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

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

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

Quantum dot A quantum dot is a nanometer-sized semiconductor. It consists of 10 3 ~10 6 atoms. Quantum effects appear due to three dimensionally confined excitons. The energy levels are discrete. E E E DOS bulk Quantum well Quantum dotQuantum wire bulk well wire dot E DOS (density of state): 状態密度

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

Quantum size effect size energy Weak-confinementStrong-confinement The energy of exciton in quantum dot E g : the band gap energy G x : the bonding energy of exciton m e : the mass of electron : m h : the mass of hole r: the quantum dot radius a ex : the exciton radius Higher energy than excitons in bulk The energy depends on dot size.

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

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

The absorption spectra (a) The absorption spectrum The absorption spectrum at 10 ps after excitation E XX’ 2E X E XX EXEX G Absorption change spectra with various excitation energies (b) The absorption saturation at the excitation energy >

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) E XX’ 2E X E XX EXEX G The excited biexciton state come from the spatial confinement

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

Application This unique propertyNew optical devices For example ON OFF Optical switch 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 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 biexicton ground excited exciton pump probe the excited states of excitons and biexcitons 1s state 2p state (1s,2p) state (1s,1s) state excitonbiexciton E X’ : excited exciton exciton

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. S/N ratio was not sufficient. There is not enough number of experimental data. problems

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

Photo luminescence spectrum Excitation energy 3.492eV (355nm) Excitation Intensity 1.41mJ/cm 2 Many excitons and biexcitons are generated in quantum dots. M Exciton :PL of exciton :PL of biexciton

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

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

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.