Combining reprogrammable optical networks with complementary metal-oxide semiconductor (CMOS) electronics is expected to provide a platform for technological developments in on-chip integrated optoelectronics. We demonstrate how opto-electro-mechanical effects in micrometer-scale hybrid photonic-plasmonic structures enable light switching under CMOS voltages and low optical losses (0.1 decibel). Rapid (for example, tens of nanoseconds) switching is achieved by an electrostatic, nanometer-scale perturbation of a thin, and thus low-mass, gold membrane that forms an air-gap hybrid photonic-plasmonic waveguide. Confinement of the plasmonic portion of the light to the variable-height air gap yields a strong opto-electro-mechanical effect, while photonic confinement of the rest of the light minimizes optical losses. The demonstrated hybrid architecture provides a route to develop applications for CMOS-integrated, reprogrammable optical systems such as optical neural networks for deep learning.
Electrically reconfigurable photonic netE works have the potential to enable technological advances in many fields such as optical neural networks used to process information with low power at the speed of light (
Electro-optical switches typically rely on interferometric waveguide configurations to divert light to different outputs by means of constructive or destructive interference. This is achieved by changing the refractive index (Dn) of the waveguide material. State-of-the-art networks control Dn by the electro-thermo-optical effect (
Opto-electro-mechanical (OEM) switches provide an alternative way to control the flow of light by mechanically changing the waveguide geometry rather than modulating the material’s intrinsic refractive index (
Here, we introduce a hybrid photonicplasmonic (HPP) OEM technology that benefits a strong plasmonic OEM-effect to fully switch light with a CMOS-level voltage (≈1.4 V), low optical losses (0.1 dB), and a compact footprint (≈10 mm2).
Figure 1A illustrates the dynamic routing of light by two nano-OEM (NOEM) switches, respectively biased to two different resonance states (wavelength lres): drop state (foreground device, 0 V) and through state (background device, 1 V). Incident light (wavelength l0) guided in the through port is transmitted (lres ≠ l0) or dropped (lres = l0) depending on the individual, bias-dependent lres of the encountered resonant switches. The HPP resonator comprises a thin gold membrane partially suspended above a silicon disc forming an air gap (z0) (see Fig. 1B) (
Figure 1D indicates that the large tunability of the resonance wavelength and dz as small as 4 nm already provide Dlres larger than the resonance’s loaded full-width half-maximum (FWHM). Low-loss coupling to the drop port requires that the waveguide-resonator coupling rates are larger than the resonator’s plasmonic loss rate (e.g., intrinsic FWHM) (fig. S3) (
This strong tuning can be understood by separating the OEM effect into its two subprocesses. First, the opto-mechanical coupling (GOM º dlres/dz) increases for decreasing gaps because of the plasmonic confinement of light to the gap (fig. S2) (
Furthermore, the dynamics of the NOEM switch are determined by its geometrical parameters similar to those of a ruler that extends beyond the edge of a table. Shorter suspensions (i.e., stiffer spring) and a lighter mass result in faster ruler oscillations (fres). Here, we make the overhang as short and thin as possible. The combination of small moving mass, large forces, and small mechanical quality (Q) factors enables tens of nanosecond switching at CMOS driving voltages.
The fabricated resonators are shown in Fig. 2. The drop port was omitted to probe the resonatorÕs intrinsic OEM properties. Vertical HPP waveguide geometries were uniformly created by depositing and selectively removing a sacrificial alumina layer, by wet-etching to a typical undercut value of ≈1.1 mm. Here, atomic layer deposition provides z0 with atomic-level precision. The critical feature size is the lateral waveguide-disc separation (w > 120 nm), which is achievable with low-cost photolithography.
The cavityÕs intrinsic Q factor (º1/FWHM) was measured by varying w (Fig. 2C). At critical coupling, Qintrinsic = 2·Qloaded ≈ 7000, which translates to propagation lengths and losses of 395 ± 70 mm and 0.01 ± 0.002 dB/mm, respectively (fig. S6) (
Characterization of the switching capability (see Fig. 2D) yields a Dlres > 6 nm, which equals five times the FWHM. The nonlinear red shift is expected from electro-mechanical effects as the growing proximity of the metal membrane increases Dneff (
The suspended membrane features a fres of ≈12 MHz. The small mechanical Q factor and the roll-off in modulation at lower frequencies is attributed to squeeze-film damping and stiffening; for example, air compression increases the stiffness at higher frequencies and smaller gaps and thus reduces the actuation (
Subsequently, we performed 1-by-2 switching experiments (see Fig. 4A). The through-and drop-port transmission spectra are plotted in Fig. 4B. Coupling the resonator to the drop port broadens the FWHM (optical bandwidth) from ≈1 nm (125 GHz) to ≈2.5 nm (350 GHz) (see Fig. 4B). Still, a 1.4-V driving voltage yielded a Dlres (≈6.2 nm) that exceeded multiple FWHMs. This enabled light routing with a cross-talk below Ð15 dB, drop-port insertion loss (ILD) of ≈2 dB, and through-port insertion loss (ILT) of ≈0.1 dB (fig. S10) (
We present devices that challenge the common presumption that opto-electro-mechanics is a slow and bulky technology that requires high driving voltages. We demonstrate NOEM switches whose distinct compactness paves the way for high-density optical switch fabrics that are directly cointegrated with CMOS driving circuits. For instance, 200 switches and their electrical drivers could be integrated on an area as small as the cross section of a single human hair. Beyond that, the strong OEM interaction and low-loss could enable nonresonant functional units such as phase shifters and intensity modulators for purposes such as light detection and ranging (LIDAR) applications. The performance of phase shifters is typically evaluated by the voltage-length product VpL, which states the minimal combination of p phase-shift voltage times device length. The HPP prototypes demonstrated here already feature a VpL = 27 ± 4 V·mm, which, in combination with low propagation losses (a = 0.026 ± 0.006 dB/mm), represents a substantial improvement over the state-of the-art electro-optical switches (fig. S1) (
We thank C. Hierold, J. Strait, M. Davanco, J. A. Liddle, N. Zhitenev, and U. Drechsler for valuable discussions. Funding: ERC grant PLASILOR (640478). C.H. acknowledges support under a Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Physical Measurement Laboratory, award 70NANB14H209, through the University of Maryland. C.H. acknowledges support by the Hans-Eggenberger Foundation. M.B. acknowledges funding from the SNSF Ambizione grant (173996). Author contributions: C.H. conceived the concept and supervised the project. C.H., A.J., F.M., and D.C. designed the switch and performed numerical optimization. C.H., A.J., and Y.F. fabricated the modulator and developed the required process technology. C.H., A.H., M.D., M.B., M.M., and V.A.A. designed and performed the experiments. All authors discussed and analyzed the data. C.H., A.J., M.M., H.J.L., V.A.A., and J.L. wrote the manuscript. Competing interests: J.L. is involved in activities toward commercializing high-speed plasmonic modulators at Polariton Technologies Ltd. Part of the work is subject to a patent application by C.H., V.A.A., H.J.L., D.C., J.L., M.M., and A.J. The remaining authors declare no competing interests. Data and materials availability: All data are available in the manuscript or the supplementary materials.
science.sciencemag.org/content/366/6467/860/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S10
Table S1
References (31–64)
- 5 August 2019; accepted 21 October 2019
- 10.1126/science.aay8645
DIAGRAM: Fig. 1. Operating principle of plasmonic NOEM networks. (A) Incident light guided in the through port is switched to a drop port if its wavelength (l0) matches the node’s resonance wavelength (lres), whereas off-resonance (lres ≠ l0) light continues along the waveguide and bypasses the plasmonic resonator, thereby avoiding ohmic losses (
DIAGRAM: Fig. 2. False-colored scanning electron microscopy images and measured device properties. (A) Perspective view and transmission spectrum. The small cavity volume results in a free spectral range (FSR) of 45 nm. (B) Focused–ion beam cross section. Air gaps (z0) of 35 or 55 nm have been realized. Gap length, 600 nm. The inset shows a simulated optical field, which is strongest in the gap. Enorm, absolute value of the electric field; a.u., arbitrary units. (C) ER (blue triangles) and loaded Q factor (red circles) versus waveguide-disc separation (w) for z0 ≈ 55 nm. The ER peaks at ≈200 nm, indicating critical coupling. (D) Dlres (blue to green) and FWHM (red to yellow) as a function of voltage for z0 ≈ 35 nm. The inset illustrates complete optical switching with a 200-mV difference. For (C) and (D), the 95% confidence intervals are approximately equal to the symbol size.
DIAGRAM: Fig. 3. Time dynamics. (A) Modulation response for a sinusoidal driving signal. The inset shows the mode shape of the fundamental mechanical eigenfrequency. k, thousand; M, million. (B) Utilizing more complex driving signals (red) enables optical (blue) rise and fall times on the order of tens of nanoseconds. The optical contrast between on and off state exceeds 90%.
DIAGRAM: Fig. 4. Performance of 1-by-2 NOEMS. (A) Perspective false-colored scanning electron microscopy image of two fabricated NOEMS. GND, electrical ground. (B) Measured power spectrum of light coupled to the through port (blue) and the drop port (red) under 0 V (solid) and 1.4 V (dashed) bias. (C) Through-port (blue circles) and drop-port (red crosses) transmittance over voltage. (D) Low through-port losses are beneficial for switching architectures such as cross-grid networks envisioned here, where light (various rainbow colors) only needs to be switched once to a drop port while propagating through a 15-by-15 network. In (C), the 95% confidence interval is smaller than the symbol size. l0, probing wavelength.