High-Field Terahertz Spectroscopy of Nanoscale Materials

This dissertation covers my work relating to the application of strong terahertz (THz) radiation to study the nonlinear properties of nano-scale material. It presents experimental and numerical studies on the optical and electronic properties of various material system including single-layer graphene, multi-walled carbon nanotubes (MWCNTs), nanoantenna-patterned gallium arsenide (GaAs) and planar metamaterial on GaAs. The THz generation and detection system was developed by our group through several years both in hardware and software to employ strong and short THz pulses for time-domain pump-probe THz/optical spectroscopy. Due to the low photon energy (4 meV for 1THz) the broadband single-cycle THz pulses are useful for noncontact, non-invasive probing of the fundamental electronic properties of material such as conductivity, permittivity and photocarrier relaxation dynamics. The images of carrier conductivity of single-layer graphene samples were obtained by recording the THz transmission via raster scanning of the samples. The theory to extract the sheet conductivity of a single-layer graphene is based on the Drude model, analyzing the THz transmission with the thin-film Fresnel coefficients. Time-resolved measurements of photoexcited single-layer graphene were also performed. The time-resolved data from the pump-probe experiment reveal that both the strong THz field and photoexcitationenhance the THz transmission due to the increase of the carrier scattering rate, however, the relaxation of photocarriers shows opposite effects of the THz fields and photoexcitation. The photoexcitation increases the relaxation time via the reabsorption of optical phonon by photocarriers, while strong THz fields reduce the relaxation time because the fields redistribute the empty states in the conduction band and open up more phonon emission path. THz raster-scan imaging on aligned MWCNTs shows strong anisotropic linear and nonlinear THz responses. Strong nonlinear absorption was observed when THz field polarization was parallel to the MWCNTs axis, on the other hand, no nonlinear effect was observed for perpendicular polarization. The THz waveforms transmitted through the sample obtained by THz time-domain spectroscopy (THz-TDS) reveal phase shifts. Numerical analysis on the waveforms to extract the complex refractive index of material shows that strong THz field enhances the permittivity of the MWCNTs by inducing strong nonlinear electron dynamics. Time-resolved measurements on optically excited MWCNTs show that the photoexcitation induces interband transition in the sample. For the low and high THz regime (E_THz 653 kV/cm) an increase of the carrier density by generating the hot carriers via interband transitions was observed. On the contrary in the moderate THz field strength (538 < E_THz < 653 kV/cm) the dominant sub-band scattering gives rise to more effective mass for the carriers, and it reduces their mobility. As a result, a switching feature in the THz field response of the optically excited sample was observed. THz plasmonic structures, consisting of a nanoantenna array fabricated on GaAs, have been tested for ultrafast optical switching by strong THz pulses. Photocarrier injection by optical excitation immediately turns off the plasmon resonance of the antenna, while the strong and short THz pulses instantly revive the antenna resonance. The strong THz fields drive intervalley scattering and interband tunneling of the photocarriers and consequently reduce the transient conductivity of the photoexcited GaAs leading to the revival of the antenna resonance. Time-resolved THz transmission of bare GaAs sample compared to that of the patterned GaAs shows that the carrier recovery rate is much faster in the patterned sample.Lastly, we studied a metamaterial structure fabricated on intrinsic GaAs to observe the dynamics of the plasmon induced transparency (PIT) when the sample is optically excited. The PIT structure shows a narrow spectral window of transparency due to the destructive interference of a super-radiant resonator (a central dipole antenna) to the sub-radiant resonator (split-ring resonator). We observed that the response of the PIT structure to the incident THz field does not show any nonlinear response. However, optical excitations resulted in the damping of the PIT resonance. Adjusting the timedelay between the excitation pulse and the THz incident field reveals a pulsation response in the transmitted THz pulses along with self-phase modulation and red-shift in the PIT response

High-resolution EBSD characterisation of friction stir welded nickel-copper alloy:Effect of the initial microstructure on microstructural evolution and mechanical properties

The effect of the initial structure on the microstructure and texture of the friction stir welded Monel 400 alloy joints was investigated. For this aim, Monel 400 alloy of two initially treated conditions, annealed and rolled, was studied. Orientation imaging microscopy with a step size of 70nm was used to characterise the microstructures of the joints. The mechanical properties were evaluated using Vickers hardness and nanoindentation tests. In the joint of the initial annealed alloy, continuous dynamic recrystallisation governed the grain structure formation. However, in the case of the initial rolled base material, discontinuous dynamic recrystallisation also occurred to cause more random texture, smaller grains, much more high-angle grain boundaries and higher dislocation densities. In the stir zone, the hardness and yielding strength increased respectively from 177 HV to 192 HV and 215.9 to 238.8MPa by changing the base metal from the initial annealed to rolled condition. In addition, the microstructural features of the different zones containing the texture components and grain boundaries were scrutinised thoroughly

Application of response surface methodology for thorium(IV) removal using Amberlite IR-120 and IRA-400: Ion exchange equilibrium and kinetics

In this work, thorium (IV) removal from aqueous solutions was investigated in batch systems of cationic and anionic resins of Amberlite IR-120 and IRA-400. In this way, the effects of pH, initial Th(IV) concentration and the amount of adsorbent were investigated. A Central Composite Design (CCD) under Response Surface Methodology (RSM) was employed to determine the optimized condition. The results showed that the maximum removal efficiency of Th(IV) onto IR-120 and IRA-400 either discretely or in combination, albeit with equal mass fraction, was determined as follows: 98.09% , 65.70% and 72.19% at pH=3.23, 6 and 4.07, initial Th(IV) concentration of 78.2, 30 and 55.4 mg.L-1 and 2.08, 2.5 and 2.2 g.L-1 of resin, respectively. The kinetic and equilibrium data were accurately described by the pseudo-second order and Langmuir models. The results showed that IR-120 is a suitable adsorbent for thorium removal from aqueous solutions

Development of an indirect sandwich ELISA for detection of urinary antigen, using Legionella pneumophila PAL protein

Legionella pneumophila peptidoglycan-associated lipoprotein (PAL) protein is an extremely conserved antigen among Legionella species. In this study, rabbit and rat anti-PAL immunoglobulin G antibodies were produced by immunization with purified, recombinant PAL (r-PAL) protein of L. pneumophila serogroup 1 and used as capture and detection antibodies in the PAL antigen-based enzyme-linked immunosorbent assay (ELISA) to detect urinary PAL antigen. Urine samples were obtained from rats experimentally infected with L. pneumophila serogroup 1. The PAL antigen was measured in urine samples of 40 infected and 40 uninfected rats. After choosing the cut-off value of 0.192, the sensitivity and specificity of the PAL antigen-based ELISA were 87.5 and 97.5 %, respectively. The results obtained by PAL antigen base ELISA were compared with those obtained by Biotest. The PAL antigen was detected efficiently by both of the assays and all of the control human urine samples were negative by the ELISA test. The PAL antigen-based ELISA assay was relatively simple to perform, precise, highly sensitive and specific, and reproducible. Based on our data the PAL antigen-based ELISA described here is the first indirect sandwich ELISA for urinary antigen detection which could easily be applied for diagnosis of Legionnaires disease

The Effect of a Cognitive-behavioral Intervention Based on Lifestyle Modification on the Left Ventricular Ejection Fraction of Patients after Coronary Artery Bypass Graft Surgery

Background & Objective: The promotion of health behaviors plays a key role in maintaining cardiac function and reducing cardiac complications. The present study aimed to investigate the effect of a lifestyle-based behavioral intervention on the left ventricular ejection fraction of patients after coronary artery bypass graft surgery. Materials and Methods: This quasi-experimental study was conducted on 80 patients undergoing coronary artery bypass graft surgery in Kashani Hospital affiliated to Shahrekord University of Medical Sciences in Shahrekord, Iran in 2019. The patients were selected via convenience sampling and randomly divided into two groups of case and control (40 per each). Data were collected using a demographic questionnaire and checklist of left ventricular ejection fraction, which were completed in two stages before and three months after the intervention by both groups. The training intervention was performed based on modifications in lifestyle, physical activity, beliefs, communication, and nutrition in eight sessions by a cardiology nurse and a clinical psychologist. Data analysis was performed in SPSS version 17 using descriptive and analytical tests, such as paired t-test, independent t-test, and Fisherchr('39')s exact, at the significance level of P<0.05. Results: The mean percentage of the left ventricular ejection fraction before the psychological intervention had no significant difference between the case and control groups (P=0.33), while a significant difference was observed between the groups after the intervention (P=0.05). The mean percentage of the left ventricular ejection fraction in the case and control groups from before to after the intervention was significantly different (P=0.05) as the ejection fraction increased in both group; however, the difference was more significant in the case group. Conclusion: According to the results, the cognitive-behavioral intervention based on lifestyle modification could effectively enhance the efficacy of the cardiac surgery in improving the left ventricular ejection fraction. Keywords: Cognitive-behavioral Intervention, Lifestyle, Left Ventricular Ejection Fraction, Coronary Artery Bypass Graf

Effects of an Orem-based Self-care Program on the Orem's Pattern of Lower-extremity Edema in Patients Undergoing Coronary Artery Bypass Grafting

Background & Objective: Coronary artery disease (CAD) is a cardiovascular disorder associated with long-term constraints in the patients' life. Open heart surgery is an effective treatment for CAD, and the postoperative complications require extensive care, especially by the patient. Self-care education plays a key role in this regard, particularly based on Orem self-care model. The present study aimed to investigating the effects of an Orem-based self-care program on Orem's pattern of lower-extremity edema in patients undergoing coronary artery bypass grafting (CABG). Materials and Methods: This clinical trial was conducted on 74 patients undergoing CABG at Kashani Hospital in Shahrekord, Iran, who were randomly allocated to the intervention and control groups using software. Data were collected using the need assessment form based on the Orem model, and edema was measured using a meter. Data analysis was performed in SPSS version 21. Results: The Orem-based self-care program reduced lower-extremity edema in the intervention group (before: 34.31±31, after: 32.14±2.87), while the changes in the control group were not significant (P>0.05). Conclusion: According to the results, nursing patterns (e.g., Orem's model) could be used to enhance self-care abilities in patients requiring long-term care. Keywords: Self-care, Orem Model, Edema, Coronary Artery Bypass Graftin

Terahertz induced transparency in single-layer graphene

We show that the transmission of a terahertz (THz) pulse through single-layer graphene is strongly nonlinear. As the peak electric field of the THz pulse exceeds 50 kV/cm, the graphene becomes increasingly transparent to the THz radiation. When field strength reaches 800 kV/cm, the increased transparency corresponds to a two-fold decrease in the time-average sheet conductivity of the graphene (time averaged over the duration of the pulse). Time-resolved measurements reveal that the leading portion of the pulse creates transparency for the trailing portion, with a 10-fold suppression in sheet conductivity at the tail of the strongest THz pulse. Comparing the THz-induced transparency phenomena in different sample geometries shows that substrate-free graphene is the best geometry for maximizing the nonlinear transparency effect.Keywords: Transistors, Spectroscop

Anisotropic high-field terahertz response of free-standing carbon nanotubes

We demonstrate that unidirectionally aligned, free-standing multi-walled carbon nanotubes (CNTs) exhibit highly anisotropic linear and nonlinear terahertz (THz) responses. For the polarization parallel to the CNT axis, strong THz pulses induce nonlinear absorption in the quasi-one-dimensional conducting media, while no nonlinear effect is observed in the perpendicular polarization configuration. Time-resolved measurements of transmitted THz pulses and a theoretical analysis of the data reveal that intense THz fields enhance permittivity in carbon nanotubes by generating charge carriers