MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Xin Zhang, Ph.D. Professor, Department of Mechanical Engineering Distinguished Faculty Fellow, College of Engineering Boston University August 22, 2014
Dr. B.-L. Les Lee, AFOSR; Co-PI: Richard Averitt Background: Metamaterials
• Artificial, engineered materials • Similar properties may/may not be found in nature • Arises from structural properties rather than composition (or both) • Uses inhomogeneities to create effective macroscopic behavior
Conventional material with Metamaterial with positive Poisson’s ratio negative Poisson’s ratio
As long as we are able to create and engineer building blocks relatively smaller than the interaction Bückmann, Tiemo, et al. Tailored 3D Mechanical length of the outside stimuli, some artificial materials Metamaterials Made by Dip‐in Direct‐Laser‐Writing can be built using these building blocks with Optical Lithography. Advanced Materials 24.20 (2012): 2710-2714. engineered properties. Background: Metamaterials
- - -
+ + + Electric response (ε) originating from molecular shape Cut wire Disk
Magnetic response (µ) originating from electron spinning, orbiting, etc. Split ring resonator We can mimic natural electric and magnetic polarizibilities of materials using engineered structures such as a dipole antenna or a split ring resonator. Background: Metamaterials
The unit structure should be much smaller than Natural Material the wavelength for the homogenization.
Metamaterial By bundling these metamaolecules we can come up with an engineered macroscopic electromagnetic property. “Terahertz Gap”
Microwave: electron Terahertz gap Infrared and visible: photon
Electronics: Antenna, high Moderate progress in sources and Photonics: speed transistor circuits for detectors, functional devices such as Source: Lasers, LEDs microwave generation, filters, switches, modulators largely Detector: Photodiodes detection, control and do not exist; Functional: Lens, polarizer, manipulation optical switch Practical applications are limited. Applications: Wireless Applications: Optical fiber communications, radar… 1 THz 300 µm 4 meV 33cm-1 47 K communications… The term terahertz gap refers to the lack of emitters/sources and detectors in the spectrum. Neither traditional optical nor microwave techniques work well in the THz region, and new methods/materials have yet to be explored.
Higher power source; Wish List More sensitive and cheaper detectors; Compact way to tune/modulate the radiation. Why Terahertz?
Wavelength (m)
Frequency (Hz)
THz radiation is non-ionizing, safe to use on humans; Could penetrate many visually opaque materials such as clothing, paper, cardboard, wood, plastic, ceramics, useful to safety scanning; Vibration and rotation molecular excitation for simultaneously investigation of both physical and chemical properties of a material. Terahertz Metamaterials
A metamaterial unit cell is to terahertz wave, as an atom is to visible light Metamaterials can be easily tuned to desired electromagnetic properties (much easier than finding the right natural material) Size of terahertz metamaterials is a perfect match for microfabrication techniques
MEMS & Metamaterials: A perfect marriage at THz frequencies Metamaterials are sub-wavelength structures in array form. 1 Terahertz corresponds to 300 microns. Sub-wavelength of terahertz is around tens of micron. MEMS is a very powerful tool in terms of fabrication. THz TDS (Time Domain Spectroscopy)
We use terahertz time domain spectroscopy to characterize our samples.
We have a femto- second laser pulse to excite the optical crystal to get our terahertz radiation and then we focus the terahertz pulse onto our sample, and then we measure the sample response in the time domain.
By using Fourier Transform, we can get the response at To better understand the resonant frequency domain. properties at the fundamental resonance, numerical simulations are conducted using full wave EM Simulated circulating surface simulations with CST Microwave current density at the fundamental resonance StudioTM MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Single Planar Metamaterials on GaAs Substrate
Planar metamaterial arrays fabricated consist of 200 nm thick Au split ring resonators (SRRs) fabricated on GaAs substrates.
Semi-insulating GaAs wafers were chosen because they are highly transmitting at terahertz frequencies.
Optics Express, 16(23) THz Wallpaper Metamaterials with Multiple Resonances
Metamaterial Persian carpets Vol. 456, Nature
We present novel metamaterial structures based upon various planar wallpaper groups, in both hexagonal and square unit cells.
Our results verify that multiple element metamaterials can be successfully designed, fabricated, and measured at terahertz frequencies. Optics Express, 16(23) MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally, optically) Stand-up metamaterials at terahertz frequencies (capacitance, 3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band, stealth applications) Optically modulated perfect absorbers Nonlinear terahertz absorbers Substrate-free semiconductor metamaterials absorbers Metamaterials on Ultrathin Silicon Nitride Substrates
Most of devices are fabricated on high-permittivity substrate such as GaAs or high resistance silicon, which contributes a large capacitance to the resonator, diminishing the changes in capacitance induced by the targets.
SRR-metamaterials fabricated on thin film substrates show significantly better performance than identical SRR-metamaterials fabricated on bulk silicon substrates paving the way for improved biological and chemical sensing applications. Applied Physics Letters, 97(26) Flexible Metamaterials at THz Frequencies
Fabricating resonant THz metamaterials on free-standing polyimide substrates, which are highly mechanically flexible and transparent to THz radiation. The low-loss polyimide substrates can be as thin as 5.5 m yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. These results pave the way for creating multilayered non-planar electromagnetic composites.
Journal of Physics D: Applied Physics, 41(23) MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Metamaterials on Paper as a Sensing Platform
Experimentally measured transmission spectra of the paper metamaterial samples coated with a series of glucose (a) and urea (b) solutions with varying concentrations as function of frequency from 0.4 THz to 1.4 THz.
Advanced Materials, 23(28)
There is increasing interest in the development of cost-effective, practical, portable, and disposable diagnostic devices suited to on-site detection and analysis applications, which hold great promise for global health care, environmental monitoring, water and food safety, as well as medical and threat reductions. Silk Metamaterials at THz Frequencies
Advanced Materials, 22(32) Directly spray large area metamaterial structures on biocompatible silk substrates which exhibit strong resonances at desired frequencies, opening opportunities for new bioelectric and biophotonic applications including in vivo bio-tracking, bio- mimicry, silk electronics, and implantable biosensors and biodetectors. MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Frequency Tunable Terahertz Metamaterials
Electrical excitation Magnetic excitation
Hybridization in BC-SRRs
Frequency tunable MM designs at THz frequencies using broadside coupled split- ring resonator (BC-SRR) arrays: - Frequency tuning, arising from changes in near-field coupling, is obtained by in- Simulation Experimental results plane displacement of the two SRR layers. - For electrical excitation, the resonance frequency continuously redshifts as a function of displacement. - The maximum frequency shift occurs for vertical displacement of half a unit cell, resulting in a shift of 663 GHz (51% f0).
Physical Review B, 83(19) Structurally Tunable THz Metamaterials
• We demonstrate reconfigurable Magnetic anisotropic metamaterials at resonance terahertz frequencies where artificial ‘‘atoms’’ reorient within unit cells in response to an H external stimulus. E • This is accomplished by K fabricating planar arrays of split ring resonators on bimaterial cantilevers designed to bend out of plane in response to a thermal stimulus. Electric resonance • We observe a marked tunability of the electric and magnetic response as the split ring resonators reorient within their unit cells. E H • Our results demonstrate that K adaptive metamaterials offer significant potential to realize novel electromagnetic functionality ranging from thermal detection to reconfigurable Physical Review Letters, cloaks or absorbers. 103(14) MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) 3D Broadband Tunable Terahertz Metamaterials ‐ Design
Silicon pad
Double splits ring resonator Physical Review B - Rapid Communications, 87(16) - A silicon pad is patterned between the bottom gap of a double splits ring resonator. - Without photoexcitation, in the circuit model, two capacitances are connected in series. There’s a circulation current in the ring showing the LC resonance. - Under the normal incidence of THz wave with magnetic field normal to the plane, a LC resonance is induced in this ring. - Under a certain photoexcitation, the carriers in the silicon is excited so that the capacitance of the bottom in the LC circuit is shorted. - Then the resonant frequency shifts to lower frequency. 3D Broadband Tunable Terahertz Metamaterials ‐ Fabrication
Physical Review B - Rapid Communications, 87(16) 3D Broadband Tunable Terahertz Metamaterials ‐ Operation
- Photoexcitation of free carriers in the silicon was achieved using optical excitation with 35-fs ultrafast pulses with a center wavelength of 800 nm.
- This optical pump pulse was set to arrive 10 ps before the THz probe beam ensuring a near steady- state accumulation of carriers due to their long lifetime in silicon.
- Over 30% of tunability of the resonance frequency is achieved by photoexcitation of 3D metamaterials. Physical Review B - Rapid Communications, 87(16) MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Microwave and Terahertz Wave Sensing with Metamaterials
Absorbing - The samples are split ring resonators (SRR) fabricated on thin Metamaterials Detector SiNx and supported by bi-material cantilever legs. + - The materials in the cantilever legs have different coefficients Bimaterial of thermal expansion, which cause the legs, and subsequently Cantilever the SRR, to deflect with a change in temperature. - This change is induced by strong absorption in the SRR upon exposure to the appropriate frequency radiation. - To detect this deflection, a reflecting pad has been fabricated in the interior of the SRR. - A HeNe laser beam is focused upon this pad and the reflected beam is aligned to a position sensitive photo-detector (PSD). 95 GHz 693 GHz
Metamaterials combined with MEMS cantilever technology. Metamaterials are spectrally selective. SiNx/Au bimaterials cantilevers provide thermo‐mechanical response. Single pixel detector scalability is wafer level design and processing. Optics Express, Metamaterial absorbers at THz frequencies are compatible with MEMS processing. 19(22) Microwave and Terahertz Wave Sensing with Metamaterials
Photoresponse of the 693 GHz pixel. (a). Response of the detector as a function of incident power. The nonzero intercept results from residual vibrations. (Inset, right) Oscilloscope observed temporal responses of the 95 GHz at 8 Hz (blue) and 10 Hz (red), respectively. (Inset, left) THz-TDS characterized transmission spectrum of the detector showing a resonance at ~ 693 GHz. (b). Image of the incident THz beam profile using the metamaterial enhanced THz detector. Smaller sensor on the way? Metamaterials Optics Express, 19(22) to See in THz, Vol. 334, Science MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Electromagnetic Wave Absorbers
Microwave, military Other wavelengths and fields Stealth Bolometers, detectors, cameras, energy
Nonlinear absorbers Anechoic chambers Ultrafast optics, sensor/eye protection
Emitters, sensors, spatial light modulators, etc… Transmission/Reflection from a Surface
Wave impedance
Permeability
Ei, Bi Er, Br Permittivity
θi θr Reflection coefficient for TM Z1, εr,1, µr,1
Z2, εr,2, µr,2
Reflection coefficient for normal incidence θt Et, Bt
In the boundary between two materials the reflection is governed by Fresnel’s formulas. The problem can be simply represented as an impedance matching problem where the impedance is a function of macroscopic material properties: permittivity and permeability. However, meeting this matching condition is not very straight forward since material properties are complex functions of frequency. Transmission/Reflection from a Surface
Reflection and transmission coefficients
E0
θi θr r1E0 -t t E P2 Z0, ε0, µ0 1 2 0 … Z , ε , µ -t E P2 d d d 1 0 Propagation Phase θ d t …
3 t1E0P -t1r2E0P An effective impedance will look like
In practice we don’t work with infinitely long materials, therefore a termination is needed and a perfect electric conductor is a usual choice for the termination. In this case, we can represent the created slab in terms of an input impedance or effective impedance. As apparent from the formula, this impedance mainly depends on macroscopic behaviors of the dielectric material. Electromagnetic Wave Absorbers
For impedance matching: -Many materials have been studied -High refractive index and losses at the same time -Others: Salisbury, Jauman, Resistive Gradient material, Ferrites … Air Sheet Air Metal -Pyramidal absorbers (spatial impedance matching) Propagation Direction
/4 Electromagnetic Wave Absorbers
+
Carbon
Horse hair
SPONGEX Reflection from a Metamaterial Slab
Slab thickness MM resonator layer 0= 300 m
Dielectric slab
Conductive ground plane
An array of split ring resonators can produce an effective resonant permittivity as represented by the formula. This behavior including the resonance frequency and oscillation strength can be precisely controlled by the geometric parameters. At an optimized thickness A strong reflection dip around the resonance. Using a common conductor and a dielectric spacer. Easily scale the geometry, making it work for most of the electromagnetic spectrum. THz Metamaterial “Perfect” Absorbers
(a) (b) (c) (d) (a) Electric resonator on the top of a polyimide spacer; (b) Cut wire on GaAs wafer; (c) Single unit cell showing the direction of propagation of incident EM wave. (d) The experimental absorptivity (in blue) reaches a maximum value of 70% at 1.3 THz. This is one of the first examples of metamaterial absorbers. The absorber structures are usually quite similar which is composed of 1) a resonant metamaterial array, 2) a dielectric spacer, and 3) a conductive ground plane. They show a great absorption performance as we see in the experimental results here. This absorber is designed for normal incidence but it works good in oblique incidences, too. Filling the THz Gap, Vol. 329, Science Near-perfect ‘black’, Vol. 453, Nature Optics Express, 16(10) Flexible THz Wide Angle “Perfect” Absorbers
TE TE
TM TM
Terahertz metamaterial absorber consisting of two metallic layers and two dielectric layers Design, fabrication, and characterization of a metamaterial absorber which is resonant at terahertz frequencies; experimentally demonstrate an absorptivity of 0.97 at 1.6 THz. The absorber is only 16 m thick, resulting in a highly flexible material that, further, operates over a wide range of angles of incidence for both transverse electric (TE) and transverse magnetic (TM) radiation. Physical Review B – Rapid Communication, 78(24) Dual Band Terahertz Absorbers
1 + 1 = 2
- Dual band terahertz metamaterial absorber consisting of a dual band Single SRR electric-field-coupled (ELC) resonator and a metallic ground plane, separated by an 8 μm thick dielectric layer. - The two resonance responses can be tuned and optimized independently at desired frequencies with comparably high absorptivity as with single band metamaterial absorbers. - This feature provides more flexibility Dual-band ELC in multi-band absorber designs and can be readily extended to infrared and visible frequency ranges. Journal of Physics D: Applied Physics, 43(22) Flexible Metamaterial Absorbers for Stealth Applications
Optics Express, 20(1) We have wrapped metallic cylinders with strongly absorbing metamaterials. These resonant structures, which are patterned on flexible substrates, smoothly coat the cylinder and give it an electromagnetic response designed to minimize its radar cross section. We demonstrate a near-400-fold reduction of the radar cross section at the design frequency of 0.87 THz. Optically Modulated Perfect Absorbers
Ground plane
Polyimide
Silicon+ SRR
Sapphire
Hybridize SRRs with silicon islands, so that using another optical beam we can generate carriers in the SRR gap. This changes the conductivity or the capacitance of the SRR gap which in turn changes the resonance behavior. Nonlinear Terahertz Absorbers
Ground plane
Polyimide
n-InAs disks
SI-GaAs
We use InAs as the nonlinear oscillators to create our absorbers. To do this, we form a similar geometry as I showed before and after forming the metamaterial layer we cover it with a polyimide layer and finish with a conductive ground plane. Substrate‐Free Semiconductor Metamaterials Absorbers
Ground plane
Polyimide n-InAs disk array Sacrificial layer (AlInSb) SI-GaAs
It is possible to create substrate-free devices using a technique called transfer printing by using a sacrificial layer between the semiconductor metamaterial layer and the substrate. Simply etching the sacrificial layer will free the metamaterial layer from the substrate. MEMS and Metamaterials: A Perfect Marriage at Terahertz Frequencies
Single planar metamaterials on GaAs substrate THz wallpaper metamaterilas with multiple resonances
Metamaterilas in ultrathin silicon nitride substrates Flexible metamaterials at terahertz frequencies
Metamaterials on paper as a sensing platform Silk metamaterilas at terahertz frequencies
Tunable metamaterials at terahertz frequencies (frequency, structurally)
Stand-up metamaterials at terahertz frequencies (3D broadband tuning)
Microwave and terahertz wave sensing with metamaterials
THz metamaterial ‘perfect’ absorbers (flexible, wide angle, dual band) Functional Metamaterials at Terahertz Frequencies
Nanoscribe Nanoscribe
Metamaterials (Blurring Distinction Between Materials and Devices): • A useful toolbox for engineering and science • Many optimization options even using the same geometry • One step closer to real world applications • Nature Highlight: 2 Advanced Optical Materials (1) • Science Highlight: 2 Applied Physics Letters (2) • MRS Bulletin: 1 Journal of Physics D: Applied • Journal Covers: 3 Physics (2) • PhD Dissertations: 3 Journal of Micromechanics & • Best Thesis Awards: 2 Microengineering (1) • Journal papers: 30 IEEE Terahertz Science & Technology (1+1*) Nature (1) Journal of Infrared, Millimeter, Physical Review Letters (2) and Terahertz Waves (2*) Physical Review B - Rapid Journal of Selected Topics in Communication (2) Quantum Electronics (1*) Physical Review B (3) Journal of Modern Optics (1*) Advanced Materials (2) (*denotes invited review papers) PhD Dissertations (Advisor: Xin Zhang): Optics Express (8) Hu Tao, "MEMS Enhanced Metamaterials: Towards Filling the Terahertz Gap," 2010. Kebin Fan, "Three-Dimensional and Nonlinear Metamaterials at Terahertz Frequencies," 2012. Huseyin Seren, "Functional Metamaterial Absorbers In the Terahertz Regime," 2014.