Size Dependence of Second Harmonic Generation in CdSe Nanocrystals
- PDF / 363,570 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 13 Downloads / 240 Views
(1)
with ,8, the first hyperpolarizibilty, determining the second order optical nonlinearities of the molecule. We investigate second harmonic generation (SHG) and the evolution with size of /8from CdSe nanocrystals as an interesting intermediate regime between molecules and the bulk. SHG in nanocrystals is interesting from an additional point of view: SHG is allowed only in a non-centrosymmetric environment. On a surface, centrosymmetry is intrinsically broken and this has led to widespread utility of SHG as a bulk-surface probe. Nanocrystals are characterized by a large surface to volume ratio, and the surface is important in describing their properties; e.g., trapped emission, and luminescence intermittency [5,6]. But the dimensions of the nanocrystals are substantially small as compared to the wavelength of light. This raises a question concerning the possibility of a surface contribution to SHG in nanocrystals. It is possible that due to symmetry, the surface contribution to SHG from a nanocrystal will be cancelled out. Our size dependent study of SHG from CdSe nanocrystals directly addresses the importance of a contribution to the SHG from the nanocrystal surface.
253 Mat. Res. Soc. Symp. Proc. Vol. 571 ©2000 Materials Research Society
EXPERIMENT CdSe nanocrystals were synthesized by solution-phase pyrolytic reaction of organometallic precursors [7,8]. The wurtzite crystals have mean diameters ranging from 20-80 A with a standard deviation of -5-10% and are slightly prolate (aspect ratio is size dependent). Absorption spectra of the solutions were measured with a Shimadzu UV1601 spectrometer To measure fi, we used the Hyper Rayleigh scattering technique (HRS) [9]. This technique is a useful method to measure SHG of nonlinear chromophores directly in solution. A cuvette containing the solution with the nanocrystals (solvent is toluene) was irradiated by a continuous wave modelocked femtosecond Ti-Sapphire laser (Coherent Mira, pulse width 120 fsec as measured by autocorrelation, vertically polarized). The laser spot diameter in the cuvette was approximately 0.5 mm. The intensity of the laser was controlled using a circular neutral density filter of varying optical density, and the excitation power was measured with a power meter. The scattered second harmonic signal was collected at right angle, filtered with a short pass filter, dispersed using a monochromator, and detected with a liquid nitrogen cooled CCD detector. RESULTS Figure 1 displays the absorption of a CdSe sample, with 28 A radius. The fundamental inducing laser wavelength (820 nm), is below the band gap, while the second harmonic (410 nm), is well above the band gap, and in deep resonance with higher excited transitions. This is the case for all the studied nanocrystal samples. Upon excitation with the 820 nm fs laser, we observed intense two photon-excited luminescence (Fig. 1). The luminescence intensity was found to depend quadratically on the intensity of the inducing field I((o), and is assigned to two photon-excited fluorescence. It is s
Data Loading...
