Femtosecond photoluminescence spectroscopy of silicon nanocrystals

Abstract

We report on the ultrafast photoluminescence dynamics in silicon nanocrystals measured by femtosecond up-conversion technique. The samples were prepared by embedding porous silicon grains in a sol-gel derived SiO 2 matrix. Efficient initial relaxation of the excess energy of photoexcited carriers with the effective rate ≥ 4 eV/ps was observed. The ultrafast photoluminescence decay (about 400 fs) was interpreted in terms of quenching the interior exciton radiative recombination by fast carrier trapping on the nanocrystal surface. We also present the observation of the photoluminescence excited by two-photon absorption at 830 nm and open aperture Z-scan measurements to demonstrate a strong optical nonlinearity in silicon nanocrystals. We have found the value of two-photon absorption coefficient β = 1.7 m/GW which corresponds to the imaginary part of the third-order nonlinear susceptibility Im χ (3) = -1 × 10 -10 esu.

Ultrafast photoluminescence dynamics of blue‐emitting silicon nanostructures

“…6. We reported the same behaviour in our previous work [17]. On the other hand, for the (F) sample, the TPA and OPA excited PL spectra differ substantially.…”

“…The decay rate depends in this case on the PL wavelength - PL decays to 1/e of its maximal value in 0.9 ps and 3.5 ps for wavelength 540 nm and 640 nm, respectively. This behaviour has been described in detail in our previous articles [16,17] and we shall not pay further attention to the (S) and (SF) samples PL decay.…”

“…The ultrafast PL dynamics were measured using a standard up-conversion technique, as described in detail in our previous article [17]. We used a frequency-doubled Ti:sapphire laser Tsunami (Spectra Physics, rep. rate 82 MHz, pulse duration 80 fs FWHM) at 405 nm as an excitation source and its fundamental wavelength 810 nm for optical gating.…”

“…In our previous articles [16,17], we studied the Si NCs with a dominating red PL band in time-integrated PL. In these NCs we observed a rapid rise of the PL signal followed by a decay with a two-exponential character.…”

“…Here we focus on the ultrafast PL evolution of the blue-emitting porous silicon with sub-picosecond time resolution which, to the best of our knowledge, have not been reported yet. This article follows up our previous measurements of the ultrafast PL decay in the S-band dominating porous Si [16,17] and it is aimed at investigating differences between the ultrafast PL of Si nanostructures with dominating S-band emission (red spectral region) and F-band emission (blue spectral region). Besides the time-resolved measurements, we carried out also other optical experiments, which could help us to identify the blue PL origin.…”