Xiang-Hui Wang

Tunable nonreciprocal terahertz transmission and enhancement based on metal/magneto-optic plasmonic lens

A tunable metal/magneto-optic plasmonic lens for terahertz isolator is demonstrated. Based on the magneto-optical effect of the semiconductor material and non-symmetrical structure, this plasmonic lens has not only the focusing feature but also nonreciprocal transmission property. Moreover, a transmission enhancement through this device greatly larger than that of the ordinary metallic slit arrays is contributed by the extraordinary optical transmission effect of the magneto surface plasmon polaritons. The results show that the proposed isolator has an isolation bandwidth of larger than 0.4THz and the maximum isolation of higher than 110dB, and its operating frequency also can be broadly tuned by changing the external magnetic field or temperature. This low-loss, high isolation, broadband tunable nonreciprocal terahertz transmission mechanism has a great potential for terahertz application systems.

Real-time quantitative terahertz microfluidic sensing based on photonic crystal pillar array

The microfluidic sensing based on the photonic crystal (PC) pillar array was investigated in the terahertz (THz) region. We fabricated the silicon PC sensors, and experimentally and theoretically demonstrated their resonances by using the THz time-domain spectroscopy system. According to the corresponding changes of the resonances dependent on the different microfluidics on the PC sensor, the qualitative monitoring can be achieved. By establishing the relation between the experimental data and simulations, the amount of microfluidic at each moment on the different PC sensors can be exactly determined. These PC sensors have great promising potential in the real-time quantitative sensing.

State conversion based on terahertz plasmonics with vanadium dioxide coating controlled by optical pumping

The state conversion and terahertz (THz) wave modulation based on a plasmonic device composed of silicon column arrays with vanadium dioxide (VO2) coating were experimentally demonstrated. For double 45° tilted optical pumping, a state conversion from dielectric photonic crystal (PC) to metallic PC was demonstrated due to the insulator-metal transition (IMT) of VO2 with the pump power increasing. In this process, a broadband intensity modulation with 70% modulation depth was achieved. Furthermore, for normally incident optical pumping, another state conversion from dielectric PC to plasmonic device was also demonstrated due to the partial IMT of VO2, and the out of plane PC resonance gradually changed to be plasmonic resonances. This device and its modulation scheme will be of great significance for potential THz applications.

Magnetically tunable terahertz magnetoplasmons in ferrofluid-filled photonic crystals

We investigated terahertz (THz) magneto-optical properties of a ferrofluid and a ferrofluid-filled photonic crystal (FFPC) by using the THz time-domain spectroscopy. A magnetoplasmon resonance splitting and an induced THz transparency phenomenon were demonstrated in the FFPC. The further investigation reveals that the induced transparency originates from the interference between magnetoplasmon modes in the hybrid magneto-optical system of FFPC, and the THz modulation with a 40% intensity modulation depth can be realized in this induced transparency frequency band. This device structure and its tunabilty scheme will have great potential applications in THz filtering, modulation and sensing.

Magnetically Tunable Terahertz Isolator Based on Structured Semiconductor Magneto Plasmonics

A tunable terahertz (THz) isolator based on a periodically structured semiconductor magneto plasmonics is proposed. The unique photonic band-gap and one-way transmission property of this structure with different magnetic fields and temperature are investigated in the THz regime. The numerical results show the proposed isolator has a bandwidth of 80 GHz with the maximum isolation of higher than 90 dB and a low insertion loss of 5%. The central operating frequency of this isolator can be broadly tuned from 1.4 to 0.9 THz by changing the external magnetic field from 0.6 to 1.6 Tesla at 195 K. This low-loss high isolation broadband nonreciprocal THz transmission mechanism has great potential applications in promoting the performances of THz application systems.

Terahertz modulator based on insulator–metal transition in photonic crystal waveguide

A terahertz modulator based on the insulator-metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8-1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

Terahertz isolator based on nonreciprocal magneto-metasurface

A magneto-metasurface with nonreciprocal terahertz (THz) transmission has been proposed to form a THz isolator. Importantly, we have discussed the two necessary conditions for THz nonreciprocal transmission in the metasurface: (1) There should be magneto-optical responses for THz waves in the metasurface; (2) The transmission system of the metasurface needs to be asymmetric for forward and backward waves. These two conditions lead to the time reversal symmetry breaking of system, and the magnetoplasmon mode splitting and nonreciprocal resonance enhancement can be observed in the asymmetry magneto-metasurface. Moreover, the isolation dependences and tunability on the external magnetic field and temperature have also been investigated, which shows that the best operating state with a high isolation can be designed. The numerical simulations show a maximum isolation of 43 dB and a 10 dB operating bandwidth of 20 GHz under an external magnetic field of 0.3 T, and the insertion loss is smaller than 1.79 dB. This low-loss, high isolation, easy coupling THz isolator has broadly potentials for THz application systems.

Terahertz Refractive Index Sensing Based on Photonic Column Array

Terahertz (THz) silicon photonic column (PC) array has been designed and fabricated, and its resonance effect has been experimentally and numerically investigated. Using this resonance effect, the refractive index sensing for liquids has been demonstrated in this device by the theoretical model calculation, numerical simulation, and experimental measurement. Moreover, resonance dependence on different incident angles in this device has also been shown. These results confirm that this THz PC array can be used as a THz refractive index sensor with high sensitivity and robustness. This sensing strategy shows that the structured surface of this device is compatible for liquid monitoring and easy to couple and transmit THz waves, and therefore can be more easily implemented in liquid refractive index sensing.

Tunable optical and magneto-optical properties of ferrofluid in the terahertz regime

The dielectric property and magneto-optical effects of ferrofluids have been investigated in the terahertz (THz) regime by using THz time-domain spectroscopy. The experiment results show that the refractive index and absorption coefficient of ferrofluid for THz waves rise up with the increase of nanoparticle concentration in the ferrofluid. Moreover, two different THz magneto-optical effects have been found with different external magnetic fields, of which mechanisms have been theoretically explained well by microscopic structure induced refractive index change in the magnetization process and the transverse magneto-optical effect after the saturation magnetization, respectively. This work suggests that ferrofluid is a promising magneto-optical material in the THz regime which has widely potential applications in THz functional devices for THz sensing, modulation, phase retardation, and polarization control.

Terahertz Single-Polarization Single-Mode Hollow-Core Fiber Based on Index-Matching Coupling

We have proposed and analyzed a novel terahertz (THz) single-polarization single-mode (SPSM) hollow-core fiber based on the index-matching coupling method. However, its coupling principle is different from the solid-core SPSM fiber. The confinement loss of the index-matched polarization state is increased slightly, while that of the other polarization state is enhanced by nearly two orders of magnitude. In particular, the confinement loss of the index-matched polarization state is only 0.004 dB/m at f=1.675 THz.