[Paper Review] Milliwatt average power, MHz-repetition rate, broadband THz generation in organic crystal BNA with diamond substrate

Samira Mansourzadeh, Tim Vogel+3 more|arXiv (Cornell University)|Jun 26, 2021

Terahertz technology and applications37 references35 citations

TL;DR

This study demonstrates broadband, milliwatt-level average power terahertz (THz) generation in the organic crystal BNA using a 13.3 MHz repetition rate, 85 fs, 1030 nm Yb-doped thin-disk laser. By integrating a diamond substrate as a heat sink, the authors achieve 0.95 mW THz average power with a 4×10⁻⁴ optical-to-THz conversion efficiency and a spectral bandwidth exceeding 6 THz at −50 dB, representing the highest reported THz power from BNA under MHz-repetition-rate excitation.

ABSTRACT

We demonstrate a 13.3 MHz repetition rate, broadband THz source with milliwatt-average power, obtained by collinear optical rectification of a high-power Yb-doped thin-disk laser in the organic crystal BNA (N-benzyl-2-methyl-4-nitroaniline). Our source reaches a maximum THz average power of 0.95 mW with an optical-to-THz efficiency of 4e-4 and a spectral bandwidth spanning up to 6 THz at -50 dB, driven by 2.4 W average power (after an optical chopper with duty cycle of 10%), 85 fs-pulses. This high average power excitation was possible without damaging the crystal by using a diamond-heatsinked crystal with significantly improved thermal properties. To the best of our knowledge, this result represents the highest THz average power reported so far using the commercially available organic crystal BNA, showing the potential of these crystals for high average power, high repetition rate femtosecond excitation. The combination of high power, high dynamic range, high repetition rate and broadband spectrum makes the demonstrated THz source highly attractive to improve various time-domain spectroscopy applications. Furthermore, we present a first exploration of the thermal behavior of BNA in this excitation regime, showing that thermal effects are the main limitation in average power scaling in these crystals.

Motivation & Objective

• To achieve high average power THz generation in commercially available organic crystal BNA at MHz repetition rates.
• To overcome thermal limitations that have previously restricted average power scaling in organic crystals.
• To investigate the thermal behavior and damage thresholds of BNA under high-repetition-rate, high-average-power excitation.
• To demonstrate a scalable, high-dynamic-range, broadband THz source suitable for time-domain spectroscopy applications.

Proposed method

• Utilization of a mode-locked Yb-doped thin-disk oscillator delivering 106 W average power at 13.3 MHz repetition rate and 85 fs pulse duration.
• Employment of a Herriott-type multi-pass cell for spectral broadening and pulse compression to achieve 85 fs pulses with 96% transmission efficiency.
• Implementation of an optical chopper with adjustable duty cycle (10–50%) to control average pump power and thermal load independently.
• Mounting the BNA crystal on a diamond substrate to enhance thermal management and prevent damage at high average power.
• Use of electro-optic sampling with a 0.2 mm GaP crystal and lock-in detection for time-domain THz field measurement under dry nitrogen purging.
• Numerical simulation of THz generation using a split-step Fourier method, incorporating phase matching, pump depletion, nonlinear susceptibility, and detection system transfer functions.

Experimental results

Research questions

• RQ1 What is the maximum achievable average THz power from BNA at MHz repetition rates when thermal effects are mitigated?
• RQ2 How does the thermal load from high-average-power excitation affect the performance and integrity of BNA crystals?
• RQ3 Can the optical-to-THz conversion efficiency in BNA be significantly improved under MHz-repetition-rate, high-average-power excitation using a diamond heat sink?
• RQ4 What is the spectral bandwidth and dynamic range of THz pulses generated in BNA when pumped at 1030 nm with 85 fs pulses?
• RQ5 To what extent do thermal effects limit the scalability of THz generation in organic crystals like BNA?

Key findings

• The study achieves a maximum THz average power of 0.95 mW with a 4×10⁻⁴ optical-to-THz conversion efficiency, the highest reported for BNA under MHz-repetition-rate operation.
• The THz spectrum spans up to 6 THz at −50 dB, with a dynamic range exceeding 50 dB, demonstrating a smooth, broadband response without the 2 THz dip observed in prior 800 nm and 1150–1550 nm pumped BNA experiments.
• The diamond-heatsinked configuration enables operation at 2.4 W average pump power (2500 kW/cm² intensity), with a maximum crystal temperature of 63°C, significantly improving thermal management compared to non-heatsinked setups.
• Thermal effects are identified as the primary limitation, with irreversible damage occurring at temperatures above 68°C, indicating a safe operating window below 60°C for stable, long-term use.
• Numerical simulations based on the split-step Fourier method show excellent agreement with measured spectra, validating the model and confirming the role of phase matching and material properties in shaping the broadband output.
• The results suggest that further power scaling to the multi-10 mW regime is feasible with improved pulse compression (sub-10 fs) and active cooling, enabling higher peak power without increased thermal load.