Itoh Lab. Hiroaki SAWADA

1. Observation of Excited Biexciton States in CuCl Quantum Dots : Control of the Quantum Dot Energy by a Photon (references) Michio IKEZAWA and Yasuaki MASUMOTO Jpn. J. Appl. Phys. Vol. 36 (1997) pp. 4191-4193 Itoh Lab. Hiroaki SAWADA

2. Contents • Introduction Exciton and biexciton Quantum-dot • Experiments • Summary • My work

3. Exciton and biexciton exciton electron An electron-hole pair combined by the Coulomb force hole Discrete energy levels like those of the hydrogen atom The distance of electron-hole is called the bohr radius (aB). biexciton A bound two exciton state Exciton: 励起子 Biexciton: 励起子分子

4. Quantum dot • A quantum dot is a nanometer-sized semiconductor. It consists of 103~106 atoms. • Quantum effects appear due to three dimensionally confined excitons. • The energy levels are discrete. DOS bulk E DOS well E DOS wire E Quantum well bulk DOS dot E Quantum wire Quantum dot DOS(density of state): 状態密度

5. Quantum confinement effect Consider the effect on an exciton in a spherical dot. Weak-confinement Strong-confinement aB≪a aB：Bohr radius a:dot radius aB≫a CdS QD and CdSe QD etc. excited state CuCl QD in NaCl matrix etc. lowest state excited state lowest state lowest state electron excited state hole 2a 2a Center-of-mass motion is confined. Motions of electron and hole are confined individually.

6. Quantum size effect The energy of exciton in quantum dot Higher energy than excitons in bulk The energy depends on dot size. Weak-confinement Strong-confinement Eg: the band gap energy Gx: the bonding energy of exciton me: the mass of electron: mh: the mass of hole r: the quantum dot radius aex: the exciton radius size energy

7. NaCl CuCl Purpose EXX’ Biexciton 2EX EXX nonlinear optical responses EX To date strong-confinement case pump G CdS and CdSe quantum dots G: ground state EX: exciton state EXX: biexciton state 2EX: two exciton EXX’:excited biexciton state This report weak-confinement case CuCl quantum dots in NaCl J=2 EXX’:excited biexciton state two exciton Rydberg 1s state J=0 EXX: biexciton state Subpicosecond pump probe spectroscopy

8. Experimental setup Ti sapphire laser Ti sapphire regenerative amplifier CuCl QDs in NaCl T = 77K SHG crystal Pump beam CCD Pulse duration: 1.2ps FWHM: 1.7meV apparatus Pulse duration: 300fs Repetition rate: 1kHz Energy: 200μJ/pulse cryostat Probe beam White continuum Pure water optical delay

9. EXX’ 2EX EXX EX > G The absorption spectra (a) The absorption spectrum The absorption spectrum at 10 ps after excitation Absorption change spectra with various excitation energies (b) The absorption saturation at the excitation energy

10. EXX’ 2EX EXX EX G Excitation energy dependence The slope : 2.0 :the transition from the exciton state to excited biexciton Crossing at the exciton energy of bulk CuCl (3.218 eV) The excited biexciton state come from the spatial confinement

11. fast decay component the creation of biexciton by two excitons : main decay component : 480 ps same : 480 ps exciton radiative lifetime Temporal evolution of absorption change peaks Excitation photon energy : 3.269 eV

12. Application This unique property New optical devices For example Optical switch ON OFF ON/OFF control with a single photon

13. Summary • The transient absorption change of CuCl quantum dots embedded in a NaCl crystal was measured by means of the pump-probe method. • Strong induced absorption was observed at higher energy side of the exciton resonance. • It is attributed to the transition from the exciton to the excited biexciton state and it depends on the excitation photon energy. • The result of the temporal evolution of the transient absorption supported the identification of the excited biexciton states.

14. About my work the excited states of excitons and biexcitons Pump-probe spectroscopy with tunable picosecond pulse laser CuCl quantum dots in NaCl (avarage dot radius ~4nm）tranverse Bridgman method PL spectrum Temporal profile of transient absorption Transient absorption spectrum excited biexciton two excitons 2p state probe biexicton pump excited exciton (1s,2p) state probe exciton (1s,1s) state pump pump 1s state ground EX’: excited exciton exciton biexciton

15. Previous work Temporal profile of transient absorption Transient absorption spectrum Infrared transient absorption of CuCl quantum dots has two decay component. Exciton and biexciton, respectively. problems S/N ratio was not sufficient. There is not enough number of experimental data.

16. Monochro- mator Photo diode MCT Experimental setup ･ Detection of the signal shot by shot by a box car integrators ･Improvement of spectral resolution （closely shifting the wavelength of probe beam） Boxcar AD Converter ~2 ps, 1 kHz Boxcar signal O P A O P A PC Pump pulse FHG 355nm Probe pulse DFG 2.6μm ~ 8μm Chopper 500Hz cryostat Long Pass Filter ND Filter CuCl QDs in NaCl T = 70K optical delay

17. :PL of exciton :PL of biexciton Photo luminescence spectrum Exciton Excitation energy 3.492eV (355nm) M Many excitons and biexcitons are generated in quantum dots. Excitation Intensity1.41mJ/cm2

18. Temporal profile of transient absorption Probe photon energy of 309 meV (4000 nm) Two decay components Fast decay : 50 ps Biexciton Slow decay： 800 ps Exciton

19. Transient absorption spectrum Absorption peak energies • biexciton : 340 meV • exciton 　： 320 meV Transition energy for the biexciton is higher than that for the single exciton.

20. Summary • We measured the infrared transient absorption of CuCl quantum dots embedded in a NaCl crystal. • The absorption peaks are derived from the transition of Rydberg 1s-2p state for exciton and the transition to the excited-states of biexciton composing of 1s and 2p excitons for biexciton. • Transition energy of biexciton is higher than that of the exciton. In the near future • The dependence of dot size of the excited states of the biexcitons and excitons will be presented under the size-selective excitation by two photon excitation.