Re: BV10382
    Optical properties of spherical and anisotropic gold shell
    colloids
    by J.J. Penninkhof, A. Moroz, A. van Blaaderen, A. Polman

Dr. J.J. Penninkhof
FOM-Institute AMOLF
Kruislaan 407
NL-1098 SJ Amsterdam
PO Box 41883
1009 DB Amsterdam, NETHERLANDS

Dear Dr. Penninkhof,

The above manuscript has been reviewed by two of our referees.
Comments of the referees are enclosed for your consideration.

You will note that one of the referees has suggested that your
paper would be more appropriate for another journal.  We cannot
act as your agent in forwarding the paper elsewhere.  However,
should you choose to submit your manuscript to another journal,
and its editor so requests, we would be happy to supply the editor with
the identity and comments of our referees.

If you do choose to submit your manuscript to another journal,
the information we supply will be more useful to the editor if you have
indicated what revisions have been made in response to the
referee's report.  It should be pointed out that another journal's editor
may not agree with our choice of referees or accept their
recommendations, and may choose to have the paper reviewed further.

Yours sincerely,

Bradley Rubin
Assistant Editor
Physical Review B
Email: prb@ridge.aps.org
Fax: 631-591-4141
http://prb.aps.org/

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Second Report of the Second Referee -- BV10382/Penninkhof
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The manuscript has been rewritten and indeed improved. However, I
maintain my previous judgment that this article is not suitable for
Physical Review B. In my opinion, the main interest of this article lies
in the preparation of oblate spheroidal gold shell colloidal
particles which are aligned on a glass slide. However, the sample
preparation method using ion irradiation has already been published by
some of the authors, and the optical extinction spectra analysis appears
to me of only average interest. I cannot agree with authors claim that
``By using a novel method to fabricate large arrays of
near-perfect ellipsoidally shaped and uniaxially aligned particles, we are
able for the first time to make a direct comparison between extinction
experiments and exact extinction calculations for these anisotropic
particles'' (page 3, line 8 from bottom). I will not give the detail of
the references, but one can find in the literature
a significant number of articles devoted to the spectroscopic
investigation of (anisotropic) core shell particles. Besides this
general comment, the authors could consider the following remarks
and suggestions when submitting this article to a journal devoted
to physical chemistry, like The Journal of Chemical Physics or the Journal
of Physical Chemistry C.

Typing error in the abstract, line 3: ``coloids'' instead of
colloids.

Page 4 in Fabrication, line 8 from bottom. Authors could explain
why they cover the silica gold shell particles with an outer
silica shell.

Page 6 in Extinction of spherical Au-shell particles section
where the authors compare Mie calculation with experimental
extinction spectra.  The label ``Extinction'' of the ordinate
in Fig. 1d is misleading: it is the extinction efficiency factor
$Q_{\mathrm{ext}}=C_{\mathrm{ext}}/\pi r^2$ where $C_{\mathrm{ext}}$ is
the extinction cross section of a single particle and r the
radius of the particle for calculated spectra. The determination
of the efficiency factor from the experimental spectrum requires
the knowledge of the number of particles by unit area. Thus, how
did the authors proceed ?

The authors discuss qualitatively the behavior of the peak wavelength
resonance on the basis of the quasistatic approximation. However,
as pointed out by the authors page 11, second paragraph,
this approximation is not valid for particles for which size
(approximately 150 nm) is not very small with respect to the incident
wavelength of light. Thus, there is some inconsistency.

Page 8, first paragraph: Authors calculate the surface plasmon
dephasing time T_2 for core-shell particles and compare it with
data for homogeneous particles. Since the ``dephasing time T_2 is
the key factor determining the local field enhancement factor |f|
etc.'', they conclude that a strong enhancement of non linear effect and
Raman signal (SERS) should be expected. It might be remembered that B.
Messinger and co-workers (PRB, Vol. 24, pages 649-657,
1981) have calculated the local field for homogeneous spherical
particles.  Therefore, the authors could compare their results for the
enhancement they find from T_2 with that directly calculated.

There is a slight ambiguity pages 8 - 9, last sentence: The index
of the sample is n=1.5150, while the index of the silica core and
outer layer is 1.45 (epsilon=2.1). Therefore a slight image effect due to
the substrate should result from this index difference, but the authors
indicate that ``no noticeable difference is observed,
as expected''.

Page 9, second and third paragraphs: the discussion appear unclear to me
and should be revised. For elongated spheroids, there are two plasmon
modes, one along the major axis (longitudinal) and the other along the
minor axis (transverse). Spectra recorded under various
incidence angles reflect merely the relative excitation of these
two modes.

Page 10, last paragraph: Again, the authors discuss the plasmon
resonance shift on the basis of the quasistatic approximation which is not
applicable in the present case.

Page 11, line 11 from bottom: The verb has been omitted in the
sentence ``Clear differences are observed between the spectra ... of
particles of this size (verb) not etc.''

In conclusion, in my opinion this article should be submitted to
a more specific journal of chemical physics, like The Journal of
Chemical Physics, the Journal of Physical Chemistry C or another
similar journal.





----------------------------------------------------------------------
Second Report of the First Referee -- BV10382/Penninkhof
----------------------------------------------------------------------

This is my second review of this manuscript. My general assessment is that
the authors have taken the comments from the previous round of reviews
seriously and that the manuscript has undergone a substantive revision.
Most importantly, the authors have addressed the questions of the novelty
and physical content. Material that could be viewed as trivial has been
removed from the manuscript. I did not change
my point of view that the main achievement demonstrated by the
authors is development of a new experimental technique. However,
the authors apply this technique to study some interesting questions that
could not be addressed before, at least experimentally, and
make this point much clearer.

I therefore recommend publication of the revised manuscript in PRB.

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