Enhanced molecular vibrational spectroscopy with phononic boron nitride antennas

IntroductionSurface enhanced infrared spectroscopy (SEIRA) is a powerful strategy to increase the weak vibrational signature of molecules in FTIR spectroscopy, by means of the confined and enhanced field on the surface of... [ view full abstract ]

Introduction

Surface enhanced infrared spectroscopy (SEIRA) is a powerful strategy to increase the weak vibrational signature of molecules in FTIR spectroscopy, by means of the confined and enhanced field on the surface of plasmonic objects [1]. Another possibility to enhance and confine infrared light into subwavelength scale is to exploit phonon polaritons in polar crystals [2] or layered materials, such as van der Waals crystals. In particular, the vdW material hexagonal boron nitride (hBN) hosts low-loss hyperbolic phonon polaritons with extremely high momenta, in the reststrahlen band frequency range (1360-1610 cm^-1) [3] With hBN nanocones and nanorods, extremely narrow resonances (Q up to 283) have been already demonstrated experimentally [4].

In this work we employ for the first time well-defined phononic antennas, hBN ribbons, to detect small amounts of organic molecules. 

Methods

We fabricated hBN ribbons by e-beam lithography and etching. FTIR spectroscopy was employed to record infrared spectra of bare ribbons and ribbons with a thermally evaporated thin layer of organic molecules. 

Results and discussion

Transmission FTIR spectra demonstrate SEIRA of thin organic molecular layers using hBN ribbons, reaching femtomolar sensitivity. We observe a remarkable shift of the phonon-polariton resonance and a strong anti-absorption feature in correspondence of the molecular vibration (see Fig 1). Our results show that phonon polaritons in engineered nanostructures represent a novel platform for SEIRA of thin molecular layers. 

[1] Aroca, Ricardo. Surface-enhanced vibrational spectroscopy. John Wiley & Sons, 2006. 

[2] Hillenbrand, R., Taubner, T., & Keilmann, F. (2002). Phonon-enhanced light–matter interaction at the nanometre scale. Nature, 418(6894), 159-162. 

[3] Caldwell, J. D., & Novoselov, K. S. (2015). Van der Waals heterostructures: Mid-infrared nanophotonics. Nature materials, 14(4), 364-366. 

[4] Caldwell, J. D. et al. (2014). Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nature Communications, 5, 5221.