Bulk saturable absorption in topological insulator thin films

Abstract

We present nonlinear optical absorption properties of pulsed laser deposited thin films of topological insulator (TI), Bi2Se3 on quartz substrate, using open aperture Z -scan technique. The saturable intensity of as deposited thin films has been found remarkably improved by an order of magnitude compared to the values reported earlier in the literature. Past results from the literature are inconclusive in establishing whether the saturable absorption is coming from surface states or the bulk. Specifically designed experiments with magnetically doped TI samples allow us to attribute the saturable absorption characteristic of TI to the bulk states. Detailed experimental procedures and possible explanation of observed results have been discussed. INTRODUCTION:A new phase of matter known as topological insulators (TI) has emerged in the last few years. Considerable attention has been given to these new phases of matter, both from the perspective of fundamental studies and possible novel device applications point of view. Theoretically predicted and experimentally observed, these topological materials have paved the way towards observing unusual and novel physical phenomena like Majorana fermions, topological magneto electric effect and axion electrodynamics 1-3 . These materials exhibit topological order in the bulk band gap and owing to that there exists metallic surface Dirac states. Due to the combined effect of strong spinorbit coupling and the time reversal symmetry, these systems display linear Dirac like energy-momentum dispersion in the bulk band gap. TI's are, therefore characterized by bulk insulating band-gap and spin-momentum locked surface states spanning the gap. These surface states are protected from the non-magnetic impurities in a two-fold manner; owing to the presence of odd number of Dirac cones and π Berry phase. These surface states have been investigated by surface sensitive probes like angle resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy and magneto-transport experiments by several groups 4-7 . Second generation binary materials such as Bi2Se3 (BS), Bi2Te3 (BT) and Sb2Te3 (ST)

Nonlinear Optical Properties of Emerging Nano‐ and Microcrystalline Materials

“…[154] For Fe-doped Bi 2 Se 3 topological insulator in nonlinear optical Z-scan experiment interesting hypotheses were introduced. [155] It has been experimentally proved that not only surface states of the topological edge are responsible for saturation absorption behavior, but also the bulk state. As a proof, the open aperture Z-scan measurement on both undoped Bi 2 Se 3 samples and Fe-doped Bi 2 Se 3 samples (without topological protection of the surface states) did not show any appreciably different values of saturation intensities.…”