Liang-Hui Du

High-Q Fano-like resonance based on a symmetric dimer structure and its terahertz sensing application

High quality factor (Q) Fano-like resonance with intense electromagnetic confinement is essential for ultrasensitive refractive index label-free sensors. Ordinarily, planar metamaterials with asymmetric configurations are the main approaches to demonstrate these extremely narrow line-width Fano-like resonances. In this work we present a high-Q Fano-like resonance with a symmetric structure consisting of a bilayer split-ring resonator (SRR) dimer structure with identical SRR array on each layer. We find that the interference between the resonances arising from the top and bottom layers of SRR array could also give rise to Fano-like resonance with more than twenty times enhanced Q -factor (Q = 19.28) comparing to that of a single layer SRR array (Q = 0.8). The figure of merit of this Fano-like resonance (FOM = 1.79) is about ten times larger than that of the ordinary electric dipole resonance (FOM = 0.186) when applied to terahertz sensing. Our symmetric structure opens up a new way to realize the high Q Fano-like resonances, which would facilitate the design of ultrasensitive chemical and biomolecular sensing platform at terahertz frequency.

A high-performance broadband terahertz absorber based on sawtooth-shape doped-silicon

Perfect absorbers with broadband absorption of terahertz (THz) radiation are promising for applications in imaging and detection to enhance the contrast and sensitivity, as well as to provide concealment. Different from previous two-dimensional structures, here we put forward a new type of THz absorber based on sawtooth-shape doped-silicon with near-unit absorption across a broad spectral range. Absorbance over 99% is observed numerically from 1.2 to 3 THz by optimizing the geometric parameters of the sawtooth structure. Our absorbers can operate over a wide range of incident angle and are polarization insensitive. The underlying mechanisms due to the combination of an air-cavity mode and mode-matching resonance on the air-sawtooth interface are analyzed in terms of the field patterns and electromagnetic power loss features.

Near-perfect terahertz wave amplitude modulation enabled by impedance matching in VO2 thin films

We present a terahertz (THz) amplitude modulation method with near perfect E-field amplitude modulation depths that is based on impedance matching in VO2 thin films during the thermally induced insulator-metal transition (IMT). It was observed that the impedance matching-induced THz amplitude modulation was sensitive to the resistance switching characteristics of the VO2 thin films. By designing the VO2 thin films to have four orders of magnitude of change in resistance during the IMT, we experimentally achieved an E-field amplitude modulation depth of 94.5% (intensity modulation depth of 99.7%) between the insulator phase of VO2 and the impedance matching state, and an E-field amplitude modulation depth of 97.6% (intensity modulation depth of 99.94%) between the impedance matching state and the metallic phase of VO2 at 0.5 THz. The experimental results were consistent with the results of simulations based on the transmission matrix model.

Label-free monitoring of cell death induced by oxidative stress in living human cells using terahertz ATR spectroscopy

We demonstrated that attenuated total reflectance terahertz time-domain spectroscopy (ATR THz-TDS) is able to monitor oxidative stress response of living human cells, which is proven in this work that it is an efficient non-invasive, label-free, real-time and in-situ monitoring of cell death. Furthermore, the dielectric constant and dielectric loss of cultured living human breast epithelial cells, and along with their evolution under oxidative stress response induced by high concentration of H2O2, were quantitatively determined in the work. Our observation and results were finally confirmed using standard fluorescence-labeled flow cytometry measurements and visible fluorescence imaging.

Terahertz Spectroscopic Diagnosis of Myelin Deficit Brain in Mice and Rhesus Monkey with Chemometric Techniques

While myelin deficit of the central nervous system leads to several severe diseases, the definitive diagnostic means are lacking. We proposed and performed terahertz time-domain spectroscopy (THz-TDS) combined with chemometric techniques to discriminate and evaluate the severity of myelin deficit in mouse and rhesus monkey brains. The THz refractive index and absorption coefficient of paraffin-embedded brain tissues from both normal and mutant dysmyelinating mice are shown. Principal component analysis of time-domain THz signal (PCA-tdTHz) and absorption-refractive index relation of THz spectrum identified myelin deficit without exogenous labeling or any pretreatment. Further, with the established PCA-tdTHz, we evaluated the severity of myelin deficit lesions in rhesus monkey brain induced by experimental autoimmune encephalomyelitis, which is the most-studied animal model of multiple sclerosis. The results well matched the pathological analysis, indicating that PCA-tdTHz is a quick, powerful, evolving tool for identification and evaluation myelin deficit in preclinical animals and potentially in para-clinical human biopsy.

Dual-mode tunable terahertz generation in lithium niobate driven by spatially shaped femtosecond laser

A new approach for dual-mode (namely broadband mode and narrowband mode) THz pulses generation in a single lithium niobate (LN) crystal excited by spatially shaped tilted-pulse-front femtosecond (fs) laser pulse was proposed and experimentally demonstrated. The two THz emission modes are generated simultaneously while spatially separated. Both central frequency and bandwidth of narrowband THz emission is controllable by in-situ tuning the spatial modulation period and beam size of the fs-laser, and the broadband (0.1-1.5 THz) THz emission keeps almost unchanged while tuning the narrowband emission. Further optimization achieves the narrowband THz emission with energy spectral density up to

Optimization of terahertz generation from LiNbO 3 under intense laser excitation with the effect of three-photon absorption

0.27 \ \mu \mathrm{J}/\mathrm{THz}

Giant impact of self-photothermal on light-induced ultrafast insulator-to-metal transition in VO2 nanofilms at terahertz frequency

and with bandwidth narrowly down to 23 GHz.