Speaker:
Dr. Akanksha Dubey
Technion Israel Institute of Technology, Israel
Attosecond Science- A Pathway to Quantum World
Probing quantum systems has always been a challenge since the advent of quantum physics. The fundamental curiosity to observe electronic motion through the quantum world on its inherent temporal scale- attosecond (1 attosecond = 10^-18 seconds) scale of time, has driven continuous pioneers and discoverers to craft ways/methods for probing it on such ultrafast realms. However, it could be realized only at the dawn of the 21st century. The advancements in the contemporary laser technology made it possible to generate the first attosecond pulses in the year 2001, marking the beginning of a new era- ‘Attosecond Science’; also called ‘Attoscience’. As the name suggests, this new research area deals with study of electron dynamics in matter on attosecond time scale. The importance of this new science is already recognized by the award of Nobel Prize in Physics for the year 2023 to the pioneers of attosecond pulse generation. Steering the electron through the quantum world using intense ultrafast lasers, we are now capable of observing electron dynamics in real-time. This provides unprecedented control on matter, which was not possible two decades ago. Attosecond
science encompasses all research domains across science and engineering streams, where electron dynamics is probed on its intrinsic temporal scale.
In this seminar, I shall take a tour of ultrafast quantum dynamics in molecules and carbon fullerenes; when such systems interact with intense ultrafast laser fields of different polarizations. Using numerical experiments, I shall elucidate some highly nonlinear optical phenomenon and ultrafast processes, such as- high harmonic generation (HHG), charge migration and light-induced quantum
correlations, time-resolved observables using the method of pump-probe spectroscopy, molecular photoemission, above-threshold ionization (ATI) and photoelectron spectrum (PES). I shall explore the origin and underlying physical mechanisms of these ultrafast quantum dynamics in the systems of interest. I shall highlight the importance and applications of such ultrafast processes in the relevant
areas and conclude the seminar by elaborating on the future perspectives and goals.
Speaker:
Dr. Naga Prathibha Jasti
JNCASR, Bangalore
Defect Tolerance in Halide Perovskites: From Concept to Direct Experimental Evidence
The rapid rise of lead-Halide perovskites (Pb-HaPs) to prominence is unprecedented in the history of semiconductor academia and industry, especially in photovoltaic and LED applications. HaP thin films, even when deposited at near-room temperature and ambient pressure from solution or by printing, a process expected to yield high structural and crystallographic defect concentrations, nonetheless exhibit excellent optoelectronic properties and yield highly efficient devices. This presents a paradox, suggesting Pb-HaPs possess intrinsic material properties that either limit defect formation or effectively tolerate their presence. Understanding these properties is therefore of paramount importance for both fundamental insight and the development of HaP materials and devices.
In this talk, I will discuss the concept of “Defect Tolerance” (DT) that is often invoked to explain the exceptional optoelectronic properties of Pb-HaPs, and their devices. While DT became a widely accepted notion in the field, direct experimental evidence for it has been lacking. In semiconductors, DT implies tolerance to structural defects without the usual electrical and optical effects (e.g., traps) associated with such defects. I will present the first direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2D, 2D-3D, and 3D Pb-HaP crystals with their optoelectronic characteristics using X-ray diffraction, photoluminescence, and high-sensitivity sub-bandgap surface photovoltage spectroscopy. These techniques probe the bulk of the material (with penetration depths of several hundred nanometres to a few micrometres), allowing us to assess intrinsic bulk—rather than just the surface properties. Our observations provide a solid experimental basis to rationalize defect tolerance in Pb-HaPs and will help guide the search for and design of other semiconductors with similar tolerance behaviour.
