Nanoresonators Enhance Light-Molecule Interactions

New results of European researchers open new avenues for funda­mental studies of vibra­tional strong coupling, as well as for the develop­ment of novel infrared sensors for chemical recognition of very small amounts of molecules. The inte­raction of light and matter at the nanoscale is a key element for many funda­mental studies and techno­logical appli­cations, ranging from light harves­ting to the detection of small amounts of molecules.

This is an illustration of the interaction between molecular vibrations and phonon polaritons in a boron nitride nanoresonator. (Source: Ella Maru Studio, Inc.)

During the last decades, many stra­tegies have been imple­mented in order to enhance nanoscale light-matter inter­actions. One approach is based on concen­trating light with the help of propa­gating and loca­lized surface plasmon pola­ritons, which are collec­tive electron oscil­lations in metals or semiconductors that are coupled to light. These electro­magnetic excita­tions can concen­trate light into nano­scale hotspots. At mid-infrared fre­quencies, they enable, for example, the detec­tion of tiny amounts of molecules. But with this surface-enhanced infrared absorp­tion (SEIRA) spectro­scopy typical mid-infra­red plasmonic structures suffer from large losses and do not achieve ultimate light concen­tration.

An interes­ting but much less explored approach for enhancing nanoscale light-matter inter­action is based on infrared-phononic materials, in which light couples to crystal lattice vibrations to form phonon pola­ritons. “Phonon-polariton reso­nators offer much lower losses and field confine­ment than their mid-infrared plasmonic counter­parts. For that reason, we decided to develop and apply infrared-phononic reso­nators to enhance the coupling of infrared light to molecular vibra­tions,” says postdoc Marta Autore at nanoGUNE.

In order to develop a method – phononic SEIRA – the researchers fabri­cated a set of ribbon arrays made of hexa­gonal-boron nitride (h-BN) flakes. By infrared trans­mission spectro­scopy they indeed observed narrow phonon pola­riton reso­nances. Then, they deposited thin layers of an organic molecule onto the ribbons. It led to a strong modi­fication of the phonon pola­riton reso­nance, which could be used to detect ultra-small amounts of mole­cules that were not detectable when deposited on conven­tional substrates.

“Interes­tingly, when we depo­sited thicker layers of molecules onto the ribbons, we observed a splitting of the phonon polariton resonance. This is a typical signature of a pheno­menon that is known as strong coupling. In this regime, the inter­action of light and matter is so strong that exciting phenomena such as modi­fication of chemical reactions, polariton conden­sation or long-range and ultrafast energy transfer can occur,” says Rainer Hillen­brand, group leader at nanoGUNE who led the work. “In the future we want to have a closer look into phonon-enhanced strong coupling and what we could do with it.”

The findings show the poten­tial of phonon polariton reso­nators to become a new platform for mid-infrared sensing of ultra-small quantities of materials and for exploring strong coupling at the nano­scale, opening the way for future funda­mental studies of quantum pheno­mena or appli­cations such as local modi­fication of chemical bond strength and selec­tive catalysis at the nano­scale. (Source: Elhuyar Foundation)

Reference: M. Autore et al.: Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit, Light: Sci. & Appl. 7, 17172 (2018); DOI: 10.1038/lsa.2017.172

Link: CIC nanoGUNE, Donostia-San Sebastián, Spain

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