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.
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.
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.
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.