On-Chip Terahertz-Frequency Measurements of Liquids

Quantitative analysis of aqueous biomolecular mixtures by THz spectroscopy based on high-power LiNbO3 radiation source

The rapid and accurate identification of the types and contents of early pathological markers by THz technology are of particular importance for the prevention and treatment of major diseases. Nevertheless, these markers usually contain interference from water and other non-target molecules, resulting in low signal-to-noise ratio (SNR) and making identification and quantitative analysis difficult. Here, based on THz spectroscopy from a high-power THz source radiated by LiNbO₃, we perform quantitative and real-time THz detection of mixtures (α-lactose monohydrate and 4-aminobenzoic acid) in liquids. The results demonstrate that the absorption spectra of the aqueous biomolecular mixtures exhibit an accumulation of THz features of each pure product, i.e., the amplitude of the absorption peaks is proportional to the mixing ratio, while the corresponding absorption baseline increases with decreasing concentration. Furthermore, the content of the target substance can be calculated from the linear relationship between the absorption spectra of pure and mixed samples. This technology will support the future application of THz-TDS in early disease diagnosis under complex states and environments.

Efficient free-space to on-chip coupling of THz-bandwidth pulses for biomolecule fingerprint sensing

Wide bandwidth THz pulses can be used to record the distinctive spectral fingerprints related to the vibrational or rotational modes of polycrystalline biomolecules, and can be used to resolve the time-dependent dynamics of such systems. Waveguides, owing to their tight spatial confinement of the electromagnetic fields and the longer interaction distance, are promising platforms with which to study small volumes of such systems. The efficient input of sub-ps THz pulses into waveguides is challenging owing to the wide bandwidth of the THz signal. Here, we propose a sensing chip comprised of a pair of back-to-back Vivaldi antennas feeding into, and out from, a 90° bent slotline waveguide to overcome this problem. The effective operating bandwidth of the sensing chip ranges from 0.2 to 1.15 THz, and the free-space to on-chip coupling efficiency is as high as 51% at 0.44 THz. Over the entire band, the THz signal is ∼42 dB above the noise level at room temperature, with a peak of ∼73 dB above the noise. In order to demonstrate the use of the chip, we have measured the characteristic fingerprint of α-lactose monohydrate, and its sharp absorption peak at ∼0.53 THz was successfully observed, demonstrating the promise of our technique. The chip has the merits of efficient in-plane coupling, ultra-wide bandwidth, ease-of-integration, and simple fabrication. It has the potential for large-scale manufacture, and can be a strong candidate for integration into other THz light-matter interaction platforms.

Quantitative detection of THz-ATR spectra of aqueous samples under strong-field terahertz wave

Owing to the characteristics of THz wave, the terahertz time-domain spectral (THz-TDS) system has potentials in the field of biological macromolecule detection. However, water with strong absorption effect on THz wave exists in most biological detection, so the research focus in this field is to study aqueous samples. In view of these, THz spectroscopy system has research value for qualitative and quantitative detection of α-lactose and its water-containing samples. This research used a THz-TDS system with LiNbO₃ crystal to generate strong THz wave that was used to test 0.29 mmol α-lactose samples with water content of 15 μL-930 μL by using attenuating total reflection (ATR) prism. The absorption peak at 0.53 THz is detected, and with the increase of water content, the curve of absorption spectrum is observed to move up on the whole. This research has a guiding role for the test and improvement of water content limit in this field.

Label-free detection of living cervical cells based on microfluidic device with terahertz spectroscopy

Early diagnosis of cervical cancer is essential for a good prognosis. Terahertz wave detection technology is a nondestructive and label-free physical detection technology, which can detect and monitor the cancer cells in real time, especially for patients with deep or inaccessible tumors. In this study, a single-cell-layer microfluidic device was developed. After replacing the optical clearing agent, the characteristics of H8, HeLa and SiHa cell lines in adherent and suspended states were detected. Additionally, the absorption increased with increasing cell density. For the mixed suspension cell samples, principal component analysis-support vector machine method was used to identify benign and malignant cell component. After living cells formaldehyde, changes in cell membrane permeability were evaluated to identify the cell survival status (i.e., dead or living) based on terahertz spectroscopy amplitude differences. Therefore, extending the terahertz spectrum detection to the molecular level can characterize the life essence of cells and tissues.

Enhanced on-chip terahertz vibrational absorption spectroscopy using evanescent fields in silicon waveguide structures

In this study, we demonstrate on-chip terahertz absorption spectroscopy using dielectric waveguide structures. The structures' evanescent fields interact with the sample material surrounding the waveguide, enabling the absorption signature of the material to be captured. The ability of fabricated terahertz dielectric waveguide structures, based on the newly developed silicon-BCB-quartz platform, to capture the fingerprint of α-lactose powder (as an example material) at 532 GHz is examined. Enhancement of the spectroscopy sensitivity through techniques such as tapering the waveguide, confining the field in a slot dielectric waveguide, and increasing the interaction length using a spiral-shaped waveguide are investigated experimentally. The proposed on-chip spectroscopy structures outperform conventional and state-of-the-art approaches in terms of sensitivity and compactness.

Terahertz sensor based on a three-dimensional double I-type metamaterial integrated microfluidic channel

Terahertz metamaterial sensors have received extensive attention in biosensing applications. However, sensitivity toward terahertz frequencies emitted by liquid samples remains challenging because of the strong absorption of terahertz waves by water. Here, we present a highly sensitive terahertz sensor based on a three-dimensional double I-type metamaterial integrated microfluidic channel. The designed sensor produces an inductive-capacitive (LC) resonance with a high quality factor of approximately 72, while demonstrating a maximum sensitivity of 832 GHz/RIU. Furthermore, we analyzed the relationship between the resonance frequency and ethanol concentration. These findings would promote the application of terahertz technology in label-free and rapid biomedical sensing as well as substance detection.

A novel THz molecule-selective sensing strategy in aqueous environments: THz-ATR spectroscopy integrated with a smart hydrogel

Terahertz (THz) spectroscopy, with fascinating advantages for biomedical applications, is still in its infancy in terms of the selective detection of aqueous biomolecules because the strong absorption of solvent water always obscures the THz spectroscopic features of biomolecules. Nevertheless, solvent water is not a passive spectator but a useful indicator, as this proposed strategy describes. This strategy utilizes THz attenuated total reflection (THz-ATR) spectroscopy to probe the glucose-induced hydration state changes of smart hydrogels for label-free and selective detection of aqueous glucose. A notable dramatic increase in both the THz absorption coefficient and hydration state (calculated by weighing) of the smart hydrogel was observed with increasing aqueous glucose concentration, which was further verified by a simple two-component model. For aqueous glucose sensing, this method surpasses individual THz-ATR devices and exhibits suitable sensitivity, ideal selectivity and excellent reusability. Moreover, the proposed strategy may provide an alternative option for the selective detection of various aqueous molecules by THz spectroscopy.

Substrate integrated Bragg waveguide: an octave-bandwidth single-mode hybrid transmission line for millimeter-wave applications

We demonstrate an air-core single-mode hollow hybrid waveguide that uses Bragg reflector structures in place of the vertical metal walls of the standard rectangular waveguide or via holes of the so-called substrate integrated waveguide. The high-order modes in the waveguide are substantially suppressed by a modal-filtering effect, making the waveguide operate in the fundamental mode over more than one octave. Numerical simulations show that the propagation loss of the proposed waveguide can be lower than that of classic hollow metallic rectangular waveguides at terahertz frequencies, benefiting from a significant reduction in Ohmic loss. To facilitate fabrication and characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated, which verifies the properties and advantages of the proposed waveguide. A zero group-velocity dispersion point is observed at near the middle of the operating band, which is ideal for reducing signal distortion. This work offers a step towards a hybrid transmission-line medium that can be used in a variety of functional components for multilayer integration and broadband applications.

Determination of Permittivity of Dielectric Analytes in the Terahertz Frequency Range Using Split Ring Resonator Elements Integrated with On-Chip Waveguide

We investigate the use of finite-element simulations as a novel method for determining the dielectric property of target materials in the terahertz (THz) frequency range using split-ring resonator (SRR) sensing elements integrated into a planar Goubau line (PGL) waveguide. Five such SRRs were designed to support resonances at specific target frequencies. The origin of resonance modes was identified by investigating the electric field distribution and surface current modes in each SRR. Red-shifts were found in the resonances upon deposition of overlaid test dielectric layers that saturated for thicknesses above 10 µm. We also confirmed that the SRRs can work as independent sensors by depositing the analyte onto each individually. The relation between the permittivity of the target material and the saturated resonant frequency was obtained in each case, and was used to extract the permittivity of a test dielectric layer at six different frequencies in the range of 200–700 GHz as an example application. Our approach enables the permittivity of small volumes of analytes to be determined at a series of discrete frequencies up to ~1 THz.

THz Spectroscopy for a Rapid and Label-Free Cell Viability Assay in a Microfluidic Chip Based on an Optical Clearing Agent

Simple, rapid, and efficient cell viability assays play a fundamental role in much of biomedical research, including cell toxicology investigations and antitumor drug screening. Here, we demonstrate for the first time a rapid and label-free cell viability assay using THz spectroscopy in combination with a new optical clearing agent (OCA) and microfluidic technology. This strategy uses a considerably less absorptive OCA to replace the highly absorptive water molecules around the living cells and thus to decrease the background signal interference. Three low-viscosity oils were screened as potential OCA candidates, among which fluorinated oil was selected because of its lower absorption and lowest cytotoxicity. After the liquid medium was replaced with fluorinated oil in a microfluidic chip, an obvious THz spectral difference was observed between the fluorinated oils with and without living cells. This change in THz response was preliminarily attributed to the distinguishable signals between the cells and the fluorinated oil. In addition, we applied this method to cell viability assays of human breast cancer cells (MDA-MB-231) after treatment with different antitumor drugs. The results indicated that THz spectroscopy with the aid of the proposed water-replacement strategy presented excellent quantification of cell viability with the advantages of a rapid, label-free, nondestructive microassay, which offers significant potential to developing a convenient and practical cell analysis platform.