L. Michael Hayden

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.

Wideband 15THz response using organic electro-optic polymer emitter-sensor pairs at telecommunication wavelengths

Poled polymer films are employed in terahertz emission and detection using telecommunication wavelength femtosecond pulses. Films 10–15μm thick with electro-optic coefficients as high as 160pm∕V at 1300nm are used to generate and sense subpicosecond pulses with continuous bandwidth up to 15THz. Terahertz emission from a poled polymer film is compared to terahertz emission from the organic crystal DAST. The use of a poled polymer as the terahertz sensor allows the identification of DAST phonons at 1.1, 3.0, 5.3, 8.5, and 12.5THz.

Resonance enhanced THz generation in electro-optic polymers near the absorption maximum

The electro-optic (EO) coefficient of an organic nonlinear material exhibits a sharp resonance near the absorption maximum of the material. Due to this resonance, we experimentally observe the amplitude of the THz field generated from a 3.1-μm-thick EO polymer composite to be larger than that emitted from a 1000-μm-thick crystal of ZnTe. This comparison allows us to estimate the resonance enhanced EO coefficient of the polymer composite to be over 1250pm∕V at 800nm.

Generation and detection of terahertz radiation with multilayered electro-optic polymer films

We use optical rectification and electro-optic sensing to generate and detect terahertz radiation, using individual and multilayered stacks of electro-optic polymer films. For comparable thickness, the polymer films are more efficient generators and detectors of terahertz radiation than ZnTe crystals.

Terahertz scattering from granular material

Terahertz (THz) imaging is emerging as a potentially powerful method of detecting explosive devices, even in the presence of occluding materials. However, the characteristic spectral signatures of pure explosive materials may be altered or obscured by electromagnetic scattering caused by their granular nature. This paper presents THz transmission measurements of granular systems representative of explosives and presents results from dense media theory that accurately explain the observed scattering response.

Fully atomistic modeling of an electric field poled guest-host nonlinear optical polymer

Fully atomistic molecular modeling is performed on an electric field poled guest-host nonlinear optical polymer system at two different densities. The higher density (HD) represents a sub-Tg system, while the lower density (LD) corresponds to a system above Tg. The electric field alignment of dopants was studied by calculating 〈cos θ〉 and 〈cos3 θ〉 and comparing with the theoretically predicted order parameters. For the LD system, the dipole orientational order achieved during 1.5 ns of NVT dynamics at 500 K was similar to the theoretical prediction. The applied field was also found to constrain the side group of the host, poly(methyl methacrylate) (PMMA). The shape of the volume swept by the dopants was found to be less cylindrical as the field was increased. The partial radial distribution functions indicated that the dopants were likely to be located near the host side group. No significant differences in the static conformational properties of the host were observed between poled and unpoled structures...

Broadband and gap-free response of a terahertz system based on a poled polymer emitter-sensor pair

We present a terahertz (THz) system based on a poled electro-optic (EO) polymer emitter-sensor pair for the generation and detection of broadband electromagnetic pulses free of spectral gaps. The gap-free THz spectrum obtained from the system is attributed to the amorphous form of the EO polymer films. Using ∼50-fs laser pulses centered at ∼800nm, a ∼100-μm thick emitter, and a ∼180-μm thick sensor, the observable bandwidth is ∼12THz. We also present an experimental study, in a systematic manner, on the effect of phase mismatch in the polymer emitter-sensor pair.

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.

Launching of guided waves in a photorefractive polymer by two-beam coupling

We report the launching of guided waves in a slab waveguide made from a photorefractive polymer. The guided modes are launched when we optically pump the surface of the waveguide. The coupling efficiency increases when the angle of incidence and the power of the pump are increased. Also, the rise time decreases when the pump power is increased.

Temperature dependence of the activation volume in a nonlinear optical polymer : Evidence for chromophore reorientation induced by Sub-Tg relaxations

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of 4-(diethylamino)-4′-nitrotolane (DEANT) in poly(methyl methacrylate) (PMMA). The decay of the SHG signal from films of DEANT/PMMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25°C below the glass transition temperature to 35°C above it. The activation volume, ΔV*αβ associated with the long range α-type motion of the polymer remained constant at 213 ± 10 A3 between Tg − 25°C and Tg + 10°C. At higher temperatures, ΔV*αβ decreased linearly with increasing temperature. The activation volume, ΔV*αβ, associated with short range secondary relaxations was constant over the entire temperature range with a value of 77 ± 10 A3. The data suggest that above Tg chromophore reorientation is coupled to both the long range and local motions of the polymer; whereas, well below Tg chromophore reorientation is closely coupled to the local relaxations of the polymer.