High-performance Ge-on-insulator lateral p-i-n waveguide photodetectors for electronic-photonic integrated circuits at telecommunication wavelengths.

We report high-performance germanium-on-insulator (GeOI) waveguide photodetectors (WGPDs) for electronic-photonic integrated circuits (EPICs) operating at telecommunication wavelengths. The GeOI samples were fabricated using layer transfer and wafer-bonding techniques, and a high-quality Ge active layer was achieved. Planar lateral p-i-n WGPDs were fabricated and characterized, and they exhibited a low dark current of 0.1 µA. Strain-induced alterations in the optical properties were observed, resulting in an extended photodetection range up to λ = 1638 nm. This range encompasses crucial telecommunication bands. The WGPDs exhibited a high responsivity of 0.56 A/W and a high detectivity of D ∗ = 1.87 ×109cmHz1/2W - 1 at 1550 nm. A frequency-response analysis revealed that increasing the bias voltage from -1 to -9 V enhances the 3-dB bandwidth from 31 to 49 MHz. This study offers a comprehensive understanding of GeOI WGPDs, fostering high-performance EPICs with implications for telecommunications and beyond.

Theoretical Analysis of GeSn Quantum Dots for Photodetection Applications.

GeSn alloys have recently emerged as complementary metal-oxide-semiconductor (CMOS)-compatible materials for optoelectronic applications. Although various photonic devices based on GeSn thin films have been developed, low-dimensional GeSn quantum structures with improved efficiencies hold great promise for optoelectronic applications. This study theoretically analyses Ge-capped GeSn pyramid quantum dots (QDs) on Ge substrates to explore their potential for such applications. Theoretical models are presented to calculate the effects of the Sn content and the sizes of the GeSn QDs on the strain distributions caused by lattice mismatch, the band structures, transition energies, wavefunctions of confined electrons and holes, and transition probabilities. The bandgap energies of the GeSn QDs decrease with the increasing Sn content, leading to higher band offsets and improved carrier confinement, in addition to electron-hole wavefunction overlap. The GeSn QDs on the Ge substrate provide crucial type-I alignment, but with a limited band offset, thereby decreasing carrier confinement. However, the GeSn QDs on the Ge substrate show a direct bandgap at higher Sn compositions and exhibit a ground-state transition energy of ~0.8 eV, rendering this system suitable for applications in the telecommunication window (1550 nm). These results provide important insights into the practical feasibility of GeSn QD systems for optoelectronic applications.

Ultrasensitive and Rapid Detection of Procalcitonin via Waveguide-Enhanced Nanogold-Linked Immunosorbent Assay for Early Sepsis Diagnosis.

Sepsis, a life-threatening inflammatory response, demands economical, accurate, and rapid detection of biomarkers during the critical "golden hour" to reduce the patient mortality rate. Here, we demonstrate a cost-effective waveguide-enhanced nanogold-linked immunosorbent assay (WENLISA) based on nanoplasmonic waveguide biosensors for the rapid and sensitive detection of procalcitonin (PCT), a sepsis-related inflammatory biomarker. To enhance the limit of detection (LOD), we employed sandwich assays using immobilized capture antibodies and detection antibodies conjugated to gold nanoparticles to bind the target analyte, leading to a significant evanescent wave redistribution and strong nanoplasmonic absorption near the waveguide surface. Experimentally, we detected PCT for a wide linear response range of 0.1 pg/mL to 1 ng/mL with a record-low LOD of 48.7 fg/mL (3.74 fM) in 8 min. Furthermore, WENLISA has successfully identified PCT levels in the blood plasma of patients with sepsis and healthy individuals, offering a promising technology for early sepsis diagnosis.

"GeSn Rule-23"-The Performance Limit of GeSn Infrared Photodiodes.

Group-IV GeSn photodetectors (PDs) compatible with standard complementary metal-oxide-semiconductor (CMOS) processing have emerged as a new and non-toxic infrared detection technology to enable a wide range of infrared applications. The performance of GeSn PDs is highly dependent on the Sn composition and operation temperature. Here, we develop theoretical models to establish a simple rule of thumb, namely "GeSn-rule 23", to describe GeSn PDs' dark current density in terms of operation temperature, cutoff wavelength, and Sn composition. In addition, analysis of GeSn PDs' performance shows that the responsivity, detectivity, and bandwidth are highly dependent on operation temperature. This rule provides a simple and convenient indicator for device developers to estimate the device performance at various conditions for practical applications.

Dark Current Analysis on GeSn p-i-n Photodetectors.

Group IV alloys of GeSn have been extensively investigated as a competing material alternative in shortwave-to-mid-infrared photodetectors (PDs). The relatively large defect densities present in GeSn alloys are the major challenge in developing practical devices, owing to the low-temperature growth and lattice mismatch with Si or Ge substrates. In this paper, we comprehensively analyze the impact of defects on the performance of GeSn p-i-n homojunction PDs. We first present our theoretical models to calculate various contributing components of the dark current, including minority carrier diffusion in p- and n-regions, carrier generation-recombination in the active intrinsic region, and the tunneling effect. We then analyze the effect of defect density in the GeSn active region on carrier mobilities, scattering times, and the dark current. A higher defect density increases the dark current, resulting in a reduction in the detectivity of GeSn p-i-n PDs. In addition, at low Sn concentrations, defect-related dark current density is dominant, while the generation dark current becomes dominant at a higher Sn content. These results point to the importance of minimizing defect densities in the GeSn material growth and device processing, particularly for higher Sn compositions necessary to expand the cutoff wavelength to mid- and long-wave infrared regime. Moreover, a comparative study indicates that further improvement of the material quality and optimization of device structure reduces the dark current and thereby increases the detectivity. This study provides more realistic expectations and guidelines for evaluating GeSn p-i-n PDs as a competitor to the III-V- and II-VI-based infrared PDs currently on the commercial market.

Investigation of the cap layer for improved GeSn multiple quantum well laser performance.

The study of all-group-IV SiGeSn lasers has opened a new avenue to Si-based light sources. SiGeSn heterostructure and quantum well lasers have been successfully demonstrated in the past few years. It has been reported that, for multiple quantum well lasers, the optical confinement factor plays an important role in the net modal gain. In previous studies, adding a cap layer was proposed to increase the optical mode overlap with the active region and thereby improve the optical confinement factor of Fabry-Perot cavity lasers. In this work, SiGeSn/GeSn multiple quantum well (4-well) devices with various cap layer thicknesses, i.e., 0 (no cap), 190, 250, and 290 nm, are grown using a chemical vapor deposition reactor and characterized via optical pumping. While no-cap and thinner-cap devices only show spontaneous emission, the two thicker-cap devices exhibit lasing up to 77 K, with an emission peak at 2440 nm and a threshold of 214 kW/cm2 (250 nm cap device). The clear trend in device performance disclosed in this work provides guidance in device design for electrically injected SiGeSn quantum well lasers.

Autonomous Microlasers for Profiling Extracellular Vesicles from Cancer Spheroids.

Self-propelled micro/nanomotors are emergent intelligent sensors for analyzing extracellular biomarkers in circulating biological fluids. Conventional luminescent motors are often masked by a highly dynamic and scattered environment, creating challenges to characterize biomarkers or subtle binding dynamics. Here we introduce a strategy to amplify subtle signals by coupling strong light-matter interactions on micromotors. A smart whispering-gallery-mode microlaser that can self-propel and analyze extracellular biomarkers is demonstrated through a liquid crystal microdroplet. Lasing spectral responses induced by cavity energy transfer were employed to reflect the abundance of protein biomarkers, generating exclusive molecular labels for cellular profiling of exosomes derived from 3D multicellular cancer spheroids. Finally, a microfluidic biosystem with different tumor-derived exosomes was employed to elaborate its sensing capability in complex environments. The proposed autonomous microlaser exhibits a promising method for both fundamental biological science and applications in drug screening, phenotyping, and organ-on-chip applications.

Transfer-printing-enabled GeSn flexible resonant-cavity-enhanced photodetectors with strain-amplified mid-infrared optical responses.

Mid-infrared (MIR) flexible photodetectors (FPDs) constitute an essential element for wearable applications, including health-care monitoring and biomedical detection. Compared with organic materials, inorganic semiconductors are promising candidates for FPDs owing to their superior performance as well as optoelectronic properties. Herein, for the first time, we present the use of transfer-printing techniques to enable a cost-effective, nontoxic GeSn MIR resonant-cavity-enhanced FPDs (RCE-FPDs) with strain-amplified optical responses. A narrow bandgap nontoxic GeSn nanomembrane was employed as the active layer, which was grown on a silicon-on-insulator substrate and then transfer-printed onto a polyethylene terephthalate (PET) substrate, eliminating the unwanted defects and residual compressive strain, to yield the MIR RCE-FPDs. In addition, a vertical cavity was created for the GeSn active layer to enhance the optical responsivity. Under bending conditions, significant tensile strain up to 0.274% was introduced into the GeSn active layer to effectively modulate the band structure, extend the photodetection in the MIR region, and substantially enhance the optical responsivity to 0.292 A W-1 at λ = 1770 nm, corresponding to an enhancement of 323% compared with the device under flat conditions. Moreover, theoretical simulations were performed to confirm the strain effect on the device performance. The results demonstrated high-performance, nontoxic MIR RCE-FPDs for applications in flexible photodetection.

Longwave IR lattice matched L-valley Ge/GeSiSn waveguide quantum cascade detector.

We propose a lattice-matched Ge/GeSiSn quantum cascade detector (QCD) capable of operating in the longwave infrared. The optical absorption and carrier transport based on intersubband transitions all occur within the L-valley of the conduction band of the group-IV material system using N-doped quantum wells (QWs). The waveguided lattice matched structure can be deposited strain free on top of a Ge buffer grown on Si substrate, and is end-coupled to low-loss on-chip Ge waveguides. We optimized the QCD structure through the analysis of the photoresponsivity and detectivity D*. The QCD operates in photovoltaic mode with narrow spectral response that is peaked anywhere in the 9 to 16 µm range, tunable by design. This work aims to push the optical response of the photodetectors made from the SiGeSn material system to longer wavelengths. The study suggests the QCD response can indeed significantly extend the spectral range beyond that of the photodiodes and photoconductors made from the same group-IV system for a wide variety of applications in imaging, sensing, lidar, and space-and-fiber communications.

Slab waveguide-based particle plasmon resonance optofluidic biosensor for rapid and label-free detection.

An effective bio-sensing platform that would meet the criteria of rapid, simple, and sensitive detection is crucial to translate bench research to clinical applications. However, simultaneously rapid and sensitive biosensing remains challenging for practical biomedical applications. In this study, for the first time, we demonstrate a cost-effective, label-free, real-time, and sensitive slab waveguide-based particle plasmon resonance (WGPPR) biosensor for practical clinical applications. A suspended glass slab waveguide structure with excellent optical confinement properties was designed and fabricated as the biosensor. Gold nanoparticles (AuNPs) were deposited on the top surface of the waveguide layer to significantly enhance the optical near field through the localized surface plasmon resonance (LSPR) effect. When light travels through the waveguide, the change in the local refractive index (RI) near the surface of the AuNPs can be transformed into changes in the intensity of transmitted light, thereby enabling sensitive and real-time detection. The RI sensing experiment shows a good sensor resolution of 1.43 × 10-4 RIU, which represents a 395% enhancement compared to that of the sensor without AuNPs. Through biochemical detection experiments, we measured IgG and determined the detection limit (LOD) at 614 ng mL-1 in ∼4 min, thereby proving the feasibility of the bio-detection sensing functionality. This study demonstrates a new type of WGPPR biosensor, which offers several unique advantages such as simple structure, high sensitivity, and rapid bio-sensing for practical bio-medical sensing applications. The new biosensor also fulfils point-of-care (POC) requirements.

Design and Optimization of GeSn Waveguide Photodetectors for 2-µm Band Silicon Photonics.

Silicon photonics is emerging as a competitive platform for electronic-photonic integrated circuits (EPICs) in the 2 µm wavelength band where GeSn photodetectors (PDs) have proven to be efficient PDs. In this paper, we present a comprehensive theoretical study of GeSn vertical p-i-n homojunction waveguide photodetectors (WGPDs) that have a strain-free and defect-free GeSn active layer for 2 µm Si-based EPICs. The use of a narrow-gap GeSn alloy as the active layer can fully cover entire the 2 µm wavelength band. The waveguide structure allows for decoupling the photon-absorbing path and the carrier collection path, thereby allowing for the simultaneous achievement of high-responsivity and high-bandwidth (BW) operation at the 2 µm wavelength band. We present the theoretical models to calculate the carrier saturation velocities, optical absorption coefficient, responsivity, 3-dB bandwidth, zero-bias resistance, and detectivity, and optimize this device structure to achieve highest performance at the 2 µm wavelength band. The results indicate that the performance of the GeSn WGPD has a strong dependence on the Sn composition and geometric parameters. The optimally designed GeSn WGPD with a 10% Sn concentration can give responsivity of 1.55 A/W, detectivity of 6.12 × 1010 cmHz½W-1 at 2 µm wavelength, and ~97 GHz BW. Therefore, this optimally designed GeSn WGPD is a potential candidate for silicon photonic EPICs offering high-speed optical communications.

Rapid and Highly Sensitive Detection of C-Reaction Protein Using Robust Self-Compensated Guided-Mode Resonance BioSensing System for Point-of-Care Applications.

The rapid and sensitive detection of human C-reactive protein (CRP) in a point-of-care (POC) may be conducive to the early diagnosis of various diseases. Biosensors have emerged as a new technology for rapid and accurate detection of CRP for POC applications. Here, we propose a rapid and highly stable guided-mode resonance (GMR) optofluidic biosensing system based on intensity detection with self-compensation, which substantially reduces the instability caused by environmental factors for a long detection time. In addition, a low-cost LED serving as the light source and a photodetector are used for intensity detection and real-time biosensing, and the system compactness facilitates POC applications. Self-compensation relies on a polarizing beam splitter to separate the transverse-magnetic-polarized light and transverse-electric-polarized light from the light source. The transverse-electric-polarized light is used as a background signal for compensating noise, while the transverse-magnetic-polarized light is used as the light source for the GMR biosensor. After compensation, noise is drastically reduced, and both the stability and performance of the system are enhanced over a long period. Refractive index experiments revealed a resolution improvement by 181% when using the proposed system with compensation. In addition, the system was successfully applied to CRP detection, and an outstanding limit of detection of 1.95 × 10-8 g/mL was achieved, validating the proposed measurement system for biochemical reaction detection. The proposed GMR biosensing sensing system can provide a low-cost, compact, rapid, sensitive, and highly stable solution for a variety of point-of-care applications.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well photoconductors on silicon.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well (MQW) photoconductors (PCs) on silicon substrate were investigated. The high quality GeSn/Ge MQW epitaxial structure was grown on a silicon substrate using low temperature molecular beam epitaxy techniques with atomically precise thickness control. Surface-illuminated GeSn/Ge MQW PCs were fabricated using complementary metal-oxide-semiconductor-compatible processing and characterized in a wide temperature range of 55-320 K. The photodetection range was extended to λ=2235nm at T=320K due to bandgap shrinkage with Sn alloying. Measured spectral responsivity was enhanced at reduced temperatures. These results provide better understanding of GeSn/Ge MQW structures for efficient short-wave infrared photodetection.

Planar GeSn lateral p-i-n resonant-cavity-enhanced photodetectors for short-wave infrared integrated photonics.

We report normal-incidence planar GeSn resonant-cavity-enhanced photodetectors (RCE-PDs) with a lateral p-i-n homojunction configuration on a silicon-on-insulator (SOI) platform for short-wave infrared (SWIR) integrated photonics. The buried oxide of the SOI platform and the deposited SiO2 layer serve as the bottom and top reflectors, respectively, creating a vertical cavity for enhancing the optical responsivity. The planar p-i-n diode structure is favorable for complementary-metal-oxide-semiconductor-compatible, large-scale integration. With the bandgap reduction enabled by the 4.2% Sn incorporation into the GeSn active layer, the photodetection range extends to 1960 nm. The promising results demonstrate that the developed planar GeSn RCE-PDs are potential candidates for SWIR integrated photonics.

Light-Harvesting in Biophotonic Optofluidic Microcavities via Whispering-Gallery Modes.

Phycobiliproteins are a class of light-harvesting fluorescent proteins existing in cyanobacteria and microalgae, which harvest light and convert it into electricity. Owing to recent demands on environmental-friendly and renewable apparatuses, phycobiliproteins have attracted substantial interest in bioenergy and sustainable devices. However, converting energy from biological materials remains challenging to date. Herein, we report a novel scheme to enhance biological light-harvesting through light-matter interactions at the biointerface of whispering-gallery modes (WGMs), where phycobiliproteins were employed as the active gain material. By exploiting microdroplets as a carrier for light-harvesting biomaterials, strong local electric field enhancement and photon confinement at the cavity interface resulted in significantly enhanced bio-photoelectricity. A threshold-like behavior was discovered in photocurrent enhancement and the WGM modulated fluorescence. Systematic studies of biologically produced photoelectricity and optical mode resonance were carried out to illustrate the impact of the cavity quality factor, structural geometry, and refractive indices. Finally, a biomimetic system was investigated by exploiting cascade energy transfer in phycobiliprotein assembly composed of three light-harvesting proteins. The key findings not only highlight the critical role of optical cavity in light-harvesting but also offer deep insights into light energy coupling in biomaterials.

Programmable Rainbow-Colored Optofluidic Fiber Laser Encoded with Topologically Structured Chiral Droplets.

Optofluidic lasers are emerging building blocks with immense potential in the development of miniaturized light sources, integrated photonics, and sensors. The capability of on-demand lasing output with programmable and continuous wavelength tunability over a broad spectral range enables key functionalities in wavelength-division multiplexing and manipulation of light-matter interactions. However, the ability to control multicolor lasing characteristics within a small mode volume with high reconfigurability remains challenging. The color gamut is also restricted by the number of dyes and emission wavelength of existing materials. In this study, we introduce a fully programmable multicolor laser by encapsulating organic-dye-doped cholesteric liquid crystal microdroplet lasers in an optofluidic fiber. A mechanism for tuning laser emission wavelengths was proposed by manipulating the topologically induced nanoshell structures in microdroplets with different chiral dopant concentrations. Precision control of distinctive lasing wavelengths and colors covering the entire visible spectra was achieved, including monochromatic lasing, dual-color lasing, tri-color lasing, and white colored lasing with tunable color temperatures. Our findings revealed a CIE color map with 145% more perceptible colors than the standard RGB space, shedding light on the development of programmable lasers, multiplexed encoding, and biomedical detection.

Raman scattering study of GeSn under 〈1 0 0〉 and 〈1 1 0〉 uniaxial stress.

The application of strain into GeSn alloys can effectively modulate the band structures, thus creating novel electronic and photonic devices. Raman spectroscopy is a powerful tool for characterizing strain; however, the lack of Raman coefficient makes it difficult for accurate determination of strain in GeSn alloys. Here, we have investigated the Raman-strain function of Ge1-xSnxalong 〈1 0 0〉 and 〈1 1 0〉 directions. GeSn nanomembranes (NMs) with different Sn compositions are transfer-printed on polyethylene terephthalate substrates. External strain is introduced by bending fixtures with different radii, leading to uniaxial tensile strain up to 0.44%. Strain analysis of flexible GeSn NMs bent along 〈1 0 0〉 and 〈1 1 0〉 directions are performed by Raman spectroscopy. The linear coefficients of Raman-strain for Ge0.96Sn0.04are measured to be -1.81 and -2.60 cm-1, while those of Ge0.94Sn0.06are decreased to be -2.69 and -3.82 cm-1along 〈1 0 0〉 and 〈1 1 0〉 directions, respectively. As a result, the experimental ratio of linear coefficient (ROLC) of Ge, Ge0.96Sn0.04and Ge0.94Sn0.06are 1.34, 1.44 and 1.42, which agree well with theoretical ROLC values calculated by elastic compliances and phonon deformation potentials (PDPs). In addition, the compositional dependence of PDPs is analyzed qualitatively. These fundamental parameters are important in designing high performance strained GeSn electronic and photonic devices.

GeSn lateral p-i-n waveguide photodetectors for mid-infrared integrated photonics.

In this Letter, we demonstrate mid-infrared (MIR) lateral p-i-n GeSn waveguide photodetectors (WGPDs) on silicon, to the best of our knowledge for the first time, as a key enabler of MIR electronic-photonic integrated circuits (EPICs). Narrow-bandgap GeSn alloys were employed as the active material to enable efficient photodetection in the MIR region. A lateral p-i-n homojunction diode was designed and fabricated to significantly enhance the optical confinement factor of the guided modes and thus enhance the optical responsivity. Thus, a photodetection range of up to 1950 nm and a good responsivity of 0.292 A/W at 1800 nm were achieved. These results demonstrate the feasibility of planar GeSn WGPDs for monolithic MIR EPICs on silicon.

Ge-on-insulator lateral p-i-n waveguide photodetectors for optical communication.

We report high-performance lateral p-i-n Ge waveguide photodetectors (WGPDs) on a Ge-on-insulator (GOI) platform that could be integrated with electronic-photonic integrated circuits (EPICs) for communication applications. The high-quality Ge layer affords a low absolute dark current. A tensile strain of 0.144% in the Ge active layers narrows the direct bandgap to enable efficient photodetection over the entire range of C- and L-bands. The low-index insulator layer enhances optical confinement, resulting in a good optical responsivity. These results demonstrate the feasibility of planar Ge WGPDs for monolithic GOI-based EPICs.

Low-cost planar waveguide-based optofluidic sensor for real-time refractive index sensing.

We report on the design, fabrication, and characterization of mass-producible, sensitive, intensity-detection-based planar waveguide sensors for rapid refractive index (RI) sensing; the sensors comprise suspended glass planar waveguides on glass substrates, and are integrated with microfluidic channels. They are facilely and cost-effectively constructed via vacuum-less processes. They yield a high throughput, enabling mass production. The sensors respond to solutions with different RIs via variations in the transmitted optical power due to coupling loss in the sensing region, facilitating real-time and simple RI detection. Experiments yield a good resolution of 5.65 × 10-4 RIU. This work has major implications for several RI-sensing-based applications.