Rama Chari - Academia.edu (original) (raw)

Papers by Rama Chari

Applied Optics, 1986

Detailed results of the parametric behavior of the prominent seven visible transitions in a pulse... more Detailed results of the parametric behavior of the prominent seven visible transitions in a pulsed Xe IV laser are reported. The various laser parameters considered are gas pressure, excitation voltage, energy storage capacitor, and the diameter and length of the plasma tube. Based on our results we believe that lasing is favored in larger bore tubes as wall collisions play a significant role in quenching the laser action.

Applied Physics Letters, 2016

Internationalquantum Electronics Conference 2004, May 21, 2004

An experimental study of the optical Stark effect in ZnSe-based quantum wells using pump-probe sp... more An experimental study of the optical Stark effect in ZnSe-based quantum wells using pump-probe spectroscopy is presented. The Stark shift at both the heavy-hole and the light-hole resonances is measured as a function of pump detuning for samples with different heavy-hole light-hole separations. The coupling between the heavy hole and the light hole has to be taken into account to explain the observed dependence on the pump wavelength detuning and polarization of pump and probe pulses. For a range of samples the Stark shift behavior is characterized by the ratio of the detuning to the heavy-hole to light-hole separation; higher order Coulomb correlations play a prominent role when the ratio is less than 2.

Journal of Luminescence, 2015

Applied Physics Letters, 2015

Force and Motion is considered one of the fundamental topics in science and is therefore taught i... more Force and Motion is considered one of the fundamental topics in science and is therefore taught in middle and high schools. However, the topic is treated in a perfunctory manner, forgetting that the current understanding of motion, and the development of concepts like speed and acceleration took place over a period of two thousand years. We have tried to tackle these difficult concepts in the modules on Force and Motion. In this, the first module, we have concentrated on describing motion in a scientific manner. The module is primarily aimed at teachers. It presents a sequence of activities worked out on the basis of detailed discussions with subject experts and feedback from teacher-training sessions that lead to an understanding of Motion.

ICONO 2001: Ultrafast Phenomena and Strong Laser Fields, 2002

ABSTRACT We describe how spectral analysis of an ultrashort pulse after propagation in a single m... more ABSTRACT We describe how spectral analysis of an ultrashort pulse after propagation in a single mode fiber can be used to characterize picosecond laser pulses. The technique is demonstrated for a cw mode locked Nd-YVO4 laser.

Optical Engineering, 2007

ABSTRACT Propagation along an optical fiber modifies the spectrum of an ultrashort laser pulse du... more ABSTRACT Propagation along an optical fiber modifies the spectrum of an ultrashort laser pulse due to interaction of the pulse with the fiber nonlinearity. These spectral modifications can be correlated with the input pulse parameters like the pulse power, pulse width, chirp, and pulse shape. We have shown earlier by numerical calculation that a small change in any of the pulse parameters is magnified in the fiber-modified spectrum due to the nonlinear nature of the interaction. We had proposed a differential spectrum monitoring technique for detecting such small changes. We now show experimentally that this technique can provide a sensitive and practical method for monitoring small changes in ultrashort pulses. Thus it can serve as a practical on-line monitor for laser stability during experiments with ultrashort pulses. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/... more Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/AlGaAs single quantum well is studied. Similar anomalous oscillations are also observed in layered manganite [K. Kouyama et.al. J. Phys. Soc. Jpn. 76:123702(1-3), 2007]. We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studied carried out on reflectivity of different materials under identical conditions shows that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

Chemical Physics Letters, 2014

ABSTRACT Optical limiting behavior of three ferrocenyl substituted calixarenes (FSCs) where ferro... more ABSTRACT Optical limiting behavior of three ferrocenyl substituted calixarenes (FSCs) where ferrocenyl group is attached via ethynyl(a), meta-(b) and para-(c) ethynyl phenyl spacers studied with nanosecond pulses at 532 nm is in the order c > a > b. While, with femtosecond laser pulses at 800 nm strength of nonlinearity is in the order c > b > a. Largest nonlinear refraction (n2 = 9.5 × 10−16 cm2/W) is estimated for the FSC c. Nonlinear absorption in these FSCs is attributed to three-photon absorption. Optical limiting behavior with nanosecond laser pulses is mainly due to thermal nonlinearity and reverse saturable absorption, whereas, with femtosecond laser pulses mainly intrinsic electronic nonlinearity contributes.

Applied Physics Letters, 2014

ABSTRACT We report tunneling assisted beating of carriers in a near-surface single GaAsP/AlGaAs q... more ABSTRACT We report tunneling assisted beating of carriers in a near-surface single GaAsP/AlGaAs quantum well using transient reflectivity measurement. The observed damped oscillating signal has a period of 120 ± 6 fs which corresponds to the energy difference between lh1 and hh2 hole states in the quantum well. Comparing the transient reflectivity signal at different photon energies and with a buried quantum well sample, we show that the beating is caused by the coherent coupling between surface state and the hole states (lh1 and hh2) in the near-surface quantum well. The dependence of decay of coherence of these tunneling carriers on the excitation fluence is also reported. This observation on the coherent tunneling of carrier is important for future quantum device applications.

International Conference on Fibre Optics and Photonics, 2012

Surface Review and Letters, 2014

Semiconductor Science and Technology, 2013

ABSTRACT Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spec... more ABSTRACT Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ~500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

Pramana, 2014

ABSTRACT We show that in a GaAs0.86P0.14/Al0.7Ga0.3As near-surface quantum well, there is coheren... more ABSTRACT We show that in a GaAs0.86P0.14/Al0.7Ga0.3As near-surface quantum well, there is coherent oscillation of holes observed in time-resolved reflectivity signal when the top barrier of the quantum well is sufficiently thin. The quantum well states interact with the surface states under the influence of the surface electric field. The time period of the observed oscillation is 120 +/- 10 fs.

Physical Review B, 2004

ABSTRACT

Optical Engineering, 2006