Eu3+ doped (100-x) SiO2 - x SnO2 (x = 8, 16 and 25 mol %) glass-ceramic waveguides were fabricated by sol-gel
technique with dip-coating processing at 800 °C. Higher thermal treatments led to the formation and growth of SnO2
nanocrystals in the matrix, enhancing the 5D0 → 7F2 emission of Eu3+. Spectroscopic investigations reveal the evolution
of a centro-symmetric local environment around the Eu3+ ion. This property of glass-ceramic waveguides can be
exploited for obtaining a higher rare-earth concentration in a smaller volume. The glass-ceramic waveguides exhibited
low-losses (0.8 ± 0.2 dB/cm at 632.8 nm) and thus are promising for the development of high-gain integrated optical
amplifiers.
In this paper, we report the statistical characteristics of Stratospheric Sudden Warming (SSW) events observed over a
low latitude station, Gadanki; 13.5°N, 79.2°E. The study uses 7 years (1998 to 2004) of quasi-continuous nighttime
LiDAR temperature measurements, which corresponds to 312 observations. The statistical characteristics are presented
in terms of major or minor, magnitude of warming, height of occurrence and stratopause descent with reference to the
mean climatological profile. The warming events are classified into major or minor warming with respect to the observed
warm temperature magnitude and reversal in the zonal wind direction in the polar region using National Centre for
Environmental Prediction (NCEP) reanalysis data. In total, 14 SSW events observed and have been classified into 2
(14.3 %) major and 12 (85.7 %) minor warming events. The magnitudes of warm temperatures with respect to the mean
winter temperature is in the range from 8.2 K to 18.1 K. Occurrence of SSWs are observed to accompany with the
descent of stratopause layer from 0 km to 6.3 km with respect to the calculated mean winter stratopause height.
This paper describes a newly developed resonance lidar system at Gadanki (13.5°N, 79.2°E), India. The lidar system is set up to probe the natural layer of neutral sodium atoms existing between 80 and 110 km. The lidar employs a YAG pumped dye laser as a transmitter. The laser is tuned to the sodium D2 line at 589.0 nm. The lidar achieved first light on 10 January 2005 and made the first mesospheric sodium measurements from India. The lidar-measured vertical profiles of resonant backscatter are found to have sufficient signal strength for deriving the mesospheric sodium concentration profiles. Using the system, sodium concentration profiles are obtained with a vertical resolution of 300 m and a time integration of 120 s. These features allow the system to detect the time and space variability of Na concentration profiles. During the initial six nights of observation, the average nocturnal columnar abundances were in the range (2 to 8.3)×109 cm−2. The nightly mean centroid heights range between 92.7 and 95.1 km, and the rms widths vary between 4.3 and 4.9 km. The preliminary analysis of Na layer dynamics on 10 January 2005 shows the presence of wavelike structures with characteristics similar to those of propagating gravity waves. The preliminary analyses of data show a considerable variability in Na concentration distribution in the mesopause region during a sporadic Na event.
We present in this paper sporadic sodium layers (SSLs), which we observe with a Resonance Lidar at Gadanki,
India (13.5°N, 79.2°E). The SSLs were observed on a total of 63 occasions during 464 hours of Na lidar
observations from January 2005 to February 2006. The SSL occurrence rate of 1 event/7 h at Gadanki was
obtained. These results show that the rate of occurrence of SSLs fall between 20°N and 2°S. The most
prominent sporadic layer, which formed on the night of Feb'12, 2005 exhibited a peak density of 60722 c.m-3
near 92 km. At our Gadanki site, SSLs have the following properties (1) they develop between 88 and 98 km
with average height of 94 km (2) they develop maximum in the early morning between 0200 and 0500 LT (3)
The ratio of the maximum peak density to the average Na density is normally 3 to 5, but values as high as 11
have been observed in the most outstanding cases (4) The events last from a few minutes to several hours. Most
of the Sporadic Sodium layer events were downward phase.
Altitude profiles of middle atmospheric temperature data using Rayleigh Lidar at Gadanki have been utilized to study the
gravity wave characteristics. The wave activity during the period November 2002-April 2005 is investigated. The
vertical propagation characteristics show waves with maximum amplitude of ~5-8 K and vertical wavelength of ~10 km.
Potential energy density of these two bands of periodicities in the altitude regions 30-60 km is estimated for different
seasons. Equinoctial enhancement in the wave activity is observed. Momentum fluxes of these two bands of periodicities
of gravity waves also exhibit seasonal variation with maximum around equinox and minimum in solstial months. A high
correlation exists between the gravity wave activity and one of its major sources namely convection.
A climatological study of cirrus occurrence has been carried out using the ground-based lidar observations over Gadanki
(13.5°N, 79.2° E) during 1998-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) measurements on
the Terra spacecraft are also used for remote sensing of high clouds cirrus from space during 2001-2004. The interannual
study using LIDAR and MODIS shows an enhancement in the cirrus occurrence during 2001 and fewer amount during
2002. The interseasonal variation of cirrus occurrence frequencies shows much of the occurrences during the monsoon
season. Further lidar observations shows that the cirrus cloud tops typically extended to near the 16.53 km, the average
tropical tropopause height. The distribution of maximum cloud base height frequency is confined to 10-12km. Frequency
of occurrence of cloud physical thickness with respect to cloud base height (Zb) gives a higher occurrence between 11-
15 km and typically the thickness of 2-4 km. At the cloud base height Zb>15km, which is in the vicinity of tropopause,
the cirrus is found to have lesser thickness. A significant observation from this statistical study over this latitude shows
appearance of cirrus at two different altitudes because of different formation mechanisms. We will also discuss the
formation mechanisms for the occurrence of tropical cirrus at this latitude.
In the present study Lower Atmospheric Wind Profiler (LAWP) operating at a frequency of 1.3 GHz is installed at
Gadanki (13.50°N, 79.20°E) is used to study the characteristics of radar Bright Band. Radar observations of the melting
layer of precipitation have been made from few decades. It has been known since then that the melting of precipitation is
often associated with an increase of the reflectivity of weather targets. The primary cause for this is a rapid increase in
the dielectric constant of hydrometeors at the top of the melting layer followed by an increase of the fall velocities of
melting snowflakes towards the end of the melting process. VHF/UHF radars can also be used to study radar bright band,
a strong enhancement in reflectivity at around 0°C isotherm level. Using VHF/UHF radars bright band structure are
studied particularly in the tropics. Though lot of work has been done on the radar signature of the melting layer, still
microphysics behind its formation possesses a challenging task to the atmospheric scientists. The ability of the profilers
to clearly resolve and identify the melting layer when it is present gives very good information regarding the
classification of various precipitating systems within the Tropical Mesoscale Convective Systems (TMCS). VHF/UHF
radars are the wonderful tools in classifying the systems. LAWP data during the period of March 1999 to September
2000 used for the present study. Dependency of bright band height and thickness on various parameters is investigated.
An automated bright band detection algorithm is made to detect the height and thickness of bright band. It is compared
well with the radiosonde observations. A statistical study has been done in correlating bright band thickness with that of
the precipitation intensity. Further in the present paper altitude variation of bright band with the seasons are calculated.
Spectroscopic properties of Ag/Er co-doped thin plates of silicate and phosphate glass were investigated with the aim of assessing the effective role of silver as a sensitizer for erbium. Additive heat treatments in air at different temperatures were performed on both a silver-exchanged and a silver-free plate in order to promote the formation of silver nanoparticles in the former and to refer to the later in the spectroscopic characterization. Absorption as well as photoluminescence measurements in the region of the 4I13/2 -> 4I15/2 transition of the Er3+ ion were performed; excitation wavelengths in the range from 360 to 750 nm were used. For the silicate glasses enhancement of the Er3+ luminescence at 1.53 mm was observed when the excitation wavelength was in the blue region. This spectral range typically coincides with the excitation energy of the surface plasmon resonance of nanometer-sized spherical silver particles.
Several modified silicate glass samples activated with erbium ions at two different doping rates, namely 0.2 and 0.5 mol%, exhibit at room temperature a fine-structured emission band around 1.5 mm. The decay of luminescence from the Er3+4I13/2 metastable level is found to evolve according to a single-exponential law and a lifetime as long as 14.2 ms is measured from the glass with the lower erbium concentration. An estimation of the corresponding radiative lifetime, τrad, is achieved on the basis of various theoretical models. Internal gain curves resulting from absorption and stimulated emission cross sections are also shown. A 75% quantum efficiency is deduced for the less Er3+ concentrated glass, which is nearly the highest one obtained from silica based doped-glasses.
4 -[4 - (Dimethylamino) styryl) - 1 - docosyl pyridinum bromide ](abbreviated as DASPB) is a nonlinear dye which shows strong two-photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared radiation. The dye possesses a much larger TPA cross section and much stronger upconversion fluorescence emission than those of common organic dyes (such as rhodamine). In order to increase the concentration of absorptive or fluorescent centers as well as the optochemical and optophysical stability and effective use of this highly nonlinear dye, it has doped in methyl methacrylate - methacrylic acid co-polymer (PMMA-MA). Linear absorption spectrum, single photon induced fluorescence spectrum and two photon induced fluorescence at different wavelength are studied. The nonlinear optical transmission at both linear absorption region and linear transmission region are studied using nano and pico second laser pulses. The optical limiting behavior of the dye is also studied in cw, pico and femto second region. Using Z-scan studies it is found that in linear absorption region, the DASPB doped in PMMA-PA shows saturation absorption at lower intensity and reverse saturated absorption at higher intensity. At transmission region above 800 nm, it has shown the absorption due to two photon induced fluorescence as well as reverse saturation absorption. With experimental results, the molecular TPA cross-sections and TPF cross-section of the sample are calculated.
With the use of high power lasers in varied applications, search for optical limiting materials has become very important to protect the eyes and other sensors. These materials are based on the principle of reverse saturable absorption (RSA) when absorption cross-section for excited state becomes more than the ground state. In such cases transmittance goes down as we increase the intensity. In this paper, we report the experimental results on two dye doped polymer films - Disperse Orange-25 and Disperse Yellow-7 prepared using hot-press technique. For both the dyes Polymehtylmethacrylate-Methacrylic acid (PMMA-MA) has been chosen as a matrix because of its easy availability and easiness of processing. We have used Z-scan technique with CW laser @ 532 nm to study nonlinear optical properties of these materials. The closed aperture Z-scan shows positive nonlinear refraction for both the materials. While the open aperture Z-scan gives RSA at higher intensities for both the samples, making them candidates for optical limiting.
The optical transmission processes for a nonlinear optical (NLO) material are influenced by the properties of its environment. NLO properties such as intersystem crossing rates may be altered by characteristics of the host such as polarity. The effects of the host material on the optical transmission of the NLO material is investigated using numerical laser beam propagation modeling (LBPM) techniques. Numerical simulations are reported for the optical transmission for zinc meso-tetra(p-methoxyphenyl)tetrabenzporphyrin (ZnTMPTBP) in liquids of differing polarity such as toluene, tetrahydrofuran (THF), dichloromethane (DCM), acetone, and pyridine. In addition to investigating the effect of the solvent on transmission, these calculations explore the effect on transmission of two different singlet lifetimes which have been reported in the literature. Transmission curves are calculated using separately determined parameters obtained from curve fitting using zscan data. Calculated results are compared with experimental data for all cases. Z-scans are also calculated for several cases and the results compared to experimental data. Numerical simulations provide a valuable tool to study the optical transmission behavior of NLO materials such as ZnTMPTBP.
We have been working on rainfall observation project at tropical site in India, in order to study tropical storm structure and raindrop size distribution (DSD) characteristics for improving the current PR rainfall retrieval algorithm, and for making the ground validation of TRMM PR observation. At Gadanki (53-MHz VHF MST radar, an L-band lower atmospheric wind profiler (LAWP), a disdrometer and an optical rain gauge (ORG) are set up to obtain more knowledge on vertical properties of DSD and rain structure during monsoon season. Measurements of drop size distribution (DSD) with disdrometer have been providing the information to study basic DSD characteristics in tropical India. We have found a clear seasonal dependence in Reflectivity (Z) - Rainfall (R) relations (i.e. DSD characteristics) in India. Our results indicate that there are about 3-times differences peak-to-peak in estimates of rain rate using a single Z-R relation. It seems that this type of seasonal dependence should be taken into account to improve the accuracy of the PR algorithm. Drop size distribution characteristics were retrieved in moderate/heavy precipitation using VHF wind profiler. The retrieved drop size parameters were compared to corresponding disdrometer data and found that there is reasonably good agreement between the measurements, lending credence to the profiler retrievals of DSD parameters. Preliminary study on the ground validation of TRMM PR shows fairly good agreement between the disdrometer and TRMM precipitation radar measurements.
We carried out systematic experiments to study the effect of solvents on power limiting in zinc meso-tetra (p- methoxyphenyl) tetrabenzoporphyrin (ZnTBP). We used Toluene, pyridine, acetone, dichloromethane (DCM) and a toluene pyridine mixture to evaluate polarity and complex formation of the solvent and solute molecules. Z-scan data were generated using picosecond and nanosecond laser pulses at 532 nm. These data were then curvefit to a 5-level model to obtain excited state and two photon absorption coefficients, and the intersystem crossing rates. An f/5 optical limiting setup was used to determine limiting thresholds. Results indicate that toluene and DCM give the lowest limiting thresholds and that pyridine ligation can be used as a means of molecular modification for future molecular studies.
Nonlinear absorption, 1D self-focusing and 1D self-trapping of optical beams are studied in 150 ppm Fe-doped Bi12SiO20 crystal using single beam Z-scan technique in nano second regime using coherent and incoherent laser pluses with out applying any external electric field across the crystal. When the crystal is placed in expanding beam 1D self-trapping is observed due to 1D self-focusing. The nonlinear absorption is also observed at high intensities due to trap assisted excited state absorption as well as two photon absorption. The ground state absorption and increased nonlinear absorption at 532 nm and 595 nm compared to pure BSO indicates the increased impurity traps in the crystal lattice due to iron incorporation. The studies reveal that incorporation of iron in BSO making it an excellent optical limiter.
We studied the nonlinear absorption properties of tetrabenzporphyrins (TBP). Two dimensional delocalized pi- electron systems such as the macrocyclic dyes tetrabenzporphyrins exhibit nonlinear absorption. Their nonlinear absorption and refraction are due mainly to a mechanism that involves the formation of large populations of excited states. This takes place in the spectral regions between B and Q bands. The relevant time frame in which the nonlinear absorption takes place is determined by the dipole dephasing times in the case of the excited singlet states (which is of order picosecond). TBPs have been proposed as candidates for applications in optical power limiting. We have monitored the nature of the nonlinear absorption of TBP samples with picosecond laser pulses at 532 nm wavelength. Open aperture picosecond Z-scan measurements obtained with solutions of TBP in tetrahydrofuran (THF) exhibit reverse saturable absorption (RSA). Our experiments also reveal RSA for a thin film of TBP in a polymethyl methacrylate (PMMA) matrix. On the other hand we observed that for films deposited using the Langmuir-Blodgett (LB) technique the RSA is decreased. A simple extrapolation of solution measurements to thin film samples might result in erroneous conclusions. We investigated the nonlinear absorption of TBP in THF-water mixture to elucidate the role of molecular aggregates in the nonlinear properties of TBP samples. The RSA observed in the case of THF solutions is lost in the mixture.
The photochromic protein bacteriorhodopsin (bR) obtained from the purple membrane of the halocbacterium halobium has attracted considerable amount of interest recently. Several applications in photonics technology have already been proposed. bR lends itself to structural alterations at the molecular level by use of bioengineering and chemical synthesis methods. It is very stable under hard environmental conditions of temperature, salinity, etc. The films that were used in our experiments are stable over a period of four years. The lifetimes of some of the intermediate states of bR photocycle can be altered over a wide range by genetic manipulations of some of the amino acids that compose the bR membrane or by controlling the pH of the host materials. All of this makes bR a very attractive material for applications in optical computers and information processing. We present a technique that utilizes the molecular states of a bR thin film to implement an all-optical switch and all-optical logic AND and Or gates. A two-color backward degenerate four-wave mixing geometry with wild-type and chemically stabilized films of bacteriorhodopsin constitute the experimental setup. The saturation intensity, sensitivity and excited state lifetime (M state lifetime) of the films are very different. We use red light to form a holographic grating,due to the B to M transition and blue light to form a grating due to the fast photochemical transition from.M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gates and the phase conjugate signal beam is the output of the gates.
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