1. Observation of ultrafast response by optical Kerr effect in high-quality CuCl thin films 2013.12.4 Asida Lab. Takayuki Umakoshi

2. Contents 1. Introduction ・ All-optical switching device ・ Exciton confinement effect ・ Calculation of radiative corrections ・ Motivation 2. Experiment ・ Sample （ CuCl ） ・ Growth of high-quality CuCl thin films ・ Previous work － Degenerate four-wave mixing － ・ My work － Optical Kerr effect － 3. Summary 4. Future plan

3. All-optical switching device Electronic router Light → Electrocity → Light Optical router Light → Light Need high-speed and power saving of information-communication Change of communication traffic and power consumption year Power consumption (TWh) 40% increase in a year Future Present

4. All-optical switching device ・ high efficiency and ・ high speed ＜ Requirement for all-optical switching device ＞ Non-linear optical response with Exciton resonance encourages high efficiency ※ General exciton lifetime : 100 ps ～ ns ←Slow Switching speed : 10 ps order Speeding up optical response by exciton confinement effect Exciton

5. Exciton confinement effect n = 4 n = 3 n = 2 n = 1 Nanostructure － Long wavelength approximation regime － Ref ： T. Itoh, M. Furumiya, T. Ikehara, and C. Gourdon, Solid State Commun. 73, 271 (1990). Radius dependence of exciton radiative decay time in CuCl quantum dots satuated The n=1 exciton dominantly interacts with light. Exciton radiative decay rate is enhanced with increase of system size, but satuated in larger region.

6. Exciton confinement effect n = 4 n = 3 n = 2 n = 1 Nanostructure － Long wavelength approximation regime － System where exciton wave functions are coherently extended to the whole volume The n=1 exciton dominantly interacts with light. Multinode-type excitons complicatedly interact with light. Exciton radiative decay rate is enhanced with increase of system size, but satuated in larger region. Size-dependent enhancement more than LWA regime is expected.

7. Calculation of Radiative Corrections ＜ Coupled-mode of CuCl exciton ＞ EigenenergyRadiative width (Γ n ) Ref ： H. Ishihara, J. Kishimoto and K. Sugihara, J. Lumin. 108, 343 (2004). 3.194 eV 24 meV PL Intensity n=3 E ・ Radiative corrections of each modes indicate complicated thickness dependence. ・ The modes with large radiative width can decay ultrafastly. 150 nm

8. Motivation Realizing non-linear optical with high efficiency and ultrafast using the exciton mode with large radiation width ・ Make high-quality thin films in the middle region of nanostructure and bulk crystal ・ Measure the speed and efficiency of non-linear response

9. Sample ( CuCl ) ＜ Features of CuCl ＞ ・ Simple band structure ・ Huge exciton binding energy (197 meV) ・ Strong non-linear optical effect Appropriate for basic research & Application possibility for optical devices k E Г6Г6 Г7Г7 Г8Г8 Valence band Conduction band Z 3 Exciton Z 1,2 Exciton

10. Growth of high-quality CuCl thin films Growth method: Substrate Screen RHEED Shutter Vacuum pump Crystal oscillator CaF 2 CuCl Shutter Molecular beam epitaxy (MBE) + Electron beam irradiation CaF 2 (111) substrate CaF 2 buffer layer CuCl layer ＜ Sample Structure ＞ Electron beam 1 mm 40 nm

11. Growth of high-quality CuCl thin films Surface morphology is extremely improved by electron beam irradiation to the sample earlier than growing CuCl. With electron beam irradiation CaF 2 (111) substrate CaF 2 buffer layer CuCl layer 1 mm 40 nm Without electron beam irradiation

12. Previous work － Degenerate four-wave mixing － ＜ Degenerate four-wave mixing （ DFWM ）：縮退四光波混合 ＞ Probe pulse Pump pulses Non-linear medium ：非線形媒 体 Transient grating (TG) TG signal ①Pump pulses → Transient grating (TG) is induced ②Probe pulse → Diffracted light (TG signal) is generated

13. DFWM Spectrum Particular peak structures appear, which have never been observed in samples grown by ordinary methods. The observed peaks are in good agreement with the calculated eigenenergies. TG signal ⇒ detect Ref: M. Ichimiya, M. Ashida, H. Yasuda, H. Ishihara, and T. Itoh, Phys. Rev. Lett. 103, 257401 (2009).

14. Radiative decay profile in DFWM Pump Probe TG signal ⇒ detect Delay time Exceptionally short radiative decay time of as low as 100 fs is observed. Measured decay profiles agree well with the calculated results. Ref: M. Ichimiya, M. Ashida, H. Yasuda, H. Ishihara, and T. Itoh, Phys. Rev. Lett. 103, 257401 (2009).

15. My work － Optical Kerr effect － ＜ Measurement of optical Kerr effect ＞ ・ Use in practical applications to the optical switch is active. ・ The both speed and efficiency of non-linear response can be evaluated. Advantage Non-linear medium Probe pulse Pump pulse x x xy y y Detect x-component by polarizer Polarizer θ

16. Spectrum of optical Kerr signal Similar to the spectrum of DFWM, shows unique peak structures, the each peaks are in good agreement with the eigenenergy value after radiation correction. Relative intensity of the peak as large mode of the radiation width is larger than DFWM spectrum. Larger width

17. Time delay dependence of optical Kerr response ・ Ultrafast optical Kerr response of 100 fs order has been observed in the modes with large radiative width. ・ Measured decay profiles agree well with the calculated results that takes into account the radiation width. Reflect exciton radiative decay

18. Summary We made high-quality CuCl thin films by new method based on molecular beam epitaxy. The peak energies of spectrum in optical Kerr signal has been confirmed to be in good agreement with the calculated results as well as spectrum in DFWM measurement. Ultrafast optical Kerr response of 100 fs order has been observed, and it reflects exciton radiative decay time.

19. Future plan Comparison with the time delay dependence of the optical Kerr response in CuCl bulk crystal To evaluate the non-linearity from Kerr rotation angle Higher crystalline quality and more accurate thickness control in the growth of CuCl thin films Enhancement of light-exciton interaction Experiments using a laser with ultrashort pulse duration Efficient excitation of the modes with huge radiative width