Integrated nano-opto-electro-mechanical sensor for spectrometry and nanometrology - PubMed (original) (raw)

doi: 10.1038/s41467-017-02392-5.

Rob W van der Heijden 2, Maurangelo Petruzzella 2, Francesco Pagliano 2, Rick Leijssen 3, Tian Xia 2, Leonardo Midolo 4, Michele Cotrufo 2, YongJin Cho 2, Frank W M van Otten 2, Ewold Verhagen 3, Andrea Fiore 2

Affiliations

Integrated nano-opto-electro-mechanical sensor for spectrometry and nanometrology

Žarko Zobenica et al. Nat Commun. 2017.

Abstract

Spectrometry is widely used for the characterization of materials, tissues, and gases, and the need for size and cost scaling is driving the development of mini and microspectrometers. While nanophotonic devices provide narrowband filtering that can be used for spectrometry, their practical application has been hampered by the difficulty of integrating tuning and read-out structures. Here, a nano-opto-electro-mechanical system is presented where the three functionalities of transduction, actuation, and detection are integrated, resulting in a high-resolution spectrometer with a micrometer-scale footprint. The system consists of an electromechanically tunable double-membrane photonic crystal cavity with an integrated quantum dot photodiode. Using this structure, we demonstrate a resonance modulation spectroscopy technique that provides subpicometer wavelength resolution. We show its application in the measurement of narrow gas absorption lines and in the interrogation of fiber Bragg gratings. We also explore its operation as displacement-to-photocurrent transducer, demonstrating optomechanical displacement sensing with integrated photocurrent read-out.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Fig. 1

Fig. 1

Overview of the sensor design. a Sketch of the device with electrical contacts and a visible cross-section with p- (blue) and n- (red) doped layers. QDs are located in the middle of the top membrane. Sensor actuation is possible by applying a reverse bias voltage (V T) to the tuning diode (on the right side), whereas the read-out is done by measuring the photocurrent from the photodiode (left side). b, Simulated optical mode wavelength dependence on the membrane separation for two modes that are symmetric (S) or antisymmetric (As) with respect to the out-of-plane direction. c False-colored SEM image of a typical device (top view) with contact pads to both sensing and actuation diodes. d Zoom-in SEM image showing the active part of the sensor: a four-arm bridge of dimensions 16 × 12 μm containing a photonic crystal cavity suspended above a fixed photonic crystal membrane. Inset: SEM image of the patterned L3 cavity design modified for high Q factor and large free-spectral range in a double-membrane structure. Optimization was done by displacing horizontally outwards and reducing the radius of six holes (green) and displacing four holes vertically (red)

Fig. 2

Fig. 2

μ-spectrometer measurements. a, b A photocurrent spectrum of the fundamental antisymmetric (symmetric) mode of the three missing holes cavity (L3) modified for high Q factor in double membrane, with a linewidth of 132 pm (76 pm) and a Q factor of Q As~9900 (Q S~17,000) is shown in blue (red). Data was taken by measuring the photocurrent while a tuneable laser (P in = 125 μW) was swept across the cavity mode. c Color-coded photocurrent spectra (P in = 25 μW) showing the fundamental antisymmetric optical modes of an optimized H0 cavity spectrally tuned over 30 nm (x axis) by increasing the reverse tuning bias V T from 0 to 5.6 V (y axis) without reaching pull-in. The tuning range is approximately equal to one free spectral range in this case. d Data traces of photocurrent collected by voltage-tuning the optical mode (fundamental antisymmetric mode of the L3 cavity) over a fixed laser wavelength, then changing the laser wavelength by 1 nm and continuing the voltage tuning. The laser power was coupled into the cavity from top through a 0.45 NA objective, with the power incident on the sample being 12.5 μW. The scale on the bottom axis in the figure is obtained from a piecewise linear fit of the voltages at the maximum photocurrent versus wavelength. The cavity linewidth provides a spectral resolution of ~200 pm, and the FSR of ~13 nm is limited by the crossing with another cavity mode. The decrease in responsivity with decreasing inter-membrane distance (decreasing wavelength) is attributed to an asymmetry in membrane thickness, the upper membrane being 15 nm thicker

Fig. 3

Fig. 3

Resonance modulation spectroscopy. a Sketch of the circuit used for resonance modulation spectroscopy. On the tuning probe input (right side), a modulated signal (V AC) at frequency f coming from the lock-in is superimposed on a tuneable DC voltage (V DC). On the detector output (left side), the photocurrent (I ph) produces a voltage drop on a load resistor (R L) and its in-phase component (_X_-channel) and its phase (φ) at the frequency (f) are measured by the lock-in amplifier. b Comparison of a laser line recorded using two operation modes of the sensor: spectrometer mode described earlier (red dots) and resonance modulation mode (blue dots). Both measurements were performed simultaneously, by sweeping the tuning voltage and reading the photocurrent DC value (red) and the in-phase component measured by the lock-in amplifier (blue), using a load resistor R L = 30 kΩ. Since the blue curve is proportional to the derivative of the red curve, the constant background is eliminated. The cavity is modulated with V AC = 5 mVpp at f = 608 Hz, and excited at a fixed laser frequency of 1322 nm (the power incident on the sample is estimated to be 125 μW); Inset: zoom-in of the zero crossing in the resonance modulation mode

Fig. 4

Fig. 4

Applications of resonance modulation spectroscopy. a Gas sensing demonstration, where an HF absorption line at 1312.591 nm is measured in transmission using the resonance modulation scheme previously described. Voltage was translated into wavelength using a calibration curve obtained from independent PL cavity mode tuning data. A superluminescent diode (SLED) fiber-coupled to an HF gas cell (at p = 50 Torr), and filtered with a 12 nm wide 1310 nm band pass filter is used for excitation. A filter is needed to isolate a single absorption line, and a single cavity mode; inset: optical spectrum analyzer (OSA) spectrum of the SLED with the HF cell inserted, showing the same absorption line. b Wavemeter measurement of a FBG resonance in reflection performed using the resonance modulation scheme. A SLED is connected to the first input of a 2 × 2 fiber beam-splitter with the FBG on one of the outputs, and the second input (reflection) is coupled to the cavity through a NA = 0.45 objective (P in ≈ 1.6 μW). The FBG peak (100 pm wide) is read by sweeping a cavity mode (δλ c = 237 pm, δV c = 270 mV), having a sensitivity (slope at zero crossing) of S I = 3.53 nA V−1. The noise measured at the zero crossing is δI noise = 3.6 pA Hz−1/2 that translates to a peak wavelength uncertainty of δλ noise = (δI noise/S I) × (δλ c /δV c) = 0.9 pm Hz−1/2. Inset left: the OSA spectrum of the FBG filter in reflection. Inset right: detuning of the FBG peak (left axis) and corresponding temperature shift (right axis) over a period of 120 s measured using the cavity sensor. The peak visible at t = 10 s is induced by convective heating from a heat-gun 50 cm away from the FBG. The current signal (lock-in output) is translated to displacement (∆λ) using the slope S I. The FBG temperature sensitivity is taken to be δλ B /δT = 8.5 pm K−1 (specified by the manufacturer)

Fig. 5

Fig. 5

Brownian motion detected via photocurrent noise. a Sketch of the measurement circuit, with indicated device, input laser, RF probe, amplifier, and the ESA. An RF probe is used to measure the on-chip photodiode, and the signal is sent via an transimpedance amplifier (A in the setup sketch) to the input of the ESA. The device was mounted in a vacuum chamber (pressure below 10−4 mbar). In this experiment, no bias is supplied to the tuning diode. b Photocurrent measurement of the cavity optical mode for the laser input power P in = 100 μW. c ESA spectrum of the photocurrent noise where the fundamental mechanical mode is visible in the output power (red dots) and control measurement with laser off (black dots). The right axis shows the calibrated power spectral density of motion. The CW laser was coupled into the cavity and the laser wavelength was red-detuned from resonance to the wavelength where the photocurrent varies maximally with detuning (λ L = 1314.29 nm) and its power was kept low enough so as not to excite self-oscillations (P in = 100 μW). The two other sharp features present in both measurements originate from the environmental RF noise; inset: 3D displacement plot of the 4-arm bridge fundamental mechanical mode with frequency f 1 = 2.18 MHz simulated using Comsol

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