Intense, carrier frequency and bandwidth tunable quasi single-cycle pulses from an organic emitter covering the Terahertz frequency gap

Abstract

In Terahertz (THz) science, one of the long-standing challenges has been the formation of spectrally dense, single-cycle pulses with tunable duration and spectrum across the frequency range of 0.1–15 THz (THz gap). This frequency band, lying between the electronically and optically accessible spectra hosts important molecular fingerprints and collective modes which cannot be fully controlled by present strong-field THz sources. We present a method that provides powerful single-cycle THz pulses in the THz gap with a stable absolute phase whose duration can be continuously selected between 68 fs and 1100 fs. The loss-free and chirp-free technique is based on optical rectification of a wavelength-tunable pump pulse in the organic emitter HMQ-TMS that allows for tuning of the spectral bandwidth from 1 to more than 7 octaves over the entire THz gap. The presented source tunability of the temporal carrier frequency and spectrum expands the scope of spectrally dense THz sources to time-resolved nonlinear THz spectroscopy in the entire THz gap. This opens new opportunities towards ultrafast coherent control over matter and light.

Laser‐Driven Strong‐Field Terahertz Sources

“…Organic THz emitters such as DAST, [88][89][90][91][92] DSTMS, [92,93] OH1, [92,94] HMQ-TMS [38] can provide high optical-to-THz conversion efficiencies at room temperature and octave-spanning, or even multi-octave THz spectra. [90,96] For phase-matched OR, organic crystals typically require infrared pump wavelengths in the 1.2-1.6 µm spectral range (Figure 17), where a collinear geometry can be used for THz generation. In case of DAST, e.g., it is d eff = 615 pm V −1 .…”

“…In case of DAST, e.g., it is d eff = 615 pm V −1 . [96] Consequently, the spectral intensity ( Figure 17b) and phase structure of the generated THz pulses becomes complex, often leading to multicycle waveforms. [90,96] For phase-matched OR, organic crystals typically require infrared pump wavelengths in the 1.2-1.6 µm spectral range (Figure 17), where a collinear geometry can be used for THz generation.…”

“…[90,96] For phase-matched OR, organic crystals typically require infrared pump wavelengths in the 1.2-1.6 µm spectral range (Figure 17), where a collinear geometry can be used for THz generation. [96] Owing to the collinear phase matching geometry, the generated THz radiation is naturally collimated, enabling a good focusability of the beam for achieving high field strengths. Organic crystals typically have several phonon absorption bands in the mid-frequency THz spectral range.…”

“…Reproduced under the terms of the Creative Commons Attribution 4.0 License [96]. b) Experimentally recorded THz spectra for various pump wavelengths.…”

Efficient Gap‐Free Broadband Terahertz Generators Based on New Organic Quinolinium Single Crystals

“…While the upper cut‐off frequency of THz generation as well as detection for ZnTe crystals is practically limited to <3 THz owing to strong phonon absorption above this frequency, benchmark organic electro‐optic crystals exhibit a broader spectral response for both THz wave generation and detection. For example, organic 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) and 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium (HMQ) crystals showing good phase matching ability between optical and THz frequencies with near‐infrared (NIR) pumping exhibit broadband THz wave generation characteristics with an upper cut‐off frequency over 8–10 THz in a relatively simple collinear experimental scheme using a standard pump configuration . Note that although inorganic LiNbO 3 crystals can generate intense THz waves, the generated THz bandwidth is practically lower than 3 THz and a sophisticated tilted‐pulse‐fronts configuration is required .…”