Litty, Mathew Irimpan; Radhakrishnan, P; V P N Nampoori(International School of Photonics, June , 2008)
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Abstract:
The spectral and nonlinear optical characteristics of nano ZnO and its composites are investigated. The fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength and there is a red shift in emission peak with excitation wavelength. Apart from the observation of the reported ultra violet and green emissions, our results reveal that additional blue emissions at 420 nm and 490 nm are developed with increasing particle size. Systematic studies on nano ZnO have indicated the presence of luminescence due to excitonic emissions when excited with 255 nm as well as significant contribution from surface defect states when excited with 325 nm. In the weak confinement regime, the third-order optical susceptibility χ(3) increases with increasing particle size (R) and annealing temperature (T) and a R2 and T2.5 dependence of χ(3) is obtained for nano ZnO. ZnO nanocolloids exhibit induced absorption whereas the self assembled films of ZnO exhibit saturable absorption due to saturation of linear absorption of ZnO defect states and electronic effects. ZnO nanocomposites exhibit negative nonlinear index of refraction which can be attributed to two photon absorption followed by weak free carrier absorption. The increase of the third-order nonlinearity in the composites can be attributed to the enhancement of exciton oscillator strength. The nonlinear response of ZnO nanocomposites is wavelength dependent and switching from induced absorption to saturable absorption has been observed at resonant wavelengths. Such a change-over is related to the interplay of plasmon/exciton band bleach and optical limiting mechanisms. This study is important in identifying the spectral range and the composition over which the nonlinear material acts as an optical limiter. ZnO based nanocomposites are potential materials for enhanced and tunable light emission and for the development of nonlinear optical devices with a relatively small optical limiting threshold.
Ajimsha, R S; Jayaraj, M K(Department of Physics, February , 2008)
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Abstract:
Transparent conducting oxides (TCO’s) have been known and used for technologically important applications for more than 50 years. The oxide materials such as In2O3, SnO2 and impurity doped SnO2: Sb, SnO2: F and In2O3: Sn (indium tin oxide) were primarily used as TCO’s. Indium based oxides had been widely used as TCO’s for the past few decades. But the current increase in the cost of indium and scarcity of this material created the difficulty in obtaining low cost TCO’s. Hence the search for alternative TCO material has been a topic of active research for the last few decades. This resulted in the development of various binary and ternary compounds. But the advantages of using binary oxides are the easiness to control the composition and deposition parameters. ZnO has been identified as the one of the promising candidate for transparent electronic applications owing to its exciting optoelectronic properties. Some optoelectronics applications of ZnO overlap with that of GaN, another wide band gap semiconductor which is widely used for the production of green, blue-violet and white light emitting devices. However ZnO has some advantages over GaN among which are the availability of fairly high quality ZnO bulk single crystals and large excitonic binding energy. ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO based devices.
Most of the TCO’s are n-type semiconductors and are utilized as transparent electrodes in variety of commercial applications such as photovoltaics, electrochromic windows, flat panel displays. TCO’s provide a great potential for realizing diverse range of active functions, novel functions can be integrated into the materials according to the requirement. However the application of TCO’s has been restricted to transparent electrodes,
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notwithstanding the fact that TCO’s are n-type semiconductors. The basic reason is the lack of p-type TCO, many of the active functions in semiconductor originate from the nature of pn-junction. In 1997, H. Kawazoe et al reported the CuAlO2 as the first p-type TCO along with the chemical design concept for the exploration of other p-type TCO’s. This has led to the fabrication of all transparent diode and transistors.
Fabrication of nanostructures of TCO has been a focus of an ever-increasing number of researchers world wide, mainly due to their unique optical and electronic properties which makes them ideal for a wide spectrum of applications ranging from flexible displays, quantum well lasers to in vivo biological imaging and therapeutic agents. ZnO is a highly multifunctional material system with highly promising application potential for UV light emitting diodes, diode lasers, sensors, etc. ZnO nanocrystals and nanorods doped with transition metal impurities have also attracted great interest, recently, for their spin-electronic applications
This thesis summarizes the results on the growth and characterization of ZnO based diodes and nanostructures by pulsed laser ablation. Various ZnO based heterojunction diodes have been fabricated using pulsed laser deposition (PLD) and their electrical characteristics were interpreted using existing models. Pulsed laser ablation has been employed to fabricate ZnO quantum dots, ZnO nanorods and ZnMgO/ZnO multiple quantum well structures with the aim of studying the luminescent properties.
Jijo, P U; Dr. Nandakumaran, V M(Cochin University of Science & Technology, August , 2008)
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Abstract:
This thesis presents analytical and numerical results from studies based on the multiple
quantum well laser rate equation model. We address the problem of controlling chaos produced
by direct modulation of laser diodes. We consider the delay feedback control methods
for this purpose and study their performance using numerical simulation. Besides the control
of chaos, control of other nonlinear effects such as quasiperiodicity and bistability using
delay feedback methods are also investigated.A number of secure communication schemes based on synchronization of chaos semiconductor
lasers have been successfully demonstrated theoretically and experimentally. The current
investigations in these field include the study of practical issues on the implementations of
such encryption schemes. We theoretically study the issues such as channel delay, phase mismatch
and frequency detuning on the synchronization of chaos in directly modulated laser
diodes. It would be helpful for designing and implementing chaotic encryption schemes using
synchronization of chaos in modulated semiconductor lasers.
Description:
International School of Photonics,
Cochin university of Science & Technology
Sobha, Cyrus; Dr.Jose, Babu T(Cochin University of Science & Technology, July , 2008)
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Abstract:
The present study aimed at critically looking at the current practice of the
installation of compacted clay liner using bentonite enhanced sand (BES).
The application of bentonite is currently the most accepted practice for
lining purposes. The ideal bentonite sand combination, which satisfies the liner
requirements is 20% bentonite and 80% sand, was selected as one of the liner
materials for the investigation of development of desiccation cracks. Locally
available sundried marine clay and its combination with bentonite were also
included in the study. The desiccation tests on liner materials were conducted for
wet/dry cycles to simulate the seasonal variations. Digital image processing
techniques were used to measure the crack intensity factor (CIF), a
useful and effective parameter for quantification of desiccation cracking.
The repeatability of the tests could be well established, as the variation in CIF values
of identical samples had a very narrow range of 0 to 2%. The studies on the
development of desiccation cracks showed that the CIF of bentonite enhanced
sand mixture (BES) was 18.09%, 39.75% and 21.22% for the
first, second and third cycles respectively, while it was only 9.83%, 7.52% and
4.58% respectively for sun dried marine clay (SMC). Thus the locally available,
alternate liner material suggested, viz SMC, is far superior to BES, when
subjected to alternate wet/dry cycles. Further, the improvement of these liner materials when amended with randomly distributed fibre reinforcements was also investigated. Three types of
fibres ,namely nylon fibre, polypropylene monofilament and polypropylene fibre
mesh were used for the study of fibre amended BES and SMC.The influence of
these amendments on the properties of the above liner materials is also studied.
The results showed that there is definite improvement in the properties of the liner
materials when it is reinforced with discrete random fibres. The study also proved
that the desiccation cracks could be controlled with the help of fibre
reinforcement.
Description:
School of Engineering, Cochin University of Science and Technology