Highly compact laser sources with low threshold, exceptional directivity, and single-mode operation are in great demand for on-chip integrated photonics. Photonic bound states in the continuum (BIC) are peculiar nonradiative localized modes that have theoretically infinite lifetime within the radiation continuum, making it a favorable candidate for pursuing single-mode, low-threshold, and surface-emitting lasers. In this presentation, I will introduce several electrically pumped Terahertz semiconductor lasers we have developed based on the BIC concepts for achieving high Q and low laser thresholds in compact cavities while achieving single mode operations, which would be promising as monolithically integrated laser sources.

The quantum cascade laser pumped molecular laser (QPML) is a polyvalent source spanning the THz gap, with demonstrated operation from 200 GHz up to more than 5 THz. Using various models for molecular relaxation permits the derivation of a laser model that describes the pressure dependent behavior of the QPML. Here, we will discuss new designs that enable high performance operation.

In the quest towards room temperature THz Quantum Cascade Laser (QCL), reducing the loss due to electron-phonon scattering is a key strategy. With that goal, we present here a novel THz QCL design based on direct bandgap GeSn/SiGeSn semiconductor alloys. In contrast to the previously proposed THz QCL based on Ge/SiGe, these direct bandgap materials with low electron effective masses combine two major advantages for room temperature THz QCL. They offer not only weak electron-phonon scattering presented in non-polar group-IV semiconductors but also a high optical gain. Nextnano NEGF simulations predict the presence of gain and thus a possibility of lasing in the proposed GeSn/SiGeSn QCL up to room temperature using a conventional metal-metal waveguide.

We investigated a new generation of terahertz optoelectronics that can be monolithically integrated using quantum well structures. A first-generation terahertz transceiver based on GaAs/AlGaAs quantum well structures was designed, fabricated and characterized. The transceiver chip includes a semiconductor optical amplifier (SOA) integrated with a p-i-n diode based on the same quantum well structure, enabling both generation and detection of frequency-tunable terahertz radiation in response to a heterodyning optical pump beam with a terahertz beat frequency. This structure can be directly transferred to InP based material system working at telecom wavelengths. The optical source can be integrated on the same substrate along with optical intensity/phase modulators, paving the way for fully integrated terahertz imaging, spectroscopy, and communication systems.

Utilizing plasmonic nanoantennas capable of concentrating light in the infrared spectrum and serving as terahertz antennas, we have engineered a terahertz focal-plane array (THz-FPA) suitable for integration into terahertz pulsed imaging systems. Comprising 64 pixels, this detector array enables scanning of a 5 cm line width. With its rapid scanning capability and expansive field-of-view, the THz-FPA has the potential to elevate terahertz pulsed imaging systems beyond mere metrology tools, rendering them high-throughput instruments applicable in industrial environments for diverse non-destructive evaluation purposes.

High-frequency on-chip communication necessitates compact plasmonic network designs, shrinking electromagnetic waves to subwavelength scales. While metallic metasurface pathways handle Spoof Surface Plasmon Polaritons (SSPPs) at microwave and terahertz frequencies, real-time reconfigurability remains challenging. We introduce a dynamically tunable metasurface employing a fishbone structure with electrostatically deflectable microbridges. Voltage application lowers the cut-off frequency and shifts the dispersion curve. Being scalable beyond 120 GHz, this leverages micro-electromechanical systems for active manipulation. We developed a tunable SSPP low-pass filter that effectively blocks SSPP transmission upon activation at a SSPP frequency of 106 GHz.

The development of planar metamaterials, specifically metalenses, has gained attention for creating ultra-thin optical components by manipulating subwavelength structures. This is crucial for reducing the size of traditional bulky high-NA lenses, especially in the terahertz range. Current metasurface fabrication relies on complex lithography techniques in clean rooms, but we are exploring ultrashort pulsed laser processing as a simpler alternative. We have successfully utilized femtosecond laser processing technology to fabricate terahertz membrane metalenses at 0.8 THz by forming controlled-diameter through-holes on a high-resistance silicon substrate. The focusing performance of the fabricated terahertz metalenses was found to be the same as that of meta-lenses fabricated by photolithography. This one-step process has a potential simplification in terahertz optics fabrication.

Heterodyne receivers, based on plasmonic photomixers have demonstrated high-sensitivity and broadband terahertz detection with high spectral resolution. They consist of a terahertz antenna with plasmonic contact electrodes fabricated on a photoconductive substrate. When the photomixer is pumped by two Continuous-Wave (CW) lasers with a terahertz beat frequency, the received terahertz signal by the antenna is down-converted to an intermediate frequency (IF) equal to the difference between the received terahertz frequency and optical beat frequency. The IF signal power increases quadratically with the optical pump power at low optical pump powers. However, the dependence of the IF signal on the optical pump power deviated from a quadratic behavior at high optical powers and could eventually saturate, limiting the maximum responsivity and Signal-to-Noise Ratio (SNR). We demonstrate that large area nanoantenna arrays, based on plasmonic gratings expand the device active area, raising the ceiling on optical power and, therefore, sensitivity.

Vector beams, enabling spatially dependent polarization states in the radial and azimuthal directions, have been intensively studied for various applications such as imaging, communication, and optical manipulation of magnetic materials. While liquid crystal-based Q-plates have been routinely employed to convert linearly polarized light to vector beams efficiently, their utility diminishes in the terahertz (THz) spectrum due to excessive absorption and large wavelengths. Alternatively, resonant metasurface-based Q-plates have been employed to demonstrate THz-vector beam generation, however, these Q-plates suffer from narrow operational bandwidth. We propose a method to design and fabricate twisted effective media-based Q-plate generating broadband terahertz vector beams. The twisted media consisted of stacked multiple layers of 270-um thick Si substrates with rotated line and space patterns following a specific twisting power- the angle per unit length along the beam propagation direction. By calculating the effective media with Berreman 4x4 method, we obtained the operating bandwidth of 0.5-1.5 THz (0.5-2.5 THz) with the twisting power of 22.5°/mm (4.5°/mm).

A high-speed line imaging scanner for walk-through security gates has been developed by combining a THz radar technology by using 275-305 GHz single transceiver with a high-speed mechanical beam scanner. This body scanner successfully visualizes concealed objects carried by pedestrians walking at more than 4 km hr^-1 with a single transceiver. Our other recent topics will be also presented.

We propose a fusion terahertz deep learning computed tomography framework designed to precisely reconstruct object 3D geometric information from THz temporal-spatio-spectral signals acquired through a terahertz time-domain spectroscopy system. This Unet-based fusion framework utilizes multi-scale branches for extracting spatio-spectral features, which undergo processing through an element-wise filter adaptive convolutional layer, resulting in high-quality restoration of THz 3D images. Furthermore, the proposed framework offers high scalability and adjustability, allowing users to choose their processing signal domains and seamlessly integrate their own modified fusion network.

We show how lightwave-driven terahertz scanning tunneling microscopy can be used as a platform for atomic-scale terahertz time-domain spectroscopy. We apply our new technique to silicon-vacancy centers at the surface of GaAs and discover a single-atom resonator with features reminiscent of the technologically important DX centers.

Non-linear optical phenomena, such as parametric detection and amplification, manifest themselves in materials such as lithium niobate (LN) under the influence of a powerful optical pump beam. These processes have facilitated the practical realization of femtosecond (fs) pulse sources in the visible (VIS) and near infrared (NIR) spectra. They are also central to quantum detection, promising extremely sensitive detection of low-energy photons, particularly in the terahertz (THz) frequency range. To explore this innovative detection approach, we used an intense and powerful THz source taking advantage of optical rectification in lithium niobate (LN) crystals with an inclined-pulse front-end pumping configuration. By taking advantage of the high brightness of this source, we can acquire NIR signals in real time by upconversion and broadband using a standard CCD camera. In this presentation, we will look at the technical intricacies of the source and detection methodologies, as well as our goal of achieving single THz photon detection capability in the near future, all in the context of using ytterbium lasers.

Up to 10% beam-to-radiation energy conversion efficiencies have been obtained at the UCLA THz FEL using a strongly tapered helical undulator at the zero-slippage resonant condition, where a circular waveguide is used to match the radiation group velocity to the electron beam longitudinal velocity. This results in short, broad bandwidth pulses with high peak power that are measured with single shot electro-optic sampling. This allows full reconstruction of the THz temporal field profile at varying beam energies, giving insight into the complex dynamics within the FEL.