The optical and electrical properties of several composites, formed by filling the high density polyethylene (HDPE) with similar amount of carbon black (CB), multi-walled carbon nanotubes (MWNT) and fullerene (C(60)), respectively, were characterized using a THz-TDS setup. It was found that the optical parameters and the details of their variation with frequency differ significantly for different kinds of carbon materials. The results are analyzed by using Cole-Cole formula of dipole relaxation under the assumption that carbon particles dispersed in the matrix behave like dipoles and contribute mainly to the dielectric loss in the THz frequency range. Fitting results indicate that MWNT and CB filled composites have a broader distribution of the relaxation time compared with C(60) which possesses a nearly single relaxation time. Compared with CB and C(60), MWNT filled composite possesses the larger relaxation strength due to its higher electron density and larger conductive clusters. The real part of conductivity for three kinds of composites all follows power law behavior with respect to frequency but the exponents are quite different. These phenomena may be related to the special properties of the fillers as well as their particulate structures, such as aspect ratio, particle size, and aggregate structure, etc.

Optical properties of carbon materials filled HDPE composites in THz region

We propose a magnetically switchable terahertz (THz) mode selector based on four transverse mode resonances in corrugated waveguides. The theoretical and numerical results show that two passbands can be obtained in the transmission spectra around the resonance points. The passband of 0.9611–1.0006 THz outputs a single second-order transverse mode with the highest ratio of 99.03% while the passband of 0.8712–0.9111 THz contains the multiple transverse mode output with the first two modes. A tunable THz mode selector with frequency band of 0.5572–0.5926 THz is realized by filling the waveguide core with a liquid crystal (E7). The widest working frequency range of the mode selector reaches 28.4 GHz when the molecular steering angle θ lies between 71.22° and 90°.

Terahertz mode selector based on multimode resonances in corrugated waveguides

The invention discloses a method for generating a microwave frequency comb based on a vertical cavity surface emitting laser. According to the method, nonlinear dynamic characteristics presented by avertical cavity surface emitting laser under the action of photoelectric feedback are utilized, and by adjusting photoelectric feedback intensity and photoelectric feedback time in a photoelectric feedback system, two paths of linear polarization microwave frequency combs which are balanced in power, pure in comb line, ultra-wide in band and orthogonal in polarization direction and have two formsof optical signals and electric signals are generated at the same time; the photoelectric feedback system is composed of a vertical cavity surface emitting laser (VCSEL), an optical isolator (OI), a variable optical attenuator (VOA), an optical beam splitter (FC), a single mode fiber (SMF), a photoelectric detector (PD), an electric amplifier (EA) and an electric coupler (EC) which are connected in sequence. The microwave frequency comb obtained through adoption of the method has the advantages of being balanced in power, pure in comb line, ultra-wide in band, orthogonal in polarization direction and the like, and can meet the application requirements of more fields.

Method for generating microwave frequency comb based on vertical cavity surface emitting laser

Based on the insulator to metal transition (IMT) characteristics of vanadium dioxide (VO2), a highly active tunable terahertz (THz) metasurface resonator patterned VO2 cross structures is proposed. The simulation results show that the transmission of the proposed structure at low temperature is higher than 0.7 because of the VO2 in its insulation phase. With the increasing temperature, a strong transmission dip appears at 0.76 THz due to the metal phase of VO2, which indicates the onset of a new resonant mode. The maximum tunable range of transmission for THz wave is 0.06-0.86. Simultaneously, we also demonstrate that the resonant responses of the heating and cooling processes are quite different and the heating process is more sensitive. The transition temperature is close to room temperature and the device can achieve good modulation effect on both TE and TM waves. The TE and TM mode resonances are selective by altering the cross arms, which is almost impossible for other THz devices. So, the resonator can greatly promote practical applications of THz functional devices such as filters, sensors, modulators, and switches.

Huan Liu

Papers

Terahertz mode selector based on multimode resonances in corrugated waveguides

Method for generating microwave frequency comb based on vertical cavity surface emitting laser

Tunable terahertz metasurface resonator based on phase transition of VO2 crosses