High performance tunable absorbers for terahertz (THz) frequencies will be crucial in advancing applications such as single-pixel imaging and spectroscopy. Metamaterials provide many new possibilities for manipulating electromagnetic waves at the subwavelength scale. Due to the limited response of natural materials to terahertz radiation, metamaterials in this frequency band are of particular interest.
The realization of a high-performance tunable (THz) absorber based on microelectromechanical system (MEMS) is challenging, primarily due to the severe mismatch between the actuation range of most MEMS (on the order of 1-10 microns) and THz wavelengths on the order of 100-1000 microns. Based on a metamaterial design that has an electromagnetic response that is extremely position sensitive, we combine meta-atoms with suspended at membranes that can be driven electrostatically. This is demonstrated by using near-field coupling of the meta-atoms to create a substantial change in the resonant frequency.
The devices created in this manner are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness ( 1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and with a fast switching speed ( 27 us). In the snap-down state, the resonance shifts by γ >200% of the linewidth (14% of the initial resonance frequency), and the absorption modulation measured at the initial resonance can reach 65%.
All-dielectric metasurfaces are a versatile platform to investigate a host of exotic electromagnetic responses. Effects including high absorption, bound-states-in-the-continuum (BIC), and Huygens’ surfaces have been shown. However, conventional dielectric metasurfaces achieve their properties through geometry alone, and are consequently static. The usefulness for realistic applications is thus inherently limited. In order to overcome the limitations of static all-dielectric metasurfaces, we utilize optical photodoping to attain precise and ultrafast control. We demonstrate the optical control of Huygens’ metasurface (HMS) absorbers, and a dynamic BIC at terahertz frequencies. The BIC realizes a high-quality factor resonance Q ~ 8700 which may be modified by over 2 orders of magnitude by photodoping with bandgap light. The HMS absorber achieves an intensity transmission modulation depth of 99.93% and an associated phase change of greater than π/2 rad. Coupled mode theory and S-parameter simulations are used to elucidate the mechanism underlying the dynamics of the metasurfaces. Similar to metal-based metamaterials, both systems may be scaled in size to operate in nearly any band of the electromagnetic spectrum. The dynamic photonic systems studied here show wide tunability and versatility which are not limited to the spectral range demonstrated, offering a new path for reconfigurable metasurface applications. Our demonstration of dynamic control can be leveraged for applications requiring ultrafast response, or spatial filtering, leading to more compact, efficient, and versatile photonic components.
Mechanism of the two-port one input mirror-symmetric all-dielectric disk array perfect absorbers is studied by temporal coupled mode theory. The perfect absorption is resulted from the degenerate critical coupling of EH111 and HE111 modes of the dielectric disks. Analytical absorption with parameters extracted from Eigen-frequency analysis matches well with that by Scattering parameter simulations, we also show that the asymmetric total field is due to the different symmetry of the two modes. The effect of the geometric parameters and material loss tangent is also investigated, which can guide the design of such all-dielectric perfect absorbers.
We present a single pixel frequency division multiplexing imaging system with two metamaterial spatial light modulators (SLMs) for THz light field imaging. One SLM is used for slicing/modulating the 4D light field from various sub-apertures, while the second one together with a single pixel detector to implement 2D multiplexing measurement. This system is in essential a programming aperture in the frequency domain of carrier signals in our categorization of the traditional light field acquisition systems. We propose a prototype adaption design with available elements for THz imaging. Besides, we present the frequency selection method and reconstruction algorithm for square wave modulation/carrier signals for at most 4096-voxel light field dataset.
Proc. SPIE. 10194, Micro- and Nanotechnology Sensors, Systems, and Applications IX
KEYWORDS: Metamaterials, Infrared radiation, Microelectromechanical systems, Thermography, Control systems, Temperature metrology, Electromagnetism, Nanotechnology, Sensors, Current controlled current source
The union of mico / nano electromechanical systems with metamaterials offers a new route to achieve reconfigurable devices which control the emission of energy. Here we propose and demonstrate the idea of metamaterial MEMS capable of modifying the emitted energy without changing the temperature. Rather our device only alters the spectral emissivity, thus realize a differential emissivity equivalent to a nearly 20-degree Celsius temperature change across the thermal infrared. We pixelate our device and thereby achieve a spatio-temporal emitter capable of displaying thermal infrared patterns up to speeds of 110 kHz. Our results are not limited to the thermal infrared band, but may be scaled to nearly any sub-optical range of the electromagnetic spectrum, and verify the potential of MEMS metamaterials to operate as reconfigurable multifunctional devices with unprecedented energy control capabilities.
The utilization of metamaterials as spatial light modulators (SLMs) offers a new route to achieve reconfigurable single pixel imaging systems. Here we demonstrate a metamaterial SLM hat permits high speed imaging at terahertz frequencies using communications engineering techniques. Specifically we implement quadrature amplitude modulation (QAM) with our all-electronic metamaterial SLM, which enables a doubling of the imaging frame rate. A second independent technique of frequency diverse imaging uses a number of sub-carriers to permit parallelization of the imaging process, which results in an increase in imaging speed only limited by signal to noise. Our results are not limited to the terahertz band, but may be scaled to nearly any sub-optical range of the electromagnetic spectrum, and verify the potential of metamaterials to operate as reconfigurable multifunctional devices which will enable next generation imaging systems.
Metamaterials are subwavelength man-made plasmonic or dielectric structures designed to realize specific effects on the electromagnetic radiation interacting with the material. Here we aim to rotate the polarization of an incident terahertz beam using chiral metamaterials, while suppressing the circular dichroism which induces ellipticity of the output beam. An incident linearly polarized wave can be decomposed into left-circularly and right-circularly polarized waves, and the difference in propagation phase will result in rotation of the plane of polarization. Since chiral structures couple electric and magnetic fields, they are often implemented in complex geometries such as spirals or bi-layered plasmonic structures, which can achieve carefully balanced responses to the two fields. The important feature of the bi-layered plasmonic structure is the cross-coupling between the resonances of the two layers. It is precisely this coupling between the layers that induces currents in the structures that are mutually dependent producing chirality within the structure. By coupling a metallic structure to its complement, we are able to achieve strong transmission in the region of maximum polarization rotation, and relatively low ellipticity of the output state. Three different structures were fabricated for this work that will be referred to as: the plain crosses, crossed arrowheads, and crossed arcs, pictured in the figure below. The terahertz responses of the structures were compared using terahertz time-domain spectroscopy and numerical simulations using CST Microwave Studio software.
Electromagnetic metamaterials have demonstrated unprecedented control over light matter interactions and have realized exotic responses difficult to achieve with natural materials. The ability to achieve real-time control of novel responses exhibited by electromagnetic metamaterials has led to the realization of metadevices and metasystems. Here we experimentally demonstrate two realizations of single pixel imaging systems that rely entirely on all-electronic metamaterial spatial light modulators. The metasystem enables images to be digitally encoded with various measurement matrix coefficients, thus permitting high speed and fidelity imaging.
In this paper, we report a computational and experimental study using tunable infrared (IR) metamaterial absorbers (MMAs) to demonstrate frequency tunable (7%) and amplitude modulation (61%) designs. The dynamic tuning of each structure was achieved through the addition of an active material—liquid crystals (LC) or vanadium dioxide (VO2)--within the unit cell of the MMA architecture. In both systems, an applied stimulus (electric field or temperature) induced a dielectric change in the active material and subsequent variation in the absorption and reflection properties of the MMA in the mid- to long-wavelength region of the IR (MWIR and LWIR, respectively). These changes were observed to be reversible for both systems and dynamic in the LC-based structure.
Single pixel cameras are useful imaging devices where it is difficult or infeasible to fashion focal plan arrays. For example in the Far Infrared (FIR) it is difficult to perform imaging by conventional detector arrays, owing to the cost and size of such an array. The typical single pixel camera uses a spatial light modulator (SLM) - placed in the conjugate image plane – and is used to sample various portions of the image. The spatially modulated light emerging from the SLM is then sent to a single detector where the light is condensed with suitable optics for detection. Conventional SLMs are either based on liquid crystals or digital mirror devices. As such these devices are limited in modulation speeds of order 30 kHz. Further there is little control over the type of light that is modulated.
We present metamaterial based spatial light modulators which provide the ability to digitally encode images – with various measurement matrix coefficients – thus permitting high speed and fidelity imaging capability. In particular we use the Hadamard matrix and related S-matrix to encode images for single pixel imaging. Metamaterials thus permit imaging in regimes of the electromagnetic spectrum where conventional SLMs are not available. Additionally, metamaterials offer several salient features that are not available with commercial SLMs. For example, metamaterials may be used to enable hyperspectral, polarimetric, and phase sensitive imaging. We present the theory and experimental results of single pixel imaging with digital metamaterials in the far infrared and highlight the future of this exciting field.
A critical electromagnetic response of a self-complementary structure was investigated. The nearly perfect selfcomplementary checkerboard patterns were fabricated by the electron-beam lithography and their electromagnetic responses are measured in the terahertz region. The electromagnetic responses are affected drastically by the small structural difference even though the differences are less than 0.1% of the wavelength of the incident electromagnetic waves. The sample most close to the self-complementary checkerboard pattern shows a less frequency dependent response, which is expected for the self-complementary structures. In this sample, the metallic squares seem to be connected randomly from the SEM observation. The effect of the structural randomness in metal mesh structures is also investigated to obtain the common electromagnetic properties in randomly connected systems.
Imaging in long wavelength regimes holds huge potential in many fields, from security to skin cancer detection. However, it is often difficult to image at these frequencies – the so called ‘THz gap1’ is no exception. Current techniques generally involve mechanically raster scanning a single detector to gain spatial information2, or utilization of a THz focal plane array (FPA)3. However, raster scanning results in slow image acquisition times and FPAs are relatively insensitive to THz radiation, requiring the use of high powered sources. In a different approach, a single pixel detector can be used in which radiation from an object is spatially modulated with a coded aperture to gain spatial information. This multiplexing technique has not fully taken off in the THz regime due to the lack of efficient coded apertures, or spatial light modulators (SLMs), that operate in this regime. Here we present the implementation of a single pixel THz camera using an active SLM. We use metamaterials to create an electronically controllable SLM, permitting the acquisition of high-fidelity THz images. We gain a signal-to-noise advantage over raster scanning schemes through a multiplexing technique4. We also use a source that is orders of magnitude lower in power than most THz FPA implementations3,5. We are able to utilize compressive sensing algorithms to reduce the number of measurements needed to reconstruct an image, and hence increase our frame rate to 1 Hz. This first generation device represents a significant step towards the realization of a single pixel THz camera.
In this paper, we present two different types of THz spatial light modulators (SLMs) that use dynamic metamaterials (MMs) to enable multiplex imaging. One imaging setup consists of a doped semiconducting MM as the SLM, with multi-color super-pixels composed of arrays of electronically controlled metamaterial absorbers (MMAs). Our device is capable of modulation of THz radiation at frequencies up to 12 MHz with maximum modulation depths over 50%. We have also implemented a different system enabling high resolution, high-fidelity, multiplex single pixel THz imaging. We use optical photoexcitation of semiconductors to dynamically tune the electromagnetic properties of MMs. By copropagating a THz and collimated optical laser beam through a high-resistivity silicon (Si) wafer with a MM patterned on the surface, we may modify the THz transmission in real-time by modifying the optical power. By further encoding a spatial pattern on the optical beam, with a digital micro-mirror device (DMD), we may write masks for THz radiation.
Metamaterial and plasmonic composites have led to the realization that new possibilities abound for creating materials
displaying functional electromagnetic properties not realized by nature. Recently, we have extended these ideas by
combining metamaterial elements - specifically, split ring resonators - with MEMS technology. This has enabled the
creation of non-planar flexible composites and micromechanically active structures where the orientation of the
electromagnetically resonant elements can be precisely controlled with respect to the incident field. Such adaptive
structures are the starting point for the development of a host of new functional electromagnetic devices which take
advantage of designed and tunable anisotropy.
In this paper we present our recent developments in terahertz (THz) metamaterials and devices. Planar THz metamaterials and their complementary structures fabricated on suitable substrates have shown electric resonant response, which causes the band-pass or band-stop property in THz transmission and reflection. The operational frequency can be further tuned up to 20% upon photoexcitation of an integrated semiconductor region in the split-ring resonators as the metamaterial elements. On the other hand, the use of semiconductors as metamaterial substrates enables dynamical control of metamaterial resonances through photoexcitation, and reducing the substrate carrier lifetime further enables an ultrafast switching recovery. The metamaterial resonances can also be actively controlled by application of a voltage bias when they are fabricated on semiconductor substrates with appropriate doping concentration and thickness. Using this electrically driven approach, THz modulation depth up to 80% and modulation speed of 2 MHz at room temperature have been demonstrated, which suggests practical THz applications.
We demonstrate external control of metamaterials operating at terahertz frequencies. Through photodoping of
semiconducting substrates, used to support metamaterial arrays, we show ultrafast switching times. New metamaterial
"grids" are presented, which may be formed by the union of electric metamaterials arrays. Metamaterial
grids are then utilized to form a Schottky contact are used to demonstrate voltage switching of the metamaterials
resonance. Both devices presented may be utilized to form novel devices at terahertz frequencies and also scaled
to other energy regimes of interest.
Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental
technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or
photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed
for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is
important overcome such material limitations by searching for new materials, or by constructing artificial materials with
a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties
that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a
resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route
towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly,
the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials.
We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted
in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz
Tunable electromagnetic metamaterials can be designed through the incorporation of semiconducting materials.
We present theory, simulation, and experimental results of metamaterials operating at terahertz frequencies.
Specific emphasis is placed on the demonstration of external control of planar arrays of metamaterials patterned
on semiconducting substrates with terahertz time domain spectroscopy used to characterize device performance.
Dynamical control is achieved via photoexcitation of free carriers in the substrate. Active control is achieved by
creating a Schottkey diode, which enables modulation of THz Transmission by 50 percent, an order of magnitude
improvement over existing devices. Because of the universality of metamaterial response over many decades of
frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly
play a key role in future demonstrations of novel high-performance devices.
We present S and P polarized measurements of artificial bianisotropic magnetic metamaterials with resonant behavior at infrared frequencies. These metamaterials consist of an array of micron sized (~40μm) copper rings fabricated upon a quartz substrate. Simulation of the reflectance is obtained through a combination of electromagnetic eigenmode simulation and Jones matrix analysis, and we find excellent agreement with the experimental data. It is shown that although the artificial magnetic materials do indeed exhibit a magnetic response, care must be taken to avoid an undesirable electric dipole resonance, due to lack of reflection symmetry in one orientation. The effects of bianisotropy on negative index are detailed and shown to be beneficial for certain configurations of the material parameters.