Phase Stabilization of a Terahertz Wave Using Mach–Zehnder Interference Detection

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.

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.

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.

By using two-dimensional particle-in-cell simulations, we demonstrate a terahertz (THz) wave propagating in the stagnation region of a reentry plasma sheath. When a THz wave propagates in the plasma sheath, the frequency blue shift is initiated as the wavelength is compressed. Attenuation is observed due to energy transfer from the THz wave to the plasma internal energy. Electrons are first accelerated mostly via oscillating in the electric field and partially via the wake field and then deliver their energies to the other particles via collisions. Detailed discussions show that increasing the intensity and frequency of the THz wave results in a higher transmission coefficient, which indicates that high power and high frequency methods are useful for communication blackout mitigation. This investigation of THz wave transmission in plasma will facilitate the mitigation of radio frequency blackout in spacecraft reentry in earth’s atmosphere communication.

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

For nonlinear optical materials to be applicable as efficient broadband terahertz (THz) wave generators, low absorption with wide transparency in the THz frequency range is highly important. In this study, we report efficient organic THz wave generators, 2-(4-hydroxystyryl)-1-methylquinolinium 4-bromobenzenesulfonate (OHQ-BBS) single crystals. Interestingly, the OHQ-BBS crystals exhibit a wide molecular vibrational mode-free range in the THz frequency region from 1.7 to 5.1 THz with an absorption coefficient of <20 mm−1. By optical rectification employing 130 fs pump pulses at 1300 nm wavelength, the OHQ-BBS crystals generate extremely broad, dimple-free THz waves in the range of 1.2–5.5 THz. Additionally, a THz electric field that is 20 times higher than the field generated from the widely used ZnTe inorganic crystal is achieved. Therefore, the OHQ-BBS single crystals are highly promising materials for diverse THz photonic applications.

The transmission characteristics of terahertz (THz) waves in a non-uniform microplasma are investigated by using the scattering matrix method. The electron density distribution in microplasma is simulated by Epstein and parabolic models. The effects of physical parameters, such as the incidence angle of THz waves, microplasma size, electron density, and collision frequency, on the propagation of THz waves are numerically analyzed. The results show that lower frequency THz waves are difficult to penetrate the microplasma with high electron density and high collision frequency. The microplasma density distribution, especially the gradient variation of the density in the first layer, has a large effect on the reflection of THz waves. Thus, THz waves can be used to diagnose the physical parameters of microplasmas.

Enhancement of terahertz wave emission from air plasma excited by harmonic three-color laser fields

Terahertz (THz) wave generation based on ultrafast laser are one of the most predominant and popular technologies. In particular, for THz generation by photoconductor antenna and optical rectification, higher optical to THz conversion efficiency usually requires ultrafast laser excitation into semiconductor or nonlinear crystal, therefore it is of great desire to develop ultrafast laser with high performance. CW Yb-doped lasers at 980 nm have been applied to generate THz wave range from 0.5 to 6 THz by DFG technique. To meet the demand of some THz wave generation (e.g. BNA crystal, gold nanoplasma), 980 nm ultrafast fiber laser are ideal pump sources and in desire to be developed. Here we demonstrate a 978 nm robust all-polarization-maintaining (PM) picosecond fiber laser mode- locked by phase-biased nonlinear amplifying loop mirror (PB- NALM). By introducing a nonreciprocal - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula> /2 phase shifter into the fiber loop, a self-starting stable mode-locking is obtained with the center wavelength of 978.14 nm and a 3-dB bandwidth of 0.43 nm. The pulse duration is 8.1 ps corresponding to the time bandwidth product (TBP) of 1.09. The maximum average output power is 14.5 mW with a repetition rate of 45.438 MHz. After a master oscillator power amplification system, the 978 nm average output power reaches 2.12 W, which is then injected into a PPLN crystal for second harmonic generation and 52 mW 489 nm ultrashort lasers is obtained. This is, to the best of our knowledge, the first report of PB-NALM-based mode-locked fiber laser at 978 nm. The 978 nm ultrafast fiber laser and SHG to 489 nm have great potential in terahertz emission by shorter NIR and blue light.

Second-order nonlinear optical wavelength-conversion has been attractive for generating terahertz (THz) wave with high peak-power and for THz-wave detection with high sensitivity. Over 50 kilowatt peak-power THz-wave radiation and sensitive THz-wave detection down to several tens of atto-joule using LiNbO 3 or 4-dimethylamino- N’ -methyl-4’- stilbazolium tosylate (DAST) crystals have been demonstrated. LiNbO 3 crystal is an eminent nonlinear crystal for converting wavelengths between THz wave and near infrared (NIR) at frequency range from 1 to 3 THz. Mixing THz wave with an intense NIR pump beam in the LiNbO 3 provides generation of a signal light at a different frequency because of efficient figure of merit. Additionally, sensitivity of up-conversion detection working at room temperature is more than that of cryogenically cooled THz detector. Here, we report on a sensitive THz-wave detection based on novel design using a slant-stripe-type periodically poled Mg doped lithium niobate (PPMgLN) for practical use. The efficient scheme that two optical waves, the pump and up-conversion signal beams, propagate collinearly in the PPMgLN to achieve effective parametric amplification for the signal beam was designed. Minimum THz-wave detection was achieved down to energy about 100 aJ at the frequency of 1.6 THz. The result leads to a novel THz detector based on fiber and integrated optics with high sensitivity, robustness, and easy handling. The nonlinear optical up-conversion detection is promising and broadening THz horizons.

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.

We propose a novel lithium-niobate ribbon waveguide for difference-frequency generation of terahertz (THz) wave from infrared light sources. By using the proposed waveguide whose thickness is less than the wavelength of the THz wave, the collinear phase-matching condition can be realized. The efficient THz-wave generation is verified by numerical simulations.

We proposed a vacuum electronic device based on fleld emission mechanism for the generation of terahertz (THz) waves in a previous work (JVST B 23(2), 849, 2005). The preliminary simulation results showed that an electronic e-ciency up to 4% can be achieved with no external magnetic flelds applied. However, the interaction mechanism is not clear. In the present work, the interaction mechanisms are studied. The MAGIC code is used to investigate the interactions between the electrons and the THz waves. To understand the interaction mechanism, the cathode has been trimmed to emit electrons. The simulation results show that the e-ciency of the case corresponding to the trimmed cathode is higher than that of the original planar cathode. The interaction regions are located among the gaps between the cathode and the anode. The AC electric flelds of the THz waves not only velocity-modulate the electron beam but also cause the density modulation of the fleld emission current. This pre-bunching efiect provides the feedback loop as required by an oscillator.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the deposition of amyloid-β (Aβ), neurofibrillary tangles, neuroinflammation, and neurodegeneration in the brain. In recent years, considering the unsatisfied benefits of pharmacological therapies, non-pharmacological therapy has become a research hotspot for AD intervention. Terahertz (THz) waves with a range between microwave and infrared regions in the electromagnetic spectrum and high permeability to a wide range of materials have great potential in the bioengineering field. However, its biological impacts on the central nervous system, under either physiological or pathological conditions, are poorly investigated. In this study, we first measured the 0.14 THz waves penetration across the skull of a C57BL/6 mouse and found the percentage of THz penetration to be ~70%, guaranteeing that THz waves can reach the relevant brain regions. We then exposed the APPSWE/PS1DE9 mouse model of AD to repeated low-frequency THz waves on the head. We demonstrated that THz waves treatment significantly improved the cognitive impairment and alleviated AD neuropathology including Aβ deposition and tau hyperphosphorylation in the AD mice. Moreover, THz waves treatment effectively attenuated mitochondrial impairment, neuroinflammation, and neuronal loss in the AD mouse brain. Our findings reveal previously unappreciated beneficial effects of THz waves treatment in AD and suggest that THz waves may have the potential to be used as a novel therapeutic intervention for this devastating disease.