Nonimaging optics

The field diaphragm controls the area of the circle of light illuminating your specimen, while the aperture diaphragm controls the angular aperture of the cone of light from the condenser.

Colloidal quantum dots are tiny crystals—you could fit a billion into the period at the end of this sentence—that emit different colors of light depending on how big you make them. They’re very efficient and easy to make and are already being used in commercial technology; you might already have bought a quantum-dot TV without knowing it.

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As light enters the diffuser, it's scattered in many directions resulting in a uniform glow exiting the diffuser. This allows your device to take an accurate ...

Scientists with the University of Chicago have demonstrated a way to create infrared light using colloidal quantum dots. The researchers said the method demonstrates great promise; the dots are already as efficient as existing conventional methods, even though the experiments are still in early stages.

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You’ll find the field diaphragm at the top of your illuminator (it’s usually controlled by a little lever on the side or front).

To set up for Koehler, head for the (usually neglected) bottom half of your microscope because that’s where you’ll find all the components you’ll need to adjust – two diaphragms and your condenser.

It can be frustrating adjusting your controls each time the objective is changed – especially for critical work. But it quickly becomes second nature.

Infrared lasers now are made through a method called molecular epitaxy, which works well but is labor- and cost-intensive. The scientists thought there might be another way.

Citation: “Mid-infrared cascade intraband electroluminescence with HgSe–CdSe core–shell colloidal quantum dots.” Shen, Kamath, and Guyot-Sionnest, Nature Photonics, Aug. 10, 2023.

Dispersion is defined to be the spreading of white light into its full spectrum of wavelengths. Refraction is responsible for dispersion in rainbows and many ...

In fact, they found that the method was already as efficient as other, conventional ways to produce infrared light, even in exploratory experiments. With further tinkering, the scientists said, the method could easily surpass existing methods.

The researchers used the University of Chicago Materials Research Science and Engineering Center and the Pritzker Nanofabrication Facility for the research.

The dots could someday form the basis of infrared lasers as well as small and cost-effective sensors, such as those used in exhaust emissions tests or breathalyzers.

Most modern microscopes are capable of using Koehler illumination or are at least capable of being retrofitted as long as certain criteria can be met.

But infrared light has a lot of uses. In particular, it is very useful for making sensors. If you want to know whether harmful gases are coming out of your car exhaust, or test whether your breath is above the legal alcohol limit, or make sure methane gas isn’t coming out of your drill plant, for example, you use infrared light. That’s because different types of molecules will each absorb infrared light at a very specific wavelength, so they’re easy to tell apart.

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“Right now the performance for these dots is close to existing commercial infrared light sources, and we have reason to believe we could significantly improve that,” said Philippe Guyot-Sionnest, a professor of physics and chemistry at the University of Chicago, member of the James Frank Institute, and one of three authors on the paper published in Nature Photonics. “We’re very excited for the possibilities.”

Aligning your microscope for Koehler illumination is easy and will give you clearer images with much better contrast, so you can see (and capture) all the important details.

In this “cascade” technique, researchers run an electrical current across a device, which sends millions of electrons traveling across it. If the architecture of the device is just right, the electrons will travel through a series of distinct energy levels, like falling down a series of waterfalls. Each time the electron falls down an energy level, it has the chance to emit some of that energy as light.

Criticalillumination

The researchers wondered if they could create the same effect using quantum dots. They created a black “ink” of trillions of tiny nanocrystals, spread it onto a surface and sent an electrical current through.

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“I think it’s one of the best examples of a potential application for quantum dots,” said Guyot-Sionnest. “Many other applications could be achieved with other materials, but this architecture really only works because of the quantum mechanics. I think it’s pushing the field forward in a really interesting way.”

They hope the discovery could lead to significantly cheaper infrared lights and lasers, which could open up new applications.

While larger pieces of polarizing material are preferred, the demonstration works with smaller pieces. Presenter Brief: Light is a transverse wave because its ...

Guyot-Sionnest and his team have been experimenting with quantum dots and infrared technology for years. Building on their previous inventions, they set out to try to recreate a “cascade” technique that is widely used to make lasers, but had never been achieved with colloidal quantum dots.

When your microscope is aligned with Kohler illumination properly, the condenser is at the correct height relative to the sample, the field diaphragm is adjusted to eliminate reflections and glare, and the aperture is moved to an optimal position to provide contrast without producing shading artifacts.

Dispersion refers to the separation of white light into its constituent colours (spectrum) due to the different frequencies (colours) of light in a medium.

Using this method, your images will have much greater clarity and contrast, with little or no glare. And if you’re into microscopy, you’ll be able to meet journal quality standards – in less than a minute of your time!

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“We thought it would be likely to work, but we were really surprised by how well it worked,” said Guyot-Sionnest. “Right away, from the first time we tried it, we saw light.”

You’ll find the aperture diaphragm at the bottom of the condenser (also usually controlled by a small level on the side).

However, those quantum dots are being used to make light in the visible wavelength—the part of the spectrum humans can see. If you wanted quantum dot light in the infrared wavelength, you’ve mostly been out of luck.

“So a cost-effective and easy-to-use method to make infrared light with quantum dots could be very useful,” explained Xingyu Shen, a graduate student and first author on the new study.

The maximum magnification of a light microscope is around x2000. However, most of the microscopes that are used in schools can only reach x400 magnification.

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It’s pretty simple to set up and it should only take a minute or two before you’re using your microscope to its full potential.