMAG films enabled THz metasurfaces for sensing applications. Taking advantage of the resonant field enhancement arising from the plasmonic metasurface structures and the strong interactions between analyte molecules and nMAG films, we successfully detected diphenylamine with a limit of detection of 4.2 pg. Those wafer-scale nMAG films present promising potential in high-performance THz electronics, photonics, and sensors.

Defect-rich graphene-coated metamaterial device for pesticide sensing in rice.

Performing sensitive and selective detection in a mixture is challenging for terahertz (THz) sensors. In light of this, many methods have been developed to detect molecules in complex samples using THz technology. Here we demonstrate a defect-rich monolayer graphene-coated metamaterial operating in the THz regime for pesticide sensing in a mixture through strong local interactions between graphene and external molecules. The monolayer graphene induces a 50% change in the resonant peak excited by the metamaterial absorber that could be easily distinguished by THz imaging. We experimentally show that the Fermi level of the graphene can be tuned by the addition of molecules, which agrees well with our simulation results. Taking chlorpyrifos methyl in the lixivium of rice as a sample, we further show the molecular sensing potential of this device, regardless of whether the target is in a mixture or not.

Pesticide detection with covalent-organic-framework nanofilms at terahertz band.

Development of rapid molecular detection technologies is critical for the safer and smarter urban agriculture, medicine, and pro-environment. The emergent terahertz (THz) spectroscopy has its distinct advantages of being non-destructive, label-free and able to trace intermolecular and intramolecular vibrations, yet it suffers from the low performance of sensing materials available and their high fabrication cost. Here, we introduce a reticular material -- two dimensional (2D) covalent organic frameworks (COFs) and prepare their nanofilms as the lossy layer for THz absorbers. The COF film can be directly deposited on the dielectric layer of THz absorbers via an in-situ wet-chemistry growth. It possesses designable hierarchical structures, high specific areas of 736-971 m2/g, and precise nanopores of 1.6-2.1 nm, depending on its 2D COF constituents. The resulting THz absorber has been tested for pesticide detection. It presents a limit of detection at 2.2 ng and a selective response of 2.7-7.8 times that of interferents such as saccharides, antibiotics, and dyes, satisfying the need for practical application. Such flexible filmy sensor can measure the pesticide residue on the surface of apple for practical application. The THz sensor also demonstrates high stability over 1000 cycles of bending. Use of reticular nanofilm as the responsive layer may permit the future development of high-performance THz absorbers and other sensors for rapid molecular recognition.

Multifunctional Macroassembled Graphene Nanofilms with High Crystallinity.

A "cooling-contraction" method to separate large-area (up to 4.2 cm in lateral size) graphene oxide (GO)-assembled films (of nanoscale thickness) from substrates is reported. Heat treatment at 3000 °C of such free-standing macroscale films yields highly crystalline "macroassembled graphene nanofilms" (nMAGs) with 16-48 nm thickness. These nMAGs present tensile strength of 5.5-11.3 GPa (with ≈3 µm gauge length), electrical conductivity of 1.8-2.1 MS m-1 , thermal conductivity of 2027-2820 W m-1 K-1 , and carrier relaxation time up to ≈23 ps. As a demonstration application, an nMAG-based sound-generator shows a 30 µs response and sound pressure level of 89 dB at 1 W cm-2 . A THz metasurface fabricated from nMAG has a light response of 8.2% for 0.159 W mm-2 and can detect down to 0.01 ppm of glucose. The approach provides a straightforward way to form highly crystallized graphene nanofilms from low-cost GO sheets.

Label-free terahertz microfluidic biosensor for sensitive DNA detection using graphene-metasurface hybrid structures.

Metasurface assisted terahertz (THz) real-time and label-free biosensors have attracted intense attention. However, it is still challenging for specific detection of highly absorptive liquid samples with high sensitivity in the THz range. Here, we incorporated graphene with THz metasurface into a microfluidic cell for sensitive biosensing. The proposed THz graphene-metasurface microfluidic platform can effectively reduce the volume of the sample solution and boost the interaction between biomolecules and THz waves, thus enhancing the sensitivity. As a proof of concept, comparative experiments using other three kinds of microfluidic cells (pure microfluidic cell, metasurface-based microfluidic cell and graphene-based microfluidic cell) were conducted to explore and verify the sensing mechanism, which evidences the high sensitivity of delicate sensing based on the hybrid graphene-metasurface THz microfluidic device. Furthermore, to perform biosensing applications on that basis, specific aptamers were modified on the graphene-metasurface, enabling DNA sequences of foodborne pathogen Escherichia coli O157:H7 to be recognized. Based on the THz microfluidic biosensor, 100 nM DNA short sequences can be successfully detected. The sensing results of antibiotics and DNA based on the graphene-metasurface microfluidic biosensor confirm the superiority of the proposed design and considerable promise in THz biosensing. The novel sensing platform provides the merits of enabling highly sensitive, label-free, low-cost, easy to use, reusable, and real-time biosensing, which opens an exciting prospect for nanomaterial-metasurface hybrid structure assisted THz label-free biosensing in liquid environment.

Metamaterial-Free Flexible Graphene-Enabled Terahertz Sensors for Pesticide Detection at Bio-Interface.

There is an increasing recognition that terahertz (THz) spectroscopy can be used for high-sensitivity molecular sensing. Therefore, in recent years, much work has been devoted to developing flexible, compact, and high-sensitivity THz sensors. However, most designs employ metamaterials, which require complicated, and often expensive, fabrication procedures. Also, the metamaterial structures create a gap between the sensor surface and the target surface, which decreases the effective contact area between them, resulting in reduced sensing performance. Here, we fabricated a metamaterial-free graphene-based THz sensor with user-designed patterns for sensing at bio-interfaces. External molecules can strongly interact with π electrons in graphene, which moves the Fermi level and changes the amount of THz absorption. We used this sensor to successfully detect chlorpyrifos methyl with a limit of detection at 0.13 mg/L. We also detected pesticide molecules of a concentration of 0.60 mg/L on the surface of an apple, revealing the flexibility of this sensor. The flexible graphene THz sensor showed high sensing stability and robustness over 1000 cycles of bending. These results show that our graphene-based thin-film sensors are easy to fabricate, flexible, versatile, and suited for a wide range of sensing applications.

Ultrahigh-Sensitivity Molecular Sensing with Carbon Nanotube Terahertz Metamaterials.

Terahertz (THz) electromagnetic waves strongly interact with complex molecules, making THz spectroscopy a promising tool for high-sensitivity molecular detection, especially for biomedical applications. Metamaterials are typically used for enhancing THz-molecule interactions to achieve higher sensitivities. However, a primary challenge in THz molecular sensing based on metallic metamaterials is the limited tunability of optical constants of metals. Here, we present an ultrahigh-sensitivity molecular sensor based on carbon nanotube (CNT) THz metamaterials. The sensor, consisting of a CNT cut-wire array on a Si substrate prepared by a novel two-step method, exhibits a reflectance resonance whose frequency strongly varies with the substrate composition, geometries of periodic arrays, and analyte composition. We used this sensor to detect glucose, lactose, and chlorpyrifos-methyl molecules, achieving limit-of-detection values of 30, 40, and 10 ng/mL (S/N = 3), respectively, higher than that of metallic metamaterials by 2 orders of magnitude. We attribute this ultrahigh sensitivity to the high conductivity of CNTs and the efficient adsorption of the target analyte by CNTs through van der Waals forces and π-π stacking. These easy-to-fabricate CNT-based THz metamaterials pave the way for versatile and reliable ultrahigh-sensitivity THz molecular detection.

Biological applications of terahertz technology based on nanomaterials and nanostructures.

Terahertz (THz) technology is now drawing increasing attention around the world; it has been considered as an efficient non-destructive, non-contact and label-free optical method for biological detection. In this field, nanomaterials and nanostructures have been constantly advancing the development of THz technology. Here, we proposed some latest applications of nanotechnology to improve THz biological detection capability for providing progressive THz systems, thus enabling outstanding detection performance utilizing THz spectroscopy and imaging; these will encourage broader interest in various fields. The uniqueness, limitations, and future prospects of THz biological applications based on nanomaterials and nanostructures will also be reviewed in light of recent developments.

Metallic mesh devices-based terahertz parallel-plate resonators: characteristics and applications.

The capability to design, fabricate, and optimize metamaterials based on various structures and material platforms has been crucial for the rapid development of modern terahertz (THz) technology. While the detailed structures of artificial unit cells within a metamaterial is certainly worth investigating, there has been increasing demand to integrate novel metamaterials with a traditional functional photonic device to form a hybrid device, whose performance is so significantly improved as to be promising for real-world applications. In this study, we proposed, for the first time, a THz parallel-plate resonator based on metallic mesh devices (MMDs) for chemical sensing applications. We studied the influences of various structural parameters through simulations, fabricated MMD-based resonator devices, and fully characterized the device performance through THz spectroscopy experiments. Furthermore, we experimentally demonstrated that our device can detect a doxycycline hydrochloride aqueous solution whose concentrations is as low as 1 mg L-1 through resonance frequency shifts, evidencing the device sensitivity capable of delicate chemical sensing tasks. Our work presents a practical and low cost architecture for chemical sensing using THz radiation, which opens new avenues for numerous useful THz devices based on metamaterials.

Mechanisms and applications of terahertz metamaterial sensing: a review.

Terahertz (THz) technology has attracted great worldwide interest and novel high-intensity THz sources and plasmonics are two of the most active fields of recent research. Being situated between infrared light and microwave radiation, the absorption of THz rays in molecular and biomolecular systems is dominated by the excitation of intramolecular and intermolecular vibrations. This indicates that THz technology is an effective tool for sensing applications. However, the low sensitivity of free-space THz detection limits the sensing applications, which gives a great opportunity to metamaterials. Metamaterials are periodic artificial electromagnetic media structured with a size scale smaller than the wavelength of external stimuli. They present localized electric field enhancement and large values of quality factor (Q factor) and show high sensitivity to minor environment changes. In the present work, the mechanism of THz metamaterial sensing and dry sample and microfluidic sensing applications based on metamaterials are introduced. Moreover, new directions of THz metamaterial sensing advancement and introduction of two-dimensional materials and nanoparticles for future THz applications are summarized and discussed.

Terahertz sensing of chlorpyrifos-methyl using metamaterials.

By squeezing electromagnetic energy into small volumes near a metal-dielectric interface, plasmonics provide many routes to enhance and manipulate light-matter interactions, which presents new strategies for signal enhancing technologies. As an extension of the ideas of plasmonics to the terahertz (THz) range, metamaterials have shown great potential in sensing applications. In this study, terahertz time-domain spectroscopy (THz-TDS) combined with metamaterials was used to detect chlorpyrifos-methyl (CM), which is one type of the broad-spectrum organophosphorus pesticides. The results demonstrate that sensitivity is greatly improved using THz metamaterials, with the limit of detection (LOD) of CM reaching 0.204mgL-1, which is lower than the World Health Organization's provisional guideline limit for CM in vegetables (1mgL-1). The results indicated that THz spectroscopy combined with metamaterials could be a valuable method for highly sensitive THz applications, presenting a new strategy for food quality and safety control in the future.

Discrimination of Transgenic Rice containing the Cry1Ab Protein using Terahertz Spectroscopy and Chemometrics.

Spectroscopic techniques combined with chemometrics methods have proven to be effective tools for the discrimination of objects with similar properties. In this work, terahertz time-domain spectroscopy (THz-TDS) combined with discriminate analysis (DA) and principal component analysis (PCA) with derivative pretreatments was performed to differentiate transgenic rice (Hua Hui 1, containing the Cry1Ab protein) from its parent (Ming Hui 63). Both rice samples and the Cry1Ab protein were ground and pressed into pellets for terahertz (THz) measurements. The resulting time-domain spectra were transformed into frequency-domain spectra, and then, the transmittances of the rice and Cry1Ab protein were calculated. By applying the first derivative of the THz spectra in conjunction with the DA model, the discrimination of transgenic from non-transgenic rice was possible with accuracies up to 89.4% and 85.0% for the calibration set and validation set, respectively. The results indicated that THz spectroscopic techniques and chemometrics methods could be new feasible ways to differentiate transgenic rice.