Paul D. Cunningham

Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials

We report broad bandwidth, 0.1–10 THz time-domain spectroscopy of linear and electro-optic polymers. The common THz optical component materials high-density polyethylene, polytetrafluoroethylene, polyimide (Kapton), and polyethylene cyclic olefin copolymer (Topas) were evaluated for broadband THz applications. Host polymers polymethyl methacrylate, polystyrene, and two types of amorphous polycarbonate were also examined for suitability as host for several important chromophores in guest-host electro-optic polymer composites for use as broadband THz emitters and sensors.

Optical properties of DAST in the THz range

We report the far-infrared properties of the organic crystal DAST, a popular terahertz emitter, from 0.6-12 THz through use of a THz spectrometer incorporating air-plasma THz generation and electro-optic (EO) sampling in a poled EO polymer. We identify absorption features at 1.1, 3.1, 5.2, 7.1, 8.4, 11, and 12.3 THz and at 1.1, 1.3, 1.6, 2.2, 3, 5.2, 7.2, 9.6 and 11.7 THz for a-axis and b-axis polarized THz radiation respectively. These results allow for more accurate prediction of the optimum crystal thickness for broadband THz emission via optical rectification and difference frequency generation.

Resonance energy transfer in DNA duplexes labeled with localized dyes.

The growing maturity of DNA-based architectures has raised considerable interest in applying them to create photoactive light harvesting and sensing devices. Toward optimizing efficiency in such structures, resonant energy transfer was systematically examined in a series of dye-labeled DNA duplexes where donor-acceptor separation was incrementally changed from 0 to 16 base pairs. Cyanine dyes were localized on the DNA using double phosphoramidite attachment chemistry. Steady state spectroscopy, single-pair fluorescence, time-resolved fluorescence, and ultrafast two-color pump-probe methods were utilized to examine the energy transfer processes. Energy transfer rates were found to be more sensitive to the distance between the Cy3 donor and Cy5 acceptor dye molecules than efficiency measurements. Picosecond energy transfer and near-unity efficiencies were observed for the closest separations. Comparison between our measurements and the predictions of Förster theory based on structural modeling of the dye-labeled DNA duplex suggest that the double phosphoramidite linkage leads to a distribution of intercalated and nonintercalated dye orientations. Deviations from the predictions of Förster theory point to a failure of the point dipole approximation for separations of less than 10 base pairs. Interactions between the dyes that alter their optical properties and violate the weak-coupling assumption of Förster theory were observed for separations of less than four base pairs, suggesting the removal of nucleobases causes DNA deformation and leads to enhanced dye-dye interaction.

Charge carrier dynamics in metalated polymers investigated by optical-pump terahertz-probe spectroscopy.

We report charge carrier dynamics in solid films of a series of metalated polymers based on Pt- and 4,7-di-2-thienyl-2,1,3,-benzothiadiazole or 4,7-di-2-thienothienyl-2,1,3,-benzothiadiazole upon photoexcitation of the pi-pi* transition using optical-pump terahertz-probe spectroscopy. Subpicosecond generated charge carriers recombine within 100 ps, but bound excitons persist. Application of the Drude-Smith model allows for estimation of the intrinsic mobility and internal quantum yield of charge carrier generation in these films. Thermal annealing is found to have no effect on nanometer scale charge transport.

Photoinduced Bandgap Renormalization and Exciton Binding Energy Reduction in WS2

Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS2 that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 ± 30 to 220 ± 20 meV as the excitation density is increased from 3 × 1011 to 1.2 × 1012 photons/cm2. This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator-metal transition in monolayer transition metal dichalcogenides.

Pulsed-THz Characterization of Hg-Based, High-Temperature Superconductors

We report on ultrafast THz-pulse time-domain spectroscopy (TDS) and femtosecond optical-pump THz-probe (OPTP) studies of Hg-Ba-Ca-Cu-O (HBCCO) high-temperature, superconducting thin films. Our 500-nm-thick films were prepared by rf-magnetron sputtering of Re-Ba-Ca-Cu-O precursor films, followed by an ex-situ, high-temperature mercuration process. The resulting films were c axis oriented with a predominant Hg-1212 (plus some Hg-1223) phase. Their transition temperature T c had an onset at 122 K and zero resistance at 110 K. The THz TDS measurements demonstrated a sharp drop in the transmitted THz signal when the sample temperature was decreased below T c, which we directly related to a change in the imaginary component of the film complex conductivity. Simultaneously, the peak of the temperature-dependent real part of the conductivity was shifted toward lower frequencies at lower temperatures. The time-resolved OPTP spectroscopy experiments showed that the quasiparticle relaxation process exhibited an intrinsic single-picosecond dynamics with no phonon bottleneck, which is a unique feature among superconductors and makes the HBCCO material promising for ultrafast radiation detector applications.

Optical Properties of Vibronically Coupled Cy3 Dimers on DNA Scaffolds

We examine the effect of electronic coupling on the optical properties of Cy3 dimers attached to DNA duplexes as a function of base pair (bp) separation using steady-state and time-resolved spectroscopy. For close Cy3-Cy3 separations, 0 and 1 bp between dyes, intermediate to strong electronic coupling is revealed by modulation of the absorption and fluorescence properties including spectral band shape, peak wavelength, and excited-state lifetime. Using a vibronic exciton model, we estimate coupling strengths of 150 and 266 cm-1 for the 1 and 0 bp separations, respectively, which are comparable to those found in natural light-harvesting complexes. For the strongest electronic coupling (0 bp separation), we observe that the absorption band shape is strongly affected by the base pairs that surround the dyes, where more strongly hydrogen-bonded G-C pairs produce a red-shifted absorption spectrum consistent with a J-type dimer. This effect is studied theoretically using molecular dynamics simulation, which predicts an in-line dye configuration that is consistent with the experimental J-type spectrum. When the Cy3 dimers are in a standard aqueous buffer, the presence of relatively strong electronic coupling is accompanied by decreased fluorescence lifetime, suggesting that it promotes nonradiative relaxation in cyanine dyes. However, we show that the use of a viscous solvent can suppress this nonradiative recombination and thereby restore the dimer fluorescent emission. Ultrafast transient absorption measurements of Cy3 dimers in both standard aqueous buffer and viscous glycerol buffer suggest that sufficiently strong electronic coupling increases the probability of excited-state relaxation through a dark state that is related to Cy3 torsional motion.

Optical-pump-THz-probe studies of carrier dynamics in Hg-based high-temperature superconducting thin films

We report time-resolved carrier dynamics in Hg-Ba-Ca-Cu-O high-temperature superconductors using optical excitation and THz probe femtosecond spectroscopy. Picosecond quasiparticle dynamics observed in our experiments suggests that Hg-based materials are attractive candidates for high-speed photodetectors.

Terahertz science and applications based on poled electro-optic polymers

We review recent research using amorphous electrooptic (EO) polymers for generation and detection of broadband terahertz radiation (0.3 THz -30 THz). The advantages of amorphous EO polymers over other materials for broadband THz generation (via optical rectification) and detection (via EO sampling) include a lack of phonon absorption (good transparency) in the THz regime, high EO coefficient and good phase-matching properties, and, of course, easy fabrication (low cost). Our ~12-THz, spectral gap-free THz system based on a polymer emitter-sensor pair is an excellent demonstration of the advantages of the use of EO polymers. This system has been employed as a wideband spectrometer to study dielectric materials in the THz regime.

Terahertz Probing of Carrier Dynamics in Hg-Based High- Temperature Superconducting Thin Films

We report on our investigation of time-resolved carrier dynamics in a Hg-based high-temperature superconducting film (Hg-Ba-Ca-Cu-O), using optical excitation and THz probing. The observed picosecond time-scale photoresponse suggests the material’s potential applications for high-speed photodetectors.