Rapid and cheap prototyping of a microfluidic cell sorter.

Development of a microfluidic device is generally based on fabrication-design-fabrication loop, as, unlike the microelectronics design, there is no rigorous simulation-based verification of the chip before fabrication. This usually results in extremely long, and hence expensive, product development cycle if micro/nano fabrication facilities are used from the beginning of the cycle. Here, we illustrate a novel approach of device prototyping that is fast, cheap, reliable, and most importantly, this technique can be adopted even if no state-of-the-art microfabrication facility is available. A water-jet machine is used to cut the desired microfluidic channels into a thin steel plate which is then used as a template to cut the channels into a thin sheet of a transparent and cheap polymer material named Surlyn® by using a Hot Knife™. The feature-inscribed Surlyn sheet is bonded in between two microscope glass slides by utilizing the techniques which has been being used in curing polymer film between dual layer automotive glasses for years. Optical fibers are inserted from the sides of chip and are bonded by UV epoxy. To study the applicability of this prototyping approach, we made a basic microfluidic sorter and tested its functionalities. Sample containing microparticles is injected into the chip. Light from a 532-nm diode laser is coupled into the optical fiber that delivers light to the interrogation region in the channel. The emitted light from the particle is collected by a photodiode (PD) placed over the detection window. The device sorts the particles into the sorted or waste outlets depending on the level of the PD signal. We used fluorescent latex beads to test the detection and sorting functionalities of the device. We found that the system could detect all the beads that passed through its geometric observation region and could sort almost all the beads it detected.

Hollow Bragg waveguides fabricated by controlled buckling of Si/SiO2 multilayers.

We describe integrated air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO2 multilayers. Thin film deposition parameters were tailored to produce a desired amount of compressive stress, and a patterned, embedded fluorocarbon layer was used to define regions of reduced adhesion. Self-assembled air channels formed either spontaneously or upon heating-induced decomposition of the patterned film. Preliminary optical experiments confirmed that light is confined to the air channels by a photonic band-gap guidance mechanism, with loss ~5 dB/cm in the 1550 nm wavelength region. The waveguides employ standard silicon processes and have potential applications in MEMS and lab-on-chip systems.

Thermal tuning of hollow waveguides fabricated by controlled thin-film buckling.

We describe the thermal tuning of air-core Bragg waveguides, fabricated by controlled formation of delamination buckles within a multilayer stack of chalcogenide glass and polymer. The upper cladding mirror is a flexible membrane comprising high thermal expansion materials, enabling large tuning of the air-core dimensions for small changes in temperature. Measurements on the temperature dependence of feature heights showed good agreement with theoretical predictions. We applied this mechanism to the thermal tuning of modal cutoff conditions in waveguides with a tapered core profile. Due to the omnidirectional nature of the cladding mirrors, these tapers can be viewed as waveguide-coupled, tunable Fabry-Perot filters.

Chip-scale spectrometry based on tapered hollow Bragg waveguides.

We describe a micro-spectrometer that exploits out-of-plane radiation at mode cutoff in a tapered leaky waveguide clad by omnidirectional Bragg reflectors. The device can be viewed as a side-coupled, tapered Fabry-Perot cavity. An effective-index transfer-matrix model reveals that optimal resolution is dependent on the reduction or mitigation of back-reflection and standing waves leading up to the cutoff point. We address this by insertion of low numerical aperture optics between the taper and the detector, and demonstrate an experimental resolution as small as approximately 1 nm and operating bandwidth >100 nm in the 1550 nm range, from a tapered waveguide with footprint approximately 50 microm x 500 microm. The device combines the small size of a Fabry-Perot instrument with the detector array compatibility and fixed optics of a grating-based instrument.

Inexpensive, universal serial bus-powered and fully portable lab-on-a-chip-based capillary electrophoresis instrument.

Capillary electrophoresis is a cornerstone of lab-on-a-chip (LOC) implementations for medical diagnostics. However, the infrastructure needed to operate electrophoretic LOC implementations tends to be large and expensive, hindering the development of portable or low-cost systems. A custom-designed and highly integrated microelectronic chip for high-voltage generation switching and interfacing is recently developed. Here, the authors integrate the microelectronic chip with a microfluidic chip, a solid-state laser, filter, lens and several dollars worth of electronic components to form an inexpensive and portable platform, which is the size of a mobile telephone. This compact system has such reduced power requirements that the complete platform can be operated using a universal serial bus link to a computer. It is believed that this system represents a significant advancement in practical LOC implementations for point-of-care medical diagnostics.

Single-step fabrication of refractive microlens arrays.

Arrays of submillimeter microlenses are made from droplets of UV-curable optical adhesive dispensed from a pressurized syringe under computer control. Measurements of the focal length uniformity, the minimum focused spot size, and the spherical aberration are presented. An excellent lens diameter and focal length uniformity are achieved over 100 element arrays.

V-groove gratings for infrared beam splitting: reply.

It is acknowledged that scalar diffraction theory may not be used to design accurately gratings with submicrometer features. This does not negate the concept of the v-groove beam-splitting grating.

V-groove gratings on silicon for infrared beam splitting.

Infrared beam-splitting transmission gratings that utilize anisotropically etched v-grooves on silicon wafers are proposed. With scalar diffraction theory to find the amplitudes of the different diffraction orders, a numerical search is used to find optimum designs for 1:3, 1:5, and 1:7 splitters with efficiencies greater than 70% with a standard deviation in intensity of no more than 7%.

The ABCD matrix in graded index tapers used for beam expansion and compression.

A new form is proposed for the ABCD matrix in a graded index taper with a large variation in cross section such as might be used for single-mode beam expansion. Expressions are given for the loss of power from the fundamental mode and the coupling efficiency between fibers when two tapers are used in an expanded beam connector. Exact solutions are found for linear tapers and for a class of tapers with zero slope ends. The distinction between adiabatic and nonadiabatic tapers is made clear from the functional form of the matrix in the linear case. Comparisons are made with previously published results and the effect of taper shape on the coupling efficiency is discussed.