Christopher M. Collier

Nonlinear Dual-Phase Multiplexing in Digital Microfluidic Architectures

A 16 × 16 digital microfluidic multiplexer is demonstrated. The device makes use of dual-phase AC activation in a bi-layered electrode structure for actuating microdrops independently. A switching arrangement is employed to localize two out-of-phase AC waveforms in one overlapped region of the two-dimensional multiplexer grid. The superimposed AC waveforms overcome the threshold voltage for motion of a local microdrop. The demonstrated dual-phase activation and nonlinear threshold-based motion overcomes the previously-reported microdrop interference effect, as it successfully actuates individual microdrops in systems with multiple neighbouring microdrops. The device is demonstrated with an integrated centre-tap transformer using a 10.0 Vrms input voltage and minimal power consumption.

Ultrafast photoconductivity of crystalline, polycrystalline, and nanocomposite ZnSe material systems for terahertz applications

Ultrafast photoconductivity is studied for crystalline (bulk), polycrystalline (microstructure), and nanocomposite (nanostructure) ZnSe material systems. Spectral transmission analyses show a pronounced red-shift of the absorption edge for only the nanocomposite ZnSe (being comprised of 500 nm nanoparticles in a polymer host). Ultrafast transient analyses show respective 6 ns, 1.5 ns, and 95 ps charge-carrier lifetimes for the respective material systems. The results are interpreted with a diffusion-recombination model, showing distinct regimes for bulk diffusion and surface recombination. Nanocomposite ZnSe is shown to be particularly advantageous for terahertz applications seeking ultrafast photoconductivity with high dielectric breakdown strengths and ultrashort charge-carrier lifetimes.

On-chip digital microfluidic architectures for enhanced actuation and sensing

An on-chip system is presented with integrated architectures for digital microfluidic actuation and sensing. Localized actuation is brought about by a digital microfluidic multiplexer layout that overcomes the challenges of multi-microdrop interference, and complete two-dimensional motion is shown for microdrops on a 14 × 14 grid with minimized complexity by way of 14+14 inputs. At the same time, microdrop sensing is demonstrated in a folded-cavity design for enhanced optical intensity probing of internal fluid refractive indices. The heightened intensities from this on-chip refractometer are shown to have a linear response to the underlying fluid refractive index. An electro-dispensing technique is used to fabricate the folded-cavity optical architecture in a format that is tuned for the desired refractive index range and sensitivity. The overall lab-on-a-chip system is successful in integrating localized microdrop actuation and sensing.

Ultrafast Photoexcitation and Transient Mobility of GaP for Photoconductive Terahertz Emission

The prospects for photoconductive (PC) terahertz (THz) generation are studied for wide-bandgap semiconductors exhibiting transient mobility. Such semiconductors offer practical benefits (by resisting dielectric breakdown and minimizing Joule heating) as well as improved frequency responses (by accentuating high-frequency PC THz emission). It is shown that GaP can offer these wide-bandgap and transient mobility characteristics. The ultrafast photoexcitation and subsequent transient mobility are investigated for a GaP PC THz emitter with photoexcitation fluences of 18, 36, and 72 μJ/cm2. The 100 fs rise and 700 fs fall in the transient photocurrent, due to the respective photoexcitation and transient mobility responses, yields far-field THz emission that improves upon that of the well-established GaAs PC THz emitter. It is ultimately found that semiconductors with both wide-bandgap and transient mobility characteristics can offer strategic improvements for emerging high-power PC THz technologies.

Ultrafast Refractometry for Characterization of Nanocomposite Material Systems

An ultrafast refractometry technique is introduced for direct characterization of refractive indices, absorption coefficients, and refractive index variances in nanocomposite (NC) assemblies. The system samples bulk NC optical characteristics by probing phase delay, absorption, and spatial coherence effects on an ultrashort laser probe pulse. The integrated system is demonstrated for representative samples of 20-nm SiC nanoparticles in a polymer host and is found to successfully sample the desired optical characteristics.

Ultrafast transient responses of optical wireless communication detectors

In this work, fundamental ultrafast transient responses are studied for optical wireless communication (OWC) detectors. It is shown that material impulse responses, associated with transient photoconductivity, and geometrical input responses, associated with transient optical power, must be considered in tandem when OWC photodetection is pursued with broad spectral and directional characteristics. An OWC detector, composed of GaAs photoconductive gaps in a corner-cube geometry, is fabricated and analyzed. The GaAs material response times are investigated experimentally and found to range from approximately 3 ps to 200 fs for 390 nm (violet) to 780 nm (red) photoexcitation. The geometrical response times are investigated theoretically and found to range from approximately 2 to 20 ps for device dimensions from 1 to 10 mm. The overall response times manifest themselves in two distinct dimensional regimes, with differing levels of wavelength and dimension dependence. The relevance of these findings is discussed for future ultrafast OWC detectors.

Optical sensing for on-chip digital microfluidics

A digital microfluidic architecture is introduced for micron-scale localized fluid actuation and in in-situ optical sensing. Contemporary device integration challenges related to localization and device scalability are overcome through the introduction of a bi-layered digital microfluidic multiplexer. Trinary inputs are applied through differential combinations of voltage signals between upper (column) electrodes and lower (row) electrodes. The ultimate layout provides increased scalability for massively parallel microfluidic actuation applications with a minimal number of inputs. The on-chip sensing technique employed here incorporates a microlens in a folded-cavity arrangement (fabricated by a new voltage-tuned polymer electro-dispensing technique). Such a geometry heightens the sensitivity between the optical probe and fluid refractive properties and allows the device to probe the refractive index of the internal fluid. This optical refractometry sensing technique is merged with the actuation capabilities of the digital microfluidic multiplexer on a single lab-on-a-chip device.

Optimization processes for pulsed terahertz systems

In this work, a pulsed terahertz (THz) system is designed and implemented. The work introduces a methodology for implementing such THz systems through three design processes: collineation, autocorrelation, and electro-optic. The collineation process establishes spatial alignment between the overlapped pump and probe beams, to ensure that there is similar spatial alignment between the subsequent THz and probe beams. The autocorrelation process characterizes the optical path difference between pulses in the THz and probe beams to define the precise temporal zero-time of the THz system. The electro-optic process optimizes the polarization-sensitive optics in the THz system to maximize the THz-induced modulation on the probe polarization. The processes are applied to design and implement a successful THz system.

Ultrafast charge-carrier and phonon dynamics in GaP

The ultrafast energy relaxation of GaP is analyzed through charge-carrier and phonon dynamics. Early timescales show hot electron intervalley scattering from the Γ valley into the X sidevalley, with 700 and 4000 fs time constants for scattering to and from the X7 valley. Later timescales show carrier-phonon interactions in the X6 valley with hot phonon and screening effects. Fluence-dependent relaxation is observed over 30 to 52 ps for 2.3 to 72 μJ/cm2 fluences. The prolonged relaxation of GaP is due to impeded (hot) phonon decay and screening at low and high fluences, respectively.

Photoconductive terahertz generation from textured semiconductor materials

Photoconductive (PC) terahertz (THz) emitters are often limited by ohmic loss and Joule heating—as these effects can lead to thermal runaway and premature device breakdown. To address this, the proposed work introduces PC THz emitters based on textured InP materials. The enhanced surface recombination and decreased charge-carrier lifetimes of the textured InP materials reduce residual photocurrents, following the picosecond THz waveform generation, and this diminishes Joule heating in the emitters. A non-textured InP material is used as a baseline for studies of fine- and coarse-textured InP materials. Ultrafast pump-probe and THz setups are used to measure the charge-carrier lifetimes and THz response/photocurrent consumption of the respective materials and emitters. It is found that similar temporal and spectral characteristics can be achieved with the THz emitters, but the level of photocurrent consumption (yielding Joule heating) is greatly reduced in the textured materials.