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... the free- standing AgNPs -PMMA films were peeled off and collected The thickness of the AgNPs -PMMA films was measured using micrometer caliper and found to be 100 μm The free- standing AuNPs -PMMA. .. and the nonlinear optical properties of two kinds of flexible free standing films have been investigated These two kinds of flexible free standing films demonstrate strong broadband optical limiting. .. Spectra of Au/ Ag- PMMA films in different weight ratios; pure PMMA; AuNP solution and AgNP solution The peaks of Ag and Au NP solutions are 418 nm and 520 nm respectively After impregnated with PMMA,
OPTICAL LIMITING PROPERTIES OF GO-PVA AND AU/AG-PMMA FREE STANDING FILMS MA RIZHAO (B.SC, Fudan University, China) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2014 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety, under the supervision of Xu-Qinghua, (in the laboratory of Ultrafast Spectroscopy), Chemistry Department, National University of Singapore, between Jan.2012 and May.2014. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. i ACKNOWLEDGEMENTS I express sincere gratitude to my supervisor, Professor Xu Qing-Hua for his helpful advice and guidance. My senior, Ms. Jiang Xiaofang, Mr. Gao Nengyue and Mr. Polavarapu Lakshminarayana contributed a lot to this thesis. I am also grateful to all the members of Ultrafast Spectroscopy for their kind support and assistance. I am also thankful to the Department of Chemistry, NUS and all staff for providing me an opportunity to study here. ii CONTENTS THESIS DECLARATION...........................................................................................i ACKNOWLEDGEMENTS.........................................................................................ii TABLE OF CONTENTS............................................................................................iii SUMMARY...................................................................................................................v LIST OF TABLES ......................................................................................................vi LIST OF FIGURES…………………………………………………………………vii 1. INTRODUCTION 1.1 Optical limiting and optical limiting materials 1 1 1.1.1 Optical limiting 1 1.1.2 Graphene oxide with PVA matirx 2 1.1.3 Noble metal nanoparticles 5 1.1.4 Saturable Absorption and Reverse Saturable Absorption 6 1.2 Experimental techniques 10 1.2.1 Open-aperture Z-scan measurement 10 1.2.2 Pump-probe measurement 11 1.3 Overview of this thesis 13 2. HIGHLY EFFICIENT FLEXIBLE BROADBAND NONLINEAR OPTICAL DEVICES BASED ON GRAPHENE OXIDE IMPREGNATED POLYMER GLASS 15 2.1 Chemicals and Materials 15 2.2 Preparation of graphene oxide impregnated polymer glass materials 15 iii 2.3 Instrumentation 16 2.4 Result and Discussion 17 2.5 Summary 24 3. THE FREE STANDING FILMS OF NOBLE METAL NANOPARTICLES WITH PMMA 25 3.1 Chemicals and Materials 25 3.2 Preparation of free standing films of noble metal nanoparticles with PMMA 25 3.3 Instrumentation 26 3.4 Result and Discussion 27 3.5 Summary 34 4. CONCLUSION 35 5. REFERENCES 36 6. APPENDICES 38 iv SUMMARY The development of low-cost flexible broadband optical limiting materials is vital for the fabrication of eye and optical sensors protection devices from intense light-induced damage. Here, we presented a simple method for the fabrication of two kinds of flexible optical limiting devices (GO-PVA free standing film and Noble metal nanoparticle-PMMA free standing film) by solution process. The linear and nonlinear optical properties of these prepared flexible films could be controlled by simply varying the concentration of optical limiting materials in the polymer film. The optical limiting activity at 400 nm and 800 nm femtosecond laser pulses have been investigated and it was found that they exhibit highly efficient optical limiting activity with very low limiting thresholds. Femtosecond pump-probe spectroscopy measurements performed on these films and reveal that the nonlinear absorption responsible for the observed optical limiting activity. Furthermore, the amazing stability and flexibility of these films open a door for the practical applications. v LIST OF TABLES Table 1 The optical limiting onsets (Fon, the input fluency when the transmittance starts decreasing) and optical limiting thresholds (F50, the incident fluence at which the transmittance falls to 50% of linear transmittance) of GO-PVA films with 21 different GO concentrations vi LIST OF FIGURES Figure 1 Schematic representation of the behavior of an ideal optical limiter. 2 Figure 2 Energy level diagram for reverse saturable absorption (RSA) process. 10 Figure 3 Experimental Setup of the Z-scan measurement. D1 and D2: photodiodes. 11 Figure 4 Experimental setup of the pump-probe experiment. 13 Figure 5 Schematic of preparing GO free-standing films. (PVA = Polyvinyl alcohol) 16 Figure 6 Raman spectra and UV-vis-NIR transmittance spectra of flexible GO-PVA 17 films in different weight ratios. Figure 7 Z-scan results of sample weight ratio GO:PVA=0.0025:1 at 400 nm and 800 nm excitations and Z-scan data on different weight ratio samples with same thickness (480μm ). 19 Figure 8 Pump-probe result of the GO-PVA film (weight ratio=0.0025). 22 vii Figure 9 Comparison of in situ reduction GO-PVA films. 23 Figure 10 Preparation processes of Au/Ag-PMMA films 26 Figure 11 TEM images of silver nanoparticles and gold nanoparticles. 27 Figure 12 UV-vis. Spectra of Au/Ag-PMMA films in different weight ratios; pure PMMA; AuNP solution and AgNP solution 28 Figure 13 Open-aperture Z-scan measurements of Au-PMMA (A) and Ag-PMMA (B) films (Weight ratio=0.001). 29 Figure 14 Z-scan measurement results of Ag-PMMA film (weight ratio=0.001). 31 Figure 15 Fin Vs Trans curve on Ag in PMMA (weight ratio=0.0004) 33 Figure 16 Pump-probe result of the Ag-PMMA film (weight ratio=0.001). 34 viii 1. INTRODUCTION This chapter gives a general overview of nonlinear optical materials and their properties at the beginning to provide a framework for my research work in this thesis, followed by an introduction of relevant experimental techniques used in this thesis. 1.1. Optical limiting and optical limiting materials 1.1.1 Optical limiting The advances in the development of lasers have led to revolutionary changes and applications of lasers in various technological and science applications. The ultra short light pulses from nanosecond, picosecond and femtosecond laser sources can easily cause very high power density. High-powered pulsed lasers have found many applications in academic research as well as in many industrial and military applications like laser weapons. With the advent of such high power laser sources operating over wide ranges of wavelengths and pulse durations, the necessity for protection of sensors, optical components and human eyes from laser inflicted damages has increased enormously over the last few years1. Under this context, Optical limiters become important because they can decrease the power density of laser fluence or irradiance. Fast response optical limiting materials with low thresholds can be used for protection of eyes and sensitive optical devices from laser-induced damage.Significant 1 research efforts have been devoted for the development of broadband optical limiting materials and related devices over the past decade. In an ideal optical limiter, the transmittance change abruptly at some critical input intensity or threshold and therefore exhibits an inverse dependence on the intensity; the output is thus clamped at a certain value (Figure 1). If this value is below the minimum that can damage the particular equipment, the optical limiter becomes an efficient safety device. The effective optical limiting materials should have low limiting threshold and high optical damage threshold and stability, leading to a large dynamic range, sensitive broadband response to long and short pulses, fast response time, and high lineartransmittance, optical clarity, and robustness2. Researchers have observed effective optical limiting behavior in various nanomaterials such as carbon nanotube (CNT) 3,4 fullerenes5, quantum dots6 and metal nanoparticles (Au & Ag)7,8. Recently it has been found that suspensions of graphene9,10, graphene oxide11,12 and their composites with other materials13-17exhibit broadband optical limiting properties. 2 Figure 1: Schematic representation of the behavior of an ideal optical limiter. 1.1.2 Graphene oxide with PVA matirx Most of the reported optical limiting studies on various nanomaterials were performed where they were dispersed in different solvents and they attributed the optical limiting due to solvent micro bubbles-induced nonlinear scattering at higher intensities1,3,5, however, they might show strong saturable absorption when made into thin films 8,18. For instance dispersions of metal nanoparticles5,6, graphene and graphene-polymer composites19,20,21 shows strong broadband optical limiting properties, whereas, thin films of such materials exhibit strong saturable absorption behavior8,16,17,22. Based on the research work that has been performed so far on optical limiting of various nanomaterials concludes that nonlinear scattering and nonlinear absorption are the mechanisms responsible for optical limiting3,6,9. One of the future challenges ahead of researchers is the fabrication of stable and flexible thin films based optical limiting devices for real-world applications based on the previous knowledge. Flexible materials that exhibit high broadband nonlinear absorption are of ideal choice towards this field of research. Recently, Lim et al.14 reported giant broadband nonlinear optical response for nanosecond laser pulses on functionalized graphene oxide nanosheets dispersion. Very recently, our group has shown that spin coated graphene oxide thin films on glass or plastic substrates exhibit tunable broadband optical limiting response for femtosecond laser pulses and moreover, the nonlinear optical 3 response of graphene oxide could be tuned from nonlinear absorption to saturable absorption by reduction of graphene oxide23. From the previous studies it is clear that graphene oxide could be an interesting material for the development of flexible optical limiting devices in spite the fact that graphene oxide sheets are easy to prepare in large scale, highly flexible and highly soluble in various solvents. Herein, we demonstrate a simple method for the fabrication of flexible nonlinear optical devices by impregnating graphene oxide into PVA polymer matrix by solution process. The prepared GO-PVA films have been characterized by UV-visible transmittance, fluorescence spectroscopy and microscopy techniques. Broadband optical limiting properties of as prepared flexible GO-PVA films made of different GO/PVA ratios have been investigated by femtosecond Z-scan measurements at laser wavelengths of 400 and 800 nm. We have found that the flexible GO-PVA films exhibit excellent optical limiting properties. Femtosecond pump-probe results suggests that nonlinear absorption (Excited state absorption or multi-photon absorption) of GO play an important role in the observed strong optical limiting activity. Previous studies revealed that GO sheets contain sp3 domains and sp2 domains. With the increasing of sp2 domains, the nonradiative recombination also increased.24 Furthermore, because of the complex energy band structures, GO films display very strong two-photon absorption coefficient at 400 nm and strong two- and three-photon absorption at 800 nm, which make them excellent candidates for broadband optical-limiting materials for femtosecond laser pulses. 23 4 Graphene is one of the crystalline forms of carbon, carbon atoms in graphene are arranged in a regular hexagonal pattern. Intrinsic graphene is a kind of semi-metal or zero-gap semiconductor due to the linear energy dispersion relation24,25. Since graphene has been isolated from graphite by mechanical exfoliation first time in 200426, the studies on graphene was burst out because their potential in technological applications for optoelectronics and also for a fundamental scientific understanding of their surprising optical and electronic properties23. Several methods have been developed to prepare graphene such as chemical vapor deposition (CVD)27, physical exfoliation24, epitaxial growth28, solvent assisted exfoliation29,30, longitudinal “unzipping” of carbon nanotubes(CNTs)31 and reduction of graphene derivatives32-35. Among all these method, the use of graphene oxide(GO) as a precursor for graphene production offer a route towards solution processed applications owing to the high solubility of GO compared to graphene in various solvents33. GO can be easily converted into reduced graphene oxide (RGO) which is a kind of semiconductor or graphene-like semi-metal by reduction31. As a result of the unique properties and potential applications of GO-based materials, numerous studies have been made in this field. However, research toward the mechanism and application has just begun. Many challenges and opportunities are still remaining. 5 1.1.3 Noble metal nanoparticles Noble metal nanoparticles represent an intriguing class of materials due to their fascinating optical properties arising from their surface plasmon resonances (SPR). But so far, there are only a few reports on the optical limiting properties of gold and silver nanoparticles like Goodson et al.36 reported a strong optical limiting ability in metal-dendrimer nanocomposites with the nanosecond laser pulses. Philip et al.37 reported optical limiting effects in monolayer protected gold nanoparticles with picoseconds laser pulses. Our group also reported the optical limiting properties of oleylamine-capped gold nanoparticles for both femtosecond and nanosecond laser pulses. From previous reports, the optical limiting properties of metal nanoparticles depend on their size, shape and surrounding environment like polymer matrix or solvent38,39. Francois et al.40 observed that the threshold of the optical limiting effect decreased with increasing particle size when they studied the optical limiting behavior of gold nanoparticles at 530 nm using picoseconds laser pulses. They suggested this behavior was responsible for nonlinear scattering. Wang and Sun41 found that aggregated gold nanoparticles showed strong optical limiting properties even though individual gold nanoparticles exhibited no optical limiting behavior. From these previous reports,38,39,40,41 the mechanism can be summarized that when the noble metal nanoparticles excited by medium power laser pulses, the energy transfer from the excited nanoparticles to the solvent generates solvent bubbles that scatter light. With high power laser the metal particle itself will expand by melting/vaporization of 6 surface atoms and the expanded particles act as another type of scattering center and limit the light transmission during the laser pulse duration. In this work, we have prepared oleylamine-capped gold/silver nanoparticles in Dichloromethane (DCM) and incorporate them into PMMA solution to fabricate free standing films and studied the optical limiting properties of these films. The optical limiting properties of oleylamine-capped gold nanoparticles were studied by our group previously and these nanoparticles show strong broad band optical limiting effects for nanosecond laser pulses at 532 and 1064 nm and femtosecond laser pulses at 780 nm.6 1.1.4 Saturable Absorption and Reverse Saturable Absorption Saturable absorption is a third order nonlinear optical process happening in materials which exhibit a decrease in light absorption with increasing light intensity. The electrons in the ground state of a saturable absorber are excited into an upper energy state at a rate that is faster than their subsequent relaxation back to the ground state when the incident light intensity is high enough42. This leads to a depletion of electrons in the ground state, also known as ground state bleaching, and is experimentally exposed as a rise in transmittance. Saturable absorption has applied in passive mode-locking and Q switching of lasers 7 for the generation of short laser pulses43. The saturable absorbers are also used for nonlinear filtering to clean up pulse shapes and to process optical signals. Recently, saturable absorption has been exhibited as a means for information storages44 which opens a broad application space. There are many types of saturable absorbers like semiconductor saturable absorber mirrors, PbS quantum dots, GaAs and Cr4+:YAG crystals which widely used in lasers. The absorption cross-sections of saturable absorbers in excited state are usually smaller than in ground state. If the absorption cross-section in excited state larger than in ground state, the material will be less transmissive with increasing light intensities. This phenomenon is called as reverse saturable absorption, because it is an opposite effect of saturable absorption. RSA can be caused by several nonlinear optical processes including multi-photon absorption, excited state absorption, nonlinear scattering and free carrier absorption45. The multi-photon absorption commonly observed from semiconductor quantum dots, organic dyes and noble metal nanostructures under the irradiation by a laser pulse. Recently, graphene and graphene oxide have been reported as good multi-photon absorption materials also. The process of multi-photon absorption can be sequential or instantaneous, depending on the system. Excited state absorption happens when an excited state is significantly populated as a consequence of intense excitation. Ground state electrons are promoted to an excited state when excitation happens and in 8 excited state the electrons only remain for a short period of time. The excited state electrons can be excited to a higher energy level, decay back to the original ground state or undergo intersystem crossing to a different spin state. The performance of excited state absorption is similar to the sequential absorption of multi-photon absorption. Nonlinear scattering is normally observed from materials with large scattering cross sections, of which noble metal nanocrystals are good nonlinear scattering material46. The noble metal nanocrystals can either absorb or scatter the light when they are excited by light. From previous reports, the size is important to the scattering cross sections of the noble metal nanocrystals, larger metal nanocrystals are more efficient at scattering light than smaller ones47. This nonlinear scattering phenomenon also leads to a decreased transmittance which corresponding to reverse saturable absorption. Free carrier absorption occurs when electrons are promoted to the conduction band when excited by photons with energy greater than the band of material 48, these electrons turn into free carriers in the conduction band. These free carriers can absorb more photons when irradiated by high enough intensity lasers, which lead to nonlinear absorption. Both SA and RSA are normally associated with the third order nonlinearity of 9 materials. Materials like graphene and graphene oxide have been reported as RSA materials before. Noble metal nanoparticles have been shown to demonstrate both SA and RSA at various excitation fluencies, acting like optical switches. At low excitation fluencies, SA dominates due to ground state bleaching of the surface plasmons in noble metal nanoparticles, but with the increasing laser power, RSA becomes more dominant because the nonlinear absorption and scattering processes play increasingly significant roles. Figure 2: Energy level diagram for reverse saturable absorption (RSA) process. 1.2 Experimental techniques 1.2.1 Open-aperture Z-scan measurement The Z-scan measurement is a technique for measuring degenerate nonlinearities 10 presented by M.Sheik-Bahae et al. at 199049. The nonlinear absorption coefficient of nonlinear materials can be determined from an open-aperture z-scan experiment. The saturation irradiance can also be determined from the open-aperture Z-scan experiment if saturable absorption happens. Excitation source of this system is a Spectra-Physics Ti:sapphire amplifier laser system. The laser output has a central wavelength of 800 nm and pulse duration of ~100 fs at a repetition rate of 1 kHz. The laser pulses are focused onto the sample by using a lens with a focal length of 15 cm, giving a focal spot size of ~40 μm (radius). In a z-scan measurement, the transmittance of the sample is measured as the sample is moved towards and away from the beam focus. By collecting all the transmitted energy, information on nonlinear optical absorption will be obtained. The z-scan experiment setup is tested by measuring the third-order nonlinear absorption coefficient of a standard sample (bulk ZnSe). Because of the simplicity, accuracy and sensitivity, the open-aperture Z-scan measurement is now a popular technique to characterize optical limiting and saturable absorption of the nonlinear optical materials. The experiment setup of the Z-scan measurement shows below in Figure 3. 11 Figure 3: Experimental Setup of the Z-scan measurement. D1 and D2: photodiodes. 1.2.2 Pump-probe measurements The electronic relaxation of nonlinear materials can be measured by using femtosecond pump-probe spectroscopy. The transient absorption (TA) and single wavelength dynamics of nonlinear materials can be obtained from pump-probe measurements also. The TA spectra provide an insight on the behavior of excited electrons as a function of time over a range of wavelengths probed. On the other hand, single wavelength dynamics measurements allow one to study the electronic relaxation rates at the specific wavelength probed by fitting the time-resolved decay curves obtained. A typical optical set up for a pump-probe experiment using a Spectra-Physics femtosecond Ti:sapphire laser system is as shown in figure 4. The amplifier laser system gives output pulse energy of 2 mJ at 800 nm with a repetition rate of 1 kHz. The 800 nm laser beam is split into two portions. The larger portion of the 800 nm 12 beam passed through a BBO crystal for the generation of 400 nm pump beam by second harmonic generation. The smaller portion of the laser beam is used to generate white light continuum in a 1 mm sapphire plate. The white light beam is further split into two portions, one as the probe and another as the reference to correct pulse-to-pulse intensity fluctuations. The pump beam is focused onto the sample with a beam size of 300 μm and overlaps the smaller probe beam (100 μm in diameter). The delay between the pump and probe pulses is varied by a computer-controlled translation stage (Newport, ESP 300). Figure 4: Experimental setup of the pump-probe experiment. Pump-probe experiments are carried out at room temperature and the pump and probe beams are keeping low energies to minimize photodamage to the samples. In a pump-probe scan, the value of the normalized pump-induced absorption change (ln (T/T0)) is determined as a function of the delay time between the pump and probe pulses. The transient absorption spectra at different delay times are measured by 13 passing the probe beam through a monochromator before a photodiode detector that is connected to the lock-in amplifier. 1.3 Overview of this thesis In this M.sc. thesis, the study of two kinds of nonlinear optical material will be presented. We have found a very simple way for the fabrication of flexible free standing nonlinear optical devices by impregnating graphene oxide into PVA polymer matrix and noble metal nanoparticle into PMMA polymer matrix by solution process. The prepared flexible free standing films have been characterized by UV-visible transmittance, fluorescence spectroscopy and microscopy techniques. Broadband optical limiting properties of different GO/PVA ratios ratios have been investigated by femtosecond Z-scan measurements at laser wavelengths of 400 and 800 nm. We have found that these flexible free standing films exhibit excellent optical limiting properties. Femtosecond pump-probe results suggest that nonlinear absorption (Excited state absorption or multi-photon absorption) of GO play an important role in the observed strong optical limiting activity of GO/PVA films. The optical limiting properties of Au/Ag-PMMA films are also investigated by femtosecond Z-scan measurements at laser wavelengths from 600 nm to 1100 nm. They also demonstrate strong optical limiting ability and the femtosecond pump-probe results suggest the optical limiting can be attributed to the nonlinear absorption. Furthermore, lattice matrix is used to improve the optical limiting ability of Au/Ag-PMMA films. 14 2 HIGHLY EFFICIENT FLEXIBLE BROADBAND NONLINEAR OPTICAL DEVICES BASED ON GRAPHENE OXIDE IMPREGNATED POLYMER GLASS 2.1 Chemicals and Materials Graphite flakes (Asbury Carbons Ltd.), NaNO3, KMnO4, H2SO4, HCl, H2O2 (30%) and polyvinyl alcohol (PVA, molecular weight 10000) were purchased from Sigma Aldrich. All solvents are of analytical grade and used as received without further purification. All aqueous solutions are prepared in deionized water. 2.2 Preparation of graphene oxide impregnated polymer glass GO was prepared from graphite via a modified Hummers and Offeman method33. The gained GO was further purified by washing with ethanol and water for multiple times and then completely dried by using a rotary evaporator. The dried GO was dispersed in water for preservation. Free-standing GO thin films can be fabricated by incorporating GO into polyvinyl alcohol (PVA) polymer matrix. PVA was added to deionized water to a concentration of 200 mg·mL-1. The solution was mixed and heated overnight at 80 °C to dissolve the PVA. Next, GO sheets were dispersed in deionized water to a concentration of 0.5 mg·mL-1. 15 In order to fabricate GO-PVA films of different weight ratio, 1 mL, 2 mL, 4 mL and 10 mL of 0.5 mg·mL-1 GO solution was added to 10 mL of 200 mg·mL-1 in 4 different glass tubes respectively. The mixture in the 4 tubes was shaken to ensure homogeneity before transferring into 4 different petri dishes (Figure 3a). The petri dishes were dried by heating at 40 °C overnight. The free-standing GO-PVA thin films were then peeled off and collected. The thickness of the GO-PVA films was measured using micrometer caliper and found to be 480 μm. Figure 5 below demonstrates the preparation process of GO free-standing film (weight ratio=0.00025). 1μm PVA 200mg/mL GO 0.5mg/mL mix Pour into Dry at 40℃ petri dish 12 hours Figure 5: Schematic of preparing GO free-standing films. (PVA = Polyvinyl alcohol) 2.3 Instrumentation 16 Extinction spectra of GO-PVA film was measured by using a SHIMADZU UV-2550 spectrophotometer. The Z-scan measurements were performed by using a Spectra Physics Ti:sapphire oscillator seeded regenerative amplifier laser system at 800 nm and 400 nm (800 nm laser through a BBO crystal) with a repetition rate of 1kHz. Pump-probe experiments were used a Spectra-Physics femtosecond Ti:sapphire laser system also. 2.4 Result and Discussion GO GO in PVA Intensity(a.u.) Transmittance 100 80 60 GO : PVA 40 0:1 0.00025:1 0.005:1 0.001:1 0.0025:1 20 0 1000 1250 1500 1750 -1 Raman shift (cm ) 400 500 600 700 800 900 Wavelength (nm) Figure 6: Raman spectra and UV-vis-NIR transmittance spectra of flexible GO-PVA films in different weight ratios. Figure 6 shows the UV-vis-NIR transmittance spectra of flexible GO-PVA films within different concentrations of GO in PVA matrix. The pure PVA film demonstrates high transparency in the vis-NIR range from 400 nm to 1000 nm just as glass. However, the GO-PVA films exhibit high transparency in the infrared area and the transmittance decreases gradually to UV region. The transmittance of the 17 GO-PVA films decreased with increasing GO concentration in the PVA matrix as showed in Fig 4. The Raman spectra of GO before and after the impregnation into PVA matrix is shown in Fig 4. Two characteristic bands of GO were found at 1354 cm-1 and 1595 cm-1 correspond to D (local defect band created by epoxide groups and hydroxyl on carbon plane) and G (E2g photon mode of sp2 carbon atoms) bands respectively. The ratio of D/G bands increases from 0.73 to 0.92 after the incorporation of GO into PVA matrix, which indicates the strong interaction of hydroxyl groups of GO with PVA. The nonlinear optical properties of prepared GO-PVA films have been investigated by using open-aperture femtosecond Z-scan technique. Briefly, femtosecond laser pulses of wavelength 800 nm, pulse duration 100 fs and repetition rate of 1 kHz have been focused onto these films. The samples were moved towards and away from the focus by the translational stage to find out the input-fluence dependent transmission of GO-PVA films. 400 nm femtosecond laser pulses were generated by focusing 800nm laser pulses into frequency doubling crystal for similar Z-scan measurements under 400 nm. 18 A C B D Figure 7: Z-scan results of sample weight ratio GO:PVA=0.0025:1 at 400 nm (A&B) and 800 nm excitations and Z-scan data on different weight ratio samples (C&D) with same thickness (480μm ). Figure 7 shows the excitation-fluence dependent normalized transmittance of GO-PVA film (GO:PVA=0.0025:1, weight ratio) obtained from open-aperture Z-scan measurements at 400 nm and 800 nm respectively. As exhibited in Figure X, the normalized transmittance at 400 nm decreased as the sample moved towards the focus (Z=0) at all excitation fluencies. It is very interesting because in fact the GO-PVA films have high linear absorption at 400 nm. Usually the materials with high linear absorption demonstrate saturable absorption at low excitation intensities due to the effect of ground state bleaching. However, in the GO-PVA case, no saturable absorption was observed even in very low intensities at 400 nm excitation, which 19 means that GO-PVA films show very high nonlinear absorption at 400 nm, which dominates the ground state bleaching signal. However, in the case of 800 nm excitation (Figure X), the Z-scan results show that the normalized transmittance increased as the sample moved towards focus at low excitation intensities, which indicates saturable absorption at low excitation fluencies. The Z-scan signals shows reverse saturable absorption at low excitation fluencies. The z-scan signals shows reverse saturable absorption with the increase of excitation fluence, indicating multi-photon absorption or reverse saturable absorption. The Z-scan results obtained from GO-PVA films is quite similar to previous observation of GO on glass substrates, which indicate PVA may have no influence on the optical limiting properties of GO and more over it protects GO from outside environment. In previous study, our group have observed laser induced reduction of GO when GO was exposed to air, in contrast, no laser induced reduction of GO was observed when GO was impregnated into PVA films and this is very important for preparation of stable optical limiting devices. Figure 7C&7D shows the input-fluence dependent normalized transmittance of GO-PVA films (different GO/PVA ratios) at 400 and 800 nm respectively, which were extracted from Z-scan results at their respective wavelengths. The results show that the transmittance of GO-PVA films at both 400 and 800 nm decreased with the increase of input fluency, exhibiting broadband optical limiting. The Z-scan measurement on pure PVA film has also performed and compared with other GO-PVA films. The results exhibit that pure PVA films have saturable absorption at high input fluencies with 400 nm due to ground state bleaching as PVA have very 20 high linear absorption in the UV region. From the UV-vis-NIR transmittance measurements, both GO and PVA have significant absorption at 400 nm, but the composite films show optical limiting activity, which more confirms the strong nonlinear absorption of GO at 400 nm. In contrast, when the laser pulses change to 800 nm, pure PVA alone does not either saturable absorption or nonlinear absorption, indicating no influence of PVA on the optical limiting properties of GO at 800nm. The optical limiting onsets (Fon, the input fluency when the transmittance starts decreasing) and optical limiting thresholds (F50, the incident fluence at which the transmittance falls to 50% of linear transmittance) of GO-PVA films with different GO concentrations are summarized in table 1 below. The optical limiting thresholds at both 400 and 800 nm decreased with the increasing of the concentration of GO in PVA matrix. The F50 values of GO-PVA films are lower than the values of previously reported different materials. 800 nm Sample Linear Transmittance Weight ratio T% (GO:PVA) 400 nm Optical Optical Limiting Limiting Onset Threshold Fon F50 [mJcm-2] [mJcm-2] Linear Optical Transmittance Limiting Onset T% Optical Limiting Threshold Fon F50 [mJcm-2] [mJcm-2] 0.00025:1 97 4.0 73 63 3.7E-02 98 0.0005:1 88 3.1 60 33 3.4E-02 27 0.001:1 79 2.4 48 20 3.3E-02 7.7 0.0025:1 40 2.2 40 2 2.7E-02 1.6 Table 1: The optical limiting onsets (Fon, the input fluency when the transmittance 21 starts decreasing) and optical limiting thresholds (F50, the incident fluence at which the transmittance falls to 50% of linear transmittance) of GO-PVA films with different GO concentrations Previously it has been reported that the optical limiting properties of GO and GO-polymer composite solutions are mainly depends on nonlinear scattering9,18. But recently, several groups and our group proposed that the strong nonlinear absorption of GO is the reason for the optical limiting effects. To investigate the mechanism responsible for the observed optical limiting of GO-PVA films, we have used pump-probe measurements on the GO-PVA film at 800 nm pump and probe. rise=80fs (equal to the pulse duration) 0.002 T/T 0.000 -0.002 1ps (55%) Pump@800 nm Probe@800 nm 2ps (45%) -0.004 -0.006 0 10 20 30 Delay time (ps) 40 50 Figure 8: Pump-probe result of the GO-PVA film (weight ratio=0.0025). Figure 8 shows the transient decay kinetics of GO-PVA film after the excitation and the results indicate transient signal (negative T/T) is dominated by nonlinear 22 absorption. The data can be well fitted with tri exponential decay with time constants of risefs, 1=0.6 ps and 2=6.6 ps. The observed time constants rise1 and 2 are attributed to the electron-electron scattering, carrier-acoustic phonon scattering and inter-band carrier recombination of quasi-fluorescent π-conjugated sp2 clusters in GO sheets. The observed nonlinear absorption of GO is likely attributed to the excited-state absorption and multi-photon absorption of broad range of quasi-molecular fluorophores present in the GO sheets21. Furthermore, from previous report21 , the optical limiting activity of graphene oxide could be tuned by the extent of reduction. Thus the in situ reduction of the Graphene oxide in the free standing film has been tried. After heating in 110 °C at different periods, GO in the free standing film can be easily converted rGO by thermal effect. As shown in figure 9, the color of GO film becomes darker and darker by increasing the heating time. This kind of in situ reduction can prevent re-aggregation of rGO sheets to retain a homogeneous distribution and the obtained rGO-PVA film can be preserved for several months. Figure 9: Comparison of in situ reduction GO-PVA films. 23 2.5 Summary In summary, this chapter demonstrates a very simple method for the fabrication of flexible, highly stable and low-cost optical limiting devices by impregnating GO into PVA polymer matrix by one step solution process. The flexible GO-PVA films exhibited strong broadband optical limiting activity with very low optical limiting thresholds even lower than many reported materials. By varying the concentration of GO in PVA matrix, the linear and nonlinear optical properties of the composite film can be adjusted and the optical limiting activity of these films can be tuned by simple heating. Femtosecond pump-probe measurements on GO-PVA films indicate that the nonlinear absorption including excited state and the multi-photon absorption plays an important role in the observed optical limiting activity of GO-PVA films. This kind of stable and flexible optical limiting device opens a door for the practical applications because it can be fixed to any optical glasses or sensitive optical detectors to prevent intense light induced damage and this simple method could also be applied to produce flexible saturable absorbers by impregnating reduced graphene or pristine graphene into polymer matrices for laser mode-locking applications. Compare to the reported optical limiting materials like pure GO, the flexibility and stability are the greatest advantages. 24 3 THE FREE STANDING FILMS OF NOBLE METAL NANOPARTICLES WITH PMMA 3.1 Chemicals and Materials Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9%), Silver nitrate(AgNO3, 99%), Poly(methyl methacrylate) (PMMA), oleylamine and sodium borohydrate (NaBH4, 98%) were purchased from Aldrich. All chemicals were used as received without further purification. All solvents are of analytical grade and used as received without further purification. All aqueous solutions are prepared in deionized water. 3.2 Preparation of free standing films of noble metal nanoparticles with PMMA The silver and gold nanoparticles were synthesized by the method we reported before. 50 Silver and gold nanoparticles were preserved in DCM with a concentration of 20 mg·mL-1. Free-standing AgNPs-PMMA thin films can be fabricated by incorporating AgNPs into poly(methyl methacrylate). PMMA solution was prepared in dichloromethane (DCM) with a concentration of 50mg·mL-1. The solution was mixed and stirred overnight at room temperature to dissolve the PMMA. In order to fabricate AgNPs-PMMA films of different weight ratios (x=0.0004, 0.001, 0.002), 10μL, 25μL and 50μL of 20 mg·mL-1 AgNP solution was added to 10 mL of 25 50 mg·mL-1 in three different glass tubes respectively. The mixture in the three tubes was shaked to ensure homogeneity before transferring into three different petri dishes. The petri dishes were dried in room temperature for one hour. Then the free-standing AgNPs-PMMA films were peeled off and collected. The thickness of the AgNPs-PMMA films was measured using micrometer caliper and found to be 100 μm. The free-standing AuNPs-PMMA thin films were prepared in the same method. The whole preparation processes of Au/Ag-PMMA films are demonstrated in figure 10 below. Figure 10: Preparation processes of Au/Ag-PMMA films 3.3 Instrumentation Extinction spectra of Au/Ag-PMMA films were measured by using a SHIMADZU UV-2550 spectrophotometer. The Z-scan measurements were performed by using a Spectra Physics Ti:sapphire oscillator seeded regenerative amplifier laser system at 800nm and 400nm (800nm laser through a BBO crystal) with a repetition rate of 26 1kHz. Pump-probe experiments were used a Spectra-Physics femtosecond Ti:sapphire laser system also. Fabrication of lattice matrix was used z-scan measurement system. 3.4 Result and Discussion Figure 11 shows the particle size distribution of metal nanoparticles. From the TEM images we can confirm these particles are substantially similar size (5 nm). Figure 11: TEM images of silver nanoparticles and gold nanoparticles. In figure 12, the sharp peaks of Au/AgNP solutions in UV-vis-NIR spectra also verified the substantially similar size of particles. 27 Extinction 0.4 0.2 0.0 Weight ratio= Au:PMMA 1.5 600 800 Wavelength (nm) 1000 0.0002 0.0004 0.001 0.002 0.003 1.0 0.5 0.0 400 PMMA Ag NPs solution 2.0 Extinction PMMA Au NPs solution Weigth ration of PMMA:Au 0.0002 0.0004 0.001 0.002 0.003 0.6 400 600 800 1000 Wavelength (nm) Figure 12: UV-vis. Spectra of Au/Ag-PMMA films in different weight ratios; pure PMMA; AuNP solution and AgNP solution The peaks of Ag and Au NP solutions are 418 nm and 520 nm respectively. After impregnated with PMMA, both Ag and Au NPs have been red shifted and the absorption peaks become broader by increasing the weight of metal nanoparticle, which can be attributed to the aggregation of nanoparticles. With the increasing weight ratio of metal nanoparticles, we can see the colors of films slightly changed and become deeper and deeper by naked eyes (Figure 10). Pure PMMA film demonstrates very high transparency from 350nm to 1000nm, the same as glass, which indicates PMMA do not influence the optical properties of mixed films under visible-NIR range. The red shifted peaks show the aggregation of metal nanoparticles in PMMA matrix and after several months the peaks have maintained, which demonstrates the stability of mixed films. The PMMA matrix helps to protect the metal nanoparticles from outside environment, especially important to silver nanoparticles because they are easily oxidized in several hours when exposed to atmosphere. 28 The nonlinear optical properties of mixed films have been studied by open-aperture femtosecond Z-scan technique and pump-probe technique. The details of the experimental setup and experiment process have been introduced in previous 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 209uW 300uW 350uW 390uW 420uW 500uW 584uW 650uW 715uW 750uW A Au: PMMA = 0.001 linear T% at 800nm = 86% -10 -8 -6 -4 -2 0 2 Z (mm) 4 6 8 10 Normalized Transmittance Normalized Transmittance paragraph. 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 85uW 135uW 222uW 323uW 411uW 500uW 615uW 683uW 732uW B Ag:PMMA = 0.001 linear T%@800nm= 87% -10 -8 -6 -4 -2 0 2 Z (mm) 4 6 8 10 Figure 13: Open-aperture Z-scan measurements of Au-PMMA (A) and Ag-PMMA (B) films (Weight ratio=0.001). The figure 11 above shows the normalized transmittance of Au/Ag-PMMA films (weight ratio= 0.001) in the case of 800 nm excitation under different laser powers. From this figure of normalized transmittance at 800 nm, the saturable absorption signal of AuNPs-PMMA composite can be neglected but an optical switching effect has been found on AgNPs-PMMA composite film. AgNPs-PMMA composite film displayed SA behavior at a relatively lower excitation fluence, while switching to RSA as the excitation fluence increases. The SA behaviors were observed when excitation fluence at was 181 GW/cm2 or below for the composite film (Ag:PMMA=0.001). As the film moved into the beam focus, the pump fluence 29 increased; the transmittance was found to increase correspondingly, indicating an optically induced transparency in the film. The observed SA behavior could be attributed to the ground state Plasmon bleaching. Bleaching of ground state plasmon band will be resulted by the increasing pump fluence as the sample moved into beam focus. Very interestingly, when the excitation intensity increased, the film exhibited an additional RSA component first, as a valley (decreased transmittance) in the middle with two humps (increased transmittance) in both sides. With the further increasing excitation intensities, the z-scan traces were dominated by the RSA characteristics. AuNPs-PMMA exhibited same RSA behavior under high excitation intensities. The observation of RSA behavior under high excitation intensities suggests strong nonlinear absorption contribution in the prepared composite films. The blank experiment on the pure PMMA film did not present any RSA signal under the excitation intensities used for composite films. Which suggests the strong nonlinear optical activity arises from noble metal nanoparticles. The nonlinear absorption and the RSA behavior in metal nanoparticles were usually believed due to nonlinear absorption, including excited-state absorption and plasmon-enhanced multi-photon absorption, which is a result of increased electronic temperature under high excitation intensities. Upon photoexcitation, the electrons are excited to energy levels higher than the Fermi level, resulting in an increased electronic temperature and this phenomenon will lead to a strong bleaching signal near the plasmon maximum 30 (increase in transmittance) and transient absorption signals (decrease in transmittance) in the wings of the band. This phenomenon has been theoretically predicted and experimentally demonstrated for many metal nanoparticles. Furthermore, the transient absorption experiments (pump-probe experiments) and z-scan measurements of these composite films under different excitation wavelengths and different pulse duration have also been done. The input fluence dependent transmittance results of one film (Ag:PMMA=0.001) under different wavelengths were extracted from the z-scan measurement results and Normalized Transmittance have been presented in figure 14 below. Optical Limiting of Ag Film 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 600 nm 700 nm 750 nm 800 nm 900 nm 1000 nm 1100 nm 1E-4 50% 1E-3 0.01 Fluence (J/cm2) Figure 14: Z-scan measurement results of Ag-PMMA film (weight ratio=0.001). The transmittance of this film decreased with increasing input fluence, exhibiting 31 promising optical limiting activity for femtosecond laser pulses at all selected wavelength. But the optical limiting threshold values (F50, defined as the incident fluence at which the transmittance falls to 50% of the linear transmittance) are slightly different under different wavelengths. The optical limiting threshold values of the Ag-PMMA film was increased first and then decreased with the increasing wavelength of excitation light from 600 nm to 1100 nm. This result has confirmed the optical limiting function of this film is applicable in a broad range. The results of pulse duration dependent Z-scan measurements of selected Ag-PMMA film (weight ratio=0.0004) have shown in figure 14. With the extension of laser pulse duration, the optical limiting ability of this film clearly decreasing. This phenomenon suggests that the fast optical limiting response of composite film is dominated by two-photon induced absorption. Because the extension of laser pulse duration will reduce the peak power of the laser pulse and significant influence the two-photon absorption. 32 Normalized Transmittance 1.1 Ag:PMMA=0.0004 1.0 0.9 0.8 0.7 109fs 300fs 0.6 649fs 0.5 1009fs 2073fs 0.4 0.3 0.2 0.01 2 Input Fluence (J/cm ) 0.1 Figure 15: Fin Vs Trans curve on Ag in PMMA (weight ratio=0.0004) The pump-probe experiments at 800 nm pump and probe have also been done on this film (Ag:PMMA=0.001) and the results have been shown in figure 16. Figure 16 demonstrates the transient decay kinetics of AgNP-PMMA film after the excitation and the results indicate transient signal (negative T/T) is dominated by nonlinear absorption. The nonlinear absorption of AgNP-PMMA film is likely attributed to the excited-state absorption and multi-photon absorption. 33 0.001 T/T 0.000 -0.001 -0.002 Ag : PMMA =0.001 Probe = 545uW@800nm Probe @ 800nm -0.003 -0.004 0 10 20 30 Delay time (ps) 40 50 Figure 16: Pump-probe result of the Ag-PMMA film (weight ratio=0.001). 3.5 Summary In summary, here we demonstrate a simple way for fabrication of flexible, low-cost and highly stable optical switching and optical limiting devices by mixing Au/Ag nanoparticles with PMMA polymer matrix in solution process. This kind of composite films shows strong broadband optical limiting ability from the wavelength 600nm to 1100 nm with very low optical limiting thresholds. The optical limiting thresholds can be lowered by lattice matrix on the film surface, much lower than many reported materials. These composite films also exhibit optical switching ability, which can be used to optical switch applications like optical signal switching and routing, optical network monitoring and other applications. The linear and nonlinear optical properties 34 have been controlled by varying the concentration of Au/Ag in PMMA matrix. The degenerated pump-probe experiments and pulse duration dependent Z-scan measurements on composite films suggest that the optical limiting can be attributed to the nonlinear absorption including the excited state and two-photon absorption. Such stable and flexible optical limiting devices are easy for practical applications like to be fixed to any expensive or sensitive optical equipment such as sensors to limit the amount of optical power from sudden intense light thus prevent photo-induced damage. 4 CONCLUSION In this Msc. 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Image of GO-PVA film under bright field microscope Weight ratio (GO:PVA) 0.00025:1 Linear Two-photon Transmittance Absorption at 400 nm Coefficient βeff T% [cmGW-1] 63 1.15±0.04 Optical Optical Limiting Limiting Onset [b] Threshold Fon F50 [mJcm-2] [mJcm-2] 3.7E-02 98 0.0005:1 33 2.44±0.10 3.4E-02 27 0.001:1 20 4.71±0.16 3.3E-02 7.7 0.0025:1 2 16.1±0.69 2.7E-02 1.6 Sample Table S1. Linear transmittance (T), effective two-photon absorption coefficients (βeff), optical limiting onsets (Fon) and thresholds (F50) of GO in PVA films at 400 nm. 38 Sample Linear Multi-photon Absorption Transmittance Coefficient at 800 nm Weight ratio T% (GO:PVA) βeff γeff [cmGW-1] [cm3GW-2] 0.118± 0.0024 0.069± 0.003 0.046± 0.0006 0.024± 0.0007 0.00025:1 97 0.0005:1 88 0.001:1 79 0.0025:1 40 0.001± 0.0003 0.0005± 0.0001 0.00035± 0.00008 0.00016± 0.00003 Optical Optical Limiting Limiting Onset Threshold Fon [mJcm-2] F50 [mJcm-2] 4.0 73 3.1 60 2.4 48 2.2 40 Table S2. Linear transmittance (T), effective two- and three-photon absorption coefficients (βeff and γeff), optical limiting onsets (Fon) and thresholds (F50) of laser reduced GO in PVA films at 800 nm F50 (Ag) F50 (Au) 2 Threshold Fluence(J/cm ) 0.12 0.11 0.10 0.09 0.08 0 200 400 600 800 Pulse duration (fs) 1000 Figure S2. Fin Vs Trans curve on Au in PMMA and Ag in PMMA (weight ratio = 0.0004) 39 [...]... The free- standing GO- PVA thin films were then peeled off and collected The thickness of the GO- PVA films was measured using micrometer caliper and found to be 480 μm Figure 5 below demonstrates the preparation process of GO free- standing film (weight ratio=0.00025) 1μm PVA 200mg/mL GO 0.5mg/mL mix Pour into Dry at 40℃ petri dish 12 hours Figure 5: Schematic of preparing GO free- standing films (PVA. .. multi-photon absorption) of GO play an important role in the observed strong optical limiting activity of GO/ PVA films The optical limiting properties of Au/ Ag- PMMA films are also investigated by femtosecond Z-scan measurements at laser wavelengths from 600 nm to 1100 nm They also demonstrate strong optical limiting ability and the femtosecond pump-probe results suggest the optical limiting can be attributed... decreasing) and optical limiting thresholds (F50, the incident fluence at which the transmittance falls to 50% of linear transmittance) of GO- PVA films with different GO concentrations are summarized in table 1 below The optical limiting thresholds at both 400 and 800 nm decreased with the increasing of the concentration of GO in PVA matrix The F50 values of GO- PVA films are lower than the values of previously... films was measured using micrometer caliper and found to be 100 μm The free- standing AuNPs -PMMA thin films were prepared in the same method The whole preparation processes of Au/ Ag- PMMA films are demonstrated in figure 10 below Figure 10: Preparation processes of Au/ Ag- PMMA films 3.3 Instrumentation Extinction spectra of Au/ Ag- PMMA films were measured by using a SHIMADZU UV-2550 spectrophotometer The... exhibited strong broadband optical limiting activity with very low optical limiting thresholds even lower than many reported materials By varying the concentration of GO in PVA matrix, the linear and nonlinear optical properties of the composite film can be adjusted and the optical limiting activity of these films can be tuned by simple heating Femtosecond pump-probe measurements on GO- PVA films indicate that... are of analytical grade and used as received without further purification All aqueous solutions are prepared in deionized water 3.2 Preparation of free standing films of noble metal nanoparticles with PMMA The silver and gold nanoparticles were synthesized by the method we reported before 50 Silver and gold nanoparticles were preserved in DCM with a concentration of 20 mg·mL-1 Free- standing AgNPs -PMMA. .. have observed laser induced reduction of GO when GO was exposed to air, in contrast, no laser induced reduction of GO was observed when GO was impregnated into PVA films and this is very important for preparation of stable optical limiting devices Figure 7C&7D shows the input-fluence dependent normalized transmittance of GO- PVA films (different GO/ PVA ratios) at 400 and 800 nm respectively, which were... simple method for the fabrication of flexible nonlinear optical devices by impregnating graphene oxide into PVA polymer matrix by solution process The prepared GO- PVA films have been characterized by UV-visible transmittance, fluorescence spectroscopy and microscopy techniques Broadband optical limiting properties of as prepared flexible GO- PVA films made of different GO/ PVA ratios have been investigated... both GO and PVA have significant absorption at 400 nm, but the composite films show optical limiting activity, which more confirms the strong nonlinear absorption of GO at 400 nm In contrast, when the laser pulses change to 800 nm, pure PVA alone does not either saturable absorption or nonlinear absorption, indicating no influence of PVA on the optical limiting properties of GO at 800nm The optical limiting. .. expand by melting/vaporization of 6 surface atoms and the expanded particles act as another type of scattering center and limit the light transmission during the laser pulse duration In this work, we have prepared oleylamine-capped gold/silver nanoparticles in Dichloromethane (DCM) and incorporate them into PMMA solution to fabricate free standing films and studied the optical limiting properties of