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

Observation of ultrafast response by optical Kerr effect in high-quality CuCl thin films. 2013.12.4 Asida Lab. Takayuki Umakoshi. Contents. 1. Introduction ・ All-optical switching device ・ Exciton confinement effect ・ Calculation of r adiative corrections  ・ Motivation

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

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  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

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