Femtosecond time domain spectroscopy and nonlinear optical properties of iron pnictide superconductors and nanosyatems

Abstract

In a broad sense, ultrafast laser pulses (pulsewidth ?t < 1 ps) have three important applications:

(i) Inducing a perturbation in a system of study within a time much shorter than the characteristic lifetimes 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 (~10¹? 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 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 a variety 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-carbon-nitrogen), 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 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 z-scan technique, and terahertz time-domain spectroscopy using sub-picosecond terahertz pulses.

The remainder of the thesis has been organized into three parts:

Part I: Femtosecond time-resolved measurements on recently discovered spin density wave and superconducting Ca(Fe???Co?)?As? iron pnictides.

Part II: Pump-probe and z-scan measurements on graphene, BCN, silicon nanowires, and gold nanoparticles.

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???Co?)?As? iron pnictides with x = 0 (parent compound) and optimally doped x = 0.056. The undoped crystal shows a spin density wave phase transition at T_SDW ~ 165 K with a concurrent structural transition from high-symmetry tetragonal to low-symmetry orthorhombic phase. The optimally doped crystal shows superconducting transition at T_SC ~ 20 K and spin density wave transition at T_SDW ~ 85 K, and high-temperature tetragonal to low-temperature orthorhombic phase transition at about 88 K. 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 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 behavior as a function of temperature.

We have used the Rothwarf-Taylor (R-T) phonon-bottleneck relaxation model for gapped systems to understand our experimental results for the carrier dynamics. In Chapter 3, 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 in the doped superconducting compound, we have to invoke the strong phonon-bottleneck regime of the model (Chapter 4).

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, show 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 is the subject 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 (?) used in our experiments.

In the degenerate pump-probe experiments (? = 790 nm), we usually see photobleaching (PB) for nanorods with ?_LSP > ?, and photo-induced absorption (PA) for nanorods with ?_LSP < ?, which can be understood from our simulations. The unusual behavior of the gold nanorods is in terms of transition from PB to PA for the same nanorod sample, which otherwise should have always shown PB (?_LSP > ?). In nondegenerate pump-probe experiments (395 nm pump and 790 nm probe), the probe fluence can be used as a control parameter to show a gradual change from PB to PA for the gold nanorod samples with ?_LSP > ?. Concurrently, around the threshold value of the probe fluence in such a crossover, the carrier relaxation time increases significantly. We have given a physical reasoning for this switching behavior, 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 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.