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PHOTONIC MEMS: FROM LASER PHYSICS TO CELL BIOLOGY

A. Q. Liuand X. M. Zhang (Invited paper)

School of Electrical & Electronic Engineering, Nanyang Technological University, SINGAPORE 639798 (Email: eaqliu@ntu.edu.sg; Tel: (65) 6790-4336; Fax: (65) 6793-3318)

Abstract: This paper reviews the recent progress from optical MEMS to photonic MEMS, nano-photonics and biophotonic MEMS, which represents a latest trend of the expansion and penetration of the MEMS technology to the nano and bio areas. Different types of devices are demonstrated as the examples of the natural synergy of MEMS with the photonics, including thermo-optic switches, tunable lasers, injection-locked laser systems, nano-photonic bandgap devices and biophotonic cell chips. The use of MEMS not only produces better integration, robustness and compactness, but also improves the functionalities and specifications of the devices.

Keywords: MEMS, Photonic crystal bandgap, Biophotonics, Thermo-optics.

1. INTRODUCTION

Over the past decade, a rapid development in the field of microelectromechanical systems (MEMS) technology has been witnessed. Similar to the integrated circuit chip process which has enabled the electronics revolution, MEMS fabrication process also makes use of photolithography and other batch processes, so as to enjoy the advantages of high accuracy (~ 0.1 µm), fast response (~ 1 ms), small volume (~ 1 mm3), lightweight (~ 10 g), single-chip integration, easy IC integration and low cost implementation [1]. This is particularly true when the technology platform is compared with traditional systems making use of only discrete component systems, which are typically bulky, expensive and of lower performance.

Hence, inspired by the inherent advantages of such technology, we have been working particularly in the area of photonic MEMS devices and biophotonic chips, and have developed a series of new generation devices, such as thermo-optic switches and different types of tunable lasers, which make use of the micromachined prisms/mirrors/gratings for switching and wavelength tuning [2-13]. The same principle is also extended to the polymer material microfluidic chips for the detection of living cells [14-16]. Using deep UV lithography and electron beam, the feature size can go down to 0.1-µm level and thus facilitates the nano-photonic devices [11]

whose bandgap crystal property opens an new area for photonic integrated circuit and controlling one light beam using another light beam [12, 13].

The objective of this paper is to give a general image of the recent progress of optical & photonic MEMS, with particular foci on thermo-optic switches and tunable lasers in Section 2 and biophotonic cell chips in Section 3.

(b) Infrared snapshots of two switching states

Fig. 1 Micromachined thermo-optic switch that employs the total internal reflection of a silicon prism .

(a) SEM of the integrated device

Input

Output 2

Output 1 Mirror

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2. OPTICAL & PHOTONIC MEMS DEVICES 2.1 Micromachined thermo-optic switches

Previous micromechanical optical switches suffer from the problems of mechanical reliability/stability and response limit (~ 1 ms). To circumvent these problems, a thermo-optic switch was demonstrated recently by utilizing the thermo-optic effect (TOE) and total internal reflection (TIR) of a silicon prism [5] as shown in Fig.1. The key idea is to change the refractive index of the prism via TOE to switch the light from the transmission state to the TIR state. It requires a temperature change of 69 K to switch from the transmission to the TIR states, which measure isolations of 15.6 dB and 40.1 dB, respectively. This switch is unique in switch mechanism and free of mechanical instability.

2.2 Photonic MEMS tunable lasersAn early version of MEMS tunable laser was

demonstrated in 2001 [3]. Although it is simple and quite rough, it is later found to be one of the ground-breaking work in the field of photonic MEMS. Fig. 1 shows an improved design [8]. The MEMS tunable laser consists of a laser chip for optical gain and a curved mirror for wavelength tuning. It is fabricated on a silicon-on-insulator wafer with a 75-µm-thick structural layer. In a recent work, a 3-dimensional (3D) optical coupling system is introduced to improve the optical coupling in the external cavity. It makes use of an optical fiber as the rod lens to collimate the light in the vertical plane and a curved mirror in the horizontal plane [4]. This is the first realization of 3D coupling in single-chip MEMS lasers. With such 3D system, the tunable laser has obtained an efficiency of 47% and a tuning range

of 53.2 nm, significantly higher than the typical values of 8% and 10 nm, making it possible for many advanced applications. Successive work has focused on new designs to improve the coupling efficiency and the wavelength tuning range [4-9].

2.3 MEMS injection-locked laser systems Another photonic device was also first

presented by the integration of several MEMS components into a functional subsystem [10]. It is a miniaturized injection-locked laser (ILL) that consists of a MEMS grated-tuned laser and a Fabry-Pérot multimode laser within dimensions of 3 mm × 2 mm × 0.6 mm as shown in Fig. 3. Single- and multiple- injections to the slave laser are both tested, achieving a side mode suppression ratio of 55 dB, a range of fully locked state of 0.16 nm and a rate of all optical switching at 100 MHz. New theoretical model has also been established to explain some observed phenomena, such as the wave mixing and detuning hysteresis. The miniaturization and theoretical study help pave the way for the ILLs for emerging applications, such as packet switching, coherent communications etc.

Fig. 2 SEM of MEMS tunable laser that makes use of a curved mirror as the external reflector.

Curved mirror

Laser chip

Actuator

Injection0.06 nm

0.20 nm

0.23 nm

Injection

(b) Evolution of the locking state of the slave Fig. 3 MEMS injection-locked laser, in which the slave laser can be fully locked by the injection from the master laser by reduction of wavelength detuning..

(a) SEM of the integrated device

Comb drive actuator

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FP slave laser

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2.4 Nano-photonic light-tuned-light switchesNovel nano-photonic crystal bandgap devices

were realized in record high resolution and aspect ratio. By use of nano pillar structures, photonic bandgap waveguiding was realized without vertical confinement in the optical wavelengths [11, 12]. We had successfully demonstrated optical interconnects as small as 5 × 5 µm2 in size, sub-wavelength waveguides and also light-induced signal modulation. Fig. 4 shows a scanning electron micrograph (SEM) of the optical intersection, which is patterned by deep UV to have photonic crystal pillars with reduced radius of sub-100 nm [13]. By applying the spatially-localized strong pump radiation onto the central photonic crystal resonator, effective modulations was observed. In experimental, it measures an signal change of 15 dB when subjected to an pump light of 70 MW/cm2. This work demonstrates the great potential for light speed control and photonic integrated circuit.

3. BIOPHOTONIC CHIPS

3.1 Cell detection chips using laser cavity Biophotonic chip for detecting living cells is

another interesting field. We developed a series of new method for measuring the effective refractive index of single living cell using an external cavity laser [14-16]. An early work is shown in Fig. 5. The measurement system is embedded in a monolithic chip by integrating an external cavity laser, a microlens set and several microfluidic channels [14]. In the experiment, 5 types of cancerous cells are delivered via the microfluidic channels into the laser cavity, which measures the cells’ refractive index and size by observing the wavelength shift and output power variation. The measurements are calibrated against at least two standard fluorescence beads with known refractive indices. The measured refractive indices of cancerous cells range from 1.392 to 1.401, which are consistently larger than the typical value of 1.35 - 1.37 for normal cells. Particular advantages of this method include its capability to detect the living cells in the liquid environment without need for labeling. Hence, this method could be developed into new toolkits for cancer diagnoses and single cell studies.

3.2 Cell refractometrer using Fabry-Pérot cavity A more accurate method for measuring single

living cells’ refractive index (RI) was recently developed using an on-chip fiber-based Fabry-Pérot cavity by a differential method [15, 16] as shown in Fig. 6. In experiment a single cell is captured into the cavity, then the spectral shift in response to the buffer change and the cell

Table 1 List of the measured sizes and refractive indices of various beams and cancerous cells.

Cells l (µm) I (nm) RI

PBS Buffer - - - 1.330 Culture medium - - - 1.350

HeLa 17.66 1 0.08 1.392 PC12 11.11 0 0.03 1.395 MDA-MB-231 18.72 1 0.03 1.399

MCF-7 17.48 1 0.02 1.401 Jurkat 12.82 0 0.07 1.390

Glass substrate

Laser chip

Actuator Mirror

PDMS

Microlens set

Microchannel

Cells

Fig. 5 Schematic diagram of the biophotonic chip for single living cell detection.

Fig. 4 Light-speed optical control using a nano-photonic bandgap intersection, which has sub-100 nm resolution waveguides.

Sub-100nm single line photonic crystal waveguides

Super dense bandgap lattice

Tapered waveguides

500 nm

Resonant cavity

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presence/absence can be used to determine the cell’s RI and size. Experiment on kidney cancer cells measures an effective RI of 1.399 at 0.1% accuracy. Compared with other approaches, this differential method eliminates uncertain factors and thus ensures high accuracy.

5. CONCLUSIONS

This paper demonstrates various novel MEMS devices such as thermo-optic switches, tunable lasers, nano-photonic crystal devices and cell detection chips. The fusion of MEMS tunability and photonic principles is opening up new chances for developing next-generation devices for not only laser applications but also cell studies.

REFERENCES

[1] H. Fujita, “Microactuators and Micromachines,” Proc. IEEE, vol. 86, pp. 1721-1732, 1998.

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[3] A. Q. Liu, X. M. Zhang, V. M. Murukeshan, and Y. L. Lam, “A novel device level micromachined tunable

laser using polysilicon 3D mirror,” IEEE Photon. Technol. Lett., vol. 13, no. 5, pp. 427-429, 2001.

[4] X. M. Zhang, H. Cai, C. Lu, C. K. Chen and A. Q. Liu, “Design and experiment of 3-dimensional micro-optical system for MEMS tunable lasers,” Proc. MEMS 2006, 22-26 Jan. 2006, Istanbul, Turkey, pp. 830-833, paper MP45.

[5] J. Li, A. Q. Liu, X. M. Zhang, and T. Zhong, “Light switching via thermo-optic effect of micromachined silicon prism,” Appl. Phys. Lett., vol. 88, no. 24, 243501, 2006.

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[10] A. Q. Liu, X. M. Zhang, H. Cai, D. Y. Tang and C. Lu, “Miniaturized injection-locked laser using microelectromechanical systems technology,” Appl. Phys. Lett., vol. 87, no. 10, 101101, 2005.

[11] E. H. Khoo, A. Q. Liu, J. H. Wu, J. Li and D. Pinjala, “Modified step-theory for investigating mode coupling mechanism in photonic crystal waveguide taper,” Opt. Exp., vol. 14, no. 13, 2006.

[12] Selin H. G. Teo, A. Q. Liu, J. B. Zhang, and M. H. Hong, “Induced free carriers modulation of photonic crystal optical intersection via localized optical absorption effect,” Appl. Phys. Lett., vol. 89, 091910, 2006.

[13] Selin H. G. Teo, A. Q. Liu, M. B. Yu, and J. Singh, “Fabrication & demonstration of square lattice two-dimensional rod-type photonic bandgap crystal optical intersections,” Photon. Nanostruct. Fundament. Applicat., vol. 4, no. 2, pp. 103-115, 2006.

[14] X. J. Liang, A. Q. Liu, C. S. Lim, T. C. Ayi, P. H. Yap, “Determining refractive index of single living cell using integrated microchip,” Sens. Actuators A, vol. 133, pp. 349-354, 2007.

[15] W. Z. Song, X. M. Zhang, A. Q. Liu, C. S. Lim, P. H. Yap and Habib Mir M. Hosseini, “Refractive index measurement of single living cells using on-chip Fabry-Pérot cavity,” Appl. Phys. Lett., vol. 89, no. 20, 203901, 2006.

[16] W. Z. Song, X. M. Zhang, L. K. Chin, C. S. Lim, A. Q. Liu, P. H. Yap And H. M. Hosseini, “Optical detection of living cells' refractive index via buffer modulation of microfluidic chip,” Proc. µTAS 2006, 5-9 Nov., 2006, Tokyo, Japan, pp. 528-530.

(b) Measured spectral shift with a cell in the cavityFig. 6 Refractive index measurement of single living cells using an on-chip Fabry-Pérot cavity.

(a) Working principle of refractive index detection

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lcCell holder Dnc

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