We report on THz MEMS sensors suitable for large focal plane arrays and readout schemes compatible with real-time imaging. Terahertz absorption near 100 %, optimized to particular monochromatic quantum cascade laser (QCL) illumination sources, was achieved using metal-dielectric metasurfaces. MEMS devices were designed using metasurface absorbers as structural components, allowing for streamlined fabrication of very efficient detectors in two different configurations. In the first scheme, bi-material sensors were used, where the heat from the absorbers is converted into mechanical deformation. The angular displacement, proportional to the absorbed THz radiation, was then optically probed. In the second configuration, THz to IR conversion was achieved whereas the front side of the metasurface absorbs THz and the backside served as an efficient infrared emitter, allowing its temperature to be probed directly by a commercial, thermal (infrared) camera. The devices are comprised of ultrathin films of silicon-rich silicon oxide and aluminum, deposited on silicon substrates and were fabricated standard MEMS processes including Bosh deep reactive ion etching to remove the substrate. The sensors were fabricated in a matrix configuration and individually characterized. The main figures of merit, such as spectral response, thermal time constant and sensitivity are controlled by the geometry and can be modified by design according to the application demands.
We propose a new approach for efficient detection of terahertz (THz) radiation in biomedical imaging applications. A double-layered absorber consisting of a 32-nm-thick aluminum (Al) metallic layer, located on a glass medium (SiO2) of 1 mm thickness, was fabricated and used to design a fine-tuned absorber through a theoretical and finite element modeling process. The results indicate that the proposed low-cost, double-layered absorber can be tuned based on the metal layer sheet resistance and the thickness of various glass media. This can be done in a way that takes advantage of the diversity of the absorption of the metal films in the desired THz domain (6 to 10 THz). It was found that the composite absorber could absorb up to 86% (a percentage exceeding the 50%, previously shown to be the highest achievable when using single thin metal layer) and reflect <1% of the incident THz power. This approach will enable monitoring of the transmission coefficient (THz transmission fingerprint) of the biosample with high accuracy, while also making the proposed double-layered absorber a good candidate for a microbolometer pixel’s active element.
We investigate the feasibility of negative photoresist as a structural material in metal-organic hybrid THz imaging detectors using SU-8. We will discuss design of metamaterials for MEMS-based terahertz (THz) thermal sensors and design and microfabrication process for building SU8-based MEMS THz focal plane arrays. Metamaterials of this kind, exhibiting absorption properties comparable to those of resonant metamaterials made using traditional thin films, coupled with the applicability of SU-8 as a structural material, offer possibilities for quick, simple microfabrication of focal plane arrays of THz imaging detectors. SU-8 is a low-cost material that can quickly be spun onto a substrate at a wide range of thicknesses and photolithographically patterned into a variety of structures. This removes the need for both PECVD deposition and plasma etching, dramatically increasing the speed and lowering the cost of production of such FPAs. We further investigate feasibility of use of such detectors as band translators rather than traditional bimaterial devices. Translators would be optically probed with an infrared (IR) camera. Individual pixels would absorb THz radiation, heat up and the thermal image would be projected onto an infrared camera, effectively translating the image from THz into IR.
One attractive option to achieve real-time terahertz (THz) imaging is a microelectromechanical systems (MEMS) bimaterial sensor with embedded metamaterial absorbers. We have demonstrated that metamaterial films can be designed using standard MEMS materials such as silicon oxide (SiOx), silicon oxinitrate (SiOxNy), and aluminum (Al) to achieve nearly 100% resonant absorption matched to the illumination source, providing structural support, desired thermomechanical properties and access to external optical readout. The metamaterial structure absorbs the incident THz radiation and transfers the heat to bimaterial microcantilevers that are connected to the substrate, which acts as a heat sink via thermal insulating legs, allowing the overall structure to deform proportionally to the absorbed power. The amount of deformation can be probed by measuring the displacement of a laser beam reflected from the sensor’s metallic ground plane. Several sensor configurations have been designed, fabricated, and characterized to optimize responsivity and speed of operation and to minimize structural residual stress. Measured responsivity values as high as 1.2 deg/μW and time constants as low as 20 ms with detectable power on the order of 10 nW were obtained, indicating that the THz MEMS sensors have a great potential for real-time imaging.
In this article we demonstrate a method based on Transfer Matrix (TMM) that can be used to analyze optical properties of multilayered thin films and planar metamaterials for terahertz (THz) detection. Producing and testing such films require host substrates that can be up to 4 orders of magnitude thicker than the THz-sensitive films. Therefore, the ability to efficiently model, simulate and accurately predict the optical properties of multilayered structures, with significant differences in thickness, is crucial to designing sensors with maximized absorption. This method, which provides an analytical tool, less computationally intensive then finite element modeling, can be used for films composed of any number of layers with arbitrary thicknesses, aspect ratios and arbitrary angles of incidence. Homogeneous or patterned (metamaterials) films can be modeled enabling accurate analysis of positive and negative index materials indistinctly. Reflection, transmission and absorption of metallic/dielectric nanolaminates, metallic thin films and planar metamaterial films are analyzed and compared with experimental measurements and FE simulations. Results show good agreement for a wide range of structures, materials and frequencies and indicate that the method has a great potential for design and optimization of sophisticated multilayered structures for THz detection and beyond.
There has been a continued interest in the terahertz (THz) imaging due to penetration and non-ionizing properties. Realtime imaging in this spectral range has been demonstrated using infrared microbolometer technology with external illumination by quantum cascade lasers (QCL). However, to achieve high sensitivity, it is necessary to develop focal plane arrays using enhanced THz-absorbing materials. One attractive option to achieve real time THz imaging is MEMS bi-material sensor with embedded metamaterial absorbers, consisting of a periodic array of metallic squared elements separated from a homogeneous metallic ground plane by a dielectric layer. We have demonstrated that the metamaterial films can be designed using standard MEMS materials such as silicon oxide (SiO<sub>x</sub>), silicon oxinitrate (SiO<sub>x</sub>N<sub>y</sub>) and aluminum (Al), to achieve nearly 100 % resonant absorption matched to the illumination source, while providing structural support, desired thermomechanical properties and access to external optical readout. The metamaterial structure absorbs the incident THz radiation and transfers the heat to bi-material microcantilevers that are connected to the substrate, which acts as a heat sink, via thermal insulating legs. A temperature gradient builds up in the legs, allowing the overall structure to deform proportionally to the absorbed power. The amount of deformation can be probed by measuring the displacement of a laser beam reflected from the sensor’s metallic ground plane. Several sensor configurations have been designed, fabricated and characterized to optimize responsivity, speed of operation and minimize structural residual stress. Measured figures of merit indicate that the THz MEMS sensors have a great potential for real-time imaging.
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/cm 2 output.
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
(SiO<sub>x</sub>) layer. The absorbing element is connected to two Al/SiO<sub>x</sub> 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 SiO<sub>x</sub> 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
Continued progress in terahertz (THz) research has emphasized the need for both improved THz sources and
detectors. One approach to generate a narrowband THz radiation is to use metamaterial absorbers as thermal
emitters. We present metamaterial based THz emitters consisting of a 100 nm aluminum layer patterned into
squares separated from a ground plane of aluminum by a thin layer of silicon oxide (<2 μm) fabricated using
standard microfabrication techniques. These metamaterials were designed to emit in one, two, and three different
bands of the 4-8 THz range and demonstrate clearly definable separate peaks with bandwidths of approximately 1
THz. Modifying the multiple band configurations can produce relatively broad emission peak if desired. Single
band emitters designed for 4.1, 5.4, and 7.8 THz were observed to emit, respectively, 11, 18, and 36 W/m<sup>2</sup> at 400 °C in accordance with Kirchhoff's law of thermal radiation. Coating a 4-inch wafer with these materials and heating it
to 400 °C would produce an estimated 86, 145, and 280 mW of power, respectively. Additionally, emitted power
increased linearly with temperature, as expected from the Planck’s radiation law in the THz spectral region at
elevated temperatures. Emissivity of the metamaterial did not change significantly when heated, indicating that the
constituent materials did not significantly change their optical or geometric properties.
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.
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.
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 (SiO<sub>x</sub>) 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.
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
SiO<sub>2</sub> 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.
Our work aims to identify nano-scale metal films with enhanced absorption between 1 to 10 THz for use in thermal
imagers operating in this spectral band. Absorption measurements of chromium and nickel films with different
thicknesses (8 - 30 nm) revealed absorption as high as 40% (Cr) and 27% (Ni) between 3 and 9 THz. Further analysis
showed that it is possible to optimize absorption by controlling conductivity of metal films by patterning them to reduce
fill factor. This design flexibility allows tailoring of the absorbing layer to reduce residual stress of membranes used in
microbolmeter and bi-material thermal sensors.
Recently, there has been a significant interest in Terahertz (THz) technology, primarily for its potential applications in
detection of concealed objects as well as in medical imaging for non-invasive diagnostics. This region of the spectrum
has not been fully utilized due to lack of compact and efficient THz sources and detectors. However, there are several
reports recently on real-time THz imaging using uncooled microbolometer camera and quantum cascade laser (QCL)
operating as a THz illuminator. The cameras used in these studies are optimized for infrared wavelengths and do not
provide optimal sensitivity in the THz spectral range. The fabrication of microbolometer focal plane arrays (FPAs) is
relatively complex due to the required monolithic integration of readout electronics with the MEMS pixels. The recent
developments in bi-material based infrared FPAs, utilizing optical readout, substantially simplifies the FPA fabrication
process by decoupling readout and sensing. In this paper, design and fabrication of a bi-material based FPAs, optimized
for the THz wavelengths, as well as design and integration of the readout optical system for real-time imaging will be
MEMS thermal transducers offer a promising technological platform for uncooled IR imaging. We report on the fabrication and performance of a 256x256 MEMS IR FPA based on bimaterial microcantilever. The FPA readout is performed using a simple and efficient optical readout scheme. The response time of the bimaterial microcantilever was <15 ms and the thermal isolation was calculated to be < 4x10<sup>-7</sup> W/K. Using these FPAs we obtained IR images of room temperature objects. Image quality is improved by automatic post-processing of artifacts arising from noise and non-responsive pixels. An iterative Curvelet denoising and inpainting procedure is successfully applied to image output. We present our results and discuss the factors that determine the ultimate performance of the FPA. One of the unique advantages of the present approach is the scalability to larger imaging arrays.
We report on the fabrication and characterization of microcantilever based uncooled focal plane array (FPA) for infrared imaging. By combining a streamlined design of microcantilever thermal transducers with a highly efficient optical readout, we minimized the fabrication complexity while achieving a competitive level of imaging performance. The microcantilever FPAs were fabricated using a straightforward fabrication process that involved only three photolithographic steps (i.e. three masks). A designed and constructed prototype of an IR imager employed a simple optical readout based on a noncoherent low-power light source. The main figures of merit of the IR imager were found to be comparable to those of uncooled MEMS infrared detectors with substantially higher degree of fabrication complexity. In particular, the <i>NETD </i>and the response time of the implemented MEMS IR detector were measured to be
as low as 0.5K and 6 ms, respectively. The potential of the implemented designs can also be concluded from the fact that the constructed prototype enabled IR imaging of close to room temperature objects without the use of any advanced data processing. The most unique and practically valuable feature of the implemented FPAs, however, is their scalability to high resolution formats, such as 2000x2000, without progressively growing device complexity and cost.