Organic Broadband TeraHertz Sources and Sensors

Abstract

We review recent research using organic materials for generation and detection of broadband terahertz radiation (0.3 THz−30 THz). The main focus is on amorphous electrooptic (EO) polymers, with semiconducting polymers, molecular salt EO crystals, and molecular solutions briefly discussed. 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 using of EO polymers. We also present a model that can predict the performance of a polymer-based THz system. Both the dielectric properties of an EO polymer and laser pulse related parameters are included in the model, making the simulations close to real conditions. From our modeling work, the roles the dielectric properties play in the THz generation and detection are clearly seen, providing us with a good guide to select and design suitable EO polymers in the future.

Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

“…For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.…”

“…For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.The so far most promising sources covering the full THz window are photocurrents in transient gas plasmas 9,10,25,26,27,28,29 . The downside of this appealing approach is that the underlying ionization process usually requires amplified laser pulses with high threshold energies on the order of 0.1 mJ.…”

Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis

“…Key to the success of THz TDS is its capability of measuring complex refractive indices of samples over bandwidths as large as 100 THz, due to its intrinsic ability to resolve the electric field amplitude of broadband THz pulses coherently and with subpicosecond resolution [6], as well as its insensitivity to thermal background radiation [7]. However, THz TDS systems in general have signal-to-noise ratios (SNRs) that are practically useful only below ∼3 THz [8] but have been reported with much higher bandwidths [9][10][11][12]. Furthermore, their highest spectral resolution is typically reported between ∼5-7 GHz [13,14] (worse in high-bandwidth systems), and they are restricted to low THz powers on the order of 10-100 µW for commonly used optically-pumped photoconductive emitters.…”

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

“…Electro-optic (EO) guest-host polymers have been utilized as broadband THz emitters and sensors for a number of years, owing to their tunable properties, high nonlinearity, and lack of phonon absorptions. 7 Free-space EO sampling (FSEOS) in poled polymers represents a simple alternative to THz field-induced second harmonic generation (TFISH) 1,2 for broadband THz sensing. These applications require knowledge of the THz refractive index to address phase matching requirements and the THz absorption to guide the selection of appropriate material composition and device geometry.…”