Ariel Kigel

Nanocrystals of PbSe core, PbSe/PbS, and PbSe/PbSe x S 1-x core/shell as saturable absorbers in passively Q-switched near-infrared lasers

The saturable optical absorption properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbScore/shell and PbSe/PbSexS(1-x) core/alloyed-shell NCs, were examined at lambda = 1.54 microm. Saturation intensities of approximately 100 MW/cm2 were obtained. The NCs act as passive Q switches in near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of 40-55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carried out according to a model that includes ground-state as well as excited-state absorption. For pulses approximately 10 ns long, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption cross section of 10-16-10-15 cm2, an excited-state absorption cross section of sigma(es) is congruent to 10(-16) cm2, and an effective lifetime of tau(eff) is congruent to 5 x 10(-12) s.

Optoelectronic properties of polymer-nanocrystal composites active at near-infrared wavelengths

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared IR emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs/ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high 66% energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology nanocrystal distribution control.

IV-VI semiconductor nanocrystals for passive Q-switch in IR

The use of semiconductor nanocrystals as a passive Q-switch in an eye-safe laser system is demonstrated. These lasers recently became popular in laser radar, three-dimensional scanning, targeting, and communication applications. Such applications require the laser to operate under Q-switching, generating a laser pulse with duration on the order of tens of nanoseconds, and a peak power on the order of a megawatt. Semiconductor nanocrystals exhibit unique physical properties, associated with the quantum size effect. The PbS and PbSe nanocrystals show a size-tunable absorption resonance in the near IR spectral region (1000-3000 nm), saturable absorbing properties, suitable as a functional Q-switch in eye-safe lasers. The quantum confinement effect and the saturable absorption can be manifested only in high quality nanocrystals with a narrow size distribution and passivated surfaces. Thus, a special synthetic procedures have been used for the preparation of PbSe core, PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystals. Then, a passively Q-switched Er:glass laser has been assembled, while the laser output energy, Q-switch threshold energies, and pulse width have been measured.

Emission processes in colloidal PbSe/PbS core-shell quantum dots

The present study describes a thorough investigation of the ground-state exciton emission of PbSe/PbS core-shell colloidal quantum dots (CQDs) with different shell-thickness/core-radius ratio, by recording the photoluminescence (PL) spectra and the corresponding emission decay processes over a temperature range from 1.4 K to 300 K. These spectra were compared with that of the corresponding PbSe core CQDs. All PL spectra showed variation in the emission intensity and peak energy, including two distinct abrupt transitions, upon the increase of the temperature. However, the transitions appeared at different temperatures in the core-shell structures with respect to that of the core. Further on, the PL-decay processes in the core materials showed a single exponent behavior, while the emission of the core-shell CQDs, decayed as a stretched exponent, with an effective lifetime of ~1 μs, at room temperature, slightly longer than that of the corresponding core.

Synthesis and properties of PbSexS1-x alloyed nanocrystals

We present a comparative study of colloidal PbSexS1-x alloyed nanocrystals (NCs) with variation of chemical composition and different structure (core-shell and homogeneous) prepared by a single-injection procedure with respect to corresponding PbSe core NCs and PbSe/PbS core-shell NCs prepared by the traditional two-injection procedure. The narrow band edge of absorption and photoluminescence 1S-exciton energy of PbSexS1-x alloyed NCs were tuned (blue shifted) from the band edge of the same size PbSe NCs to the band edge of PbS NCs by controlling the Se+S/Pb molar ratio in the synthetic mixture. The magnitude of the Stokes shift was found to be dependent upon the size of NCs and on the core(shell) chemical composition. The largest Stokes shift (100 meV) was observed in the smaller PbSe NCs, while PbSexS1-x core-alloyed shell NCs prepared by a single-injection procedure show the vanishing small Stokes shift.

Synthesis, characterization and the use of PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystals as saturable absorbers in passively switched near infra-red lasers

The saturable absorbing properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbS core-shell and PbSe/PbSeS core-alloyed shell NCs, were examined at an energy of 1.54 micron. These NCs act as an efficient passive Q-switch in near infra-red pulsed lasers. Saturation fluence values in the order of a few hundreds mJ/cm2 were obtained, leading to a laser power output of 2.0-3.5 mJ with a pulse duration of 40-53 nsec. We demonstrated a substantial increase of the emission quantum yield and a slight decrease of the saturation fluence values, when using PbSe/PbS and PbSe/PbSexS1-x core-shells structures, in comparison with the corresponding PbSe core NCs. A quasi-three level energy manifold was used for the simulation of saturation fluence curves and of the absorption cross sections. All samples were prepared in a novel colloidal synthetic method.