Long-Lifetime Nonlinear Absorption of PbS Quantum Dots
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E13.23.1
Long-Lifetime Nonlinear Absorption of PbS Quantum Dots Feiran Huang, Aleksey Filin, Pratima G.N. Rao1, Robert H. Doremus1, and Peter D. Persans Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, U.S.A 1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Tory, New York 12180, U.S.A ABSTRACT We have fabricated PbS particles in glass of average size from 1.2 nm to 3 nm. This allows us to tune the energy of the lowest electron-hole pair transition from 1.7 eV to 0.8 eV respectively. Optical absorption spectra show a pronounced quantum peak, implying a narrow size distribution. We have performed pump-probe spectroscopy where the probe is cw and the pump is nanosecond; this allows us to investigate a wide time scale. We have observed bleaching of the lowest quantum peak with lifetimes of ~2 microseconds. We interpret this in terms of trapped carriers. Phenomenological description of observed effect is also presented. INTRODUCTION Nanocrystals of semiconductor materials have been in the center of much interest in the last decade not only because possible applications but due to very interesting fundamental properties [1-3]. PbS (lead sulfide) is one of the promising semiconductors for fundamental studies because of its simple band structure near the direct edge and comparably large Bohr radius of both electron and hole. It results in an opportunity to reach the strong quantum confinement in relatively large particles. PbS is also at interest in applications related to optical switching and mode-locking lasers at 1.55 µm. EXPERIMENT In this paper, we report nanosecond pump-probe experiment on nonlinear optical absorption of PbS nanoparticles in glass samples. We focus on the long time bleaching. Earlier pump-probe studies have focused on short-time scales from 100 fs to 10’s of picoseconds [4,5,6]. In some of those studies [4,5] the repetition rate of the excitation laser was faster than the relaxation time we observed here. In our experiment, we are using a low-repetition rate dye laser, so we are able to take an opportunity to observe events on the nanosecond and microsecond scale. Growth of PbS nanoparticles in glass for optical absorption study in visible and NIR range has been reported earlier [7]. In our preparation process, PbS nanoparticles are grown from quenched PbS-doped borosilicate samples by heat-treating at the temperature range from 540 to 580°C for 12 to 36 hours. In quenched glass, particles are smaller than 1 nm [8]. We have prepared a set of samples in which the particles size is varied from 1.2 nm to 3 nm. The optical absorption spectra of prepared samples are presented in Figure 1. Particle sizes were deduced by using the Four-Band Envelope-Function of Kang and Wise [3,8]. Broadening of the peak is caused by the particle size distribution. In the remainder of this paper, we discuss exclusively a sample with pronounced absorption peak at 0.8 eV (1550 nm), corresponding to average particles size of 3 nm.
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