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 Contents • Introduction Exciton and biexciton Quantum-dot • Experiments • Summary • My work Exciton and biexciton exciton An electron-hole pair combined by the Coulomb force Discrete energy levels like those of the hydrogen atom electron hole 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 Quantum dot DOS DOS DOS wire dot Quantum wire well bulk DOS bulk • 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. Quantum well Quantum dot E E E E DOS (density of state): 状態密度 Quantum confinement effect Consider the effect on an exciton in a spherical dot. Weak-confinement aB≪a Strong-confinement aB≫a aB：Bohr radius CdS QD and CdSe QD etc. a:dot radius excited state lowest state CuCl QD in NaCl matrix etc. excited state lowest state electron hole 2a lowest state excited state 2a Center-of-mass motion is confined. Motions of electron and hole are confined individually. Quantum size effect The energy of exciton in quantum dot Higher energy than excitons in bulk The energy depends on dot size. Weak-confinement 2 E (r ) Eg Gx 2M Strong-confinement * r 2 M me mh r * r - a ex ( 1) 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 2 e2 E (r ) Eg * 1.786 0.248Gx 2 r r 2 1 1 1 me m h size energy Purpose EXX’ 2EX EXX Biexciton nonlinear optical responses EX To date strong-confinement case pump G CdS and CdSe quantum dots This report weak-confinement case CuCl quantum dots in NaCl G: ground state EX: exciton state EXX: biexciton state 2EX: two exciton EXX’:excited biexciton state J=2 EXX’:excited biexciton state two exciton Rydberg 1s state NaCl J=0 EXX: biexciton state Subpicosecond pump probe spectroscopy CuCl Experimental setup Ti sapphire laser Ti sapphire regenerative amplifier CuCl QDs in NaCl SHG crystal Pump beam T = 77K CCD Pulse duration: 1.2ps FWHM: 1.7meV Pulse duration: 300fs Repetition rate: 1kHz Energy: 200μJ/pulse optical delay apparatus cryostat Pure water Probe beam White continuum The absorption spectra (a) The absorption spectrum The absorption spectrum at 10 ps after excitation (b) Absorption change spectra with various excitation energies The absorption saturation at the excitation energy > 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) EXX’ 2EX EXX EX The excited biexciton state come from the spatial confinement G Temporal evolution of absorption change peaks Excitation photon energy : 3.269 eV main decay component : 480 ps same : 480 ps exciton radiative lifetime fast decay component : the creation of biexciton by two excitons Application This unique property New optical devices For example Optical switch ON OFF ON/OFF control with a single photon 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. About my work the excited states of excitons and biexcitons Pump-probe spectroscopy with tunable picosecond pulse laser PL spectrum Temporal profile of transient absorption Transient absorption spectrum 2p state excited exciton probe CuCl quantum dots in NaCl (avarage dot radius ~4nm） tranverse Bridgman method excited biexciton probetwo excitons biexicton pump (1s,2p) state exciton 1s state pump (1s,1s) state pump ground exciton biexciton EX’: excited exciton Previous work Temporal profile of transient absorption 0.07 Transient absorption spectrum 0.15 Probe 250meV (4.9m) biexciton exciton 0.06 O.D. 0.04 0.03 O.D. experiment fitting curve fast decay ~56ps slow decay ~490ps 0.05 0.10 0.05 0.02 0.01 0.00 0 100 200 300 400 Time (ps) 500 600 700 0.00 100 200 300 400 500 Photon Energy (meV) 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. 600 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 Photo diode ~2 ps, 1 kHz Boxcar AD Converter signal O P A Pump pulse FHG O P A 355nm Chopper 500Hz ND Filter MCT Monochromator PC Probe pulse DFG 2.6μm ~ 8μm cryostat Long Pass Filter CuCl QDs in NaCl optical delay T = 70K Photo luminescence spectrum Excitation energy PL Intensity Exciton 3.492eV (355nm) :PL of exciton M 3.15 3.20 3.25 Photon Energy (eV) Excitation Intensity 1.41mJ/cm2 :PL of biexciton 3.30 Many excitons and biexcitons are generated in quantum dots. Temporal profile of transient absorption Probe photon energy of 309 meV (4000 nm) Two decay components 0.20 experiment fitting curve fast decay 50 ps slow decay 800 ps Δαl 0.15 0.10 Fast decay : 50 ps Biexciton Slow decay： 800 ps 0.05 Exciton 0.00 -200 0 200 400 600 Delay Time (ps) 800 Transient absorption spectrum biexciton exciton Absorption peak energies Δαl (a.u.) •biexciton : 340 meV •exciton ： 320 meV Transition energy for the biexciton is higher than that for the single exciton. 0.000 150 200 250 300 350 400 450 500 Photon Energy (meV) 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 sizeselective excitation by two photon excitation.
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