Towards low-noise tunable terahertz waves generation via photomixing

In nonlinear optical (NLO) crystals, the selection of pump light wavelengths for the generation of terahertz (THz) waves is limited due to problems associated with coherence length, refractive index, and absorption by the crystal. Relaxation of this limitation would open up potential light sources for THz generation. One such solution, Cherenkov phase matching, removes the coherence length constraint. In this study, we attempted to generate THz waves from an NLO crystal using femtosecond pulses of various wavelengths. Specifically, 805-nm and 1560-nm femtosecond pulses were used to pump a prism-coupled LiNbO3 crystal. Broadband THz-wave generation and a THz-wave output proportional to the square of the pump light intensity were observed at both wavelengths. The generation of THz waves by prism-coupled Cherenkov phase matching was not limited by the wavelength of the pump light. Moreover, THz-wave generation at even greater intensities may be possible by optimizing the pump source and coupling to an NLO crystal.

Tunable terahertz (THz)-wave generation from 0.5 to 4.5 THz has been achieved in a GaP crystal by collinear difference frequency mixing of dual wavelengths in the range 940–1000 nm. THz output energy of 5 nJ/pulse at 2.6 THz was obtained when the pump energy was 1.2 mJ/pulse.

In this paper, we numerically demonstrate a tunable terahertz single-mode generator, which mainly consists of a cylindrical waveguide structure with periodically corrugated walls. The liquid crystals (LCs) are filled in the waveguide and sealed by a dielectric layer. By using the mechanism of multi-transverse mode resonances, the variation periodicity of the wall is designed to realize the generation and transmission of a single second-order mode of terahertz radiation. The simulated results exhibit that, based on the magnetic characteristics of the filled LC molecules (E7), the frequency tunablity of such a terahertz single-mode generator can be achieved by adjusting the external magnetic fields. The proposed tunable terahertz single-mode generator is very conducive to its system integration and applications.

Understanding the physics of terahertz (THz) wave generation from water is essential for developing liquid THz sources. This letter reports detailed experimental measurements of THz wave emission by focusing intense laser pulses onto water films. The simulation based on a ponderomotive force-induced dipole is supported by the observation of the THz intensity dependence on the laser incidence angle. This work provides fundamental insights into the THz wave generation process in water and an alternative perspective for studying laser-induced breakdown in liquids.

A new scheme to generate narrow-band tunable terahertz (THz) radiation using a variant of the echo-enabled harmonic generation is analyzed. We show that by using an energy chirped beam, THz density modulation in the beam phase space can be produced with two lasers having the same wavelength. This removes the need for an optical parametric amplifier system to provide a wavelength-tunable laser to vary the central frequency of the THz radiation. The practical feasibility and applications of this scheme are demonstrated numerically with a start-to-end simulation using the beam parameters at the Shanghai Deep Ultraviolet Free-Electron Laser facility (SDUV). The central frequency of the density modulation can be continuously tuned by either varying the chirp of the beam or the momentum compactions of the chicanes. The influence of nonlinear rf chirp and longitudinal space charge effect have also been studied in our article. The methods to generate the THz radiation in SDUV with the new scheme and the estimation of the radiation power are also discussed briefly.

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystals. With DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.

Widely tunable surface-emitted monochromatic terahertz-wave generation beyond the Reststrahlen band

A scheme of terahertz (THz) radiation generation is investigated by photo-mixing of two super Gaussian laser beams having different frequencies (ω1, ω2) and wave numbers (k→1, k→2) in a performed corrugated plasma embedded with transverse dc magnetic field. Lasers exert a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons that couples with the density corrugations (n′=nα0eiαz) to generate a strong transient nonlinear current, that resonantly derives THz radiation of frequency ∼ωh (upper hybrid frequency). The periodicity of density corrugations is suitably chosen to transfer maximum momentum from lasers to THz radiation at phase matching conditions ω=ω1−ω2 and k→=k→1−k→2+α→. The efficiency, power, beam quality, and tunability of the present scheme exhibit high dependency upon the applied transverse dc magnetic field along with q-indices and beam width parameters (a0) of super Gaussian lasers. In the present scheme, efficiency ∼10−2 is achieved with the optimization of all these parameters.

Concept of Internal Mixing in Semiconductor Lasers and Optical Amplifiers for Room-Temperature Generation of Tunable Continuous TeraHertz Waves

A scheme is presented for generation of powerful coherent terahertz radiation by multiphoton resonant excitation of three-level hydrogenlike atoms and homonuclear diatomic molecules or molecular ions with strong laser pulses. The results of analytical and numerical investigations of the problem show that with the proposed mechanism one can achieve the implementation of widely tunable powerful terahertz sources, which may open new perspectives for terahertz science and its applications.

A complete analysis of terahertz(THZ) wave generation and detection of silicon nanowire gated field effect transistor (SNFET) is presented in this paper. Based on the developed SNFET THZ device theory, the dependence of THZ detection of SNFET on bias and structure parameters are obtained and illustrated. The THZ generation condition and various unique characteristics are also demonstrated and analyzed in details. The numerical skills utilized in this paper are also described. Based on the developed numerical tool, the evolution processes of plasma wave in generation and detection mode are also presented.

The dynamically tunable terahertz (THz) waves and electromagnetically induced transparency (EIT) in coupled hybrid superconducting niobium-graphene split-ring resonator arrays are investigated. Active modulation of THz waves is studied through two different approaches. Thermal tuning of THz amplitude and group delay is observed due to the temperature sensitivity of the niobium superconductor. Stronger photoresponses are observed when niobium is superconducting. The electrical tuning of the integrated hybrid device is accomplished through the integration of graphene patches with the superconducting circuit. The modulation of resonance strength and group delay is observed due to damping of the dark mode resonance in coupled split-ring resonator arrays. The proposed chip-scale device provides a route toward the implementation of active cryogenic THz devices.

In this research, the focus is on harnessing Terahertz (THz) waves for next-generation wireless communication, specifically for 6G networks.THz waves have the potential to significantly boost data rates beyond what is possible with current 5G technology.However, the main challenge lies in generating THz waves efficiently.Traditional electronic THz emitters, which use frequency multiplication, face limitations when dealing with the high carrier frequencies required for 6G.One major issue is increased phase noise due to higher-order frequency multiplication.The paper explores the use of photonic technology, particularly optical frequency combs (OFC), as a solution to reduce phase noise.Soliton microcombs, small and cost-effective OFCs, are introduced.These microcombs have a wide frequency spacing, making them suitable for 6G carrier frequencies.By directly using adjacent OFC modes for photomixing, low phase noise THz waves are generated [1].We demonstrated the generation of THz waves at 60 GHz using soliton microcombs and Uni-travelling-carrier photodiodes (UTC-PD).We have previously achieved wireless data transmission at 2 Gbit/s in the 560-GHz band [2].While the quality of the data transmission was acceptable, further improvement in the Q-factor is desired for higher-quality data transmission.To address this, optical injection locking (OIL) of high-power CW lasers to soliton microcombs is proposed [3].The experimental setup involves using distributed feedback laser diodes (DFBs) injection-locked to microcomb modes, enabling advanced modulation formats such as BPSK, QPSK, and 16QAM.Results show near error-free data transmission with a Q-value of 6.23 for 1 Gbit/s OOK data transmission at 560 GHz.Advanced modulation formats, including BPSK and QPSK, exhibit BER values below the FEC limit, ensuring error-free operation.Additionally, 16QAM data transmission meets IEEE standards.In conclusion, the paper presents a promising approach for generating high-quality THz waves for 6G wireless communication by using soliton microcombs and optical injection locking, enabling advanced modulation formats and achieving error-free data transmission.

We present calculations concerning the generation of terahertz waves by difference frequency generation (DFG) using mid-infrared (MIR) radiation. We investigate phase matching conditions using a waveguide design for semiconductor materials like GaAs, GaP, InP or InAs which are transparent for both MIR and THz radiation and exhibit a large nonlinear coefficient.

We have realized tunable terahertz wave parametric generator by using microchip Nd:YAG laser and observed output terahertz wave with tuning range of 0.9 - 3 THz, peak power of about 100 mW, and detected by schottky barrier diode.