Thesis (M.S.)--Oregon State University, 1995. / Typescript (photocopy). Includes bibliographical references (leaf 45). Also available via the World Wide Web.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, June 2005. / Thesis Advisor(s): Gamani Karunasiri, Douglas Fouts. Includes bibliographical references (p. 73-74). Also available online.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2006. / F. Levent Degertekin, Committee Chair ; David S. Citrin, Committee Member ; Paul E. Hasler, Committee Member ; Peter J. Hesketh, Committee Member ; Zhiping Zhou, Committee Member.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Report (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Includes bibliographical references (leaves 32-34). Also available via the Internet.

Near infrared (NIR) light detection has drawn substantial attentions in a variety of applications. NIR detectors prepared by the solution processable organic semiconductors and organic-inorganic hybrid material systems have the advantages for achieving flexible, light weight and large area NIR detection devices. The aim of this research work is to study the modulation of the photo-generated charge carriers in perovskite and polymer systems for attaining high performing solution-processable NIR phototransistors (PTs) and photodiodes (PDs). In the first part, an organo-lead halide perovskite/polymer bi-layer channel PT is discussed. Compared to the performance of the polymer only PTs, the PTs with a perovskite/polymer hybrid channel exhibit a profound broadband enhancement in photoresponsivity over the light wavelength range from UV to NIR. The improved performance in the hybrid perovskite/polymer PTs is closely associated with the efficient charge separation and transfer between the perovskite and polymer functional layers in the heterojunction. With the success in developing the bi-layer perovskite/polymer PTs, the enhancement in NIR detection is further realized in PTs by incorporating a bulk heterojunction (BHJ) channel. The BHJ channel is formed by blending NIR sensitive polymer donor with three different acceptor materials for enhancing exciton dissociation and charge separation efficiency. It is shown that the use of donor/acceptor BHJ greatly facilitates the dissociation of photo generated excitons. The efficient exciton dissociation helps to boost the utilization of the photo generated excitons, and thereby improves the photoresponsivity. Although the PTs with a BHJ channel possess high photoresponsivity, the photosensitivity of these NIR PTs is still less than satisfactory. The results reveal that a higher activation energy of charge carriers is favorable for a lower carrier density in the channel. While under NIR light illumination, the mobile charge carriers, created due to the dissociation of the photo generated excitons, result in a dramatic increase in carrier density in the active channel. The increase in carrier density under NIR illumination, gives rise to a higher channel current, thereby a higher photosensitivity. A pixel-less NIR imaging device based on light up-conversion from near-infrared to green is demonstrated. A polymer donor/non-fullerene acceptor BHJ serving as the NIR sensitive unit is integrated monolithically with a perovskite-based light emitting diode (LED) unit for achieving light up-conversion from NIR to visible light. The BHJ serves as an NIR sensitive hole injection layer in the perovskite LEDs. Therefore, the efficient electroluminescence in area where NIR-induced efficient charge injection occurs in the LEDs can be displayed clearly, producing the visible light image. The photoresponsivity of perovskite based UV-Visible-NIR photodiodes (PDs) is discussed in the last part of the thesis. A periodic nano-grating hole transporting layer (HTL), formed by the nano-imprinting, is adopted for enhancing the photoresponsivity of the PDs. Theoretical simulation reveals that the periodic nano-grating HTL helps to improve light absorption in the active layer, caused by the enhanced light in-coupling effect. In the meantime, the nano-grating HTL based PDs exhibit weak angular dependency as compared to that of the planar control ones which is appealing for wide angle light detections.

M.Eng. (Electrical and Electronic Engineering) / An in-fibre Bragg grating is a period perturbation of the refractive index of the core of an optical fibre, where the length of the grating is much longer than the period of the grating. These gratings reflect light in a narrow band around a certain optical frequency. This narrow band is shifted in wavelength when the element is strained and is an absolute measure of the strain state of the grating. One way to interrogate this type of sensor is by using it as a mirror of an optical fibre laser, therefby forcing the laser emission band to coincide with the resonant frequency of the grating. In this thesis a theoretical model was developed to describe the operation of optically pumped Er+-doped optical fibre lasers. An explicit expression for the ouput power for a Fabry Perot fibre laser, was derived for the first time. The intracavity power propagation was also solved numerically. An optical fibre laser consisting out of an Er+-doped optical fibre optically pumped at 980nm, was built. A multilayer dielectric stack mirror was deposited on the one fibre endpoint of the laser. The other reflector is an in-fibre Bragg grating with a peak reflectance of 94% at 1548.5nm and a bandwith of <O.3nm. The buildup of the output power of the laser during the deposition, showed good correlation with the predicted values. The length of the active fibre was also varied, and again the theory could correctly predict the output power as well as the laser pump threshold. A scanning type of Sagnac interferometer was investigated as a possible wavelength demodulation system for the fibre laser sensor. With some alterations this system shows merit as an inexpensive and effective way to monitor the wavelength shifts in in-fibre Bragg gratings.

No description available.

A MOS optical sensor that utilizes avalanche multiplication in silicon is proposed and investigated both theoretically and experimentally. The above-breakdown operating regime is discussed and it is shown how a MOS photosensor may be operated in a photon counting mode by pulsing it into very deep depletion, beyond the point where avalanche "breakdown normally occurs. Avalanche discharges in such a MOS sensor are self-quenching due to the formation of an inversion layer. This self-quenching property suggests that a monolithic self-scanned array of MOS photon counting sensors should be possible. It is described how specially designed charge-coupled arrays (PC-CCD's) could be operated in this new regime. The high response of silicon in the visible and near infrared, compared with the responsive quantum efficiency of the commonly-used photocathode materials, gives the proposed imager a distinct advantage over presently-existing photon counting sensors in these spectral regions. It is shown that a PC-CCD must be fabricated on a p-type silicon substrate and illuminated from the back side in order to obtain a high avalanche initiation probability for the photogenerated carriers. It is also shown that all thermally activated, steady-state dark generation of carriers can be reduced to a negligible level by cooling the sensor to 100 K or less, while the generation due to interband tunneling may be reduced to an acceptable level by ensuring that the peak fields within the depletion region remain below approximately 4.3 x 10⁵ Vcm⁻¹. The dark generation due to band-to-band tunneling via trap states may make it necessary to restrict the peak fields to even lower values. Re-triggering following a breakdown pulse, due to charge trapping or impact ionization of these traps during the avalanche, is also analysed. Optical coupling due to light emission during the avalanche discharges is discussed and two methods for the prevention of this coupling between the image elements in linear arrays are described. MOS gates that break down either at the Si-SiO₂, interface, or in the bulk at a n-p junction created by a buried n-channel, have been fabricated and operated above breakdown. The surface breakdown devices were operated in a charge-injection mode while the bulk breakdown devices were operated in a charge transfer mode similar to that which would occur in a full PC-CCD imager. The surface breakdown devices exhibited excessive dark count rates that were attributed to the high electric fields at the Si-SiO₂ interface. The bulk breakdown detectors were found to be far superior. They had very sharply peaked pulse height distributions and considerably lower dark pulse rates. Operation up to 12 volts above breakdown with a corresponding avalanche initiation probability greater than 0.9 was possible with these devices. Only a very weak temperature dependence of the dark pulse rate was observed, suggesting that a tunneling mechanism of dark carrier generation was limiting the performance of the bulk-breakdown devices. The magnitude of the dark count rate agreed with that expected for band-to-band tunneling through mid-gap states. These states, through a change in their occupancy during breakdown, were also believed to cause the re-triggering of avalanches that was observed when operating at high, above-breakdown voltages. These limitations on performance can be expected to be removed by employing improved processing techniques which would reduce the mid-gap trap levels by one or two orders of magnitude. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Filter-free spectral tunable photodetectors (PDs) are critical for a plethora of applications in imaging, indoor light fidelity (Li-Fi), and light communications. The present band-selective light detection is realized by incorporating different optical filters with broadband inorganic semiconductor-based PDs. However, the use of the optical filters reduces the overall performance of these PDs and is not appliable in the emerging flexible and wearable applications. The rapid advancement of the organic semiconductors offers an exciting opportunity for the development of high-performance filter-free spectral tunable organic photodetectors (OPDs). The development of OPDs has attracted tremendous interests because of the tailored optoelectronic properties of the π-conjugated organic semiconductors and the solution fabrication process of the OPDs. Apart from the rapid progresses made in improving the responsivity and detectivity of OPDs, the spectral properties of OPDs also receive intense attention. This Ph.D. research work has been focused on developing a universal strategy to achieve high-performance filter-free band-selective and spectral tunable OPDs. The correlation between the optical profile and responsivity spectrum of the novel OPDs with a bilayer photoactive layer has been investigated. It suggests that the responsivity spectrum of the OPDs can be effectively modulated by managing the optical profile in the bilayer and multilayer photoactive layer. A filter-free band-selective OPD model, comprising a bilayer shorter-wavelength light depletion layer/longer-wavelength light-absorbing layer architecture photoactive layer, has been developed. The depletion layer in the filter-free OPDs has a dual-function serving as a shorter-wavelength light-absorbing layer and a hole-transporting layer. The photodetection spectrum window of the filter-free band-selective OPDs, defined by the difference in wavelengths between the transmission cutoff of the shorter-wavelength light depletion layer and the absorption edge of the longer-wavelength light-absorbing layer, can then be tuned over the different wavelength ranges by using an appropriate combination of the shorter-wavelength light depletion layer and the longer-wavelength light-absorbing layer. A dual-mode OPD, having a trilayer visible light absorber/optical spacer/near-infrared (NIR) light absorber configuration photoactive layer, has been proposed. The dual-mode OPD exhibits electrically switchable NIR response operated under a reverse bias and visible light response operated under a forward bias. In the presence of NIR light, the trap-assisted charge-injection behavior at the organic/cathode interface in the OPDs operated under a reverse bias. The photocurrent is produced in the visible light-absorbing layer, enabled by the trap-assisted charge injection at the anode/organic interface under a forward bias. The developed filter-free band-selective OPDs and electrically switchable dual-mode OPDs provided an attractive alternative optical detection technology to the conventional panchromatic and single-mode OPDs. The spectral tunable photodetection thus demonstrated offers a promising option for new OPD applications.