1. Looking Inside Hidden Excitons with THz Radiation Tim Gfroerer Davidson College Supported by the American Chemical Society – Petroleum Research Fund

2. Outline Motivation: Excitons Using THz radiation –Beyond broadening –Internal transitions Proposed Experiment: –Optically-detected THz resonance (ODTR) Application Example: GaAs:N

3. What are excitons? Hydrogen-like electron-hole pairs bound by the Coulomb interaction. + -

4. Reduced effective mass Exciton Hamiltonian: Reduced effective mass: Energy levels: K E E 1,K E 2,K EgEg Free states Bound states

5. Reduced dimensionality in quantum wells Quantum confinement squeezes the exciton (even in the plane of the quantum well), increasing the binding energy. Conduction band Valence band Barrier Well Barrier + -

6. Optical vs. THz Transitions K E E 1s,K Free states Bound states EgEg Transitions: Optical Phonon THz E 2p,K

7. Proposed Experiment: ODTR

8. Typical ODTR Traces M. S. Salib et al., PRL 77, 1135 (1996).

9. Application Example: GaAs:N Original interest: 1eV gap for quad junction solar cell Caveat: multitude of localized states Potential still exists for broadband light-emitting devices

10. [N]-dependent spectra Yong Zhang et al., Phys. Stat. Sol. B 240, 396 (2003).

11. Electron Effective Mass Y. Zhang et al., PRB 61, 7479 (2000). E.D. Jones et al., PRB 62, 7144 (2000). P.N. Hai et al., APL 77, 1843 (2000).

12. Conclusions Excitons are critical in modern opto-electronic devices –Reduced dimensionality increases binding energy so excitons exist at room temperature THz is an ideal excitonic probe –Sees through problematic inhomogeneous broadening –Induces internal transitions not accessible in ordinary PL THz studies of GaAs:N may elucidate this important alloy