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Photonic crystals show a great deal of promise for applications in numerous types of devices in 1, 2, and 3D structures. One of the simplest techniques of fabricating photonic crystals involves colloidal self-assembly, wherein, monodisperse colloidal spheres will spontaneously assemble into periodic arrays under certain circumstances. Zinc Oxide is a promising candidate for optically-active self-assembled photonic crystals.
ZnO monodisperse colloidal spheres were made by a modified polyol synthesis procedure by reacting zinc acetate with diethylene glycol under controlled heating conditions. Varying the amount of supernatant (obtained from the primary reaction mixture) added to the secondary reaction mixture enabled us to control the size of spheres. The scanning electron micrograph (SEM) shown in figure 1 consists of monodisperse colloidal spheres. Photonic crystals were produced from the ZnO colloidal spheres using a sedimentation self assembly process by the technique of drop casting onto a preheated glass substrate.
Third order optical nonlinearity studies of these self assembled spheres as well as the colloidal solution was carried by the single beam z-scan technique using 8 ns pulses from Nd:YAG laser at 532 nm as well at variable wavelengths. The magnitude of nonlinearity of the material was found to reverse for the same input energy and wavelength from saturable absorption behaviour in self assembled ZnO to reverse saturable absorption in colloidal ZnO. This behaviour can be attributed to the changing nanoparticle filling factor when the colloidal solution is transformed into self assembled monolayers. The nonlinear absorption coefficient of monolayer ZnO calculated using a theoretical fit to the experimental curve shown in figure 2 was found to be 1.5x10-5 m/W. The nature of nonlinear absorption of these samples was investigated over the visible region (450 nm- 700 nm) and the sample was found to exhibit saturable absorption behaviour over the entire range scanned. A theoretical study using the five-level model should enable the evaluation of its excited state absorption cross-section and two-photon absorption cross-section which is underway.
The increased use of transparent conducting oxides (TCO) in various optoelectronic applications such as solar cells, liquid crystal displays, heat mirrors, photothermal conversion systems has spurned the search for better and cheaper materials. Aluminum doped zinc oxide films shows high transmittance from UV to near IR and nearly metallic electrical conductivity and hence is used as transparent electrodes in solar cells. In addition p type ZnO can be used as hole injectors in thin film electroluminescent devices. Pulsed laser deposition enables the growth of high quality thin film relatively at low substrate temperature.
We have prepared c axis oriented ZnO:Al thin films on plastic substrates at room temperature by PLD using Nd:YAG. These films show good transmittance (>85%) in the visible region and are of fairly low resistivity (~10-4).
Photopyroelectric technique is one of the photothermal techniques involving detection of temperature change as a result of modulated excitation and subsequent de-excitation. The voltage or current signal obtained as a result of temperature change from the pyroelectric signal can be easily related to the optical and thermal properties of the sample. We are in the initial stages of analyzing pulsed pyroelectric signal for the evaluation of excited state absorption and life time of the excited state.
Laser pulses of 8 ns from an Nd: YAG laser operating at a wavelength of 532 nm is used to excite Rhodamine 6G dissolved in water. The pyroelectric sensor used for the detection of heat released is a PVDF film of thickness 28m coated with nickel. The voltage output from the sensor is connected to 500MHz Tektronix oscilloscope. Experiments are done for different intensities at different wavelengths.
The experimental results are analyzed on the basis of a five level model, with the rate equations being solved for the transient case. To the best of our knowledge, photopyroelectric technique is used for the first time for studying excited state absorption and lifetime, though studies have already been reported using the piezoelectric property of such materials as well as using photoacoustic technique. Rhodamine 6G is a well studied material and hence a good choice for standardizing new techniques.
The increased use of transparent conducting oxides (TCO) in various optoelectronic applications such as solar cells, liquid crystal displays, heat mirrors, photothermal conversion systems has spurned the search for better and cheaper materials. Aluminum doped zinc oxide films shows high transmittance from UV to near IR and nearly metallic electrical conductivity and hence is used as transparent electrodes in solar cells. In addition p type ZnO can be used as hole injectors in thin film electroluminescent devices. Pulsed laser deposition enables the growth of high quality thin film relatively at low substrate temperature.
We have prepared c axis oriented ZnO:Al thin films on plastic substrates at room temperature by PLD using Nd:YAG. These films show good transmittance (>85%) in the visible region and are of fairly low resistivity (~10-4).
Photopyroelectric technique is one of the photothermal techniques involving detection of temperature change as a result of modulated excitation and subsequent de-excitation. The voltage or current signal obtained as a result of temperature change from the pyroelectric signal can be easily related to the optical and thermal properties of the sample. We are in the initial stages of analyzing pulsed pyroelectric signal for the evaluation of excited state absorption and life time of the excited state.
Laser pulses of 8 ns from an Nd: YAG laser operating at a wavelength of 532 nm is used to excite Rhodamine 6G dissolved in water. The pyroelectric sensor used for the detection of heat released is a PVDF film of thickness 28m coated with nickel. The voltage output from the sensor is connected to 500MHz Tektronix oscilloscope. Experiments are done for different intensities at different wavelengths.
The experimental results are analyzed on the basis of a five level model, with the rate equations being solved for the transient case. To the best of our knowledge, photopyroelectric technique is used for the first time for studying excited state absorption and lifetime, though studies have already been reported using the piezoelectric property of such materials as well as using photoacoustic technique. Rhodamine 6G is a well studied material and hence a good choice for standardizing new techniques.
The observed experimental signals are analyzed on the basis of the standard five level model. The important transitions we took into our model are as shown in figure 3. The dotted lines represent the spontaneous relaxation. The experimental pyroelectric signal obtained for different intensities of pump is shown in Figure 4.
The above figure shows the dependence of pyroelectric signal on the intensity of the excitation source. The different slopes at high intensities suggest the presence of excited state absorption and multiphoton absorption.
In order to proceed with various studies involving wavelength scan including the band tail measurements, urbach energy, defect states etc using pyroelectric technique, we charted a wavelength dependence of the PVDF sensor. The spectrum charted is shown in figure 5.
The above figure shows the dependence of pyroelectric signal on the intensity of the excitation source. The different slopes at high intensities suggest the presence of excited state absorption and multiphoton absorption.
In order to proceed with various studies involving wavelength scan including the band tail measurements, urbach energy, defect states etc using pyroelectric technique, we charted a wavelength dependence of the PVDF sensor. The spectrum charted is shown in figure 5.
To test the correctness of the spectra, the spectrum of Rhodamine 6G is recorded using photopyroelectric technique and is shown in figure 6.
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