Ultrafast Spectroscopy of Quantum Dots QDs Ulrike Woggon

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Ultrafast Spectroscopy of Quantum Dots (QDs) Ulrike Woggon FB Physik, Universität Dortmund With thanks

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: • 1. Types of QDs and Techniques of Ultrafast Spectroscopy

Outline: • 2. Application Aspects: Dynamics of Amplification in QD-Lasers Monitoring of high-frequency optical

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“

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

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

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. 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

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 Ultrafast Spectroscopy Usually: J. Shah, Ultrafast Spectroscopy

Femtosecond Heterodyne FWM- and PP-Spectroscopy Usually: Here:

Femtosecond Heterodyne FWM- and PP-Spectroscopy Usually: Here:

150 fs 76 MHz AOM 1 laser 79 MHz delay AOM 2 80 MHz

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.

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

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

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 ~

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

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 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

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

„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,

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

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:

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>

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

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

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

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

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 =

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

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>

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

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

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