Looking on Subwavelength-Size Objects With Nanobulbs

Nanoparticles – size amounts to 150 nanometers – are used as nanobulbs to improve light microscopy. (Source: S. Makarov et al.)

Scientists from ITMO Univer­sity in Saint Petersburg have proven that a silicon-gold nano­particle can act as an effective source of white light when agitated by a pulse laser in IR band. One such nanobulb was inte­grated into a standard probe micro­scope, which allowed the researchers to overcome the diffraction limit and examine subwave­length-size objects. The new tech­nology makes modern near-field microscopy cheaper and simpler, also being poten­tially useful in medicine.

In order to examine an object using a regular optical micro­scope, visual light is focused using special lenses. However, if the object is less than a wave­length in size, it cannot be observed in detail. There are several techno­logies today capable of overcoming this dif­fraction limit – for instance, near-field optical micro­scopy. In that case, the electro­magnetic field of the object is measured in near field by using a special probe that can interact with the localized electro­magnetic field and disperse it into the area where it can be registered by common detectors. Yet to get information about an object with a subwave­length resolution in a broad spectrum; researchers often spend hours scanning at different wave­lengths until they cover the whole spectrum.

Researchers from ITMO Univer­sity solved this problem by using the nanobulb – a miniature light source based on a silicon-and-gold nano­particle. Its main charac­teristic is that it emits light in an im­mensely wide wave­length band from 400 to 1,000 nm. A single nanobulb can register and analyze the optical response of all kinds of subwave­length nano­structures in the whole visible spectrum at the same time. This increases the effi­ciency and speed of micro­scopy by several times. To create the nanobulb, scientists from the Depart­ment of Nano­photonics and Meta­materials printed a silicon-and-gold nano­particle. To make it emit light it is lit with a femto­second IR laser. Thanks to this energy, electrons first attain higher energy levels, and then slide towards the bottom of silicon’s conduction band, emitting photons on different wave­lengths.

“Silicon, being a nondirect-gap semi­conductor, is a poor material for gene­rating emission. In other words, if you light it with a laser, it will absorb maybe a million photons and emit just one. Yet, it is very cheap, you can literally make it from sand. This is why humanity aspires to find as many appli­cations for it as possible: in photo­voltaics, micro­electronics, and other fields. We have found a most unexpected appli­cation, using its main drawback – its indirect band-gap – to create a nanosize source of white light that can emit photons of energy 3.4 to 1.1 eV,” says Sergei Makarov.

“What is more, at the boundary of these two materials, gold and silicon, inter­faces emerge that provide for an even better radiative recom­bination of electrons. Lots of physical mechanisms that we are yet to research are at work here, so there’s a lot of theo­retical work that we will have to do to improve our nanobulb, including creating an emittance model,” comments PhD student Ivan Sinev. He notes that another positive feature of the nanobulb is that it uses and IR-band laser to generate visible light. This means that extra noise in the optical signal can be removed by way of filtering the dissi­pated IR light, which improves the effec­tiveness with which the actual signal is registered.

At the sugges­tion of Anton Samusev the nanobulb was placed on a common probe of an atomic-force micro­scope. The probe allows bringing the source of visible light imme­diately near the test subject, which greatly amplifies the inter­action of the near fields. The signal from this nano­particle’s emission is regis­­tered and separated on a spectrum using a regular spectro­meter. Thus, a nanobulb may be inte­grated into standard micro­scopic equipment; in other words, it can be attached to any probe and used to record its signals with regular photo­detectors – all while receiving information on a nanoobjects’s near-field in the entire visible spectrum band. This means that silicon-gold nano­particles make micro­scopy more flexible and cheaper.

“We are also developing an idea of using the nanobulb as a nano­laser. If we place such a particle into a resonator capable of changing the wave’s operating length, we can end up with a tunable laser, one that can function at any set wave­length in the visible spectrum. Besides, the nanobulb may also see use in biology for purposes such as illu­minating cells and detecting substances that are sensi­tive to parti­cular wave­lengths,” adds Sinev. (Source: ITMO)

Reference: S. V. Makarov et al.: Nanoscale Generation of White Light for Ultrabroadband Nanospectroscopy, Nano Lett. 18, 535 (2018); DOI: 10.1021/acs.nanolett.7b04542

Link: Dept. of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, Russia


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