A thorough investigation of copper oxide, specifically cupric oxide (CuO), is performed in the following work with a focus on CuO’s ultrafast free-carrier dynamics and bandstructure. An above-bandgap control beam and below-bandgap signal beam are utilized in transient absorption spectroscopy to gain insight on CuO nanocrystals’ recombination and relaxation dynamics at varying control beam fluences. The authors witnessed three distinct time constants, the first of which changed with control beam fluence between 330 and 630 fs, while the second and third remained constant at 2 ps and 50 ps, respectively. The first time constant is attributed to momentum relaxation from valence band carrier-carrier scattering and exciton-exciton annihilation. The second time constant is attributed to energy relaxation from valence band carrier-phonon scattering. The third time constant is attributed to trapping and recombination as a result of the CuO nanocrystals’ increased trap state density. The findings of this work provide a basis for future research on this emerging CuO nanocrystal system.
Optical technology is crucial to the heads-up displays used in wearable virtual reality and augmented reality goggles. However, the implemented technology can be challenging to implement. It must form a magnified image, focused at infinity, for imaging with the relaxed human eye, and it must be compact. The proposed work does this using coupled microlens arrays. The coupled plano-concave and plano-convex microlens arrays together act as a superlens that meets the requirements of heads-up displays. The required plano-concave and plano-convex microlens arrays are implemented using a technique of dispensing and in situ ultraviolet-curing of polymer microdroplets. It is shown that the planoconcave and plano-convex microlens arrays can be formed to enable imaging in a compact package with an excellent resolution, corresponding to a spatial modulation transfer function beyond 20 cycles/mm.
A challenge is emerging for utilities having a significant reliance on solar power generation. Such a reliance leads to substantial solar power generation in the midday hours and insufficient solar power generation during the late-day hours. This yields high demand and an upswing in electrical power prices at the end of the day when the solar generating capacity is low. The upswing is titled the "Duck Curve" and it is a growing concern—particularly for California. This work tackles this challenge by introducing an engineered solar cell array that traps light over a broadened range of incident angles, which leads to increased late-day solar power generation. The proposed U-Groove Array consists of a periodic array of U-shaped grooves that are parallel and macroscopic in size. This work shows that a U-Groove Array, when engineered with the appropriate orientation and aspect ratio, promotes multiple reflection and absorption processes for low-angle light. This broadens the duration over which the array collects solar power, extending the generation into the high-value evening hours. The accumulated value of the solar power generated by the U-Groove Array is simulated and compared to that of two contemporary structures: a standard array of flat solar cells and an analogous V-Groove Array (which has been the subject of numerous recent investigations). It is shown that future implementations of the UGroove Array can outperform these contemporary structures and yield improved solar power generation.
Flat silicon solar cells are the standard for solar technology implementations, due to the simplicity of its form and the low cost of its material. However, the bare material of such a technology is known to suffer from high reflectivity and low solar conversion efficiency. Engineered surfaces (e.g., textured) and bulk structures (e.g., quantum dots) are often used to diminish the reflection and enhance the efficiency, but such processes come at the expense of complexity and cost. The proposed work responds to these challenges by introducing a new architecture for traditional silicon solar technology. The architecture takes the form of a Smart Solar Sensing (S-Cubed) Array. It consists of a macroscopic close-packed array of corner-cube- (CC-) shaped solar cells. Each CC-cell has three silicon solar cells lining its interior corners. The three silicon solar cells establish multiple internal reflections for enhanced overall absorption. At the same time, the impedances of the three silicon solar cells in each CC-cell of the S-Cubed Array can be sensed and independently matched to a common load. This allows for maximized electric power transfer to the load over a broad range of illumination conditions. It is shown in this work that the collected energy density of the CC-cell array, over the course of a day, can be increased by 33.02% when compared to an array of conventional (flat) silicon solar cells. Such findings can lay the groundwork for future implementations of high-efficiency solar technology.
This work puts forward new technologies for free-space optical communications, with emphasis on deployments between ground and aerial transceivers. The proposed system targets the challenges of these aerial-ground links by applying direct laser transmission for the ground-to-aerial active uplink and applying all-optical retro-modulation (AORM) for the aerial-to-ground passive downlink. It is shown that such a system can function with multiple ground transceivers, over wide service coverage, and one aerial transceiver, with low demands for its mass and power. The AORM architecture applied in the passive downlink implements glass S-LAH79 hemispheres for effective retroreflection and CuO nanocrystal semiconductor thin film layer for all-optical modulation on ultrafast timescale. The fabricated AORM architecture is demonstrated to have an system response time of 770 fs, which limits the aggregate data rate. Such a fast system response establishes the possibility of terabit-per-second data rates. Ultimately, these findings can lay the foundation for future laser-based terabit-per-second links between satellites, unmanned aerial vehicles, and high-altitude platforms.
Optical wireless (OW) technologies are an emerging field utilizing optical sources to replace existing radio wavelength technologies. The vast majority of work in OW focuses on communication; however, one smaller emerging field is indoor OW positioning. This emerging field essentially aims to replace GPS indoors. One of the primary competing methods in indoor OW positioning is angle-of-arrival (AOA). AOA positioning uses the received vectors from several optical beacons to triangulate its position. The reliability of this triangulation is fundamentally based on two aspects: the geometry of the optical receiver’s location compared to the optical beacon locations, and the ability for the optical receiver to resolve the incident vectors correctly. The optical receiver is quantified based on the standard deviation of the azimuthal and polar angles that define the measured vector. The quality of the optical beacon geometry is quantified using dilution of precision (DOP). This proceeding discusses the AOA standard deviation of an ultra-wide field-of-view (FOV) lens along with the DOP characteristics for several optical beacon geometries. The optical beacon geometries used were simple triangle, square, and hexagon optical beacon geometries. To assist the implementation of large optical beacon geometries it is proposed to use both frequency and wavelength division multiplexing. It is found that with an ultra-wide FOV lens, coupled with the appropriately sized optical beacon geometry, allow for high accuracy positioning over a large area. The results of this work will enable reliable OW positioning deployments.
The potential of terabit-per-second fibre optics can be unlocked with emerging all-optical networks and processors employing all-optical switching. To be effective, all-optical switching must support operations with femtojoule switching energies and femtosecond switching times. With this in mind, this work studies geometrical and material characteristics for all-optical switching and develops a new all-optical switching architecture. A nanojet focal geometry is applied, in the form of dielectric spheres, to direct high-intensity photonic nanojets into peripheral semiconductors. Theoretical and experimental analyses demonstrate photonic nanojets, enabling femtojoule switching energies through localized photoinjection, and semiconductor nanoparticles, enabling femtosecond switching times through localized recombination.
A practical all-optical switch is necessary to alleviate electronic bottlenecks in fibre optic networks. Thus, a new alloptical switch is introduced here—exhibiting femtojoule switching energies and femtosecond switching times. The alloptical switches use 40 μm dielectric spheres to direct high-intensity photonic nanojets into peripheral coatings of semiconductor nanoparticles. Semiconductor nanoparticle coatings of Si, CdTe, InP, and CuO are studied and found to yield switching energies of approximately 1 pJ, 500 fJ, 400 fJ, and 300 fJ with switching times of 2 ps, 2.3 ps, 900 fs, and 350 fs, respectively.
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 <i>n</i> ≈ 2 and a Kerr nonlinear index of <i>n</i><sub>2</sub> ≈ (1.8 ± 0.1) × 10<sup>-15</sup> cm<sup>2</sup>/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.
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.
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.
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 X<sub>7</sub> sidevalley over 700 fs and X<sub>6</sub> 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.
Customized high-contact-angle microlenses are presented for optical wireless communication (OWC) and optical
wireless location (OWL) applications. These microlenses are fabricated by way of an electro-dispensing technique to
establish wide field-of-views (FOVs). Each microlens is formed from dispensed UV-curable polymer with pressurecontrol
defining the microlens volume and a voltage on the metal needle tip defining the microlens shape (by way of
electrowetting). UV curing is then applied. Microlenses with FOVs up to 90° are fabricated for high-density integration
above a CMOS imaging sensor for wide-FOV operation in emerging OWC and OWL applications. Both theoretical raytracing
analyses and experimental imaging results are presented with good agreement.
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.
An integrated photoconductive (PC) element is introduced as a new optoelectronic device in free-space optical (FSO)
wireless applications. The device is a fundamental extension of the standard PC switch, as it has the capabilities for both
local optoelectronic signal reception and active directional angle of arrival (AOA) sensing. This second capability is
brought about through the use of a three-phase differential technique through three triangular PC switches arranged in a
corner-cube architecture. Each PC switch is comprised of 50/150 nm Cr/Au electrodes, patterned on either side of a 200-
micron GaAs PC gap, and is biased with the superposition of common DC and AC three-phase (120° phase-shifted) bias
voltages. The DC bias forms a summed signal photocurrent on the central vertex output electrode and facilitates data
reception; the AC three-phase bias facilitates link reliability for diversity reception in optical wireless communication
systems. Complete theoretical and experimental angular characteristics of this device are presented in this work.
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.
There are critical implementation challenges to consider for new digital microfluidic technologies. In dynamic
applications, properties of both the system and the microdroplet are changing in time due to the adsorption of
microdroplet species at the solid-liquid and liquid-vapor interfaces. In this paper, these digital microfluidic dynamic
challenges are overcome through the introduction of real-time sensing submodules. Dynamic sampling has been added
to the actuation mechanisms of the digital microfluidic system, and it is shown that the complete fluid system can be
characterized simultaneously by measurements of the microfluidic system capacitance and the microdroplet contact
A bi-layer digital microfluidic structure is introduced. The integrated design employs a two-dimensional structure with
perpendicular linear electrode arrays controlling x- and y- directional actuation. The introduced structure is capable of
electrowetting-based fluid control, and it is presented here as an implementation capable of electrical sensing of the state
of fluids within the device. The state of conductive fluids within the bi-layer structure is sampled through differential
measurements of conductance values between the x- and y-channels. It is shown here that the features of microdroplets
within the device can be effectively mapped onto these differential conductance measurements. An electronic acquisition
system is ultimately employed to sense these conductance states and extract the position and size of microdroplets within
the device. The complete system is demonstrated here for both single microdroplet and multiple microdroplet
Digital microfluidic systems (DMFS) are poised to provide fully automated, high-throughput, dynamically
reconfigurable sensing devices superior to those available today. Efficient droplet routing algorithms for these systems
have not yet been established, though several solutions have been proposed. Such algorithms are ultimately required to
generate droplet movement schedules and must be robust enough to handle the inevitable increases in problem
complexity that will come as this technology matures. We have proposed a new solution based on a classic VLSI lineprobe
algorithm to meet these demands for the detailed routing of droplets within a multi-stage algorithm. The most
significant addition includes a sub-algorithm that calculates the routing complexity for any DMFS configuration based
on the size, shape, number, type, and distribution of rectilinear obstacles throughout a DMFS biochip surface. By
determining the complexity of the routing of each droplet, routing schedules may be prioritized, minimizing the number
of fluidic and time constraint violations that affect high priority droplet routes. The complexity characterizations
generated by our algorithm may also be used to create consistent, standardized benchmarks for the evaluation of existing
droplet routing solutions. The efficiency of the proposed algorithm has been verified using the simulation presented in
We present theoretical and experimental results for the generation of both transmission-line-coupled terahertz electrical transients and free-space terahertz radiation. Time-resolved reflectivity experiments are used to gain insight into the ultrafast carrier dynamics and fundamental limitations of conventional photoconductive (PC) switches. Self-switching and frozen wave generation are introduced as effective sources of terahertz electrical transients, and the PC response of these techniques is seen to be independent of the charge carrier lifetime in the semiconductor substrate. The concept of PC switching is further extended to the launching of free-space terahertz radiation, and electro-optic sampling techniques using polycrystalline sensors are shown to be viable sensors for free-space terahertz radiation.
We report on a novel photoconductive (PC) switching technique, capable of generating ultrashort electrical pulses. The PC geometry of the switch is such that the incident optical pulse can both switch 'on' and later switch 'off' the electrical transient. The method of electrical pulse generation is, therefore, independent of the charge- carrier lifetime in the semiconductor.
We report on the application of a photoconductive (PC) frozen wave generator (FWG) for the generation of two-cycle THz radiation. A bipolar PC array comprised of four electrodes is employed in the production of THz electrical transients. Variations in the uniformity of the optical excitation intensity across the FWG PC gaps are found to provide a means of controlling the temporal chirp and power spectral bandwidth of the resulting electrical signals. Upon generating the electrical transients, the signals are launched, and later received, with integrated dipole antennas.
We present a novel optical-optical switching technique for modulation of infrared radiation. The modulation response is based upon the optical perturbation of semiconductor layers within an air-filled metal-clad semiconductor waveguide. Generation of the electron-hole plasma within these layers is via femtosecond pulses of above bandgap radiation (800 nm). The propagation characteristics of this five-layer structure are analyzed through the coupling of quasi-static electromagnetic analysis to the time-varying optical properties of the semiconductor layers. It is found that the device is able to modulate radiation at various frequencies, though we specifically investigate modulation of 10.6 micrometers radiation. At this wavelength, an electron-hole photoinjection density of approximately 1 X 10<SUP>18</SUP> cm<SUP>-3</SUP> in the semiconductor layers provides an extinction ratio of 30 dB. The significance of this modulation depth and possible applications to all-optical Mach Zehnder metal-clad semiconductor modulators and self- limiting switches are discussed.
A plasma-dielectric waveguide is presented as a possible optical element for enhancing the phase matching condition in high harmonic generation processes. The phase velocity of the pump wave, in a partially ionized plasma, is controlled by its interaction with dielectric walls within the waveguide. The phase velocity is adjusted by changing the thickness and spacing of the dielectric slab walls.