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

R. Fenollosa; F. Ramiro-Manzano; M. Garín; F. Meseguer

doi:10.1002/adom.202300135

Research · Publication & note

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

Journal article

R. Fenollosa, F. Ramiro-Manzano, M. Garín and F. Meseguer

Advanced Optical Materials Volume 11 Issue 15 2300135 2023

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.

How to cite

Initials first

R. Fenollosa, F. Ramiro-Manzano, M. Garín and F. Meseguer. Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. Advanced Optical Materials 11(15), 2300135 (2023). DOI: 10.1002/adom.202300135.

Family name first

Fenollosa, R., Ramiro-Manzano, F., Garín, M., & Meseguer, F. (2023). Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared. Advanced Optical Materials, 11(15), 2300135. https://doi.org/10.1002/adom.202300135

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.

Figures

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

Figure 2 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared — **Figure 2.** 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. Visible light (not shown) scattered at 90° by the microsphere and a visible camera helps positioning a 36× Cassegrain objective (see scattering pattern in the inset). Mid-infrared light emitted by the microsphere is guided by two parabolic mirrors to an interferometer consisting of two retroreflectors, one of them movable by a step motor, and a ZnSe beam splitter. At the same time, light coming from an HeNe laser is introduced in the interferometer for calibration purposes of the retroreflector displacement. Light emitted by the microsphere is focused on a MCT detector by a 15× Cassegrain objective at the output of the interferometer while the HeNe laser light is guided to a Si detector. Both lasers are modulated by a chopper at 630 Hz, and the corresponding signals acquired by using two lock-in amplifiers.

Figure 3 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared — **Figure 3.** a) Measured thermal emission spectrum (black line) of a silicon microsphere with 2080 nm in diameter. The red curve corresponds to the fit of the experimental data to Equation (1) with a fitted temperature of 660 °C (M1 in Table 1). The blue line is the calculated emission of a blackbody that has an area equal to the geometric projected area of the microsphere. The modes associated to each peak are indicated. b) Same as (a) but for a 3730 nm in diameter microsphere with a fitted temperature of 560 °C (M2 in Table 1).

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

Figure 5 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared — **Figure 5.** The zoomed spectral zone corresponding to modes a_5,1 and b_6,1 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.

Figure 6 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared — **Figure 6.** Calculated scattering efficiency, q_sca, of a silicon microsphere with a diameter of 2080 nm (equal to that of M1) at zero absorption. The dashed red line indicates the limit above which a resonance can yield emission above the Planck limit as long as it is at Q-matching condition. These resonances have been indicated beside their corresponding peak.

Figure 7 from Silicon Microspheres for Super-Planckian Light Sources in the Mid Infrared — **Figure 7.** Calculated sphere diameter (black line) that maximizes the absorption efficiency, q_abs, (red line) at the temperature of the M1 experiment (660 °C). It is achieved in several spectral sections through different indicated resonances. The dashed line specifies the sphere diameter of M1 (2080 nm). The intersections with the continuous black line, indicated by the blue arrows, correspond with resonances b_2,1 and b_3,1 of Figure 3a and they indicate that maximized emission has been achieved at those spectral positions.