FEMTOSECOND TIME-DOMAIN SPECTROSCOPY AND NONLINEAR OPTICAL PROPERTIES OF IRON-PNICTIDE SUPERCONDUCTORS AND NANOSYSTEMS A Thesis Submitted for the degree of Doctor of Philosophy IN THE FACULTY OF SCIENCE by Sunil Kumar Department of Physics Indian Institute of Science BANGALORE 560 012 February, 2012 i
(Sunil Kumar), 2012 All rights reserved ii
THESIS SYNOPSIS In a broad sense, ultrafast laser pulses (pulsewidth t < 1ps) have three important applications: (i) inducing a perturbation in a system of study within a time much shorter than the characteristic life-times of the system-constituents such as charge-carriers, phonons and other excitations. The temporal evolution and the channels through which the system restores to its equilibrium state can be studied by using a much weaker second pulse. (ii) Electromagnetic energy contained in the ultrafast laser-pulse results in high peak power (~10 10 Watt) which is sufficient to induce nonlinear effects in materials, primarily electronic in nature as compared to thermal phenomena in the nanosecond time domain. (iii) Thirdly, the spectral width of the femtosecond laser pulses is very broad ( 1/ t) resulting in generation of new radiation such as terahertz radiation by nonlinear frequency-mixing in a nonlinear crystal. In the present thesis we have taken advantage of all these three features of femtosecond laser pulses and have studied varieties of novel systems in condensed matter. We have experimentally investigated electron and phonon dynamics in newly discovered iron pnictide superconductors, graphene and analogous BCN (boron-carbonnitrogen) and other semiconducting and metallic nanosystems. Third-order optical nonlinear coefficients of graphene, BCN, silicon nanowires and gold nanoparticles have been estimated using nonlinear transmission measurements. Also, investigations of the electronic and vibrational characteristics of carbon nanotubes and silver nanoparticles embedded in a polymer matrix have been carried out using terahertz time-domain spectroscopy. Chapter 1 reviews the important physical principles involved in the ultrafast processes in condensed matter, in general, describing the above mentioned three attributes of femtosecond laser pulses. The physics to be derived from time-resolved pump-probe spectroscopy, nonlinear transmission experiments and terahertz time-domain spectroscopy has been discussed in detail. Background study of the electronic and phononic properties of the systems studied is given in the last section of the chapter. In Chapter 2, the experimental techniques and related tools used in the present thesis are described. These include generation and detection of femtosecond laser pulses, femtosecond time-resolved pump-probe spectroscopy, nonlinear transmission single beam iii
z-scan technique, and terahertz time-domain spectroscopy using sub-picosecond terahertz pulses. The remainder of the thesis has been organised into three parts. Part I: Femtosecond time-resolved measurements on recently discovered spin density wave and superconducting Ca(Fe 1-x Co x ) 2 As 2 iron pnictides; Part II: Pump-probe and z-scan measurements on graphene, BCN, silicon nanowires and gold-nanoparticles, and Part III: Terahertz time-domain spectroscopy of single-walled and double-walled carbon nanotubes and silver nanoparticles embedded in polymer films. In Part I (Chapters 3 and 4) we have presented results on quasiparticle dynamics in femtosecond photoexcited Ca(Fe 1-x Co x ) 2 As 2 iron pnictides with x = 0 (parent compound) and optimally doped x = 0.056. The undoped crystal shows spin density wave phase transition at T SDW ~ 165K with a concurrent structural transition from high symmetric tetragonal to low symmetry orthorhombic phase. Optimally doped crystal shows superconducting transition at T SC ~ 20K and spin density wave transition at T SDW ~ 85K, and high-temperature tetragonal to low temperature orthorhombic phase transition at about 88K. We have carried out detailed temperature (3.5 K to 300 K) and laser-fluence dependent studies on these compounds. It is observed that the photoexcited carrier dynamics evolves with three relaxation components in both the spin density wave and the superconducting states, showing large variations in their amplitudes and time-constants. We observed coherent longitudinal acoustic phonons (LAM) in the undoped crystal whereas both the longitudinal and transverse acoustic phonons (TAM) along with a high frequency single optical phonon mode at frequency ~5.6 THz were detected in the doped crystal. The temperature-dependence has been studied in the whole temperature range of 3.5 K to 300 K. Using the thermal and/or electronic stress induced strain pulse propagation for the generation and detection of the acoustic phonons in the crystals we estimate the elastic behaviour as a function of temperature. We have used Routhwarf-Taylor (R-T) phonon-bottleneck relaxation model for gapped systems to understand our experimental results for the carrier dynamics. We will see in Chapter 3 that the temperature evolution of the fast (sub-picosecond) electronic relaxation parameters in the parent compound can be understood by considering the weak phonon-bottleneck description in the R-T model, whereas that in the doped superconducting compound, we have to invoke the strong phonon-bottleneck regime of the model (Chapter 4). iv
In Part II (Chapters 5, 6 and 7), the femtosecond photophysics of nanosystems including graphene have been discussed where results on the ultrafast carrier-dynamics and optical nonlinearities of these systems have been presented. In Chapter 5, we have presented results of pump-probe measurements on graphene suspensions as well as their thin films deposited on glass plates or indium-tin-oxide (ITO) coated glass plates. The dependence of the carrier dynamics on the laser-fluence and the pump-wavelength has been studied in detail. Our pump-probe measurements in conjunction with nonlinear transmission z-scan show saturable absorption in graphene. Optical responses from BCN (boron-carbon-nitrogen) (two to three layers), an analogue of graphene shows photobleaching at 395 nm pump and 790 nm probe, and optical limiting at 395 nm. The evolution of pump-induced changes in the differential reflectivity and transmission from a silicon nanowire film consisting of crystalline-core amorphous-shell silicon nanowires is reported in Chapter 6. Comparing our results on these nanowires with those obtained separately on crystalline silicon-on-sapphire under similar conditions, we infer that the multicomponent relaxation of the differential reflectivity or transmission from the core-shell nanowires has contributions at larger time-scales (> 100 ps) from the electrons in the crystalline core and at faster time-scales (< 10 ps) from the electrons in the amorphous shell. Unusual femtosecond photophysics of gold nanoparticles makes the subject study of Chapter 7. We have taken two examples of gold nanoparticles for our study: the gold nanorods and 15-atom gold clusters. Gold nanorods are prepared such that they have the longitudinal surface plasmon peak ( LSP ) varying from 660 nm to 849 nm (depending on the nanorods aspect ratio), which lie on either side of the laser wavelength ( L ) used in our experiments. In the degenerate pump-probe experiments ( L = 790 nm) we usually see photobleaching (PB) for nanorods with LSP > L, and photo-induced absorption (PA) for nanorods with LSP < L which can be understood from our simulations. The unusual behaviour of the gold nanorods is in terms of transition from PB to PA for the same nanorods sample which otherwise should have always shown PB ( LSP > L ). In nondegenerate pump-probe experiments (395 nm pump and 790 nm probe) the probefluence can be used as a control parameter to show a gradual change from PB to PA for the gold nanorods samples with LSP > L. Concurrently, around the threshold value of the probe-fluence in such a cross-over, the carrier relaxation time increases significantly. We v
have given a physical reasoning for this switching behaviour, hand in hand with the understanding of switching from photo-bleaching to photo-induced absorption in terms of two-photon absorption from the probe. Next, the 15-atom gold clusters deposited on an ITO coated glass plate show significant enhancement of the third-order optical susceptibility as compared to the clusters deposited on a glass plate, as observed from z-scan and pump-probe experiments. Noting that these 15-atom gold clusters do not show any surface plasmon resonance band in the optical absorption, we have qualitatively attributed the enhancement effect to excited state charge transfer between the gold cluster and ITO film. In Part III (Chapters 8 and 9) we have discussed the low-energy electronic and phononic features of carbon nanotubes, semi-crystalline polymer and silver nanoparticles as investigated using terahertz time-domain spectroscopy. We show that the experimentally measured real and imaginary parts of the frequency-dependent dielectric function have signatures in the experimental frequency range of 0.1 3.0 THz for all the three nanosystems. In carbon nanotubes, these low frequency resonances are attributed to flexural modes which have been predicted theoretically but not observed directly in any scattering or absorption experiments so far. For silver nanoparticles, these resonances arise due to infrared active confined acoustic phonons. Similarly, in a poly(vinyl alcohol) film, the observed single resonance feature at ~1.2 THz is due to longitudinal acoustic mode of vibrations localized along the length of the crystalline lamellae in the polymer. vi
TABLE OF CONTENTS Declaration Acknowledgement Synopsis Publications i iii v ix Chapters 1 Introduction 1.1 Response of a material under optical excitation 1.1.1 Linear and nonlinear optical properties. 1.1.2 Second order optical nonlinearities 1.1.3 Third order optical nonlinearities 1.1.4 z-scan method to estimate and 1.2 Time-resolved pump-probe spectroscopy 1.2.1 Photoexcited carrier dynamics 1.2.2 Coherent optical phonons 1.2.3 Thermo-elastic strain pulse propagation in solid media 1.3 Coherent terahertz radiation for spectroscopy 1.3.1 Photoconductive antennas 1.3.2 Optical rectification and electro-optical sampling 1.3.3 Principles of THz time domain spectroscopy 1.4 Condensed matter systems studied: a background survey 1.4.1 Graphene: electronic and phonon structures 1.4.2 Carbon nanotubes: low-energy electronic and phononic structures 1.4.3 Metal nanoparticles 1.4.3.1 Surface plasmon resonance 1.4.3.2 Lattice vibrations of a small particle 1.4.3.2.1Elastic vibrations of a spherical particle 1.4.3.2.2Elastic vibrations of a cylindrical rod 1.4.4 Iron pnictide superconductors 1.4.4.1 The 122 family of iron pnictides 1.4.4.1.1Electronic structure 1.4.4.1.2Optical phonons 2 Experimental tools and procedures. 2.1 Femtosecond laser pulses as experimental tools 2.1.1 Low-pulse-energy high-repetition-rate femtosecond oscillator 2.1.2 High pulse-energy low-repetition-rate femtosecond regenerative amplifier system 2.1.3 Pulse-width Measurements by autocorrelation techniques vii 1 2 2 3 5 6 7 10 11 14 16 17 20 22 24 25 28 35 37 38 39 41 43 43 48 57 57 59 60 61
2.1.3.1 Intensity autocorrelation by second harmonic generation 2.1.3.2 Autocorrelation by two-photon absorption 2.2 Experimental Techniques 2.2.1 Femtosecond time-resolved pump-probe spectroscopy 2.2.1.1 Degenerate pump-probe spectroscopy 2.2.1.2 Nondegenerate pump-probe spectroscopy 2.2.2 Single beam z-scan method 2.2.3 Designing a terahertz time-domain spectrometer 2.2.3.1 Photoconductive antenna based spectrometer 2.2.3.2 ZnTe based spectrometer 3 Gap dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe 2 As 2 across the spin density wave phase transition 3.1 Introduction 3.2 Experimental details 3.3 Results and discussion 3.3.1 Carrier dynamics 3.3.2 Generation and detection of coherent longitudinal acoustic phonons 3.4 Conclusions 4 Charged quasiparticle dynamics, coherent optical and acoustic phonons studied by time-resolved spectroscopy in superconducting iron-pnictide Ca(Fe 0.944 Co 0.056 ) 2 As 2 4.1 Introduction 4.2 Experimental details 4.3 Results and discussion 4.3.1 Carrier dynamics 4.3.1.1 Dynamics below T SC 4.3.1.2 Dynamics in the region T SC < T < T SDW 4.3.1.3 Fluence-dependent dynamics 4.3.2 Coherent acoustic phonons 4.3.3 Coherent optical phonons 4.4 Conclusions 5 Femtosecond photoresponse from graphene and analogous BCN.. 5.1 Graphene under ultrafast laser excitation 5.1.1 Carrier dynamics in graphene flakes suspended in various solvents 5.1.1.1 Preparation of colloidal suspensions of graphene 5.1.1.2 Degenerate time-resolved differential transmission spectroscopy 5.1.1.3 Results and discussion 5.1.2 Comparison of pump-fluence dependent carrier-dynamics in reduced-graphene-oxide suspensions and films 5.1.3 Two-color pump-probe spectroscopy of reduced-graphene oxide layers in suspensions and thin films 5.1.4 Nonlinear optics on graphene by z-scan 5.1.4.1 Saturable absorption in graphene-suspensions at 790 nm excitation 62 64 66 67 68 69 71 72 72 77 78 79 81 81 86 90 95 96 97 98 101 101 103 104 106 110 112 117 118 119 120 120 121 123 126 131 133 viii
5.1.4.2 Saturable absorption in graphene-suspensions at 395 nm excitation 5.2 Graphene analogue BCN 5.2.1 Hot carrier dynamics in few layer BCN 5.2.2 Optical-limiting and nonlinear refraction in BCN-suspension at 395 nm 5.3 Conclusions 6 Probing ultrafast carrier dynamics and nonlinear absorption and refraction in core-shell silicon nanowires. 6.1 Introduction 6.2 Experimental details 6.3 Results and discussion 6.4 Conclusions 7 Unusual femtosecond photophysics of (a) gold nanorods and (b) excited state charge transfer enhanced optical nonlinearity in 15-atom gold clusters. 7.1 Gold nanorods 7.1.1 Preparation and characterization of gold nanorod suspensions 7.1.2 Time-resolved differential transmission measurements on gold nanorods 7.1.3 Tuning between photo-bleaching and photo-induced absorption by selective longitudinal surface plasmon resonance 7.1.4 Transition from photo-bleaching to photo-induced absorption triggered by probe fluence 7.1.5 Coherent extensional vibrational mode of nanorods 7.2 Ultrafast optical limiting in gold nanoclusters 7.2.1 Gold 15-atom clusters 7.2.1.1 Nonlinear transmission measurements 7.2.1.2 Time-resolved differential transmission measurements 7.2.1.3 Discussion 7.3 Conclusions 8 Direct observation of confined acoustic phonons in silver nanoparticles and elongated polymeric chains in poly(vinyl alcohol) free standing films.. 8.1 Spectroscopy of metal nanoparticles 8.2 Terahertz time-domain spectroscopy 8.2.1 Samples and experimental conditions 8.2.2 Results for pristine PVA film 8.2.2.1 Longitudinal acoustic modes of crystalline lamellae in PVA 8.2.3 Results for silver nanoparticles-embedded PVA film 8.2.3.1 Confined acoustic phonons in silver nanoparticles 8.3 Conclusions 134 136 138 141 143 149 150 151 153 161 165 166 167 168 170 173 178 179 180 182 184 186 187 193 194 194 195 195 199 200 202 204 9 Experimental evidence of ultralow frequency vibrations of single-walled And double-walled carbon nanotubes 209 ix
9.1 History of ultralow frequency vibrations of carbon nanotubes 9.2 Terahertz time-domain spectroscopy of single-walled carbon nanotubes 9.2.1 Sample details and experimental conditions 9.2.2 Results and discussion 9.2.2.1 Low frequency phonons of single walled carbon nanotubes 9.3 Terahertz time-domain spectroscopy of double-walled carbon nanotubes 9.3.1 Details of sample and experimental conditions 9.3.2 Results and discussion 9.4 Conclusions 210 211 212 213 215 216 217 219 223 x