OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Wavelength-division multiplexing of nanooptomechanical doubly clamped beam systems Vincent T. K. Sauer,1,2 Zhu Diao,2,3 Mark R. Freeman,2,3 and Wayne K. Hiebert2,3,* 1
Department of Electrical and Computer Engineering, University of Alberta, ECERF 2nd Floor, Edmonton, Alberta T6G 2V4, Canada 2 National Institute for Nanotechnology, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada 3
Department of Physics, University of Alberta, 4-181 CCIS, Edmonton T6G 2E1, Canada *Corresponding author: [email protected]
‑cnrc.gc.ca Received January 26, 2015; revised March 20, 2015; accepted April 3, 2015; posted April 3, 2015 (Doc. ID 233082); published April 20, 2015
Wavelength-division multiplexing is demonstrated for a set of two doubly clamped beams. Using a single input/ output waveguide in a nanophotonic detection system, the two mechanical beams are independently addressable using different wavelength channels as determined by their respective racetrack resonator detection cavities. The two cavities slightly overlap, which also enables the mechanical frequency of both beams to be detected simultaneously with a single wavelength. Finally, to physically map which wavelength channel corresponds to which specific device, a heating laser is targeted individually on each beam to create a reversible mechanical frequency shift. This multiplexing method would allow for the simpler detection of large arrays of nanomechanical devices in a sensor system. OCIS codes: (130.3120) Integrated optics devices; (130.3990) Micro-optical devices; (130.6010) Sensors; (220.4880) Optomechanics; (230.4685) Optical microelectromechanical devices; (350.4238) Nanophotonics and photonic crystals. http://dx.doi.org/10.1364/OL.40.001948
Nanomechanical beam resonators have taken great strides toward real-life applications in particle detection and measurement systems. They have been utilized both in gas chromatography [1,2] and mass spectrometry systems [3–5], and have measured particles in a fluid [6–8] and gas phase . They have measured masses at the atomic scale [10–12], and their mass sensitivity has reached the single proton level [13,14] confirming their potential in future measurement systems. A critique of nanomechanical resonator sensors arises from their tiny size  and hence very small sensing area. This could make their practical implementation difficult, as a sensor will not function if the analyte cannot reach the detector. Luckily, nanomechanical sensors have been developed using traditional integrated circuit technologies, so this limited capture area can be offset using large parallel sensor arrays. Detecting large arrays of devices can become complicated if each individual device requires an independent connection. A full-field interferometer optical-detection method has been used to circumvent this issue , but it is limited by the requirement that the full sensor array is visible in the detector’s optical field with sufficient resolution. Alternatively, a series-parallel electronic configuration can be used to detect an entire array with a single electrical input and output . The drawback to this method is the array operates as a single sensor so each individual event is averaged across the entire array. An alternative method may be required to detect large arrays of beams individually. Integrated nano-optomechanical systems (NOMS) have emerged as a promising detection method for nanomechanical resonators. The high displacement sensitivity and operational bandwidth [17,18] match well with ever smaller nanomechanical resonators . The detection method involves a mechanical resonator interacting with the evanescent field of a nanophotonic waveguide [20–22] or optical cavity [23–27]. Multiplexing using nanophotonics has been demonstrated using an array
of 63 nanomechanical cantilevers detected with a single optical fiber . There is still room for improvement since this detection method uses light scattering, instead of phase interactions, meaning each device added to the array will cause incremental signal losses. Here, we circumvent this issue by demonstrating the nanophotonic phase detection of multiple nanomechanical cantilevers using wavelength-division multiplexing (WDM). One powerful property of optical fiber communication systems is the ability to use WDM. A single communication fiber has the ability to send multiple signals simultaneously using different wavelength carrier channels . This property is used in integrated optical systems as well especially with optical ring resonator cavities . Applying this technique to detect individual nanomechanical beams in a large array would be useful for a practical sensor system. The low capture efficiency of a single beam could be offset by using an array that would still be addressable with a single input/output. The independence of each nanomechanical beam’s signal would be maintained on its own wavelength channel. Additionally, spatial information for mass loading events on the sensor could be obtained since the signal from a specific wavelength channel could be matched to a distinct optical cavity on the sensor chip. Coupling WDM with nanomechanical beam transduction would work well since both can utilize integrated optical cavities. Nanophotonic device multiplexing is demonstrated using two adjacent optical racetrack resonator cavities implemented in an all-pass configuration. Each optical racetrack detects its own adjacent nanomechanical doubly clamped beam as shown in Fig. 1(a). Each racetrack is designed identically with a 3-μm straight portion and end curves of 5-μm radius. The mechanical beams are each about 7.5 μm long and 160 nm wide. The mechanical beams oscillate toward and away from the racetrack cavity, so this width functions as its vibrational thickness. The devices are fabricated on standard integrated photonic silicon-on-insulator wafers having a 220-nm-thick
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS 5 µm
Fig. 1. (a) Colorized SEM of a multiplex NOMS. Two separate racetrack resonator NOMS devices (red, green) are connected to the same waveguide bus (blue). The different optical resonance frequencies of each cavity allow the separate mechanical beams to be probed individually at their respective cavity resonance mode. (b) DC optical-transmission spectrum of a multiplexed racetrack resonator nanomechanical beam-transducer system. Each dip corresponds to the optical resonance frequency of each individual racetrack resonator. The labels indicate probe wavelength values used in the experiment. The inset shows a similar spectrum taken before (solid-blue) and after (dotted-red) a 1064-nm heating laser is incident on the sample. The shifts for each resonance mode are each ∼170 pm.
device layer and 2-μm buried oxide layer and using stepper lithography. The gap spacing between the beam and cavity is 110 nm. The devices were fabricated through CMC Microsystems. After the integrated photonic chip is fabricated, a timed buffered oxide etch is used to release the mechanical beams to allow them to vibrate freely. Since the beams are much thinner than the 430nm wide waveguides, the beams release while the waveguides remain anchored to the chip. This fabrication process follows what is used in Refs. [22,27]. Although both optical racetracks are designed using identical dimensions, the optical resonance wavelength and optical quality factor of each cavity is slightly different due to fabrication variations. This also holds true for the mechanical frequencies of the doubly clamped beams. The difference in the optical resonances is illustrated in the DC optical transmission spectrum in Fig. 1(b), which shows the throughput power at varying probe wavelength. The “w” shape follows from the overlapping of the bandwidth of the two cavity resonances. Further separation could be achieved with design changes between the two cavities. Although the cavity quality factor for each racetrack is poor compared to typical integrated photonics cavity filters, as illustrated by
the width of the optical dips, it is adequate for the nanomechanical beam detection demonstrated here. The phase detection scheme used here operates by the mechanical motion of the doubly clamped beam interacting with the evanescent field of the cavity. This changes the effective index of a portion of the cavity length, which in turn alters the cavity’s resonant frequency. The mechanical beam motion is detected by monitoring the throughput power set at a constant measurement wavelength slightly detuned from the cavity resonance. At the detuned wavelength, the transmitted power is extremely sensitive to the cavity mode shift. A detailed description of this detection mechanism is described in Ref. . In this experiment, each individual doubly clamped beam will only modulate the optical resonance of the racetrack it is directly adjacent to. Each isolated cavity wavelength can be used to individually detect each mechanical beam. The WDM property of this two-device set is tested by monitoring the thermomechanical (TM) noise frequency response of the beams while scanning the probe wavelength across the two optical cavities. The devices are measured under vacuum to remove any air damping using a Zurich lock-in amplifier (200 Hz BW, 4× avg). The probe wavelength is set to different carrier channels of 1550.4 nm (A), 1550.7 nm (B), 1551.5 nm (C), and 1552.0 nm (D). These wavelength channels correspond respectively to the side of one optical resonance, the minimum of that resonance, the middle peak of the overlapping cavities, and the side of the second resonance as seen in Fig. 1(b). The TM noise response at each probe wavelength channel is shown in Fig. 2. In a phasedetected system, the mechanical signal strength is dependent on the slope of the DC optical transmission curve where the probe is sitting. This explains the lack of signal at B, which is plotted offset by −10 nW for clarity.
Fig. 2. TM noise response of a two-device multiplex system probed at different wavelength channels. As the probe wavelength is changed, the signal shifts from one mechanical resonance to the next. The overlap of the two optical resonances allows a single wavelength to probe the mechanical beams on both racetracks simultaneously (blue diamonds).
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
The overlapping optical frequencies of the two cavities allow the system to detect both mechanical resonator signals at a single probe wavelength. This suggests that multiple mechanical devices can also be probed using the same optical cavity in a phase-detection system as demonstrated in Ref. . This could further decrease transducer complexity by detecting multiple devices at the same wavelength. If this is done, care must be taken to track each mechanical frequency individually since this frequency differentiates each nanomechanical sensor. This is especially true if the devices are functionalized to detect different analytes. WDM NOMS allow a set probe wavelength to differentiate between various devices in a nanomechanical beam array as opposed to the mechanical beam frequencies that change at each measurement event. With very careful design and fabrication, the channel wavelength of each ring, and hence each mechanical device, could be determined. However, it is still difficult due to fabrication variations affecting the cavity resonance mode. Alternatively, the cavity resonance frequency can be permanently tuned after fabrication using a femtosecond laser . A simpler method can be used to determine which mechanical resonance corresponds to which optical cavity. This is achieved using a heating laser to reversibly alter a targeted beam’s mechanical frequency. The mechanical frequency shift is induced by a softening of the mechanical beam with higher temperature. The reversible targeted heating induced on the mechanical beams is achieved using a 0.5-mW 1064-nm laser focused onto the substrate surface with ∼150 μm diameter. This laser is manually scanned across the multiplex devices while monitoring the mechanical frequency response. As the laser is preferentially aimed toward the left device (red) in Fig. 1(a), the lower mechanical frequency, corresponding to the 1550.4-nm cavity mode, is shifted downward due to its decreased mechanical spring constant. Conversely, while targeting the right most (green) device in the figure, the higher mechanical frequency decreases. This is shown in Fig. 3. This experimentally links the 1550.4-nm probe channel device to the left-most doubly clamped beam (18.55 MHz mechanical frequency), and the 1552.0-nm probe channel device to the right-most doubly clamped beam (18.615 MHz). Both doubly clamped beams shift in frequency ∼3 kHz upon heating, and they slightly differ most likely due to the position of the heating laser. Estimating a mechanical frequency change due to temperature of −30 ppm∕°C for silicon, this indicates an approximate change of 5°C for each beam when heated. The heating laser also changes the optical frequency of the racetrack cavities, so each measurement is taken after these frequencies have stabilized. They are stabilized by targeting the heating laser ∼200 μm from the devices and allowing the heat to conduct through the substrate. Both optical cavity modes shift approximately equally and remain stable as the position of the heating laser is scanned across the devices. Both optical resonance modes undergo a redshift of ∼170 pm as seen in the inset of Fig. 1(b) indicating an estimated temperature change of 2°C assuming an 80 pm/°C shift. The lower temperature change compared to the beams could be
Fig. 3. Simultaneous monitoring of both mechanical frequencies while the device is not heated (blue circles), the left device in Fig. 1(a) is heated preferentially (red triangles), and the right device is heated preferentially (green inverted triangles). The arrows above the peaks indicate the direction of the frequency shift upon heating. A 3.1-kHz shift for the lower peak and 2.6-kHz shift for the upper peak indicate a temperature change in the beams of ∼5°C.
a result of the beams’ poorer heat conduction compared to the rest of the sample. To acquire Fig. 3, the overlapping optical cavity frequencies are exploited by tuning the detection wavelength to simultaneously detect each mechanical response with approximately equal magnitude. To avoid potential interference caused by the probes optical spring effect on the nanomechanical beam, the probe laser output is decreased by a factor of 3 compared to earlier measurements. To compensate for the resultant signal loss, the devices are mechanically actuated using a shear-mode piezoelectric crystal as in Ref. . The devices are both driven and detected using an HP 8752C network analyzer with 300-Hz BW and 4× averaging. Any future mass loading experiments could now use the optical channel as the device differentiator instead of the mechanical frequency. Wavelength-division multiplexing is demonstrated for two independent NOMS devices using two racetrack resonator optical cavities on the same waveguide. Each racetrack can detect the motion of its respective doubly clamped beam using a wavelength channel within its optical resonance mode. Additionally, overlapping of the two cavities’ optical modes allow both mechanical beams to be detected using the same probe wavelength. Finally, the wavelength channel corresponding to each physical device is determined by selectively heating either mechanical beam. Knowing the set wavelength channel for each independent mechanical resonator allows the mechanical responses to be easily tracked along with the location of any mass loading events. Multiplexing offers a possible solution to the small capture area of individual nanomechanical sensors, and NOMS implementation is an excellent candidate for future sensor systems.
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
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