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Terahertz quantum-cascade vertical external cavity surface emitting laser (VECSELs) are made possible through the development of amplifying reflectarray metasurfaces. The metasurface is made up of sub-wavelength arrays of antenna coupled sub-cavities loaded with quantum-cascade active material. The QC-VECSEL approach allows scaling of laser power while maintaining a high quality, near diffraction limited beam - something which has been a long standing challenge for THz quantum-cascade lasers with sub-wavelength metallic waveguides. The latest results of cavity and metasurface engineering are presented, including the demonstration of a focusing reflectarray metasurface that enables a "flat-optics" hemispherical VECSEL cavity, with improved geometric stability and a Gaussian profile beam with beam quality parameter of M2=1.3.

Vertical-external-cavity surface-emitting lasers (VECSELs) have been successfully used in the visible and near-infrared to achieve high output power with excellent Gaussian beam quality. However, the concept of VECSEL has been impossible to implement for quantum-cascade (QC) lasers due to the "intersubband selection rule". We have recently demonstrated the first VECSEL in the terahertz range. The enabling component for the QC-VECSEL is an amplifying metasurface reflector composed of a sparse array of metallic sub-cavities, which allows the normally incident radiation to interact with the electrically pumped QC gain medium. In this work, we presented multiple design variations based on the first demonstrated THz QC-VECSEL, regarding the lasing frequencies, the output coupler and the intra-cavity aperture. Our work on THz QC-VECSEL initiates a new approach towards achieving scalable output power in combination with a diffraction-limited beam pattern for THz QC-lasers. The design variations presented in this work further demonstrate the practicality and potential of VECSEL approach to make ideal terahertz QC-laser sources.

The velocity-matched distributed photodetectors (VMDP) with high saturation power and large bandwidth have been proposed and demonstrated. The theoretical analysis on the trade-off between saturation power and bandwidth shows that VMDP provides fundamental advantages over conventional photodetectors. The theory for designing and simulating the performance of the VMDP is developed comprehensively from the aspect of microwave transmission line, optical waveguide, and the VMDP structure. The VMDP with very high saturation photocurrent (56 mA) and instrument-limited bandwidth (49 GHz) is demonstrated experimentally. The theoretical analysis and experimental results show that VMDP is attractive for high-performance microwave photonics links and high-power optical microwave applications.

As the requirement for microwave engineering has been quickly changing, the research community must also change. Rather than observing the entire microwave research, this paper picks up a subjective view in the form of challenge to the electromagnetics for new requirement for microwaves. This challenge comes from quickly changing environment prompted by a renewed interest in "radio" engineering combined with defense cut. The classical electromagnetic theory must change to be able to provide constructive roles toward the modem radio engineering. Some of the features needed toward this goal is to capture interdisciplinary areas from the electromagnetics point of view. They include close couple to microelectronics, visualization, and system applications. Several subjective views and examples will be presented.

A new ultrafast photodetector with high bandwidth and high efficiency is proposed and analyzed. The new device, called optical-to-microwave transformer (OMT), consists of an input optical waveguide with periodically loaded photodiodes and a velocity-matched output microwave transmission line. Theoretical simulation shows that a bandwidth greater than 300 GHz and an external quantum efficiency of 46% can be achieved.

This paper summarizes the recent progress of engineering efforts in the area of active circuit techniques for quasi-optical applications. Typical grid type structures are described. In addition, a number of schemes for locking the output for a coherent power combining are reviewed. Some methods for characterizing these components are reviewed.

The high T sub c superconducting microstrip line and coplanar waveguide are compared in terms of the loss characteristics and the design aspects. The quality factor Q values for each structure are compared in respect to the same characteristic impedance with the comparable dimensions of the center conductor of the coplanar waveguide and the strip of the microstrip line. Also, the advantages and disadvantages for each structure are discussed in respect to passive microwave circuit applications.

This paper describes how various concepts and ideas developed in integrated optics structures can be and have been applied toward development of new components and devices for millimeter-wave integrated circuits. Methods of analyzing dielectric waveguides are given and examples of passive and active components made of dielectric waveguides are presented. We discuss possibilities of developing new active and nonreciprocal structures based on the distributed interaction of the guided waves with matters. Some of the difficulties involved in development of these devices are included.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electrically thick layered substrates using the resonant structures discussed in [2-4] offer the advantage of providing simple and efficient support for millimeter wave printed circuit antennas. They can also produce desirable beam patterns through the proper choice of dielectrics. There has been theoretical study of these layered structures [2-4], but experimental investigation has been limited. Also, the primary focus has been on the use of dipole antennas with these structures, while slot antennas have received little attention. Here we give results for efficiency calculations for both slots and dipoles, and experimental pattern measurements for slots on layered substrates which show that they can produce desirable beam patterns with high front-to-back ratio. We find good agreement between theory and lOGHz modeling experiments.

A small-signal and large-signal analysis of a traveling wave structure implemented by periodically loading a parallel-plate waveguide with QWITT diodes is presented. The analysis starts with a self-consistent, large-signal model for the diode. The diode model is then coupled to a model for the waveguide circuit and solved using the harmonic balance technique. The output power as a function of frequency for three harmonics for a QWITT oscillator is determined. Our results emphasize the importance of obtaining the ac voltage modulation self-consistently from the circuit when calculating the output power obtained from a QWITT oscillator. The oscillation frequency of the multiple-diode circuit is determined primarily by the periodicity of the structure. The physical dimensions of the waveguide facilitate monolithic fabrication of the oscillator circuit.

A number of waveguiding structures for millimeter-wave use are compared from the characteristic and application points of view.

The dominant loss mechanism is the conductor loss in planar MIC waveguides on low-loss substrates. Superconductors can dramati­ cally reduce the conductor loss, so further reductions in the total loss must come in the dielectric or radiation loss. This paper inves­ tigates the use of ~90 K superconductors to reduce the conductor loss. A rigorous mode-matching procedure is used to analyze two microstrip-like transmission lines for their dielectric loss. Total loss reductions are on the order of 10 to 100 times at 10 GHz when using superconductors and dielectric loss reduction techniques. Reductions at other frequencies below 100 GHz should be similar.1. INTRODUCTIONLosses in waveguides are generated from three sources: conductor loss, dielectric loss, and radiation. For the typical planar transmis­ sion lines used in microwave integrated circuit (MIC) and monolithic microwave integrated circuit (MMIC) applications, the conductor loss is dominant when low-loss substrates are used. Efforts to reduce the total loss have concentrated on conductor loss through two avenues of research: novel structures, and structural modifications to typical waveguides.Novel structures that have low conductor loss include image line [1] dielectric strip waveguide [2], nonradiative waveguide [3], and Micro- slab 14]. These structures reduce the conductor loss by replacing metallic confinement with dielectric confinement, so they typically work best at mid-to high-millimeter- wave frequencies. The primary disadvantage of these waveguides is their non-planar nature and the resulting high cost of high-volume manufacturing.From the manufacturing point-of-view, planar waveguides that can be fabricated using standard MIC technology [5] are highly de­ sirable. Structural modifications generally compatible with pho­ tolithographic production techniques have been shown to result in lower conductor losses. In microstrip, conductor-loss reduction has been shown by reducing the strip metallization thickness [6]. To­ tal loss reductions have been shown for inverted microstrip [7] and suspended-substrate microstrip [8]. Inverting and suspending the substrate induces wider lines, which have lower conductor loss. In­ verted microstrip might allow the use of SrTiO^ substrates, where the high dielectric constant (300-1800) and high dielectric loss tan­ gent (0.001 - 0.02) [9] would not as severely affect the performance since the fields lie primarily in the air gap between the strip and the ground plane. For coplanar waveguide, etching or ion milling to

A number of recent efforts in the area of dielectric waveguide techniques are reviewed for millimeter-wave circuits. Several forms of dielectric waveguides are discussed along with passive components. An isolator based on the nonreciprocal coupling is described in an image guide form. Some attempts for developing active components are included.