Microfluidics

Printing integrated photonic circuits; lab-on-a-chip technology; microfluidic devices for tuning photonic circuits.

Introduction: Microfluidics and Silicon Photonics

The semiconductor industry today is incredibly developed and can be leveraged into producing photonics chips using the techniques already available in the production of electronics. This is dubbed silicon photonics and is an area of intense research within the field of optics. Silicon photonics research and development promise powerful integrated optics platforms where high-bandwidth devices can transition from prototype to production very quickly.

As these photonic circuits grow and complexity however, manufacturing variability can drastically alter the performance of the circuit. The common means of solving this problem is through tuning the circuit post fabrication using heating pads built into the circuit. By heating select areas of the chip the effective index of refraction of the waveguide changes. This can be power costly and increases the complexity of a circuit.

As a research group we are currently working on another method involving the use of microfluidic chips that can be 3D printed directly onto a silicon photonics chip. Microfluidic chips precisely control the flow of various fluids at sub-millimeter precision. These chips can be used to pass fluid with varying indices of refraction over the waveguides of our photonics circuit, changing the effective index of refraction of the waveguide and thereby tuning the circuit.

3D Printed Microfluidics

Microfluidics has a broad range of applications in point-of-care diagnostics, drug discovery, biomarkers, tissue engineering, and many others. Optofluidics uses fluidic technology and optics in an integrated system, and has found many applications including lab-on-a-chip devices, fluid-based and controlled lenses, optical sensors for fluids and for suspended particles, biosensors, imaging tools, etc. Merging optofluidics technologies with silicon photonics may offer unprecedented sophistication and control in optofluidic systems, but is currently limited by the scaling and fabrication limitations of microfluidic technologies.

Traditional microfluidic device fabrication technology uses polydimethylsiloxane (PDMS), glass, silicon, embossed or injection molded plastics. These technologies require access to cleanroom equipment and involve steps such as photolithographic microfabrication, molding and release, and careful alignment and bonding of each layer to fabricate a complete device. The achievable device dimensions are far larger than typical silicon photonic elements, and the fabrication steps are often cumbersome and time consuming, requiring days to even weeks to create a successful microfluidic device. We have teamed up with Dr. Nordin's lab at BYU to instead use custom 3D printing technology to interface directly with silicon photonic devices. This eliminates the need to fabricate and bond discrete layers individually, and also enables the utilization of device volume in all 3 dimensions for highly integrated component placement and fluidic routing. Moreover, this technology allows rapid fabrication (less than 15 minutes for most devices), modification, and testing of devices. Unlike commercially available 3D printing technologies, this printer demonstrates the ability to fabricate true microfluidic (< 100 μm) structures. Previous work has demonstrated the capability to print microfluidic devices with channels as small as 18 μm x 20 μm , valves with only 150 μm diameter, and highly integrated pumps and mixers. This approaches the scale of many functional silicon photonic devices.

Microfluidic Tuning of Ring Resonators

The working principle behind how we tune silicon photonic circuits is that fact that the light propagating in a waveguide is affected by the index of refraction of the material just outside of the waveguide. This allows us to construct microfluidic devices that can control a fluid that is exposed to certain elements in a photonic circuit. For a ring resonator, we can tune the resonant wavelength by adjusting the effective index of the device. This can be obtain by varying the concentration of sodium chloride (NaCl) in water. Since a concentrated solution of NaCl has a higher index of refraction (~1.5) than pure water (~1.3).

The design and working principle of one of our microfluidic devices is illustrated in figure above, which shows two distinct chips: a silicon photonic chip (left) containing an array of microring resonators varying in size from 10 μm to 80 μm, and a 3D-printed microfluidic chip (middle and right) that clamps directly on the top of the silicon chip. The microfluidic chip mixes water and saline with reconfigurable concentrations, and then passes the resulting solution over the microring resonators, thereby tuning the resonance. To allow for microfluidic access while also protecting the photonic structures from clamping damage, a 2 μm thick SiO2 cladding layer with 160 X 140 μm etched windows over the ring resonators is deposited on top of the silicon guiding layer.

The microfluidic mixing and delivery mechanism can be described by following the fluids from entry to exit of the microfluidic chip. First, the unmixed fluids are prepared in two 20 cc plastic syringes pumped using a dual drive pump controlled by a computer. The fluids enter the microfluidic device via two separate pinholes (a) using 0.5 mm inner diameter flexible PTFE tubing. After entering the device the fluids pass through a pair of pneumatically controlled valves (b) with the supply of pneumatic pressure coming from the leftmost pinhole. They then combine at a dilution serpentine channel (c) for mixing. After mixing, the fluid passes through a channel with dimensions calibrated for optical examination (d))to validate the mixing efficacy, which is discussed in further detail below. The mixed fluid then passes through a 40 μm deep interface channel (e) where it comes into contact with the microring resonators. A tight seal with the silicon photonics chip is ensured by using a 20 μm tall micro-gasket around the channel. Before exiting the device through the rightmost pinhole, the fluid passes through another pneumatically controlled valve which, when used in combination with the other valves, can stop the flow of fluid and create an isolated chip free from contamination with external fluids. In order to ensure precise alignment during clamping, an alignment tab (f) is designed so that it sits flush with the photonics chip. Below is pictured a fully assembled chip clamped together with a 3d printed stage and acrylic top.

Automated Tuning of Ring Resonators

An automated control system operates the mixer and the calibration probe laser, and measures the output power of the add or drop ports. To control the mixing we operate a dual channel syringe pump. The two different channels control the flow rate of two syringes filled with different fluids, de-ionized water (fluid A) and 26.3% concentration aqueous solution of NaCl by mass (fluid B) which is the saturation limit of NaCl in water. We initialize the mixer with flow rates of fluid A and B at 50 μL/min. We then sweep the laser a through a free spectral range (~2.3 nm) and fit the measured dropped power to a rate-equation model of the optical resonance. The automated control loop then calculates the concentration needed and adjusts the flow rates of the inputs to tune the resonance. When the resonance is tuned to the target frequency, the program turns off the pumps and actuates the pneumatic valves to prevent any further mixing, thus stabilizing the resonance at the target frequency.

The figure above (top) shows the initial and final spectral response resulting from the automatic tuning of one of the microring resonators in the array. It shows the resonance shifting from ~1600.8 nm to ~1600.0 nm. The resonances are captured at two different times during the experiment as indicated in the above figure (bottom) by vertical red lines. The first vertical red line corresponds to when the program sequence that controls the resonance was initiated. The second vertical red line corresponds to when the program no longer makes further adjustments to the resonance. The target resonant frequency was achieved and maintained with an accuracy of +/- 50 pm. The system has an effective tuning range of 4 nm, which is 1.7 times the FSR. The tuning range was calculated measuring the change in optical resonant wavelength of the microring resonator after adjusting the concentration of the fluid passing over the resonator from 0% to 26.3% concentration by mass of an aqueous solution of NaCl. This represents the maximum tuning range as 26.3% concentration is approximately the saturation limit of NaCl in water.

Conclusion

In summary, automatic control and tuning of microphotonic elements using 3D-printed optofludic devices can be a great way to tune silicon photonic circuits. While we have worked mostly with microring resonators, other photon photonic circuit elements such as Bragg gratings, photonic crystals, waveguides, etc. may also benefit from 3D-printed microfluidics. The 3D-printed microfluidics control method we have developed may enable large-scale control, tuning, and reconfigurablility of photonic circuits. For example, owing to the small feature size and precision of the 3D-printing technique, the overlay geometry of the microfluidics interface can target multiple elements individually. Furthermore, multiple interfacing channels and mixing modules can address multiple photonic elements for reconfigurable circuits.