1 Room temperature slow light with 27 GHz bandwidth in semiconductor quantum dots Giovanni Piredda, Aaron Schweinsberg, and Robert W. Boyd The Institute of Optics, University of Rochester, Rochester, NY Optical Society of America Slow and Fast Light Topical Meeting Monday, July 24, 2006

2 Outline Theory of slow / fast light in fast saturable absorbers Characteristics of the quantum dot system Demonstration of pulse delay Plans for future work Conclusion

3 Theory In a medium that exhibits saturable absorption, it is possible for the leading part of a pulse to saturate the absorption of the medium, allowing the trailing part to see increased transmission relative to the front. This very particular reshaping creates a delayed output pulse. intensity distance delay input output

4 Theory (cont.) Recovery time must be comparable to pulse duration. If it is not, the result will be a sub-optimal delay and possibly a distorted pulse Intererested in high-bandwidth applications seek a medium with a recovery time ~ tens of picoseconds. Quantum dot nanocrystals - absorption recovery time of ~ 30 ps. This recovery is caused by carrier scattering into defect states, rather than a full return of the system to the ground state. 1 [1] A. Dementjev et al., “Mode-locking of neodymium lasers by glasses doped with PbS nanocrystals,” Appl. Phys. B 77, 595 (2003).

5 The quantum dot material system Our dots are PbS nanocrystals, manufactured by Evident Technologies. Dots have a diameter of 2.9 nm and are used in a solution of toluene. Absorption spectrum measured with spectrophotometer shows first exciton peak near 800 nm.

6 The quantum dot system (cont.) Advantages for slow light High bandwidth: recovery time ~30 ps Room temperature operation Pulses can be self-delayed or a separate control field can be used Customizable to different operating wavelengths – different dot sizes Disadvantages: High optical intensities required (measured saturation fluence of ~4 mJ / cm 2 ) Less pulse delay than some other techniques - still potentially useful for applications such as recentering.

7 Experimental Setup Pulses from OPA are 12 ps long with a center wavelength of 795 nm To measure delay, a cross-correlation operation is performed. Pulses sent through the sample are combined with a reference pulse that goes through a scanning delay in the other arm, and focused on a 2-photon detector.

8 Observation of pulse delay Cross-correlation measurement shows pulse delay of 17% of the output FWHM. The pulse broadens from 12 to 16 ps.

9 Plans for future work Shift work to more technologically interesting wavelengths Dots with the first exciton absorption feature near 1550 nm can be used. PbSe dots in tetrachloroethylene, provided by the Krauss group. QD sample preparation and absorbance measurement by Jeff Peterson

10 Plans for future work(cont.) Dots on (or in) a waveguide: field strength kept high for the length of the guide to allow larger delays at lower optical powers. Collaboration with group of Chee Wei Wong at Columbia. Additional credit to James McMillan for field simulations using full-vectorial eigensolver. Based on slotted waveguide design of Lipson group (Opt. Lett. 2004) and Scherer group (Appl. Phys. Lett. 2005).

11 Plans for future work (cont.) Dots suffer from degradation largely due to reaction with oxygen - improve stability by encapsulation in a solid host (resin or glass) Tuning the delay: separate beams for a pump / probe setup

12 Conclusions Obtained slow light of 27 GHz bandwidth pulses in a quantum dot solution A fractional pulse delay of 17% of the FWHM has been measured Future work is likely to focus on improving dot stability, integration into telecom-type systems Support for this work has been provided by the slow light program of DARPA / DSO