Paul D Cunningham

Electro-optic polymers for THz applications

The main advantage to using electro-optic poled guest-host polymers as THz emitters and sensors is the lack of phonon absorption. Crystalline materials have a period structure, giving rise to collective lattice vibrations, known as phonons, which typically occur in the THz regime. THz light couples to transverse-optical phonons, leading to absorption features, or spectral gaps, in the emitted and detected spectra. These absorption features also lead to changes in the index of refraction in the THz regime. As effective generation and detection require the THz pulse to travel at the same speed as an infrared generating or detecting pulse, i.e. phase matching, changes in index of refraction decrease efficiency. Polymers have low dispersion, i.e. relatively constant indicies of refraction, and thus good phase matching properties. Electro-optic polymers can also be engineered to be highly nonlinear, reaching nonlinear optical coefficients much larger than inorganic crystals, and thus more efficient at THz generation and detection.

THz spectroscopy of polymers

In order to determine which polymer systems are best for THz applications, one must measure their THz properties. For possible emitters and sensors, we look for low absorption and a relatively flat index of refraction across a broad bandwidth. We also compare the THz index of refraction, to the group index of refraction in the visible range. The wavelength at which they are most similar will be were the best phase matching is present. Here, very thick polymers can be utilized for increased efficiency.

The THz regime is often host to molecular vibrational modes and can be used to study molecular motions. We found and modeled, via Density Functional Theory, the collective molecular modes in the commercial polymer Teflon, which consist of a backbone "twisting" motion at 6.1THz and a "rocking" motion of the CF₂ groups at 8.3THz.

Broadband spectroscopy of THz Emitters

In order to determine the optimum operation conditions for THz emitters and sensors, one must measure their THz properties. We measure the absorption and index of refraction across the THz band. We can compare the THz index of refraction, to the group index of refraction in the visible range to determine the coherence length. From the THz dielectric function the non-linear susceptibility, phase-mismatch, and detector response function can be estimated.

THz optical rectification in polymers

We have examined the THz emission from numerous poled polymer systems. Many of which, are capable of broadband emission. However, as the polymer chains are main longer, in order to increase their nonlinearity, the linear absorption band shifts to lower energy (higher wavelength). As such, these materials become resonantly enhanced at 800nm, at the price of poor phase matching. For example, given an index of refraction of about 1.7, the polymer showed above must have an effective electro-optic coefficient of 4300 pm/V to produce such strong emission. However, given the strong infrared absorption and poor phase matching, the emission does not scale with increased film thickness. When operated at telcom wavelengths, polymers like the one shown above have electro-optic coefficients of ~100 pm/V and show emission out to 15 THz for films as thin as 15 microns, with amplitude only 10x smaller than that from 160 micron DAST emitter.

Free-space electro-optic sampling in polymers

We have studied THz sensing via free-space electro-optic sampling in a variety of poled polymer systems. The improved phase matching allows for broad band, typically >10THz, detection. When used with a DAST emitter, we see multiple absorption bands from phonons in DAST. The overall emitted spectrum is limited by the emission properties of DAST, though ~ 15 THz bandwidth has been demonstrated at telacom wavelengths. Air-plasma THz generation shows bright, broadband emission that is ideal for spectroscopic applications.