Figure 3a,b shows photomicrographs of a single flavin mononucleotide (FMN) microdroplet formed in a hydrophobic microwell.[64] The droplet was also formed by spraying the FMN solution. The core (blue) is visible at the edge of the droplet. d) Emission spectrum of a 45 µm adipocyte showing typical WGM multimodes. The inset represents the fluorescence image of the corresponding cell while irradiated above the radiation threshold. e) Droplet output energy versus different pump energies. f) Confocal fluorescence image of adipocytes in situ in subcutaneous adipose tissue after injection of Nile red dye (yellow) and pumping with a pulsed green laser. g) Laser radiation from inside the tissue is observed during optical excitation with an optical fiber. h) Laser spectrum obtained from tissue through an optical fiber.
One of the pioneering works on polymer microlasers, a microring laser, was developed by Kuwata-Gonokami et al.[87] The microring was formed by immersing a standard glass fiber in a dyed polymer solution, allowing the polymer droplet to cover the fiber by surface tension.
Soft-Matter Microlasers Developed from Biomaterials The use of biological materials in laser devices started in the
Conductive polymers hold great promise for electrically pumped lasers because of their charge transport capability. Electrically pumped lasers are more compact and therefore more attractive compared to traditional optically pumped devices. Much research has been done into the realization of electrically pumped polymer lasers.
Very recently, organic semiconductor lasers have been demonstrated operating in the quasi-continuous wave regime. [146] The use of semiconducting conjugated polymer lasers, which mainly operate in the continuous wave regime, is a crucial step towards the realization of optoelectronic devices based on these fascinating materials. Soft-Matter microlasers developed from biomaterials The use of biological materials in laser devices began in the. Because the laser operated in the diffusive regime, the laser spectrum was quite broad and sharp peaks were not visible.
Random lasing based on biological materials/structures has potential for a variety of applications, such as effective tools for distinguishing malignant tissue from healthy tissue, [169] probing nanoscale structural changes in bone, [170] pH [173] and dopamine sensing . [176]. Although lasing can be achieved in a variety of configurations, including biological structures, it is surprising to see laser emission in something that is alive. Soft-Matter Microlasers: Typical and Emerging Materials As discussed in the previous sections, materials play a crucial role.
Soft-Matter Microlasers: Typical and Novel Materials As discussed in the previous sections, materials play essential
Interestingly, an exciton-polariton laser based on eGFPs has been realized. The finding demonstrates the efficiency of eGFP as an active optical material. The inset shows the laser output energy of a DFB laser as a function of pump fluence. The inset shows the close-up view of the air cavities and silk structure.
Since 1980, PS microspheres have been studied for the generation of WGMs.[49] Lasing emission was reported from dyed PS microspheres in the early 1980s.[191] PS microspheres offer high Q-factor, enabling low-threshold microlasers with a diameter as small as 10 μm.[192] Recently, PS microspheres have been integrated into single cells for intracellular lasers.[45,46] Another interesting polymer is PMMA known as acrylic glass.[193] The chemical structure of PMMA is shown in Figure 14a. LCs are a class of soft matter that combines liquid-like behavior with solid crystal-like ordering that has been used for a wide range of optical applications. in different mesophases such as chiral nematic, chiral smectic and blue phase (Figure 14b).[44] One of the great properties of LCs is the great adaptability to the electric field, making them Moreover, the temperature tunability of LCs is also high, allowing a large shift of the lasing wavelength to be obtained based on a small temperature change.[71] In particular, LC dot lasers are potentially for laser-based display applications (discussed in Sect.
Another biomaterial of interest is DNA - the molecule of life, which has been shown to be a promising photonic material due to its biocompatibility, high transmittance at visible and near-infrared wavelengths.[213] To obtain active components, an amplification medium is needed to incorporate into the DNA. In the early 2000s, Ogata's group was one of the pioneers attempting to use DNA-based lasers and observe amplified spontaneous emission from dye-doped DNA films. [214,215]. In addition, since the architectures of the DFB lasers can dissolve in water, they could observe the physical transition from DFB lasers to random lasers.
Soft-Matter Microlasers: Fabrication Technologies
- Self-Assembling Technique
- Solution Printing Technique
- Direct Laser Writing Technique
- Molding Technique
- Electrospinning and Direct Drawing Technique
It can be provided by simply mixing DNA with luminescent molecules or by binding luminescent molecules to the DNA helix. However, this shortcoming can be overcome by new techniques such as ink-jet printing and direct laser writing, which are presented in the next section. It is expected that with the rapid development of solution printing techniques, it will be possible to fabricate more complex photonic microstructures for practical devices.
A droplet of the resist layer was inserted. Figure 15. a) Microscopic images of a CdZnS/ZnS CQD/PMMA nanocomposite droplet (41 wt%) during the drying process. Reproduced with permission.[231] Copyright 2015, American Chemical Society. e) Microscopic images of the self-assembly process that breaks the polymer fiber into many hemispheres. The commercial DLW system is equipped with a femtosecond fiber laser (780 nm wavelength, sub-150 fs pulse duration, 100 MHz repetition rate) to illuminate the photoresist.
Forming takes the rewards of advanced semiconductor nanofabrication and the mechanical flexibility of polymer materials, providing an alternative method for fabricating microlasers. At a critical value, the electrostatic forces can overcome the surface tension of the polymer solution, leading to the ejection of a liquid stream from the top of the roll. Using this simple technique, fibers with diameters from tens of nanometers to several micrometers can be achieved.
Soft-Matter Microlasers: Applications
- Wavelength-Tuning Applications
- Biosensing and Cell Tracking Applications
- Physical and Chemical Sensing Applications
- Display Applications
In both works, the wavelength tuning mechanism was based on modifying the refractive index of the laser cavity. Consequently, the laser mode shifted to shorter wavelengths with the increase of the applied field (Figure 18d).[42] Larger droplets showed better wavelength tuning and a relative wavelength shift of about 3% was recorded from a 16 µm diameter droplet. Due to the inherent high Q-factor, WGM optical microcavities and microlasers are suitable for label-free biosensing applications. For WGM lasers, as shown in Equation (1), the resonance wavelength depends on the cavity size and the refractive index of the surrounding medium.
Experimentally, the authors observed the fluctuation of output spectra with time which revealed the dynamic variations of the cellular stress in living cells (Figure 19a). For example, Sun's group demonstrated strain-induced bidirectional tuning of the WGM resonances, enabling the possibility of bend sensing applications. performed temperature sensing using colored doped oil droplets immersed in aqueous solution.[261]. The presence of SDS affects the orientation of liquid crystal molecules in the microdroplet, which leads to the changes of the laser spectrum.
All laser modes switched by the same amount (Figure 20c). The blue-shift of the laser modes can be due to two main factors: i) the reduction of the effective refractive index of the microbubble and ii) the reduction of the size of the microbubble, both caused by the presence of water vapor. Due to the structural robustness and high photostability, the reversible shift of the laser peaks was highly reproducible (Figure 20e). As a result, the change in laser intensity and eventually the complete suppression of lasing (instead of the change in laser wavelength) are indications of the pH variation.
Summary and Future Prospects
Besides WGM structures, other configurations such as DFB lasers and random lasers are also suitable for certain sensing applications. In general, the applications of soft matter microlasers are very diverse, ranging from tunable laser sources to biosensing and display applications. In terms of wavelength tuning, soft-matter lasers offer better flexibility compared to their conventional semiconductor counterparts.
Moreover, microlasers based on living cells and bacteria are appealing, but they require external cavities.[177,178] Last but not least, most of the available biolasers are of millimeter-scale size (except the microsphere or droplet lasers) at this moment, which hinders their applications for implantation. and biointegration. We also believe that soft matter microcavities and microlasers expand their applications beyond what we have. First, the polymer cavities may be promising for studies of optical chaos[276] and directional emission lasers.[277] These studies rely on the deformed cavities such as elliptical structures that can be easily obtained from soft matter spherical cavities by mechanical deformation.[24] Very recently, Zhang et al.
In these hybrid lasers, the photostability of conventional dye-doped soft matter lasers, especially biolasers, is greatly improved while maintaining their inherent properties such as biocompatibility. Fourth, the incorporation of responsive dyes into microlasers for continuous analyte monitoring in vivo would be very beneficial for biomedical applications.[280] Finally, soft matter microlasers fabricated from disordered media would be an exciting topic for research. So far, this type of small random lasers has primarily been obtained using semiconductor materials rather than soft matter [281,282] It is expected that such micrometer-sized random lasers can be much more cost-effective compared to conventional microlasers.
Acknowledgements
For example, the laser wavelength of LC lasers can be easily tuned by mechanical strain, temperature and electric field [44]. For sensing applications, the ability to produce lasers inside living cells and use them as intracellular probes is unique [45]. Recently, intracellular plasmonic nanolasers, [273] semiconductor nanowires [274] and nanodiscs [275] have also been applied for detecting cellular environmental changes and labeling cells.
With the development of new low-threshold and efficient laser sources, it is expected that microlasers will soon be applied for in vitro and in vivo biomedical applications. For example, despite its unique properties, such as biocompatibility and biodegradability, biomaterials used for photonic applications typically require complicated synthesis steps, leading to relatively high costs. Another problem is that the emitting biomaterials generally have low quantum yield and photoinstability. Second, the study of strong coupling between exciton and photon is also an interesting topic.
Conflict of Interest
Keywords
