Table 1 summarizes the properties of the liquid used.

Therefore, given the stability of the fabrication technique, and given the application as a tunable WGM microlaser, we have carried out self-formation of the microdroplets by enriching the reference RhB:DEG solution with different dielectric materials at different volume fractions. We have used a solution of RhB in DEG (RhB: mM) as the reference liquid for the microcavity self-formation. The energetic conditions, thus defined, allow a reproducibility and stability in the droplet formation, which we have tested for diameters ranging from hundreds of microns to a few millimeters. Therefore, the droplet takes an ellipsoidal shape for γ L ≤ γ PDMS, while a spherical one for γ L ≫ γ PDMS. In fact, considering the Young–Laplace equation (40) in the G surface term of eq 1, we can obtain an inequality on the contact angle θ value, −1 ≤ cos θ < 0. Similarly, the droplet shape and the condition of sphericity, mandatory for the WGM resonances, are guaranteed by a surface tension of the liquid much higher than that of PDMS, γ L ≫ γ PDMS. Indeed, our findings attest that the required microdroplet occurs when the density of the liquid is greater than that of the PDMS, ρ L > ρ PDMS, and its position within the flexible matrix is defined by the liquid density value. Uncured liquid PDMS (ρ PDMS = 0.97 g/ml and γ PDMS = 22–25 mN/m) was used as a flexible support matrix. Specifically, we have used DEG (ρ DEG = 1.12 g/mL and γ DEG = 44.8 mN/m) and water (ρ W = 1.00 g/mL and γ W = 72.8 mN/m) for the liquid phases at room temperature. Here, we have appropriately selected the properties of different liquid phases in order to obtain spherical microdroplets, symmetrically embedded in the liquid PDMS. (30) suggests, in fact, that an interplay between the densities ρ of the different liquid phases and their surface tensions γ allows us to obtain liquid droplets with different properties. Taking these considerations into account, we have carefully chosen the properties of the liquids to obtain the microlasers with the desired shape and composition. (30) All the force terms involved in the formation of the droplet are shown in Figure 1C. The development of the various terms of eq 1 suggests that the interplay between the density and surface tension of the drop solution and the liquid PDMS has a great influence on the encapsulation of the droplet within the flexible matrix. The condition for a spherical shape of the lasing droplet is guaranteed by the density difference (ρ L > ρ PDMS) and the interplay of surface tensions (γ L ≫ γ PDMS). (C) Spontaneous, single-step droplet formation due to a viscoelastic dewetting between two immiscible liquid phases: a gain solution of dye-doped diethylene glycol and a liquid slab of polydimethylsiloxane (PDMS). The pictures show the free-standing structure of both labels which can be realized on multiple substrates, detachable and reusable. (B) Images of the WGM device realized on a polymeric matrix (top) and on a clothes’ tag (bottom). Below, the 3D reconstruction of the optical images of a droplet section outside the elastomeric matrix before encapsulation. Such a polymeric device is realized by a random sequence of microdroplets characterized by different sizes and different compositions of dielectric particles included into the gain medium and compatible with a barcode reading. (A) Photograph of the smart WGM lasing device obtained via the spontaneous self-formation of 3D active microdroplets of a gain solution in a liquid elastomeric matrix. The wide possibility of modulating the emission wavelength of the microlasers by acting on different parameters, such as the cavity size, type and volume fraction of the dielectrics, and gain medium, offers a multitude of spectroscopic encoding schemes for the realization of photonic barcodes and labels to be employed in anticounterfeiting applications and multiplexed bioassays.įigure 1. The innovative lasing emission mechanism proposed here is based on whispering gallery mode emission coupled to random lasing, the latter prompted by the inclusion of dielectric compounds into the active gain medium. We design a flexible device to be used as a smart photonic label which is detachable and reusable on various types of substrates such as paper or fabric. In this work, we realize liquid microlasers with tunable emission by exploiting the self-formation of three-dimensional liquid droplets into a polymeric matrix driven by viscoelastic dewetting. Microscopic lasers represent a promising tool for the development of cutting-edge photonic devices thanks to their ability to enhance light–matter interaction at the microscale.

Spectra logic