There are severe limitations that photoconductive (PC) terahertz (THz) antennas experience due to Joule heating and ohmic losses, which cause premature device breakdown through thermal runaway. In response, this work introduces PC THz antennas utilizing textured InP semiconductors. These textured InP semiconductors exhibit high surface recombination properties and have shortened carrier lifetimes which limit residual photocurrents in the picoseconds following THz pulse emission—ultimately reducing Joule heating and ohmic losses. Fine- and coarse-textured InP semiconductors are studied and compared to a smooth-textured InP semiconductor, which provides a baseline. The surface area ratio (measuring roughness) of the smooth-, fine-, and coarse-textured InP semiconductors is resolved through a computational analysis of SEM images and found as 1.0 ± 0.1, 2.9 ± 0.4, and 4.3 ± 0.6, respectively. The carrier lifetimes of the smooth-, fine-, and coarse-textured InP semiconductors are found as respective values of 200 ± 6, 100 ± 10, and 20 ± 3 ps when measured with a pump-probe experimental system. The emitted THz electric fields and corresponding consumption of photocurrent are measured with a THz experimental setup. The temporal and spectral responses of PC THz antennas made with each of the textured InP semiconductors are found to be similar; however, the consumption of photocurrent (relating to Joule heating and ohmic losses) is greatly diminished for the semiconductors that are textured. The findings of this work can assist in engineering of small-scale PC THz antennas for high-power operation, where they are extremely vulnerable to premature device breakdown through thermal runaway.
Optical wireless communications (OWC) offers the potential for high-speed and mobile operation in indoor networks. Such OWC systems often employ a fixed transmitter grid and mobile transceivers, with the mobile transceivers carrying out bi-directional communication via active downlinks (ideally with high-speed signal detection) and passive uplinks (ideally with broad angular retroreflection and high-speed modulation). It can be challenging to integrate all of these bidirectional communication capabilities within the mobile transceivers, however, as there is a simultaneous desire for compact packaging. With this in mind, the work presented here introduces a new form of transceiver for bi-directional OWC systems. The transceiver incorporates radial photoconductive switches (for high-speed signal detection) and a spherical retro-modulator (for broad angular retroreflection and high-speed all-optical modulation). All-optical retromodulation are investigated by way of theoretical models and experimental testing, for spherical retro-modulators comprised of three glasses, N-BK7, N-LASF9, and S-LAH79, having differing levels of refraction and nonlinearity. It is found that the spherical retro-modulator comprised of S-LAH79, with a refractive index of n ≈ 2 and a Kerr nonlinear index of n2 ≈ (1.8 ± 0.1) × 10-15 cm2/W, yields both broad angular retroreflection (over a solid angle of 2π steradians) and ultrafast modulation (over a duration of 120 fs). Such transceivers can become important elements for all-optical implementations in future bi-directional OWC systems.
The development and ultimate operation of a nanocomposite high-aspect-ratio photoinjection (HARP) device is presented in this work. The device makes use of a nanocomposite material as the optically active layer and the device achieves a large optical penetration depth with a high aspect ratio which provides a strong actuation force far away from the point of photoinjection. The nanocomposite material can be continuously illuminated and the position of the microdroplets can, therefore, be controlled to diffraction limited resolution. The nanocomposite HARP device shows great potential for future on-chip applications.
Microfluidics technologies have received great attention and appear in many bioanalyses applications. A recent microfluidics subset has appeared as droplet-based digital microfluidics (DMF). Here, microdroplets are manipulated in a two-dimensional on-chip plane using electric fields, contrasting the one-dimensional pressure-based channel flow of continuous flow microfluidics. These DMF systems fundamentally offer reconfigurability, whereby one device performs many bioanalysis tasks. A subset of DMF systems called optoelectrowetting is also of recent interest due to its ability for intricate microdroplet routing processes in the on-chip plane. For an optoelectrowetting chip, the DMF structure is modified with optically triggered electrodes with arrayed photoconductive switches. The arrayed photoconductive switches are optically-activated so microdroplets in the vicinity are routed to the illuminated switch. Unfortunately, such systems still require intricate electrode arrays, limiting microdroplet actuation resolution by the electrode size. This work proposes an on-chip optofluidic device with a continuous and planar semiconductor layer as the photoconductive mechanism. An illuminated section of the semiconductor layer acts as a localized electrode, with the photogenerated charge-carriers attracting nearby microdroplets. Given this planar topology, the illuminating beam is used to move the microdroplets continuously over the on-chip plane with precise optical control. The resolution for such a process is ultimately limited by charge-carrier diffusion, so an alternative material, a nanocomposite, is introduced to the on-chip device design. The nanocomposite consists of 20 nm semiconductor nanoparticles embedded in an insulative polymer host. This gives restricted diffusion length, being on the nanometer-scale of the nanoparticle diameter. Experimental device operation is demonstrated.
Digital (droplet-based) microfluidic systems apply electromagnetic characteristics as the fundamental fluid actuation mechanism. These systems are often implemented in two-dimensional architectures, overcoming one-dimensional continuous flow channel practical issues. The fundamental operation for digital microfluidics requires the creation of an electric field distribution to achieve desired fluid actuation. The electric field distribution is typically non-uniform, enabling creation of net dielectrophoresis (DEP) force. The DEP force magnitude is proportional to the difference between microdroplet and surrounding medium complex dielectric constants, and the gradient of the electric field magnitude squared. Force sign/direction can be manipulated to achieve a force towards higher (positive DEP) or lower (negative DEP) electrostatic energy by tailoring the relative difference between microdroplet and surrounding medium complex dielectric constants through careful selection of the devices fabrication materials. The DEP force magnitudes and directions are applied here for well-controlled and high-speed microdroplet actuation. Control and speed characteristics arise from significant differences in the microdroplet/medium conductivity and the use of a micropin architecture with strong electric field gradients. The implementation, referred to here as a DEP microjet, establishes especially strong axial propulsion forces. Single- and double-micropin topologies achieve strong axial propulsion force, but only the double-micropin topology creates transverse converging forces for stable and controlled microdroplet actuation. Electric field distributions for each topology are investigated and linked to axial and transverse forces. Experimental results are presented for both topologies. The double-micropin topology is tested with biological fluids. Microdroplet actuation speeds up to 25 cm/s are achieved—comparable to the fastest speeds to-date.
Biosensing is important for detection and characterization of microorganisms. When the detection and characterization of targeted microorganisms require micron-scale resolutions, optical biosensing techniques are especially beneficial. Optical biosensing can be applied through direct or indirect optical sensing techniques. The latter have demonstrated especially high sensitivities for the detection of targeted microorganisms with labeling. Unfortunately, such systems rely on high-resolution microscopy with microscopic sampling areas to image the labeled target microorganisms. This leads to long characterization times for applications such as pathogen detection in water quality monitoring where users must scan the micron-scale sampling areas across millimeter- or even centimeter-scale samples. This work introduces retroreflector labels for the detection and characterization of microorganisms for macroscopic sample sizes. The demonstrated retroreflective imaging system uses a laser source to illuminate the sample, in lieu of the fluorescent excitation source, and micron-scale retroreflector labels, in lieu of fluorescent stains/proteins. Antibodies are used to bind retroreflectors to targeted microorganisms. The presence of these microscopic retroreflector-microorganism pairs is monitored in a retroreflected image that is captured by a distant image sensor which shows a well-localized retroreflected beamspot for each pair. Characteristics of an appropriately-designed retroreflective imaging system which provide a quantifiable record of microorganism-coupled retroreflectors across macroscopic sample sizes are presented. Retroreflection directionality, collimation, and contrast are investigated for both corner-cube retroreflectors and spherical retroreflectors (of varying refractive indices). It is ultimately found that such a system is an effective tool for the detection and characterization of microorganism targets, down to a single-target detection limit.
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.
An integrated photoconductive (PC) sensor is introduced as an optoelectronic element for visible light optical wireless communications (OWC) links. The sensor applies the standard PC switch, being a biased metal-semiconductor-metal gap, in a three-fold-symmetric corner-cube architecture with a summed output photocurrent at the vertex. Such a form facilitates bidirectional retroreflective communications to meet fundamental OWC requirements for broad directional and broad spectral capabilities. The ultimate OWC capability, for ultrafast optoelectronic switching times, is studied here for material response and transit time response, and it is shown that ultrafast (picosecond) optoelectronic switching times can be achieved and the general device design consideration is discussed for emerging visible light OWC systems.
GaP is investigated for photoconductive terahertz (THz) generation. It is shown that the atypical bandstructure of GaP,
with a central high-mobility valley and low-mobility sidevalleys, can be exploited to form a transient high-mobility state.
The subsequent scattering and relaxation of hot electrons into and within the lower-mobility sidevalleys leaves the
material in a relaxed low-conduction state. The experimental and theoretical study shows that ultrafast transient mobility,
occurring over 800 fs, can create broadband THz pulses with reduced recovery times (and low leakage currents). The
impacts of these findings are discussed for efficient and portable next-generation THz systems.
The capabilities for practical all-optical switch (AOS) operation, being picosecond switching times and femtojoule
switching energies, are investigated in this work. Two distinct nanophotonic architectures are introduced. The first
nanophotonic architecture uses nanostructures, in the form of semiconductor nanoparticles, to enhance the rate of surface
recombination and provide picosecond switching times. Switching times down to 4.5 ps are demonstrated. The second
architecture uses photonic nanoinjection, with high refractive index spheres, to create high-intensity pump-probe beam
interaction at a GaAs surface. This architecture offers 10 ps switching times with switching energies as low as 50 fJ.
Nanophotonic architectures such as these can provide the capabilities needed for future AOS implementations.
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.
A new technique, ultrafast refractometry, is introduced for probing the refractive, absorptive, and diffractive conditions
in nanocomposite assemblies. It is shown that the physical characteristics of nanocomposites-nanoparticle sizes and
volumetric ratios in the polymer host-are decisive factors in determining the material's overall optical properties.
Ultrashort optical pulses (100 fs) act as an in-situ probe for temporal phase, optical attenuation, and spatial coherence in
these discrete materials. This technique is demonstrated for numerous samples of 20 nm SiC nanoparticle/polymer
nanocomposites. A close link between the physical properties and the ultimate refraction, absorption, and diffraction
characteristics of these nanocomposite optical materials is shown.
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.