A NANO-ACTUATOR VIA CAVITY-ENHANCED OPTICAL DIPOLE FORCE

J. F. Tao1, 2, 3, J. Wu1, H. Cai3, Q. X. Zhang3, X. Kun1, J. M. Tsai3, D. L. Kwong3 and A. Q. Liu2† 1State Key Laboratory of Information Photonics and Optical communication, Beijing University of

Posts and Telecommunications, Beijing 100876, China 2School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798

3Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore 117685

ABSTRACT

In this paper, we demonstrate a nano-actuator using a silicon-based monolithic cavity nano-opto-mechanical system. The nano-actuator is constructed by a special designed nano-scale silicon suspended cantilever which is efficiently driven by the optical gradient force. In experiment, the actuator obtains a tuning range up to 52 nm. The optical power consumption is reduced to 0.04 mW/nm, which is much smaller than typical value of 3mW/nm in optomechanical systems. INTRODUCTION

With nano-technology development, mechanical actuators capable of working with nanoscale precision positioning are essential for active devices which are compatible for high density on-chip integration [1-4]. However, it is very difficult to realize nanoscare actuation by conventional micro-machined actuators using piezoelectric [5], thermal expansion [6], magnetic [7] and surface acoustic waves (SAW) [8]. Conventional actuators have big working volume which results in the difficulties for on-chip level integration. Tough advanced in high-resolution and miniature size, micro-machined electrostatic actuators are greatly limited by high driven voltage and instability when their feature sizes are down to sub-micron scale [9-12].

Light can trap small particles using its electromagnetic field which can be used as optical tweezers [13-14]. With the development of nano-fabrication technology, optical force in nano-mechanical structures has been observed and experimentally demonstrated in recently research on nano-opto-mechanical systems [NOMS] [15-21]. This type of optical force is induced by strong field gradient in the near-field of guided wave structures. Therefore, it is named as optical gradient force in order to distinguish from the traditional optical radiation force. The produced optical gradient force is up to approximately 20 pN/mW/μm for milli-watt level optical input power by using micro-ring [17], micro-disk [18-19] or photonics crystal [20-21] to enhance the light. In general, optical gradient force is more precise and stable than conventional forces for the nano-scaled actuation.

Here, for the first time, the optical gradient force is applied to demonstrate a nano-actuator in which a double-disk resonator system and a suspending cantilever beam with ultra-small spring constant are special designed. In experiment, it successful achieves tens of nanometers displacement with high position accuracy and high

resolution. DESIGN AND SIMULATION

Figure 1: Schematic of nano-actuator driven by optical force. The optical force between the suspension beam and the micro-disk resonator is generated by the evanescent light wave.

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Figure 2: (a) Magnitude of the E-filed; (b) optical energy transfer into mechanical energy through optical force.

The design and working principle of the proposed

nano-actuator is illustrated in Fig. 1. It consists of an input bus-waveguide, a pair of micro-disk resonators, and a moveable suspension beam with a sharp sensing tip. The nano-scale suspension beam is fully released so that it can be actuated by optical gradient force between micro-disk and suspension beam. When light is input and resonates in the micro-disks, its enhanced evanescent field excites

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978-1-4673-0325-5/12/$31.00 ©2012 IEEE 1125 MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012

dipoles in the adjacent suspension beam and generates an attractive optical gradient force, which attracts the suspension beam near the micro-disk. The displacement of the sensing tip is determined by this optical force which is proportional to the input optical power and the coupling strength between optical energy and mechanical energy. To enhance the coupling strength, it requires a large coupling area while maintaining a small coupling gap. In this work, the suspension beam is designed as a curve sharp around the micro-disk so that it interacts with almost half of the micro-disk circumference. The displacement of the sensing tip is adjusted by changing the power of incident light. The spring constant is reduced by scaling down the dimensions of the suspension beam. Therefore, the nano-actuator can be precisely driven by optical gradient force.

The micro-disk resonator has a radius of 5 μm and a 200-nm working gap from the released suspension beam. The simulated projection of the electric field intensity (|E(x)|2) along the diameter direction (x-axis) and the contour plot of (|E(x)|2) at the cross section are shown in Fig. 2(a) and (b), respectively. The fields of both the micro-disk resonator and input waveguide decay exponentially outside their geometric boundaries owing to the evanescent nature of the electric field. By using this E-field distribution, we employ Maxwell stress tensor formula to calculate the optical force

where, f is the force per unit volume exerted by the

electromagnetic fields; S is the pointing vector presenting the energy flux; ε0 is the permittivity of free space; μ0 is the vacuum permeability; V is the volume of the force loading part; T is the Maxwell Stress Tensor which is expressed as

where, Ei (Ej) and Bi(Bj) are electric filed and magnetic field; Based on these equations, the optical force is calculated. The estimated optical force is approximately 16.2 pN/μm/mW when the gap is 200 nm. This force is about 31 times larger than the previously demonstrated optical force on a single waveguide coupled to silicon dioxide substrate.

The displacement of the actuator can be easily tracked using optical method. Since the mechanical displacement of suspension beam changes the effective mode index of the disk resonator and induces a resonance wavelength shift. Therefore, the displacement value can be estimated by measuring the resonance wavelength shift or the intensity change of the output light. The on-resonance transmission depends on the coupling efficiency and the loss rate, which can be expressed as

where Υe is the extrinc coupling rate which is determined by the coupling gap (g) between the input bus-waveguide and the disk resonator; that is Υe =Υe0 exp(-ŋg), where isΥe0 nominally the ‘zero-gap’ coupling rate and ŋ is the decay constant. Υi is the intrinsic cavity loss rate which is determined by the effective index of the disk resonator [22]. When the suspension beam is displaced in the evanescent field of the disk resonator, the resonant condition of the disk resonator is mechanical modulated, namely the mechanical-optic effect. Therefore when the gap between the suspension beam and disk resonator changes, the resonance wavelength is shifted and results in the change of output optical power for certain wavelength. The nano-scaled displacement can be tracked automatically using optical method by measuring this wavelength shift or power changing. Figure 3 shows the transmission states when the gap is 200 nm, 180 nm and 160 nm respectively.

Figure 3: Ez filed contour map of the 2D simulation at different gap (g) between micro-disk resonator and suspended curving beam when the input wavelength is fixed.

At the initial state, the input wavelength is on resonant when the gap is 200 nm. With the gap decreases, the power in the disk becomes lower and lower. When the gap becomes 160 nm, the output optical power reaches the peak and the resonant power in the disk is almost zero. Thus it provides a flexible way to label the displacement using all-optical method. FABRICATION

The scanning electron micrographs (SEMs) of the nano-actuator are shown in Fig. 4. The actuator with a footprint of 20 μm × 45 μm is fabricated on silicon-on-insulator (SOI) wafer with a structure layer of 2200 nm. The actuator is patterned by deep UV lithography and etched by plasma dry etching. The waveguide is covered by a layer of SiO2 cladding (2 μm thick) which is deposit by using plasma enhanced chemical vapor deposition (PECVD). In release process, a 50-nm amorphous silicon layer is used as the hard mask to protect the structures which are not supposed to be released. Then the buried-oxide layer is removed using HF-vapor with precisely time control. The width of the suspension beam is 200 nm and total length is 38 μm, so that it can provide an ultra-small spring constant and more sensitivity to the driven force. The input waveguide is fixed by using a waveguide holder to protect the released waveguide to keep a constant coupling gap. The radii of the two disks are 5 μm. Although the designed radii are same, the resonance

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wavelengths are a slight different mainly induced by the release process and dry etching. Unlike ring resonator, the disk resonator has both the high optical Q-factor and the high thermal distribution efficiency [17]. A pair of inversed mode converters was used to couple light into and out of the device on waveguide alignment system platform. The tested fiber-to-chip coupling efficiency loss is small than 1.85 dB. Another coupling between the input waveguide and the disk is depended on the gap between them. We determined the critical coupling gap between input waveguide and the disk to be 170 nm by measuring devices with varying geometries.

Figure 4: SEM of the fabricated optical nanoactuator.

EXPERIMENTS AND DISCUSSIONS

The device is tested on an optical alignment system (Model: KUGE KS-502-M) in which a tipper-fiber with a mode field diameter of 4 μm is used to couple the light into the bus-waveguide with taper tips. The normalized transmission spectrum of the devices for input light with TE polarization is shown in Fig. 5. Some resonance dips are obtained in the range of 40 nm from 1560 nm to 1600 nm. Although these two micro-disks are designed with same radius, the fabrication and release conditions can generates a tiny difference on the real resonant conditions

which makes the resonance frequency splitting a little. However, the measured Q-factor (~ 12, 000) is almost the same which guarantees the equal driving forces by two disks. In order to drive the actuator and track the displacement concurrently, a wideband pump light covering the resonance wavelength region from 1582 nm to 1586 nm is used to driven the actuator while an optical spectrum analyzer (OSA, model: ANDO-AQ6317) is used to detect the wavelength shift. The actuation displacement is read out by monitoring the output dip wavelength (resonance wavelength).

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Figure 5. Transmission spectrum of the TE-mode. Pump is a broadband light acting as the driven light from 1582 nm to 1586 nm.

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Figure 6: Resonance wavelength is red-shifted when the input optical power is increased.

The resonance wavelength tuning is observed when the input optical power increases from 0.1 mW to 2.4 mW, as shown in Fig. 6. The actuation displacement is linearly increased with the increasing of input pumping power. The displacement up to 60 nm is obtained when the input optical power is increased to 2.4 mW as shown in Fig. 7. The detected detuning of the resonance wavelength is mainly induced by the gap decreasing. The detected transmission spectrum changes according to the displacement. When the input optical power increases, the attractive optical force between the suspension beam and micro-disk is linearly enhanced. Therefore the suspension beam can be actuated close to the micro-disk. The displacement tracking also can be demonstrated by measuring the output power value. The dip frequency change of the output is sufficient to sense a 0.4-nm

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displacement since the typical sensitivity of the optical spectrum analyzer is approximately 0.005 nm.

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Figure 7: Input pumping optical power vs Displacement. CONCLUSIONS

In conclusion, a nano-actuator based on optomechanical force is designed, fabricated and experimentally demonstrated by using NOMS technology on an integrated silicon platform. The cavity enhanced gradient optical force is used to drive the special designed suspension beam in nano-scale integration. Compared with electrostatic force, it provides a larger driven force, more stable performance and more suitable for electrical free working environment. Moreover, this optical nano-actuator has the merits of a large tuning range (52.8 nm), low power consumption (0.04 mW/nm), and small footprint (20 μm × 45 μm). This new type of nano-actuator has promising applications in areas of nano-manipulation and scanning probe microscopy. REFERENCES [1] O. Custance, R. Perez and S. Morita, “Atom force

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CONTACT *A. Q. Liu, tel: +65-67904336; eaqliu@ntu.edu.sg

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