Terahertz Direct Modulation Techniques for High-speed Communication Systems

Terahertz communication technology can provide abundant frequency resources,strong confidentiality,antijamming capability,communication tracking capability and the ability to achieve highspeed data transmissions and can serve as an important technical method for high-speed communications in the future.Among these terahertz communication technologies,terahertz direct modulation technology is a key means to achieve low system complexity and power consumption.In this paper,a review and outlook of terahertz direct modulation technology are proposed from the aspects of high-electron-mobilitytransistor-based terahertz direct modulation,parallelswitch terahertz direct modulation,diode-based terahertz direct modulation,quantum cascade laser-based terahertz direct modulation and new-material-based terahertz direct modulation.We hope through this paper that more readers can gain knowledge about the development and challenges of terahertz direct modulation technology for high-speed communication systems,thus promoting the development of high-speed terahertz communication technology based on direct modulation.

Symmetry-broken silicon disk array as an efficient terahertz switch working with ultra-low optical pump power

The advancement of terahertz technology in recent years and its applications in various fields lead to an urgent need for functional terahertz components,among which a terahertz switch is one example of the most importance because it provides an effective interface between terahertz signals and information in another physical quantity.To date many types of terahertz switches have been investigated mainly in the form of metamaterials made from metallic structures and optically-active medium.However,these reported terahertz switches usually suffer from an inferior performance,e.g.,requiring a high pump laser power density due to a low quality factor of the metallic metamaterial resonances.In this paper,we report and numerically investigate a symmetry-broken silicon disk based terahertz resonator array which exhibits one resonance with ultrahigh quality factor for normal incidence of the terahertz radiations.This resonance,which can never be excited for regular circular Si disks,can help to realize a superior terahertz switch with which only an ultra-low optical pump power density is required to modify the free carrier concentration in Si and its refractive index in the terahertz band.Our findings demonstrate that to realize a high terahertz transmittance change from 0 to above 50%,the required optical pump power density is more than 3 orders of magnitude smaller than that required for a split-ring resonator(SRR)based terahertz switch reported in the literature.

Ultrabroadband and multiband infrared/terahertz photodetectors with high sensitivity

Broadband response is pursued in both infrared(IR)and terahertz(THz)detection technologies,which find their applications in both terrestrial and astronomical realms.Herein,we report an ultrabroadband and multiband IR/THz detector based on blocked-impurity-band detecting principle.The detectors are prepared by implanting phosphorus into germanium(Ge:P),where photoresponses with a P impurity band,a self-interstitial defect band,and a vacancy-P(V-P)pair defect band are realized simultaneously.The response spectra of the detectors show ultrabroad and dual response bands in a range of 3-28μm(IR band)and 40-165μm(THz band),respectively.Additionally,a tiny mid-IR(MIR)band within 3-4.2μm is embedded in the IR band.The THz band arises from the P impurity band,whereas the IR and the MIR bands are ascribed to the two defect bands.At150 m V and 4.5 K,the peak detectivities of the three bands are obtained as 2.9×10^(12) Jones(at 3.9μm),6.8×10^(12) Jones(at 16.3μm),and 9.9×10^(12) Jones(at 116.5μm),respectively.The impressive coverage andsensitivity of the detectors are promising for applications in IR and THz detection technologies.

A reflecting-type highly efficient terahertz cross-polarization converter based on metamaterials

We propose and experimentally demonstrate a wideband linear polarization converter in a reflection mode operating from 2.4 to 4.2 THz with conversion efficiency of more than 80%. Our device can expand the applications to a higher frequency band. A numerical simulation is performed for this metamaterial converter, which shows a good agreement with experimental results. Importantly, a concise and intuitive calculating model is proposed for the Fabry–Pérot cavity. The theoretical results indicate that the underlying reason for the enhanced polarization conversion is the additional phase difference induced by the resonance of the meta-structure and multiple reflections within the Fabry–Pérot cavity.

Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation

In this paper,we presented single mode terahertz quantum cascade lasers(THz QCLs)with sampled lateral grating emitting approximately 3.4 THz.Due to strong mode selection,the implementation of sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB.The measured peak power of the grating laser is improved by about 11.8%compared to the power of devices with uniform distributed feedback gratings.Furthermore,the far-field pattern of the presented device is uninfluenced by grating structures.

Terahertz imaging using an optical frequency comb source

Optical frequency combs, which are generated by the cascade of a phase modulator and a Mach–Zehnder intensity modulator, are used as a polychromatic signal source in the terahertz imaging system to improve imaging quality. The interference effect caused by the monochromatic wave has been greatly suppressed.The required optical power in the presented system is as low as～30% of that in the system using the Er-doped fiber amplifier as a source, which can reduce cost and protect photodiodes from damage. This work provides an effective, low power consumption, low cost, and easy way to realize terahertz imaging with high quality and can be used in future security inspections.