Helmholtz optical resonators for ultra sensitive and specific sensing of explosives – HORUS

The key point of the HORUS project is to use a new concept of nano-antenna, called Helmholtz optical resonator by analogy with acoustics, to exalt the SEIRA signatures of explosive molecules (TNT) and to propose a sensitive, specific and versatile sensor. The key properties of the Helmholtz resonator for SEIRA detection are the following
i) It increases the electric field in a hot volume by six orders of magnitude. This is 2 orders of magnitude more than other antennas which are often limited to an exaltation at hot spots.
ii) A single resonator can absorb the entire flux of a focused beam at the diffraction limit, there is no need for periodicity, and resonators at different wavelengths can be juxtaposed to improve detector specificity and versatility.
iii) When molecules are deposited on the resonator, it gives a dark signal.
iv) The Helmholtz resonator has no harmonic resonances, avoiding spurious signals at other wavelengths.
v) The resonance wavelength can be modulated from the lateral geometrical parameters.

The success criteria of the project are:
1) Fabrication of a Helmholtz resonator with a 10 nm slit,
2) Achieve total absorption under plane wave and focused field,
3) Demonstrate a sensitivity of a few hundred zeptomoles of DTT,
4) Demonstrate the specificity and versatility of the sensor based on a resonator array on several families of explosives

The Helmholtz optical nanoresonator is a structure consisting of a slot on top of a box, both of nanometric dimensions, all in a metallic material. The slot will behave like a capacitor and the box like a coil, whose dimensions will determine a resonance wavelength that will be much larger than the geometrical dimensions involved. In particular, the size of the slit can be arbitrarily small (but still greater than 1 nm) but still concentrate all the incident flux, a characteristic that is conducive to the exaltation of absorption of molecules.

These resonators were experimentally demonstrated during the project in the mid-infrared range, and their very good angular tolerance was demonstrated. Their manufacture was simplified, and they were then used in the characterisation of nitro-aromatic molecules. The work focused on 2,4-dinitrotoluene, whose infrared permittivity was characterised and modelled, making it possible to make theoretical predictions of resonator responses in the presence of these molecules, and to compare theory and experience.

The HORUS project made it possible to make progress on the applicability of the Helmholtz resonator to the detection of molecules, in particular by moving to a simplified version of the resonator. But also by developing a set of permittivity models for explosive molecules, an essential step for the optimisation of optical resonators to detect them, and then for analysing the results of an ensemble of resonators.

There are still several challenges that need to be addressed after the HORUS project:
- Increasing the SEIRA signal.
- Taking advantage of both the SEIRA effect and the SPR effect in a dispersive material
- Improving the deposition of small amounts of molecules
- The physics of the boxless Helmholtz resonator is not fully understood, in particular the influence of periodicity which defines an equivalent volume for the box.
- Consider an alternative to spectroscopic measurement (with a limited number of light sources) that is compact, portable, low power.
- Several «incremental« improvements are also to be considered, such as the use of non-polarised resonators.
These different points will be addressed in a new project called Dartagnan, which will also study new families of molecules and new resonator configurations. The aim of this new project is to produce a demonstrator (TRL 3).

During the first part of the project, a patent was filed on the Helmholtz resonator and its application to infrared detection. An article on the experimental demonstration of this resonator was published in Applied Physics Letters (selected as Editor's Picks). The results obtained in the second half of the project, devoted to the infrared spectroscopy of explosive molecules, are the subject of an article published in Optics Express in 2020 and another under review, as well as a patent application filed at the end of 2020 with the INPI. The results of the project have been the subject of oral communications in several national and international conferences.

Identification of biochemical molecules is a major stakes for many applications, ranging from biology, sanitary control, medicine, air quality monitoring or detection of NRBC-E components. Infrared spectroscopy is a technique with a high potential to specifically identify these molecules, particularly explosives, because they have characteristic absorption bands in the infrared related to rotational or vibrational modes.

The explosive detection devices usually consist of a part that collect the surrounding air and pre-concentrate the explosive molecules; in a second time a sensor will detect the released molecules, and there is a final step in analysis and reporting of results. The Horus project focuses primarily on the sensor part of explosives detectors, and to a lesser extent on the analysis. The most common techniques and most reliable are commercially mass spectrometry and ion mobility spectrometry, but also dogs. However, none of these techniques is low cost, and have additional operability constraints, which limit their use to a few strategic sites.
As for infrared spectroscopy techniques, they face several challenges:
1) The infrared signal related to the absorption of the molecules is very low.
2) Several molecules have similar signatures and can trigger false alarms.
3) There are several explosives families it is important to detect.

Many studies have proposed the use of periodic nanostructures to increase the infrared signal from the surface molecules (SEIRA - Surface enhanced infrared absorption), but the proof of concepts have been limited to the enhancement of the field at a given wavelength, limiting de facto specificity and versatility. Furthermore, they require large areas due to the periodicity, and the signal remains low.

The key point of the Horus project is to use a new concept of nano-antenna, called the optical Helmholtz resonator by analogy with acoustics, to enhance the SEIRA signatures of molecules of explosives (TNT) and to develop a sensor being at the same time sensitive, specific and versatile.
Key properties of the Helmholtz resonator for SEIRA detection are the following:
i) It increases the electric field by six orders of magnitude in a hot volume. This is 2 orders of magnitude more than other antennas that are often limited to the enhancement in size limited hot spots.
ii) A single resonator can absorb the entire flow of a beam focused at the diffraction limit, there is no need for a periodicity of the resonator, and so variously shaped resonators can be juxtaposed, each having a different resonance wavelength, in order to improve the specificity and the versatility of the detectors.
iii) When the molecules are deposited on the resonator, it gives a signal out of a dark background.
iv) The Helmholtz resonator presents no harmonic resonances, avoiding spurious signals at other wavelengths.
v) The resonant wavelength may be modulated from lateral geometric parameters.

The success criteria of the project are:

1) Manufacture of a Helmholtz resonator with a 10 nm slit,
2) Get a total absorption in plane wave and focused field
3) Demonstrate a sensitivity of a few hundred zeptomoles of TNT,
4) To demonstrate the specificity and the versatility of the sensor based on a matrix of resonators on several explosives families.

The development of this sensor opens important prospects to other applications (control of COx and NOx combustion, air quality in homes.) From a more fundamental point of view, it promises advances on the Helmholtz resonator concept itself, as well as the information processing algorithm of a matrix of sensors.