Broadband high-contrast-grating-type waveplates for the terahertz range.

The high-contrast-grating waveplates utilizing high contrast between silicon and air refractive indexes were developed in order to perform as a quarter wave and a half wave plate in the selected THz frequency range. The waveplates possessed anti-reflective properties due to the specific inclination of the walls both in parallel and in perpendicular direction to grating axis, efficiently suppressing the reflection losses caused by air-dielectric interface for both transverse magnetic and transverse electric polarizations. Moreover, significant reduction of the transmittance gap was achieved between both polarizations while mitigating overall Fabry-Perot effect. Validation of the concepts was carried out by measuring transmission amplitude and phase spectra of the fabricated samples in a broadband of THz time-domain spectroscopy and vector-network-analysis systems considering also some real applications.

150 Gbps THz Chipscale Topological Photonic Diplexer.

Photonic diplexers are being widely investigated for high data transfer rates in on-chip communication. However, dividing the available spectrum into nonoverlapping multicarrier frequency sub-bands has remained a challenge in designing frequency-selective time-invariant channels. Here, an on-chip topological diplexer is reported exhibiting terahertz frequency band filtering through Klein tunneling of topological edge modes. The silicon topological diplexer chip facilitates two high-speed channels with quadrature amplitude modulation (QAM) over a broad bandwidth of 12.5 GHz each. These channels operate at carrier frequencies of 305 and 321.6 GHz, achieving a combined diplexer capacity of 150 Gbit s-1. To ensure minimal interference between adjacent channels, a guard band is implemented. Topologically protected edge modes suppress the frequency selective fading of the broadband signals and hold promise for diverse integrated photonic applications spanning terahertz and telecommunication realms, including the design of lossless topological multiplexers, interconnects, antennas, and modulators for the sixth to X generation (6G to XG) wireless.

Frequency stable and low phase noise THz synthesis for precision spectroscopy.

We present a robust approach to generate a continuously tunable, low phase noise, Hz linewidth and mHz/s stability THz emission in the 0.1 THz to 1.4 THz range. This is achieved by photomixing two commercial telecom, distributed feedback lasers locked by optical-feedback onto a single highly stable V-shaped optical cavity. The phase noise is evaluated up to 1.2 THz, demonstrating Hz-level linewidth. To illustrate the spectral performances and agility of the source, low pressure absorption lines of methanol and water vapors have been recorded up to 1.4 THz. In addition, the hyperfine structure of a water line at 556.9 GHz, obtained by saturation spectroscopy, is also reported, resolving spectral features displaying a full-width at half-maximum of 10 kHz. The present results unambiguously establish the performances of this source for ultra-high resolution molecular physics.

Valley-conserved topological integrated antenna for 100-Gbps THz 6G wireless.

The topological phase revolutionized wave transport, enabling integrated photonic interconnects with sharp light bending on a chip. However, the persistent challenge of momentum mismatch during intermedium topological mode transitions due to material impedance inconsistency remains. We present a 100-Gbps topological wireless communication link using integrated photonic devices that conserve valley momentum. The valley-conserved silicon topological waveguide antenna achieves a 12.2-dBi gain, constant group delay across a 30-GHz bandwidth and enables active beam steering within a 36° angular range. The complementary metal oxide semiconductor-compatible valley-conserved devices represent a major milestone in hybrid electronic-photonic-based topological wireless communications, enabling terabit-per-second backhaul communication, high throughput, and intermedium transport of information carriers, vital for the future of communication from the sixth to X generation.

Phototunable chip-scale topological photonics: 160 Gbps waveguide and demultiplexer for THz 6G communication.

The revolutionary 5G cellular systems represent a breakthrough in the communication network design to provide a single platform for enabling enhanced broadband communications, virtual reality, autonomous driving, and the internet of everything. However, the ongoing massive deployment of 5G networks has unveiled inherent limitations that have stimulated the demand for innovative technologies with a vision toward 6G communications. Terahertz (0.1-10 THz) technology has been identified as a critical enabler for 6G communications with the prospect of massive capacity and connectivity. Nonetheless, existing terahertz on-chip communication devices suffer from crosstalk, scattering losses, limited data speed, and insufficient tunability. Here, we demonstrate a new class of phototunable, on-chip topological terahertz devices consisting of a broadband single-channel 160 Gbit/s communication link and a silicon Valley Photonic Crystal based demultiplexer. The optically controllable demultiplexing of two different carriers modulated signals without crosstalk is enabled by the topological protection and a critically coupled high-quality (Q) cavity. As a proof of concept, we demultiplexed high spectral efficiency 40 Gbit/s signals and demonstrated real-time streaming of uncompressed high-definition (HD) video (1.5 Gbit/s) using the topological photonic chip. Phototunable silicon topological photonics will augment complementary metal oxide semiconductor (CMOS) compatible terahertz technologies, vital for accelerating the development of futuristic 6G and 7G communication era driving the real-time terabits per second wireless connectivity for network sensing, holographic communication, and cognitive internet of everything.

Active Ultrahigh-Q (0.2 × 106 ) THz Topological Cavities on a Chip.

Rapid scaling of semiconductor devices has led to an increase in the number of processor cores and integrated functionalities onto a single chip to support the growing demands of high-speed and large-volume consumer electronics. To meet this burgeoning demand, an improved interconnect capacity in terms of bandwidth density and active tunability is required for enhanced throughput and energy efficiency. Low-loss terahertz silicon interconnects with larger bandwidth offer a solution for the existing inter-/intrachip bandwidth density and energy-efficiency bottleneck. Here, a low-loss terahertz topological interconnect-cavity system is presented that can actively route signals through sharp bends, by critically coupling to a topological cavity with an ultrahigh-quality (Q) factor of 0.2 × 106 . The topologically protected large Q factor cavity enables energy-efficient optical control showing 60 dB modulation. Dynamic control is further demonstrated of the critical coupling between the topological interconnect-cavity for on-chip active tailoring of the cavity resonance linewidth, frequency, and modulation through complete suppression of the back reflection. The silicon topological cavity is complementary metal-oxide-semiconductor (CMOS)-compatible and highly desirable for hybrid electronic-photonic technologies for sixth (6G) generation terahertz communication devices. Ultrahigh-Q cavity also paves the path for designing ultrasensitive topological sensors, terahertz topological integrated circuits, and nonlinear topological photonic devices.

Short bends using curved mirrors in silicon waveguides for terahertz waves.

Dielectric waveguides are capable of confining and guiding terahertz waves along sub-wavelength sized structures. A small feature size allows for a denser integration of different photonic components such as modulators, beam-splitters, wavelength (de)multiplexers and more. The integration of components on a small scale requires bending of the waveguides. In this paper we demonstrate a very short silicon 90°-bend, based on total internal reflection on an elliptically curved outer facet and a rounding of the inner corner joining two waveguides, with an average loss of 0.14 dB per bend in the 600-750 GHz range.

Frequency-division multiplexer and demultiplexer for terahertz wireless links.

The development of components for terahertz wireless communications networks has become an active and growing research field. However, in most cases these components have been studied using a continuous or broadband-pulsed terahertz source, not using a modulated data stream. This limitation may mask important aspects of the performance of the device in a realistic system configuration. We report the characterization of one such device, a frequency multiplexer, using modulated data at rates up to 10 gigabits per second. We also demonstrate simultaneous error-free transmission of two signals at different carrier frequencies, with an aggregate data rate of 50 gigabits per second. We observe that the far-field spatial variation of the bit error rate is different from that of the emitted power, due to a small nonuniformity in the angular detection sensitivity. This is likely to be a common feature of any terahertz communication system in which signals propagate as diffracting beams not omnidirectional broadcasts.There is growing interest in the development of components to facilitate wireless communications in the terahertz but the characterization of these systems involve an unmodulated input. Here the authors demonstrate multiplexing and demultiplexing of data streams in the terahertz range using a real data link.

Mode-hopping suppression in long Brillouin fiber laser with non-resonant pumping.

We propose a reliable method for stabilizing narrow linewidth Brillouin fiber lasers with non-resonant pumping. Mode-hopping is suppressed by means of a phase-locked loop that locks the pump-Stokes detuning to a local radio-frequency (RF) oscillator. Stable single-mode operation of a 110-m-long Brillouin fiber laser oscillating at 1.55 µm is demonstrated for several hours. The beat note between two independent Stokes waves presents a phase noise level of -60 dBc/Hz at 100 Hz with a -20 dB/decade slope, and a FWHM linewidth lower than 50 Hz.

Heterogeneously integrated III-V/silicon dual-mode distributed feedback laser array for terahertz generation.

We demonstrate an integrated distributed feedback (DFB) laser array as a dual-wavelength source for narrowband terahertz (THz) generation. The laser array is composed of four heterogeneously integrated III-V-on-silicon DFB lasers with different lengths enabling dual-mode lasing tolerant to process variations, bias fluctuations, and ambient temperature variations. By optical heterodyning the two modes emitted by the dual-wavelength DFB laser in the laser array using a THz photomixer composed of an uni-traveling carrier photodiode (UTC-PD), a narrow and stable carrier signal with a frequency of 0.357 THz is generated. The central operating frequency and the emitted terahertz wave linewidth are analyzed, along with their dependency on the bias current applied to the laser diode and ambient temperature.

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy.

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements.