Skip to main content

Research · Publication & note

April 28, 2023 September 9, 2026

Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared - Notes

Research publication featured image

Published abstract

Silicon microspheres with a diameter in the range of 2-3 micrometers constitute photonic nanocavities that emit light through their Mie resonances when heated at high temperatures. At 500-600 °C these microresonators show a particular mid-infrared (MIR) emission dominated by the lowest order modes. Such resonances feature a large free spectral range, about 600 cm−1, and a high proximity to the critical coupling condition. In fact, resonances with high-quality factor, around 160 are found. It corresponds to the limit of detection of their measuring setup, being 600 the theoretical value. Most importantly, several modes emit light above the calculated blackbody limit because they feature an optical absorption cross-section larger than their geometric one. All these characteristics set silicon microspheres as very promising zero-dimensional materials for developing micrometric and sub-wavelength light sources in the MIR.

Figures

Figure 1 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared
Figure 1. Summary. SEM image showing a side view of as-synthesized polycrystalline silicon microspheres.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

Figure 2 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared
Figure 2. Summary. Schematic of the experimental setup for thermal emission measurements on individual silicon microspheres. A blue laser focused on the microsphere works as a heat source by taking advantage of the absorption of light by silicon at that wavelength.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

Panels a and b compare the measured mid-infrared thermal emission of two silicon microspheres (black) with the Mie-theory fit (red). The blue curve is the calculated Planckian emission of a black body whose area equals the projected geometric area of each sphere. The labelled Mie resonances explain the narrow spectral peaks, and peaks rising above the blue reference identify super-Planckian emission at selected frequencies.
Figure 3. Summary. Panels a and b compare the measured mid-infrared thermal emission of two silicon microspheres (black) with the Mie-theory fit (red). The blue curve is the calculated Planckian emission of a black body whose area equals the projected geometric area of each sphere. The labelled Mie resonances explain the narrow spectral peaks, and peaks rising above the blue reference identify super-Planckian emission at selected frequencies.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

Figure 4 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared
Figure 4. Summary. Calculated quality factor associated with absorption, Qabs, (blue and red lines), and with intrinsic radiative curvature losses, Qrad (blue and red dots) for several modes of M1 and M2 respectively. They are close to the critical coupling condition (Qrad = Qabs).

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

Figure 5 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared
Figure 5. Summary. The zoomed spectral zone corresponding to modes a5,1 and b6,1 of M2. The Q-values obtained by fitting the experimental (black line) and theoretical (red line) spectra to a curve consisting of the summation of two Lorentzians are indicated beside each peak.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

The calculated zero-absorption scattering efficiency of the 2080 nm silicon microsphere is used to identify resonances capable of super-Planckian emission. At the quality-factor matching condition, peaks above the dashed red threshold at a scattering efficiency of four can attain an absorption efficiency greater than unity and exceed the Planckian reference defined by the projected area. The labelled low-order Mie modes are the most experimentally accessible candidates because their radiative losses can be matched to material absorption without requiring the very low temperatures associated with higher-quality-factor modes.
Figure 6. Summary. The calculated zero-absorption scattering efficiency of the 2080 nm silicon microsphere is used to identify resonances capable of super-Planckian emission. At the quality-factor matching condition, peaks above the dashed red threshold at a scattering efficiency of four can attain an absorption efficiency greater than unity and exceed the Planckian reference defined by the projected area. The labelled low-order Mie modes are the most experimentally accessible candidates because their radiative losses can be matched to material absorption without requiring the very low temperatures associated with higher-quality-factor modes.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

Figure 7 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared
Figure 7. Summary. Calculated sphere diameter (black line) that maximizes the absorption efficiency, qabs, (red line) at the temperature of the M1 experiment (660 °C). It is achieved in several spectral sections through different indicated resonances.

© 2023 The Authors. Source: R. Fenollosa et al., Advanced Optical Materials 11(15), 2300135 (2023), DOI 10.1002/adom.202300135. Licensed under CC BY 4.0.

How to cite

Article access and reuse

The Version of Record is openly available from Wiley. © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH. The article is distributed under the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium provided that the original work is properly cited. The figures below reproduce the complete original compositions from the article PDF without modification.

Related publications

Connections within F. Ramiro-Manzano's publication record, selected from shared research topics and evidence reviewed in the source papers.

Original embedded field-emission scanning electron micrograph of a typical hydrogenated amorphous silicon microsphere with a one micrometre scale bar
Journal article
Int. J. Therm. Sci. 213, 109829 (2025)
Thermal emission of hydrogenated amorphous silicon microspheres in the mid-infrared
R. Fenollosa and F. Ramiro-Manzano

What the related paper adds

This later paper extends the research direction developed in Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. This paper distinguishes a new mid-infrared emission mechanism in hydrogenated amorphous silicon spheres. Vibrations of silicon-hydrogen bonds create a pronounced phononic band that can couple to the sphere resonances, while controlled laser heating reveals bond reconfiguration and the onset of transformation towards polycrystalline silicon.

What this paper adds to the related work

This paper provides an earlier basis for the later development reported in Thermal emission of hydrogenated amorphous silicon microspheres in the mid-infrared. This work carries resonant thermal emission into the mid infrared, where the lowest-order modes of two-to-three-micrometre silicon spheres are widely separated and approach the absorption-emission matching condition. Several peaks exceed the calculated black-body limit, establishing the spheres as compact, spectrally selective mid-infrared sources.

Figure 1: experimental setup and temperature
Journal article
ACS Photonics 6(12), 3174-3179 (2019)
Thermal Emission of Silicon at Near-Infrared Frequencies Mediated by Mie Resonances
R. Fenollosa, F. Ramiro-Manzano, M. Garín and R. Alcubilla

What the related paper adds

This earlier paper establishes a scientific basis for Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. This study turns a single silicon microsphere into a resonant thermal emitter in the near infrared. Heating produces discrete Mie peaks instead of a smooth black-body spectrum, with quality factors around 400 and a predicted microsecond response, and points towards emission above the Planck limit in selected spectral intervals.

What this paper adds to the related work

Building on the earlier contribution Thermal Emission of Silicon at Near-Infrared Frequencies Mediated by Mie Resonances, this paper advances the research line. This work carries resonant thermal emission into the mid infrared, where the lowest-order modes of two-to-three-micrometre silicon spheres are widely separated and approach the absorption-emission matching condition. Several peaks exceed the calculated black-body limit, establishing the spheres as compact, spectrally selective mid-infrared sources.

Protocol Exchange
Protocol
Protocol Exchange (2013)
Processing and Characterization of Monodisperse Silicon Colloids
L. Shi, X. Lu, R. Fenollosa, I. Rodriguez, J. T. Harris, F. Ramiro-Manzano, B. A. Korgel and F. Meseguer

What the related paper adds

This earlier paper establishes a scientific basis for Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. This protocol makes the silicon-colloid platform reproducible at smaller and more uniform sizes. It describes how to fabricate monodisperse spheres in the 300 to 500 nanometre range and how to characterize individual particles with a purpose-built confocal microscope, linking controlled processing to well-resolved electric and magnetic Mie modes.

What this paper adds to the related work

Building on the earlier contribution Processing and Characterization of Monodisperse Silicon Colloids, this paper advances the research line. This work carries resonant thermal emission into the mid infrared, where the lowest-order modes of two-to-three-micrometre silicon spheres are widely separated and approach the absorption-emission matching condition. Several peaks exceed the calculated black-body limit, establishing the spheres as compact, spectrally selective mid-infrared sources.

Two silicon nanocavities illustrating mirror-image-induced magnetic modes
Article
ACS Nano 7(1), 664-668 (2013)
Mirror-Image-Induced Magnetic Modes
E. Xifré-Pérez, L. Shi, U. Tuzer, R. Fenollosa, F. Ramiro-Manzano, R. Quidant and F. Meseguer

What the related paper adds

This earlier paper establishes a scientific basis for Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. This paper shows that a silicon nanocavity can acquire a strongly enhanced magnetic-like optical response through interaction with its own image in a metallic mirror. The real particle and its virtual counterpart form an effective magnetic dipole, providing a route to amplify scattering without assembling two perfectly matched physical particles.

What this paper adds to the related work

Building on the earlier contribution Mirror-Image-Induced Magnetic Modes, this paper advances the research line. This work carries resonant thermal emission into the mid infrared, where the lowest-order modes of two-to-three-micrometre silicon spheres are widely separated and approach the absorption-emission matching condition. Several peaks exceed the calculated black-body limit, establishing the spheres as compact, spectrally selective mid-infrared sources.

Complete original Figure 2 composition extracted directly from the article PDF
Journal article
J. Appl. Phys. 109, 102424 (2011)
Silicon colloids: A new enabling nanomaterial
F. Meseguer, R. Fenollosa, I. Rodriguez, E. Xifre-Perez, F. Ramiro-Manzano, M. Garin and M. Tymczenko

What the related paper adds

This earlier paper establishes a scientific basis for Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. This paper defines silicon colloids as a material platform rather than a single device. It surveys polycrystalline, amorphous and porous spherical particles, explains their strong low-order Mie resonances and infrared light confinement, and opens applications ranging from optical filtering to light-controlled forces between particles.

What this paper adds to the related work

Building on the earlier contribution Silicon colloids: A new enabling nanomaterial, this paper advances the research line. This work carries resonant thermal emission into the mid infrared, where the lowest-order modes of two-to-three-micrometre silicon spheres are widely separated and approach the absorption-emission matching condition. Several peaks exceed the calculated black-body limit, establishing the spheres as compact, spectrally selective mid-infrared sources.

Research fields

Bottom-up

Micrometre-scale polycrystalline silicon spheres form the resonant emitters. Their smooth spherical geometry is obtained without lithographically defining a cavity, allowing the particle diameter itself to set the mid-infrared mode structure.

Simulations & fits

Measured thermal spectra are fitted with Mie theory to determine the particle temperature and assign individual electric and magnetic resonances. The same model evaluates absorption efficiency, radiative losses and the conditions under which emission can exceed the projected-area blackbody limit.

Characterization

A dedicated single-particle interferometric setup heats each sphere with a focused blue laser and collects its mid-infrared radiation through a Cassegrain objective. Correlated SEM sizing and spectral measurements resolve high-Q peaks across the atmospheric transparency windows.

RESEARCH TOPICS

EmissionThe measured mid-infrared spectrum contains narrow thermal-emission peaks at the Mie resonances of each silicon microsphere. Several peaks rise above the Planckian reference for the projected particle area, showing how a wavelength-scale emitter can exceed that reference within selected spectral intervals.MaterialsCrystalline silicon combines a high refractive index with temperature-dependent absorption, allowing a single microsphere to confine and emit mid-infrared light resonantly. The material response and the spherical geometry therefore work together to set the position and strength of the thermal-emission peaks.ResonatorsEach silicon microsphere behaves as a three-dimensional Mie resonator whose low-order modes remain spectrally distinct in the mid infrared. Their proximity to critical coupling explains the strong, selective emission and provides a route towards compact resonant thermal sources.SetupsA purpose-built Fourier-transform infrared system isolates the weak emission of one laser-heated microsphere from the thermal background. Phase-calibrated interferometry and synchronous detection make it possible to resolve the resonances and compare them quantitatively with Mie theory and the Planckian reference.
F. Ramiro-Manzano website
Home Publications News Gallery Misc About
Site designed by F. Ramiro-Manzano. You are welcome to share, adapt and use my original content, including commercially, under CC BY 4.0, with citation to F. Ramiro-Manzano. Publisher and third-party materials remain subject to their own policies.