Brandon Born

Electrodispensing of Microspheroids for Lateral Refractive and Reflective Photonic Elements

An electrodispensing fabrication process is introduced in this work for the fabrication of microspheroid photonic elements. The fabrication process is suitable for in situ patterning of microdroplet spheroids whose optical responses can be tailored for use in on-chip focusing or retroreflecting. Theoretical ray-based and wave-based analyses are used to characterize the optical responses of these microspheroid systems. Design-based details are extracted from these theoretical conclusions, and the electrodispensing fabrication process needed to pattern the structures is presented. Polymer microspheroids with diameters down to 150 μm and contact angles up to 160° are fabricated, and the focusing/retroreflecting characteristics of these structures are experimentally characterized.

Integration of photonic nanojets and semiconductor nanoparticles for enhanced all-optical switching

All-optical switching is the foundation of emerging all-optical (terabit-per-second) networks and processors. All-optical switching has attracted considerable attention, but it must ultimately support operation with femtojoule switching energies and femtosecond switching times to be effective. Here we introduce an all-optical switch architecture in the form of a dielectric sphere that focuses a high-intensity photonic nanojet into a peripheral coating of semiconductor nanoparticles. Milli-scale spheres coated with Si and SiC nanoparticles yield switching energies of 200 and 100 fJ with switching times of 10 ps and 350 fs, respectively. Micro-scale spheres coated with Si and SiC nanoparticles yield switching energies of 1 pJ and 20 fJ with switching times of 2 ps and 270 fs, respectively. We show that femtojoule switching energies are enabled by localized photoinjection from the photonic nanojets and that femtosecond switching times are enabled by localized recombination within the semiconductor nanoparticles.

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.

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 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.

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.

Ultrafast spectroscopy of hot electron and hole dynamics in GaP

This work analyzes ultrafast carrier dynamics in GaP under intense photoexcitation. The dynamics are initially dominated by hot electron scattering from the central Γ valley to the X7 sidevalley over 700 fs and X6 sidevalley over 4 ps. Subsequent pump-fluence-dependent relaxation is observed over 30 to 52 ps for as pump fluence increases. This prolonged energy relaxation is ascribed to impeded phonon decay. Experimental and theoretical results are shown to provide evidence for a hot phonon bottleneck at the high fluences. The implications of these ultrafast carrier dynamics are discussed for emerging GaP applications.

Integrated photonic retroreflectors for lateral cross-connects and interconnects

A new photonic architecture for integrated retroreflection is presented. High contact angle polymer microdroplets are dispensed in ambient filler solutions to create spheroid microdroplets, for which incident beams can be made to focus, back-reflect and retroreflect in planar topologies. Results are shown for microdroplet retroreflectors with radii ranging from 50 μm to 2 mm. It is found that a balancing of surface tensions between the filler fluid, liquid, and solid substrate can be used to modify the structures eccentricity and produce the required geometrical profile for lateral beam retroreflection. Such structures will be well-suited to integrated photonic topologies employing cross-connects and interconnects in planar switching/routing applications.