Metalon: Optical Etalons, Metagratings, & Metallenes

• Metalon is a context-dependent term describing distinct metal-based systems, including solid Fabry–Pérot etalons, all-metallic metagratings for beam steering, and loosely defined atomically thin metallenes.
• In spectroscopy, Fabry–Pérot metalons employ broadband metallic coatings to balance finesse and throughput, achieving stable calibration through controlled thermal responses.
• In metasurface applications, engineered metallic grooves enable anomalous reflection and dual-polarization control, while metallene studies focus on 2D elemental metals with unique electronic properties.

Metalon is a context-dependent technical term rather than a single standardized object. In astronomical instrumentation, it denotes a solid fused-silica Fabry–Pérot etalon whose two reflecting surfaces are broadband metallic mirrors rather than multilayer dielectric mirrors (Ghosh et al., 30 Jul 2025). In metasurface electromagnetics, the supplied technical summary uses “Metalon” for dielectric-free, fully metallic metagratings composed of rectangular grooves in a metal slab and designed for anomalous reflection (Rabinovich et al., 2020). In the literature on atomically thin two-dimensional metals, the established term is metallene; one paper states explicitly that “Metalon” does not appear in the paper and that the subject is metallenes, i.e. atomically thin, nonlayered two-dimensional elemental metals (Bagheri et al., 8 Jan 2026). This suggests that “Metalon” functions less as a universal scientific designation than as a domain-specific label applied to distinct metal-based optical or materials platforms.

1. Terminological scope

Usage	Definition	Source
Fabry–Pérot metalon	Solid fused-silica etalon with broadband metallic mirrors	(Ghosh et al., 30 Jul 2025)
All-metallic “Metalon”	Periodic metallic metagrating with rectangular grooves for anomalous reflection	(Rabinovich et al., 2020)
Loose metallene-related usage	Shorthand interpretation for atomically thin metallic monolayers; exact term absent in one paper	(Abidi et al., 27 Jun 2025, Bagheri et al., 8 Jan 2026)

The most precise usage in the supplied corpus is the spectrographic one. There, the device is explicitly described as a solid fused silica Fabry–Pérot metalon (FPM): the cavity is the fused-silica slab itself, with metallic coatings on the two parallel faces. The defining distinction is that the mirrors are metallic—specifically silver deposition with a protective SiO$_2$ overcoat—rather than dielectric multilayers. The term is therefore anchored to a particular resonant optical component rather than to metallicity in a generic sense (Ghosh et al., 30 Jul 2025).

The metasurface usage is also technically precise, but it refers to a different class of objects: monolithic metallic diffractive structures that realize beam steering through groove-waveguide modes and Floquet-channel control. In the supplied summary, this is presented as directly relevant to all-metallic metasurfaces or metagratings (“Metalon”), not as a Fabry–Pérot resonator (Rabinovich et al., 2020).

The metallene-related usage is explicitly looser. One supplied summary interprets “Metalon” as shorthand for atomically thin metallic materials and then discusses metallenes: one-atom-thick or nearly one-atom-thick crystalline metal sheets (Abidi et al., 27 Jun 2025). Another states more strictly that the paper discusses metallenes, not a distinct “Metalon” phase or platform (Bagheri et al., 8 Jan 2026). A terminological caution is therefore necessary: in condensed-matter contexts, metallene is the formal term.

2. Solid fused-silica Fabry–Pérot metalons in spectroscopy

In the spectrographic sense, a metalon is a solid fused-silica Fabry–Pérot etalon with metallic coatings on the two reflecting faces. The studied devices were commercially available LightMachinery parts, designated OP-8565-T, with fused silica as the substrate and cavity material, 20 mm clear aperture, quoted finesse $\sim 2.5$, transmission $<50\%$, and modulation of about 30% from 350 to 900 nm. Two cavity spacings were examined: a thin FPM with thickness $Z_0 = 3371~\mu\text{m}$, corresponding to FSR $= 1/\text{cm}$ $\approx 30$ GHz, and a thick FPM with thickness $Z_0 = 6743~\mu\text{m}$, corresponding to FSR $= 0.5/\text{cm}$ $\approx 15$ GHz (Ghosh et al., 30 Jul 2025).

The optical physics is that of a solid Fabry–Pérot resonator. The phase difference between successive reflected beams is

$\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$

and constructive interference is written as

$\sim 2.5$0

At normal incidence,

$\sim 2.5$1

The wavelength free spectral range is therefore

$\sim 2.5$2

The device thus retains the standard Fabry–Pérot formalism, but with the important practical substitution of metallic coatings for broadband operation (Ghosh et al., 30 Jul 2025).

This substitution changes the usual design trade-off. Dielectric-coated air-spaced etalons can reach very high reflectivity, typically around $\sim 2.5$3, and hence high finesse, but their phase response is more chromatic and the coatings are harder to make truly broadband. The metallic-coated solid etalon instead trades away finesse and some throughput in exchange for broad bandwidth, simpler construction, mechanical rigidity, immunity to pressure fluctuations, lower cost, and a coating response that is less wavelength-sensitive in both reflectance and phase (Ghosh et al., 30 Jul 2025).

The corresponding throughput penalty is measurable but not prohibitive. Measured average system transmittance near 6650 Å was about 27% for the thin FPM and 24% for the thick FPM. Using the paper’s approximation that the peak transmittance is approximately the average transmittance multiplied by the finesse, the estimated peak transmittances were near 67% and 60%, respectively. Representative Lorentzian fits for the thin FPM gave line centers and widths of $\sim 2.5$4 Å with FWHM $\sim 2.5$5 Å, $\sim 2.5$6 Å with FWHM $\sim 2.5$7 Å, and $\sim 2.5$8 Å with FWHM $\sim 2.5$9 Å; the inferred finesse values were about $<50\%$0 at $<50\%$1 Å and $<50\%$2 at $<50\%$3 Å and $<50\%$4 Å (Ghosh et al., 30 Jul 2025).

3. Thermal response and calibration performance

The central practical issue for Fabry–Pérot metalons is thermal drift. The paper takes both the cavity thickness $<50\%$5 and refractive index $<50\%$6 to vary with temperature, using

$<50\%$7

and

$<50\%$8

Differentiating the FSR gives

$<50\%$9

and the wavelength drift of an individual peak is

$Z_0 = 3371~\mu\text{m}$0

Because both coefficients are positive for fused silica in the regime studied, increasing temperature increases $Z_0 = 3371~\mu\text{m}$1, decreases the wavelength FSR, and shifts transmission peaks linearly with $Z_0 = 3371~\mu\text{m}$2. The thermo-optic contribution dominates the expansion contribution (Ghosh et al., 30 Jul 2025).

At 6550 Å for the thin FPM, the theoretical drift was evaluated as

$Z_0 = 3371~\mu\text{m}$3

which implies a temperature-stability requirement of roughly

$Z_0 = 3371~\mu\text{m}$4

to achieve 1 m s$Z_0 = 3371~\mu\text{m}$5 calibration precision. Experimentally, the smallest observed drift from the 20–22°C comparison was about

$Z_0 = 3371~\mu\text{m}$6

implying about

$Z_0 = 3371~\mu\text{m}$7

for 1 m s$Z_0 = 3371~\mu\text{m}$8. The practical target was therefore stated as about 0.5–1.5 mK (Ghosh et al., 30 Jul 2025).

Two enclosure designs were tested. Metalon v1.0, a custom insulated acrylic box with active temperature control, achieved only about 4 mK stability at the FPM box. Metalon v2.0, based on an off-the-shelf Dilvac MS222 dewar flask with a redesigned actively controlled lid, improved the short-term thermal stability to 0.8 mK. The spectrograph characterization was performed on EXOhSPEC, a fiber-fed high-resolution echelle covering 460–880 nm at spectral resolution about 82,500; data reduction used HiFLEx, and relative drift measurements were derived with TERRA and SERVAL (Ghosh et al., 30 Jul 2025).

The reported performance is that the metalon provided higher signal-to-noise calibration and better nightly drift measurement relative to ThAr in the wavelength range between 470 nm and 780 nm. In the most stable subset of the Metalon v1.0 run, simultaneous drift standard deviations were 17 m s$Z_0 = 3371~\mu\text{m}$9 and 15 m s$= 1/\text{cm}$0 for ThAr versus 8 m s$= 1/\text{cm}$1 and 8 m s$= 1/\text{cm}$2 for the FPM, using TERRA and SERVAL respectively. For Metalon v2.0, over about 12 hours, the simultaneous drift measurements gave 17 m s$= 1/\text{cm}$3 and 37 m s$= 1/\text{cm}$4 for ThAr, versus 8 m s$= 1/\text{cm}$5 and 8 m s$= 1/\text{cm}$6 for the FPM. If a polynomial is fit to and removed from the FPM drift curve, the residual drift over 4700–7800 Å falls to about 5 m s$= 1/\text{cm}$7 (Ghosh et al., 30 Jul 2025).

These gains are bounded by explicit limitations. The metallic coatings yield lower finesse and lower throughput than optimized dielectric mirrors; the measured average transmission of 24–27% and finesse of only $= 1/\text{cm}$8 are central constraints. The study also identifies alignment sensitivity, fiber-coupling sensitivity, modal noise, contamination between channels in the bifurcated fiber arrangement, and the absence of established long-term aging data. The paper’s conclusion is therefore qualified: the device is feasible and practically useful, especially as a simultaneous drift reference, but long-term monitoring remains necessary (Ghosh et al., 30 Jul 2025).

4. All-metallic metagratings and metasurface “Metalon” platforms

In the metasurface usage, “Metalon” denotes an all-metallic metagrating composed of periodic rectangular grooves in a metallic half-space. The device occupies $= 1/\text{cm}$9, is modeled as a PEC in theory, and the surface $\approx 30$0 is perforated by grooves repeated with periods $\approx 30$1 and $\approx 30$2. Each groove is a short-circuited rectangular waveguide section with center $\approx 30$3, widths $\approx 30$4 and $\approx 30$5, and depth $\approx 30$6. The central synthesis problem is perfect anomalous reflection: redirecting essentially all power from an incident plane wave at angle $\approx 30$7 into a selected reflected Floquet–Bloch order corresponding to $\approx 30$8, while suppressing all other propagating orders (Rabinovich et al., 2020).

The formulation is semianalytical. Above the grating, the fields are expanded in Floquet–Bloch harmonics with

$\approx 30$9

and

$Z_0 = 6743~\mu\text{m}$0

Inside each groove, the fields are expanded in rectangular waveguide eigenmodes with modal propagation constant

$Z_0 = 6743~\mu\text{m}$1

After truncation, the boundary-value problem becomes a finite linear system

$Z_0 = 6743~\mu\text{m}$2

which the paper uses for both analysis and synthesis. Full-wave optimization is not required for the design stage (Rabinovich et al., 2020).

The geometry is tied directly to the target diffraction channels. For a chosen operating frequency and anomalous reflection angle, the period is selected using

$Z_0 = 6743~\mu\text{m}$3

The anomalous reflection efficiencies for a selected order $Z_0 = 6743~\mu\text{m}$4 are written as

$Z_0 = 6743~\mu\text{m}$5

$Z_0 = 6743~\mu\text{m}$6

The paper’s main design insight is that a finite set of groove degrees of freedom can be matched to the number of propagating channels that must be suppressed (Rabinovich et al., 2020).

A distinctive feature of this platform is dual-polarization capability. Because the grooves are finite in both $Z_0 = 6743~\mu\text{m}$7 and $Z_0 = 6743~\mu\text{m}$8, their internal mode spectrum depends on both $Z_0 = 6743~\mu\text{m}$9 and $= 0.5/\text{cm}$0, allowing simultaneous control of TE and TM scattering. This differs from common PCB loaded-wire microwave metagratings, which are predominantly polarization-selective. The same geometry must, however, satisfy TE and TM constraints jointly rather than independently, so dual-polarized working points generally differ from the single-polarization optima (Rabinovich et al., 2020).

The paper also identifies a physical limitation: for TE-polarized anomalous reflection, perfect performance is not achievable for arbitrary wide angles in this all-metallic groove platform. The numerically observed bound follows the local passive lossless anomalous-reflector relation

$= 0.5/\text{cm}$1

The interpretation given is that the structure does not readily excite the transversely guided auxiliary surface-wave channels needed to bypass the local impedance and power-conservation constraint for TE operation at large angles (Rabinovich et al., 2020).

Three CNC-machined aluminum prototypes at 20 GHz were used for validation. The first, a TM two-channel design from $= 0.5/\text{cm}$2 to $= 0.5/\text{cm}$3, used one groove per period with $= 0.5/\text{cm}$4 and $= 0.5/\text{cm}$5, giving 99.9% analytical prediction, 99.6% full-wave simulation, and 98.7% measured peak total efficiency at 20.28 GHz. The second, a TM three-channel design from $= 0.5/\text{cm}$6 to $= 0.5/\text{cm}$7, used two grooves per period and yielded 98% simulated efficiency to the target order, 91.2% with rounded-corner simulation, and 88% measured efficiency at 20 GHz. The third, a dual-polarized design from $= 0.5/\text{cm}$8 to $= 0.5/\text{cm}$9, used one groove per period with $\approx 15$0, $\approx 15$1, and $\approx 15$2, giving 98% simulated TM efficiency, 90% simulated TE efficiency, and measured peak efficiencies of 88% for TM and 81% for TE (Rabinovich et al., 2020).

5. Metallene-related usage: structural stability, physisorption, and interfaces

Where “Metalon” is used loosely for atomically thin metallic membranes, the relevant technical subject is the metallene. A metallene is defined as an atomically thin, nonlayered two-dimensional elemental metal, or more broadly as an atomically thin elemental metallic monolayer realized either freestanding or with external stabilization such as substrates, confinement, or encapsulation (Bagheri et al., 8 Jan 2026). A large part of the recent literature is organized around 45 elemental metals and six monolayer lattice types—planar honeycomb (hc), square (sq), and hexagonal (hex), plus buckled honeycomb (bhc), buckled square (bsq), and buckled hexagonal (bhex)—giving 270 element–lattice systems (Abidi et al., 24 Feb 2025).

A central theme is that most metallenes are not intrinsically flat. In the substrate-focused physisorption study, the unsupported ground state is usually buckled honeycomb (bhc), with Ti, Zr, V, Nb, and Fe as exceptions favoring buckled square (bsq). The total energy of an adsorbed metallene is modeled as

$\approx 15$3

where the adhesion term is derived from an integrated Lennard–Jones potential,

$\approx 15$4

The flattening criterion is summarized by the rule of thumb

$\approx 15$5

Within realistic physisorption, the practically flattenable set is mainly Na, K, Rb, Ag, Au, Cd, with Zn and Tl entering the accessible window under biaxial tensile strain of about $\approx 15$6 (Abidi et al., 27 Jun 2025).

The same paper identifies two external control knobs. Biaxial tensile strain lowers the adhesion strength required to flatten a buckled sheet. A perpendicular electric field does the opposite because the buckled form is slightly more polarizable. The field-dependent total energy is written as

$\approx 15$7

and the threshold field for restoring buckling is

$\approx 15$8

An especially strong structural consequence is that buckling reduces the area of finite metallene patches by almost 50%, so field control is also a morphology-control mechanism (Abidi et al., 27 Jun 2025).

A second stabilization route is pore confinement by graphene. In the interface study, 45 metallenes were combined with graphene edges in four profiles—zz/str, zz/sta, ac/str, and ac/sta—and the central thermodynamic quantity was the interface energy

$\approx 15$9

Across all 45 metals and four profile classes, interface energies at minimal lattice mismatch lay in the range

$\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$0

The dominant result is that smooth interfaces are favored: zz/str is the prototypical smooth profile and usually the most stable, while staggered interfaces are more ragged and less stable. Transition metals, especially early and middle transition metals, show the lowest interface energies overall (Bagheri et al., 8 Jan 2026).

That smooth-interface rule reappears in several observables. Static reconstruction is smaller for zz/str than for zz/sta; armchair interfaces are, on average, about 70% more reconstructed than zigzag interfaces; many zz/sta interfaces become unstable after only a few picoseconds in 300 K ML-based molecular dynamics; metal vacancy formation energies lie in the range 2.1–4.7 eV, while carbon vacancy formation energies lie in the range 7.7–10.9 eV; and all selected interfaces remained stable up to 7% strain with tensile strengths greater than 0.07 eV/Å (Bagheri et al., 8 Jan 2026).

Strictly speaking, these papers describe metallenes rather than a formally established “Metalon” phase. A plausible implication is that when the term is used in this area, it functions as an informal umbrella for atomically thin metallic 2D systems whose stability is governed by buckling energetics, substrate adhesion, and edge/interface engineering.

6. Electronic structure, classification, and unresolved questions

The electronic-structure literature on metallenes extends the structural picture into a periodic-table-scale classification. One catalog of 270 metallenes emphasizes coordination as the dominant organizing variable. The fixed coordination numbers are $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$1, $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$2, $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$3, $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$4, $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$5, and $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$6. The robust energetic trend is that $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$7 is usually the least stable and $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$8 is usually the most stable, because buckling increases coordination most strongly in the $\delta = \left(\frac{2\pi}{\lambda}\right) 2 n Z,$9 transformation. The same study also states that simple metals and noble metals lack a distinct buckled hexagonal lattice: for those families, $\sim 2.5$00 and $\sim 2.5$01 relax to the same structure (Abidi et al., 24 Feb 2025).

That coordination-centered view is complemented by a fermiology classification of the same 45 elements across the same six monolayer lattice families. The paper’s main result is that lattice type primarily fixes the gross Fermi-line geometry—its shape and radial placement within the first Brillouin zone—while buckling acts as a controlled perturbation that shortens long straight Fermi-line segments, opens mini-gaps, and can create, remove, split, merge, or rearrange small pockets. The orbital family then determines whether the states at $\sim 2.5$02 are mainly $\sim 2.5$03-, $\sim 2.5$04-, or $\sim 2.5$05-dominated (Abidi et al., 20 Feb 2026).

The study compresses this picture into four descriptors—crossing density $\sim 2.5$06, flatness $\sim 2.5$07, Fermi-line anisotropy $\sim 2.5$08, and $\sim 2.5$09-centricity $\sim 2.5$10—and then into a composite element-level score called pocketness $\sim 2.5$11. The main text does not print the explicit algebraic formula for $\sim 2.5$12, stating only that it is a weighted combination of the four oriented descriptors. High $\sim 2.5$13 corresponds to compact, round, $\sim 2.5$14-centered pockets with fewer crossings; low $\sim 2.5$15 corresponds to frequent crossings and elongated anisotropic contours near the Brillouin-zone boundary (Abidi et al., 20 Feb 2026).

Several family-level trends are explicit. Among simple fermiologies, alkali metallenes are archetypal low-complexity systems with simple $\sim 2.5$16-dominated loops. The late transition metals are the most intricate: for example, $\sim 2.5$17 reaches $\sim 2.5$18 with $\sim 2.5$19, and $\sim 2.5$20 reaches $\sim 2.5$21 with $\sim 2.5$22. Hg is a special outlier: HSE predicts it to be gapped in all six lattices, with band gaps of 1.55 eV (bsq), 1.70 eV (bhc), 1.97 eV (sq), 2.47 eV (bhex), 2.49 eV (hex), and 3.55 eV (hc) (Abidi et al., 20 Feb 2026).

For application-oriented screening, the implications are concrete. High-$\sim 2.5$23 systems are proposed as candidates for clean ARPES, controlled Lifshitz transitions, and more isotropic transport. Hexagonal families with higher $\sim 2.5$24 are suggested for anisotropic responses and larger angle-dependent magnetoresistance. Earlier structural papers connect the same metallene families to catalysis, sensing, plasmonics, nanoelectronics, and energy conversion/storage (Abidi et al., 20 Feb 2026).

The unresolved issues are equally explicit. The electronic classification is a freestanding or idealized baseline rather than a complete description of supported samples; real substrates, moiré potentials, disorder, and many-body corrections such as GW/DMFT can shift quantitative values. Explicit SOC-resolved spin textures are not presented in the fermiology study. On the structural side, physisorption models are intended for weak adsorption rather than chemisorption, and the graphene-interface study does not address growth kinetics, ambient chemistry, or long timescales. In the strict terminological sense, these open questions belong to metallenes rather than to a universally recognized “Metalon” material class (Abidi et al., 20 Feb 2026).

Taken together, the supplied literature establishes three technically distinct meanings of Metalon. In spectroscopy it is a broadband metallic-mirror Fabry–Pérot etalon; in electromagnetic metasurface design it is an all-metallic grooved metagrating; and in loose condensed-matter usage it points toward the metallene family of atomically thin elemental metals. The term’s scientific content therefore depends entirely on disciplinary context.