Charan M. Shah

Dielectric resonator nanoantennas at visible frequencies

Drawing inspiration from radio-frequency technologies, we propose a realization of nano-scale optical dielectric resonator antennas (DRAs) functioning in their fundamental mode. These DRAs operate via displacement current in a low-loss high-permittivity dielectric, resulting in reduced energy dissipation in the resonators. The designed nonuniform planar DRA array on a metallic plane imparts a sequence of phase shifts across the wavefront to create beam deflection off the direction of specular reflection. The realized array clearly demonstrates beam deflection at 633 nm. Despite the loss introduced by field interaction with the metal substrate, the proposed low-loss resonator concept is a first step towards nanoantennas with enhanced efficiency. The compact planar structure and technologically relevant materials promise monolithic circuit integration of DRAs.

Flexible metasurfaces and metamaterials: A review of materials and fabrication processes at micro- and nano-scales

The ability to bend, stretch, and roll metamaterial devices on flexible substrates adds a new dimension to aspects of manipulating electromagnetic waves and promises a new wave of device designs and functionalities. This work reviews terahertz and optical metamaterials realized on flexible and elastomeric substrates, along with techniques and approaches to lend tunability to the devices. Substrate electromagnetic and mechanical characteristics suitable for flexible metamaterials are summarized for readers, followed by fabrication and processing techniques, and finally novel approaches used to-date to attain tunability. Future directions and emerging areas of interests are identified with these promising to transform metamaterial design and translate metamaterials into practical devices.

Mechanically tunable terahertz metamaterials

Electromagnetic device design and flexible electronics fabrication are combined to demonstrate mechanically tunable metamaterials operating at terahertz frequencies. Each metamaterial comprises a planar array of resonators on a highly elastic polydimethylsiloxane substrate. The resonance of the metamaterials is controllable through substrate deformation. Applying a stretching force to the substrate changes the inter-cell capacitance and hence the resonance frequency of the resonators. In the experiment, greater than 8% of the tuning range is achieved with good repeatability over several stretching-relaxing cycles. This study promises applications in remote strain sensing and other controllable metamaterial-based devices.

Elastomeric silicone substrates for terahertz fishnet metamaterials

In this work, we characterize the electromagnetic properties of polydimethylsiloxane(PDMS) and use this as a free-standing substrate for the realization of flexible fishnet metamaterials at terahertz frequencies. Across the 0.2–2.5 THz band, the refractive index and absorption coefficient of PDMS are estimated as 1.55 and 0–22 cm−1, respectively. Electromagnetic modeling, multi-layer flexible electronics microfabrication, and terahertz time-domain spectroscopy are used in the design, fabrication, and characterization of the metamaterials, respectively. The properties of PDMS add a degree of freedom to terahertz metamaterials, with the potential for tuning by elastic deformation or integrated microfluidics.

Efficiency and Scalability of Dielectric Resonator Antennas at Optical Frequencies

Dielectric resonators have been foreseen as a pathway for the realization of highly efficient nanoantennas and metamaterials at optical frequencies. In this paper, we study the resonant behavior of dielectric nanocylinders located on a metal plane, which in combination create dielectric resonator antennas operating in reflection mode. By implementing appropriate resonator models, the field distributions, the scaling behavior, and the efficiency of dielectric resonator antennas are studied across the spectrum from the microwave toward visible frequency bands. Numerical results confirm that a radiation efficiency above 80% can be retained up to the near-infrared with metal-backed dielectric resonators. This paper establishes fundamental knowledge toward development of high efficiency dielectric resonator antennas and reflection metasurfaces at optical frequencies. These dielectric resonators can be incorporated as basic elements in emerging applications, e.g., flat optical components, quantum dot emitters, and subwavelength sensors.

Flexible terahertz metamaterials for dual-axis strain sensing

Utilizing an elastic polymer, we design and experimentally demonstrate a four-fold symmetric flexible metamaterial operating at terahertz frequencies. The fabricated metamaterials exhibit good stretchability and recoverability. Two independent resonances can be observed when the structure is probed with linearly polarized terahertz waves in two orthogonal axes. Applying a stretching force along a main axis causes an observable frequency shift in the corresponding resonance, with minimal effect on the other. This study suggests a possible application of flexible metamaterials for dual-axis strain sensing.

Elastomer-Based Pneumatic Switch for Radio Frequency Microdevices

This paper reports the realization and characterization of a pneumatic microswitch integrated with a high-frequency radio frequency (RF) transmission line on an elastomer substrate. A process for the fabrication of low-loss RF coplanar transmission lines on flexible elastomeric polydimethylsiloxane (PDMS) substrates was developed, and devices realized using this process were used to determine the characteristics of PDMS as an RF substrate with uniform low loss and low dielectric constant being measured. To demonstrate the capabilities of this elastomer-based RF platform, a micromechanical switch exploiting a pneumatic membrane valve was integrated with the PDMS RF transmission line. Repeatable switching was observed with greater than 20-dB suppression in the “off” state and minimal degradation of the transmission line characteristics in the “on” state being achieved over a multioctave 2-20-GHz bandwidth. These valve-integrated transmission lines had an insertion loss of 0.16 dB·mm-1 at 20 GHz. This proof-of-concept device represents a novel combination of the areas of micropneumatics, flexible electronics, and broadband microwave devices with excellent RF properties, low interference, bias-free pneumatic switching, and relatively simple fabrication.

Spectral and angular characteristics of dielectric resonator metasurface at optical frequencies

The capability of manipulating light at subwavelength scale has fostered the applications of flat metasurfaces in various fields. Compared to metallic structure, metasurfaces made of high permittivity low-loss dielectric resonators hold the promise of high efficiency by avoiding high conductive losses of metals at optical frequencies. This letter investigates the spectral and angular characteristics of a dielectric resonator metasurface composed of periodic sub-arrays of resonators with a linearly varying phase response. The far-field response of the metasurface can be decomposed into the response of a single grating element (sub-array) and the grating arrangement response. The analysis also reveals that coupling between resonators has a non-negligible impact on the angular response. Over a wide wavelength range, the simulated and measured angular characteristics of the metasurface provide a definite illustration of how different grating diffraction orders can be selectively suppressed or enhanced through antenna sub-array design.

Strain-resistance relationship in gold conductors for elastomeric-based flexible devices

Flexible electronic devices rely on effective conductors integrated with elastomeric substrates. This work reports on characterization of thin gold layers on flexible polymers as a platform for further research into their use in flexible electronic and microsystems. This work utilizes standard microfabrication techniques and a biocompatible, silicone polymer (polydimethylsiloxane) as the flexible substrate material. Flexible conductors defined by gold have been realized, and the dependence of resistance on geometry has been characterized. The results follow theoretical resistance dependence on geometry while showing an increase in the resistivity of the gold layer, a direct effect of deposition on elastomer causing wrinkles or striations in the metal layer. This work also discusses the effect of uniaxial mechanical deformation on thin film conductors and defines a procedure for creating and testing them in a repeatable manner. The ability to stretch the resistors by 10%, with full recovery to original resistance value is demonstrated. This work has implications for flexible device performance and provides a platform for integrated applications. Future work will explore combinations with piezoelectric thin films to enable conversion of mechanical to electrical energy, as this flexible platform will enhance the functionality of such energy generators.

Large area metal-silicone flexible electronic structures

Conductive structures on flexible substrates are expected to be the foundation of the next generation of electronics, displays, and solar cells. This work presents a simplified fabrication process for definition of electrode structures on silicone polymer. Flexible gold thin film structures on polydimethyl siloxane (PDMS) were successfully realised. This process is conducive to large area fabrication of flexible electrode structures, for incorporation with microfluidics, and for conformal adhesion to non-planar surfaces.