Marina Sirota

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.

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.

Magneto-Optical Measurements of Chromophore/Semiconductor Nanocrystalline Superstructures

The magneto-optical properties of chromophore/semiconductor nanocrystalline superstructures were examined using optically detected magnetic resonance (ODMR) and time-resolved photo-luminescence (PL) spectroscopy. The samples consisted of CdS or PbS nanocrystals separated by conjugated organic chains, forming three-dimensional superstructures. The ODMR measurements revealed that the magnetooptical properties of the superstructures are mainly controlled by the individual characteristic of the nanocrystals. The ODMR spectra were compared with simulated curves generated by the diagonalization of a spin Hamiltonian, indicating the existence of the following luminescence events. a) Recombination between electron-hole pairs trapped at a stoichiometric defect (metal or sulfur vacancies) or oxygen adatom sites at the surface of the nanocrystals. These electron-hole pairs showed anistropic g-factors and weak exchange interactions. b) Bound exciton emission, from strongly coupled electron-hole pairs trapped at intrinsic stoichiometric or structural defects at the core of the nanocrystals. The existence of the two overlapping luminescence events is further confirmed by the acceptance of biexponential PL decay processes.

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.

IV-VI semiconductor nanocrystals for passive Q-switching of eye-safe laser

Laser, operating in the range of 1-2 μm (NIR), is currently an attractive candidate for various applications include ranging, 3D scanning laser radar, communication and other areas where human contact with the laser radiation is possible. The present work is focused on application of PbSe or PbSe/PbS semiconductor nanometer-sized crystals (NCs) for passive Q-switching of NIR laser. Owing to narrow band gap and large exciton Bohr radius of the bulk materials, the NCs of PbSe and PbS acquire unique properties: The quantization effects are strongly pronounced in PbSe and PbS NCs, the strong quantum confinement is thus easily obtained, and the ground-state absorption edge can be tuned over a wide wavelength range (from the visible to infrared). The NCs gain properties of saturable absorber, which allow using them as optical switches. We propose a colloidal synthesis procedure for the preparation of size-selected NCs, suitable for Q-switching of NIR laser. Colloidal synthesis allows simple control over the size of the crystals, and therefore, provides a possibility to produce the samples with desired absorption band position. This method is also very effective for stabilization of NCs and a passivation of their surface with the help of organic ligands.