where Θ(ω -ω c ) is the Heaviside step function, ∆k ∥ is the spacing of the in-plane wavevector k ∥ (or the k-space lattice constant). Note that g 1D (ω) has a singularity at ω = ω c , which is known as (the first type of) the van-Hove-type singularity . The DOS for a 2D FP cavity is expressed as g