Suppression of relaxation-oscillations in intra-cavity optical parametric oscillators by two-photon absorption.

The applications of continuous-wave (cw), intra-cavity optical parametric oscillators (ICOPO) in molecular sensing and spectroscopy have been hampered by their relaxation-oscillation and power-stability problems. To solve these problems, we propose a two-photon-absorption (TPA) mechanism into ICOPOs. In a proof-of-principle experiment, we inserted a CdTe plate into an ICOPO as a TPA medium and demonstrated efficient suppression of relaxation-oscillations, obtaining an intensity-noise reduction of over 70 dB at the relaxation-oscillation frequency. To the best of our knowledge, this is the first demonstration of relaxation-oscillation suppression in ICOPOs based on TPA.

Widely tunable, continuous-wave, intra-cavity optical parametric oscillator based on an Yb-doped fiber laser.

We report an optical parametric oscillator (OPO) intra-cavity pumped by an Yb-doped fiber laser. In comparison to an intra-cavity OPO based on a solid-state laser gain medium, the benefits of using a fiber-based scheme include a superior beam quality of the generated mid-infrared idler light at high power levels, a more efficient process of nonlinear frequency conversion, and the prospect of scaling to higher power. In a preliminary experiment, we obtained a slope efficiency of the down-converted power of as high as 66% with respect to the absorbed laser diode power. To the best of our knowledge, this is the first demonstration of an intra-cavity OPO based on a fiber laser.

Continuous-wave, singly-resonant, intracavity optical parametric oscillator based on a single-mode-laser-diode-pumped Yb:KYW laser.

We report a continuous-wave (cw), intracavity, singly resonant optical parametric oscillator (ICSRO) based on an Yb:KYW laser pumped by a single-mode laser diode (LD). Pumping the ICSRO by a low-noise single-mode LD, combined with the reduced heat generation due to the lower quantum defect of the Yb:KYW laser system, effectively eliminated the onset of relaxation oscillations, which have been a long-standing problem in previous multimode-LD-pumped ICSROs, and resulted in a cw ICSRO being operated free of relaxation oscillations. At an LD power of 515 mW, the generated idler power was 21 mW at ∼3500 nm. To the best of our knowledge, this is the first demonstration of a single-mode-LD-pumped ICSRO.

Laser-treated hydrophobic paper: an inexpensive microfluidic platform.

We report a method for fabricating inexpensive microfluidic platforms on paper using laser treatment. Any paper with a hydrophobic surface coating (e.g., parchment paper, wax paper, palette paper) can be used for this purpose. We were able to selectively modify the surface structure and property (hydrophobic to hydrophilic) of several such papers using a CO(2) laser. We created patterns down to a minimum feature size of 62±1 µm. The modified surface exhibited a highly porous structure which helped to trap/localize chemical and biological aqueous reagents for analysis. The treated surfaces were stable over time and were used to self-assemble arrays of aqueous droplets. Furthermore, we selectively deposited silica microparticles on patterned areas to allow lateral diffusion from one end of a channel to the other. Finally, we demonstrated the applicability of this platform to perform chemical reactions using luminol-based hemoglobin detection.

Squeeze-film hydrogel deposition and dry micropatterning.

In this technical note, we demonstrate a squeeze-film based spacer-free method for creating controllable submicrometer hydrogel films on planar substrates that can be used to photolithographically fabricate hydrogel microstructures. This new technique improves the photolithographic resolution and yield by providing a uniform and low-defect hydrogel film. The optimum polymerization initiation time for achieving such a layer was determined to be around 1 min. For patterning, the dried hydrogel film was coated with a parylene-C masking layer. Subsequent etching in oxygen plasma was used to transfer selected patterns of hydrogel to the substrate in a batch scale.

Selective nanofiber deposition through field-enhanced electrospinning.

In this paper, we demonstrate a nanofiber patterning technique using field-enhanced electrospinning. Polyethylene oxide (PEO) nanofibers were electrospun on an elastomeric substrate with gold-coated pyramidal protrusions with the majority of fibers being deposited at the tips. The deposited nanofiber spots ranged from 8 x 8 microm(2) to 60 x 60 microm(2) in size, uniformly covering an area of 5 x 10 mm(2). Our experiments also indicate that nanofiber pattern selectivity is highly dependent on the separation/size ratio of the pyramidal protrusions with a ratio of <1, resulting in a superior selectivity.

Micromachined hydrogel stamper for soft printing of biomolecules with adjustable feature dimensions.

We report on the development of a hydrogel stamper with built-in reservoirs for soft-printing of biomolecules with adjustable feature dimensions. The stamper consists of potassium hydroxide (KOH)-etched silicon cavities onto which biomolecule-soaked low cross-linked density hydrogel with large pores and low mechanical strength is loaded. Application of weight to the top surface allows for a controllable protrusion of hydrogel from the opposite nozzles. Such protrusion combined with a suitable spacer between the stamper and a substrate provides a means for printing features with dimensions depending on the applied weight. Utilizing the above method, we successfully stamped bovine serum albumin conjugated with fluorescein isothiocyanate (BSA-FITC) model proteins on hydrophilic silicon substrates with a feature dimension ratio of 20:1 using a single stamper.