Recently, Harrisson and Ben-Yakar [1

R. K. Harrisson and A. Ben-Yakar, “Role of near-field enhancement in plasmonic laser nanoablation using gold nanorods on a silicon substrate,” Opt. Express 18(21), 22556–22571 (2010). [CrossRef]

] presented a paper on the role of near-field enhancement in laser ablation of a silicon surface using gold nanorods. While very interesting experimental results and calculation are presented, the paper reaches the conclusion that the field enhancement of the Poynting vector rather than the enhancement of the electric field is responsible for laser ablation. We argue in this comment that the use of the Poynting vector is incorrect and in fact not necessary. It can be found in any standard textbook on electromagnetism [3

J. D. Jackson, “Poynting’s Theorem for Harmonic Fields,” in Classical Electrodynamics , 3rd edition, (John Wiley & Sons Inc., 1999), pp 264–265.

] that, in the case of an harmonic field, the time-averaged rate of work W done on a material contained in a volume V, representing the conversion of electromagnetic energy into mechanical or thermal energy, is given by where J = σE is the current distribution in the material. W is obviously proportionnal to |E|² when the material is ohmic. Using Maxwell’s equations, basic vector identities and assuming linear and lossless medium, one gets an harmonic version of the Poynting’s theorem which reads where S= 12E× H* is the complex Poynting vector and E and H are the complex amplitude of the electric and magnetic field respectively. Equation (2) clearly shows that |S| is not in general a measure of W in the case of an harmonic field. However, ∇ · S would provide the correct result. In the particular case of a plane wave solution propagating in an absorbing medium, ∇ · S is proportional to the Poynting vector itself and W = α|S| where α is the absorption coefficient. The use of |S| to evaluate energy source term for laser ablation is thus only justified in the cases involving plane wave, including for example the interaction of a focused laser on a surface such as in [2

J. Chen and J. Beraun, “Numerical study of ultrashort laser pulse interaction with metal films,” Numer. Heat Transfer, Part A 40, 1–20 (2001). [CrossRef]

]. In the situations involving the interaction with plasmonic nanostructures, the resulting field is by far not a plane wave, as one can easily see by looking at the Mie solution for a sphere in a dielectric medium, and |S| is not a good metric to evaluate energy absorption. This is also true for a plasmonic nanostructure resting on a surface. In [1

R. K. Harrisson and A. Ben-Yakar, “Role of near-field enhancement in plasmonic laser nanoablation using gold nanorods on a silicon substrate,” Opt. Express 18(21), 22556–22571 (2010). [CrossRef]

], both the enhancement of |S| and of |E|² are evaluated to discuss the power absorption in the nanorod. Two completely different results are found by the authors as seen in Fig. 2. This is not surprising because the fields involved here, especially close to the nanostructure, are not plane waves and the disagreement between the two results only shows that |S| cannot be used in that particular case in evaluating the rate of work done by the field on the material.