Imperial College London

Henry Rzepa

The title here is from an article on
metalenses\[cite\]10.1021/acs.nanolett.6b01897\[/cite\] which caught my
eye.

Metalenses are planar and optically thin layers which can be
manufactured using a single-step lithographic process. This contrasts
with traditional lenses that are not flat and where the optical
properties result from very accurately engineered curvatures, which in
turn are expensive to manufacture. Metalenses can have built into them
many interesting optical properties, including light polarisation and
dispersion. Nanoengineering has now
resulted\[cite\]10.1021/acs.nanolett.6b01897\[/cite\] in a metalens
which can simultaneously present two images of opposite helicity of an
object within the same field of view.

What is the relevance to chemistry? Well, a well-known chiroptical
technique is known as electronic circular dichroism (ECD). At its
simplest, it probes the difference in absorption by a chiral molecule of
UV and visible light with opposite circular polarisation. This
difference plotted as a function of the wavelength of the light is known
as the ECD response. Importantly, this response can also be calculated
for either enantiomer of the chiral molecule and hence the absolute
configuration can be assigned on the basis of which calculated response
matches the observed spectrum. Because the difference in response to the
two polarisations of the light (Δε) is actually very small, the ECD
technique is intrinsically less sensitive than *e.g.* normal UV/Visible
spectra and this requires the use of expensive instruments to record
that small difference. Chiral metalenses offer an interesting future
opportunity to create new forms of ECD instrument, perhaps ones that are
far more sensitive. In turn, this could lower the costs of acquiring ECD
functionality in the standard laboratory (see
\[cite\]10.1021/ed500398e\[/cite\] for an application in teaching
laboratories). Very possibly, the most expensive component would in fact
then be the computational simulations required to match up with the
experimental spectrum!

When metalenses were first introduced, they were only able to lens a
limited range of wavelengths. In another article by the same
group\[cite\]10.1038/s41565-017-0034-6\[/cite\] they now announce a new
generation of metalens that covers the region 470 to 670 nm. This
excludes the UV regions (\<300nm) or the IR regions (\>1200nm). The
latter covers another important chiroptical instrumental technique known
as vibrational circular dichroism, or VCD. As with ECD, the VCD response
of a chiral molecule can be pretty well calculated using quantum
chemistry and indeed often the VCD method is the only one that can
successfully be used to assign absolute molecular
configurations.\[cite\]10.1002/anie.201700977\[/cite\] Unfortunately,
VCD instruments are even more expensive than ECD ones, again largely due
to the intrinsic insensitivity and the need to accumulate data using
Fourier Transform methods over many hours. Few chemistry departments
have such an instrument. So I will keep an eye out for when an effective
chiral metalens operating in infra-red regions is announced! The
prospect of routine VCD analyses is tantalising! 

 

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The title here is from an article on metalenses[cite]10.1021/acs.nanolett.6b01897[/cite] which caught my eye. Metalenses are planar and optically thin layers which can be manufactured using a single-step lithographic process. This contrasts with traditional lenses that are not flat and where the optical properties result from very accurately engineered curvatures, which in turn are expensive to manufacture.

Messaggi di Rogue Scholar

Multispectral Chiral Imaging with a Metalens.