Peter Domachuk

Over 4000 nm Bandwidth of Mid-IR Supercontinuum Generation in sub-centimeter Segments of Highly Nonlinear Tellurite PCFs

We report broad bandwidth, mid-IR supercontinuum generation using a sub-cm (8 mm) length of highly nonlinear tellurite microstructured photonic crystal fiber (PCF). We pump the fiber at telecommunication wavelengths by using 1550 nm, 100 fs pulses of energy E=1.9 nJ. When coupled in the PCF, these pulses result in a supercontinuum (SC) bandwidth of 4080 nm extending from 789 to 4870 nm measured at 20 dBm below the peak spectral power. This bandwidth is comparable or in excess of previously reported spectra for other nonlinear glass fiber formulations despite the significantly shorter fiber length. In addition, besides offering a convenient pump wavelength, short fiber lengths enable smoother SC spectra, lower dispersion, and reduced material absorption at longer wavelengths making the use of this PCF particularly interesting.

Bio-microfluidics: biomaterials and biomimetic designs.

Bio-microfluidics applies biomaterials and biologically inspired structural designs (biomimetics) to microfluidic devices. Microfluidics, the techniques for constraining fluids on the micrometer and sub-micrometer scale, offer applications ranging from lab-on-a-chip to optofluidics. Despite this wealth of applications, the design of typical microfluidic devices imparts relatively simple, laminar behavior on fluids and is realized using materials and techniques from silicon planar fabrication. On the other hand, highly complex microfluidic behavior is commonplace in nature, where fluids with nonlinear rheology flow through chaotic vasculature composed from a range of biopolymers. In this Review, the current state of bio-microfluidic materials, designs and applications are examined. Biopolymers enable bio-microfluidic devices with versatile functionalization chemistries, flexibility in fabrication, and biocompatibility in vitro and in vivo. Polymeric materials such as alginate, collagen, chitosan, and silk are being explored as bulk and film materials for bio-microfluidics. Hydrogels offer options for mechanically functional devices for microfluidic systems such as self-regulating valves, microlens arrays and drug release systems, vital for integrated bio-microfluidic devices. These devices including growth factor gradients to study cell responses, blood analysis, biomimetic capillary designs, and blood vessel tissue culture systems, as some recent examples of inroads in the field that should lead the way in a new generation of microfluidic devices for bio-related needs and applications. Perhaps one of the most intriguing directions for the future will be fully implantable microfluidic devices that will also integrate with existing vasculature and slowly degrade to fully recapitulate native tissue structure and function, yet serve critical interim functions, such as tissue maintenance, drug release, mechanical support, and cell delivery.

Rapid Nanoimprinting of Silk Fibroin Films for Biophotonic Applications

With soft microand nanopatterned materials becoming increasingly useful for various optical, mechanical, electronic, microfluidic, and optofluidic devices, the extension of this paradigm to a pure protein-based material substrate would provide entirely new options for such devices. Silk fibroin is an appealing biopolymer for forming such devices because of its optical properties, mechanical properties, all aqueous processing, relatively easy chemical and biological functionalization, and biocompatibility. Biologically functionalized silk fibroin films can be patterned on the micro and nanoscale using a soft lithography casting technique while maintaining the biological activity of the embedded proteins. The combination of these properties could enable a new class of active optofluidic devices that merge high-quality photonic structures whose very material constituent responds, through the embedded proteins, to analytes infused through integrated microfluidics. However, the silk fibroin casting process takes 12–36 h, hindering the ability to rapidly produce multiple devices and the resulting silk structures contain artifacts due to drying and liftoff. In this communication, we will show that silk has the properties of an ideal nanoimprint resist enabling rapid device fabrication, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines nanophotonics, biopolymeric and biocompatible materials. Optofluidics, though a relatively new field, is already undergoing evolution, finding applications to an ever-increasing range of problems, including varieties of biological sensing and detection. Initially optofluidics was developed as a fusion of microfluidics and photonics to enable compact, novel optical modulation technologies. The union of optical and fluidic confining structures, however, led optofluidic devices to be applied to sensing problems especially looking toward highly parallel, sensitive and low analyte volume applications. A further development of the optofluidic paradigm, introduced here through the use of silk, is to ‘‘activate’’ the constituent material of the device to make it chemically sensitive to species flowed past it. Typically, optofluidic devices are fabricated from materials usually found in photonics or microfluidics such as silica, silicon, polydimethylsiloxane or polymethacrylmethacrylate and other polymers. These materials, while possessing suitable and well-characterized optical and material properties are not inherently chemically sensitive or specific. It is possible to functionalize the surfaces of these materials with chemical reagents, however, a much broader range of sensitivities and specificities can be achieved if proteins or enzymes are used as the sensitizing agents. The use of proteins presents an issue in itself. Binding proteins (or chemicals receptive to them) to inorganic or synthetic polymer surfaces is complex. Ideally, a material such as silk fibroin that posseses excellent optical and mechanical qualities can be formed into a variety of optofluidic geometries and maintains the activity of embedded proteins is needed for realizing active optofluidic devices. A proof of concept presented here is to build a self-sensing nanoscale imprinted optofluidic device based on imprinted silk doped with lysed red blood cells. The device can be thought of as ‘‘self-analyzing’’ in that the single optofluidic component provides both chemical and spectral analysis due to the activation of the constituent imprinted silk. Nanoimprinting is a high-throughput lithography technique in which a mold is pressed onto a thermoplastic material heated above its glass-transition temperature. The softened material conforms to the mold due to applied pressure. Sub-100 nm structures by nanoimprint lithography were first demonstrated in polymethylmethacrylate (PMMA) and now structures as small as 10 nm are routinely achieved in PMMA. An ideal nanoimprint resist combines rapid imprinting times with low temperature and pressure as well as low surface energy to aid in mold removal. As such, the mold is often coated with a low surface energy surfactant. Nanoimprinting of biopolymers presents additional challenges because of a restricted parameter space that limits the ranges of temperature and pressures usable. However, in this communication, we demonstrate that silk fibroin films exhibit many characteristics of an ideal nanoimprint resist, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines

Compact resonant integrated microfluidic refractometer

We introduce a class of highly compact refractometers integrated onto a planar microfluidic geometry that demonstrates high resolution refractive index measurements in 50μm fluid channels utilizing a Fabry–Perot cavity formed between resonant Bragg grating reflectors. This cavity forms a resonant peak in the transmission spectrum which is dependent upon the refractive index of the fluid in the microfluidic channel. We demonstrate this class of refractometer using optical fiber Bragg gratings; to provide high resolution, intracavity losses are minimized using integrated collimating optics. The refractometer can resolve refractive index changes of 0.2% and is simulated using coupled mode theory.

Microfluidic tunable photonic band-gap device

We introduce a method for tuning a photonic band-gap material by means of displacing microfluidic plugs. The fluid is introduced into air voids that constitute the structure of the photonic crystal and is displaced using a capillary heater. The photonic crystal geometry is obtained using a microstructured optical fiber, comprising a periodically spaced array of air holes that is interrogated in the transverse direction, creating a “tall microchip.” Optical spectra are compared to band structure calculations of an idealized band-gap material.

Compact tunable microfluidic interferometer

We demonstrate a compact tunable filter based on a novel microfluidic single beam Mach-Zehnder interferometer. The optical path difference occurs during propagation across a fluid-air interface (meniscus), the inherent mobility of which provides tunability. Optical losses are minimized by optimizing the meniscus shape through surface treatment. Optical spectra are compared to a 3D beam propagation method simulations and good agreement is found. Tunability, low insertion loss and strength of the resonance are well reproduced. The device performance displays a resonance depth of -28 dB and insertion loss maintained at -4 dB.

Application of optical trapping to beam manipulation in optofluidics.

We introduce a novel method of attaining all-optical beam control in an optofluidic device by displacing an optically trapped microsphere through a light beam. The micro-sphere causes the beam to be refracted by various degrees as a function of the sphere position, providing tunable attenuation and beam-steering in the device. The device itself consists of the manipulated light beam extending between two buried waveguides which are on either side of a microfluidic channel. This channel contains the micro-spheres which are suspended in water. We simulate this geometry using the Finite Difference Time Domain method and find good agreement between simulation and experiment.

Thinnest optical waveguide: experimental test.

A thin dielectric waveguide with a subwavelength diameter can exhibit very small transmission loss only if its diameter is greater than a threshold value, while for smaller diameters, waveguide loss grows dramatically. The threshold diameter of transition between these waveguiding and nonwaveguiding regimes is primarily determined by the wavelength of propagating light and, to a much lesser degree, by the characteristic length of the waveguides long-range nonuniformity. For this reason, the transmission spectrum of a thin waveguide allows immediate and quite accurate determination of its thickness. An experimental test of these facts is performed for a tapered microfiber. Good agreement with the recently developed theory of adiabatic microfiber tapers is demonstrated.

Transverse characterization of high air-fill fraction tapered photonic crystal fiber

We demonstrate tapering of a high air-fill fraction photonic crystal fiber by using the flame-brushing technique. Transverse probing along the taper allows us to ascertain how the microstructure is preserved during tapering. Experimental results are compared with numerical simulations performed with the finite-difference time-domain and plane-wave expansion methods. Through this investigation we find that the fiber geometry is well preserved throughout the tapering process and we resolve the apparent discrepancies between simulation and experiment that arise through the finite extent of the fiber microstructure.

Application of optical trapping to beam manipulation in optofluidics

We introduce a novel method of attaining all-optical beam control in an optofluidic device by displacing an optically trapped silica micro-sphere though a light beam. The micro-sphere causes the beam to be refracted by various degrees as a function of the sphere position, providing tunable attenuation and beam-steering in the device. The device itself consists of the manipulated light beam extending between two buried waveguides which are on either side of a microfluidic channel. This channel contains the micro-spheres which are suspended in water. We simulate this geometry using the Finite Difference Time Domain method and find good agreement between simulation and experiment.