Optomechanical mass spectrometry

Abstract

Nanomechanical mass spectrometry has proven to be well suited for the analysis of high mass species such as viruses. Still, the use of one-dimensional devices such as vibrating beams forces a trade-off between analysis time and mass resolution. Complex readout schemes are also required to simultaneously monitor multiple resonance modes, which degrades resolution. These issues restrict nanomechanical MS to specific species. We demonstrate here single-particle mass spectrometry with nano-optomechanical resonators fabricated with a Very Large Scale Integration process. The unique motion sensitivity of optomechanics allows designs that are impervious to particle position, stiffness or shape, opening the way to the analysis of large aspect ratio biological objects of great significance such as viruses with a tail or fibrils. Compared to top-down beam resonators with electrical read-out and state-of-the-art mass resolution, we show a three-fold improvement in capture area with no resolution degradation, despite the use of a single resonance mode.

Fig. 1. Single-mode optomechanical resonator for mass spectrometry.

a In-plane rigid-body vibration mode of interest of the nanomechanical resonator (see Supplementary Fig. 2 for the other modes). b Finite-element color map of normalized frequency sensitivity to added point mass, showing that the frequency shift due to particle adsorption does not depend on particle position on the platform. c False-colored scanning electron microscope images of the device, general view (left), and zoom-in on the nano-ram (right). The platform is 1.5 µm wide and 3 μm long, with 80 × 500 nm support beams. The optical ring diameter is 20 μm, and the optical ring-to-platform gap is 100 nm. Close to 1.55 µm wavelength light is coupled in and out of the ring by optical waveguides through a 200 nm gap. Electrostatic actuation is performed with a side-gate 250 nm away from the nanoresonator. d Cross-section showing the different components of the device. The silicon-on-insulator (SOI) top layer is 220 nm thick, partially etched to realize the optical grating couplers. The nanoresonator is etched down to 60 nm. The crystalline Si layer is highly doped locally for low metal-to-silicon contact resistance. A 200 nm amorphous silicon layer is deposited above a planarized silicon oxide layer for protection and etched open above the grating couplers and the nano-ram.

Fig. 2. Optical, optomechanical, and frequency stability measurements of the nanoresonator at ~10⁻⁵ Torr and 77 K.

a Optomechanical readout scheme. FPC stands for fiber-polarization controller, PD for photodetector, and LIA for lock-in amplifier. Red and dark blue lines are optical and electrical signals, respectively. The nanomechanical resonator vibrating at frequency f₀ modulates the resonance wavelength of the optical cavity and consequently the light output power at f₀. b Narrow-band optical transmission spectrum around one resonance of the ring resonator measured with 0.5 mW output laser power. A slight thermo-optic behavior is observed. The loaded optical quality factor is ~5 × 10⁴, and the contrast is 0.23. c Optomechanical spectra of the first in-plane mode close to 44.9 MHz with increasing drive voltages. The optical input power is set to 10 mW (Supplementary Fig. 4). The mechanical quality factor is ~1700. Inset: thermomechanical noise of the resonator. The noise floor (green) is set by the photodetector’s amplifier input noise, equivalent to a displacement resolution of 2.8 × 10⁻¹⁴ ${\mathrm{m}\mathrm{Hz}}^{-\frac{1}{2}}$. d Allan deviation of the resonator, using a PLL. When increasing the drive voltage, the stability falls into a 1 ppm limit over the whole integration time range, which is consistent with the presence of mechanical resonance frequency fluctuations.

Fig. 3. Vacuum system used for mass deposition and optically packaged optomechanical device.

a The set-up consists in a sputtering source capable of generating metallic clusters of controlled size and mass, and a time-of-flight spectrometer. Vacuum feedthroughs carry electrical and optical signals in and out of the system. The optomechanical device is placed on a retractable sample holder at the end of a cryostat (liquid nitrogen). By moving the sample holder in and out of the cluster beam, a given cluster population can be measured with the nanoresonator and TOF detector sequentially. b Light is coupled in and out of the optomechanical chips using grating couplers with a pitch of 0.6 μm and a width of 0.3 μm, designed for maximum transmission close to a 1550 nm wavelength and an input angle of 10°. c Quasi in-plane optical packaging by waveguide-to-fiber-transposer chips (measuring ~1 × 2 × 20 mm), aligned and glued to the grating couplers. The electrical inputs/outputs are obtained by standard wire-bonding on metallic pads (see also Supplementary Fig. 9). d SEM image of the amorphous silicon protection layer covering all optical and electrical elements so that mass deposition only occurs on the sensing platform.

Fig. 4. Single-particle optomechanical mass spectrometry of tantalum clusters.

a Frequency trace of the optomechanical resonator for the light green (5.7 MDa) cluster population deposition. For this particular measurement, 1140 events were recorded in ~5 min, for a total deposited mass close to 1% of the mass of the resonator. The sampling time is here 50 μs, the PLL bandwidth is 10 ms and the plotted frequency trace is averaged to a 10 ms integration time. Spectra are plotted from detected frequency jump heights (Methods). Inset: detail showing several frequency jumps from individual cluster depositions. b Normalized nanoresonator’s and c TOF mass spectra for four different tantalum cluster populations fitted with a log-normal function (dark blue lines) with mean masses ranging from 2.7 to 7.7 MDa (for optomechanical MS), equivalent to particle diameters from 8 to 11.3 nm. The number of events in the nanoresonator’s spectra range from 534 to 1384, for measurement times up to 300 sec in each case.