We present third-order optical nonlinear absorption in CdSe quantum dots (QDs) with particle sizes in the range of 4.16–5.25 nm which has been evaluated by the
Before the advent of lasers, transparent optical materials were assumed to be essentially passive unaffected by the light travelling through them. The high powers of laser beams made it possible to observe that the effect of light on a medium can indeed change its properties such as refractive index or absorption. These are optical nonlinear phenomena. When this happens, the light itself gets affected by this change in a nonlinear way; for example, the nonlinear response of the material can convert the laser light into new colours, both harmonics of the optical frequency and sum and difference frequencies. With the development of optical communication networks, various nonlinear optical (NLO) devices such as optical switches, optical limiters, optical detectors, and optical sensors have attracted considerable attention because of their widespread usage for scientific and industrial purposes. Among all the NLO properties, optical limiting is one of the most promising practical applications, as it can protect the human eye and photosensitive components from damage caused by intense optical radiation [
In recent years, interest in the synthesis, characterization, and application of colloidal quantum dot (QD) semiconductor materials has grown markedly. QDs of cadmium selenide (CdSe) are by far the most studied system among all the semiconducting nanocrystals. The bulk CdSe has a direct band gap of 1.74 eV at 300 K and a typical Bohr exciton diameter of around 5.6 nm [
In the past decade, there has been increasing interest in the luminescent and nonlinear optical properties of these nanometer-sized QDs. Large optical nonlinearities in CdSe QDs have been reported using different techniques. These techniques include degenerate four-wave mixing and
In this paper, we present the study of the effect of size on the nonlinear optical properties of CdSe QDs and estimate the nonlinear absorption coefficient
In the present work, CdSe NPs were prepared by a modified aqueous method using reflux [
Absorption spectra of the prepared CdSe QDs were recorded using UV-Visible Spectrophotometer (Jasco V-570 UV/VIS/IR). Fluorescence spectra of QDs were obtained on a Cary Eclipse Fluorescence Spectrophotometer (Varian). The excitation wavelength was kept at 390 nm, and the emission spectra were recorded. All absorption and fluorescence spectra were measured without any postpreparative size separation. The structural properties of the samples were investigated by X-ray diffraction (XRD) on a Bruker AXS D8 Advance X-ray diffractometer with Ni-filtered Cu K
For detailed study, we used samples taken after reflux times of 10 minutes, 1 hour, and 3 hours and they are denoted by C1, C2, and C3, respectively. The variation in absorption peak and the corresponding optical energy band gap (inset) of samples (C1, C2, and C3) are shown in Figures
((a), (b), and (c)) The variation in absorption peak and the corresponding optical energy band gap (inset) of samples (C1, C2, and C3).
Energy band gap of the QDs shows a variation from 2.51 eV to 2.34 eV as the particle size increased during the reflux time from 10 minutes to 3 hours. It is observed that as particle size increases, absorption peak shifts to higher wavelength side and the band gap is enhanced with decrease in particle size. The emission spectra of the C1, C2, and C3 samples are also shown in Figure
The emission spectra of C1, C2, and C3 samples under the excitation wavelength of 390 nm.
The particle size is calculated with effective mass approximation [
Data showing particle size, energy band gap, nonlinear absorption coefficient (
Sample | Particle size (nm) | Energy B.G (eV) |
|
Optical limiting threshold (GW/cm2) | Im |
---|---|---|---|---|---|
C1 | 4.17 | 2.52 | 0.27 |
0.57 | 0.16 |
C2 | 4.96 | 2.38 | 0.69 |
0.48 | 0.41 |
C3 | 5.25 | 2.34 | 2.47 |
0.35 | 1.45 |
XRD spectra of C3 sample with the average radius of the particle being 4 nm.
The calculated average particle size is about 4 nm for the powdered sample. The peak observed at
Figure
(a): Histogram and TEM image of C3 sample with average particle size of 4.09 nm. (b) HRTEM image of C3 sample.
FTIR spectrum of the C3 sample is shown in Figure
FTIR spectrum of the C3 sample.
The nonlinear optical characterization of the samples was carried out using the
The open-aperture curve exhibits a normalized transmittance valley at the focal point, indicating the presence of reverse saturable absorption (RSA) in the nanocolloidal solutions. The open aperture
The open aperture
The data can be fitted well by assuming two-photon absorption (TPA) in the nonlinear optical absorption process. From this fit, we can confirm that the basic mechanism involved in the nonlinear absorption of nanocolloidal solutions of CdSe QDs is TPA process because the photon energy of the 532 nm laser is within the range
C1 and C2 samples have band gap very near to 2.4 (2.39 ± 0.02) and 2.5 (2.52 ± 0.02), respectively, which is more than one-photon energy corresponding to 532 nm. The data fitting shows that the nonlinear optical absorption is through two-photon absorption process in these samples. Our studies show that sample C3 has large optical nonlinearity (2.47 × 10−10 m/W) enhancement, compared to other samples as seen in Figure
Band gap corresponding to this sample is 2.33 eV to 2.33 ± 0.02 eV which is very near to the one-photon energy corresponding to 532 nm (2.33 eV). The fact that optical nonlinearity in this case is also due to the two-photon absorption reveals that two-photon excitation cross section is enhanced due to resonant one-photon absorption level. A small mismatch between band gap of sample C3 and one by photon energy will be compensated by phonon-assisted excitation. The enhancement also arises due to increased optical nonlinear interaction between the radiations and the particles. This can be explained by the fact that, with increase in particle size, there is an increase in the multiple scattering from the QDs. This increase in multiple scattering leads to large effective interaction length which in turn results in an enhancement in nonlinear absorption [
The open
To study the optical limiting property of the sample, the nonlinear transmission of the sample is measured as a function of input fluence. Optical power limiting is effected through the nonlinear optical processes of the sample. An important term in the optical limiting study is the optical limiting threshold. Optical limiters are essentially those systems which transmit light at low input fluence or intensities but become opaque at high inputs. The optical limiting property is mainly found to be absorptive nonlinearity, which corresponds to the imaginary part of third-order susceptibility [
Optical limiting response of the samples (C1, C2, and C3).
The line in Figure
To conclude, a simple synthetic route described in this paper permits the synthesis of high-quality CdSe QDs of different particle size in aqueous solution. The MSA-capped CdSe QDs possess a tuning effect in particle size and in band gap. Linear and nonlinear optical properties were studied using optical absorption, fluorescence spectroscopy, and open aperture
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge DST for financial assistance and SAIF-STIC for XRD analysis.