Sanjit successfully defended his PhD dissertation on February 1, 2012.

TITLE:
Ghost Imaging with Sunlight

ABSTRACT:
The main result of this dissertation is the first successful experimental demonstration of ghost imaging using the sun as a light source. This result supports the quantum theory of near-field thermal light ghost imaging and also clarifes the physics of near-field thermal light ghost imaging from the fundamental level. The quantum theory of two-photon interference is the key to understanding non-local ghost imaging with thermal light sources. Two-photon interference occurs between two different yet indistinguishable probability two-photon amplitudes, nonclassical entities produced by the joint-detection between two distant photo-detectors. On the other hand, the classical theory considers the reason behind thermal light ghost imaging to be an intensity fluctuation correlation. Interestingly, the physics of intensity fluctuation correlation was misled by the speckle-to-speckle picture.

The experimental demonstration of ghost imaging with sunlight suggests that the nonlocal ghost-imaging effect of thermal light is caused by quantum-mechanical two-photon interference and it also proves that the idea of 'speckles" is unnecessary in near-field thermal light ghost imaging. Most importantly, the sun does not make any speckle and it is a near-field source. The experimental studies on sunlight-based ghost imaging is discussed in two steps: (1) an experimental demonstration as well as a quantum mechanical explanation of the nontrivial intensity correlation with the sun, a natural thermal source, as a light source and (2) the demonstration of the experimental observation of ghost imaging with sunlight with its quantum-mechanical explanation. These observations with their theoretical explanation are very helpful to understanding the physics of ghost imaging from a fundamental level. From the application point of view, sunlight-based ghost imaging may achieve a spatial resolution equivalent to that of a classical imaging system taking pictures at a distance of 10 km with a lens of 92 m size.

So far ghost imaging using thermal light with one color are demonstrated. This dissertation also reports an experimental study of two-color, biphoton ghost imaging using an entangled photon pair source. The result of this experimental observation shows a ghost image with enhanced angular resolving power by means of a greater field of view compared with that of classical imaging. The experience gained in the two-color ghost imaging experiment with entangled photon pairs will be helpful to get a real color ghost image with sunlight. A proposal to achieve sunlight-based ghost imaging with real colors is also reported here. Potential real color sunlight-based ghost imaging with its nonlocal behavior and turbulence-free nature gives us a promise for its applications in distant imaging.

Neetika successfully defended her PhD Proposal on February 8, 2012.

TITLE:
Diagnostics of Photoionized Gas in Seyfert Galaxies

ABSTRACT:
As their name implies Active Galactic Nuclei (AGN) reside in the centers of many (10%) galaxies. They are active in the sense that their spectra exhibit a broad (from radio to gamma-rays) non-stellar continuum which exceeds the luminosity of the host galaxy. Furthermore at some wavelengths intensity variations have been observed on timescales less than a day (minutes in extreme cases). The ultimate cause of this tremendous outflow of energy in the form of radiation is widely believed to be due to the accretion of matter onto a supermassive black hole (BH ~ 10⁶-10⁹ times the mass of the Sun). Unfortunately since all this activity takes place in such a relatively small region (<< 3lyr) the central engine of even the closest AGN cannot be imaged directly with current technology (eg. The central region of Circinus, the nearest active galaxy at a distance of about 40 million trillion miles, has an angular size of about 0.2" that is just resolvable with VLT of spatial resolution 0.1"-0.5" ). Nevertheless spectroscopic observations can help us constrain the conditions of the gas very close to the BH. Such observations reveal that, as expected from considerations of the angular momentum of the infalling matter, such material in the circumnuclear regions of an AGN is not in a spherically-symmetric distribution. The unified model of AGN comprises of different gas components surrounding the central engine and hence that leads to the classification of AGN depending upon their oreintations with respect to the observer on the Earth.

My thesis focuses on constraining the physical conditions in the NLR of Seyfert galaxies. I propose to compare observational data with the predictions of a publically available software tool CLOUDY in order to further constrain the conditions within the circumnuclear regions of a number of nearby Seyfert Galaxies. Specifically I propose to use archival data in the soft X-ray band (6-38Å) obtained by the grating spectrometers onboard the XMM-Newton and Chandra observatories for this purpose. Some Seyfert galaxies (Seyfert 2 galaxies) emit a multitude of emission lines due to H-like and He-like ions of the cosmically abundant elements (C, N, O, Ne, Mg...etc). These line intensities are far in excess of those predicted by models of the gas in the standard Unied Model of Active Galactic Nuclei (AGN). It has been suggested that the lines are due to an additional amount of circumnuclear material. Some of this material is actually thought be co-located with the so-called Narrow-line region (NLR) of AGN. This has been dubbed the Xray NLR (XNLR) and is probably co-located with the regular NLR. Due to the intense continuum radiation, the XNLR gas is thought to be photoionized. Understanding this X-ray emitting component of gas is important, as it will help us understand the overall structure and dynamics of the circumnuclear regions of AGN.