Materials Science Section

Nano Materials Lab.

In recent years the Nano Materials Lab has been working on ultrafast spectroscopy of nanostructured materials. We now plan to demonstrate qubit (quantum bit) initiation and its manipulation in ultrafast time scales in an optimized low-dimensional physical system. Quantum computation is being seen as a means for efficiently solving classes of problems that are not possible with classical computing techniques. Among the different physical systems that are being investigated for quantum computation, one of the most promising t in terms of scalability as well as practical convenience is the ultrafast manipulation of carriers confined in low-dimensional materials. Semiconductor quantum dots and 2D materials are good candidates for studying qubit operation which may result in a solid-state quantum computer. Our research will provide the necessary understanding and generate the expertise in ultrafast handing of quantum states in low-dimensional systems for the long term goal of making an operational quantum computer. Our research will provide the necessary expertise in ultrafast coherent manipulation of quantum states in low-dimensional systems which, in near future, will become essential for secure communication and massive parallel computation.

HIGHLIGHTS OF RECENT AND CURRENT WORK

Carrier dynamics and the origin of optical nonlinearities in silver nanoplatelets

With a new two-dimensional way of analyzing conventional transient absorption measurement data the magnitude and decay time of the imaginary part of the third as well as the fifth and seventh order optical nonlinearities were extracted. The time dependent variation in the third, fifth and seventh-order nonlinearities were analyzed using two temperature model. By comparing the results of the calculation with that of the experiments, it has been shown that the higher-order nonlinearities originating from the hot-electrons of metal do contribute to the measured higher-order nonlinear absorption coefficients of the composite material.


The transient absorption/reflection setup and Time and Intensity dependence of transient absorption



Experimentally measured time dependence of second, fourth and sixth order change in absorption of silver nanoplatelets
and
Theoretical time dependence of second, fourth and sixth order change in absorption calculated using two-temperature model.



Quantum beats from near surface GaAs0.86P0.14/Al0.7Ga0.3As single quantum well

The carrier relaxation mechanisms and their respective time scales are being studied for the GaAs0.86P0.14/Al0.7Ga0.3As single quantum wells (SQW) using degenerate femtosecond pump-probe transient reflectivity (DPPR) measurements. These measurements were carried out on a near surface quantum well with 5 nm (QW5) AlGaAs top barrier thickness and a deep QW with 50 nm (QW50) thickness to study the effect of top barrier thickness on the carrier dynamics of the SQWs. In the structure under study the contributions to the measured ∆R/R is due to the carriers excited in the GaAs substrate and carriers excited in quantum well. The significant part of the observed ∆R/R arises from carriers excited in GaAs. The excited carriers in GaAs show a fast initial thermalization of the order of ~ 50 fs and a subsequent drift towards the AlGaAs/GaAs interface. This interface actually acts like a triangular quantum well. The existence of such a triangular quantum well is indicated by the electric field (~34 kV/cm) near GaAs estimated form photo-reflectivity measurements. The rise and decay observed in the ∆R/R shows that the carrier capture time for the triangular quantum well is of the order of few picoseconds while the recombination takes about few hundreds of ps. The carriers excited in the quantum well contribute a very small part to the DPPR signal from the quantum well structure. However in QW5 a signature of the interaction of surface states with the quantum well states was observed as an oscillatory structure in the decay dynamics. The period of oscillation observed in our measurement is 120 fs ± 10 fs. This corresponds to an energy level separation of 33 meV which is of the order of energy level separation between lh1 and hh2 of the quantum well. The oscillatory signature is absent in QW50.


The figure on left side shows the quantum beats measured from quantum well sample QW5at various pump powers.


We welcome collaboration with groups making novel materials.

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