[Paper Review] Comprehensive Characterization of Terahertz Generation with the Organic Crystal BNA

Isaac C. Tangen, Gabriel A. Valdivia‐Berroeta+6 more|arXiv (Cornell University)|May 12, 2020

Terahertz technology and applicationsEngineering45 references40 citations

TL;DR

This study comprehensively characterizes terahertz (THz) generation in organic BNA crystals using 800-nm and 1250–1500-nm pump lasers across thicknesses from 123 to 700 μm. It demonstrates that thin, high-quality BNA crystals produce smooth, broadband THz spectra up to 6 THz with minimal absorption features, significantly reducing the 2 THz dip seen in prior reports, and enables efficient broadband THz generation with common Ti:Sapphire lasers, offering a viable alternative to LiNbO3 sources.

ABSTRACT

We characterize the terahertz (THz) generation of N-benzyl-2-methyl-4-nitroaniline (BNA), with crystals ranging in thickness from 123-700 {\mu}m. We compare excitation using 800-nm and 1250 to 1500-nm wavelengths. Pumping BNA with 800-nm wavelengths and longer near-infrared wavelengths results in a broad spectrum, producing out to 6 THz using a 100-fs pump, provided the BNA crystal is thin enough. ~200 {\mu}m or thinner crystals are required to produce a broad spectrum with an 800-nm pump, whereas ~300 {\mu}m thick crystals are optimal for broadband THz generation using the longer wavelengths. We report the favorable THz generation and optical characteristics of our BNA crystals that make them attractive for broadband, high-field THz generation, and we also find significant differences to BNA results reported in other works.

Motivation & Objective

• To investigate the terahertz (THz) generation performance of N-benzyl-2-methyl-4-nitroaniline (BNA) crystals across varying thicknesses and pump wavelengths.
• To resolve discrepancies in reported THz spectra, particularly the persistent 2 THz dip in earlier studies, by characterizing high-quality BNA crystals grown via controlled methods.
• To determine the optical and THz properties of BNA, including refractive index, group index, and absorption coefficient, to explain spectral behavior and phase-matching conditions.
• To compare THz electric field measurements using electro-optic (EO) sampling in GaP and THz Kerr effect in diamond, assessing calibration reliability.
• To establish BNA as a viable, broadband THz source with improved spectral flatness and compatibility with standard 800-nm laser systems.

Proposed method

• Pumped BNA crystals (123–700 μm thick) with 100-fs, 1-kHz repetition-rate laser pulses at 800 nm and 1250–1500 nm using a Ti:Sapphire laser and optical parametric amplifier (OPA).
• Measured THz electric fields using electro-optic sampling with a 100-μm (110) GaP crystal bonded to a 1-mm (001) GaP crystal, with detection bandwidth limited to ~6 THz.
• Determined refractive and group indices in the THz and near-infrared (NIR) regions via spectral fitting and modeling of THz generation, using Lorentz oscillator models for complex refractive index.
• Measured THz field strengths using both GaP-based EO sampling and diamond-based Kerr effect measurements to compare calibration methods.
• Modeled THz generation spectra using measured optical properties and phase-matching conditions, comparing predictions to experimental data.
• Analyzed absorption coefficients in the THz range, identifying low absorption (<150 cm⁻¹) across 0–5.5 THz, with minor features at 2.1 and 3.3 THz.

Experimental results

Research questions

• RQ1 How does crystal thickness and pump wavelength (800 nm vs. 1250–1500 nm) affect the spectral bandwidth and shape of THz pulses generated in BNA?
• RQ2 Why does the 2 THz dip in THz generation, previously reported in BNA, differ significantly in the current study?
• RQ3 What are the accurate values of the THz refractive index, group index, and absorption coefficient in BNA, and how do they compare to prior reports?
• RQ4 To what extent do different THz field calibration methods (GaP EO vs. diamond Kerr effect) yield consistent results, and what explains discrepancies?
• RQ5 Can thin BNA crystals (≤200 μm) efficiently generate broadband THz pulses when pumped with 800-nm Ti:Sapphire lasers, and how do they compare to LiNbO3-based sources?

Key findings

• BNA crystals thinner than 200 μm produce a smooth, broadband THz spectrum extending up to 6 THz when pumped with 800-nm light, with no significant dip at 2 THz.
• For 1250–1500 nm pumping, ~300 μm thick BNA crystals yield optimal broadband THz generation, producing spectra from 0 to 6 THz with minimal spectral distortion.
• The absorption coefficient in BNA is below 150 cm⁻¹ across most of the 0–5.5 THz range, with only minor features at 2.1 THz and 3.3 THz, explaining the broad, smooth spectra.
• The refractive index in the THz range varies only slightly (1.93–2.1) and shows no strong dispersion, supporting effective phase-matching across a wide bandwidth.
• The study reports a significant discrepancy between GaP-based EO sampling (300 kV/cm peak field) and diamond Kerr effect measurements (1.6 MV/cm), suggesting potential overestimation in diamond-based calibration.
• The authors conclude that differences in BNA crystal processing lead to substantial variations in THz spectral response, and that high-quality, thin BNA crystals are a promising alternative to LiNbO3 for broadband THz generation.