Search

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. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This work at CCU was supported by the Young Scholar Fellowship Program by the Ministry of Science and Technology of Taiwan (MOST) under Grant no. MOST 111-2636-E-194-002 and MOST 110-2636-E-194-002. The work at Nanyang Technological University was supported by the A*STAR, Singapore, Advanced Manufacturing and Engineering (AME) Young Individual Research Grant (YIRG) under the Project A2084c0066, and Ministry of Education, Singapore, under the Grant ACRF Tier 2 grant (T2EP50120-0001) and Tier 1-2020-T1- 002-020 (RG136/20). The authors also acknowledge the support of the Nanyang NanoFabrication Centre (N2FC).

Ge1-xSnx photodetectors (PDs) have emerged as a new type of mid-infrared (MIR) CMOS-compatible PDs for a wide range of applications. Here we present a comprehensive theoretical study to evaluate the achievable performance of Ge1-xSnx p-i-n homojunction PDs with strain-free and defect-free Ge1-xSnx active layer for the purpose of demonstrating its potential in advancing the MIR detection technology. Starting from the Sn-composition-dependent band structures, the theoretical model calculates optical absorption, responsivity, dark current density, and detectivity. The results show that the optical responsivity can be enhanced with the Sn incorporation due to the improved optical absorption and the large mobilities and diffusion lengths of the photo-generated electrons and holes. The dark current density, however, increases with the increasing Sn composition. Our model suggests that not only the photodetection range of the Ge1-xSnx PDs can be extended to the MIR region but their detectivity at room temperature can be competitive with the existing MIR technology, and in some cases better than some commercial PDs operating at lower temperatures. This study establishes the ultimate performance that can be potentially achieved with the Ge1-xSnx MIR technology with the maturity of its material development in due time in addition to its much anticipated CMOS-compatible advantages.

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. Agency for Science, Technology and Research (A*STAR) This research is supported by A*STAR under its AME IRG Grant (Project No. A20E5c0085). Wang Z. would like to thank the support from the China Scholarship Council (Grant No. 202006250152).

Near infrared (NIR) photodetectors (PDs) have attracted great attention for their applications in the field of optical telecommunication. Ge is one of the most attractive materials for the active region of the NIR PDs due to complementary metal oxide semiconductor (CMOS) compatibility and lower bandgap energy compared to those of Si. However, they suffer from significantly reduced responsivity in the wavelength region above 1.55 μm. Here, we develop a new scheme to boost the responsivity of Ge PDs by integrating TiN and Ge on flexible platforms. Responsivity is further modified by controlling the bandgap energy via applying various tensile and compressive strains. TiN is used as a responsivity booster, showing improvement of 63% compared to flat Ge PDs due to increased absorption via plasmon resonance and reduced reflection on the surface. Moreover, a further increase in responsivity is achieved by applying 0.30% tensile strain in the active region, reaching a responsivity of 11.5 mA W−1 at 1.55 μm. This work provides an efficient way to enhance the responsivity of flexible Ge PDs via heterogeneous integration of dissimilar materials. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This work was supported by the A*STAR, Singapore, Advanced Manufacturing and Engineering (AME) Individual Research Grant (IRG) under the Project M21K2c0107, and Ministry of Education, Singapore, under the Grant ACRF Tier 2 grant (MOE-T2EP50120-0001, T2EP50121-0001 (MOE-000180-01)) and AcRF Tier 1 grant (2021-T1-002-031 (RG112/21)). G. E. C. acknowledges the support from Ministry of Science and Technology, Taiwan under Project number MOST 111-2636-E-194- 002. The authors also acknowledge the support from the Nanyang NanoFabrication Centre (N2FC).

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. Ministry of Education (MOE) Nanyang Technological University This research is supported by the Ministry of Education Singapore (Tier 1-RG 158/19(S)). We would like to thank Internal Grant NAP SUG -M4082308.040 from NTU.

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

In this work, we present a novel distributed Bragg reflector (DBR)-based resonant-cavity-enhanced (RCE) GeSn photodetector on Si substrates to achieve high-performance photodetection in terms of responsivity and 3-dB bandwidth (BW) for the short-wave infrared (SWIR) high-speed applications. The proposed structure consists of Si/SiO2 distributed Bragg reflectors (DBRs) to enhance the performance of the device. The top and bottom DBRs create a high-quality ( ${Q}$ ) optical cavity to enable multiple pass reflection schemes to increase the responsivity and also high wavelength selectivity with a sharp response. In addition, with an increase in Sn concentration in the active ( i- GeSn) layer, the photodetection range extends to longer wavelengths due to the shrinkage of bandgap energy. The calculated result shows an enhanced 3-dB BW and responsivity as compared to the existing p-i-n PDs. Therefore, the proposed DBR-based RCE GeSn PD can be a promising device for high-speed SWIR photodetection applications.

Silicon-based electronic-photonic integrated circuits, which are compatible with state-of-the-art complementary metal-oxide-semiconductor processes, offer promising opportunities for on-chip mid-infrared photonic systems. However, the lack of efficient mid-infrared optical modulators on silicon hinders the utilization of such systems. Here, we demonstrate the Franz-Keldysh effect in GeSn alloys and achieve mid-infrared electro-absorption optical modulation using GeSn heterostructures on silicon. Our experimental and theoretical results verify that the direct bandgap energy of GeSn can be widely tuned by varying the Sn content, thereby realizing wavelength-tunable optical modulation in the mid-infrared range with a figure-of-merit greater than 1.5 and a broadband operating range greater than 140 nm. In contrast to conventional silicon-photonic modulators based on the plasma dispersion effect, our GeSn heterostructure demonstrates practical and effective Franz-Keldysh mid-infrared optical modulation on silicon, helping to unlock the potential of electronic-photonic integrated circuits in a wide range of applications. Silicon-based electronic-photonic integrated circuits are promising for various applications, but their mid-infrared optical modulation is elusive. Here, tunable mid-infrared electro-absorption modulation, with broadband operation range >140 nm, is achieved in GeSn alloys on Si by controlling the Sn content.

We propose and analyze GeSn resonant-cavity-enhanced photodetectors (RCE-PDs) to achieve high-speed and high-responsivity photodetection for communication applications. The designed GeSn RCE-PDs comprises a Ge/GeSn/Ge p-i-n photodiode structure grown on a silicon-on-insulator (SOI) substrate and a Si/SiO2 distributed Bragg reflector (DBR). The incorporation of Sn into the active layer effectively reduced the direct bandgap energy, thereby extending the photodetection range to longer wavelengths and, thus, enhanced the optical responsivity. The buried oxide and the Si/SiO2 DBR act as the top and bottom mirrors, respectively, that considerably enhance the responsivity. We show our theoretical models to calculate the optical absorption coefficient, bandwidth, reflectivity, and responsivity, and we optimize the proposed devices to simultaneously achieve high-responsivity and high-speed operation at 1550 nm and 2000 nm. The results show that the optimized GeSn RCE-PDs can achieve a responsivity of 0.619 A/W for 6% Sn (1.02 A/W for 10% Sn) and high 3-dB bandwidth of ~58 GHz (59 GHz) at 1550 nm (2000 nm). These results show that the proposed GeSn RCE-PDs are very promising for high-performance photodetection in communication applications.

We study the effect of temperature and doping in Si-based GeSn heterojunction phototransistors (HPTs) for low-power-consuming, low-cost, and high-speed mid-infrared (MIR) applications. The incorporation of ${\rm{G}}{{\rm{e}}_{1 - x}}{\rm{S}}{{\rm{n}}_x}$ alloy in the base of our HPTs significantly shortens the emitter-to-collector transit time, leading to high cut-off frequency ( ${f_T}$ ) due to an increase in mobility. Furthermore, the ${\rm{G}}{{\rm{e}}_{1 - x}}{\rm{S}}{{\rm{n}}_x}$ base extends the optical detection over a wide range (up to 2500 nm) due to the shrinkage of the bandgap energy caused by alloying with Sn. Additionally, spectral responsivity increases with Sn alloying due to the increased absorption coefficient. Our results show that ${f_T}$ and responsivity are strongly dependent not only on doping but also on temperature. The impact of temperature on the noise behavior of phototransistors was also analyzed for frequencies up to 100 GHz. As the temperature increased, the signal-to-noise ratio (SNR) decreased; however, spectral responsivity significantly improved. The high SNR, responsivity, and ${f_T}$ values of the GeSn HPT make it a potential candidate for future Si-based uncooled high-speed MIR photodetection.

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.

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.

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 S i O 2 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.

We present a comprehensive analysis of practical p-n-p Ge/Ge1-xSnx/Ge heterojunction phototransistors (HPTs) for design optimization for efficient infrared detection. Our design includes a Ge1-xSnx narrow-bandgap semiconductor as the active layer in the base layer, enabling extension of the photodetection range from near-infrared to mid-infrared to perform wide-range infrared detection. We calculate the current gain, signal-to-noise ratio (SNR), and optical responsivity and investigate their dependences on the structural parameters to optimize the proposed Ge1-xSnx p-n-p HPTs. The results show that the SNR is strongly dependent on the operation frequency and that the introduction of Sn into the base layer can improve the SNR in the high-frequency region. In addition, the current gain strongly depends on the Sn content in the Ge1-xSnx base layer, and a Sn content of 6%-9% maximizes the optical responsivity achievable in the infrared range. These results provide useful guidelines for designing and optimizing practical p-n-p Ge1-xSnx HPTs for high-performance infrared photodetection.

We demonstrate room-temperature, mid-infrared resonant electroluminescence from GeSn resonant-cavity LEDs with a lateral p-i-n configuration on a silicon-on-insulator substrate. A vertical cavity to enhance light emission in the GeSn active layer is formed by the low-index buried oxide and deposited SiO 2 layer. A planar lateral p-i-n diode structure favorable for CMOS-compatible, dense integration was designed and fabricated for current injection. Under continuous-wave electrical injection, room-temperature resonant electroluminescence was successfully observed at ∼ 1980 nm with a spectral emission factor of 2.2. These results could pave the way toward efficient electrically injected GeSn light emitters operating at room temperature.

Recent studies of SiGeSn materials and optoelectronic devices hold great promise for photonics integrated circuits (PICs) on Si platform featuring scalable, cost-effective, and power-efficient. Thanks to the breakthrough of low temperature material growth techniques, device-quality level materials have been grown, following by the demonstration of light-emitting diodes, photodetectors, and optically pumped and electrically injected band-to-band lasers. While the exciting developments in bulk devices were reported, the quantum wells (QWs) have been investigated targeting the dramatically improved and/or novel device performance via variety of quantum confinement effects. In this review, we summarize the recent progress on development of SiGeSn QWs, including the fundamental optical and transition studies and optoelectronic device applications. The inspirational results reveal the possibility of all-group-IV PICs with photonics and electronics monolithically integrated on a single-chip.

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.

Liquid droplets offer a great number of opportunities in biochemical and physical research studies in which droplet-based microlasers have come into play over the past decade. While the recent emergence of droplet lasers has demonstrated their powerful capabilities in amplifying subtle molecular changes inside the cavity, the optical interactions between droplet resonators and an interface remain unclear. We revealed the underlying mechanism of droplet lasers when interacting with a droplet–solid interface and explored its correlation with intermolecular forces. A vertically oriented oscillation mode—arc-like mode—was discovered, where the number of lasing modes and their Q-factors increase with the strength of interfacial hydrophobicity. Both experimental and theoretical results demonstrated that hydrophobicity characterized by contact angle and interfacial tension plays a significant role in the geometry of droplet cavity and laser mode characteristics. Finally, we demonstrated how tiny forces induced by proteins and peptides could strongly modulate the lasing output in droplet resonators. Our findings illustrate the potential of exploiting optical resonators to amplify intermolecular force changes, providing comprehensive insights into lasing actions modulated by interfaces and applications in biophysics.

We report on lateral p-i-n GeSn waveguide photodetector on silicon substrates for complementary metal-oxide-semiconductor (CMOS)-compatible mid-infrared silicon photonics.

We have demonstrated high-performance flexible germanium (Ge) vertical p-i-n photodetectors (PDs) based on a resonant cavity structure by a direct flip transfer of Ge nanomembranes on polyethylene terephthalate (PET) substrates. Finite-difference time-domain simulation proves that the vertical cavity structure composed of the bottom gold and top SU-8 layers as a reflector and an anti-reflection surface, respectively, could enhance the average absorption in the near-infrared (NIR) region (i.e., 1520-1640 nm) from 0.06 to 0.20 by 233%. Strains introduced into Ge NMs by convex and concave fixtures are measured to be 0.37 and-0.32%, respectively. The fabricated PDs exhibit a low dark current density of 9.6 mA/cm2 at-1 V and a high forward-reverse current ratio of 105 under the flat condition. Responsivity at 1550 nm increases from 52.5 to 133.8 mA/W by tensile strain, while it slightly decreases to 32.6 mA/W under comparable compressive strain. Furthermore, the devices show no degradation in their optoelectronic responses after 200 bending cycles at convex fixtures with a radius of 30 mm. Overall, such flexible Ge PDs with the capabilities of both excellent optoelectronic performance and mechanical durability represent significant advances in the field of group IV NIR optoelectronic devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Submitted/Accepted version This work was supported by the A*STAR Advanced Manufacturing and Engineering (AME) Young Individual Research Grant (YIRG) under Project A2084c0066, the Nanyang Technological University (NTU) start-up grant (M4082289.040), and partly by the National Research Foundation, Singapore, under its Competitive Research Program (CRP award NRF-CRP19-2017-01) and the Ministry of Education Tier 2 (MOE2018-T2-1-137). The work at the CCU was supported by the Ministry of Science and Technology, Taiwan, grant number MOST 109-2636-E-194- 002.

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. Agency for Science, Technology and Research (A*STAR) Nanyang Technological University This research is supported by A*STAR under its AME IRG Grant (Project No. A20E5c0085). We would like to thank the Centre of Bio-Devices and Bioinformatics and Internal Grant NAP SUG -M4082308.040 from NTU for the lab support.

GeSn-based quantum wells (QWs) are of great interests for the development of all-group-IV optoelectronic devices such as lasers. Using a GeSn buffer and SiGeSn barrier has been studied with the aim of obtaining a direct bandgap well and increasing the carrier confinement. However, the carrier collection efficiency with such a configuration remains unsatisfactory. In this work, a single QW with additional GeSn barrier inserted between the GeSn well and the SiGeSn barrier was grown and characterized. Under relatively low carrier injection, the photoluminescence results show dramatically enhanced emission from the QW compared to the reference samples with only SiGeSn barrier, indicating a significantly improved carrier collection efficiency of the well.

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.

Metal-semiconductor-metal photodetectors (MSM PDs) are effective for monolithic integration with other optical components of the photonic circuits because of the planar fabrication technique. In this article, we present design, growth, and characterization of GeSn MSM PDs that are suitable for photonic integrated circuits. The introduction of 4% Sn in the GeSn active region also reduces the direct bandgap and shows a redshift in the optical responsivity spectra, which can extend up to 1800 nm wavelength, which means it can cover the entire telecommunication bands. The spectral responsivity increases with an increase in bias voltage caused by the high electric field, which enhances the carrier generation rate and the carrier collection efficiency. Therefore, the GeSn MSM PDs can be a suitable device for a wide range of short-wave infrared (SWIR) applications. National Research Foundation (NRF) Published version National Research Foundation Singapore Competitive Research Programme under Grant NRF-CRP19-2017-01.

GeSn alloys have emerged as promising materials for silicon-based optoelectronic devices. However, the epitaxy of pseudomorphic GeSn layers on a Ge buffer is susceptible to a significant compressive strain that significantly hinders the performance of GeSn-based photonic devices. Herein, we report on a new strategy to produce strain-free GeSn nanomembranes for advanced optoelectronic applications. The GeSn alloy was grown on a silicon-on-insulator substrate using Ge buffers, and it has a residual compressive strain. By transfer-printing the GeSn/Ge/Si multi-layers, followed by etching the Si template and the Ge buffer layers, respectively, the residual compressive strain was completely removed to achieve strain-free GeSn layers. A bandgap reduction was also observed as a result of strain relaxation. Furthermore, theoretical analysis was performed to evaluate the effect of strain relaxation on the GeSn-based optoelectronic devices. The proposed approach offers a practical and viable method for preparing strain-free GeSn alloys for advanced optoelectronic applications. Ministry of Education (MOE) Nanyang Technological University Accepted version This work at CCU was supported by the Ministry of Science and Technology of Taiwan (MOST) under the grant Nos. MOST 108-2221-E-194-055 and MOST 109-2636-E-194-002, and the Advanced Institute of Manufacturing with High-tech Innovations (AIM-HI) from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The work at NTU was supported by NTU start-up grant (M4082289.040) and Singapore MOE AcRF Tier 1 grant (M4012124.040). The authors would like to thank Dr. Hui Li at National Taiwan University, Taiwan, for providing assistance in the material growth, and Dr. I-Fang Cheng at Taiwan Semiconductor Research Institute, Taiwan, for providing assistance in the Raman experiments.

The germanium-on-insulator (GOI) has recently emerged as a new platform for complementary metal-oxide-semiconductor (CMOS)-compatible photonic integrated circuits. Here we report on resonant-cavity-enhanced optical responses in Ge photodetectors on a GOI platform where conventional photodetection is difficult. A 0.16% tensile strain is introduced to the high-quality Ge active layer to extend the photodetection range to cover the entire range of telecommunication C- and L-bands (1530-1620 nm). A carefully designed vertical cavity is created utilizing the insulator layer and the deposited SiO2 layer to enhance the optical confinement and thus optical response near the direct-gap absorption edge. Experimental results show a responsivity peak at 1590 nm, confirming the resonant cavity effect. Theoretical analysis shows that the optical responsivity in the C- and L-bands is significantly enhanced. Thus, we have demonstrated a new type of Ge photodetector on a GOI platform for CMOS-compatible photonic integrated circuits for telecommunication applications. National Research Foundation (NRF) Published version This work was supported by the National Research Foundation Singapore Competitive Research Programme under Grant NRF–CRP19–2017–01.

Metal ions are known to play various roles in living organisms; therefore, the detection of metal ions in water resources is essential for monitoring health and environmental conditions. In contrast to artificially fabricated materials and devices, biological-friendly materials such as microalgae have been explored for detecting toxic chemicals by employing fluorescence emissions and biophotovoltaic fuel cells. However, complicated fabrication, long measurement time, and low sensitivity remain the greatest challenge due to the minimal amount of bioelectricity generated from whole-cell microalgae. Herein we report the novel concept of a microalgae living biosensor by enhancing photocurrent through nanocavities formed between copper (Cu) nanoparticles and the Cu-electrode beneath. The strong energy coupling between plasmon cavity modes and excited photosynthetic fluorescence from Chlorella demonstrated that photoelectrical efficiency could be significantly amplified by more than two orders of magnitude through nanocavity confinement. Simulation results reveal that substantial near-field enhancements could help confine the electric field to the electrodes. Finally, the microalgae sensor was exploited to detect various light and heavy metal ions with a breakthrough detection limit of 50 nM. This study is envisioned to provide inspirational insights on nanocavity-enhanced electrochemistry, opening new routes for biochemical detection, water monitoring, and sustainable optoelectronics. Ministry of Education (MOE) Nanyang Technological University We would like to thank the support from University Internal Grant NAP SUG - M4082308.040. We would especially like to thank financial support from the Ministry of Education Singapore AcRF Tier 1 RG 158/19-(S).

We have demonstrated flexible GeSn metal–semiconductor–metal (MSM) photodetectors (PDs) by exploring the effect of mechanical strain on their optoelectronic properties. The PDs were fabricated from transfer-printed GeSn nanomembranes on polyethylene terephthalate (PET) substrates. Strain was introduced into the GeSn PDs under bend-down (uniaxial tensile strain) and bend-up (uniaxial compressive strain) conditions and their values were measured by Raman spectroscopy. The applied strain can affect the band-structure of the GeSn alloys, leading to a modulation of the electrical and optical characteristics of the PDs. Accordingly, dark current characteristics show an increase from 8.1 to 10.3 μA under the bend-down conditions and a decrease to 7.2 μA under the bend-up conditions, respectively. The optical responsivity at a wavelength of 2 μm increased by 151% under bend-down conditions, while it decreases by 35% under bend-up conditions. A theoretical study was carried out to support the fact that the responsivity enhancement is attributed to the change in the absorption coefficient of the strained GeSn. The results offer a new pathway to modulate the optical properties of GeSn for flexible applications. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version The work was supported by the Nanyang Technological University (NTU) start-up grant (M4082289.040) and partly by the National Research Foundation, Singapore, under its Competitive Research Program (CRP Award NRF-CRP19-2017-01) and Ministry of Education, Singapore, under its Tier 2 (MOE2018-T2-1-137). The authors would like to thank Prof. Hong Wang at the NTU for providing a laser setup.