Brian Kearney

Bi-material terahertz sensors using metamaterial structures.

In this paper we report on the design, fabrication and characterization of terahertz (THz) bi-material sensors with metamaterial absorbers. MEMS fabrication-friendly SiOx and Al are used to maximize the bimetallic effect and metamaterial absorption at 3.8 THz, the frequency of a quantum cascade laser illumination source. Sensors with different configurations were fabricated and the measured absorption is near 100% and responsivity is around 1.2 deg/μW, which agree well with finite element simulations. The results indicate the potential of using these detectors to fabricate focal plane arrays for real time THz imaging.

Microelectromechanical systems bimaterial terahertz sensor with integrated metamaterial absorber

This Letter describes the fabrication of a microelectromechanical systems (MEMS) bimaterial terahertz (THz) sensor operating at 3.8 THz. The incident THz radiation is absorbed by a metamaterial structure integrated with the bimaterial. The absorber was designed with a resonant frequency matching the quantum cascade laser illumination source while simultaneously providing structural support, desired thermomechanical properties and optical readout access. Measurement showed that the fabricated absorber has nearly 90% absorption at 3.8 THz. A responsivity of 0.1°/μW and a time constant of 14 ms were observed. The use of metamaterial absorbers allows for tuning the sensor response to the desired frequency to achieve high sensitivity for potential THz imaging applications.

Strong terahertz absorption using SiO2/Al based metamaterial structures

Metamaterial absorbers with nearly 100% absorption in the terahertz (THz) spectral band have been designed and fabricated using a periodic array of aluminum (Al) squares and an Al ground plane separated by a thin silicon dioxide (SiO2) dielectric film. The entire structure is less than 1.6 mm thick making it suitable for the fabrication of microbolometers or bi-material sensors for THz imaging. Films with different dielectric layer thicknesses exhibited resonant absorption at 4.1, 4.2, and 4.5 THz with strengths of 98%, 95%, and 88%, respectively. The measured absorption spectra are in good agreement with simulations using finite element modeling.

Al/SiOx/Al single and multiband metamaterial absorbers for terahertz sensor applications

Abstract. To increase the sensitivity of uncooled thermal sensors in the terahertz (THz) spectral range (1 to 10 THz), we investigated thin metamaterial layers exhibiting resonant absorption in this region. These metamaterial films are comprised of periodic arrays of aluminum (Al) squares and an Al ground plane separated by a thin silicon-rich silicon oxide (SiOx) dielectric film. These standard MEMS materials are also suitable for fabrication of bi-material and microbolometer thermal sensors. Using SiOx instead of SiO2 reduced the residual stress of the metamaterial film. Finite element simulations were performed to establish the design criteria for very thin films with high absorption and spectral tunability. Single-band structures with varying SiOx thicknesses, square size, and periodicity were fabricated and found to absorb nearly 100% at the designed frequencies between three and eight THz. Multiband absorbing structures were fabricated with two or three distinct peaks or a single-broad absorption band. Experimental results indicate that is possible to design very efficient thin THz absorbing films to match specific applications.

Narrowband terahertz emitters using metamaterial films

In this article we report on metamaterial-based narrowband thermal terahertz (THz) emitters with a bandwidth of about 1 THz. Single band emitters designed to radiate in the 4 to 8 THz range were found to emit as high as 36 W/m(2) when operated at 400 °C. Emission into two well-separated THz bands was also demonstrated by using metamaterial structures featuring more complex unit cells. Imaging of heated emitters using a microbolometer camera fitted with THz optics clearly showed the expected higher emissivity from the metamaterial structure compared to low-emissivity of the surrounding aluminum.

Highly absorbing nano-scale metal films for terahertz applications

Our work aims to identify nano-scale metal films with enhanced absorption in the terahertz (THz) spectral range (1 to 10 THz) that can be incorporated in thermal imagers that operate in this spectral band. Absorption measurements of chromium and nickel films with different thicknesses (2.5 to 50 nm) revealed that absorption as high as 47% can be achieved by controlling the thickness of the film. The measured absorption agrees well with the predicted maximum absorption of 50% using thin metal films. The results indicate that nanometer scale metal films can provide high THz absorption for applications in thermal sensing.

Tunable THz absorption using Al/SiO x planar periodic structures

To increase the sensitivity of uncooled microbolometer-based THz imagers, absorbing structures (metamaterial films) with resonant absorption that can be tuned to a QCL illuminator frequency are investigated. The metamaterial films are comprised of periodic arrays of aluminum (Al) squares and an Al ground plane separated by a thin silicon-rich silicon oxide (SiOx) dielectric film. Finite element simulations were performed by varying the structural parameters to establish the design criteria for high absorption, spectral tunability and bandwidth. Several structures with single band and multiband absorption characteristics were fabricated. Measured absorption spectra show absorption up to 100% at designed THz frequencies and the spectral characteristics agree with simulations.

High sensitivity metamaterial based bi-material terahertz sensor

We report on the fabrication of a microelectromechanical systems (MEMS) based bi-material terahertz (THz) detector integrated with a metamaterial structure to provide high absorption at 3.8 THz. The absorbing element of the sensor was designed with a resonant frequency that matches the quantum cascade laser illumination source, while simultaneously providing structural support, desired thermomechanical properties and optical read-out access. It consists of a periodic array of aluminum squares separated from a homogeneous aluminum (Al) ground plane by a silicon-rich silicon oxide (SiOx) layer. The absorbing element is connected to two Al/SiOx microcantilevers (legs), anchored to a silicon substrate, which acts as a heat sink, allowing the sensor to return to its unperturbed position when excitation is terminated. The metamaterial structure absorbs the incident THz radiation and transfers the heat to the legs where the significant difference between thermal expansion coefficients of Al and SiOx causes the structure to deform proportionally to the absorbed power. The amount of deformation is probed optically by measuring the displacement of a laser beam reflected on the Al ground plane of the metamaterial absorber. Measurement showed that the fabricated absorber has nearly 95% absorption at 3.8 THz. The responsivity and time constant were found to be 1.2 deg/μW and 0.65 s, respectively. The minimum detectable incident power including the readout noise is around 9 nW. The obtained high sensitivity and design flexibility indicate that sensor can be further tuned to achieve the required parameters for real time THz imaging applications.

Metal-organic hybrid resonant terahertz absorbers with SU-8 photoresist dielectric layer

Abstract. We report on the characterization of metal-organic hybrid metamaterials for MEMS-based terahertz (THz) thermal sensors and on the characterization of refractive index of SU-8 in the THz band. This type of metamaterial, coupled with the applicability of SU-8 as a structural material, offers possibilities for quick, simple microfabrication of THz imagers. SU-8, a negative photoresist, is a low-cost material that can quickly be spun onto a substrate at a wide range of thicknesses, and then photolithographically patterned into a variety of structures. It is also transparent to THz radiation and thus a suitable choice for a dielectric spacer in metamaterials. We investigated metamaterials consisting of a 0.18 μm Al ground plane and 0.18-μm layer of patterned Al separated by a dielectric spacer of ∼0.5  μm of SU-8. Absorption close to 70% at around 6.1 THz was observed. A model was developed to simulate absorption spectra of several metamaterials, agreeing well with experiments. Matching simulation to measurements was used to determine the refractive index of SU-8 at THz frequencies, extending the known values from 0.1 to 1.6 THz to as far as 10 THz. Finally, Kirchoff’s law for these metamaterials was verified and their use as THz emitters demonstrated with about 0.8  mW/cm2 output.

Identification of nano-scale films for THz sensing

There is a continued interest in the terahertz (THz) spectral range due to potential applications in spectroscopy and imaging. Real-time imaging in this spectral range has been demonstrated using microbolometer technology with external illumination provided by quantum cascade laser based THz sources. To achieve high sensitivity, it is necessary to develop microbolometer pixels using enhanced THz absorbing materials. Metal films with thicknesses less than the skin depth for THz frequencies can efficiently absorb THz radiation. However, both theoretical analysis and numerical simulation show that the maximum THz absorption of the metal films is limited to 50%. Recent experiments carried out using a series of Cr and Ni films with different thicknesses showed that absorption up to the maximum value of 50% can be obtained in a broad range of THz frequencies. A further increase in absorption requires the use of resonant structures. These metamaterial structures consist of an Al ground plane, a SiO2 dielectric layer, and a patterned Al layer. Nearly 100% absorption at a specific THz frequency is observed, which strongly depends on the structural parameters. In this paper, the progress in the use of thin metal films and metamaterial structures as THz absorbers will be described.