We report an approach to achieving a tunable time response of the nonlinearity of nano-Ag:polymer nanocomposites. The response time of the nonlinearity of the nanocomposite made of Ag nanoparticles dispersed in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] matrix can be tuned by adjusting the doping concentration of CdTe0.13S0.87 quantum dots. An ultrafast response time of 14.5 ps is achieved for a doping concentration of 27%. Moreover, the third-order nonlinear susceptibility, achieving the order of 10−5 esu, is one order of magnitude larger than that of the undoped nanocomposite. An ultrafast and low-power photonic crystal all-optical switching is also realized based on the photonic bandgap shift.
©2009 Optical Society of America
Recently, photonic materials having large nonlinear optical susceptibility and fast response have attracted great attention due to their important applications in the fields of nonlinear optics and integrated photonic devices. Various approaches have been proposed to develop photonic materials with large nonlinear susceptibility, such as by forming one-dimensional metallodielectric periodic structure based on resonances in Bragg stacks or local-field enhancing nonlinearity [1,2], forming nanocomposite materials based on strong quantum confinement effect and dielectric confinement effect [3,4], and constructing photonic microresonators to enhance the interactions of light and matter [5,6]. Kishida et al.reported that large nonlinear susceptibility in the range of 10−8 to 10−5 esu could also be achieved in one-dimensional Mott-Hubbard insulators due to strong quantum confinement effect . Recently, our group also reported a polymer composite having a nonlinear susceptibility of the order of 10−7 esu and an ultrafast response time of 1.2 ps . Murzyn et al. realized a control of the nonlinear carrier response time of AlGaAs photonic crystal waveguides by adjusting sample parameters . To date, much attention has been paid to how to achieve a large nonlinear susceptibility. However, little attention was paid to the time response properties.
The target of this letter is to achieve a tunable time response of the nonlinearity for nanocomposite materials made of Ag nanoparticles dispersed in a polymer matrix. For this purpose, we adopted poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a nonlinear organic conjugated material, as the polymer matrix. CdTe0.13S0.87 quantum dots were doped in the nano-Ag:MEH-PPV nanocomposite. Semiconductor quantum dots possess excellent nonlinear optical properties due to strong quantum confinement effect and ultrafast exciton relaxation dynamics [10,11]. The time response of the nonlinearity of the nanocomposite can be tuned by adjusting the doping concentration of CdTe0.13S0.87 quantum dots. Moreover, the nonlinear susceptibility was enhanced by one order of magnitude compared with that of the undoped nanocomposite. An ultrafast and low-power photonic crystal all-optical switching was also realized based on the shift of the photonic bandgap.
2. Sample preparation and characterization
MEH-PPV powder with an average molecular weight of 200,000 was obtained from Sigma-Aldrich Company, USA. MEH-PPV powder was dissolved in chloroform with a weight ratio of 1:160 and the chloroform solution of MEH-PPV was obtained. Ag nanoparticles with an average diameter of 15 nm was prepared by the chemical reduction method . The water in the Ag colloid was removed through centrifugalization. Ag nanoparticles was added into MEH-PPV solution with a doping concentration of 32%. The composite solution was vibrated by ultrasonic wave for 120 minutes to ensure that Ag nanoparticles were monodispersed in the solution. The chloroform solution of CdTe0.13S0.87 quantum dots with an average diameter of 7 nm was obtained from URA Company, China. CdTe0.13S0.87 quantum dots were added in the composite solution with a doping concentration of 10%. The units used to measure the doping concentration for both Ag nanoparticles and CdTe0.13S0.87 quantum dots were weight percentage. Then the composite solution was vibrated by ultrasonic wave for another 120 minutes to ensure the monodispersion properties of Ag and CdTe0.13S0.87 nanoparticles. The spin coating method was used to fabricate 450-nm-thick CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV films on silicon dioxide substrates. The linear absorption spectra of CdTe0.13S0.87 quantum dots, Ag colloid, and MEH-PPV film are shown in Fig. 1 . The absorption peak of Ag colloid, originating from the excitation of surface plasmon resonance (SPR), is situated at 400 nm, which is located in the linear absorption band of MEH-PPV and CdTe0.13S0.87 quantum dots .
A focused-ion-beam (FIB) etching system (Model DB235, FEI Company, USA) was employed to prepare the periodical patterns of CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV photonic crystals. The fabrication process is detailed in Ref . Scanning-electron-microscopy (SEM) images of the photonic crystal are shown in Fig. 2 . The photonic crystal comprised square arrays of cylindrical air holes embedded in a CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV slab. The lattice constant and the diameter of the air holes were 266 nm and 212 nm, respectively. The length and the width of the patterned area were 3 μm and 100 μm, respectively. The photonic crystal was connected to two 450-nm-thick access waveguides made of the CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV film. The length and the width of each access waveguide were 5 mm and 100 μm, respectively. The access waveguide and the photonic crystal were connected directly. There was no air gap separating the photonic crystal from the access waveguide. The dark stripes on either side of the photonic crystal region in Fig. 2(a) are gold layers used for charge removal during FIB etching . The femtosecond pump and probe method was used to study the transmittance properties of the CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV photonic crystal. The experimental setup is shown in Fig. 3 . A tunable Ti:sapphire laser system (Model Mira 900F, Coherent Company, USA) was used as light source. The pulse duration and the repetition rate of the Ti:sapphire laser system were 120 fs and 76 MHz, respectively. The beam output from the laser system was split into two beams with a ratio of 1:1. One beam was used as the pump light, and the other beam the probe light. Both the probe and pump light were transverse-electric (TE) polarized waves with the electric-field vector parallel to the CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV film. A beta-barium borate (BBO) crystal was used to double the frequency of the pump light. Then the pump light was focused and normally incident on the upper surface plane of the photonic crystal with a spot size of about 100 μm. Two apertures with a diameter of 0.5 mm were used to collimate the probe light and attenuate its intensity. The energy of the probe light was coupled into the access waveguide by using the prism coupling method . The probe light propagated through the photonic crystal in the Г-X direction. A fiber monochromator (Model HR4000, Ocean Optics, USA) with a resolution of 0.25 nm was used to detect the probe light coupled out from the photonic crystal. The output signal from the fiber monochromator was collected and analyzed by a computer. A delay line was used to adjust the timing between the pump and probe pulses. The intensity of the probe light propagating through the photonic crystal was three orders of magnitude lower than that of the pump laser due to attenuation and coupling loss. Moreover, the frequency of the probe light was far away from the linear absorption band of the nanocomposite material. The refractive index change induced by the probe light was less than 1×10−6. So, influences of probe light on the optical nonlinearity were neglected. The transmittance spectra of CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV photonic crystal are plotted in Fig. 4 . The linear transmittance was normalized to a reference waveguide without patterned region, as previously done by Liguda et al. . Limited by the tunable frequency range of the laser system, only the transmittance spectrum of the dielectric bandedge was measured. The dielectric bandedge spanned a 1.5 nm range. Moreover, the transmittance changed from 20% in the photonic bandgap to 90% in the passband, which was in reasonable agreement with our calculated results by using the finite-difference time-domain method .
3. Switching experiments and discussion
To perform the all-optical switching, we measured the transmittance changes of the probe laser as a function of the time delay between the pump and probe pulses, and the results are shown in Fig. 5(a) . The wavelength of the probe laser was 798.5 nm and the pump intensity was 120 kW/cm2. The wavelength of the pump laser was 399.25 nm, which was within the linear absorption band of MEH-PPV, CdTe0.13S0.87 quantum dots, and the SPR peak of Ag nanoparticles. The signal profile shows a fast rise followed by a slow drop. The transmittance of the probe laser was at its minimum value, 23%, when the pump pulse was far away from the probe pulse. The optical switch was in the ‘OFF’ state. When the pump and probe pulse overlapped temporally, the photonic bandgap shifted in the short-wavelength direction because of the pump-induced variation of the effective refractive index of the photonic crystal. The wavelength of the probe light gradually dropped in the passband. As a result, the transmittance of the probe laser increased. The maximum transmittance, 73%, corresponds to the ‘ON’ state for optical switching. Therefore, a high switching efficiency of 50% was achieved. The value of the nonlinear susceptibility can be estimated with the relation [8,17]14]. Boyd and Sipe have pointed out that an exceptionally high third-order optical susceptibility is attainable in a nanocomposite structure when both components respond nonlinearity [18,19]. Therefore, under resonant excitation all the nonlinearity enhancement in MEH-PPV, Ag nanoparticles, and CdTeS quantum dots contribute to the large nonlinear optical susceptibility of the CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV nanocomposite material. The thermal effect does exist under excitation of a femtosecond laser with a 76 MHz repetition rate. But the thermal effect is not influent on the presented results, which document an ultrafast effect while the thermal change of refractive index remains constant during the observation window. Therefore, the thermal effect was totally omitted since it was not crucial in our experiment. Hayes et al. pointed out that the sample heating of MEH-PPV film arising from the resonant photon absorption could be discounted because of the small amount of energy presented in each femtosecond laser pulse . Martucci et al. pointed out that the contributions of the thermal effect to the optical nonlinearity of semiconductor quantum dots could be neglected under resonant excitation .
The dynamic process of the transmittance change of Fig. 5(a) reflects the time response properties of the nonlinearity of the nanocomposite, which determines the switching time of the photonic crystal all-optical switching. The fast rise is attributed to the formation of excitons followed by their subsequently vibrational relaxation onto the lowest vibrational level of the first excited electronic state in MEH-PPV and the formation of excitons in CdTe0.13S0.87 quantum dots [22–24]. The thick red line in Fig. 5(a) is the double-exponentially fitted result of the drop curve. The time constant was 1.7 ps (16%) for the fast process and 28.1 ps (84%) for the slow process, respectively. The characteristic time of the relaxation dynamics of excitation state electrons of MEH-PPV film was 145 ps . There exist two relaxation pathways for the excited states of MEH-PPV molecules in the CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV composite. For pure nano-Ag:MEH-PPV composite, the nonradiative energy transfer process from MEH-PPV molecules to Ag nanoparticles with a characteristic time of 35 ps dominated the relaxation dynamics of excited states of MEH-PPV molecules, as reported in Ref . Therefore, the first pathway is nonradiative energy transfer from MEH-PPV molecules to Ag nanoparticles. It is very clear that the response time of the nonlinearity is quickened to 28.1 ps after CdTe0.13S0.87 quantum dots are doped. It has been pointed out that there exists a photoinduced electron transfer process from excited states of MEH-PPV molecules to semiconductor nanocrystal with a characteristic time of about two picoseconds order in MEH-PPV/semiconductor nanocrystal composites [25,26]. Elim et al. pointed out that there exists a nonradiative energy transfer through Auger process from semiconductor quantum dots to metal nanoparticles with a characteristic time of less than ten picoseconds in semiconductor-metal nanohybrid composites . Therefore, there exists another relaxation pathway in the CdTe0.13S0.87 doped nano-Ag:MEH-PPV composite simultaneously: electron transfer from excited states of MEH-PPV molecules to CdTe0.13S0.87 quantum dots, and subsequent energy transfer from CdTe0.13S0.87 quantum dots to Ag nanoparticles. The SPR in Ag nanoparticles will be continuously excited during the relaxation process of excited electronic states of MEH-PPV molecules. The relaxation time of this pathway is much faster than the former one. The appearance of the fast relaxation pathway makes the response time of the nonlinearity of the nanocomposite faster. The schematic relaxation pathways of CdTe0.13S0.87 doped nano-Ag:MEH-PPV composite is shown in Fig. 6 .
While under a low doping concentration of CdTe0.13S0.87 quantum dots, the slow relaxation pathway, direct energy transfer from MEH-PPV molecules to Ag nanoparticles, still contributes a large part to the response time of the nonlinearity of the nanocomposite. To further tune the response time of the nonlinearity, we fabricated a CdTe0.13S0.87 quantum dots doped nano-Ag:MEH-PPV photonic crystal with a doping concentration of 27%, having the same structural parameters as that of the former one. The transmittance dynamics of the 797-nm probe laser were measured and the results are plotted in Fig. 5(b). The wavelength and the intensity of the pump laser were 398.5 nm and 210 kW/cm2, respectively. The fitted time constant for the drop curve was 1.8 ps (21%) for the fast process and 14.5 ps (79%) for the slow process, respectively. The time constant of 1.8 ps of the fast process corresponds to the electron transfer process from excited states of MEH-PPV molecules to CdTe0.13S0.87 quantum dots [25,26]. The time constant of 14.5 ps for the slow process was in agreement with Elim’s measured results , which corresponds to the energy transfer process from CdTe0.13S0.87 quantum dots to Ag nanoparticles. With the increase of the doping concentration, the fast relaxation pathway, electron transfer from excited states of MEH-PPV molecules to CdTe0.13S0.87 quantum dots and subsequent energy transfer to Ag nanoparticles, gradually occupies the dominant position in the competition between two relaxation pathways. The fast relaxation pathway dominates the relaxation dynamics of the nanocomposite under a large doping concentration, which results in an ultrafast response time of 14.5 ps.
To further confirm the approach of tailoring response time of the nonlinearity of nanocomposites, we fabricated a CdTe0.13S0.87:MEH-PPV nanocomposite photonic crystal. The doping concentration of CdTe0.13S0.87 quantum dots was 40%. The lattice constant and the diameter of the air holes were 274 nm and 223 nm, respectively. The transmittance dynamics of the 795-nm probe laser were measured and the results are plotted in Fig. 5(c). The wavelength and the intensity of the pump laser were 397.5 nm and 970 kW/cm2, respectively. The fitted time constant for the drop curve was 2.3 ps (29%) for the fast process and 5.6 ps (71%) for the slow process, respectively. The 2.3 ps fast process corresponds to the electron transfer from excited states of MEH-PPV molecules to CdTe0.13S0.87 quantum dots [25,26]. The pathway, the electron transfer from MEH-PPV molecules to CdTe0.13S0.87 quantum dots and subsequent Auger recombination, dominates the relaxation dynamics of the nanocomposite without Ag nanoparticles. The relaxation dynamics will be discussed in detail elsewhere. This leads to an ultrafast response time of 5.6 ps. Limited by the experimental setup, we could not perform the longer delay experiment. We did not obtain the longer decay results.
In conclusion, we have realized a tunable time response of the nonlinearity for nano-Ag:polymer nanocomposite materials. The response time of the nonlinearity varied with the doping concentration of CdTe0.13S0.87 quantum dots. An ultrafast response time of 14.5 ps is achieved for a doping concentration of 27%. Moreover, the nonlinear susceptibility was enhanced by one order of magnitude. These results may be useful references for the study of ultrafast integrated photonic devices.
This work was supported by the National Natural Science Foundation of China under grants 10874010, 10821062, and 10434020, and the National Basic Research Program of China under grant 2007CB307001.
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