Molecular Screening for Terahertz Detection with Machine-Learning-Based Methods

Abstract

The molecular requirements are explored for achieving efficient signal up-conversion in a recently developed technique for terahertz (THz) detection based on molecular optomechanics. We discuss which molecular and spectroscopic properties are most important for predicting efficient THz detection and outline a computational approach based on quantum-chemistry and machine-learning methods for calculating these properties. We validate this approach by bulk and surface-enhanced Raman scattering and infrared absorption measurements. We develop a virtual screening methodology performed on databases of millions of commercially available compounds. Quantum-chemistry calculations for about 3000 compounds are complemented by machine-learning methods to predict applicability of 93 000 organic molecules for detection. Training is performed on vibrational spectroscopic properties based on absorption and Raman scattering intensities. Our top molecules have conversion intensity two orders of magnitude higher than an average molecule from the database. We also discuss how other properties like molecular shape and self-assembling properties influence the detection efficiency. We identify molecular moieties whose presence in the molecules indicates high activity for THz detection and show an example where a simple modification of a frequently used self-assembling compound can enhance activity 85-fold. The capabilities of our screening method are demonstrated on narrow-band and broadband detection examples, and its possible applications in surface-enhanced spectroscopy are also discussed.

Molecular Vibration Explorer: an online database and toolbox for surface-enhanced frequency conversion, infrared and Raman spectroscopy

“…The creation of the database is described in Ref. [2]. DFT calculations are performed at the B3LYP+D3/def2-SVP level using the Gaussian program package.…”

“…where M refers to the set of normal modes that have frequencies in the range defined by the user. These properties are standardized as described in Ref [2]. A, R, and P can also be calculated using orientation-specific intensities, this enables fast comparison of molecular performance at different orientations.…”

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We present Molecular Vibration Explorer, a freely accessible online database and interactive tool for exploring vibrational spectra and tensorial light-vibration coupling strengths of a large collection of thiolated molecules. The 'Gold' version of the database gathers the results from density functional theory calculations on 2'800 commercially available thiol compounds linked to a gold atom, with the main motivation to screen the best molecules for THz and mid-infrared to visible upconversion [1][2][3][4]. Additionally, the 'Thiol' version of the database contains results for 1'900 unbound thiolated compounds.

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Detecting mid-infrared light by molecular frequency upconversion in dual-wavelength nanoantennas

“…To understand the frequency-selective dependence, we calculate the product [in m 3 /(mol·sr)] of infrared absorption and Raman intensity of BPT, averaged over all orientations for each normal mode (Fig. 4A and supplementary materials, section S1) ( 24 ). This clearly showed that the optimum overlap of optical and vibrational modes was at 1080 cm −1 , and that the dipoles were all well aligned with the vertical E field in the nanogap at both visible and MIR wavelengths.…”

“…S3). Prospects for multiband operation are promising [selection of optimal molecules is required to broaden the vibrational range and responsivity ( 24 )] because lower-frequency anti-Stokes emission has already been observed at 250 cm −1 (Fig. 3Β, λ = 40 μm or 7.5 THz).…”