- Slides: 32
Ultrafast Spectroscopy of Quantum Dots (QDs) Ulrike Woggon FB Physik, Universität Dortmund With thanks to: M. V. Artemyev, P. Borri, W. Langbein, B. Möller, S. Schneider Fruitful cooperations: calculations: samples: R. Wannemacher, Leipzig, D. Bimberg and coworkers, Berlin D. Hommel and coworkers, Bremen A. Forchel and coworkers, Würzburg Experimentelle Physik IIb
Outline: • 1. Types of QDs and Techniques of Ultrafast Spectroscopy
Outline: • 2. Application Aspects: Dynamics of Amplification in QD-Lasers Monitoring of high-frequency optical operation in semiconductor nanostructures by ULTRAFAST SPECTROSCOPY predicted advantages of QD-lasers: • • low threshold current density high characteristic temperature high differential gain large spectral tunability, from NIR to UV D. Bimberg and coworkers, TU Berlin
Outline: • 3. Fundamental aspects: Semiconductor QDs as artificial atoms Monitoring of the „discrete-level“ - structure of semiconductor nanostructures by ULTRAFAST SPECTROSCOPY size y g r e n e L. Banyai, S. W. Koch, Semiconductor Quantum Dots
Part 1: Types of QDs and Techniques. . . Quantum Dots: Nanocrystals and epitaxially grown Islands Precipitation of spherical nanocrystals in colloidal solution or glass, polymer etc. matrix Lattice-mismatch induced island growth
Cd. Se QDs emitting in the visible (nanocrystals) Cd. Se in glass 5 nm
In. Ga. As self-assembled islands emitting in the NIR Calculated confined eh-pair energies for In. As assuming pyramidal shape D. Gerthsen et al. , Karlsruhe Grundmann, Bimberg et al. , TU Berlin
Part 1: Types of QDs and Techniques. . . Femtosecond Heterodyne Technique w 2 w 1 signal Dt probe pump waveguide w 2 probe 2 w 2 -w 1 four-wave mixing (FWM) Ti: Sa + OPO, 80 fs. . . 2 ps
Femtosecond Ultrafast Spectroscopy Usually: J. Shah, Ultrafast Spectroscopy
Femtosecond Heterodyne FWM- and PP-Spectroscopy Usually: Here:
150 fs 76 MHz AOM 1 laser 79 MHz delay AOM 2 80 MHz probe beam AOM- reference beam 76 MHz Acousto-Optical Modulator FWM probe pump beam sample 3 MHz Idetµ erefesignal e= electric field 4 MHz _ 2 MHz HF-Lock-in delay +
Part 2: Applied aspects: QD-laser. . . Gain Dynamics in Quantum Dots In. As/In. Ga. As QDs 3 x stack, 20 nm Ga. As barrier
Gain Dynamics of In. Ga. As QDs ridge waveguide 5 x 500 mm, 3 stacked QD layers areal dot density ~2 x 1010 cm-2 optical density ~ 1. 5 (a~30 cm-1) 20 m. A 0. 5 m. A Ground State Emission (GS): 1070 nm @ 25 K, 1170 nm @ 300 K Carrier injection electrically (0. . . 20 m. A) Sample from TU Berlin, Prof. Bimberg
Gain Dynamics of In. Ga. As QDs Pump-induced gain change in a heterodyne pumpprobe experiment at maximum gain (20 m. A) and without electrical injection (0 m. A) Gain recovery in < 100 fs at 300 K ! P. Borri et al. , J. Sel. Topics Q. El. 6, p. 544 (2000); Appl. Phys. Lett. 76, p. 1380 (2000).
Gain Dynamics of Cd. Se QDs Cd. Se nanocrystals in glass matrix R ~ 2. 5 nm two . . . (4) (1), (2) Ground State Emission (1): 605 nm @ 6 K Woggon et al. , Phys. Rev. B 54, 17681 (1996), J. Lum. 70, 269 (1996). . . pairs 1 pe 1 ph 1 se 1 sh 2 se 1 sh 1 se 1 sh 1 pe 1 ph 1 se 2 sh 1 se 1 sh (3) one pair
Gain Dynamics of Cd. Se QDs Excitonic and biexcitonic contributions to optical gain Gain recovery time spectrally varying, <1. . . 100 ps Optics Lett. 21, 1043 (1996).
Gain Dynamics of Cd. Se QDs Gain spectrum inhomogeneously broadened: Spectral hole burning in gain spectrum with two fs-pump and one fs-probe beam Spectral hole width of a single gain process ~20 me. V Intrinsic limit of gain recovery below 100 fs ! Chem. Phys. 210, 71 (1996)
Part 2: Applied aspects: QD-laser. . . Quantum dots as active media in optical microcavities Cd. Se QDs linked to microspheres 5 mm Picture: M. V. Artemyev, I. Nabiev
„Dot - in - a - Dot“ - Structure l=619. 22 nm Cd. Se nanodot R=2. 2 nm Glass microsphere R=2. 77 mm R=3. 1 mm Artemyev et al. , APL 78, p. 1032 (2001), Nano Lett. 1, 309 (2001). R=2. 5 mm
Cavity Modes of a Cd. Se-doped Microsphere WGM RPD = 2. 5 mm TM, l=36, n=1 TM, l=36, n=2 Nano Lett. 1, p. 309 (2001), Appl. Phys. Lett. 80, p. 3253 (2002) RQD = 2. 5 nm
Optical Pumping of a Cd. Se-doped Microsphere RPD ~ 15 mm cw-Ar laser, 488 nm Excitation spot size 40 mm 2 T = 300 K 520 nm < lem < 640 nm 10 m. W Cd. Se nanocrystals (not on microsphere) 14 m. W See also: Artemyev, Woggon et al. Nano Letters 1, 309 (2001)
Part 3: Fundamental aspects: Artficial atoms. . . Rabi Oscillations in Quantum Dots Bloch-sphere: population oscillation
Rabi-Oscillations in Atoms Simple model: two coupled oscillators w. R. . . |e> |g> |3> |2> |1> |0> . . . |e> |g> photon field atom states |3> |2> |1> |0> Rabi frequency Two-level system in resonance with photon field Eb Ea E 0 : electromagn. field vector E 0 = hw 0 = Eb - Ea hw: 0 transition energy m : transition dipole moment
Rabi Oscillations versus Pulse Area Here pulsed excitation ! Occupation probability of the ground (excited) state Population oscillation blue = -1 red = +1 Initial conditions: for t << -t 0 in ground state No dephasing! detuning (me. V) Pulse area: time-integrated Rabi frequency (~ input field intensity) 0 2 4 6 pulse area (p) 8 10
Effect of Dephasing T 2 on Rabi oscillations The effect of a damping g =1/T 2 of polarization: w=w 0 Population flopping over many periods is possible in systems with long dephasing times and large transition dipole moments: g / w. R<<1.
Dephasing time T 2 of In. Ga. As quantum dots From 300 K to 100 K the FWM decay is dominated by a short dephasing time < 1 ps Below T=10 K a slow dephasing time > 500 ps is observed (suppression of LO-phonon scattering!) In. Ga. As - QDs Is the observed dephasing time T 2 large enough to observe population flopping, i. e. Rabioscillation in QDs ? ? ? P. Borri et al. , Phys. Rev. Lett. 87, 157401 (2001)
Rabi Oscillations in In. Ga. As Quantum Dots Experiment Use of spectrally shaped ps-pulses a sharpened distribution of the spectral intensity improves the visibility of the oscillations. Rabi oscillation: two oscillation maxima can be clearly distinguished Borri et al. , Phys. Rev. B (Rapid Comm. ), in press
Distribution in Transition Dipole Moments m in average m = 35 D s = 20% Borri et al. , Phys. Rev. B (Rapid Comm. ), in press
Part 3: Fundamental aspects: Artficial atoms. . . Quantum Beats in Quantum Dots Discrete Level-System |2> |1> |0> DE DE can be derived from beat period
Exciton-Biexciton Quantum Beats in QDs uncorr. electron and hole |1 e, 1 h> |x> Coulomb interaction |G> |G> Quantum Beats between two optical transitions: |G> |xx> |x> with EX |xx> with EXX EX - EXX = Ebin (biexciton binding) exciton |x> biexciton |xx> formation |2 x> |x+> Ebin s- s+ s+ s- |G> |x_>
Exciton-Biexciton Quantum Beats in QDs Determination of biexciton binding energy in Cd. Se/Zn. Se QDs by femtosecond quantum beat spectroscopy Biexciton binding energy DE = 21 me. V Gindele, Woggon et al. , Phys. Rev. B 60, p. 8773 (1999).
2 mm Summary Cd. Se QDs in microspheres ® In. Ga. As QDs in waveguides Types of Quantum Dots and Techniques of Ultrafast Spectroscopy ® Application Aspects: Dynamics of Amplification in Quantum Dot Lasers ® Fundamental aspects: Semiconductor Quantum Dots as Artficial Atoms