Bio-switchable optofluidic lasers based on DNA Holliday junctions

Device parameter to evaluate exciton energy transfer in organic whispering-gallery-mode microresonators and its dependence on the amplified spontaneous emission threshold

Exciton energy transfer in organic whispering-gallery-mode (WGM) resonators and its effect on the amplified spontaneous emission (ASE) threshold have been investigated using the stilbene-based energy donor 4,4'-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) and the coumarin-based energy acceptor 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino[9,9a,1gh]coumarin (C545T). Using the stacked-layer structure of BSB-Cz/C545T/BSB-Cz, we fabricated bowl-shaped microresonators on silica microspheres with a total thickness of 250 nm fixing the thickness of the C545T layer to 1 nm. The ASE threshold depended on the thicknesses of the top and bottom BSB-Cz layers, which affect the magnitude of the energy transfer. To assess the relationship between the ASE threshold and energy transfer, we developed a device parameter to evaluate the magnitude of the energy transfer by formulating the rate equations. We found that ASE easily occurs under the condition that the C545T molecules become unable to accept energy from the BSB-Cz excitons owing to the high exciton density of C545T, and that the ASE threshold decreases with decreasing device parameter. The device parameter is useful for optimizing microresonator structures in multi-component organic WGM resonators that utilize energy transfer.

Monitoring Various Bioactivities at the Molecular, Cellular, Tissue, and Organism Levels via Biological Lasers

The laser is considered one of the greatest inventions of the 20th century. Biolasers employ high signal-to-noise ratio lasing emission rather than regular fluorescence as the sensing signal, directional out-coupling of lasing and excellent biocompatibility. Meanwhile, biolasers can also be micro-sized or smaller lasers with embedded/integrated biological materials. This article presents the progress in biolasers, focusing on the work done over the past years, including the molecular, cellular, tissue, and organism levels. Furthermore, biolasers have been utilized and explored for broad applications in biosensing, labeling, tracking, bioimaging, and biomedical development due to a number of unique advantages. Finally, we provide the possible directions of biolasers and their applications in the future.

Perylene-Based Chromophore as a Versatile Dye for Light Amplification

One of the challenges for modern optoelectronics is to find versatile, easily adaptable components for novel laser-based technologies. A very attractive perylene-derivative chromophore in different organic matrices for high-performance light amplification is discussed and outlined. Our approach demonstrates the outstandingly compatible laser dye and a viable strategy to provide an effective optical gain for stimulated emission enhancement. Through structural control, we produce simple optical devices embedded in organic matrices, such as poly(methyl methacrylate), nematic liquid crystalline (NLC) mixture, and a hybrid emulsion system (poly(vinyl alcohol) PVA + NLC mesophase). Importantly, we investigate and compare the spectroscopy of differently constructed organic systems in terms of stimulated-emission thresholds and light amplification process efficiency. Moreover, we report the effects of tunability for LC cells by an applied external electric field stimulus. Future directions of laser systems are outlined with an emphasis on the role of the perylene derivative. The studies meet current challenges in the field of modern organic technologies dedicated to various optoelectronic systems, including touch screens, displays, and Li-Fi networks.

Smart Protein-Based Biolasers: An Alternative Way to Protein Conformation Detection

Detecting conformational changes in protein is imperative due to its major role in neurodegenerative disorders. Here, we propose an alternative strategy for monitoring the structural change of proteins based on biological microlasers. Smart responsive protein-based microscale biolasers were constructed by incorporating organic gain medium into the microspheres of silk fibroin via emulsion-solvent evaporation. The lasing characteristic of the biolasers exhibited a sensitive response to the structural transformation of the silk fibroin. With narrowed linewidth, the as-prepared biolasers as sensing signals enable highly sensitive protein conformation detection. These results offer an effective approach to monitoring the protein conformational changes and provide valuable guidance for a better understanding of the relationship between bio-microstructures and their photonic properties.

DNA Self-Switchable Microlaser

Advances in switchable microlasers have emerged as a building block with immense potential in controlling light-matter interactions and integrated photonics. Compared to artificially designed interfaces, a stimuli-responsive biointerface enables a higher level of functionalities and versatile ways of tailoring optical responses at the nanoscale. However, switching laser emission with biological recognition has yet to be addressed, particularly with reversibility and wavelength tunability over a broad spectral range. Here we demonstrate a self-switchable laser exploiting the biointerface between label-free DNA molecules and dye-doped liquid crystal matrix in a Fabry-Perot microcavity. Laser emission switching among different wavelengths was achieved by utilizing DNA conformation changes as the switching power, which alters the orientation of the liquid crystals. Our findings demonstrate that different concentrations of single-stranded DNA lead to different temporal switching of lasing wavelengths and intensities. The lasing wavelength could be reverted upon binding with the complementary sequence through DNA hybridization process. Both experimental and theoretical studies revealed that absorption strength is the key mechanism accounting for the laser shifting behavior. This study represents a milestone in achieving a biologically controlled laser, shedding light on the development of programmable photonic devices at the sub-nanoscale by exploiting the complexity and self-recognition of biomolecules.

Virus lasers for biological detection

The selective amplification of DNA in the polymerase chain reaction is used to exponentially increase the signal in molecular diagnostics for nucleic acids, but there are no analogous techniques for signal enhancement in clinical tests for proteins or cells. Instead, the signal from affinity-based measurements of these biomolecules depends linearly on the probe concentration. Substituting antibody-based probes tagged for fluorescent quantification with lasing detection probes would create a new platform for biomarker quantification based on optical rather than enzymatic amplification. Here, we construct a virus laser which bridges synthetic biology and laser physics, and demonstrate virus-lasing probes for biosensing. Our virus-lasing probes display an unprecedented > 10,000 times increase in signal from only a 50% increase in probe concentration, using fluorimeter-compatible optics, and can detect biomolecules at sub-100 fmol mL⁻¹ concentrations.

Many ligand-binding assays still rely on signals that scale linearly with probe concentration. The authors present lasing detection probes with a dye-labelled virus as the gain medium to optically amplify the signal, which could enable much higher signals than for fluorescent quantification.

Multi-color lasing in chemically open droplet cavities

In this paper, we demonstrate FRET-based multicolor lasing within chemically open droplet cavities that allow online modulation of the gain medium composition. To do this, we generated monodisperse microfluidic droplets loaded with coumarin 102 (donor), where the spherical droplets acted as whispering gallery mode (WGM) optical cavities in which coumarin 102 lasing (~ 470 nm) was observed. The lasing color was switched from blue to orange by the introduction of a second dye (acceptor, rhodamine 6 G) into the flowing droplet cavities; subsequent lasing from rhodamine 6 G (~ 590 nm) was observed together with the complete absence of coumarin 102 emission. The ability to control color switching online within the same droplet cavity enables sequential detection of multiple target molecules within or around the cavity. As a demonstration of this concept, we show how the presence of FITC-Dextran and methylene blue (MB) in the medium surrounding the lasing droplets can be sequentially detected by the blue and orange laser respectively. The method is simple and can be extended to a range of water-soluble dyes, thus enabling a wide spectral range for the lasing with the use of a single pump laser source.

π-Electronic Co-crystal Microcavities with Selective Vibronic-Mode Light Amplification: Toward Förster Resonance Energy Transfer Lasing

π-conjugated organic microcrystals often act as optical resonators in which the generated photons in the crystal are confined by the reflection at the crystalline facets and interfere to gain lasing action. Here, we fabricate microcrystals from a mixture of carbon-bridged oligo- para-phenylenevinylenes (COPVs) with energy-donor (D) and energy-acceptor (A) characters. Upon weak excitation of the single D-A co-crystal, Förster resonance energy transfer (FRET) takes place, exhibiting spontaneous emission from A. In contrast, upon strong pumping, stimulated emission occurs before FRET, generating lasing action from D. Lasing occurs with single- and dual-vibronic levels, and the lasing wavelength can be modulated by the doping amount of A. Time-resolved spectroscopic studies reveal that the rate constant of lasing is more than 20 times greater than that of FRET. Furthermore, microcrystals, vertically grown on a Ag-coated substrate, reduce the lasing threshold by one-fourth. This study proposes possible directions toward organic solid FRET lasers with microcrystalline resonators.

Embedding liquid lasers within or around aqueous microfluidic droplets

In this paper, we demonstrate the incorporation of dye-based liquid lasers within or around flowing aqueous microfluidic droplets. In particular, we use dye solutions in benzyl alcohol, and either disperse an ensemble of small (∼20 μm) lasing droplets within large (∼500 μm) aqueous droplets flowing in a simple glass capillary-based microfluidic device, or 'wrap' a thin (∼10 μm) lasing benzyl alcohol shell around larger (∼560 μm) microfluidic aqueous droplets. We experimentally and theoretically characterize the lasing behavior in both cases, which is supported by whispering-gallery mode (WGM) optical resonances at the droplet interfaces. We showcase a simple application of our method, which highlights the advantages of having embedded, spatially segregated laser sources within a droplet containing a model analyte solution. With this method, each microfluidic droplet now functions not only as an isolated experiment flask, but is also capable of on drop sensing that exploits WGM-based lasing, thus expanding the possibilities for online monitoring of biophysical/biochemical processes and sensitive detection of biomolecules in droplet-based microfluidics.

High-resolution and multi-range particle separation by microscopic vibration in an optofluidic chip

An optofluidic chip is demonstrated in experiments for high-resolution and multi-range particle separation through the optically-induced microscopic vibration effect, where nanoparticles are trapped in loosely overdamped optical potential wells created with combined optical and fluidic constraints. It is the first demonstration of separating single nanoparticles with diameters ranging from 60 to 100 nm with a resolution of 10 nm. Nanoparticles vibrate with an amplitude of 3-7 μm in the loosely overdamped potential wells in the microchannel. The proposed optofluidic device is capable of high-resolution particle separation at both nanoscale and microscale without reconfiguring the device. The separation of bacteria from other larger cells is accomplished using the same chip and operation conditions. The unique trapping mechanism and the superb performance in high-resolution and multi-range particle separation of the proposed optofluidic chip promise great potential for a diverse range of biomedical applications.

Optofluidic bioanalysis: fundamentals and applications

Over the past decade, optofluidics has established itself as a new and dynamic research field for exciting developments at the interface of photonics, microfluidics, and the life sciences. The strong desire for developing miniaturized bioanalytic devices and instruments, in particular, has led to novel and powerful approaches to integrating optical elements and biological fluids on the same chip-scale system. Here, we review the state-of-the-art in optofluidic research with emphasis on applications in bioanalysis and a focus on waveguide-based approaches that represent the most advanced level of integration between optics and fluidics. We discuss recent work in photonically reconfigurable devices and various application areas. We show how optofluidic approaches have been pushing the performance limits in bioanalysis, e.g. in terms of sensitivity and portability, satisfying many of the key requirements for point-of-care devices. This illustrates how the requirements for bianalysis instruments are increasingly being met by the symbiotic integration of novel photonic capabilities in a miniaturized system.

Droplet lasers: a review of current progress

It is perhaps surprising that something as fragile as a microscopic droplet could possibly form a laser. In this article we will review some of the underpinning physics as to how this might be possible, and then examine the state of the art in the field. The technology to create and manipulate droplets will be examined, as will the different classes of droplet lasers. We discuss the rapidly developing fields of droplet biolasers, liquid crystal laser droplets and explore how droplet lasers could give rise to new bio and chemical sensing and analysis. The challenges that droplet lasers face in becoming robust devices, either as sensors or as photonic components in the lab on chip devices, is assessed.

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.

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.

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.

Optofluidic FRET lasers using aqueous quantum dots as donors

An optofluidic FRET (fluorescence resonance energy transfer) laser is formed by putting FRET pairs inside a microcavity acting as a gain medium. This integration of an optofluidic laser and the FRET mechanism provides novel research frontiers, including sensitive biochemical analysis and novel photonic devices, such as on-chip coherent light sources and bio-tunable lasers. Here, we investigated an optofluidic FRET laser using quantum dots (QDs) as FRET donors. We achieved lasing from Cy5 as the acceptor in a QD-Cy5 pair upon excitation at 450 nm, where Cy5 has negligible absorption by itself. The threshold was approximately 14 μJ mm(-2). The demonstrated capability of QDs as donors in the FRET laser greatly improves the versatility of optofluidic laser operation due to the broad and large absorption cross section of the QDs in the blue and UV spectral regions. The excitation efficiency of the acceptor molecules through a FRET channel was also analyzed, showing that the energy transfer rate and the non-radiative Auger recombination rate of QDs play a significant role in FRET laser performance.

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.

Optofluidic lasers with a single molecular layer of gain

We achieve optofluidic lasers with a single molecular layer of gain, in which green fluorescent protein, dye-labeled bovine serum albumin, and dye-labeled DNA, are used as the gain medium and attached to the surface of a ring resonator via surface immobilization biochemical methods. It is estimated that the surface density of the gain molecules is on the order of 10(12) cm(-2), sufficient for lasing under pulsed optical excitation. It is further shown that the optofluidic laser can be tuned by energy transfer mechanisms through biomolecular interactions. This work not only opens a door to novel photonic devices that can be controlled at the level of a single molecular layer but also provides a promising sensing platform to analyze biochemical processes at the solid-liquid interface.

All-optical tuning of a magnetic-fluid-filled optofluidic ring resonator

An all-optical tunable optofluidic ring resonator (OFRR) is proposed and experimentally demonstrated. The all-optical control of a silica microresonator is highly attractive, but it is difficult to realize because of the relatively weak Kerr effect and the absence of a plasma dispersion effect of silica. Here, we infuse a silica microcapillary-based optofluidic ring resonator with a magnetic fluid, into which pump light is injected by a fiber taper. Iron oxide nanoparticles dispersed in the magnetic fluid produce a strong pump light absorption, and this leads to a resonance shift of the silica microresonator due to the photothermal effect. To the best of our knowledge, this is the first scheme for all-optical tuning of an OFRR. A tuning sensitivity of up to 0.15 nm mW(-1) and a tuning range of 3.3 nm are achieved. With such excellent performance, the magnetic-fluid-filled OFRR has great potential in filtering, sensing, and signal processing applications.

The potential of optofluidic biolasers

Optofluidic biolasers are emerging as a highly sensitive way to measure changes in biological molecules. Biolasers, which incorporate biological material into the gain medium and contain an optical cavity in a fluidic environment, can use the amplification that occurs during laser generation to quantify tiny changes in biological processes in the gain medium. We describe the principle of the optofluidic biolaser, review recent progress and provide our outlooks on potential applications and directions for developing this technology.