# Presentation: 119262

## Harmonic Potential Traps for Excitons and Polaritons

- 	PI: D.W. Snoke, University of Pittsburgh
- 	NSF project DMR-0404912

*Bose-Einstein condensation of atoms started with the *
*accomplishment of “optical molasses” and optical trapping,*
*methods of creating a harmonic potential for atoms at low*
*temperature. The same has been done for polaritons *
*and excitons, which are “energy particles” in semiconductor*
*structures. Stress is used to tune the band structure of*
*the semiconductor, creating an energy minimum. *

- excitons flow
- “downhill” to the
- collection point
- here

- polaritons flow
- “downhill” to the
- collection point
- here

**Polaritons:**
**effective mass **
**   ****~ 10****-5**** m****e**
**lifetime ~ 2 ps**

**Excitons:**
**effective mass ~ .3 m****e**
**lifetime ~ 10 ********s**

**Notes:**
Photons can be converted into two different types of energy particles inside certain semiconductor structures, known as excitons and polaritons. Both of these are like a photon, but unlike a photon, they have mass.  This means that we can manipulate them is ways that we can not manipulate photons.  Polaritons are more photon-like, with very light mass (several orders of magnitude less than an electron), and they turn back into photons very quickly, on times scales of picoseconds.  Excitons have heavier mass, about the same as an electron. Their lifetime can be controlled in our structures, so that in many cases they remain as long as 10 microseconds before turning back into a photon.

This project has focused on ways of controlling the motion of excitons and polaritons, trapping them in one place or sending them from one place to another. A long range goal is Bose-Einstein condensation of excitons and polaritons. This unique state, which was demonstrated in trapped atoms in 1995, earning the Nobel prize, can be called “spontaneous coherence.”  A large number of particles, in this case excitons or polaritons, can enter into a single wave state. Creating traps for the particles is an important first step.  

## Exciton and Polariton Research-- Broader Impact

- 	PI: D.W. Snoke, University of Pittsburgh
- 	NSF project DMR-0404912

- Long-Range Applications
- 	• An exciton/polariton BEC would produce
- 	_lasing without inversion_
- 	   *extremely low threshold lasers*
- 	• “Excitonic circuits”
- 	control of light in chips in a new way
*	** *

- Education
- 	• Two graduate students perform all the experiments
- 	*optics, lasers, cryogenics, computer interfacing, etc.*
- 	• Undergraduate Certificate in Photonics
- 	program at Pitt is also supported by this project,
- 	including undergraduate research experiences
- 	_www.phyast.pitt.edu/snoke/~photonics_

**Notes:**
An exciton or polariton BEC wouldbe a new type of coherent light source. It would emit coherent light similar to a laser, but unlike a laser there is no “inversion”- i.e., no need to pump a large number of electrons into excited states. This light source would make possible extremely small and efficient coherent light sources.  

In recent years, there has been great interest in controlling other degrees of freedom in circuits instead of just charge. “Spintronics” is the study of how to control the spin of the electrons.  This project gives us ways to control excitons and polaritons, in principle opening up the possibility of new types of “excitonic circuits” in which light energy is moved instead of charge.

This project is a terrific graduate education experience, since the graduate students are involved in every aspect of the work, learning a whole range of laboratory methods. We also bring undergraduate students into the lab for summer research projects, as part of the undergraduate photonics education program which was created 5 years ago with NSF support. This program is unique: students earn a Certificate in Photonics along with a physics, chemistry or electrical engineering degree, which gives them cutting edge education and experience in modern photonics.