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.

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.

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

Antimony (Sb) heavily-doped germanium (Ge)-on-silicon (Si) epitaxial films are investigated as mid-infrared (MIR) plasmonic materials. Structural, electrical, and optical properties have been improved by proper choice of dopant species (i.e., Sb) and optimization of the growth parameters (i.e., Sb flux and substrate temperature). The increased electron conductivity can be attributed to the elevated carrier concentration (1.5 × 1020 cm-3) and carrier mobility (224 cm2 V-1 s-1) in the Sb-doped Ge epilayers. The measured MIR reflectivities of the Sb-doped Ge films show free-carrier-dependent properties, which leads to tunable real and imaginary parts of permittivities. Localized surface plasmon polaritons of the bowtie antennas fabricated from the Sb-doped Ge films are demonstrated. The fabricated antennas can provide signal enhancement for the molecular vibrational spectroscopy when these vibrational lines are spectrally in proximity to the localized plasmon resonance. These CMOS-compatible Sb-doped Ge epilayers offer a platform to study the interaction of MIR plasmon with nanostructures on chips.

ITO-TiN-ITO Sandwiches for Near-Infrared Plasmonic Materials.

Indium tin oxide (ITO)-based sandwich structures with the insertion of ultrathin (<10 nm) titanium nitride (TiN) are investigated as near-infrared (NIR) plasmonic materials. The structural, electrical, and optical properties reveal the improvement of the sandwich structures stemmed from TiN insertion. TiN is a well-established alternative to noble metals such as gold, elevating the electron conductivity of sandwich structures as its thickness increases. Dielectric permittivities of TiN and top ITO layers show TiN-thickness-dependent properties, which lead to moderate and tunable effective permittivities for the sandwiches. The surface plasmon polaritons (SPP) of the ITO-TiN-ITO sandwich at the telecommunication window (1480-1570 nm) are activated by prism coupling using Kretschmann configuration. Compared with pure ITO films or sandwiches with metal insertion, the reflectivity dip for sandwiches with TiN is relatively deeper and wider, indicating the enhanced coupling ability in plasmonic materials for telecommunications. The SPP spatial profile, penetration depth, and degree of confinement, as well as the quality factors, demonstrate the applicability of such sandwiches for NIR plasmonic materials in various devices.

Optical constants acquisition and phase change properties of Ge2Sb2Te5 thin films based on spectroscopy.

In this work, phase change chalcogenide Ge2Sb2Te5 (GST) thin films were fabricated by magnetron sputtering. The optical properties, especially the optical constants (refractive index and extinction coefficient), of such alloys were systematically studied by investigating their thermally and photo-thermally induced switching between different phases. The results show that GST films are highly tunable in microstructure and optical constants, either by post-annealing at 160 °C, 200 °C, 250 °C and 350 °C, respectively, or by laser irradiation of 1 mW, 3 mW, 5 mW and 10 mW power with beam diameter of 7 µm at 532 nm, respectively. From the structural analysis, we can clearly observe different crystallinities and chemical bonding in the different post-treated GST films. The optical constants of GST films under various phases were obtained from spectrophotometry, by fitting their transmittance data with the Tauc-Lorentz (TL) dispersion model. The refractive index and extinction coefficient exhibit notable change upon annealing and laser irradiation, specifically at 1550 nm, from 3.85 (amorphous) to 6.5 (crystalline) in refractive index. The optical constants have been proved capable of fine tuning via the laser irradiation method. Hence, the pronounced adjustability in optical properties due to rapid and repeatable phase change render GST suitable for tunable photonic devices.

SiC Nanowire Film Photodetectors: A Promising Candidate Toward High Temperature Photodetectors.

In this study, UV photodetectors (PDs) based on SiC nanowire films have been successfully prepared by a simple and low-cost drip-coating method followed by sintering at 500 °C. The corresponding electrical characterizations clearly demonstrate that the SiC nanowire based PD devices can be regarded as a promising candidate for UV PDs. The PDs can exhibit the excellent performances of fast, high sensitivity, linearity, and stable response, which can thus achieve on-line monitoring of weak UV light. Furthermore, the SiC nanowire-based PDs enable us to fabricate detectors working under high temperature as high as 150 °C. The high photosensitivity and rapid photoresponse for the PDs can be attributed to the superior single crystalline quality of SiC nanowires and the ohmic contact between the electrodes and nanowires.