Optical Actuation of a Micromechanical Photodiode Via the Photovoltaic-Piezoelectric Effect A

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Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Microsystems & Nanoengineering https://doi.org/10.1038/s41378-021-00249-y www.nature.com/micronano

ARTICLE Open Access Optical actuation of a micromechanical photodiode via the photovoltaic-piezoelectric effect A. Rampal1,2 andR.N.Kleiman 1

Abstract Radiation pressure and photothermal forces have been previously used to optically actuate micro/nanomechanical structures fabricated from semiconductor piezoelectric materials such as gallium arsenide (GaAs). In these materials, coupling of the photovoltaic and piezoelectric properties has not been fully explored and leads to a new type of optical actuation that we call the photovoltaic-piezoelectric effect (PVPZ). We demonstrate this effect by electrically measuring, via the direct piezoelectric effect, the optically induced strain in a novel torsional resonator. The micron- scale torsional resonator is fabricated from a lattice-matched single-crystal molecular beam epitaxy (MBE)-grown GaAs photodiode heterostructure. We ﬁnd that the strain depends on the product of the electro-optic responsivity and piezoelectric constant of GaAs. The photovoltaic-piezoelectric effect has important potential applications, such as in the development of conﬁgurable optical circuits, which can be used in neuromorphic photonic chips, processing of big data with deep learning and the development of quantum circuits. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Introduction limited by the thermal constants of the materials. A fur- Optical actuation of micro/nanomechanical resonators ther limitation of these mechanisms is that extra steps are has been developed across various material platforms and required to integrate an electrical transducer to quantify – geometries1 23. Of particular interest is optical actuation the optically induced strain. The extra steps can come – of semiconductor materials1 15, given their technological either in the form of additional fabrication steps for ubiquity in devices such as computers, cell phones, solar integration of a transducer11,14,15 or via complicated cells, lasers, photodiodes, and micro/nanomechanical experimental setups to integrate a transducer external to – sensors. One of the main reasons for their ubiquity stems the device3,5,7 10,12. These constraints limit the use of from the capability for high-volume, high-yield manu- optical actuation to either lab settings or customized facturing of micro/nanodevices. applications in commercial settings. The most common mechanisms used for optical Here, we introduce and demonstrate a new optical actuation are radiation pressure and photothermal forces. actuation mechanism that we call the photovoltaic- Radiation pressure is a weak force; hence, large optical piezoelectric (PVPZ) effect. This effect can occur in intensities and/or complex optical setups are required, semiconductor materials that possess both piezoelectric which limits the circumstances in which these forces can and photovoltaic properties such as those in the III–V and be used. Mechanical structures actuated photothermally II–VI compound semiconductor families. The piezo- can undergo large strains, but their frequency response is electric property enables coupling between the mechanical and electrical degrees of freedom (DOFs) via the inverse and direct piezoelectric effects. In these effects, an Correspondence: A. Rampal ([email protected]) applied voltage results in a mechanical strain, and a 1Department of Engineering Physics, McMaster University, Hamilton, ON L8S 4L7, Canada. 2Present address: CircuitMind Inc, 185 Spadina Avenue, Toronto, mechanical strain results in a charge, respectively. Using ON M5T 2C6, Canada

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both effects together, mechanical strain can not only be The demonstration of the PVPZ effect extends the electrically induced but also measured. Both of these application of semiconductors to the optoelectromechanical effects are utilized in research and commercial piezo- domain, addressing the long-standing open issue of pro- electric pressure sensors24,25 and accelerometers26,27 to viding optical actuation in a semiconductor platform for all- measure the strain resulting from external forces. Simi- optical computing, processing, and telecommunication larly, resonant quartz tuning forks making use of both the applications. The advantages of using semiconductor inverse and direct piezoelectric effects are employed in materials are the ability to make use of conventional scanning probe microscopy28,29 and atomic force micro- batch fabrication methods and the opportunity to leverage scopy30,31 for imaging topographical features of material mature highly integrated on-chip optical and electronic surfaces. Building on the photovoltaic effect and the functionalities. inverse piezoelectric effect, the optical actuation is a result of a two-step process in which the incident light induces a Results and discussion photovoltage that causes mechanical strain. Making use of We experimentally demonstrate the PVPZ effect by the direct piezoelectric effect, the mechanical strain can optically actuating a novel micromechanical structure, a be electrically measured directly, hence mitigating the planar torsional resonator (Fig. 1a), and electrically need for extra fabrication steps or complicated experi- measuring the resulting optically induced strain near mental setups for the integration of a transducer. The resonance via the direct piezoelectric effect. The tor- PVPZ effect therefore couples the optical, electrical, and sional resonator is fabricatedfromanepitaxiallygrown mechanical DOFs of the material. Broadly speaking, GaAs photodiode heterostructure (Supplementary Sec- the PVPZ effect leads to relatively large forces with a tion I). The torsional deformation arises from the anti- geometry-limited frequency response. symmetric flexural bending of the two diagonally We demonstrate the PVPZ effect using a single-crystal oriented tines that comprise the structure. The driving micromechanical device fabricated from a GaAs p-n force for this antisymmetric bending is the result of (a) junction photodiode-solar cell heterostructure, with het- using the antisymmetric piezoelectric property of GaAs erostructure layers added to ensure successful release of in the plane of the device and (b) designing the photo- the mechanical resonator. GaAs is chosen because it is diode heterostructure with vertical asymmetry about the commonly used to make high-efficiency photodiode-based neutral plane to resemble a unimorph structure. The – solar cell structures32 35 and has a robust piezoelectric formercanbeunderstoodbyconsideringthepiezo- – effect36 38. Piezoelectric micromechanical devices fabri- electric matrix for a [001]-oriented wafer of GaAs, a cated from GaAs heterostructures have previously been III–V semiconductor having a zincblende crystal studied. For example, Masmanidis et al.36 demonstrated structure37: electrical tuning of the device resonant frequency in a 2 3 GaAs PIN structure. In their studies, the device was elec- 00Àβ 6 7 trically driven, and its mechanical strain was measured 6 00β 7 39 6 7 using optical interferometry. Okamoto et al. demon- 6 7 d14 6 0007 strated dynamic mechanical coupling between two GaAs dij ¼ 6 7 ð1Þ 2 6 2α 2β 0 7 bridge structures. In their studies, the devices were elec- 6 7 4 5 trically driven using the inverse piezoelectric effect, and À2β 2α 0 their motion was detected using the direct piezoelectric 000 effect, as in quartz tuning fork resonators. Our micromechanical structure is a novel planar torsional resonator where dij are the piezoelectric coefficients, β ¼ cosð2φÞ, designed to minimize clamping losses and null out residual α ¼ sinð2φÞ and φ is the rotation angleÂÃ about the photothermal actuation. We chose to use the direct pie- [001]-direction with respect to the 110 -direction, zoelectric effect to provide a direct measure of deflection φ = 0. For GaAs, d14 = 1.345 pm/V. To make use of the due to the seamless and natural integration with the device planar antisymmetry, the two tines of the torsional structure. We also use the indirect piezoelectric effect for resonator are aligned parallel to the two orthogonally electrical actuation of the device for calibration purposes. oriented [110]-directions (Fig. 1b).Accordingtothis Our device has a resonant frequency of 76.7 kHz, a room- matrix, d13 and d23 are equal in magnitude but opposite temperature Q of ~6000 and an electro-optic responsivity in sign; hence, an electric field applied in the [001]- of 0.178 A/W. We find that the optically induced strain direction results inÂÃ equal butÂÃ opposite longitudinal resulting from the PVPZ effect depends on the product of strains, ε,inthe 110 -and 110 -directions, where the electro-optic responsivity and the piezoelectric con- ε1 ¼ d13E3 and ε2 ¼ d23E3, respectively. The flexural stant and that this strain is linear with optical power, with a bending of the tines, required for driving and detecting, nonresonant tip deflection of δz ∼ 11 μm/W. is achieved by designing the GaAs heterostructure such Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 3 of 9

a Fig. 1 Design of the photodiode torsional resonator. a Fundamental undeformed torsional mode shape (semitransparent) and deformed mode shape of the resonator under optical illumination. The illustrated mode shape is determined from simulations (as shown in Supplementary Section V). The resonator is composed of a central spine terminated on the top by a rectangular pad and at the bottom by an anchor, where three contact pads are situated. Protruding from the spine at ±45° are two tines. The electrodes on the anchor facilitate measurement of the piezoelectric current. Above-bandgap light is modulated near the resonant frequency of the resonator, exciting the torsional mode via the PVPZ effect. b SEM image of a fabricated torsional resonator on a [001] GaAs wafer. The spine is parallel to the [100]-direction (not marked). The directions of the tines are chosen to exploit the antisymmetry in the III–V piezoelectric properties, resulting in antisymmetric ﬂexural b deformation. c Cross-section (not to scale) of the MBE-grown photodiode heterostructure. The section shown is the released

structure, i.e., the Al0.8Ga0.2As layer remains in place at the anchor and is removed below the spine and tines. The depletion region is intentionally placed above the neutral plane to enable unimorph-type actuation. The arrow indicates the crystal orientation. The top and bottom electrodes contacting the n+ and p+ GaAs layers are used to measure the piezoelectric current resulting from the induced strain. The electrodes can also be used to electrically drive the resonator. The center contact pad shown in Fig. 1a connects to the device top electrode, while the two adjacent contact pads connect to the device bottom electrode

c chosen to achieve successful fabrication of the torsional Top Electrode resonator40 and to maximize both the photocurrent and n+ GaAs (200 nm) – n In0.485Ga0.515P (25 nm) piezoelectric current. The torsional resonator design is n– GaAs (87 nm) chosen to null out, to a first approximation, the driving Depletion Region/Piezoelectric Region (146 nm) force from photothermal effects because uniform illu- p– GaAs (1367 nm) Bottom mination would result in symmetric photothermal – Electrode p In0.485Ga0.515P (50 nm) [001] bending of the tines. The PVPZ effect is not unique to p+ GaAs (200 nm) this geometry and could be demonstrated and utilized in Al0.80Ga0.20As (1000 nm) simpler cantilever structures since the use of both d13 and d23 is not required. p+ GaAs (50 nm) p+ GaAs – Substrate In the first set of measurements, we actuate the resonator structure electrically by making use of the inverse piezoelectric effect and measure its motion using the current from the direct piezoelectric effect, providing that the piezoelectric layer is offset from the neutral direct characterization of the device and its piezoelectric surface of the tines, in this case near the upper surface of and mechanical properties. This permits us to determine thedevice(Fig.1c). The depletion region of the p-n the electromechanical device properties entirely in the junction of the GaAs photodiode responds similarly to a electric domain using traditional analysis methods41. The dielectric material, so an applied voltage leads to a electromechanical transduction is characterized by piezoelectric stress. The n- and p-type layers are applying an AC voltage at frequencies near the resonant sufficiently doped to serve as equipotential surfaces for frequency (Fig. 2a) across the device and fitting device actuation without the need for metal contact the measured electrical response to the modified layers that would be opaque to optical illumination. The Butterworth-van Dyke (BVD) model (Fig. 2b). The mea- heterostructure is grown by molecular beam epitaxy sured response (Fig. 2c) agrees very well with the modified + μ fi ω2 ¼ =ðÞ (MBE) on a p GaAs substrate and consists of a 1.0- m- BVD model. Using the de nitions 0 1 LmCm and thick Al0.8Ga0.2As sacrificial etch layer followed by a Q ¼ 1=ðÞω0RmCm , the resonator is found to have a Q of 2.075-μm-thick photodiode structure resembling a GaAs 10,116 and a resonant frequency of f0 ¼ ω0=2π ¼ 76; 696 solar cell. The doping concentrations and thicknesses of Hz, which are consistent with expectations from finite the individual layers (Supplementary Section III) are element simulations40. The resonator dimensions for the Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 4 of 9

a Fig. 2 Electrical actuation of the photodiode torsional resonator. a Experimental setup for electrically inducing piezoelectric strain and measuring piezoelectric charge. A function generator, represented as

Vac, supplies the AC signal required to drive the resonator near its resonant frequency. The piezoelectric current is first preamplified and then measured using an SR844 Lock-In Amplifier. The resonator and the preamplifier are placed in a vacuum chamber at a pressure of Top Contact ~2 mTorr. b Equivalent circuit of the modified BVD model of a piezoelectric resonator arranged for standard two-terminal drive and

Bottom Contact detection. The motional components, Rm, Cm and Lm, are the electrical analogs of a damped harmonic oscillator representing the mechanical

dissipation, spring constant, and mass of the resonator, respectively. CJ V ac is the junction capacitance or geometric capacitance of the device, Rsh is primarily due to the shunt resistance of the diode, Rs is dominated by the series resistance of the metal-semiconductor contacts, and i is the piezoelectric current. The dashed line encompasses the equivalent circuit of the resonator or device under test (DUT). c Frequency b DUT response of the measured RMS current with a drive voltage of

66 μVrms. The solid line is a ﬁt to the data using equation (S4) derived Cm for the circuit shown in Fig. 2b, and the ﬁtted parameters are given in Rm Lm Supplementary Section VIII. The resonator dimensions for the tine, spine, and pad are 80 × 30 μm, 102.5 × 10 μm, and 40 × 15 μm, μ R CJ respectively, with a total released device thickness of 1.875 m s i

– Rsh V A large, which leads to a large offset in the current, as shown ac+ in Fig. 2c. In a second set of measurements, we calibrate the optical response of the photodiode using a DC voltage source and an infrared LED (Fig. 3a) with a peak wave- c 1.30 length of 830 nm and a photon energy just higher than the Q = 10,116 bandgap of GaAs. The electro-optic responsivity, R , f0 = 76,696 Hz resp 1.25 and efﬁciency of the photodiode in converting optical to electrical power are determined by ﬁtting the measured )

nA photocurrent, I, to the single-diode DC photovoltaic (PV) (

i 1.20 model44 shown in Fig. 3b and described by Eq. (S8). The measured photodiode I–V characteristics (Fig. 3c) agree 1.15 well with this model, and the photodiode is found to have Rresp = 0.178 A/W with an efﬁciency of approximately

76640 76660 76680 76700 76720 76740 11.5%. The responsivity is calculated as the ratio of the Frequency (H z) short-circuit current, Isc, to the incident optical power, Popt. The short-circuit current is the current measured at VDC = 0, which is closely equivalent to the incident photocurrent, Iph. The efficiency of the photodiode is calcu- tine, spine, and pad are 80 × 30 μm, 102.5 × 10 μm, and lated as the ratio of electrical power at the maximum 40 × 15 μm, respectively, with a total released device power point (The maximum power point is defined as a thickness of 1.875 μm. All measurements are performed at point on the curves in Fig. 3b at which the product of the room temperature. The high value of Q at room tem- photocurrent and photovoltage is a maximum) to the perature is expected because the resonator is designed incident optical power. In the optical experiments, the such that the torsional mode has minimal clamping losses LED is placed ~0.5 cm from the device such that the resulting from it being supported at the nodal point42,43. device is homogenously illuminated. Experimental details The motional capacitance Cm can be analytically related on the measured optical powers are given in Supple- to the mechanical mode, device geometry, and piezo- mentary Section XI. electric coefficient (Supplementary Section VII). The In a third set of measurements, we actuate the resonator junction capacitance, CJ, is dominated by the contact pad, structure optically by making use of the photovoltaic which is much larger in area than the device itself. As a effect in conjunction with the piezoelectric effect and result, the background (or feedthrough) current from CJ is measure its motion using the current from the direct Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 5 of 9

a Fig. 3 DC optical responsivity of the photodiode. a Experimental setup for measuring I–V curves. The photodiode is illuminated by a DigiKey model TSGH510-ND infrared LED, and an HP-4145B semiconductor parameter analyzer measures the generated photocurrent and applies a DC voltage. The illuminated section of the photodiode is shown in red and illustrates homogeneous illumination. Unlike the resonance measurements, the I–V curves are measured at ambient pressure. b Equivalent circuit of the single-diode DC PV model. The photocurrent, I, from the photodiode is measured at the Top contact output terminals as a function of an externally applied DC voltage,

VDC, where Iph is the incident photocurrent. The dashed line encompasses the equivalent circuit of the photodiode or DUT. c I–V Bottom contact curves of the illuminated photodiode. The solid lines are a ﬁt to the HP-4145B data using equation (S8) derived from the circuit shown in Fig. 3b, and the ﬁtted parameters are given in Supplementary Section IX. The

expected linearity of the short-circuit current Isc with the incident optical power is shown in the inset

b DUT

Rs or 3b. The equivalent circuit including the actuation and measurement components is shown in Fig. 4b. We set up a parallel RC load branch at the output terminals of the Iph A optically driven resonator and measure the current iL through CL. The optically driven resonator circuit is Rsh I highly nonlinear due to the diode properties, with a 40 – general expression for iL given in Rampal for different V + DC load conditions. The expression is strongly dependent on the DC bias point, which in this case is determined primarily by RL. When RL < Voc/Isc, the device is in the current-dominated regime of the photodiode40, where changes in photocurrent, i1, are directly related to changes c –0.4 –0.2 0.0 0.2 0.4 0.6 in optical power, as in the DC case, i.e., i1 ¼ Rresppopt and Iph ¼ RrespPopt for the respective AC and DC cases. The V (V ) DC value of RL must also be chosen so that its impedance is larger than the impedance near resonance of the capaci- –0.1 0.00 tive components, C ¼ C þ C , i.e., ωR C >1, to –0.05 T J L L T

) –0.10 maintain a sufﬁciently high bandwidth to detect the 0.3 W A

( –0.15

) 0.4 W sc I resonant device operation. Based on these considerations, A –0.20

–0.2 1.6 W ( –0.25 I R and I should be chosen such that 1/(ω C )

iLðωÞ¼i1jωCLZt ð2Þ piezoelectric effect. To illustrate the PVPZ effect and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi hence demonstrate the coupling among the optical, ðÞa À 1 Q2ðÞω2 À ω2 2þðÞωω 2 j ðωÞj ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0 0 iL i1 electrical and mechanical DOFs, we modulate the output 2 of the LED sinusoidally and vary the modulation fre- Q2 ω2 Cm þ aðÞω2 À ω2 þðÞaωω 2 0 CJ 0 0 quency. Figure 4a shows the experimental setup for ð Þ measuring the PVPZ effect. Since the photovoltaic cell is 3 in parallel with the mechanical resonator by construction, where Zt is the total impedance in parallel with the the electrical detection of the mechanical actuation must photodiode, as indicated by the dashed lines in Fig. 4a, be established differently than that shown in Figs. 2b and a 1 þ CL=CJ. Circuit analysis shows that the swing Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 6 of 9

a Fig. 4 Optical actuation of the photodiode torsional resonator. a Experimental setup for measuring the PVPZ effect. The photodiode resonator, optical source, bias T, and preampliﬁer are placed in a vacuum chamber at a pressure of ~2 mTorr. Similar to the DC measurements, the illuminated section of the resonator is shown in C red and illustrates homogeneous illumination. The same 830-nm LED L i Top contact L used to characterize the DC efﬁciency and responsivity is also used here. The bias T is composed of the load capacitance and resistance RL Bottom contact (CL and RL), and iL is the measured current. b PVPZ equivalent circuit. To the left of the diode are the AC and DC photocurrents (i1 and Iph) photoinduced by the AC and DC optical powers (popt and Popt) of the LED. To the immediate right of the diode is the BVD circuit similar to Fig. 2b. The load branch is composed of the bias T and is placed at the

b DUT output terminal or the top electrode of the photodiode. Together,

Rs these components establish a DC bias for the PV cell and impedance matching between the device and the measurement circuit. The dashed line encompasses the equivalent circuit of the photovoltaic- i1 Iph i im J iL piezoelectric resonator or DUT. c Same circuit as in Fig. 4b, with the Cm

CL components comprising the total impedance, ZT, and motional

Rsh CJ Lm RL impedance, Zm, indicated in dashed lines. d Frequency response of the measured iL for Popt of 304 nW and popt of 1.61, 3.34, and 17.6 nW. fi Rm The solid lines are ts to the data using Eq. (3) derived from Fig. 4c. A Equation (3) depends on 4 parameters [i1,Cm, Q, and ωo] related to the device and the drive/detection parameters. All four parameters are independently determined through the fits, and the parameters are provided in Supplementary Section X. The values are in reasonable c Zt agreement with those determined from purely electrical Rs measurements. The measured currents for the popt values of 1.61 and Zm 3.34 nW have been shifted up so that their background currents align i I 1 ph i im = fi J iL with the background current for popt 17.6 nW. The tted Q values Cm C are 6875, 5825, and 5625 for popt values of 1.61, 3.34, and 17.6 nW, L fl Rsh CJ Lm RL respectively. e RMS de ection of the tines due to the PVPZ effect the three different optical power levels. The linearity of the maximum fl Rm de ection with the drive power is shown in the inset as predicted by A Eq. (7)

d 2.29 in i is maximum when C = C or when a = 2. In popt = 1.61 nW L L J popt = 3.34 nW our experiments, C = 144 pF and C = 55 pF, resulting 2.28 popt = 17.6 nW L J in a ~3.62. The device is designed such that Rs is low 2.27 enough that we can consider it a short circuit and that )

nA 2.26 (

L both R and R are high enough that they can be i sh L

2.25 considered open circuits for the purpose of AC analysis at frequencies near resonance. To verify the PVPZ effect 2.24 based on the coupling among the optical, electrical and

2.23 76,620 76,640 76,660 76,680 76,700 76,720 mechanical DOFs, we drive the resonator optically by Frequency (H z) modulating the LED at frequencies near the resonant e frequency of the device. The measured response (Fig. 4d) popt = 1.61 nW 1.0 popt = 3.34 nW agrees well with expectations from the PVPZ circuit (Fig. popt = 17.6 nW

0.8 1.0 4c). The measured resonant frequency is found to be close ) nm (

to the electrically driven case (Fig. 2c), while Q is reduced )

* 0.5

0.6 nm ( to ~6100, most likely due to aging of the resonator 0.0 0 3 6 9 12 15 18 0.4 popt (nW ) resulting from lack of sidewall passivation. Similar to the

0.2 feedthrough current in Fig. 2c, the feedthrough current in Fig. 4d is due to parallel paths through CL and CJ. 0.0

76,620 76,640 76,660 76,680 76,700 76,720 As previously described, it is a good approximation to Frequency (H z) assume that the motion of the resonator in its torsional mode can be modeled by considering the deﬂection of the tines as the deﬂection of cantilevers. In this Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 7 of 9

approximation, we can determine the complex deflection resonant shape without the feedthrough terms present of the cantilever tips as given by45: due to electrical detection. Whilewehaveincorporatedelectricalconnectionsto 2 2 fl L 2xc L 2xc im verify circuit parameters and detect the resonator de ection, δzðωÞ¼À3 d23VC ¼À3 d23 t t t t jωCm this is not required for optical actuation using the PVPZ ð4Þ effect. However, for maximum amplitude, the device needs to be biased at a sufficiently low voltage such that RL < Voc/ where δz is the deflection of the tip, xc is the distance from Isc; therefore, a load resistance is still required, which could the center of the beam to the center of the p-n junction be incorporated into the device structure. While the load (here, xc = 752.5 nm), im is the current through the capacitance can be eliminated (i.e., CL = 0, leading to a = 1), motional arm (Rm, Lm, Cm), L and t are the length and ω0RLCJ > 1 is still required. By eliminating the contact pad, thickness of the tines and VC is the voltage across Cm. CJ can be substantially reduced; however, this makes the Using analysis of the PVPZ circuit (Fig. 4c), we find that latter criterion more difficult to achieve. By choosing RL ~1/ the complex deflection is given by: (ω0CJ)andIsc ~Voc/RL, both criteria are just met, and with full modulation and the p-n junction placed close to the 2 À À 2 À a L 2xc i1 iJ iL L 2xc i1 iL ðÞaÀ1 device surface, Eq. (7)simplifies to jδ ðÞjω δ ðωÞ¼À3 d ¼À3 d ÀÁ z max z 23 ω 23 ω 3 L 2 t t j Cm t t j Cm pffiffi d V Q, which is the maximum amplitude possible 2 t 23 oc ð5Þ via PVPZ optical actuation. which can be used to transform the measured iL to a In measurements (not shown), we observe no resonance determination of δz (Fig. 4e), based on the fitted values within our measurement resolution in the vicinity of the from Supplementary Section X. Figure 4e illustrates a fundamental symmetric mode, expected at 26,550 Hz, con- clear resonance (with no antiresonance), as would be firming the nulling of symmetric actuation and detection. expected for a driven simple harmonic oscillator. The We also fabricate control devices with the same geometry peak magnitude is ~1.1 nm RMS for an AC optical but with the tines of the torsional resonator aligned parallel modulation of only 17.6 nW, corresponding to a non- to the [100]- and [010]-directions, which is expected to lead resonant (Q = 1) actuation of δz 11μm=W. to no PVPZ effect. The results (not shown) indicate no Starting with Eq. (4) and using imZm ¼ i1Zt, where Zm is deflection within our measurement resolution, indicating the impedance of the motional arm of Fig. 4c, the full rejection of the torsional mode and supporting full expression for the bending of the cantilever is given rejection of symmetric modes in the original geometry. directly in terms of optical parameters, with no reference Historically, the measurement of subtle optically driven to electrical currents, by: forces, such as radiation pressure, has been obscured by the much larger optically induced photothermal for- L 2 2x ces46,47. While the PVPZ effect is much stronger than jδ ðωÞj ¼ 3 c d R p z t t 23 resp opt radiation pressure effects, the role of photothermal forces must be carefully examined and managed. Our resonator ω2Q 1 0 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiis intentionally designed for use in its fundamental anti- ωCJ 2 2 ω2 Cm þ ðÞω2 À ω2 þðÞωω 2 symmetric torsional mode to address this concern. The Q 0 C a 0 a 0 J antisymmetric nature of the torsional oscillator design ð6Þ provides actuation and detection of only antisymmetric modes, with symmetric modes leading to cancellation. where we have substituted i1 ¼ Rresppopt. Since Cm « CJ This cancellation is experimentally confirmed to first and Q » 1, the maximum amplitude, or resonant ampli- order by the two null experiments described above. The tude, is given by: illumination wavelength is chosen such that the optical absorption within the GaAs layers is quite axially uniform, 2 reducing photothermal forces. At this wavelength, the jδ ðω Þj ¼ L 2xc 1 ð Þ z max 3 d23RresppoptQ 7 fi t t ω0CT optical absorption is still suf ciently high to achieve good photovoltaic efficiency and minimize light transmission where ωmax is the resonant frequency. From Eqs. (6) and through the substrate, reducing another potential source (7), we can see clearly that the magnitude of δz depends of photothermal drive. Uniform optical illumination and on the product of the optoelectrical material property, absorption in a photothermal process would only drive Rresp, and the piezoelectric constant, d23, and a propor- symmetric modes since both tines would be actuated in tional and linear change in δz with popt is predicted, as the same way. Therefore, uniform photothermal processes shown in the inset of Fig. 4e. We also see that the would not actuate the antisymmetric mode or be detected mechanical motion is described by a conventional by the antisymmetric piezoelectric detection. However, Rampal and Kleiman Microsystems & Nanoengineering (2021) 7:29 Page 8 of 9

nonuniformities in optical actuation could in principle former is of special interest as the prevailing platform for weakly drive the antisymmetric mode photothermally. We optical communication devices, while the latter platform estimate the deflection from differential thermal expan- is now widely used in the manufacture of white light solid- sion in the z-direction as would be expected from a fully state sources and transistors for high-power electronics. nonuniform photothermal force (i.e., only illuminating a single tine) and find that it is 150 times smaller than the Materials and methods PVPZ effect (Supplementary Section XII) for a single tine. Fabrication However, due to the nulling from the antisymmetric A detailed description of the torsional resonator fabri- device design, the observed photothermal actuation would cation steps is given in Supplementary Section I. In brief, be substantially smaller, indicating that photothermal the torsional resonator geometry is first defined via wet actuation likely makes a negligible contribution to the etching, followed by formation of the bottom and top observed results. Additionally, the observed currents are electrical contact pads and then ending with a timed etch in good agreement with expected values based on known release step. There are a total of six process steps con- and calibrated piezoelectric and photovoltaic effects. We sisting of four lithography steps, a back-contact step and a also find (not shown) that the current upon actuation is resonator release step. A single chip consists of 326 modified by the bias voltage, which would only be resonators and is 14 mm × 14 mm. The devices are iso- expected for the PVPZ effect40. lated from each other by wet etching of the hetero- In conclusion, we have demonstrated the PVPZ effect in structure layers between the devices. The impetus for this a single-crystal GaAs micromechanical torsional reso- step is to minimize stray and feedthrough capacitances nator photodiode. Observation of this effect in GaAs and resistances between devices. utilizes its optical, mechanical, piezoelectric and semi- Acknowledgements conductor properties, coupling them through a device The heterostructure sample used in this work was MBE grown at McMaster that was intentionally designed to demonstrate the PVPZ University by Dr. Shahram Tavakoli. We would like to thank Dr. Matthew Minnick, effect. The novel torsional resonator design allowed for Dr. Martin Gerber and Dr. Kevin Boyd for helpful discussions. A.R. would like to thank Doris Stevanovic for providing training in using the clean room at planar antisymmetric actuation and a high quality factor, McMaster University. Natural Sciences and Engineering Research Council of exploiting the anisotropic piezoelectric properties of Canada (NSERC) (03736); Canada Foundation for Innovation (CFI) (32168). GaAs while minimizing photothermal effects. However, the PVPZ effect could be harnessed in a wide variety of Author contributions A.R. fabricated the devices and performed the measurements. A.R. and R.N.K. other optomechanical designs, including simple cantilever conceived and designed the experiment and contributed equally to writing structures. The device was fabricated using conventional the manuscript. growth and fabrication technology, which allows inte- Conflict of interest gration with other device technologies. The PVPZ effect The authors declare no competing interests. can be utilized for fast, strong optical actuation based on the intrinsic properties of GaAs with known design rules. 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