Impact of the uniaxial strain on terahertz modulation characteristics in flexible epitaxial VO2 film across the phase transition.

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO2) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO2 film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

Giant dual-mode graphene-based terahertz modulator enabled by Fabry-Perot assisted multiple reflection.

We report a high-performance terahertz (THz) modulator with dual operation mode. For the pulse operation mode, the proposed THz modulator has the advantage of high modulation depth (MD) and can operate in a broadband frequency range. We have experimentally achieved a MD larger than 90% for the fifth-order pulse THz echo at 0.8 THz, and the MD stays larger than 75% in a broadband frequency range larger than 1 THz, whereas, for the coherent operation mode, the Fabry-Perot (F-P) interference effect has been taken into consideration and a MD larger than 75% at 0.76 THz has also been realized.

Terahertz wave near-field compressive imaging with a spatial resolution of over λ/100.

We demonstrate terahertz (THz) wave near-field imaging with a spatial resolution of ∼4.5 µm using single-pixel compressive sensing enabled by femtosecond-laser (fs-laser) driven vanadium dioxide (VO2)-based spatial light modulator. By fs-laser patterning a 180 nm thick VO2 nanofilm with a digital micromirror device, we spatially encode the near-field THz evanescent waves. With single-pixel Hadamard detection of the evanescent waves, we reconstructed the THz wave near-field image of an object from a serial of encoded sequential measurements, yielding improved signal-to-noise ratio by one order of magnitude over a raster-scanning technique. Further, we demonstrate that the acquisition time was compressed by a factor of over four with 90% fidelity using a total variation minimization algorithm. The proposed THz wave near-field imaging technique inspires new and challenging applications such as cellular imaging.

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

Ultrafast detection and switching of light are key processes in high-speed optoelectronic devices. However, the performances of VO2-based optoelectronics are strongly degraded by photothermal. The mechanism of the latter is still unclear. Here, by using femtosecond-laser (fs-laser) driven kinetic terahertz wave absorption, we quantitatively separate slow photothermal response and ultrafast photodoping response (e.g. light-induced insulator-to-metal transition) from second- to picosecond-timescales, and discover the competing interplay between them. With self-photothermal (mainly determined by fs-laser pulse repetition rate and pump fluence), the ultrafast transition time was degraded by 190% from 50 ps to 95 ps, the ultrafast transition threshold was decreased to 82% from 11mJ/cm2 to 9mJ/cm2, while the amplitudes of the two photoresponse are competing. Percolation theory, along with the macroscopic conductivity response, is used to explain the competing interplay. Our findings are relevant for designing and optimizing VO2-based ultrafast optoelectronic devices.

Ultrasensitive specific terahertz sensor based on tunable plasmon induced transparency of a graphene micro-ribbon array structure.

We proposed an ultrasensitive specific terahertz sensor consisting of two sets of graphene micro-ribbon with different widths. The interference between the plasmon resonances of the wide and narrow graphene micro-ribbons gives rise to the plasmon induced transparency (PIT) effect and enables ultrasensitive sensing in terahertz region. The performances of the PIT sensor have been analyzed in detail considering the thickness and refractive index sensing applications using full wave electromagnetic simulations. Taking advantage of the electrical tunability of graphene's Fermi level, we demonstrated the specific sensing of benzoic acid with detection limit smaller than 6.35 µg/cm2. The combination of specific identification and enhanced sensitivity of the PIT sensor opens exciting prospects for bio/chemical molecules sensing in the terahertz region.

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) terahertz (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 0.27 µJ/THz and with bandwidth narrowly down to 23 GHz. Such dual-mode THz source is useful for nonlinear THz optics, such as selected resonant THz excitation with broadband THz probe spectroscopy of crystalline matters.

Optimization of terahertz generation from LiNbO3 under intense laser excitation with the effect of three-photon absorption.

We proposed a three-dimensional model to simulate terahertz generation from LiNbO3 crystal under intense laser excition (up to ~50 mJ/cm2). The impact of three-photon absorption, which leads to free carrier generation and free carrier saturation (when pump fluence above ~10 mJ/cm2) on terahertz generation was investigated. And further with this model, we stated the optimized experimental conditions (incident postion, beam diameter, and pulse duration, etc) for maximum generation efficiency in commonly-used tilted-pulse-front scheme. Red shift of spectrum, spatial distribution "splitting" effects of emitted THz beam, and primilary experimental verification under intense laser excitation are given.

Ghost spintronic THz-emitter-array microscope.

Terahertz (THz) waves show great potential in nondestructive testing, biodetection and cancer imaging. Despite recent progress in THz wave near-field probes/apertures enabling raster scanning of an object's surface, an efficient, nonscanning, noninvasive, deep subdiffraction imaging technique remains challenging. Here, we demonstrate THz near-field microscopy using a reconfigurable spintronic THz emitter array (STEA) based on the computational ghost imaging principle. By illuminating an object with the reconfigurable STEA followed by computing the correlation, we can reconstruct an image of the object with deep subdiffraction resolution. By applying an external magnetic field, in-line polarization rotation of the THz wave is realized, making the fused image contrast polarization-free. Time-of-flight (TOF) measurements of coherent THz pulses further enable objects at different distances or depths to be resolved. The demonstrated ghost spintronic THz-emitter-array microscope (GHOSTEAM) is a radically novel imaging tool for THz near-field imaging, opening paradigm-shifting opportunities for nonintrusive label-free bioimaging in a broadband frequency range from 0.1 to 30 THz (namely, 3.3-1000 cm-1).

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.