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Eitan Ehrenfreund:

Physics Department

e-mail: eitane@physics.technion.ac.il

Fax: (+972)(0)4 823-5104

Phone: (+972)(0)4 829-3610, 829-3630(secretary)


Research activities

Revised: January 2002

General subjects: A. Electro-optics and photoexcitations of conjugated polymers

B. Photo-excitations in semiconducting quantum wells and dots

Detailed description:

A.1 Polaron photo-generation in conjugated polymers studied by pico-second time resolved photo-induced absorption of infrared active vibrations.

The photogeneration of charges in conjugated polymers is studied by the novel pico-second time resolved UV-visible pump mid-infrared probe technique. The photoinduced infrared active vibrations (IRAV) of the polymer chains are used as a direct indication for the photogenerated charges. With a spectral resolution of 1 meV and time resolution of 5 psec, we are able to identify the nature of the photo-generated charges and to follow their spectral evolution.

A.2 Use of acid-sensitive bipyridine moieties as optical tuning agents in conjugated polymers.

New electro-luminescent, PPV-like, conjugated polymers containing bipyridylene- vinylene subunits were prepared and characterized with respect to their electro- optical properties. The polymers were found to exhibit reversible and tunabla optical properties depending on protonation-deprotonation processes. pH sensitivity and reversible tunability of the polymers were observed by photo- and electro- luminescence, optical absorption, photo-induced absorption and electro-absorption measurements. A luminescence red shift of 200 meV was observed upon full protonation of a free-base film.

We have also found that the photoexcitations follow a “dispersive” relaxatio process, resembling dispersive transport in amorphous materials.

A.3 Photo-induced charge separation, photo-conductivity and photo-voltaics of conjugated polymers with pendant electron-acceptor groups.

The application of conjugated polymers having viologen units pendant groups, as active photo-voltaic and photo-conducting media is investigated. Photo-induced charge species are characterized using photo-induced absorption techniques and photo- conductivity. The ability of using such media as active layers in photo-voltaic cells is demonstrated.

A.4 n-doping of poly(thiophene-pyridine) derivatives.

We have successfully characterized either chemically n-doping and/or negatively charged photogenerated species in these polymers.

B.1 Photoinduced intersubband absorption in semiconducting quantum wells: spectroscopy and recombination mechanisms.

Photoinduced intersubband absorption provides a direct measure of the carrier population (electrons, heavy and light holes or excitons) in the optically excited state, under a variety of external conditions, such as temperature, pump intensity, modulation frequency and wavelength and applied electric fields.



B.2 Photoinduced intersubband absorption in semiconducting quantum wells:

pico-second time resolved dynamics of two dimensional excitons.

Time resolved pico-second photoinduced absorption has been applied in order to study the dynamics of carriers in AlGaAs/GaAs superlattices. We studied the time evolution of the absorption line shape and found that resonantly excited excitons reach thermal equilibrium on a time scale of few psec. This time scale is inversely proportional to the density of excitons. Non-resonant excitation results in a much faster time scale. These results show that the momentum redistribution times for resonantly excited heavy-hole excitons are relatively long: few psec. This relatively long time is due to multiple exciton-exciton scattering events needed to redistribute the small exciton momentum over the entire Brillouine zone.

B.3 Multi-excitons in III-V semiconducting quantum dots: carrier-carrier correlations.

The photoluminescence from SINGLE dots is studied using low temperature confocal optical microscopy. It is demonstrated that a single dot emits light in a few very narrow spectral lines, which are the optical transitions between confined EXCITON multiplexes. The electron-electron and hole-hole exchange interactions give rise to the appearance of pairs in the emission spectrum; the pair splitting is a measure for the strength of the exchange interaction.

Recent publications:

Quantum dots:

“Multiexciton spectroscopy of a single self-assembled quantum dot” : Phys. Rev. Letters, 80, 4991-4994 (1998); Proc. of ICPS24, Jerusalem, World Scientific, Ed. D. Gershoni, Ch. VII.B.8 (1999).

“Carrier-carrier correlations in an optically excited single semiconductor quantum dot”: Phys. Rev. B61, 11009-20 (2000), B62, 11038-45 (2000).

“Radiative lifetimes of single excitons in quantum dots”, Solid Stat Communications, 117, 395-400 (2001).

“Artificial atoms and molecules of charge carriers in positively and negatively charged quantum dots”, Phys. Rev. B64, 165301 (1-7) (2001), Physica E (2001), Physica Status Solidi (2001).

“Quantum dots: a quantum light source f multicolor photons with tunable statistics”, Phys. Rev. Lett. 87, 257401 (2001).

Time resolved intersubband absorption

“Momentum redistribution times of resonantly photogenerated 2d excitons”: Phys. Rev. Letters, 78, 3919-3922 (1997); Physica E, 2, 65-69 (1998);

“Intersubband transitions…”, Ed. Li and Su, Kluwer, pp. 187-192 (1998) ); Proc. of ICPS24, Jerusalem, World Scientific, Ed. D. Gershoni, Ch. IV.F.4 (1999).

“Carrier dynamics of semiconductor lasers studied under resonant pumping”, Physica E, 7, 237-240 (2000), and Appl. Phys. Letters, 76, 2988-90 (2000).

“Intersubband resonance in a quantum well with electron-hole plasma”, Phys Rev B65, 035310 (2002).

“Temporal evolution of the excitonic distribution in GaAs/AlGaAs superlattices”,

Phys Rev B65, (2002).

Intersubband absorption in quantum wells

“Recombination dynamics in SiGe/Si quantum wells”: Phys. Rev. B, 56, 15734-39 (1997); Physica E, 2, 777-780 (1998).

“Electro-absorption spectroscopy of intersubband transitions in multi-quantum-well superlattices”, Proc. ICPS24, Jerusalem, World Scientific, Ed. D. Gershoni, Ch. IV.I.5 (1999) and Phys. Rev. B, 61 , 10972-10977 (2000).

Electro-optic polymers

“psec time resolved IRAV studies”: Synthetic Metals, 102, 1182-1185 (1999), 119, 507-510 (2001).

“Protonation of bipyridine/PPV polymers”: J. Am. Chem. Soc., 120, 10463-70 (1998); Synthetic Metals, 101, 269-270 (1999).

“Energy transfer from photoexcited conjugated oligomer to rare earth ions (Eu3+) embeded in the matrix”: Synthetic Metals, 101, 240-241 (1999).

“Di-substituted polyacetylene”: Phys. Rev. Letters, 82, 4058-4061 (1999), Synthetic Metals, 116, 91-94 (2001).

“ODMR in 2d-3d polymer systems”: Chem. Phys. Letters, 300, 626-632 (1999).

“Perylene – optical studies”: J. Phys. Chem. B, 102, 967-973 (1998).

“Poly-pyrrol/viologen systems”: J. Phys. Chem. B, 104, 770-774 (2000).

“Dispersive relaxation”: Synthetic Metals, 119, 585-586 (2001); Phys. Rev. B, 63, 125206 (1-6) (2001).

“Photocurrent response of bipyridine/PPV polymers”, J. Phys. Chem. B, 105, 7671-7677 (2001), Synthetic Metals 121, 1559 (2001).

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