In the format provided by the authors and unedited.

Zero Power Infrared Digitizers Based on Plasmonically-enhanced Micromechanical

Photoswitches

Zhenyun Qian, Sungho Kang, Vageeswar Rajaram, Cristian Cassella, Nicol E. McGruer, Matteo Rinaldi

Section 1: FEM simulation of the PMPs

The three dimensional (3D) finite element method (FEM) simulations were performed using COMSOL

Multiphysics version 5.0. The 3D model of half of the device (i.e. one cantilever) without the smallest

structures (i.e. plasmonic absorber and contact tip) was first built and analyzed for the sake of computational

simplicity. Heat transfer and solid mechanics modules were used in conjunction to simulate the temperature

distribution upon a constant input IR power and the subsequent mechanical deformation induced by the

temperature change. The space around the device was set to be ideal vacuum environment with an initial

temperature of 293.15 K. Distinctly different temperature profiles between the inner and outer bimaterial

legs upon the absorption of IR power in the head were confirmed by heat transfer simulation (Figure S1).

The overall thermal resistance considering both conductive and radiative thermal transfer was found to be

~1.5×106 K/W for the 500 nW threshold device presented in the main text.

Figure S1: Transient response of the temperature at the center of the head upon absorption of IR power. The inset shows the final temperature distribution of the cantilever.

Since the gap is fixed at 500 nm in this work, the sensitivity of the device (displacement per unit absorbed

power), defined as the product of thermal resistance and temperature sensitivity (displacement per unit

temperature change) of the bimaterial legs, directly sets the threshold (assume 100% absorption of IR

Zero-power infrared digitizers based on plasmonically enhanced micromechanical photoswitches

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radiation). The thermal expansion-induced temperature sensitivity can be effectively tuned by altering the

length of the bimaterial legs: shorter beams yield lower temperature sensitivity but higher stiffness and vice

versa. In order to demonstrate the design flexibility and investigate the trade-off between threshold power

and restoring force, devices significantly stiffer than the 500 nW threshold device were also designed and

simulated (Figure S2). The beam length of the stiffest device is designed to be half of that of the 500 nW

device (200 µm v.s. 400 µm). In addition to the bimaterial legs, the thermal isolation link also plays an

important role in the overall stiffness of the cantilever. Therefore, the stiffer devices were designed to have

shorter and wider isolation regions, which leads to a higher stiffness and lower thermal resistance. The

threshold of the stiffest device was found to be 3.1 µW based on the simulated device sensitivity of ~ 0.16

µm/µW. The simulation result also reveals that, the response time of the stiffer devices are shorter than that

of the 500 nW device owing to its lower thermal capacitance and thermal resistance. It is worth noting that,

the thermal time constant of the PMPs was not optimized in this work, and we believe it could be

significantly improved by reducing the thermal capacitance (e.g. reduce the thickness of the heads) without

scarifying the device sensitivity.

Figure S2: (a) 3D FEM simulated deflection of the two devices in response to threshold input power. (b) Displacements of the two devices upon different levels of absorbed power. (c) Comparison of the time response between the two devices.

One of the key design features of the proposed PMP is the built-in compensation mechanism structure that

makes the devices completely immune to ambient temperature changes and residual stress. Such a

compensation mechanism highly relies on 1) the axial symmetry between each adjacent inner and outer

bimaterial beam and 2) the symmetry between the two cantilevers. Theoretically, the former alone can

prevent the displacement of the head when ambient temperature changes. However, the difference between

the anchors of the inner and outer beam slightly breaks the symmetry, leading to a low but non-zero

sensitivity of the head to ambient temperature changes. The two-cantilever design further compensates the

aforementioned tiny asymmetry, making the gap between the contact terminals completely insensitive to

ambient temperature changes and residual stress. Moreover, we believe such a design can effectively

enhance the robustness of the mechanical structure when overwhelming IR power strikes on the device,

hence providing higher reliability.

The temperature and stress compensation mechanism was then evaluated with 3D FEM simulation. The

array of gold patches for the plasmonic absorber was simplified to a continuous gold layer with same

volume as the patches in the 3D model. A small sensitivity of the head to ambient temperature variations

of ~ 4.3 nm/°C for one unmatched cantilever was observed in the simulation. When matched with a second

identical cantilever facing against each other, the relative displacement between the heads were

compensated, resulting in an ambient temperature sensitivity of ~ 0 nm/°C as expected (Figure S3). These

simulations also demonstrate the residual stress compensation after device release since the mechanism of

residual stress essentially has the same effect as that of thermomechanical bending (as residual stress

originates mainly from material deposition steps at higher temperatures for different layers).

Figure S3: (a) 3D FEM simulated deflection profile of the micromechanical photoswitch under ambient temperature changes. (b) Comparison of the sensitivity to ambient temperature variations between matched and unmatched cases.

The implementation of a robust contact tip is a key enabler for MEM photoswitches with high switching

reliability. The unique bowl-shaped design not only greatly improves the stiffness of the tip but it also

reduces the maximum stress accumulated for a given point force load applied at the tip. Compared to the

conventional planar contact tip, this 3D tip has ~3× higher stiffness and ~2.5× lower stress accumulation

(Figure S4).

Figure S4: Simulation showing the improved bowl-shaped top contact structure with enhanced stiffness and better stress management compared to a planar contact under the same upward applied force.

Section 2: Simulation of the Plasmonic Absorbers

The plasmonic absorber used in this work is a metal-insulator-metal (MIM) tri-layer structure featuring a

near-unity narrowband absorption (absorptance > 95%, full width half maximum < 17%) with flexibility of

tailoring absorption wavelength in a wide spectral range (visible to far infrared) through lithography1-3. The

top metal layer of the MIM structure is an array of sub-wavelength gold square patches leading to a

polarization independent and omnidirectional absorption. The metal-insulator-metal (MIM) plasmonic

absorber, can be seen as an artificial metamaterial with effective permittivity, 𝜀𝜀𝑀𝑀𝑀𝑀𝑀𝑀(𝜔𝜔), and permeability,

𝜇𝜇𝑀𝑀𝑀𝑀𝑀𝑀(𝜔𝜔). Total absorption occurs at the frequency in which the impedance of MIM stack (𝑍𝑍𝑀𝑀𝑀𝑀𝑀𝑀(𝜔𝜔) =

�𝜇𝜇𝑀𝑀𝑀𝑀𝑀𝑀(𝜔𝜔) 𝜀𝜀𝑀𝑀𝑀𝑀𝑀𝑀(𝜔𝜔)⁄ ) matches that of free space. Upon incident light matching the defined absorption

wavelength, electromagnetic fields are fully confined within the MIM structure via localized gap plasmon

resonance. The electromagnetic energy is efficiently converted to heat through the lossy components of the

structure, namely gold patches and platinum ground plate.

The optical properties of the plasmonic absorber were simulated using a full-wave commercial software,

Computer Simulation Technology (CST) Microwave Studio. A unit cell with proper boundary settings was

first built in the software in order to simulate the periodic structure of the absorber. For the material settings,

a Brendel oscillator model was adopted, with fitting parameters obtained from Rakic, A. D. et al.4 for

platinum (Pt) and gold (Au), and Kischkat, J. et al.5 for silicon dioxide (SiO2). The incident angle of light

was set to be oblique at 25 degrees, matching that of our Fourier transform infrared (FTIR) microscope. In

order to simulate absorption of unpolarized incident light, both transverse-electric (TE) and transverse-

magnetic (TM) polarizations of incident waves were included in the simulation. The absorption of

unpolarized waves was then calculated by 𝜂𝜂𝑎𝑎𝑎𝑎𝑎𝑎 = (𝜂𝜂𝑇𝑇𝑇𝑇 + 𝜂𝜂𝑇𝑇𝑀𝑀)/2.

The optimization of the plasmonic absorber in terms of absorptance and FWHM was performed by

sweeping the geometric parameters including the thicknesses of each material layer and the pitch and width

of the gold square patches. First, the ground platinum plate was set to be 90 nm, thick enough to prevent

transmission. Then the thicknesses of gold patches and dielectric layer were set in such a way that the

confinement of electromagnetic fields was maximum. A thicker or thinner gold patch would result in a

large scattering and a low dipole resonance, respectively; whereas the dielectric layer thickness mainly

contributes to magnetic coupling. For an absorption peak targeting 5 µm, the optimized periodicity of a unit

cell (pitch, Γ in Figure 2a of main text) and patch size (a in Figure 2a of main text) were found to be 2.2

µm and 1.2 µm. The absorbers for other three different absorption wavelengths were designed accordingly

by varying the patch size with a fixed pitch (Figure S5).

Figure S5: Simulated absorption wavelength and absorptance of the absorbers with various patch size and a fixed pitch (Γ = 2.2 µm). The corresponding measured absorption spectra of fabricated plasmonic absorbers with 4 different wavelengths are also plotted in the same graph with dotted line, outlining the agreement between the experimental and theoretical results. (The same simulation and experimental results are reported with a different manner in Figure 2 of the main text.)

Section 3: Fabrication and Characterization of the Plasmonic Absorbers

E-beam lithography was used to define the gold patches due to the small dimensions and the requirement

of high accuracy. Lift-off resist (LOR) was used to assist the fabrication for a clean lift-off sidewall

minimizing the scattering effect from the edges of gold patches (Figure S6). It is worth noting that over-

developing LOR layer may result in a collapse of LOR-PMMA stack. Therefore, a precise undercut was

characterized for the sub-micron gaps. In this fabrication process, two step fabrication alterations were

performed to avoid the aforementioned problem: (1) increasing baking temperature of LOR; and (2)

developing with diluted TMAH-based developer. The etch rate of LOR was slowed down to ~ 1nm per

second, which enabled a precise control of the undercut for such densely arranged nanostructures (Figure

S7).

Figure S6: Detailed absorber fabrication process: (a) spin-coat of LOR3A at 4000 rpm and bake on a 200°C hot plate; (b) spin-coat of PMMA950K (diluted with thinner) at 5000 rpm and bake on a 180°C hot plate; (c) exposure of PMMA using SUPRA 25 SEM coupled with J.C. Nabity followed by development of PMMA in MIBK:IPA (1:3) solution for 1 minute; (d) etch of LOR in diluted AZ400K:DI (1:1) developer for 4 minutes; (e) electron beam evaporation of Ti-Au-Pt stack, where Ti is an adhesion layer and Pt is a protection layer during XeF2 release; (f) lift-off after submersion in Remover PG for 3 hours.

Figure S7: Cross sectional SEM image of PMMA-LOR stack. The sub-micron (< 150nm) width of LOR layer was realized with controlled etch of undercut.

The fabricated plasmonic absorbers were characterized using a Bruker V70 Fourier transform infrared

(FTIR) spectroscope coupled with a Hyperion 1000 Microscope. A reference measurement of reflectance

was first performed on the reflecting head with a 150 nm gold mirror. The reflectance (R) of the plasmonic

absorber was obtained by subtracting the reference from the measured reflectance in order to exclude

atmospheric absorptions such as those of water and carbon dioxide in the air. Absorptance (𝜂𝜂) was then

calculated by 𝜂𝜂 = 1 − 𝑅𝑅 − 𝑇𝑇 ≈ 1 − 𝑅𝑅, where R is the measured reflectance and T is transmittance. Note

that T is assumed to be 0 since the continuous bottom ground plate (90 nm Pt) is much thicker than 3 times

the skin depth (3𝛿𝛿𝑃𝑃𝑃𝑃 ≈ 50 nm).

Section 4: Characterization of adhesion force and subthreshold slope

Adhesion force in a mechanical switch is the attractive force between the contacts upon closure of the gap.

It is typically caused by a combination of forces acting between the two substances such as mechanical

forces and electrostatic forces 6-8. Due to the existence of adhesion force, the micromechanical photoswitch

has to be designed with a restoring force larger than the adhesion force in order to re-open the contacts once

the IR radiation is not present anymore. On the other hand, stiffer beams and larger contact gap that lead to

a larger restoring force in turn limit the achievement of lower threshold of the device. Therefore, the

adhesion force that ultimately sets the threshold ought to be carefully characterized and analyzed. Hysteresis

behavior of micromechanical switches is commonly exploited for the extraction of adhesion force. The

blackbody experimental setup presented in the main text features widely tunable wavelength of IR radiation

(using bandpass filters) and a uniform power density in a relatively large area. However, the emitting

intensity of our blackbody instrument cannot be precisely and continuously tuned, which is not favorable

for the hysteresis measurements. In this context, a power-tunable 5.3-μm quantum cascade laser (QCL)

from Thorlabs (Model QF5300CM1) was employed as the IR source and the experimental optical table

setup illustrated in Figure S8 was designed and implemented for the characterization of adhesion force and

subthreshold slope.

Figure S8: Experimental IR testing setup with QCL. 5 reflective mirrors and a dichroic filter are properly set up to co-align a red laser beam with the QCL beam to facilitate the alignment with the sample (the red laser was OFF during IR testing).

The spot size of the QCL beam was measured to be ~2 mm in diameter. The power density at the center of

the beam was calibrated at the test plane with a 100 µm diameter pinhole and a commercial thermal detector

(Thorlabs S401C). A linear relation between the center power density (ρ) and the QCL current (I) was found

(Figure S9).

Figure S9: Calibrated power density at the beam center as a function of QCL operating current. The operation current in IR test was chosen in the linear region well above the threshold current for the QCL (981 mA).

Considering the overwhelming power from the laser, a 1% neutral density (ND) filter was used when testing

devices. The device under test was first manually aligned to the red laser spot by moving the sample

positioner inside the vacuum chamber with the help of a real-time video feed from a microscope camera.

Then the red laser was turned off and the power of co-aligned QCL beam was gradually increased. The

absorbed power in this case was calculated by P=1%·95%·η·ρ·A, where 1% and 95% are the transmission

of ND filter and CaF2 window, η is the absorption at 5.3 μm wavelength, ρ is the calibrated power density

at the center of the QCL beam and A is the absorber area. It is worth noting that, the absorbed power

estimated in this way represents the maximum possible value: a minor potential misalignment between the

absorbing head of the device and the center of the QCL beam inevitably results in an over 50% lower power

density delivered to the device. Therefore, the actual absorbed power is likely to be smaller than the

estimated maximum value. A fabricated PMP with an absorption wavelength at 5.5 µm and a designed

threshold of 500 nW was tested with this experimental setup. The device turned ON for an estimated

maximum absorbed power of 950 nW and turned OFF when the power was reduced to 520 nW (Figure

S10). The discrepancy of threshold power between the designed and measured values could be due to 1)

the aforementioned misalignment, 2) a contact gap larger than the designed value (500 nm) due to

fabrication deviation, or both. Nevertheless, from this hysteresis of 430 nW (Ph = PON - POFF), an estimated

maximum contact adhesion force < 9 nN was further extracted by Fad = Ph·S·k, where S and k is the designed

sensitivity (1 μm/μW) and cantilever stiffness (0.022 N/m).

Figure S10: Measured current through the device under test (for a 1 mV applied bias) while the power of the QCL beam is gradually increased.

The subthreshold slope of a switch indicates the steepness of the OFF-to-ON transition9-11. A sharp

subthreshold slope and a high ON/OFF ratio guarantee a low OFF state leakage current when the device is

operated below threshold, hence low standby power consumption. For a photoswitch, the subthreshold

slope, Ss-th, can be defined as the logarithmic change in device conductance per unit absorbed power.

The ON-state resistance, RON, of the device was found to be ~ 3×103 Ω (ION ~3×10-7 A with 1 mV bias

voltage). The OFF-state resistance measurement was limited by the accuracy of the instrument (Hewlett-

Packard semiconductor parameter analyzer): The measured current of a stiff PMP (designed to work with

100 μW IR power) was less than 5×10-15 A while the bias voltage was varied between 0 V and 20 V, which

indicates that the OFF-state resistance of our devices is larger than 4×1015 Ω (Figure S11). PMP 1 and PMP

2 reported in the main text were not used for this measurement due to their relatively low pull-in voltages

(~ 4V). The design of stiffer device used in this experiment is the same of PMP 1 and PMP 2 (including the

~ 500 nm contact gap and the distance between the probing pads) except for the lateral dimensions of the

bimaterial legs and thermal isolation links, which does not affect the off-state current. Therefore, the actual

ON/OFF conductance ratio of our device was extracted to be > 1.3×1012.

During the contact hysteresis measurement using the QCL, the OFF-to-ON transition occurred when the

QCL current was dialed up by the minimum step of 0.1 mA, which corresponds to a change in absorbed

power ∆P ~1.3 nW. Although, this minimum power step is certainly limited by the precision of the laser

controller, an extremely high subthreshold slope of > 9 dec/nW was extracted by Ss-th=log10(ON/OFF

ratio)/∆P.

Figure S11: Measured I-V characteristics of a PMP in OFF state in vacuum (10-6 Torr).

Figure S12: 8-mask microfabrication process: (a) Mask 1 – deposition and lift-off of Pt bottom reflector of

the absorber on 1.9-μm SiO2; (b) Mask 2 – deposition and lift-off of metal routing and contact pad on 2-

μm SiO2; (c) Mask 3 – deposition and wet etch of Al; (d) Mask 4 – deposition and lift-off of gold defining

a reflector on the reflecting head and e-beam lithography and lift-off of Au/Pt defining the nano plasmonic

structures on the absorbing head; (e) Mask 5 – self-aligned co-etch of Al and SiO2 layers; (f) Mask 6 –

deposition and dry etch of a-Si sacrificial layer; (g) Mask 7 – spin coating, patterning and shaping of

planarizing photoresist plug; (h) Mask 8 – deposition and lift-off of contact tip; (i) dicing and XeF2 release.

Figure S13: a. Three-dimensional mock-up of the experimental measurement setup consisting of: (i) a

calibrated blackbody IR source with a filter wheel mounted in front of it and (ii) a vacuum chamber

containing the devices under test. The vacuum chamber is equipped with an IR-transparent (transmission ~

95% from 3 to 9 μm) calcium fluoride (CaF2) window. The pressure of the chamber during test was

maintained at ~1.0×10-6 Torr. b Transmission spectra of the bandpass IR filters and the FTIR measured

absorption spectra of PMP 1 (1.6-μm Au patches defining the plasmonic absorber) and PMP 2 (1.0-μm Au

patches defining the plasmonic absorber) respectively. It is worth noting that, the absorption peaks shifted

to slightly shorter wavelength after device release due to the slight etching of gold patches in XeF2.

Figure S14: Calibration of the blackbody beam spot with filters at the sample plane.

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