Polymeric composite for integrated photonic devices

Integrated photonic devices promise to be increasingly important especially for the fast switching required in future optical communication networks. The key characteristics of this technology are extremely small size (on the order of nanometers), ultrafast response, and very low energy consumption. Accordingly, the materials used in constructing integrated photonic devices must exhibit both a large nonlinear optical (i.e., intensity-dependent) coefficient and an ultrafast response time.¹ However, in conventional nonlinear materials, the optical coefficient is relatively small,² substantially limiting research in the area.

Recently, a number of new approaches to the problem have been proposed. One method forms composite materials based on local-field-enhancing nonlinearity, which also provides a means for engineering new materials with desired properties.^3,4 A second approach is to include photonic microresonators (devices that oscillate at specific or ‘resonant’ frequencies) to enhance the interaction of light and matter.^5,6 Yet another technique involves synthesizing materials with inherently large nonlinear responses.^7,8 However, the larger the nonlinear optical coefficient, the slower the response time.⁹ We have developed a novel means of building photonic materials possessing both a large nonlinear optical coefficient and ultrafast response that is adaptable to a wide range of applications. It is based on the enhanced optical nonlinearity associated with both excited-state interelectron and resonant-energy transfer in polymeric composite materials.^10,11

The material we use consists of dopant—either organic chromophore or metal nanoparticles—dispersed in a polymer matrix. The linear absorption bands of the dopant and polymer matrix overlap. When a pump light at a frequency within the overlapped absorption band excites the composite material, it provides a very large nonlinear optical coefficient under resonant conditions. In addition, the interelectron transfer process between the chromophore and polymer molecules—or the energy transfer process between metal nanoparticles and polymer molecules—ensures a very rapid response.

Specifically, in our case, a polymeric composite material composed of polystyrene doped with coumarin 153 (C153) was fabricated and adopted as the matrix for a 2D photonic-bandgap microcavity. Under the excitation of a pump light, the effective refractive index of the photonic crystal changes, which causes a shift in the position of the microcavity mode in the bandgap. This shift is the basis of photonic crystal all-optical switching. We used a femtosecond pump-and-probe method to measure the switching effect (see Figure 1). The wavelength of the pump light was within the linear absorption band of polystyrene and C153, and it led to near-resonant enhancement (i.e., the impedance was high) of the optical nonlinearity, resulting in a very low operating pump intensity of 0.1MW/cm², four orders of magnitude less than that obtained with undoped polystyrene.¹² The switching efficiency—i.e., transmittance contrast of on and off states—was as high as 80%.¹⁰ The switching time, estimated from the half-width of the signal profile of Figure 1, was 1.2ps and enabled by the ultrafast interelectron transfer from C153 to the polystyrene molecules.

Figure 1. Polymeric composite photonic crystal all-optical switching. The intensity of the pump light was 0.1MW/cm². The thick red line represents the exponentially fitted result. The right insert shows a schematic diagram of the excited-state interelectron transfer process. C153: Coumarin 153. e: Energy.

We then constructed a polymeric nanocomposite material out of silver (Ag) nanoparticles dispersed in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and used it to demonstrate ultrafast photonic crystal all-optical switching (see Figure 2). The wavelength of the pump light was 399.5nm, which was within the linear absorption band of MEH-PPV and close to the surface plasmon resonance (SPR) frequency of the Ag nanoparticles. (SPR is an optical phenomenon arising from the interaction between an electromagnetic wave and the conduction electrons in metal nanoparticles.) The nanocomposite nonlinear coefficient, which was very large owing to SPR enhancement under resonant excitation, resulted in operating pump intensity as low as 0.2MW/cm².¹¹ The energy-transfer process from MEH-PPV molecules to Ag nanoparticles made it possible to maintain an ultrafast switching time of 35ps.

Figure 2. Polymeric nanocomposite photonic crystal all-optical switching. The intensity of the pump light was 0.2MW/cm². The thick red line represents the exponentially fitted result. The right insert is a schematic illustration of the excited-state energy transfer process. Ag: Silver. MEH-PPV: Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene].

In summary, we have developed an approach to constructing photonic materials that have both a large nonlinear optical coefficient and ultrafast response. Future work will focus on new methods of improving the response time to femtosecond levels.

We acknowledge the support of the National Natural Science Foundation of China and the National Basic Research Program of China.