Microspectroscopy using filled holes (
This work was performed by Joseph Knab at the University of Technology Delft)
Our measurements on the behavior of the electric field near very small holes are useful in that they provide us with
a thorough understanding on how light actually passes through a hole that is much smaller than the wavelength. This information is useful in that it
helps us understand how light behaves when we actually fill the hole with something that scatters or absorbs terahertz light. Doing actual spectroscopy on something that
is much smaller than the wavelength, such as a small bare crystal, is not trivial. The problem is that light usually bends around the object so that far away from it,
the light carries little information on the properties of the object. However, we would like to be able to measure on small sample volumes, if only because in some cases
we simply don't have large volumes of a sample. Tiny holes to the rescue. The thing is, we already demonstrated our ability to sensitively measure terahertz
electric fields very close to tiny holes, so filling them with something and then measuring the electric fields close to the holes is the next logical step. The reason
that this could work is that we know that when we have a tiny hole in a metal foil, the metal itself doesn't transmit any THz light, but each tiny hole does. Not much,
but with our detection techniques AND because we measure in the immediate neighbourhood of the hole, the signal we measure is strong enough. Now here's the interesting bit:
since the hole is completely filled with the substance we're trying to measure the spectrum of, we know that all the light passing through the hole must have interacted
with the substance. So, even though the total signal is not huge, the changes induced by the substance are. That is, relative to a measurement on a bare
(meaning, not in a metal hole) sample.
The principle is illustrated in the figure below:
Terahertz light is incident on tiny holes drilled in a metal plate. Some holes are filled with something. We tried polyethylene powder, silicon powder and
D-tartaric acid, a food additive. When we look at the tranmission characteristics of an empty hole, we find the well know cut-off indicating that frequencies below
a certain well-defined frequency cannot pass through the hole.
The next figure shows how the measured terahertz electric field (E_x) is distributed immediately behind a hole:
Three different different frequencies (terahertz colors if you like) are shown. The left column shows the signal when the hole is filled with D-tartaric acid,
the right column shows the signal behind an empty hole. There's a lot to say about this measurement but one feature stands out: the filled hole
transmits more light at lower frequencies than the empty hole.
Since the two-dimensional images don't provide a lot of extra information, we now confine ourselves to one location behind the hole and do our measurements
there. This allows us to look at the entire spectrum of terahertz light transmitted by the apertures. The next figure shows the spectra of light transmitted by
an empty hole, a hole filled with silicon powder, and a hole filled with polyethylene powder:
Note that the vertical scale is logarithmic.
There are clear differences between the results for the three holes and it is now clear that filling the hole shifts the cut-off frequency to lower frequencies.
This is actually not unexpected and was observed in the past by others. The reason that more light gets through is simply because the wavelength of light inside a
material is shorter than in air. A (half)waves which wouldn't fit inside an empty hole, still fits inside a filled hole.
The oscillations, visible in the spectrum of silicon are etalon oscillations.
They are more pronounced for Si because the silicon powder has a higher effective refractive index.
The inset shows calculations that that are in pretty good agreement with the
measurements. Finally, since our goal is to perform spectroscopy, we show the spectrum transmitted by a hole filled with D-tartaric acid. D-tartaric acid has
shows absorption at distinct frequencies and the question is if we can see it also when we are dealing with only a tiny amount of material such as present
inside the hole. The image speaks for itself:
The dips in the spectrum are at the known frequencies where the D-tartaric acid absorbs the THz radiation. They are not "artifacts". This proves that we can
perform THz microspectroscopy on small sample volumes (a couple of nanoliters here). For more details, you should read the paper
(which can be found at the publications page).
In simple terms
How do you use light to measure properties of a sample
that is too small to be seen with, well,
light? Under normal circumstances, that's very difficult.
Here, we show how we did it using terahertz light.
The basic trick is to force a certain portion of the light
to go through a tiny hole that's filled with a substance
you want to measure the properties of. Even though the hole
itself may be so small that it is almost not "seen" by the light
we were able to measure light getting through anyway. The reason
this works is because we don't measure the light some
distance away from the hole, but very, very close to the hole.
Close to the hole, the light appears much stronger than far away
and that's partly because far away, we collect much less light then closeby.
(that's assuming that the light goes off in every direction, because then,
the further away you are, the less you collect). Sounds simple, doesn't it?
Well, in reality it is a bit more complicated because it requires
so-called "near-field measurements", which require some special tricks for
it to work. Suffice it to say that our techniques work quite well, thus
allowing us to measure the (terahertz) properties of very small quantities
of a substance.