Nonlinear optical and optical limiting properties of new structures of organic nonlinear optical materials for photonic applications

Abstract

The nonlinear optical (NLO) and optical limiting (OL) properties of three new structures of organic NLO guest-host Poly(N-vinylcarbozole)/disperse orange 3 (PVK/DO3), PVK/disperse orange 13 (PVK/DO13), and PVK/disperse orange 25 (PVK/DO25) as a solution at different concentrations and as a thin-film sample are studied using continuous wave z-scan system at 532 nm. The open-aperture z-scan data of the NLO materials in the solution and thin-film samples displayed two-photon and saturable absorptions, respectively. The PVK/DO13 exhibites the largest and best values of the nonlinearities, such as n2, β, χ (3) , compared with those of PVK/DO3 and PVK/DO25. This nonlinearity increases as the concentration increases. The results indicate that these NLO materials are good candidates for optical switching and OL devices. Organic nonlinear optical (NLO) materials have been exploited as bases for photonic devices. Recently, a growing interest has come up on the third-order nonlinear-ity of azo-dye-doped polymer materials because of their large optical nonlinearity, broadband spectral response, and fast response time, which are interesting features in the optical switching, optical limiting (OL), and photonic applications [1−3]. Concentration dependence has been known to play a very important role in the NLO and OL investigations. Several groups have studied the effect of concentration on the NLO and OL behavior of organic dye polymer [4−9]. Thus, the pursuit for new materials or structures suitable for constructing high-performance all-optical switching and OL has opened up new avenues in material research. In this letter, we report the experimental studies on the NLO and OL properties of new structures of disperse-dye polymer in a solution with different concentrations and in a thin-film sample using a modified z-scan system at 532 nm. Disperse orange 3 (DO3), disperse orange 13 (DO13), and disperse orange 25 (DO25) were chosen as NLO active chromophores (guests), whose broadband spectral response was doped with Poly(N-vinylcarbozole) (PVK). Finally, the results are discussed and compared with one another. The nonlinear refractive index n 2 , nonlinear absorption β, and OL of NLO polymers were investigated by a modified z-scan system. The system is shown in Fig. 1. A laser beam that propagated through the NLO polymer experienced both amplitude (intensity) and phase variations. The beam was focused by a small spot using a lens with a 5-cm focal length, and the sample was mounted on a translation stage and moved along the z-axis using a computer-controlled stepper motor. The laser beam waist ω 0 at the focus was measured as 16 µm, and the Rayleigh length z 0 was equal to 1.51 mm. Figure 1 shows that, to measure the nonlinear refrac-tive index, the transmission of the laser beam through the aperture (or closed aperture) must be placed in the far field. In this case, the transmission of the beam is sensitive to both nonlinear absorption and nonlinear refrac-tion. Meanwhile, the nonlinear absorption measurements are performed by removing the aperture (or opened aperture). Here, transmission of the beam is sensitive only to the nonlinear absorption. In both cases, the detected signal is received by a photodetection system (PDS). The PDS consists of a photodiode (PD) and its associated high-gain PD amplifier, followed by a low-noise amplifier. The output of the entire photodetection system is conveyed to a data acquisition (DAQ) controller and then processed by a computer. In the case of the open-aperture system, the transmission of the laser beam measured by the PDS is sensitive only to the intensity variations. The absorption α of the material is intensity-dependent and is given by [10] α = α 0 + βI, (1) where α 0 is the linear absorption coefficient and β is the nonlinear absorption coefficient. Fig. 1. Modified z-scan system for measuring the NLO and OL properties. BS: beam splitter.