Nonlinear optical response of Ge nanocrystals in silica matrix with excitation of femtosecond pulses

We report an investigation of third-order optical nonlinearities in Ge nanocrystals (∼ 6 nm radius) embedded in silica matrix using the Z-scan and pump-probe techniques with femtosecond laser pulses at 780-nm wavelength. The nanocrystallite Ge samples were prepared using magnetron co-sputtering and post-thermal annealing at 800 ◦C. The nonlinear absorption coefficient and refractive index of the Ge nanocrystals were determined to be in the range from 1.8×10−7 to 6.8×10−7 cm/W and 1.5×10−12 to 8.0×10−12 cm2/W, respectively, which are proportional to the Ge atomic fraction in the matrix. Relaxation of the nonlinear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms responsible for the observed nonlinear response are discussed. PACS: 42.65.An; 61.46.+w; 78.20.Ci In recent years there has been considerable interest in the linear and nonlinear optical properties of quantum-confined semiconductor nanocrystals (NCs), particularly for their potential applications in opto-electronics and signal processing. Glass is an attractive host matrix for NCs in such optical applications. In the semiconductor-doped glasses, the energy structure of semiconductor NCs, in particular the energy band gap, can be varied in a broad range by altering the average NC size. Meanwhile, three-dimensional quantum confinement results in discrete energy structures and atomic-like selection rules for interband and intraband optical transitions in semiconductor NCs. Glasses containing semiconductor NCs exhibit interesting optical properties, including tunable absorption, strong photoand electro-luminescence, and large thirdorder optical nonlinearities [1–4]. Since the observation of the efficient visible photoluminescence from porous Si [5], nanocrystallite Si and Ge structures have extensively been studied [6] because it would open a new possibility for indirect-gap semiconductors as new materials for opto-electronic applications. In comparison with electronic properties of Si, Ge has a larger dielectric constant and small effective masses for electrons and ∗Corresponding author. (Fax: +65-791/2687, E-mail: [email protected]) holes, and the energy difference (∆E = 0.12 eV) between the indirect gap (Eg = 0.66 eV at 300 K, Γ ′ 25 → L1) and the direct gap (E0 = 0.8 eV : Γ ′ 25 → Γ ′ 2) is smaller. The exciton Bohr radius of bulk Ge (24.3 nm) [7] is much larger than that of bulk Si (4.9 nm) [8], which implies that the quantum size effects will be more prominent in Ge NCs even for larger size of the crystallites. These electronic conditions lead to an expectation that it is much easier to change the electronic structure around the band gap of Ge, resulting in strong modification of its optical properties. The visible and near-infrared photoluminescence from Ge NCs embedded in silica matrices have been observed [6, 9], which are attributed to the quantum confinement mechanism. The refractive nonlinearity of Ge NCs in a silica matrix fabricated by ion-implantation and annealing has been recently reported [10] using timeresolved degenerate-four-wave-mixing (DFWM) measurements. Strong refractive nonlinearity was observed and relaxation of the nonlinear response was found to have two characteristic time constants: one < 100 fs and the other ∼ 1 ps. In this paper, we report an experimental investigation of third-order optical nonlinearities of Ge NCs embedded in silica matrix formed by magnetron co-sputtering and annealing using femtosecond laser pulses at 780-nm wavelength. Both the nonlinear absorption coefficient β and nonlinear refractive index n2 in the Ge NCs have been measured using the Z-scan technique. It has been found that the measured β and n2 values range from 1.8×10−7 to 6.8×10−7 cm/W and 1.5×10−12 to 8.0×10−12 cm2/W, respectively, which are proportional to the Ge atomic fraction in the matrix. We also present femtosecond pump-probe results at 780 nm that allow the determination of the nonlinear response time of the NCs. The mechanisms responsible for the observed nonlinear response are discussed.