Outline

1. Looking Inside Hidden Excitons with THzRadiationTim GfroererDavidson CollegeSupported 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. E E2,K Free states E1,K Eg Bound states K Reduced effective mass Exciton Hamiltonian: Reduced effective mass: Energy levels:

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

6. E E2p,K Free states E1s,K Eg Bound states Transitions: Optical Phonon THz K Optical vs. THz Transitions

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