Chip-scale laser frequency microcombs has achieved equidistant coherent frequency markers over a broad spectrum, advancing frontiers in ultrafast frequency metrology, laser spectroscopy, dense communications, and precision metrology. In this talk we will describe advances in the generation of laser frequency microcombs in dispersion-engineered microresonators with terahertz frequency spacings. We start with the understanding of the phase- and frequency-noise characteristics of a variety of microcomb states, including the generation of THz Turing patterns in optical microcombs. Referenced to external high-purity sources, the microcombs – in the mmWave and THz frequency spacings – can preserve tooth-to-tooth relative frequency stabilization to an uncertainty of 50 mHz and 2.7e−16. Subsequently, these frequency microcombs drive the chip-scale efficient plasmonic photomixers from the Jarrahi group at UCLA, where together we are able to generate coherent and broadly-tunable THz-mmWave radiation generation. Coherent terahertz radiation spanning 2.8-octaves is achieved from 330 GHz to 2.3 THz, with ~ 20 GHz cavity-mode-limited frequency tuning step and ~ 10 MHz intracavity-mode continuous frequency tuning range at each step. By stabilizing the microresonator pump power and wavelength, we observed sub-100 Hz linewidth of the terahertz radiation with 1e-15 residual frequency instability. The room-temperature coherent frequency-agile THz radiation offers unique capabilities in metrology, sensing, imaging and communications.

In this paper, the authors discuss the optimization of the Fe(FM-layer)/Pt(NM-layer) for efficient THz emission considering the following points: the FM and NM layer thickness, the optical pump wavelength, the choice of the substrate, and the out-coupling efficiency to the free-space. To improve the out-coupling efficiency, we introduce antenna structures with various shape. It has been demonstrated that a well-designed antenna structure can enhance the THz emission efficiency by several times. In addition, the authors will report a magnetic-field bias modulation, with which the signal detection efficiency can be almost doubled.

High-power terahertz (THz) sources are highly desired for applications in biomolecular and chemical sensing such as detection of DNA and protein, evaluation of pollutants and hazards, and atmosphere monitoring. THz Quantum cascade lasers (THz QCLs) are one of the most promising terahertz sources for those applications, with a vast commercial potential. Here, we present a high-power terahertz quantum cascade laser emitting at ~3.9 THz operating in continuous-wave operation. The high output power and wall-plug efficiency are achieved based on a hybrid bound-to-bound quantum active design. A record output power of 312 mW and a low threshold power density of 0.8 kW/mm3 in continuous-wave mode at 20 K is demonstrated for a 300-μm-wide and 2-mm-long device from single facet. The wall-plug efficiency is 1.38% and the slope efficiency is 684 mW/A with a differential quantum efficiency of ~120 photons per injected electron. The demonstration of this low-threshold and high-power THz laser will promote THz-based remote sensing and standoff detection for pharmaceutical and health industry applications.

I will discuss diffractive optical networks designed by deep learning to all-optically implement various complex functions as the input light diffracts through spatially-engineered surfaces. These diffractive processors designed by deep learning have various applications, e.g., all-optical image analysis, feature detection, object classification, computational imaging and seeing through diffusers, also enabling task-specific camera designs and new optical components for spatial, spectral and temporal beam shaping and spatially-controlled wavelength division multiplexing. These deep learning-designed diffractive systems can broadly impact (1) all-optical statistical inference engines, (2) computational camera and microscope designs and (3) inverse design of optical systems that are task-specific. In this talk, I will give examples of each group, enabling transformative capabilities for various applications of interest in e.g., autonomous systems, defense/security, telecommunications as well as biomedical imaging and sensing.

We present a plasmonic photoconductive terahertz focal-plane array that can provide spatial amplitude and phase, ultrafast temporal and spectral information simultaneously with a high imaging speed. Utilizing the high dynamic range (> 60 dB) and wide bandwidth (> 3 THz) in all focal-plane array pixels, we demonstrate super-resolution imaging on partially-etched high-resistivity silicon objects and reconstruct both the 2D shape and depth with a 16-fold enhancement in the space-bandwidth product and an effective number of pixels larger than 1-kilo pixels. We also demonstrate a 16-fps terahertz time-domain video, which can be further super-resolved by a holographic algorithm.

We use particle swarm algorithms to devise subwavelength waveguide array structures that serve, for example, as transmissive walls (transmittance > 88%) for microwaves with incidence angles between -80 and +80 deg or spatial filters that refract microwaves with incidence angles smaller than +/-20 deg at a refraction angle of 0 deg in the forward direction. Furthermore, we optimized radar cross section reducing (RCSR) metasurfaces by use of stimulated annealing and applied machine learning to implement an RIS, whose backward deflection angle of a normally incident wave is electrically tuned between 5 deg and 65 deg for microwaves at 31 GHz.

An understanding of electron dynamics at interfaces requires access to the angstrom length defining the interface and the femtosecond time scale characterizing their dynamics. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following optical excitation. We apply this to probe the dynamics of charge transfer in two-dimensional heterostructures, finding that charge relaxation and separation occurs on 100 fs time-scales. We also present new measurements of the twist dependence of these processes. Finally I will also describe new efforts in other classes of 2d materials including the 2d hybrid perovskites.

The ability to general strong-field THz radiation has opened the possibility to study nonequilibrium structural and energetic dynamics in chemistry, biology, condensed matter physics, and a host of other quantum phenomena in ultrafast timescales. In this talk, we will review some of these enabling THz technologies and their application to emerging research in ultrafast quantum biochemistry and quantum information science using state-of-the-art probes, spanning ultrashort X-ray, optical, and free electron probes. The resulting knowledge and methodologies are poised to become essential components for addressing critical challenges and needs ranging from personalized medicine to approaches in environmental and toxicity remediation

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near-infrared to a semimetal with the number of atomic layers. Here, we demonstrate that a controlled THz nonlinearity and circular dichroism - tuned from monolayer to bulk PtSe2 - can be realized in wafer size polycrystalline PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. These results highlight that PtSe2 is promising for THz photonic applications, ranging from THz valleytronics to harmonic generation.

Quantum paraelectrics are materials that have a lattice structure similar to ferroelectrics and show soft mode phonon behavior. However, a macroscopic ferroelectric phase transition is inhibited by the quantum mechanical ground state energy. We use intense THz pulses to resonantly excite the soft mode in the quantum paraelectrics SrTiO3 (STO) and KTaO3 (KTO) and observe the resulting lattice dynamics using the Kerr effect, second harmonic and x-ray probe pulses while varying temperature and in-plane strain. We show that there is a strong nonlinear response of the soft mode phonon and coupling to other IR and Raman active modes. In bulk KTO there we observe the hardening of the soft mode and polarization of nano domains, but no transient THz induced ferroelectric state. On the other hand ferroelectricity in STO can be induced by in-plane strain and observed by a dramatic change in the nonlinear THz response.

The injection-seeded terahertz wave parametric generator (is-TPG) technology presented here is expected to enable measurements through thick shielding due to its wide dynamic range of over 120 dB in combination with detection methods that up-convert the terahertz wave to a near-infrared beam. Furthermore, recently machine learning has been applied to the fingerprint spectral analysis of reagents, which has enhanced identification accuracy. Real-time measurement has also been achieved by multi-wavelength generation and detection with image recognition of detection Stokes beams. We will present these improvements in is-TPG technology for non-destructive testing.

Terahertz focal-plane arrays (THz-FPAs) can revolutionize the terahertz technology market by addressing all critical needs of practical terahertz pulsed imaging (TPI) systems. Such THz-FPAs would transform TPI systems from a metrology tool with a slow scan speed and limited field-of-view to a high-throughput instrument that can be used in industrial settings for various quality control applications. For this purpose, we developed a 64-pixel one-dimensional THz-FPA that can scan a line width of 5 cm to use it in scenarios where moving, continuous material needs to be inspected. The plasmonic nanoantenna technology realizing THz-FPAs helped achieving broadband and high-sensitivity operation.

In this presentation, we will review recent progress in the development of ultrafast laser sources with high average power. We will present the state-of-the-art of high-power ultrafast laser sources with potential for driving THz sources, current technological challenges in scaling THz average power, and applications areas that could potentially benefit from these novel sources.

We present new design strategies of dielectric metasurfaces for terahertz generation, detection, and manipulation. Metasurface designs can enhance two-step photon absorption process of low-temperature grown GaAs and can be used to detect terahertz radiation with 60 dB of dynamic range and 4.5 THz of bandwidth using telecom-wavelengths lasers. Moreover, combining InAs metasurfaces and unpatterned films we can realize focused terahertz beam generation using Fresnel zone plate designs without additional terahertz lenses and other optical elements. Our all-dielectric metasurface designs provides an unparalleled opportunity to realize low cost, high performance, and compact terahertz devices.

Graphene plasmonic elements enhance the light-matter interaction at resonance. Intense optical excitation results in a nonlinear response, based on two main causes: thermal effects and the Kerr effect. Here we present polarization dependent pump-probe experiments on graphene disks. Those revealed that both effects are similar in strength, though the Kerr nonlinearity is much faster, making it a candidate for efficient harmonic generation. Besides excitation with linear polarization, we investigated the impact of circularly polarized pump radiation. The circulating currents cause an effective quasi-static magnetic field perpendicular to the disks and thus Faraday rotation without the need of external magnetic field.

The skin's interstitial fluid is rich in composition and easily accessible for the monitoring of systemic biomarkers, however, THz-based molecular detection in biological media is challenging. Machine learning can provide solutions, but strict data engineering is required to avoid confounding trends and ensure large training datasets. We propose an experimental framework to mimic interstitial fluid diffusion in ex vivo pig skin to detect analytes via THz-ATR spectroscopy. We evaluate the applicability of the protocol for controlled studies of THz-ATR spectroscopy-based biomolecular detection in skin. Our findings can significantly contribute to the field of ML-reinforced biosensing.

High-brightness sub-terahertz (THz)-wave source has been required for potential THz-wave nondestructive testing. We have succeeded in developing a ubiquitous THz-wave source with a peak intensity of about 200W at 0.3 THz which is on the level of a gyrotron, aimed to robot installation. The high-brightness THz-wave emission is based on a cascaded backward optical parametric down-conversion up to third-order Stokes radiation and a threshold reduction of 63% by an injection-seeding to the idler beam. This remarkable THz-wave source expands the horizon of THz-wave research and applications.