Optofluidics - the synergistic integration of photonics and microfluidics - has recently emerged as a new analytical field that provides a number of unique characteristics for enhanced sensing performance and simplification of microsystems. In this review, we describe various optofluidic architectures developed in the past five years, emphasize the mechanisms by which optofluidics enhances bio/chemical analysis capabilities, including sensing and the precise control of biological micro/nanoparticles, and envision new research directions to which optofluidics leads.

/pdf/optofluidic-microsystems-for-chemical-and-biological-2x1od0syk0.pdf

Optofluidic microsystems for chemical and biological analysis

Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.

Single Nanoparticle Detection Using Optical Microcavities.

Microelectromechanical system (MEMS) technology, and surface micromachining in particular, have led to the development of miniaturized optical devices with a substantial impact in a large number of application areas. The reason is the unique MEMS characteristics that are advantageous in fabrication, systems integration, and operation of micro-optical systems. The precision mechanics of MEMS, microfabrication techniques, and optical functionality all make possible a wide variety of movable and tunable mirrors, lenses, filters, and other optical structures. In these systems, electrostatic, magnetic, thermal, and pneumatic actuators provide mechanical precision and control. The large number of electromagnetic modes that can be accommodated by beam-steering micromirrors and diffractive optical MEMS, combined with the precision of these types of elements, is utilized in fiber-optical switches and filters, including dispersion compensators. The potential to integrate optics with electronics and mechanics is a great advantage in biomedical instrumentation, where the integration of miniaturized optical detection systems with microfluidics enables smaller, faster, more-functional, and cheaper systems. The precise dimensions and alignment of MEMS devices, combined with the mechanical stability that comes with miniaturization, make optical MEMS sensors well suited to a variety of challenging measurements. Micro-optical systems also benefit from the addition of nanostructures to the MEMS toolbox. Photonic crystals and microcavities, which represent the ultimate in miniaturized optical components, enable further scaling of optical MEMS. [2013-0297]

Optical MEMS: From Micromirrors to Complex Systems

This critical review summarizes the developments in the integration of micro-optical elements with microfluidic platforms for facilitating detection and automation of bio-analytical applications. Micro-optical elements, made by a variety of microfabrication techniques, advantageously contribute to the performance of an analytical system, especially when the latter has microfluidic features. Indeed the easy integration of optical control and detection modules with microfluidic technology helps to bridge the gap between the macroscopic world and chip-based analysis, paving the way for automated and high-throughput applications. In our review, we start the discussion with an introduction of microfluidic systems and micro-optical components, as well as aspects of their integration. We continue with a detailed description of different microfluidic and micro-optics technologies and their applications, with an emphasis on the realization of optical waveguides and microlenses. The review continues with specific sections highlighting the advantages of integrated micro-optical components in microfluidic systems for tackling a variety of analytical problems, like cytometry, nucleic acid and protein detection, cell biology, and chemical analysis applications.

Micro-optics for microfluidic analytical applications

The integration of microactuators within a silicon photonic chip gave rise to the field of optical micro-electro-mechanical systems (MEMS) that was originally driven by the telecommunication market. Following the latter's bubble collapse in the beginning of the third millennium, new directions of research with considerable momentum appeared focusing on the realization and applications of miniaturized instrumentation in biology, chemistry, physics and materials science. At the heart of these applications light interferometry is a key optical phenomenon, in which miniaturized scanning interferometers are the manipulating optical devices. Monolithic free-space optical interferometers realized on a silicon chip take advantage of the recent progress in the microfabrication technology that is enabling accurate control of the etching depth, the aspect ratio, the verticality and the curvature of the etched surfaces. The fabrication technology, the library of micro-optical and mechanical components, the realized architectures and their characterization are described in detail in this review, followed by a discussion of the foreseen challenges.

Monolithic silicon‐micromachined free‐space optical interferometers onchip

We report a refractive index (RI) sensor based on the use of vertically etched silicon Bragg reflectors. The device is robust and performs measurements through tens of micrometers of liquid. A sensitivity of 907 nm/RIU (RI units) and a resolution of 1.7×10−5 RIU are obtained and are in good agreement with optical simulations. This resolution is the highest reported for a volume RI sensor integrated with a microfluidic system. Expected applications for the sensor in the fields of single cell characterization and chip based liquid chromatography are discussed.

All-silicon integrated Fabry―Pérot cavity for volume refractive index measurement in microfluidic systems

We propose a tunable erbium doped fiber laser based on a Fabry-Perot (F-P) cavity tuned by an electrostatic actuator. The device is made of single crystalline silicon. The F-P cavity consists of two Bragg mirrors, one being displaced by a comb-drives actuator. The F-P cavity, grooves for optical fibers and electro-mechanical structure are fabricated by deep reactive ion etching on a 70 μm silicon on insulator wafer and are integrated in a ring fiber laser. The resulting tunable fiber laser has a tuning range of 35 nm in the C-band and a spectral width of less than 0.06 nm. The maximum applied voltage for full tuning of the laser is 37 V. The mechanical resonance frequency of the actuated mirror is 14.4 kHz allowing fast tuning of the laser. The maximum output power is 1.8 mW.

Tunable Fiber Laser Using a MEMS-Based in Plane Fabry-Pérot Filter

Tunable Fiber Laser Using a MEMS-Based In Plane Fabry-P´

A method for fabricating a tunable Fabry-Perot cavity comprises etching a substrate to form two reflectors separated by an air gap and an electrostatic mechanism. One of the two reflectors is mobile and connected to the electrostatic mechanism. Therefore, operation of the electrostatic mechanism moves the mobile reflector to change the thickness of the air gap and thereby tune the Fabry-Perot cavity. A tunable Fabry-Perot cavity fabricated with the above method comprises: a substrate; two reflectors formed in the substrate and separated by an air gap having a thickness, wherein one of the two reflectors is mobile; and an electrostatic mechanism formed in the substrate and connected to the mobile reflector. The mobile reflector connected to the electrostatic mechanism is moved upon operation of the electrostatic mechanism to change the thickness of the air gap and thereby tune the Fabry-Perot cavity. Applications of the Fabry-Perot cavity may comprise a tunable doped fiber laser, a tunable dispersion compensator and an integrated microfluidic refractometer.

MEMS tunable silicon fabry-perot cavity and applications thereof

In this paper we present an innovative tunable Fabry-Perot cavity micromachined in silicon. A short summary of the
theoretical background of these filters is presented, followed by technical requirements for the design of the dielectric
mirror composing the Fabry-Perot cavity and the cavity itself. Simulations and experimental data are demonstrated to be
in good agreement. An in plane design is used to allow easy fiber alignment. The Fabry-Perot is tuned by an electrostatic
comb drive actuator supported by a set of four springs to achieve a uniform modulation of the air gap of the filter. Only
15.4 V are required to tune the Fabry-Perot over 73nm bandwidth (covering more than the whole C-band) with a FWHM
varying from 6 to 10nm. Transmission losses are -11dB.

Jonathan Masson

Papers

All-silicon integrated Fabry―Pérot cavity for volume refractive index measurement in microfluidic systems

Tunable Fiber Laser Using a MEMS-Based in Plane Fabry-Pérot Filter

Tunable Fiber Laser Using a MEMS-Based In Plane Fabry-P´

MEMS tunable silicon fabry-perot cavity and applications thereof

MEMS tunable silicon Fabry-Perot cavity