Single-mode biological distributed feedback laser

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.

Cellular lasers for cell imaging and biosensing

The possibility to produce laser action involving biomaterials, in particular (single) biological cells, has fostered the development of cellular lasers as a novel approach in biophotonics. In this respect, cells that are engineered to carry gain medium (e.g., fluorescent dyes or proteins) are placed inside an optical cavity (i.e., typically a sandwich of highly reflective mirrors), allowing the generation of stimulated emission upon sufficient optical pumping. In another scenario, micron-sized optical resonators supporting whispering-gallery mode (WGM) or semiconductor-based laser probes can be internalized by the cells and support light amplification. This review summarizes the recent advances in the fields of biolasers and cellular lasers, and most importantly, highlights their potential applications in the fields of in vitro and in vivo cell imaging and analysis. They include biosensing (e.g., in vitro detection of sodium chloride (NaCl) concentration), cancer cell imaging, laser-emission-based microscope, cell tracking, cell distinction study, and tissue contraction monitoring in zebrafish. Lastly, several fundamental issues in developing cellular lasers including laser probe fabrication, biocompatibility of the system, and alteration of local refractive index of optical cavities due to protein absorption or probe aggregation are described. Cellular lasers are foreseen as a promising tool to study numerous biological and biophysical phenomena. STATEMENT OF SIGNIFICANCE: Biolasers are generation of laser involving biological materials. Biomaterials, including single cells, can be engineered to incorporate laser probes or fluorescent proteins or fluorophores, and the resulting light emission can be coupled to optical resonator, allowing generation of cellular laser emission upon optical pumping. Unlike fluorescence, this stimulated emission is very sensitive and is capable of detecting small alterations in the optical property of the cells and their environment. In this review, recent development and applications of cellular lasers in the fields of in vitro and in vivo cell imaging, cell tracking, biosensing, and cell/tissue analysis are highlighted. Several challenges in developing cellular lasers including probe fabrication and biocompatibility as well as alteration of cellular environment are explained.

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.

Protein-based microsphere biolasers fabricated by dehydration

Biolasers made of biological materials have attracted considerable research attention due to their biocompatibility and biodegradability, and have the potential for biosensing and biointegration. However, the current fabrication methods of biolasers suffer from several limitations, such as complicated processing, time-consuming and environmentally unfriendly nature. In this study, a novel approach with green processes for fabricating solid-state microsphere biolasers has been demonstrated. By dehydration via a modified Microglassification™ technology, dye-doped bovine serum albumin (BSA) droplets could be quickly (less than 10 minutes) and easily changed into solid microspheres with diameters ranging from 10 μm to 150 μm. The size of the microspheres could be effectively controlled by changing either the concentration of the BSA solution or the diameter of the initial droplets. The fabricated microspheres could act as efficient microlasers under an optical pulse excitation. A lasing threshold of 7.8 μJ mm-2 and a quality (Q) factor of about 1700 to 3100 were obtained. The size dependence of lasing characteristics was investigated, and the results showed a good agreement with whispering gallery mode (WGM) theory. Our findings contribute an effective technique for the fabrication of high-Q factor microlasers that may be potential for applications in biological and chemical sensors.

Optical dispersion control in surfactant-free DNA thin films by vitamin B2 doping

A new route to systematically control the optical dispersion properties of surfactant-free deoxyribonucleic acid (DNA) thin solid films was developed by doping them with vitamin B2, also known as riboflavin. Surfactant-free DNA solid films of high optical quality were successfully deposited on various types of substrates by spin coating of aqueous solutions without additional chemical processes, with thicknesses ranging from 18 to 100 nm. Optical properties of the DNA films were investigated by measuring UV-visible-NIR transmission, and their refractive indices were measured using variable-angle spectroscopic ellipsometry. By doping DNA solid films with riboflavin, the refractive index was consistently increased with an index difference Δn ≥ 0.015 in the spectral range from 500 to 900 nm, which is sufficiently large to make an all-DNA optical waveguide. Detailed correlation between the optical dispersion and riboflavin concentration was experimentally investigated and thermo-optic coefficients of the DNA-riboflavin thin solid films were also experimentally measured in the temperature range from 20 to 85 °C, opening the potential to new bio-thermal sensing applications.

Flexible and ultra-lightweight polymer membrane lasers

Organic semiconductors enable the fabrication of a range of lightweight and mechanically flexible optoelectronic devices. Most organic semiconductor lasers, however, have remained rigid until now, predominantly due to the need for a support substrate. Here, we use a simple fabrication process to make membrane-based, substrate-less and extremely thin (<500 nm) organic distributed feedback lasers that offer ultralow-weight (m/A<0.5 gm⁻²) and excellent mechanical flexibility. We show operation of the lasers as free-standing membranes and transfer them onto other substrates, e.g. a banknote, where the unique lasing spectrum is readily read out and used as security feature. The pump thresholds and emission intensity of our membrane lasers are well within the permissible exposures for ocular safety and we demonstrate integration on contact lenses as wearable security tags.

Organic semiconductors provide a platform for flexible lasers, but these are still produced on rigid, thick substrates. Here, Karl et al. develop a method to make freestanding membrane lasers that can be transferred onto any substrate and show that these could be used as anti-counterfeiting labels.

A robust tissue laser platform for analysis of formalin-fixed paraffin-embedded biopsies

Laser emission-based detection and imaging technology has attracted significant interest in biomedical research due to its high sensitivity, narrow linewidth, and superior spectral and spatial resolution. Recent advances have further revealed the potential to use laser emission to investigate chromatin dynamics, as well as to diagnose cancer tissues based on nuclear biomarkers. To move the laser emission based detection technology a step further towards practical use, in this work, we developed a highly robust tissue laser platform by microfabricating an SU8 spacer with a fixed height on the top mirror of the Fabry-Pérot (FP) cavity, which allows generation of reproducible and stable lasing results regardless of tissue thickness. Then we applied this platform to achieve lasing emission from formalin-fixed, paraffin-embedded (FFPE) lung tissues, which account for an overwhelming fraction of tissues collected for research and clinical use worldwide. We further showed that the cancer and normal FFPE lung tissues can be distinguished by their respective lasing thresholds. Two different tissue thicknesses (10 μm and 5 μm) commonly used in pathological labs were explored. Finally, we tested three additional types of tissues (colon, stomach, and breast) that were prepared independently by lab technicians in a pathology lab in China and shipped to the US in order to validate the general applicability and practicality of the laser emission-based technology as well as the corresponding sample preparation protocol and the tissue laser platform. Our work will not only vastly broaden the applications of laser emission-based detection/imaging technology but also help translate it from the laboratory to an automated system for clinical practice that may eventually benefit biomedicine and biological research.

Lasing with cell-endogenous fluorophores: parameters and conditions

The notion of lasing with biologics has recently been realized and has rapidly developed with the collective objective of creating lasers in vivo. One major limitation of achieving this is the requirement of exogenous dyes and fluorescent materials. We thus investigate for the first time the possibility of lasing unlabelled cells, using just cell-endogenous fluorophores - the source of cell autofluorescence. In this work, we theoretically studied the lasing potential and efficiency of flavins and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) using a dye lasing model based on coupled rate equations. Analytical solutions for one- and two-photon pumped system were used in multi-parameter studies. We found that at physiological conditions, the more abundant NAD(P)H can be lased with a cavity quality factor of 10⁵. We then recommended the tuning of intersystem crossing to make the lasing of flavins feasible even at their low physiological concentrations. Under conditions of reduced intersystem crossing, we concluded that it is more practical to lase unlabelled cells using flavins, because lasing thresholds and cavity quality factors were both at least an order lower. We also note the higher threshold requirements and lower efficiencies of two-photon pumping, but recognize its potential for realizing lasing in vivo.

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.

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.

Optofluidic ring resonator laser with an edible liquid laser gain medium

We demonstrate a biocompatible optofluidic laser with an edible liquid laser gain medium, made of riboflavin dissolved in water. The proposed laser platform is based on a pulled-glass-capillary optofluidic ring resonator (OFRR) with a high Q-factor, resulting in a lasing threshold comparable to that of conventional organic dye lasers that are mostly harmful, despite the relatively low quantum yield of the riboflavin. The proposed biocompatible laser can be realized by not only a capillary OFRR, but also by an optical-fiber-based OFRR that offers improved mechanical stability, and is promising technology for application to in vivo bio-sensing.

Flavin mononucleotide biomolecular laser: longitudinal mode structure, polarization, and temporal characteristics as probes of local chemical environment

A detailed characterization of the flavin mononucleotide (FMN) biomolecular laser, optically pumped in a stable resonator, is reported here. Photoexcitation of the molecule at 355 nm results in lasing over the ~566.5-573.5 nm spectral region, and the threshold pump energy density is measured to be 110 ± 10 µJ/mm² for a 10 mM FMN/water solution. Over twenty longitudinal modes are observed when the cavity length L and the energy pump fluence E_p are 375 µm and 300 µJ/mm², respectively. Partial substitution of glycerol for water as the solvent results in a factor of four reduction in the threshold pump energy fluence (to < 30 µJ/mm²) and a quadrupling of the slope efficiency. This effect is attributed to the O₂ - mediated photoconversion of FMN molecules in the triplet state to the singlet species. For pump intensities a factor of 2.5 above threshold, the laser pulse width is ~2 ns FWHM, and the output intensity decays exponentially with a photon lifetime of 1.7 ns. The addition of glycerol to a FMN/water solution also suppresses s-polarized emission (yielding P = 0.78 ± 0.08), presumably as a result of the inhibition of FMN rotational diffusion. The sensitivity of the spectral and optical properties of this and other biomolecular lasers to the chemical environment underscores the value of coherent emission as a biochemical or biomedical diagnostic tool, particularly insofar as molecule-molecule interactions are concerned.

Cellular dye lasers: lasing thresholds and sensing in a planar resonator

Biological cell lasers are promising novel building blocks of future biocompatible optical systems and offer new approaches to cellular sensing and cytometry in a microfluidic setting. Here, we demonstrate a simple method for providing optical gain by using a variety of standard fluorescent dyes. The dye gain medium can be located inside or outside a cell, or in both, which gives flexibility in experimental design and makes the method applicable to all cell types. Due to the higher refractive index of the cytoplasm compared to the surrounding medium, a cell acts as a convex lens in a planar Fabry-Perot cavity. Its effect on the stability of the laser cavity is analyzed and utilized to suppress lasing outside cells. The resonance modes depend on the shape and internal structure of the cell. As proof of concept, we show how the laser output modes are affected by the osmotic pressure.

Lasing within Live Cells Containing Intracellular Optical Microresonators for Barcode-Type Cell Tagging and Tracking

We report on a laser that is fully embedded within a single live cell. By harnessing natural endocytosis of the cell, we introduce a fluorescent whispering gallery mode (WGM) microresonator into the cell cytoplasm. On pumping with nanojoule light pulses, green laser emission is generated inside the cells. Our approach can be applied to different cell types, and cells with microresonators remain viable for weeks under standard conditions. The characteristics of the lasing spectrum provide each cell with a barcode-type label which enables uniquely identifying and tracking individual migrating cells. Self-sustained lasing from cells paves the way to new forms of cell tracking, intracellular sensing, and adaptive imaging.

A fully biocompatible single-mode distributed feedback laser

A fully biocompatible laser would be attractive in many aspects of biomedical research. Here we report a single-mode biocompatible distributed feedback laser consisting of silk, riboflavin and silver in the form of a freestanding film. The distributed feedback structure has a large surface area and flexibility. The fabricated laser exhibited single-mode lasing at a wavelength of 495 nm.