## Abstract

We discuss some properties of dielectric gratings with period
comparable with the illuminating wavelength for slanted illumination
(this illumination geometry is often referred to as concical
mounting). We demonstrate the usefulness of such an illuminating
geometry. We show that the threshold period (under which only the
zeroth transmission and reflection orders are nonevanescent) can be
significantly higher, thereby easing fabrication constraints, and that
this illumination setup makes it possible to design achromatic phase
retarders. Such a design, for an achromatic quarter-wave plate with
λ/60 uniformity of the retardation phase in the
0.47–0.63-µm wavelength interval, is demonstrated.

© 2001 Optical Society of America

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### Equations (5)

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(1)
$${\mathrm{\Lambda}}_{\mathrm{th}}=\frac{\mathrm{\lambda}}{{n}_{\mathrm{I}}sin\mathrm{\theta}cos\mathrm{\varphi}+{\left(n_{\mathrm{II}}{}^{2}-n_{\mathrm{I}}{}^{2}{sin}^{2}\mathrm{\theta}{sin}^{2}\mathrm{\varphi}\right)}^{1/2}},$$
(2)
$${n}_{\mathrm{TE}}={\left[\mathit{fn}_{2}{}^{2}+\left(1-f\right)n_{1}{}^{2}\right]}^{1/2},$$
(3)
$${n}_{\mathrm{TM}}={\left[\mathit{fn}_{2}{}^{-2}+\left(1-f\right)n_{1}{}^{-2}\right]}^{-1/2},$$
(4)
$$\mathrm{\Delta}\mathrm{\psi}=\mathrm{arg}\left[{n}_{\mathrm{TE}}\left({n}_{\mathrm{I}}+{n}_{\mathrm{II}}\right)cos\left(\frac{2\mathrm{\pi}}{\mathrm{\lambda}}{n}_{\mathrm{TE}}t\right)+i\left({n}_{\mathrm{I}}{n}_{\mathrm{II}}+n_{\mathrm{TE}}{}^{2}\right)sin\left(\frac{2\mathrm{\pi}}{\mathrm{\lambda}}{n}_{\mathrm{TE}}t\right)\right]-\mathrm{arg}\left[{n}_{\mathrm{TM}}\left({n}_{\mathrm{I}}+{n}_{\mathrm{II}}\right)cos\left(\frac{2\mathrm{\pi}}{\mathrm{\lambda}}{n}_{\mathrm{TM}}t\right)+i\left({n}_{\mathrm{I}}{n}_{\mathrm{II}}+n_{\mathrm{TM}}{}^{2}\right)sin\left(\frac{2\mathrm{\pi}}{\mathrm{\lambda}}{n}_{\mathrm{TM}}t\right)\right],$$
(5)
$$\mathrm{\Delta}\mathrm{\psi}\left(\mathrm{\lambda},t\right)=\left(2\mathrm{\pi}/\mathrm{\lambda}\right)\left({n}_{\mathrm{TE}}-{n}_{\mathrm{TM}}\right)t.$$