Masayoshi Ichimiya , Masaaki Ashida , Hideki Yasuda , Hajime Ishihara, and Tadashi Itoh

1. Observation of Superradianceby Nonlocal Wave Coupling of Light and Excitonsin CuCl Thin Films Masayoshi Ichimiya, Masaaki Ashida, Hideki Yasuda, Hajime Ishihara, and Tadashi Itoh PHYSICAL REVIEW LETTERS 103, 257401 (2009) Ashida Lab. Umakoshi Takayuki

2. Introduction – Confinement of an Exciton – ＜In Quantum Well＞ Conduction band -e electron Light hole Valence band + e Exciton Exciton Energy states of an excitonare quantized, and the wave functions are coherently extended.

3. Introduction – Light-Exciton Coupling – System where exciton wave functions are coherently extended to the whole volume Nanostructure －Long wavelength approximation regime－ n = 4 n = 3 n = 2 n = 1 The n=1 exciton is dominantly coupled with light. Multinode-type excitons are coupled with light. Oscillator strength increases with the system size. Size-dependent increase is not limited. ※satuated in larger region

4. Introduction – Calculation of Radiative Corrections – Ref：H. Ishihara, J. Kishimoto and K. Sugihara, J. Lumin. 108, 343 (2004). ＜Eigenmode of the CuCl Z3Exciton＞ Inverse proportion to Radiative decay time Eigenenergy Radiative width Radiative corrections of each modes indicate complicated thickness dependence.

5. Introduction – About CuCl – ＜Feature＞ ・Simple band structure ・Huge exciton binding energy(197 meV) ・Strong non-linear optical effect E Г6 Conduction band Z3Exciton Z1,2Exciton Г7 Г8 Valence band k Appropriate for basic research & Application possibility foroptical devices

6. Introduction – Degenerate Four-Wave Mixing– ＜Degenerate Four-Wave Mixing（DFWM）：縮退四光波混合＞ Transient grating (TG) Probe pulse Pump pulses TG signal Non-linear medium：非線形媒体 All-optical switching device Application Faster and lower-power information processing

7. Motivation ＜Requirementfor All-Optical Switching Device ＞ ・High efficient nonlinear optical response ・Ultrafast radiative decay agenda ・Make high-quality CuCl thin films in non-LWA regime ・Measure radiative decay profiles by DFWM

8. Results – Growth of CuCl Thin Films – Growth method: molecular beam epitaxy (MBE) ＜Sample Structure＞ AFM Image of CuCl Thin Film electron beam CuCl CaF2 buffer layer 40 nm 1 mm CaF2 (111) substrate Surface morphology is extremely improved by electron beam irradiation to the sample earlier than growing CuCl.

9. Results– Two-Pulse DFWM Spectrum(1) – ＜DFWM Spectrum for Thickness of 261 nm＞ TG signal⇒ detect • Particular peak structures appear. • The observed peaks are in good agreement with the calculated eigenenergies. • Spectral shape reflects calculated induced polarization spectra.

10. Results – Two-Pulse DFWM Spectrum(2) – ＜DFWM Spectrum for Thickness of 190 nm＞ TG signal⇒ detect • The measured peak photon energies change with film thickness. • The experimental and calculated results coincide for every thickness. Expectation for ultrafast radiation decay based on large radiative width

11. Results – Measurement of Radiative Decay – ＜Radiative decay profiles＞ TG signal⇒ detect Delay time • Exceptionally short radiative decay time of as low as 100 fs is observed. • Much faster than the dephasing • Clear beat structures in the decay curves reveal the high excitonic coherence. • Measured decay profiles agree well with the calculated results. Probe Pump

12. Summary • We made high-quality CuCl thin films by new method. • We have experimentally elucidated the systematic level scheme of the coupled system of photons and multinode-type excitons for thicknesses above hundreds of nanometers. • We have successfully verified that the harmonized wave-wave coupling yields exceptionally high-speed superradiance that is much faster than the normal dephasing process.

13. Future Plan (My Work) • Higher crystalline quality and more accurate thickness control in the growth of CuCl thin films Improvement of the wave-wave coupling strength • Experiments using a laser with ultrashort pulse duration (the order of 10 fs) Efficient excitation of the excitonic state with huge radiative width by the broad spectral width of laser