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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.

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.

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.

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.

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.

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.

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.

We present the design and analysis of three-terminal Ge1− x Sn x /Ge multiple-quantum-well (MQW) heterojunction phototransistors (HPTs) for high-performance short-wave infrared (SWIR) photodetection. The active layer incorporates Ge1− x Sn x /Ge MQW structures between base–collector junctions pseudomorphically grown on Ge-on-Si substrates, thereby providing compatibility with CMOS platforms for low cost and scalable manufacturing technology. Introducing Sn into the MQW structures can effectively reduce the direct bandgap, thereby considerably extending the photodetection range in the SWIR spectral range with improved absorption efficiencies. In addition, the use of MQW structures as the active layer can enhance the absorption coefficient via quantum-confinement effects, thus further enhancing the photoresponses. We develop theoretical models to calculate the strained electronic band structures and optical absorption coefficients for the Ge1− x Sn x /Ge MQW structures, and the current gain is evaluated as a function of the barrier/well thicknesses, number of periods of the MQW structures, and doping concentrations of the layers for the HPT structures. The calculation results show that the absorption coefficient can be significantly enhanced by increasing the Sn content in the Ge1− x Sn x /Ge MQW active layer, and the current gain can be considerably enhanced by optimizing the HPT structures. With the enhanced absorption coefficient and current gain, high-responsivity photodetection can be achieved in the SWIR spectral range, thus making this device very promising for a wide range of SWIR photodetection applications.

GeSn lasers enable the monolithic integration of lasers on the Si platform using all-group-IV direct-bandgap material. The GeSn laser study recently moved from optical pumping into electrical injection. In this work, we present explorative investigations of GeSn heterostructure laser diodes with various layer thicknesses and material compositions. Cap layer material was studied by using Si 0.03 Ge 0.89 Sn 0.08 and Ge 0.95 Sn 0.05 , and cap layer total thickness was also compared. The 190 nm SiGeSn-cap device had threshold of 0.6 kA / cm 2 at 10 K and a maximum operating temperature ( T max ) of 100 K, compared to 1.4 kA / cm 2 and 50 K from 150 nm SiGeSn-cap device, respectively. Furthermore, the 220 nm GeSn-cap device had 10 K threshold at 2.4 kA / cm 2 and T max at 90 K, i.e., higher threshold and lower maximal operation temperature compared to the SiGeSn cap layer, indicating that enhanced electron confinement using SiGeSn can reduce the threshold considerably. The study of the active region material showed that device gain region using Ge 0.87 Sn 0.13 had a higher threshold and lower T max , compared to Ge 0.89 Sn 0.11 . The performance was affected by the metal absorption, free carrier absorption, and possibly defect density level. The maximum peak wavelength was measured as 2682 nm at 90 K by using Ge 0.87 Sn 0.13 in gain regions. The investigations provide directions to the future GeSn laser diode designs toward the full integration of group-IV photonics on a Si platform.

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 = 2235 n m at T = 320 K 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.

We present an intensity-detection-based optofluidic biosensing system with a disposable guided-mode-resonance (GMR) biosensor chip for low-cost, rapid, label-free biosensing. The developed sensing system consists of a low-cost spectrally limited light-emitting diode (LED) as the light source and a photodetector as the optical receiver. This detection instrument can transform the GMR wavelength shift induced by the change in the local refractive index (RI) near the chip’s grating surface into change in the light intensity, thus enabling label-free, real-time detection. RI measurement experiments in both the transmission and reflection modes are performed to study the corresponding sensing mechanisms and compare detection performance. The results indicate that this sensing system operating in the reflection mode can achieve a superior RI resolution of 4 . 10 × 10 - 5 RIU over a wide linear detection range of 0.04 RIU. Label-free biomolecular interaction experiments show that a low limit of detection (LOD) of 7.5 × 10−8 g/ml for dinitro-phenyl (DNP)/anti-DNP interactions can be achieved, thus confirming the our system’s sensitivity. The present investigation demonstrates a low-cost, compact, high-performance optofluidic biosensing system for rapid and sensitive biochemical detection that can be utilized for a wide variety of applications.

We present an investigation of the use of a low refractive index buffer layer to improve the sensitivity of grating coupler biosensor for different-sized bio-molecular. Our experimental results show that the bulk and surface sensitivities can be improved by optimizing the matching conditions.

We present a study of Franz-Keldysh effect in GeSn alloys for mid-infrared photonic applications. The electric-field-dependent absorption coefficient of GeSn alloys with different Sn compositions are calculated and discussed.

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.

We propose the use of Ge $_{1-x}$ Sn $_x$ heterojunction phototransistors (HPTs) as efficient optical receivers on Si substrates and analyze their performance. Our designs use n-Ge/p-Ge $_{1-x}$ Sn $_x$ /n-Ge $_{1-x}$ Sn $_x$ layers pseudomorphically grown on Si wafers via a Ge virtual substrate, which offers compatibility with complementary metal–oxide–semiconductor (CMOS) technology. By incorporating Sn into the Ge photon-absorbing layer to shrink the bandgap, the photodetection range can be significantly extended to the mid-infrared (MIR) region with a considerably enhanced optical response. The use of HPT structures provides optical conversion gain to further enhance the optical responsivity, thereby enabling efficient photodetection in the short-wave infrared region. We develop theoretical models to calculate the composition-dependent band alignments, the band structures (by taking into account the nonparabolic effect), the absorption coefficient, and the optical responsivity for the proposed GeSn HPTs. As the Sn content increases, the conduction band nonparabolicity becomes increasingly significant and considerably impacts the optical absorption coefficient. Moreover, analysis of the spectral response for the Ge $_{1-x}$ Sn $_x$ HPTs shows that efficient photodetection covering the entirety of the fiber-optic telecommunication bands, as well as the emerging 2- $\mathbf {\mu }$ m MIR communication band, can be achieved. These results indicate that the proposed Ge $_{1-x}$ Sn $_x$ HPTs are attractive for use as high-responsivity CMOS-compatible photodetectors in communication applications.

We report an investigation of the photoresponse of a GeSn film with a graphene layer placed on top and a thin GeO2 layer sandwiched between them. Both wavelength- and power-dependent amplification of the photocurrent are demonstrated. These results are associated with the spatial separation of photoexcited electrons and holes enabled by the thin oxide layer, where electrons and holes accumulate in graphene and the GeSn film, respectively. This spatial separation of negative and positive charges generates a mutual gating that increases the number of carriers in both layers, yielding the amplification observed in the measurement. A quantitative method based on an equivalent circuit model is provided, and the numerical results agree well with the experimental data. Our results represent an advance toward the realization of high-performance heterostructured photodetectors, and the modeling provides a framework for analyzing the photodetection capability of other two-dimensional materials on semiconductor films.

In article number 1903707, Yu‐Cheng Chen and co‐workers demonstrate an approach to amplify bio‐nano‐electricity through the encapsulation of living algae in an optical micro‐cavity. The strong energy coupling between the cavity mode and photosynthetic resonance reveal the potential of exploiting optical resonators to amplify biological photocurrent generation and energy‐harvesting efficiency. Potential applications include photocatalysis, electrochemistry, and sustainable biophotonics. [Image: see text]

Manipulation of laser emission offers promising opportunities for the generation of new spatial dimensions and applications, particularly in nanophotonics, super-resolution imaging, and data transfer devices. However, the ability to control laser modes and wavelength in a microcavity remains challenging. Here, a novel approach is demonstrated to control laser modes by manipulating the 3D-optical confinement, chirality, and orientations in a Fabry−Pérot microcavity with cholesteric liquid crystal droplets. Different configurations of intracavity micro-/nanostructures generate versatile dimensions of laser modes, while the significantly reduced laser mode volume further leads to single-mode lasing. Theoretical analysis is carried out to support this interesting discovery. Finally, switchable lasing wavelength with various surface anchoring forces and pH interactions is demonstrated. This novel concept not only provides a simple yet highly versatile method to manipulate laser emissions, but deepens insight into how molecules interact with and modulate laser light, laying the foundation for the development of tunable photonic devices at the molecular level. Promising applications include highly selective laser devices, laser-emission imaging, and bioinspired sensing. Nanyang Technological University The authors would like to thank the Centre of Bio-Devices and Signal Analysis for the lab support andand Nanyang Technological University, Singapore for the internal grant (NAP-SUG M4082308.404).

The 2 µm wavelength band has recently gained increased attention for potential applications in next-generation optical communication. However, it is still challenging to achieve effective photodetection in the 2 µm wavelength band using group-IV-based semiconductors. Here we present an investigation of GeSn resonant-cavity-enhanced photodetectors (RCEPDs) on silicon-on-insulator substrates for efficient photodetection in the 2 µm wavelength band. Narrow-bandgap GeSn alloys are used as the active layer to extend the photodetection range to cover the 2 µm wavelength band, and the optical responsivity is significantly enhanced by the resonant cavity effect as compared to a reference GeSn photodetector. Temperature-dependent experiments demonstrate that the GeSn RCEPDs can have a wider photodetection range and higher responsivity in the 2 µm wavelength band at higher temperatures because of the bandgap shrinkage. These results suggest that our GeSn RCEPDs are promising for complementary metal-oxide-semiconductor-compatible, efficient, uncooled optical receivers in the 2 µm wavelength band for a wide range of applications.

We demonstrate silicon-based p - n - p floating-base GeSn heterojunction phototransistors with enhanced optical responsivity for efficient short-wave infrared (SWIR) photodetection. The narrow-bandgap GeSn active layer sandwiched between the p - G e collector and n - G e base effectively extends the photodetection range in the SWIR range, and the internal gain amplifies the optical response by a factor of more than three at a low driving voltage of 0.4 V compared to that of a reference GeSn p - i - n photodetector (PD). We anticipate that our findings will be leveraged to realize complementary metal-oxide-semiconductor-compatible, sensitive, low driving voltage SWIR PDs in a wide range of applications.

We present a design of Ge 1−x Sn x waveguide electroabsorption modulators on silicon and analyze the Franz-Keldysh modulation properties. The introduction of Sn into Ge allows for considerably narrowing the direct bandgap energy, thereby extended the absorption edge toward mid-infrared region to achieve mid-infrared optical modulation. We calculate the change in absorption coefficient of Ge 1−x Sn x induced by the Franz-Keldysh effects. The results show that significant absorption coefficient variation associated with the Franz-Keldysh effect can be achieved for optical modulation in the mid-infrared region.

A n-p-n heterojunction phototransitor with Ge 1-x Sn x base is presented. The work is focussed on effect of Sn concentration on the gain performance of the device based on optimised base and collector doping.

We report mid-infrared optical modulation in GeSn photodiodes on silicon. The incorporation of Sn into Ge shrinks the direct bandgap energy, redshifting the absorption edge to longer wavelengths and enabling electro-absorption optical modulation based on Franz-Keldysh effect in the mid-infrared range.

We report cavity-enhanced direct band electroluminescence from GeSn p-i-n diodes on silicon-on-insulator substrates. A Fabry-Perot cavity was created between the buried oxide layer and the deposited oxide layer to enhance electroluminescence from the GeSn active layer.

Micromilling is a straightforward approach to the manufacture of polymer microfluidic devices for applications in chemistry and biology. This fabrication process reduces costs, provides a relatively simple user interface, and enables the fabrication of complex structures, which makes it ideal for the development of prototypes. In this study, we investigated the influence of micromilling parameters on the surface roughness of a cyclic olefin copolymer (COC) substrate. We then employed factor analysis to determine the optimal cutting conditions. The parameters used in all experiments were the spindle speed, the feed rate, and the depth of cut. Roughness was measured using a stylus profilometer. The lowest roughness was 0.173 μm at a spindle speed of 20,000 rpm, feed rate of 300 mm/min, and cut depth of 20 μm. Factor analysis revealed that the feed rate has the greatest impact on surface quality, whereas the depth of cut has the least impact.

We present a variational study of direct transitions in GeSn/Ge quantum well system for photonic applications. The exciton radiuses, binding energies, and oscillator strengths are calculated for various Sn contents are calculated and discussed.

Group-IV GeSn material systems have recently considered as a new material for sensitive photodetection in the short-wave infrared (SWIR) region. The introduction of Sn into Ge can effectively narrow the bandgap energies, thereby extending the absorption edges toward the longer wavelengths and enabling effective photodetection in SWIR region. Here we present an experimental and modeling study of GeSn/Ge quantum well (QW) photodetectors on silicon substrates for effective SRIW photodetection. Epitaxial growth of pseudomorphic GeSn/Ge QW structures was realized on Ge-buffered silicon substrates using low-temperature molecular beam epitaxy. Normal incident GeSn/Ge QW photodetectors were then fabricated and characterized. The optical responsivity experiments demonstrate that the photodetection cutoff wavelengths is extended to beyond 1800 nm, enabling effective photodetection in SWIR spectral region. We then develop theoretical models to calculate the composition-dependent strained electron band structures, oscillation strengths, and optical absorption spectra for the pseudomorphic GeSn/Ge QW structures. The results show that Ge1-xSnx well sandwiched by Ge barriers can achieve a critical type-I alignment at Γ point to provide necessary quantum confinement of carriers. With an increase in the Sn content, the band offsets between the GeSn well and Ge barreirs increases, thus enhancing the oscillation strengths of direct interband transitions. In addition, despite stronger quantum confinement with increasing Sn content, the absorption edge can be effectively shifted to longer wavelengths due to the direct bandgap reduction caused by Sn-alloying. These results suggest that GeSn/Ge QW photodetectors are promising for low-cost, high-performance SWIR photodetection applications.

An efficient Si-based laser is one of the most important components for photonic integrated circuits to break the bottleneck of data transport over optical networks. The main challenge is to create gain media based on group-IV semiconductors. Here we present an investigation of using low-dimensional Ge 1-xS n x /Ge quantum-well (QW) structures pseudomorphically grown on Ge-buffered Si substrates as optical gain media for efficient Si-based lasers. Epitaxial growth of Ge 1-xS n x /Ge QW structures on Ge-buffer Si substrate was carried out using low-temperature molecular beam epitaxy techniques. The light emission properties of the grown Ge 1-xS n x /Ge QW structure were studied using photoluminescence spectroscopy, and clear redshifts of emission peaks were observed. Theoretical analysis of band structures indicates that Ge 1-xS n x well sandwiched by Ge barriers can form type-I alignment at Г point with a sufficient potential barrier height to confine carriers in the Ge 1-xS n x well, thereby enhancing efficient electron-hole direct recombination. Our calculations also show that the energy difference between the lowest Г-conduction subband and L conduction subband can be reduced with increasing Sn content, thereby enabling optical gain. These results suggest that Ge 1-xS n x /Ge QW structures are promising for optical gain media to develop efficient Si-based light emitters.

We report on tensile-strained Ge/Sisub0.11/subGesub0.89/subquantum-well (QW) metal-semiconductor-metal (MSM) photodetectors on Si substrates. A tensile strain of 0.21% is introduced into the Ge wells by growing the QW stack on in-situ annealed Ge-on-Si virtual substrates (VS). The optical characterization of Ge/Sisub0.11/subGesub0.89/subQW MSM photodetectors indicates that the optical response increases to a wavelength of 1.5 μm or higher owing to the strain-induced direct bandgap shrinkage. Analysis of the band structure by using a k · p model suggests that by optimizing the tensile strain and Ge well width, tensile-strained Ge/SiGe QW photodetectors can be designed to cover the telecommunication C-band and beyond for optical telecommunications and on-chip interconnection.

We present a theoretical analysis of strain effect on optical gain in biaxially-stressed Ge 1−x Sn x alloys. The electronic band structure for biaxially-stressed Ge 1−x Sn x alloys is calculated using deformation potential theory and k·p method. For unstrained Ge 1−x Sn x alloys, a Sn content of 6.7% is required to achieve a direct bandgap for providing optical gain. The introduction of tensile strain can further soften the requirements for indirect-to-direct bandgap transition, thereby enhancing optical gain. On the other hand, compressive strain significantly increases the energy difference between the Γ- and L-valley conduction band edges, and hence quenching optical gain in Ge 1−x Sn x alloys.

We present design and modeling of a polarization-insensitive electro-absorption waveguide modulator operating at 1550 nm. Our design uses tensile-strained Ge-SixGeySn1-x-y quantum wells as the active material grown on a relaxed SiGeSn buffer on a silicon substrate, compatible with complementary metal-oxide semiconductor (CMOS) processes. Introducing tensile strain in the Ge wells can effectively reduce the direct bandgap for optimal modulation at the critical 1550 nm wavelength, and provide polarization-insensitive electro-absorption properties. We present a theoretical model for the electronic band structure, excitonic absorption coefficient, and polarization-dependent optical confinement factor. We also bring forth a waveguide design based on index guidance, where we calculate the optical confinement factors of various regions to determine the extinction ratio and insertion loss. The proposed modulator operating at 1550 nm can offer unique advantages for use in high-performance, CMOS-compatible electronic-photonic integrated circuits.

We propose and analyze a strain-balanced GezSn1-z-SixGeySn1-x-y multiple-quantum-well (MQW) laser. By incorporating a proper amount of -Sn into Ge, a direct-bandgap GeSn alloy can be realized to achieve population inversion in the direct conduction band. The introduction of compressive strain into the GeSn QW can effectively modify the valence band structure to reduce the threshold carrier density. We calculate the electronic band structure and the polarization-dependent optical gain of the strained GezSn1-z-SixGeySn1-x-y MQW laser taking into account the effect of the L-conduction bands. We also present our waveguide design for index guidance and calculate the optical confinement factors of various regions. Our calculation indicates that the modal gain can reach the threshold condition and lead to lasing action.

We report GeSn p-i-n resonant-cavity-enhanced photodetectors (RCEPDs) grown on silicon-on-insulator substrates. A vertical cavity, composed of a buried oxide as the bottom reflector and a deposited SiO2 layer on the top surface as the top reflector, is created for the GeSn p-i-n structure to enhance the light-matter interaction. The responsivity experiments demonstrate that the photodetection range is extended to 1820 nm, completely covering all the telecommunication bands, because of the introduction of 2.5% Sn in the photon-absorbing layer. In addition, the responsivity is significantly enhanced by the resonant cavity effects, and a responsivity of 0.376 A/W in the telecommunication C-band is achieved that is significantly higher than that of conventional GeSn-based PDs. These results demonstrate the feasibility of CMOS-compatible, high-responsivity GeSn-based PDs for shortwave infrared applications.

We propose and develop an intensity-detection-based refractive-index (RI) sensor for low-cost, rapid RI sensing. The sensor is composed of a polymer bent ridge waveguide (BRWG) structure on a low-cost glass substrate and is integrated with a microfluidic channel. Different-RI solutions flowing through the BRWG sensing region induce output optical power variations caused by optical bend losses, enabling simple and real-time RI detection. Additionally, the sensors are fabricated using rapid and cost-effective vacuum-less processes, attaining the low cost and high throughput required for mass production. A good RI solution of 5.31 10−4 × RIU−1 is achieved from the RI experiments. This study demonstrates mass-producible and compact RI sensors for rapid and sensitive chemical analysis and biomedical sensing.

Low-cost label-free bio-sensing systems have long been desired to enable rapid, sensitive, quantitative, and high-throughput biosensing for bio-medical and chemical applications. Here we present an optical bio-detection system consists of injection-molded biosensors based on double-sided grating waveguide couplers and an optical intensity-based detection platform for low-cost, real-time, and label-free biosensing. The biosensors were fabricated combining injection-molding and sputtering techniques, providing unique advantages of low-cost and reduced production time. A simple and cost-effective optical intensity-based detection system employing a low-cost light emitting diode and a simple photodetector is also developed to perform label-free biosensing. We demonstrate that a high refractive index resolution of 6.43 × 10−5 RIU is achieved with this compact bio-sensing system, showing great promises for low-cost, real-time, label-free detection in bio-medical and chemical applications.Copyright © 2015 by ASME

We report the fabrication and characterization of GeSn waveguide structures on Si substrates grown by molecular beam epitaxy for efficient light-detection and emission. For photodetectors, GeSn waveguide structures exhibit a higher optical response compared to a reference Ge device as revealed by the photocurrent experiments. For light-emission, room-temperature photoluminescence experiments show a redshifted emission wavelength for the GeSn samples compared to the Ge reference sample due to the Sn incorporation. Besides, we observe ripple characteristics in the amplified spontaneous emission spectrum of the GeSn waveguide structure, which are attributed to the waveguide modes. Those results suggest that GeSn waveguide structures are promising for high-performance Si-based lightdetectors and emitters integrable with Si electronics.