Philip W. C. Hon

Zero-Index Terahertz Quantum-Cascade Metamaterial Lasers

We propose a new one-dimensional composite right/left-handed (CRLH) transmission line active metamaterial based upon a sub-wavelength metal waveguide loaded with quantum-cascade material that provides terahertz gain via stimulated emission. Finite element simulations were performed to design and characterize a one-dimensional CRLH metamaterial supporting ¿backward¿ waves in the range of 1-2 THz. The addition of capacitive gaps in the top metal and inductive virtual current paths from top contact to the ground plane of a metal-metal quantum-cascade waveguide introduces propagating negative index and zero-index modes. We evaluate the feasibility of a zero-index terahertz quantum-cascade laser, based on a CRLH resonator, which exhibits a uniform spatial mode that is immune to spatial hole burning. An alternate balanced design for traveling-wave applications is also discussed.

Metasurface external cavity laser

A vertical-external-cavity surface-emitting-laser is demonstrated in the terahertz range, which is based upon an amplifying metasurface reflector composed of a sub-wavelength array of antenna-coupled quantum-cascade sub-cavities. Lasing is possible when the metasurface reflector is placed into a low-loss external cavity such that the external cavity—not the sub-cavities—determines the beam properties. A near-Gaussian beam of 4.3° × 5.1° divergence is observed and an output power level >5 mW is achieved. The polarized response of the metasurface allows the use of a wire-grid polarizer as an output coupler that is continuously tunable.

Terahertz quantum-cascade laser with active leaky-wave antenna

We report the demonstration of a one-dimensional waveguide for terahertz quantum-cascade (QC) lasers, which acts as a leaky-wave antenna and tailors laser radiation in one dimension to a directional beam. This scheme adapts microwave transmission-line metamaterial concepts to a planar structure realized in terahertz metal-metal waveguide technology. The active leaky-wave antenna is fed by a master oscillator QC-laser with a mode that propagates with an effective phase index smaller than unity, such that it radiates in the surface direction. The direction of emission of main beam is governed by the antenna dispersion characteristic. 25° of beam steering is observed as the lasing frequency of the QC-laser is varied from 2.65–2.81 THz.

Radiation Model for Terahertz Transmission-Line Metamaterial Quantum-Cascade Lasers

We present the use of the cavity antenna model in predicting the radiative loss, far-field polarization and far-field beam patterns of terahertz quantum-cascade (QC) lasers. Metal-metal waveguide QC-lasers, transmission-line metamaterial QC-lasers, and leaky-wave metamaterial antennas are considered. Comparison of the fundamental and first higher order lateral mode in a metal-metal waveguide QC-laser shows distinct differences in the radiation characteristics. Full-wave finite-element simulations, cavity model predictions and measurements of far-field beam patterns are compared for a one-dimensional leaky-wave antenna. Lastly, an active leaky-wave metamaterial antenna with full backward to forward wave beam steering is proposed and analyzed using the cavity antenna model.

Dispersion Analysis and Design of Planar Electromagnetic Bandgap Ground Plane for Broadband Common-Mode Suppression

A planar electromagnetic bandgap (EBG) structure designed for broadband common-mode suppression is proposed. A uniplanar compact photonic-bandgap (UC-PBG) structure with periodic center slots is etched on the ground plane to obtain broadband common mode suppression while minimally disturbing the differential signal within the designed bandwidth. Dispersion analysis has been conducted for both common (even) and differential (odd) mode signals. Good signal integrity is observed in both simulated and measured results. The fractional bandwidth of the proposed EBG ground plane is around 70% with common-mode suppression below -20 dB. The proposed EBG ground planes may be applicable for systems requiring low-cost and simple solutions in designing high-speed differential interlinks operating above 5 Gbps.

Leaky and bound modes in terahertz metasurfaces made of transmission-line metamaterials

Prism coupling and reflection spectroscopy are used to characterize bound modes within composite right/left handed terahertz metamaterial waveguides. The cavity antenna model is used to understand the polarization dependence of the radiative coupling to TM00 and TM01 waveguide modes. Furthermore, the cavity model along with transmission-line theory is used to derive a surface impedance model for a waveguide array metasurface. Qualitative agreement with the experiment is observed, including a mode splitting for p-polarized surface waves at the light line and the existence of s-polarized magnetic spoof surface plasmons.

Terahertz composite right-left handed transmission-line metamaterial waveguides

We report terahertz metamaterial waveguides based on the concept of composite right/left-handed transmission-lines. The waveguides are implemented in a metal-insulator-metal geometry fabricated with spin-coated Benzocyclobutene and contact photolithography. Angle-resolved reflection spectroscopy shows strong resonant absorption features corresponding to both right-handed and left-handed (backward wave) propagating modes within the leaky-wave bandwidth. Tuning of the waveguide dispersion is achieved by varying the effective lumped element series capacitance. The experimental results are in good agreement with full-wave finite element method simulations as well as an intuitive transmission-line circuit model.

Thermal homeostasis using microstructured phase-change materials

Humans and other warm-blooded mammals maintain their body temperature within a narrow range in a process called homeostasis. This ability to maintain an internal temperature, which is relatively insensitive to changes in the external environment or heat load is vital for all complex processes that sustain life. Without the ability to regulate temperature, materials and devices that experience large temperature gradients or temperature cycles are vulnerable to performance degradation or even catastrophic failure. Thermal control akin to the way living organisms achieve thermal homeostasis is particularly important in environments such as space, where changing solar illumination can cause large temperature variations. Various systems have been used to mitigate temperature fluctuations; however, they tend to be bulky and require power. Here, we model micropatterned phase-change materials to design an efficient, solid-state alternative, which requires no external input power. Our design is based on switchable thermal emission, which takes advantage of temperature-induced phase-change behavior in thin films of vanadium oxide on silicon microcones.

Feasibility of graphene CRLH metamaterial waveguides and leaky wave antennas

The feasibility of composite right/left-handed (CRLH) metamaterial waveguides based upon graphene plasmons is demonstrated via numerical simulation. Designs are presented that operate in the terahertz frequency range along with their various dimensions. Dispersion relations, radiative and free-carrier losses, and free-carrier based tunability are characterized. Finally, the radiative characteristics are evaluated, along with its feasibility for use as a leaky-wave antenna. While CRLH waveguides are feasible in the terahertz range, their ultimate utility will require precise nanofabrication, and excellent quality graphene to mitigate free-carrier losses.

Transmission-line metamaterial antennas for THz quantum-cascade lasers

We present a scheme for achieving active composite right/left handed (CRLH) transmission line metamaterial waveguides in the THz by loading THz QC-laser metal-metal waveguides with sub-wavelength capacitive and inductive structures. We discuss our progress in using transmission-line metamaterial concepts for the engineering of THz active leaky-wave antennas that provide amplification and exhibit beam steering.