Optofluidic lasers with a single molecular layer of gain

Highly Accurate Pneumatically Tunable Optofluidic Distributed Feedback Dye Lasers

Optofluidic dye lasers integrated into microfluidic chips are promising miniature coherent light sources for biosensing. However, achieving the accurate and efficient tuning of lasers remains challenging. This study introduces a novel pneumatically tunable optofluidic distributed feedback (DFB) dye laser in a multilayer microfluidic chip. The dye laser device integrates microfluidic channels, grating structures, and vacuum chambers. A second-order DFB grating configuration is utilized to ensure single-mode lasing. The application of vacuum pressure to the chambers stretches the soft grating layer, enabling the sensitive tuning of the lasing wavelength at a high resolution of 0.25 nm within a 7.84 nm range. The precise control of pressure and laser tuning is achieved through an electronic regulator. Additionally, the integrated microfluidic channels and optimized waveguide structure facilitate efficient dye excitation, resulting in a low pump threshold of 164 nJ/pulse. This pneumatically tunable optofluidic DFB laser, with its high-resolution wavelength tuning range, offers new possibilities for the development of integrated portable devices for biosensing and spectroscopy.

Submonolayer biolasers for ultrasensitive biomarker detection

Biomarker detection is key to identifying health risks. However, designing sensitive and single-use biosensors for early diagnosis remains a major challenge. Here, we report submonolayer lasers on optical fibers as ultrasensitive and disposable biosensors. Telecom optical fibers serve as distributed optical microcavities with high Q-factor, great repeatability, and ultralow cost, which enables whispering-gallery laser emission to detect biomarkers. It is found that the sensing performance strongly depends on the number of gain molecules. The submonolayer lasers obtained a six-order-of-magnitude improvement in the lower limit of detection (LOD) when compared to saturated monolayer lasers. We further achieve an ultrasensitive immunoassay for a Parkinson's disease biomarker, alpha-synuclein (α-syn), with a lower LOD of 0.32 pM in serum, which is three orders of magnitude lower than the α-syn concentration in the serum of Parkinson's disease patients. Our demonstration of submonolayer biolaser offers great potentials in high-throughput clinical diagnosis with ultimate sensitivity.

Ultralow-threshold green fluorescent protein laser based on high Q microbubble resonators

Biological lasers have attracted vast attention because of their potential medical application prospects, especially the low threshold biological laser that can be used for ultrasensitive biological detection while leaving the luminous gain medium undamaged by the pump light. By coupling the low concentration green fluorescent protein (GFP) solution with a high Q whispering gallery mode microbubble resonator, we managed to fabricate a miniature GFP laser with the lowest threshold and highest Q value compared to any known type of the GFP laser. The threshold energy is as low as 380 fJ, two orders of magnitude lower than any type of GFP laser at present. The Q value of the optical cavity in this biological laser is 5.3 × 10⁷, two orders higher than the highest Q value of GFP lasers. We further confirmed the long-term stability of the working characteristics of GFP laser. It can work well nearly a month in temperature 3-4°C. Finally, we measured the effects of different concentrations of fluorescent protein on laser threshold. The data show that this biological laser can be used for highly sensitive detection of GFP concentration, which is particularly useful when the GFP is used as tracers.

A sequentially bioconjugated optofluidic laser for wash-out-free and rapid biomolecular detection

Microstructures can improve both sensitivity and assay time in heterogeneous assays (such as ELISA) for biochemical analysis; however, it remains a challenge to perform the essential wash process in those microstructure-based heterogeneous assays. Here, we propose a sequential bioconjugation protocol to solve this problem and demonstrate a new type of fiber optofluidic laser for biosensing. Except for acting as an optical microresonator and a microstructured substrate, the miniaturized hollow optical fiber (HOF) is used as a microfluidic channel for storing and transferring reagents thanks to its capability in length extension. Through the capillary action, different reagents were sequentially withdrawn into the fiber for specific binding and washing purposes. By using the sequentially bioconjugated FOFL, avidin molecules are detected based on competitive binding with a limit of detection of 9.5 pM, ranging from 10 pM to 100 nM. It is demonstrated that a short incubation time of 10 min is good enough to allow the biomolecules to conjugate on the inner surface of the HOF. Owing to its miniaturized size, only 589 nL of liquid is required for incubation, which reduces the sample consumption and cost for each test. This work provides a tool to exploit the potential of microstructured optical fibers in high-performance biosensing.

Imaging-Based Optofluidic Biolaser Array Encapsulated with Dynamic Living Organisms

Optofluidic biolasers have emerged as promising tools for biomedical analysis due to their strong light-matter interactions and miniaturized size. Recent developments in optofluidic lasers have opened a new Frontier in monitoring biological processes. However, most biolasers require precise recording of the lasing spectrum at the single cavity level, which limits its application in high-throughput applications. Herein, a microdroplet laser array encapsulated with living Escherichia coli was printed on highly reflective mirrors, where laser emission images were employed to reflect the dynamic changes in living organisms. The concept of image-based lasing analysis was proposed by quantifying the integrated pixel intensity of the lasing image from whispering-gallery modes. Finally, dynamic interactions between E. coli and antibiotic drugs were compared under fluorescence and laser emission images. The amplification that occurred during laser generation enabled the quantification of tiny biological changes in the gain medium. Laser imaging presented a significant increase in integrated pixel intensity by 2 orders of magnitude. Our findings demonstrate that image-based lasing analysis is more sensitive to dynamic changes than fluorescence analysis, paving the way for high-throughput on-chip laser analysis of living organisms.

Ultralow-Threshold Continuous-Wave Room-Temperature Crystal-Fiber/Nanoperovskite Hybrid Lasers for All-Optical Photonic Integration

Continuous-wave (CW) room-temperature (RT) laser operation with low energy consumption is an ultimate goal for electrically driven lasers. A monolithically integrated perovskite laser in a chip-level fiber scheme is ideal. However, because of the well-recognized air and thermal instabilities of perovskites, laser action in a perovskite has mostly been limited to either pulsed or cryogenic-temperature operations. Most CW laser operations at RT have had poor durability. Here, crystal fibers that have robust and high-heat-load nature are shown to be the key to enabling the first demonstration of ultralow-threshold CW RT laser action in a compact, monolithic, and inexpensive crystal fiber/nanoperovskite hybrid architecture that is directly pumped with a 405 nm diode laser. Purcell-enhanced light-matter coupling between the atomically smooth fiber microcavity and the perovskite nanocrystallites gain medium enables a high Q (≈1500) and a high β (0.31). This 762 nm laser outperforms previously reported structures with a record-low threshold of 132 nW and an optical-to-optical slope conversion efficiency of 2.93%, and it delivers a stable output for CW and RT operation. These results represent a significant advancement toward monolithic all-optical integration.

Review of biosensing with whispering-gallery mode lasers

Lasers are the pillars of modern optics and sensing. Microlasers based on whispering-gallery modes (WGMs) are miniature in size and have excellent lasing characteristics suitable for biosensing. WGM lasers have been used for label-free detection of single virus particles, detection of molecular electrostatic changes at biointerfaces, and barcode-type live-cell tagging and tracking. The most recent advances in biosensing with WGM microlasers are described in this review. We cover the basic concepts of WGM resonators, the integration of gain media into various active WGM sensors and devices, and the cutting-edge advances in photonic devices for micro- and nanoprobing of biological samples that can be integrated with WGM lasers.

Bioresponsive microlasers with tunable lasing wavelength

Lasing particles are emerging tools for amplifying light-matter interactions at the biointerface by exploiting its strong intensity and miniaturized size. Recent advances in implementing laser particles into living cells and tissues have opened a new frontier in biological imaging, monitoring, and tracking. Despite remarkable progress in micro- and nanolasers, lasing particles with surface functionality remain challenging due to the low mode-volume while maintaining a high Q-factor. Herein, we report the novel concept of bioresponsive microlasers by exploiting interfacial energy transfer based on whispering-gallery-mode (WGM) microdroplet cavities. Lasing wavelengths were manipulated by energy transfer-induced changes of a gain spectrum resulting from the binding molecular concentrations at the cavity surface. Both protein-based and enzymatic-based interactions were demonstrated, shedding light on the development of functional microlasers. Finally, tunable lasing wavelengths over a broad spectral range were achieved by selecting different donor/acceptor pairs. This study not only opens new avenues for biodetection, but also provides deep insights into how molecules modulate laser light at the biointerface, laying the foundation for the development of smart bio-photonic devices at the molecular level.

Reconfigurable Integrated Optofluidic Droplet Laser Arrays

Optofluidic lasers are an emerging technology for the development of miniaturized light sources and biological and chemical sensors. However, most optofluidic lasers demonstrated to date are operated at the single optical cavity level, which limits their applications in high-throughput biochemical sensing, high-speed wavelength switching, and on-chip spectroscopic analysis. Here, we demonstrated an optofluidic droplet laser array on a silicon chip with integrated microfluidics, in which four individual droplet optical cavities are generated and controlled by a 2 × 2 nozzle array. Arrays of droplets with a diameter ranging from 115 to 475 μm can be generated, removed, and regenerated on demand. The lasing threshold of the droplet laser array is in the range of 0.63-2.02 μJ/mm². An image-based lasing threshold analysis method is developed, which enables simultaneous lasing threshold measurement for all laser units within the laser array using a low-cost camera. Compared to the conventional spectrum-based threshold analysis method, the lasing threshold obtained from the image-based method showed consistent results. Our droplet laser array is a promising technology in the development of cost-effective and integrated coherent light source on a chip for point-of-care applications.

Mass production of thin-walled hollow optical fibers enables disposable optofluidic laser immunosensors

Disposable biosensors are of great importance in disease diagnosis due to their inherent merits of no cross-contamination and ease of use. Optofluidic laser (OFL) sensors are a new category of sensitive biosensors; however, it is challenging to cost-effectively mass-produce them to achieve disposability. Here, we report a disposable optofluidic laser immunosensor based on thin-walled hollow optical fibers (HOFs). Using a fiber draw tower, the fabrication parameters, including drawing speed and gas flow rate, are explored, and the HOF geometry is precisely controlled, which allows identical laser microring resonators to be distributed along the fibers. The disposable OFL immunosensor detects the protein concentration in the HOF through a wash-free immunoassay. Enabled by the disposable sensors, the statistical characteristics of 80 tests for each concentration greatly reduces the bioassay uncertainty. A low coefficient of variation (CV) of 3.3% confirms the high reproducibility of the disposable HOF-OFL sensors, and the mean of the normal distribution of the logarithmic OFL intensity serves as the sensing output. A limit of detection of 11 nM within a short assay time of 15 min is achieved. These disposable immunosensors possess the advantages of low cost, high reproducibility, fast assay, and low-volume consumption of sample and reagents. We believe that this work will inspire disposable optofluidics through the mass production of multifunctional microstructured optical fibers.

Chromatin laser imaging reveals abnormal nuclear changes for early cancer detection

We developed and applied rapid scanning laser-emission microscopy (LEM) to detect abnormal changes in cell nuclei for early diagnosis of cancer and cancer precursors. Regulation of chromatins is essential for genetic development and normal cell functions, while abnormal nuclear changes may lead to many diseases, in particular, cancer. The capability to detect abnormal changes in "apparently normal" tissues at a stage earlier than tumor development is critical for cancer prevention. Here we report using LEM to analyze colonic tissues from mice at-risk for colon cancer (induced by a high-fat diet) by detecting pre-polyp nuclear abnormality. By imaging the lasing emissions from chromatins, we discovered that, despite the absence of observable lesions, polyps, or tumors under stereoscope, high-fat mice exhibited significantly lower lasing thresholds than low-fat mice. The low lasing threshold is, in fact, very similar to that of adenomas and is caused by abnormal cell proliferation and chromatin deregulation that can potentially lead to cancer. Our findings suggest that conventional detection methods, such as colonoscopy followed by histopathology, by itself, may be insufficient to reveal hidden or early tumors under development. We envision that this innovative work will provide new insights into LEM and support existing tools for early tumor detection in clinical diagnosis, and fundamental biological and biomedical research of chromatin changes at the biomolecular level of cancer development.

A Tunable Optofluidic Microlaser in a Photostable Conjugated Polymer

The optofluidic laser has become an important platform for biological sensing and medical diagnosis. To date, fluorescent dyes and proteins have been widely utilized as gain materials for biological analysis due to their good biocompatibility, but the limited photostability restricts their reliability and sensitivity. Here, an optofluidic microlaser with an ultralow threshold down to 7.8 µJ cm^-2 in the ultrahigh-Q whispering-gallery microcavity, which is filled with a biocompatible conjugated polymer, is demonstrated. This conjugated polymer exhibits a significant enhancement in the lasing stability compared with a typical laser dye (Nile red). In the experiment, after 20 min of illumination with the excitation intensity of 23.2 MW cm^-2 , the lasing intensity of the conjugated polymer experiences a decrease of less than 10%, while the lasing feature of Nile red completely disappears. Additionally, by mechanically stretching the resonator, the lasing frequency can be fine-tuned with the range of about 2 nm, exceeding the free spectral range of the resonator.

Tunable sub-kHz single-mode fiber laser based on a hybrid microbottle resonator

We experimentally demonstrated an all-optical tunable sub-kHz single-mode fiber laser based on an ultrahigh-quality (Q)-factor hybrid microbottle resonator. The wavelength tunability is a very important function for fiber lasers, and the all-optical tuning method has rarely been proposed. Here, we use the iron-oxide-nanoparticle-coated silica microbottle resonator with a Q factor of 1.8×10⁸ as the feedback element of the fiber ring laser and suppress the higher-order modes of the microresonator to achieve single-mode lasing with a linewidth of ∼500  Hz and a signal-to-noise ratio of 49 dB. Iron oxide nanoparticles are coated on the tapered area of the microbottle resonator and the control light is fed through the axial direction of the microbottle. The lasing wavelength of the fiber laser can be all-optically and linearly tuned with a range of 2.7 nm due to the strong photothermal effect of iron oxide nanoparticles. With such an excellent tunability and a narrow linewidth, this single-mode fiber laser has great potential in applications, such as optical spectroscopy, sensing, and signal processing.

Advances of Optofluidic Microcavities for Microlasers and Biosensors

Optofluidic microcavities with high Q factor have made rapid progress in recent years by using various micro-structures. On one hand, they are applied to microfluidic lasers with low excitation thresholds. On the other hand, they inspire the innovation of new biosensing devices with excellent performance. In this article, the recent advances in the microlaser research and the biochemical sensing field will be reviewed. The former will be categorized based on the structures of optical resonant cavities such as the Fabry⁻Pérot cavity and whispering gallery mode, and the latter will be classified based on the working principles into active sensors and passive sensors. Moreover, the difficulty of single-chip integration and recent endeavors will be briefly discussed.

Amplifying fluorescence signal contrast of aptamer-modified microspheres inspired by whispering-gallery mode lasing

We propose a new sensor design that maximizes fluorescence contrast, inspired by whispering-gallery mode lasing (WGM). Aptamer-modified glass microspheres (cf. 1–38 μm) and thrombin are used as a model sensory cavity and target protein, respectively. Two types of microsphere are prepared to compare fluorescence contrast: turn-on and turn-off types by using fluorophore-labeled target protein and fluorophore-labeled DNA duplex with thrombin-binding aptamer (TBA), respectively. For the turn-on type, a fluorescence increase was detected in the presence of 100 nM thrombin, but signal amplification upon excitation power did not occur. As for the turn-off type, in the absence of thrombin, fluorophores are densely populated around the microsphere, leading to fluorescence confinement and intensity amplification upon increasing the pump intensity. By adding thrombin, a complementary strand of TBA is dissociated from the microsphere, and the G-quadruplex structure of the thrombin aptamer recognizes and binds to thrombin. As the fluorophore density around the microsphere decreases, fluorescence amplification based on WGM resonance is halted, resulting in 29-fold enhanced contrast under increased excitation power.

We propose a new sensor design that maximizes fluorescence contrast, inspired by whispering-gallery mode lasing (WGM).

Reproducible fiber optofluidic laser for disposable and array applications

Disposable sensors are widely used in biomedical detection due to their inherent safety, ease of use and low cost. An optofluidic laser is a sensitive bioassay platform; however, demonstrating its fabrication cheaply and reproducibly enough for disposable use has been challenging. Here, we report a low-cost, reproducible fiber optofluidic laser (FOFL) using a microstructured optical fiber (MOF). The MOF not only supports the whispering gallery modes for lasing but also serves as a microfluidic channel for sampling the liquid gain medium via capillary force. Because of the precise control of its geometry (δ < 0.4%) during the fiber-drawing process, good reproducibility in laser intensity (δ = 6.5%) was demonstrated by changing 10 sections of the MOF. The strong coupling between the in-fiber resonator and gain medium enables a low threshold of 3.2 μJ mm^-2. The angular dependence of the laser emission was observed experimentally and analyzed with numerical simulations. An array of the FOFLs was also demonstrated. This technology has great potential for low-cost bioassay applications.

Biomaterial microlasers implantable in the cornea, skin, and blood

Stand-alone laser particles that are implantable into biological tissues have potential to enable novel optical imaging, diagnosis and therapy. Here we demonstrate several types of biocompatible microlasers and their lasing action within biological systems. Dye-doped polystyrene beads were embedded in the cornea and optically pumped to generate narrowband emission. We fabricated microbeads with poly(lactic-co-glycolic acid) and poly(lactic acid)-substances approved for medical use-and demonstrate lasing from within tissues and whole blood. Furthermore, we demonstrate biocompatible cholesterol-derivative microdroplet lasers via self-assembly to an onion-like radially-resonant photonic crystal structure.

Molding Photonic Boxes into Fluorescent Emitters by Direct Laser Writing

Direct laser writing of photonic boxes into active layers of biologically produced recombinant fluorescent protein in optical microcavities is demonstrated. Irradiation with laser light above the photobleaching threshold induces photonic confinement potentials on the order of 40 meV. The technique provides high spatial selectivity and enables room-temperature lasing in protein rings, and circular and elliptical pillars with customized beam shapes.

Versatile tissue lasers based on high-Q Fabry-Pérot microcavities

Biolasers are an emerging technology for next generation biochemical detection and clinical applications. Progress has recently been made to achieve lasing from biomolecules and single living cells. Tissues, which consist of cells embedded in an extracellular matrix, mimic more closely the actual complex biological environment in a living body and therefore are of more practical significance. Here, we developed a highly versatile tissue laser platform, in which tissues stained with fluorophores are sandwiched in a high-Q Fabry-Pérot microcavity. Distinct lasing emissions from muscle and adipose tissues stained respectively with fluorescein isothiocyanate (FITC) and boron-dipyrromethene (BODIPY), and hybrid muscle/adipose tissue with dual staining were achieved with a threshold of only ∼10 μJ mm-2. Additionally, we investigated how the tissue structure/geometry, tissue thickness, and staining dye concentration affect the tissue laser. Lasing emission from FITC conjugates (FITC-phalloidin) that specifically target F-actin in muscle tissues was also realized. It is further found that, despite the large fluorescence spectral overlap between FITC and BODIPY in tissues, their lasing emissions could be clearly distinguished and controlled due to their narrow lasing bands and different lasing thresholds, thus enabling highly multiplexed detection. Our tissue laser platform can be broadly applicable to various types of tissues/diseases. It provides a new tool for a wide range of biological and biomedical applications, such as diagnostics/screening of tissues and identification/monitoring of biological transformations in tissue engineering.

Whispering-gallery-mode emission from biological luminescent protein microcavity assemblies

Fluorescence and bioluminescence are widely used to study biological systems from molecular to whole organism level. However, their broadband emission is often a bottleneck for sensitive spectral measurements and multiplexing. To overcome the limitation, the emitters can be coupled with optical cavity modes to generate narrowband spectral features. Here we demonstrate several types of emitter-resonator complexes made of fluorescent or bioluminescent proteins and artificially or naturally formed optical resonators. We engineered cells to express green fluorescent protein (GFP) fused with ABHD5, which binds to oil or lipid droplets supporting whispering gallery modes (WGM). The genetically-integrated complexes feature well-defined WGM spectral peaks. We measured WGM peaks from GFP-coated BaTiO₃ beads (2.56 μm in diameter) during mitosis. Finally, we demonstrate cavity-enhanced bioluminescence using luciferase-coated beads and biochemical excitation. The ability to tailor spontaneous emission by cavity resonance inside biological systems should have applications in biological sensing, imaging and cell tagging.

Digital DNA detection based on a compact optofluidic laser with ultra-low sample consumption

DNA lasers self-amplify optical signals from a DNA analyte as well as thermodynamic differences between sequences, allowing quasi-digital DNA detection. However, these systems have drawbacks, such as relatively large sample consumption and complicated dye labelling. Moreover, although the lasing signal can detect the target DNA, it is superimposed on an unintended fluorescence background, which persists for non-target DNA samples as well. From an optical point of view, it is thus not truly digital detection and requires spectral analysis to identify the target. In this work, we propose and demonstrate an optofluidic laser that has a single layer of DNA molecules as the gain material. A target DNA produces intensive laser emission comparable to existing DNA lasers, while any unnecessary fluorescence background is successfully suppressed. As a result, the target DNA can be detected with a single laser pulse, in a truly digital manner. Since the DNA molecules cover only a single layer on the surface of the laser microcavity, the DNA sample consumption is a few orders of magnitude lower than that of existing DNA lasers. Furthermore, the DNA molecules are stained by simply immersing the microcavity in the intercalating dye solution, and thus the proposed DNA laser is free of any complex dye-labelling process prior to analysis.

Lasing in blood

Indocyanine green (ICG) is the only near-infrared dye approved by the U.S. Food and Drug Administration for clinical use. When injected in blood, ICG binds primarily to plasma proteins and lipoproteins, resulting in enhanced fluorescence. Recently, the optofluidic laser has emerged as a novel tool in bio-analysis. Laser emission has advantages over fluorescence in signal amplification, narrow linewidth, and strong intensity, leading to orders of magnitude increase in detection sensitivity and imaging contrast. Here we successfully demonstrate, to the best of our knowledge, the first ICG lasing in human serum and whole blood with the clinical ICG concentrations and the pump intensity far below the clinically permissible level. Furthermore, we systematically study ICG laser emission within each major serological component (albumins, globulins, and lipoproteins) and reveal the critical elements and conditions responsible for lasing. Our work marks a critical step toward eventual clinical and biomedical applications of optofluidic lasers using FDA approved fluorophores, which may complement or even supersede conventional fluorescence-based sensing and imaging.

An exciton-polariton laser based on biologically produced fluorescent protein

Under adequate conditions, cavity polaritons form a macroscopic coherent quantum state, known as polariton condensate. Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction with each other but stronger coupling to light, which recently enabled the first realization of a polariton condensate at room temperature. However, this required ultrafast optical pumping, which limits the applications of organic polariton condensates. We demonstrate room temperature polariton condensates of cavity polaritons in simple laminated microcavities filled with biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating polariton condensation under conventional nanosecond pumping. Condensation is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence, and the presence of a second threshold at higher excitation density that is associated with the onset of photon lasing.

Portable IR dye laser optofluidic microresonator as a temperature and chemical sensor

A compact and portable optofluidic microresonator has been fabricated and characterized. It is based on a Fabry-Perot microcavity consisting essentially of two tailored dichroic Bragg mirrors prepared by reactive magnetron sputtering deposition. The microresonator has been filled with an ethanol solution of Nile-Blue dye. Infrared laser emission has been measured with a pump threshold as low as 0.12 MW/cm² and an external energy conversion efficiency of 41%. The application of the device as a temperature and a chemical sensor is demonstrated. Small temperature variations as well as small amount of water concentrations in the liquid laser medium are detected as a shift of the resonant laser modes.

Optofluidic chlorophyll lasers

Chlorophylls are essential for photosynthesis and also one of the most abundant pigments on earth. Using an optofluidic ring resonator of extremely high Q-factors (>10(7)), we investigated the unique characteristics and underlying mechanism of chlorophyll lasers. Chlorophyll lasers with dual lasing bands at 680 nm and 730 nm were observed for the first time in isolated chlorophyll a (Chla). Particularly, a laser at the 730 nm band was realized in 0.1 mM Chla with a lasing threshold of only 8 μJ mm(-2). Additionally, we observed lasing competition between the two lasing bands. The presence of laser emission at the 680 nm band can lead to quenching or significant reduction of laser emission at the 730 nm band, effectively increasing the lasing threshold for the 730 nm band. Further concentration-dependent studies, along with theoretical analysis, elucidated the mechanism that determines when and why the laser emission band appears at one of the two bands, or concomitantly at both bands. Finally, Chla was exploited as the donor in fluorescence resonance energy transfer to extend the laser emission to the near infrared regime with an unprecedented wavelength shift as large as 380 nm. Our work will open a door to the development of novel biocompatible and biodegradable chlorophyll-based lasers for various applications such as miniaturized tunable coherent light sources and in vitro/in vivo biosensing. It will also provide important insight into the chlorophyll fluorescence and photosynthesis processes inside plants.

Integrating Nanostructured Artificial Receptors with Whispering Gallery Mode Optical Microresonators via Inorganic Molecular Imprinting Techniques

The creation of label-free biosensors capable of accurately detecting trace contaminants, particularly small organic molecules, is of significant interest for applications in environmental monitoring. This is achieved by pairing a high-sensitivity signal transducer with a biorecognition element that imparts selectivity towards the compound of interest. However, many environmental pollutants do not have corresponding biorecognition elements. Fortunately, biomimetic chemistries, such as molecular imprinting, allow for the design of artificial receptors with very high selectivity for the target. Here, we perform a proof-of-concept study to show how artificial receptors may be created from inorganic silanes using the molecular imprinting technique and paired with high-sensitivity transducers without loss of device performance. Silica microsphere Whispering Gallery Mode optical microresonators are coated with a silica thin film templated by a small fluorescent dye, fluorescein isothiocyanate, which serves as our model target. Oxygen plasma degradation and solvent extraction of the template are compared. Extracted optical devices are interacted with the template molecule to confirm successful sorption of the template. Surface characterization is accomplished via fluorescence and optical microscopy, ellipsometry, optical profilometry, and contact angle measurements. The quality factors of the devices are measured to evaluate the impact of the coating on device sensitivity. The resulting devices show uniform surface coating with no microstructural damage with Q factors above 10⁶. This is the first report demonstrating the integration of these devices with molecular imprinting techniques, and could lead to new routes to biosensor creation for environmental monitoring.

Optofluidic tunable lenses using laser-induced thermal gradient

This paper reports a new design of optofluidic tunable lens using a laser-induced thermal gradient. It makes use of two straight chromium strips at the bottom of the microfluidic chamber to absorb the continuous pump laser to heat up the moving benzyl alcohol solution, creating a 2D refractive index gradient in the entrance part between the two hot strips. This design can be regarded as a cascade of a series of refractive lenses, and is distinctively different from the reported liquid lenses that mimic the refractive lens design and the 1D gradient index lens design. CFD simulation shows that a stable thermal lens can be built up within 200 ms. Experiments were conducted to demonstrate the continuous tuning of focal length from initially infinite to the minimum 1.3 mm, as well as the off-axis focusing by offsetting the pump laser spot. Data analyses show the empirical dependences of the focal length on the pump laser intensity and the flow velocity. Compared with previous studies, this tunable lens design enjoys many merits, such as fast tuning speed, aberration-free focusing, remote control, and enabling the use of homogeneous fluids for easy integration with other optofluidic systems.

Emission shaping in fluorescent proteins: role of electrostatics and π-stacking

We obtained the fluorescence spectrum of the GFP with trajectory simulations, and revealed the role of the protein sidechains in emission shifts.

Demonstration of polarization mode selection and coupling efficiency of optofluidic ring resonator lasers

We demonstrate the polarization mode selection and the dependence of coupling efficiency on polarization state of pump light for an optofluidic ring resonator (OFRR) laser. An optical fiber is chosen to serve as the ring resonator and surrounded by rhodamine 6G dye solution of lower refractive index as the fluidic gain medium. When the ring resonator is pumped by a linearly s-polarized laser, the emitted whispering gallery mode (WGM) lasing is of parallel polarization (TM mode), while p-polarized laser excitation generates a vertically polarized lasing emission (TE mode), both TM and TE mode lasing emission coexist simultaneously if the ring resonator is pumped by the s- and p-mixed polarized light. Further investigation reveals that the lasing intensity of the TM mode is approximately twice that of the TE mode for the same pump energy density, meaning an obvious difference of coupling efficiency on the polarization state of pump light; the experimental results of coupling efficiency are well explained by an induced dipole model.

Optofluidic laser array based on stable high-Q Fabry-Pérot microcavities

We report the development of an optofluidic laser array fabricated on a chip using stable plano-concave Fabry-Pérot (FP) microcavities, which are far less susceptible to optical misalignment during device assembly than the commonly used plano-plano FP microcavities. The concave mirrors in our FP microcavities were created by first generating an array of microwells of a few micrometers in depth and a few tens of micrometers in diameter on a fused silica chip using a CO2 laser, followed by coating of distributed Bragg reflection (DBR) layers. The plano-concave FP microcavity had a Q-factor of 5.6 × 10(5) and finesse of 4 × 10(3), over 100 times higher than those for the FP microcavities in existing optofluidic lasers. 1 mM R6G dye in ethanol was used to test the plano-concave FP microcavities, showing an ultralow lasing threshold of only 90 nJ mm(-2), over 10 times lower than that in the corresponding unstable plano-plano FP microcavities formed by the same DBR coatings on the same chip. Simultaneous laser emission from the optofluidic laser array on the chip and single-mode lasing operation were also demonstrated. Our work will lead to the development of optofluidic laser-based biochemical sensors and novel on-chip photonic devices with extremely low lasing thresholds (nJ mm(-2)) and mode volumes (fL).

NaCl ion detection using a silica toroid microcavity

An optical-microcavity-based sensor is an important building block for an optofluidics system, because it allows us to fabricate small devices with high sensitivity. Here we describe the detection of NaCl and pH in water using a silica toroid microcavity. First we demonstrate the detection of NaCl particles, and show that a detection sensitivity of 0.38 mM is possible with a sample volume of 0.03 nl. Then, we report the detection of NaCl ions in liquid and demonstrate a sensitivity of 3.20 mM and also, in principle, the detection of pH with a sensitivity of 0.14 pH. Finally, we integrate a tapered optical fiber, a silica toroid microcavity, and a fluidic channel for future optofluidics applications.

Monolithic optofluidic ring resonator lasers created by femtosecond laser nanofabrication

We designed, fabricated, and characterized a monolithically integrated optofluidic ring resonator laser that is mechanically, thermally, and chemically robust. The entire device, including the ring resonator channel and sample delivery microfluidics, was created in a block of fused-silica glass using a 3-dimensional femtosecond laser writing process. The gain medium, composed of Rhodamine 6G (R6G) dissolved in quinoline, was flowed through the ring resonator. Lasing was achieved at a pump threshold of approximately 15 μJ mm(-2). Detailed analysis shows that the Q-factor of the optofluidic ring resonator is 3.3 × 10(4), which is limited by both solvent absorption and scattering loss. In particular, a Q-factor resulting from the scattering loss can be as high as 4.2 × 10(4), suggesting the feasibility of using a femtosecond laser to create high quality optical cavities.