Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials
Summary (1 min read)
I. INTRODUCTION
- Recent advances in air plasma terahertz (THz) generation have revitalized broadband THz time-domain spectroscopy (TDS), 1, 2 bringing the THz community one step closer to broadband imaging, standoff detection, materials identification and analysis.
- As such, it becomes increasingly important to catalog the optical properties of materials across the so-called THz gap, spanning 0.1-10 THz.
- 7 Free-space EO sampling in poled polymers represents a simple alternative to THz field-induced second harmonic generation 1, 2 for broadband THz sensing.
- Here the authors report the optical properties of several technologically important polymers from 0.1-10 THz.
- These materials include polyimide , poly [bisphenol A carbonateco-4,4 0 -(3,3,5-trimethyl cyclohexylidene) diphenol carbonate] (APC), poly [bisphenol A carbonate] (BPC), polymethyl methacrylate (PMMA), polystyrene (PS), poly (styrene-comethyl methacrylate) (PS-PMMA), polyethylene cyclic olefin copolymer , polytetrafluoroethylene , and high-density polyethylene (HDPE).
II. RESULTS AND DISCUSSION
- Details of their THz-TDS experimental setup based on air-plasma THz generation and FSEOS in EO polymers are reported elsewhere.
- 11 Calculations on the chromophores were done on single molecules using the 6-311G(d,p) basis set and the B3LYP functional.
- The polymers were modeled as oligomers containing eight monomers for Teflon, PMMA, and Topas and four monomers for BPC, APC, and Kapton.
- The THz refractive index and absorption coefficients are summarized in Fig. 4 , where Topas is included for comparison.
- The observed features will lead to poor phase matching and strong THz absorption in EO polymers used for THz emission or detection resulting in spectral gaps.
III. CONCLUSIONS
- By using a broadband, air-plasma/EO polymer THz-TDS system, the authors have identified absorption features in Teflon, Kapton, and PMMA that make these materials poor choices for broadband THz components such as lenses, beam splitters, optical windows, or substrates.
- Topas and PS have relatively constant refractive indices and show low loss across the THz regime, making them ideal materials for passive broadband THz optical components, even for THz guided wave applications.
- The nonpolar nature of Topas makes it difficult to heavily dope this polymer with the highly dipolar chromophores potentially precluding its use as a host polymer for guesthost THz generation and sensing.
- The addition of nonlinear optical chromophores into polymer hosts generally increases the refractive index and induces weak absorption features due to the specific structure of the chromophores.
- EO polymers based on APC and BPC, although ideal for broadband FSEOS of THz pulses in thin solid films, may exhibit too much absorption for long THz emitting waveguide structures.
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Frequently Asked Questions (15)
Q2. What is the absorption trend for HDPE?
The featureless, nearly linear absorption trend persists across the THz regime and is also observed in Fourier transform infrared spectros-copy (FTIR).
Q3. How did the spectra of the polymers be obtained?
Assignments of molecular motions for the THz absorption features the authors see in the spectra of these polymers and chromophores were made by performing density functional theory (DFT) calculations using the Gaussian 09 suite of programs.
Q4. What is the refractive index of a glass substrate?
To achieve good velocity matching between the optical and THz beams involved in optical rectification or FSEOS, the refractive index should be constant throughout the THz regime.
Q5. What is the material for a wideband THz spectroscopy?
Topas is also resistive to most acids and solvents, making it a good choice for a substrate material for broadband THz spectroscopy.
Q6. What is the effect of the observed features on the EO polymer?
The observed features will lead to poor phase matching andstrong THz absorption in EO polymers used for THz emission or detection resulting in spectral gaps.
Q7. What is the description of polyimide?
Polyimides are a class of polymers with good mechanical properties that are resistant to most solvents and frequently are used for electrical and thermal insulation.
Q8. What is the refractive index of the EO polymers?
The higher refractive index of these hosts will also contribute to higher sensitivity in FSEOS, as the detected signal is proportional to the index cubed.
Q9. What is the absorption of the CN group in the apc?
18 The absorption features at 4.9 and 5.7, in DCDHF-6-V/APC are due to similar atomic motions as found in the Lemketwo modes, one of which is an out-of-plane wagging of the lone CN group attached to the dihydrofuran ring and the other is due to an in-plane oscillation of the hexyl tails and phenyl group attached to the lone N atom.
Q10. Where would a high resistivity silicon be placed?
Where possible, high resistivity silicon (HRSi) placed at Brewster’s angle would make a better choice for a beam block, owing to its very low THz absorption.
Q11. What are common host materials for EO guest host polymers?
14Common host materials for EO guest host polymers include PS, PMMA, PS-PMMA, BPC, and APC, Fig. 3. These are also common construction materials and may also be useful for THz optical components.
Q12. What is the reason for the chromophores to be added to polymers?
The addition of nonlinear optical chromophores into polymer hosts generally increasesdue to the specific structure of the chromophores.
Q13. What is the purpose of this paper?
Because exactly reproducing the experimental spectra from theoretical calculations is beyond the scope of this paper, the authors aim, merely, to qualitatively identify the important modes for a given THz feature, which for these low energy vibrations nearly always involves the collective motion of almost all the atoms in the molecule.
Q14. What is the absorption coefficient of the AJTB203?
the addition of chromophores to BPC leads to an increase in both the refractive index and absorption coefficient, Fig. 7.
Q15. What is the difference between the chromophores of PMMA and PS?
The authors see that any guest-host polymer system using PMMA or a PMMA copolymer as a host will exhibit significant THz absorption, leading to significant losses and spectral gaps if used for THz emission or detection.