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    Time resolved high frequency spectrum of Br
    ( 2013-01-01) Yehya, Fahem ; Chaudhary, A. K.
    The paper reports the time resolved spectral distribution of higher order acoustic modes generated in Br2 molecules using pulsed Photoacoustic (PA) technique. New time resolved vibrational spectrum of Br2 molecules are recorded using a single 532 nm, pulses of 7 ns duration at 10 Hz repetition rate obtained from Q-switched Nd:YAG laser. Frank-Condon principle based assignments confirms the presence of 12 numbers of (v″-v′) vibrational transitions covered by a single 532 + 2 nm pulse profile. Inclusions of higher order zeroth modes in Bassel's function expansion series shows the probability of overlapping of different types of acoustic modes in the designed PA cells. These modes appear in the form of clusters which occupies higher frequency range. The study of decay behavior of PA signal with respect to time confirms the photolysis of Br2 at 532 nm wavelength. In addition, the shifting and clustering effect of cavity eigen modes in Br2 molecules have been studied between 1 and 10 ms time scale. The estimated Q-factor of PA cell (l=16 cm, R = 1.4 cm) is 145 ± 4 at 27 kHz frequency. © 2013 Elsevier B.V. All rights reserved.
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    A novel investigation of the thermal decomposition mechanism of (MTNI) and (KNO
    ( 2013-01-31) Yehya, F. ; Chaudhary, A. K.
    We report a novel approach based on time resolved pulsed photoacoustic (PA) technique to study the thermal decomposition mechanism of 1-methyl-2,4,5- trinitroimidazole (MTNI) and potassium nitrite (KNO2). The technique is based on the detection of free -NO2 molecules which is released due to thermal decomposition from these samples. The second harmonic i.e. λ = 532 nm pulses of 7 ns width at repetition rate 10 Hz obtained from Q-switched Nd:YAG laser is employed to record the PA spectrum. The recorded PA spectrums show the presence of first five even longitudinal acoustic modes i.e. 2nd, 4th, 6th, 8th and 10th with different intensities and frequency shift. In addition, the thermal decomposition mechanism of MTNI and KNO2 is crossed examined by TG-DTA technique which provides direct information of weight loss and heat flow thermal zones. The PA based temperature study is carried out for MNTI and KNO2 separately. We have obtained several highly resolved thermal zones which clearly demonstrate the process of thermal decomposition. Finally, the effect of pressure and input laser energy on PA signal has also been studied. © 2012 Elsevier B.V.
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    Optical characterization of GaAs photoconductive antennas for efficient generation and detection of Terahertz radiation
    ( 2014-01-01) Venkatesh, M. ; Rao, K. S. ; Abhilash, T. S. ; Tewari, S. P. ; Chaudhary, A. K.
    We report the optical characterization of low temperature (LT)-GaAs and Semi Insulating (SI)-GaAs based dipole types Photoconductive (PC) antennas with gap of 5 μm for the generation and detection of Terahertz (THz) signals. We have used tunable 140 femto-seconds laser pulses at 80 MHz repetition rate of different power and biasing voltage to evaluate the performance of our antennas as a THz generator and detector. Interestingly, we found that LT-GaAs and SI-GaAs have high photocurrent (density of charge carriers) response at some specific laser wavelengths such as 840 nm and 880 nm, respectively. We have employed these selected wavelengths for the generation and detection of THz radiation from these materials made PC antennas. The comparative study shows that the intensity of THz radiation in case of SI-GaAs is higher than the LT-GaAs made PC antennas. The THz frequency spectrum range of LT-GaAs and SI-GaAs antennas are extended up to 3.6 THz and 2.5 THz respectively. Finally, we have cross verified the reliability of our antennas to detect the THz radiation generated from ZnTe crystal. © 2013 Elsevier B.V. All rights reserved.
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    Low-limit detection of NO2 by longitudinal mode selection in a Photoacoustic resonant system
    ( 2013-09-01) Yehya, F. ; Chaudhary, A. K.
    The paper reports the pulsed laser-based photoacoustic (PA) spectroscopy of NO2 in a resonant PA cavity with special filters made of stainless steel. The PA cell along with special types of sound filters are designed and fabricated to excite only the second-order longitudinal mode inside the cavity. The second harmonic, i.e. ? = 532 nm pulse width, of 7 ns obtained from Q-switched Nd:YAG laser at 10 Hz repetition rate has been used to study the saturation behaviour of the PA signal and absorption coefficient with respect to the input laser energy. Generally, the Q-factor of longitudinal modes in the acoustic cavities is quite low. However, by modifying the design of the cell and the filter, we can achieve high value of Q = 30. The combination of special filter along with the experimental data acquisition technique helped us to achieve the minimum detection concentration of NO2 of the order of 9 ppbV which is much better than the previous value of the same PA cell without filter [Yehya and Chaudhary, Appl. Phys. B 106, 953 (2012)].
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    Study of damping, saturation and surface losses on low level detection of NO
    ( 2014-01-01) Yehya, F. ; Chaudhary, A. K.
    The time resolved pulsed photo-acoustic (PA) spectrum of atmospheric pollutant gas (NO2) buffered in two different mediums is reported. The closed window PA resonance cell made of stainless steel filled with highly pure NO2 gas mixed with air and nitrogen separately to study the role of buffer gases for the generation of radial modes of higher frequency and damping effect in the same cavity. The energy storage phenomena of the resonant cavity is explained using coupled oscillator theory. The second harmonics i.e. λ=532 nm pulses obtained from Q-switched Nd: YAG laser having 7 ns pulse width is used to excite the resonant modes of the cavity. The losses corresponding to radial and longitudinal modes are estimated experimentally and found to have a good agreement with their corresponding theoretical values. The dependence of saturation behavior of NO2 as an artifact of the PA cell along with gas molecules at different values of the incident laser energy has been discussed for the first time. In addition, we have successfully demonstrated the effect of damping on the quality factor-Q of the cavity which is not only responsible for generation of higher order modes but also decide the low level detection of the PA system. The developed PA sensor helped us to achieve minimum detection concentration of NO2 of the order of 0.213 ppbV and 1.2 ppbV. © 2013 Elsevier B.V.