Sheng-Jiang Chang

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.

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.

A Review of Magneto-Optical Microstructure Devices at Terahertz Frequencies

Recent research work on magneto-optical micro- structure devices in terahertz (THz) regime has been reviewed. Some magneto-optical materials responding at THz frequency range were introduced. Based on these magneto-optical materials, MO microstructures devices were reviewed, including magnetic photonic crystals, magneto-plasmonics, and magneto-metasurface all in the submillimetre scale. These devices can realize several functions of isolating, modulation, sensing, and directional beam scanning. Moreover, the necessary conditions of forming THz nonreciprocal transmission in magneto-optical microstructure devices were concluded, and the tunability of these devices was also analysed, which strongly depends on the magneto-optical property of material and the symmetry of structure. The unique magneto-optical effects make it play an irreplaceable role in the high performance THz applications of communication, imaging, and sensing systems.

Artificial high birefringence in all-dielectric gradient grating for broadband terahertz waves

Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6–1.4u2009THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material.

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.

Multifunctional magneto-metasurface for terahertz one-way transmission and magnetic field sensing.

A magneto-metasurface is demonstrated for one-way transmission of terahertz (THz) waves and magnetic field sensing. Due to the magneto-optical effect and the asymmetric structure of the transmission system, magnetoplasmon mode splitting for forward and backward THz waves and one-way transmission has been observed in this magneto-metasurface. Significantly, the resonance of the magneto-metasurface has been found that can remain at 0.750 THz at a temperature of 218 K, performing as a stable isolator with an isolation of larger than 30 dB within a magnetic field disturbance from 0.23 to 0.35 T. Also, since the resonance of the magneto-metasurface can be tuned by the different external magnetic fields at a temperature that is higher or lower than 218 K, the magneto-metasurface can work as a highly sensitive magnetic field sensor. The sensitivity of this device reaches S=513.05u2009u2009GHz·T(-1) when T=230u2009u2009K. This multifunctional magneto-metasurface has broad potential in THz application systems.