Observation of ultrafast response by optical Kerr effect in high-quality CuCl thin films

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

4. All-optical switching device ＜Requirement for all-optical switching device＞ ・high efficiency and ・high speed Non-linear optical response with Exciton 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

5. Exciton confinement effect • Nanostructure • －Long wavelength approximation regime－ Radius dependence of exciton radiative decay time in CuCl quantum dots n = 4 n = 3 n = 2 n = 1 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. Ref：T. Itoh, M. Furumiya, T. Ikehara, and C. Gourdon, Solid State Commun. 73, 271 (1990).

6. Exciton confinement effect • 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 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 enhancementmore than LWA regime is expected.

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

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 ・Measurethe 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 E Г6 Conduction band Z3 Exciton Z1,2 Exciton Г7 Г8 Valence band k Appropriate for basic research & Application possibility foroptical devices

10. RHEED Growth of high-quality CuCl thin films Molecular beam epitaxy (MBE) + Electron beam irradiation Growth method: Vacuum pump Substrate Screen ＜Sample Structure＞ Crystal oscillator Electron beam Shutter Shutter CuCl layer 40 nm CaF2 buffer layer CaF2 CuCl 1 mm CaF2 (111) substrate

11. Growth of high-quality CuCl thin films Without electron beam irradiation With electron beam irradiation CuCl layer 40 nm CaF2 buffer layer 1 mm CaF2 (111) substrate Surface morphology is extremely improved by electron beam irradiation to the sample earlier than growing CuCl.

12. Previous work －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 ①Pump pulses →　Transient grating (TG) is induced ②Probe pulse → Diffracted light (TG signal) is generated

13. DFWM Spectrum Ref: M. Ichimiya, M. Ashida, H. Yasuda, H. Ishihara, and T. Itoh, Phys. Rev. Lett. 103, 257401 (2009). TG signal⇒ detect • 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.

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

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

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 fsorder 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