CMOS-Compatible Antimony-Doped Germanium Epilayers for Mid-Infrared Low-Loss High-Plasma-Frequency Plasmonics

Phonon-coupled perfect absorption in photonic-plasmonic hybrid metasurfaces

The simultaneous spectral and spatial confinement of optical modes, governed by high-Q resonances and subwavelength field localization, is critical for optimizing light-matter interactions. However, the coupling mechanisms in non-local metasurfaces, particularly those involving quasi-bound states in the continuum (QBIC) and van der Waals (vdW) materials in vibrational strong coupling (VSC), remain unclear. To address this, we propose a photonic-plasmonic hybrid metasurface supporting photonic QBIC, localized surface plasmon resonances (LSPRs), and plasmonic QBIC mode. At a grating duty cycle of R = 0.3, the structure supports LSPRs under normal incidence, while oblique excitation simultaneously activates photonic QBIC and LSPR modes. Although the Q-factor of photonic QBICs decreases under tilted incidence, their electromagnetic fields redistribute toward the interface, emulating the spatial confinement of LSPRs. This interfacial localization, combined with the high Q-factor of photonic QBICs, enhances spectral and spatial overlap with hexagonal boron nitride (h-BN) phonon-polaritons, enabling robust VSC with coupling strengths surpassing those of LSPRs. Increasing the duty cycle to R = 0.7 transitions the system to plasmonic QBIC modes, which exhibit unconventional asymptotic degeneracy upon coupling with phonons, revealing distinct polaritonic interaction dynamics. Furthermore, angle-tunable QBIC modes achieve near-unity absorption under critical coupling conditions, facilitating dual applications as mid-infrared thermal radiation sources and surface-enhanced infrared spectroscopy platforms for biosensing.

Exploring the resonance absorption of subwavelength-patterned epitaxial-grown group-IV semiconductor composite structures

We experimentally and theoretically demonstrate a mid-infrared perfect absorber with all group-IV epitaxial layered composite structures. The multispectral narrowband strong absorption (>98%) is attributed to the combined effects of the asymmetric Fabry-Perot (FP) interference and the plasmonic resonance in the subwavelength-patterned metal-dielectric-metal (MDM) stack. The spectral position and intensity of the absorption resonance were analyzed by reflection and transmission. While a localized plasmon resonance in the dual-metal region was found to be modulated by both the horizontal (ribbon width) and vertical (spacer layer thickness) profile, the asymmetric FP modes were modulated merely by the vertical geometric parameters. Semi-empirical calculations show strong coupling between modes with a large Rabi-splitting energy reaching 46% of the mean energy of the plasmonic mode under proper horizontal profile. A wavelength-adjustable all-group-IV-semiconductor plasmonic perfect absorber has potential for photonic-electronic integration.

n-type Ge/Si antennas for THz sensing

Ge-on-Si plasmonics holds the promise for compact and low-cost solutions in the manipulation of THz radiation. We discuss here the plasmonic properties of doped Ge bow-tie antennas made with a low-point cost CMOS mainstream technology. These antennas display resonances between 500 and 700 GHz, probed by THz time domain spectroscopy. We show surface functionalization of the antennas with a thin layer of α-lipoic acid that red-shifts the antenna resonances by about 20 GHz. Moreover, we show that antennas protected with a silicon nitride cap layer exhibit a comparable red-shift when covered with the biolayer. This suggests that the electromagnetic fields at the hotspot extend well beyond the cap layer, enabling the possibility to use the antennas with an improved protection of the plasmonic material in conjunction with microfluidics.

Study on epsilon crossover wavelength tuning of heavily doped germanium-on-silicon in mid-infrared range

In this paper, we demonstrate that n-type heavily doped germanium (Ge) can serve as a sort of CMOS-compatible, permittivity crossover wavelength (at which the real part of permittivity changes sign) wide range adjustable epsilon-zero material in mid-infrared (MIR). The antimony (Sb) doped Ge films with high doping concentrations have been highly crystalline grown on silicon substrates with the molecular beam epitaxy (MBE) process. Our results reveal that the crossover wavelength of doped germanium is highly tunable by adjusting the carrier concentration and crystallinity of the films simultaneously. By optimizing dopant flux and substrate temperature, the maximum carrier concentration can be achieved as high as 1.6×10²⁰ cm^-3, resulting in a very short crossover wavelength of 4.31 µm, which is very difficult to realize in group IV semiconductors. The heavily doping process also enables it possible to observe the room temperature photoluminescence (PL) from direct band transition of germanium films.