2023-09-10 06:00:13
Understanding how materials absorb light at the nanometer scale is essential for applications in photonics or quantum optics. By superimposing a laser beam and an electron beam with previously unmatched precision in an electron microscope, scientists were able to study absorption (In optics, absorption refers to the process by which the energy of a photon is taken by…) of glass and polystyrene microbeads with very high precision.
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The optical properties of nanomaterials are intimately dependent on the details of their size and shape, and can vary on nanoscales. This makes them difficult to study with conventional microscopies using light. Using a transmission scanning electron microscope (STEM), atomic spatial resolutions can be achieved to image the structure of materials. material of natural or artificial origin that man shapes for…).
Using electronic energy loss spectroscopy (EELS), optical properties can be studied in parallel with a spatial resolution of the order of ten nanometers. However, EELS is limited to spectral resolutions ten to a thousand times too low for the study of photonic systems. Such spectral resolutions are easy to obtain with lasers, but unfortunately with insufficient spatial resolution.
A collaboration bringing together the Solid State Physics Laboratory (LPS, CNRS / Paris-Saclay University) and Spanish and Brazilian teams have developed an injection system (The word injection can have several meanings: ) of light in a STEM (Figure 1a), which he adapted to a latest generation monochromated electron microscope (CHROMATEM). The precision of this system makes it possible to optimize the injection of a laser into a STEM, and thus to obtain spectral resolutions of a few hundred µeV. In doing so, the technological limit predicted by the designers of the STEM-EELS systems themselves (of the order of a few meV) is largely exceeded!
As an application, the scientists studied micro-beads of silica and polystyrene with sizes between 1 and 8 µm. In these beads, light is captured by internal reflection (Figure 1a, insert), giving rise to modes propagating around a circumference and called gallery modes. The modes thus formed have very long lifetimes because they cannot escape from the balls, and their width at half height is theoretically of the order of a few µeV (to be compared with the few meV accessible in EELS). Unfortunately, it is also correlatively very difficult to inject light into it.
Figure 1 – Adapted from: Auad, Y., Dias, EJC, Tencé, M. et al., μeV electron spectromicroscopy using free-space light. Nat Commun 14, 4442 (2023)
has. Schematic diagram of an energy gain experiment. A fast electron (here, 200 kV) is focused on a sample at the point Re (see insert). In parallel, a laser beam is also focused at the Rf point. As part of the experiment, the laser beam creates a gallery mode (WGM: Whispering gallery mode). As it de-excites, it will accelerate the electron. The energy of the electrons is measured above the sample using a magnetic prism.
b. Comparison of an electronic energy loss spectrum for a silica sphere. Each peak corresponds to the energy of a mode. We see that gain spectroscopy (EEGS) is much better resolved than electron energy loss spectroscopy (EELS, see text), or cathodoluminescence spectroscopy (CL, consisting of the light emitted by a sample following electron irradiation).
vs. Gain spectrum for a polystyrene sphere. We see that very fine and partially overlapping modes are resolved. The spectral range of the spectrum corresponds to the best known spectral resolution in EELS (electronic energy loss spectroscopy).
The very precise positioning (of a few hundred nanometers in the three directions of space) of a high numerical aperture mirror allowed the injection optimal performance of a laser beam in the spheres, creating an evanescent electric field on their surface. The latter accelerates a beam of fast electrons (regarding half the speed of light) positioned at the edge of the sphere. By varying the laser energy with a precision of around ten µeV, the modes might be excited, accelerating a greater number of electrons as this energy was close to those of the gallery modes (Figure 1b). . The energy gain spectrum thus formed already makes it possible to resolve modes with a resolution exceeding by two orders of magnitude the EELS resolutions at the acceleration voltage (Acceleration commonly refers to an increase in speed; in physics,…) used, and by an order of magnitude the best EELS resolution ever reported (Figure 1c).
This is of particular interest for a variety of applications in the fields of quantum optics and photonics. In addition, this technique might open up to many other fields of investigation such as the physics of semiconductors, infrared spectroscopy or quantum optics, and will require new theoretical developments to understand all its aspects. This work is published in Nature Communications.
References
μeV electron spectromicroscopy using free-space light,
Yves Auad, Eduardo JC Dias, Marcel Tencé, Jean-Denis Blazit, Xiaoyan Li, Luiz Fernando Zagonel, Odile Stéphan, Luiz HG Tizei, F. Javier García de Abajo et Mathieu Kociak,
Nature Communications, published July 24, 2023.
Doi: 10.1038/s41467-023-39979-0
Open archive: arXiv
1694331200
#spectacular #association #quantum #optics
