Abstract
We use coupled mode theory, adequately incorporating optical losses, to model ultra-broadband terahertz (THz) waveguide emitters (0.1-20 THz) based on difference frequency generation of femtosecond infrared (IR) optical pulses. We apply the model to a generic, symmetric, five-layer, metal/cladding/core waveguide structure using transfer matrix theory. We provide a design strategy for an efficient ultra-broadband THz emitter and apply it to polymer waveguides with a nonlinear core composed of a poled guest-host electro-optic polymer composite and pumped by a pulsed fiber laser system operating at 1567 nm. The predicted bandwidths are greater than 15 THz and we find a high conversion efficiency of 1.2 × 10(-4) W(-1) by balancing both the modal phase-matching and effective mode attenuation.
Highlights
A widely used method to produce ultra-broadband terahertz (THz) pulses is to mix the frequency components of infrared (IR) femtosecond pulses through difference frequency generation (DFG) in organic and inorganic crystals [1, 2]
We report a model for ultra-broadband THz DFG with a frequency domain (FD) coupled mode theory (CMT), similar to [23]
Using a design strategy where the conversion between a single-IR-mode and a singleTHz -mode is optimized, we demonstrate an efficient broadband THz emitter (>15 THz) composed of a symmetric five-layer slab waveguide with the core composed of the guest-host EO composite DAPC
Summary
A widely used method to produce ultra-broadband terahertz (THz) pulses is to mix the frequency components of infrared (IR) femtosecond (fs) pulses through difference frequency generation (DFG) in organic and inorganic crystals [1, 2]. Poled EO guest-host polymer films are attractive since their material properties are tunable and provide an inexpensive alternative to crystal based ultra-broadband THz emitters These EO composites are somewhat absorptive in the THz range. Using a design strategy where the conversion between a single-IR-mode and a singleTHz -mode is optimized, we demonstrate an efficient broadband THz emitter (>15 THz) composed of a symmetric five-layer slab waveguide with the core composed of the guest-host EO composite DAPC (in the appendix we demonstrate how our theory allows for modeling the conversion of two IR pump modes into a single THz mode and use that result to verify the output from a previous experimental demonstration by Cao et al, [19]). Modifying the waveguide dimensions allows tuning both modal effective indices to achieve better phase-matching and lower loss conditions for this single mode to single mode DFG interaction
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