Lens-Coupled Spiral Absorbers

• The paper demonstrates a novel detector architecture where a lens-coupled spiral absorber achieves >70% aperture efficiency over broad bandwidth.
• The design integrates Ti/Al bi-layer MKIDs with both a single double spiral and a 4×4 spiral array to optimize dual-polarization and scalability.
• Simulations and laboratory tests at ~85 GHz confirm efficient optical concentration, low noise, and high detector yield for large-format applications.

Searching arXiv for the specified papers and closely related work on lens-coupled spiral absorbers. Lens-coupled spiral absorbers are quasi-optical coupling structures in which millimeter-wavelength radiation is focused by a silicon lens onto a superconducting spiral absorber that simultaneously functions as the inductor of a Microwave Kinetic Inductance Detector (MKID). In the Ti/Al bi-layer implementation reported for millimeter-wave detection, the absorber is fabricated on a silicon slab backed by a groundplane, and the lens-coupled geometry is used to realize dual-polarized response with high simulated aperture efficiency, broad optical bandwidth, and compatibility with large-format arrays (Laguna et al., 3 Oct 2025). In this context, the term denotes a detector architecture rather than a generic spiral metamaterial: the spiral is an absorptive superconducting element integrated into a lumped-element resonator, and the lens provides the quasi-optical interface between free-space radiation and the absorbing structure.

1. Definition and architectural setting

The defining feature of the architecture is the use of a lens-coupled spiral absorber as the quasi-optical coupling mechanism for millimeter-wavelength radiation detection. In the reported implementation, Ti/Al bi-layer MKIDs are combined with an absorber placed at the lower focus of an extended hemispherical Si lens designed to focus millimeter-wave radiation onto the absorber (Laguna et al., 3 Oct 2025). The absorber is therefore not an auxiliary front-end component but part of the superconducting resonator itself.

This detector class sits within the broader family of lens absorber coupled KIDs and MKIDs. In far-infrared work, lens absorber coupled MKIDs are described as more robust to misalignment and assembly issues at THz frequencies due to their incoherent detection mechanism while requiring a less complex fabrication process (Dabironezare et al., 2023). A closely related far-infrared focal-plane concept similarly uses a focusing lens to concentrate radiation onto a planar, distributed absorber coupled directly to a KID resonator for readout (Dabironezare et al., 3 Jun 2025). These related studies place the spiral-based millimeter-wave design in a wider detector lineage in which the lens defines the optical throughput and the absorber geometry determines polarization response, bandwidth, and fabrication tolerance.

A plausible implication is that the spiral implementation extends the lens-absorber concept into a dual-polarized millimeter-wave regime by using absorber topology, rather than antenna phasing, to control quasi-optical coupling. That interpretation is consistent with the reported emphasis on dual-polarization capability, simple fabrication, and scalability (Laguna et al., 3 Oct 2025).

2. Spiral absorber geometries and superconducting implementation

Two absorber geometries were developed. The first is a single double spiral, described as a square unit cell with a double spiral inductor; each spiral has $3\,\mu\textrm{m}$-wide arms, spiraling inwards and outwards, connected in series. It is used as the inductor in a lumped-element resonator, paired with an interdigitated capacitor. The second is a $4 \times 4$ spiral array, an array of these double spirals with rows connected in parallel to avoid self-resonance; it is used to increase optical bandwidth and coupling efficiency (Laguna et al., 3 Oct 2025).

The absorbers are fabricated on a silicon slab backed by a groundplane that serves both as a millimeter-wave quarter-wavelength back-reflector and as the groundplane for the MKID’s microstrip readout line. The superconducting absorber is a bi-layer of Titanium and Aluminum, $10~\textrm{nm}$ Ti + $20~\textrm{nm}$ Al, patterned via laser lithography and wet etching. Reported electrical properties are a sheet resistance $R_s\approx 1~\Omega/\square$, critical temperature $T_c \approx 800~\textrm{mK}$, kinetic inductance $\sim 1.9~\textrm{pH}/\square$, and gap frequency $\sim 59~\textrm{GHz}$, which sets the low-frequency cutoff of detection. The back reflector is $200~\textrm{nm}$ aluminum deposited on the backside of the SI wafer (Laguna et al., 3 Oct 2025).

These details place the spiral absorber in the category of absorber-coupled superconducting resonators rather than wire-grid or antenna-fed detectors. The use of rows connected in parallel to avoid self-resonance is especially significant for the spiral-array geometry because the paper directly associates that topology with increased optical bandwidth (Laguna et al., 3 Oct 2025).

Configuration	Geometry	Reported role
Single double spiral	Square unit cell; double spiral inductor; $3\,\mu\textrm{m}$-wide arms; series connection	Used as the inductor in a lumped-element resonator
$4 \times 4$0 spiral array	Array of double spirals; rows connected in parallel	Used to increase optical bandwidth and coupling efficiency

3. Lens coupling, optical interface, and dual-polarized operation

The absorber is placed at the lower focus of an extended hemispherical Si lens. Two lens configurations are reported. The single spiral is coupled with an $4 \times 4$1 lens of aperture diameter $4 \times 4$2, chosen to maximize collection area while preventing overfilling. The spiral array uses a slower $4 \times 4$3 lens to match the larger physical area of the $4 \times 4$4 absorber array. The top of the lens is patterned with vertical frustra, described as non-conformal AR structures, to provide broadband antireflection compatible with cryogenic operation and manufactured using laser ablation (Laguna et al., 3 Oct 2025).

The paper defines aperture efficiency as

$4 \times 4$5

and uses that quantity to evaluate the quasi-optical performance of the lens-absorber system (Laguna et al., 3 Oct 2025).

A central property of the design is dual-polarized response. The symmetry of the spiral designs yields nearly identical aperture efficiencies and beam patterns for orthogonal linear polarizations, which the paper identifies as essential for dual-polarized detection (Laguna et al., 3 Oct 2025). The abstract likewise states that the lens-coupled absorbers provide a 70% lens aperture efficiency in both polarizations over an octave band with a spiral array absorber and over 10% relative bandwidth with a single spiral (Laguna et al., 3 Oct 2025).

This lens-mediated coupling strategy aligns with the broader lens-absorber literature. Related far-infrared studies describe lens absorber coupled detectors as using a lens to focus radiation tightly onto an absorber structure, thereby enhancing absorption efficiency and spatial selectivity across the pixel array (Dabironezare et al., 2023). They also emphasize that incoherent absorber coupling is less sensitive to small misalignments than antenna-coupled systems (Dabironezare et al., 2023). This suggests that the spiral absorber’s lens interface is intended not only for optical concentration but also for packaging and assembly robustness.

4. Simulated quasi-optical response and measured detector performance

Simulations report high aperture efficiency for both absorber types. For the single spiral, the aperture efficiency is >70% within a 10% relative bandwidth, centered around the design frequency of approximately $4 \times 4$6. For the $4 \times 4$7 spiral array, the aperture efficiency is >70% over an octave bandwidth, that is, a factor of two in frequency. Both polarizations exhibit similar performance (Laguna et al., 3 Oct 2025).

The AR “frustra” are reported to improve lens transmission substantially across the band. The paper’s sample simulation data define efficiency as the ratio of absorbed power in the MKID to total power entering the lens aperture (Laguna et al., 3 Oct 2025). This establishes the reported efficiencies as system-level optical coupling figures for the coupled lens-absorber assembly rather than isolated absorber cross-sections.

Laboratory measurements at $4 \times 4$8 report a frequency responsivity

$4 \times 4$9

and a Noise Equivalent Temperature

$10~\textrm{nm}$0

with

$10~\textrm{nm}$1

The measured bandwidth is reported as approximately 10% for the single-spiral design and octave for the spiral-array design, confirming broadband performance as predicted. Both detector types show a significant optical response at $10~\textrm{nm}$2 (Laguna et al., 3 Oct 2025).

The principal reported performance figures are summarized below.

Quantity	Single spiral	$10~\textrm{nm}$3 spiral array
Aperture efficiency	$10~\textrm{nm}$4 within a $10~\textrm{nm}$5 relative bandwidth	$10~\textrm{nm}$6 over an octave bandwidth
Polarization response	Dual-polarized	Dual-polarized
Optical response	Significant at $10~\textrm{nm}$7	Significant at $10~\textrm{nm}$8

The paper states that the measured sensitivity is competitive with state-of-the-art MKID cameras such as NIKA (Laguna et al., 3 Oct 2025). No additional quantitative comparison is given in the provided material, so the significance of that comparison remains limited to the stated qualitative benchmark.

5. Array scaling, multiplexing, and system-level considerations

Two physical implementations were fabricated and measured. One is a $10~\textrm{nm}$9 chip with 9 pixels, used to characterize the optical response at $20~\textrm{nm}$0 of the two absorber variations. The other is a large-format demonstrator with 253 spiral-array pixels, intended to show potential toward a large-format millimeter-wavelength camera (Laguna et al., 3 Oct 2025).

For the 253-pixel demonstrator, 241 resonators were observed out of 253, corresponding to approximately 95% detector yield. Of these, 202 were considered usable because they were spectrally isolated and not severely affected by cross-talk. Reported uniformity metrics are an average frequency deviation of 5% between design and measured resonance frequencies, an average internal quality factor $20~\textrm{nm}$1, and an average coupling quality factor $20~\textrm{nm}$2. The array uses a spectral shuffling scheme for multiplexed readout to minimize cross-talk (Laguna et al., 3 Oct 2025).

The paper explicitly attributes the approach’s promise for large-format, dual-polarized, broadband MKID millimeter-wave cameras to the arrayed spiral absorber geometry, simple fabrication, and robust yield. It also states that absorber-coupled, as opposed to antenna-coupled, MKIDs are fabrication-tolerant and thus well suited for large arrays (Laguna et al., 3 Oct 2025).

Large-array behavior must also be interpreted in light of stray-radiation effects known in superconducting focal planes. In large area imaging arrays, stray radiation trapped and propagated via surface or substrate-guided modes can create a low-level, spatially broad pedestal response. For lens-coupled MKID arrays, a mesh-patterned on-chip absorber has been shown to suppress the pedestal response by more than $20~\textrm{nm}$3 with minimal impact on main-beam coupling (Yates et al., 2018). A plausible implication is that future lens-coupled spiral-absorber cameras may require similar system-level stray-light mitigation as array size increases.

6. Relation to other spiral absorbers and neighboring research lines

The phrase “spiral absorber” spans several distinct research programs, and the detector architecture described here should not be conflated with all of them. In the present millimeter-wave MKID context, the spiral is a superconducting absorber integrated into a lumped-element resonator and illuminated quasi-optically by a lens (Laguna et al., 3 Oct 2025).

A different line of work studies coupled spiral meta-atom resonators on dielectric waveguides in non-Hermitian photonics. There, exceptional points arise from unidirectional coupling of chiral dipole modes, and higher-order exceptional points broaden an extinction plateau with near-zero transmission of guided light (Zhang et al., 2024). That work concerns guided-light attenuation by scattering and absorption in waveguide-coupled resonators, not lens-focused superconducting photon detection. The overlap is therefore morphological and conceptual—spiral resonators used for absorption or extinction—rather than architectural.

Another neighboring literature concerns logarithmic spiral metasurfaces backed by a metallic surface. In that setting, scale-invariant logarithmic spiral resonators combined with Fabry-Perot-type resonances and vortex energy flow are reported to absorb more than 95% of incident microwave energy from 6 GHz to 37 GHz (Wang et al., 2018). Yet another spiral-based absorber class uses double-layered metallic spiral arrays engineered for a mu-near-zero response, achieving absorption efficiency above 90% at illumination angles up to 60 degrees in an ultrathin microwave absorber (Zhong et al., 2012). These metamaterial absorbers share broad themes of spiral-mediated absorption, metallic backing, and polarization engineering, but they are not MKID readout structures.

The detector literature on lens absorber coupled MKIDs provides the closest architectural context. For far-infrared imaging spectroscopy, lens absorber coupled MKIDs are described as candidate highly sensitive large-format detector arrays, with the performance of prototypes at 6.98 and 12 THz under investigation (Dabironezare et al., 2023). A later lens-based KID study reports that the coupling of lens absorber prototypes to an incident plane wave, that is, aperture efficiency, is indirectly validated experimentally matching the expected value of $20~\textrm{nm}$4 averaged over two polarisation, and reports a limiting noise equivalent power of $20~\textrm{nm}$5 (Dabironezare et al., 3 Jun 2025). This suggests that lens-coupled spiral absorbers belong to a broader design trajectory in which distributed absorbers, dual polarization, and lens-mediated optical concentration are being developed across the millimeter to far-infrared range.

7. Significance, limitations, and prospective development

The reported millimeter-wave results establish three core attributes of lens-coupled spiral absorbers in Ti/Al MKIDs: dual-polarization, broad bandwidth, and scalability. The spiral-array geometry delivers simulated $20~\textrm{nm}$6 aperture efficiency over an octave bandwidth in both polarizations, while the single spiral retains $20~\textrm{nm}$7 efficiency over a narrower $20~\textrm{nm}$8 relative bandwidth (Laguna et al., 3 Oct 2025). The 253-pixel demonstrator, with approximately 95% detector yield, shows that the concept is not restricted to isolated test structures (Laguna et al., 3 Oct 2025).

The limitations stated in the source are practical rather than conceptual. The paper identifies next steps as integration with lens arrays and measurement of absolute NEP and optical efficiency; it also states that improvements are possible by further reducing noise and optimizing configurations (Laguna et al., 3 Oct 2025). These directions are consistent with the broader lens-absorber KID literature, where optical coupling to incoherent distributed sources, aperture efficiency, and noise equivalent power are treated as principal figures of merit (Dabironezare et al., 3 Jun 2025).

A common misconception is that spiral geometry alone guarantees broadband or polarization-independent performance. The literature indicates otherwise. In the present MKID work, bandwidth and dual-polarization performance depend on the specific absorber topology, the lens $20~\textrm{nm}$9, the silicon optical interface, and the AR treatment (Laguna et al., 3 Oct 2025). In other spiral-resonator literatures, broadband absorption can derive from scale invariance and vortex energy flow (Wang et al., 2018), or from higher-order exceptional points enabled by unidirectional coupling (Zhang et al., 2024). The shared spiral motif does not imply a shared physical mechanism.

For detector engineering, the most conservative conclusion is that lens-coupled spiral absorbers constitute a specific absorber-coupled MKID architecture in which the optical, superconducting, and array-level properties have been jointly optimized. Within the scope of the reported work, they provide a route toward large-format millimeter-wavelength cameras with dual-polarized response, octave-class bandwidth in the arrayed geometry, and high detector yield (Laguna et al., 3 Oct 2025).