  Polaritons in 2D and van der Waals (vdW) materials have been investigated in
several recent works as an innovative platform for light-matter interaction,
rich of new physical phenomena.Hexagonal Boron Nitride (h-BN), in particular,
is an out of plane anisotropic material (while it is in-plane isotropic) with
two very strong phonon polaritons bands where the permittivity becomes
negative. In the first restrahlen band (RS1, 780-830 cm-1) the relative out of
plane permittivity is negative, while in the second restrahlen band (RS2,
1370-1610 cm-1) the relative in-plane permittivity is negative. Due to these
optical properties, thin h-BN flakes support guided modes which have been
observed experimentally both via far field and near field methods. In this
work, we show how selectively excite the more confined modes in the RS1 and RS2
bands. The supported guided modes have phase and group velocities respectively
tens and hundreds of times slower than the speed of light. We also show the
possibility of full hyperspectral nano-imaging of modes in RS1 band by means of
photo-induced force microscopy (PiFM). Moreover, a direct comparison of (PiFM)
and scattering-type near-field microscopy (s-SNOM) is obtained by imaging the
modes of the RS2 band with both techniques implemented on the same platform.
The possibility of addressing ultraslow (ultraconfined) polaritonic modes of
h-BN crystal flakes together with the possibility of optical nano-imaging in
both the restrahlen bands have many innovative aspects that can lead to
unprecedented schemes for strong light-matter interaction, slow and confined
light.
