Browsing by Subject "Terahertz"
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Item Electro-optic Polymer Devices for Terahertz Applications(2010-01-01) McLaughlin, Colin Vincent; Hayden, L. Michael; Physics; Physics, AppliedThe rapid development of technologies employing terahertz (THz) radiation has led to numerous industrial and scientific applications. Current THz technologies are limited in their frequency response because of phonon absorbance and poor phase matching in crystalline emitters and detectors, or are limited to high-power bench-top pump laser systems for air-plasma generation and detection. In contrast, amorphous electro-optic (EO) polymer composite materials have the potential for broad-bandwidth, spectral gap-free THz emission and detection while requiring a relatively low pump laser power. In this thesis a theoretical description of THz radiation emission and detection using EO polymers is reviewed, including the effects of laser spectral bandwidth, pulse distortion, and material properties of the EO media. This model is used as a guide to improve the response of a THz system employing electro-optic polymer emitters and detectors. EO polymer composites that have been engineered for terahertz applications are described. These materials show a progression of improvements for use in THz systems, including higher EO coefficients, increased photostability, and reduced aggregation and dimerization. A study of in-plane (longitudinal) and parallel-plate (transverse) poling of EO polymers for use as THz sensors is presented, including a theoretical description of detection sensitivity for each device. In-plane poling allows access to the full optical nonlinearity of the EO polymer, potentially increasing detection sensitivity by a factor of 2.4 over parallel-plate poled devices. A transmission ellipsometric EO measurement technique is developed for the in-plane poling device and is used in the experimental comparison of the two devices. EO polymer composites are employed as terahertz emitters and sensors in systems using communication-wavelength pump lasers. A 15 THz wideband response is achieved using the ALTB203/APC composite, and is compared to the organic crystal DAST and to the THz system model. Increasing emitter thickness is studied through stacking multiple EO polymer emitter films. Frequency-dependent terahertz index and absorption of the emitter and sensor films are included into the THz model for a more accurate representation of the terahertz system response. Given proper phase-matching and low absorption, EO polymer materials can potentially be used in a waveguide geometry to generate broadband THz radiation. Coupling these devices with currently-available ultra-fast fiber lasers could lead to the development of field-deployable, compact, inexpensive THz systems.Item Optical Pump Terahertz Probe Studies of Semiconducting Polymers(2010-01-01) Cunningham, Paul D; Hayden, L. Michael; Physics; Physics, AppliedOptical-pump terahertz-probe spectroscopy (OPTP) has been applied to study charge generation, transport and the evolution of the photo-induced excited states in thin film organic semiconductors, with emphasis on their relevance to photovoltaic technology. In these experiments the response of the photoexcited material to the AC electric field of a terahertz (THz) pulse was measured. From this response, the evolution of the complex conductivity in the far-infrared was monitored. OPTP presents advantages over other techniques by being an all-optical probe of the complex conductivity over nanometer scale distances with sub-picosecond resolution and exhibits particular sensitivity to carrier scattering rates, which typically lay in the THz range. Conductivity models were applied to the extracted conductivity curves in order to determine technologically relevant quantities like the charge carrier mobility and external quantum yield of charge carrier generation. We observed charge carriers generated on a subpicosecond time scale in thin films of polyhexylthiophene (P3HT). Through application of the Drude-Smith model (DSM) over the 0-2 THz band, we determined a room temperature intrinsic mobility of about 30 cm2 The temperature dependence of the conductivity dynamics showed signs of thermally activated polaron hopping influenced by torsional disorder. Both above and below gap excitation resulted in similar dynamics, showing that the majority of carriers recombine within 1 ps. We were able to observe charge transfer occurring on a sub-ps timescale to the soluble fullerene, PCBM, for both excited states, demonstrating that narrow gap polymers can be blended with PCBM for photovoltaic applications. We observed charge carrier generated on a sub-ps time scale in thin amorphous films of metalated polymers. The time evolution of the conductivity showed that charge carriers recombine and only excitons persist after 100 ps. This characteristic appears to be common to amorphous systems. An intrinsic mobility of 20 cm2 s found for the most promising material. Broadband (0-6 THz) studies of the photoconductivity in P3HT suggest that the hole mobility is lower than initially determined. They also bring into question whether the DSM can describe the conductivity effectively or whether delocalized polaron transitions at higher frequencies are the origin of the observed features.Item Ultrafast Carrier Dynamics in Monolayer Transition Metal Dichalcogenides via Time-Resolved Terahertz Spectroscopy(2021-01-01) Gustafson, Jon K.; Hayden, L. Michael; Physics; PhysicsAtomically thin transition metal dichalcogenides (TMDs) are a class of two-dimensional materials that have attracted much attention in the past decade because of their unique properties. One such property is the existence of tightly bound excitons at room temperature. These excitons are a consequence of quantum confinement and the reduced dielectric environment enhancing the Coulombic interactions between electrons and holes. The strong Coulombic interactions in TMDs are sufficient to support charged excitons (trions) as well. As people look to fabricate devices out of TMDs, a fundamental understanding of how excitons, trions, electrons, holes, and other quasiparticles behave in these two-dimensional systems is essential. In this work, we present three studies in which we characterize the behavior of charge carriers in monolayer TMDs utilizing time-resolved terahertz spectroscopy (TRTS). TRTS is a pump-probe technique in which a terahertz pulse probes the photogenerated excited state of a material. By measuring this interaction between the terahertz pulse and the excited-state carriers, we can monitor the frequency-dependent photoconductivity dynamics of a material with sub-picosecond resolution. In the first study, we investigate the carrier dynamics of monolayer WS2 in vacuum. Here, we observe a photoinduced increase in conductivity due to defect-mediated positive trion formation. Furthermore, we find that trions in monolayer WS2 contribute to the conductivity in three ways: a Drude response, a broad resonance response, and a dissociation response at the trion binding energy. We then consider the role the environment plays in modulating the photoconductivity response of monolayer TMDs. In the second study, we investigate the effect of molecular oxygen on the photoconductivity of monolayer MoS2. Here, we find that the photoconductivity shifts from negative to positive as the MoS2 environment changes from vacuum to atmospheric pressure. This transition results from physically adsorbed oxygen depleting electrons from the n-type MoS2. In the third study, we investigate the effect of the dielectric environment on the photoconductivity of monolayer WS2. Here, we find that the broad resonance response and the dissociation response at the trion binding energy shift to lower frequencies as the dielectric constant of the environment increases. This shift is due to increased electrostatic screening. These results should prove useful to those who study the ultrafast carrier dynamics in two-dimensional materials, as well as to those who look to make devices out of these nanomaterials.