Photonic Band Gap Materials: Light Control at Will




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Photoluminescence properties of the CdSe/ZnS core-shell quantum dots accompanied with rotation of the defocused wide-field fluorescence images



Qiang Li, Xiao-Jun Chen and Li-Jun Wu*


Laboratory of Photonic Information Technology, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, People’s Republic of China


Abstract


We investigate the emitting dynamics of single CdSe/ZnS core-shell quantum dots (QDs) by employing a defocused wide-field imaging method. The defocused image (DI) i.e., emission pattern of some QDs was observed to rotate after blinking-off periods for the first time. We have attributed this change to a “self-rotation” of the electron cloud around the non-emitting axis of the QDs [1]. To gain insight into this phenomenon, we have extended the observation time to 350 s and traced the DIs of a number of individual QDs under different environments. It has been found that the “active” QDs blink less frequently than the “normal” QDs and remain dark most of the observation time. When the QDs were illuminated in air, the occurrence probability of the DI-rotation was observed to be higher than that in the PVA and SU8 film by a factor of 6. In addition, the mean off-time of the QDs with DI-rotation was remarkably longer than those without. We propose that when the QDs are exposed to air upon illumination, oxygen could corrode the surface of the core and induce surface defects, which could act as traps [2,3]. These surface defect traps may influence the electron cloud whereby the “self-rotation” of the DI could likely take place more often. Furthermore, the surface defect seems able to affect the recapture of the ejected carrier and quench the emission of the QDs, whereby the photobleaching of the QDs occurs much faster in air than in the SU8 or the PVA film.


References

Xiao-Jun Chen, Yi Xu, Sheng Lan, Qiao-Feng Dai, Xu-Sheng Lin, Qi Guo and Li-Jun Wu. Phys. Rev. B.79, 115312 (2009). Rotation of defocused wide-field fluorescence images after blinking in single CdSe/ZnS core-shell quantum dots.

W. G. J. H. M. van Sark, P. L. T. M. Frederix, D. J. Van den Heuvel, and H. C. Gerritsen. J. Phys. Chem. B . 105, 8281 (2001) .Photooxidation and Photobleaching of Single CdSe/ZnS Quantum Dots Probed by Room-Temperature Time-Resolved Spectroscopy

A. Y. Nazzal, Xiao-yong Wang, Min Xiao. J. Phys. Chem. B. 108, 5507 (2004). Environmental Effects on Photoluminescence of Highly Luminescent CdSe and CdSe/ZnS Core/Shell Nanocrystals in Polymer Thin Films.


Quantum Dot Strong Coupling Dynamics in a Structured Electromagnetic Vacuum and its Application in All-optical Information Processing



Xun Ma and Sajeev John

Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, Canada, M5S 1A7

(Dated: December 4, 2009)


Abstract


We demonstrate fundamental modifications of quantum dot (Q dot) strong coupling dynamics by sharp variations in electromagnetic density of states (DOS) inside photonic crystal waveguides. In particular, we show dynamical, near-complete inversion switching of a two-level Q dot driven by picosecond optical pulses in a bimodal photonic band gap (PBG) waveguide at microwatt power levels. This is enabled by a sharp step-like discontinuity in the electromagnetic local DOS (LDOS) provided by a cutoff in one of the two waveguide modes. The atomic Bloch vector equations in this colored vacuum are derived using dressed state description of the atomic density operator in the presence of phonon mediated dephasing. Instead of assuming phenomenological decay rates, explicit expressions for decay terms of bare state atomic dipole moments and population are obtained under local Markov approximation. Giant Mollow splitting of the atom level due to sub-wavelength light localization of strong driving pulse causes the time-dependent Mollow bands to straddle the LDOS jump, leading to different radiative decay rates in the upper and lower Mollow sidebands. This results in remarkable field-dependent spontaneous emission and dipolar dephasing rates, combined with a novel “vacuum structure” term in the Bloch equations. These field-dependent relaxation rates allow dynamic control of the dissipation time scale, resulting in alterations between coherent transient time regime and steady state time regime during the passage of a single monochromatic pulse. The extended Bloch equations thus predict ultra-fast, high-contrast inversion switching that is activated and de-activated by picosecond pulse trains detuned below and above the atomic resonance, respectively. This dynamic inversion is due to the rapid rise in relaxation rates as the pulse amplitude rises, causing the Bloch vector to switch from antiparallel to parallel alignment with the pulse “torque vector” during a steady state attraction process. Subsequent near-complete inversion occurs through a coherent adiabatic following process retained long after the pulse amplitude subsides and the system reverts to slow relaxation. For a 1% inhomogeneously broadened distribution of Q dots with average 100Debye dipole moment, driven by 1.5μm picosecond pulses and coupled to a cutoff mode LDOS jump with radiative emission rates high = 2.5THz and low = 5GHz, a large average population switching contrast of 0.5 is demonstrated with a phonon dephasing rate p = 0.5THz. A 1.6 femtojoule control pulse is required per switching operation and a 30 microwatt pulse train is sufficient to maintain the inversion. This switchable gain/loss segment of the PBG waveguide can be used to controllably amplify/absorb signal pulses conveying optical information. This provides a robust mechanism for ultra-fast, multi-wavelength channel all-optical switching in PBG microchips. In the end, further theoretical implications and possibilities about structured vacuum strong coupling dynamics that is brought about by the extended Bloch equations will be discussed, with some preliminary results on multi-pulse optical logic operations shown. Keywords: Photonic crystal waveguide; quantum dot; strong coupling; all-optical switch; structured vacuum; population inversion; adiabatic following Relevant Publications:

[1] Xun Ma and Sajeev John, Phys. Rev. Lett., 103, 233601 (2009)

[2] Xun Ma and Sajeev John, Phys. Rev. A, 80, 063810 (2009)


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