1. Introduction

Microcavities that support whispering gallery modes (WGMs) hold great interest in photonics and optoelectronics because they enable strong light-matter interactions in a small footprint [1–4]. As a result, these photonic cavities have emerged as promising platforms in integrated photonic technology, including high-efficiency optical frequency comb generators [5] and symmetry-breaking-induced nonlinear optics [6]. In addition, WGM resonators are an ideal platform for sensing applications showing great sensibility down to the single-molecule level [7,8]. These cavities are usually solid-state fabricated from a wide variety of dielectric and semiconductor materials [3,9]. Plasmonic-photonic coupling is also interesting because plasmonic nanoparticles (NPs) are characterized by a much superior electromagnetic field confinement, thus enhancing light-matter interaction of WGM cavities [10,11]. Unfortunately, weak interactions of plasmonic NPs with the evanescent field of the solid-state WGM drastically limit their functionality as sensors. On the other hand, “soft” cavities created with liquids or gels enable direct sensing where an optical cavity is both the sensing unit and the sample under analysis, i.e. lab-in-a-droplet [7,12]. In addition, soft cavities could enable strong interactions of a plasmonic NP with the WGMs because of its location inside the cavity mode [11]. These resonators not only allow chemical and biochemical sensing but could also allow several other photonic functionalities, including photocatalytic water splitting [13].

Here we describe a novel method to generate WGM resonators in solution via laser-induced cavitation. A droplet is formed inside the bubble around the NP and generates lasing emission. This process is initiated by focusing a laser (λ = 400 nm) on a solution containing a mixture of dye and plasmonic nanoparticles (Fig. 1(a)). When the NP is captured by the beam, it generates small, sub-micron sized bubbles that self-assemble around the NP. The inclusion of the plasmonic NP in the WGM is a unique feature of this method. Others have suggested that the position of the plasmonic NP with respect to the cavity plays a vital role in coupling light in/out of the cavity [14]. Interestingly, WGMs can be coupled in and out of the cavity using free-space, likely because the NP serves as both a receiving antenna and scatterer. We initially used Au NPs because they have a strong, tunable absorption peaks at visible to near-infrared (NIR) wavelengths [15]. TiN NPs were also examined because it has higher optical losses which is advantageous for photothermal heating. In addition, TiN NPs has a higher melting temperature (>800 °C) and broader SPR covering the visible range and near IR centered at 710 nm [16–18].

 

Fig. 1. Self-assembly of the plasmonic WGM in solution. (a) Schematic illustrating the side-view of the process. The steps I-V are discussed in the text. Green indicates the organic dye dissolved in ethanol, blue indicates the plasmonic nanoparticle, and white indicates the air bubble. (b) Representative images of the top view from the WGM self-assembly process. The red-dashed circle of the step I, II-IV and V indicates the plasmonic NP, the bubbles and the WGM cavity isolated by an air bubble, respectively. Scale bar: 10 µm.

Download Full Size | PDF

2. Experimental section

Coumarin 500 dye was purchased from Exciton. 20-nm-diameter citrate-stabilized Au NPs were purchased from Nanopartz and used without further purification. The extinction spectrum –ln$({T/{T_0}} )$, where T is the transmittance of the NPs dispersed in ethanol, and T₀ is the transmittance of the ethanol, was measured by a spectrometer (Jasco, V670). Extinction spectrum of the Au NPs colloidal suspensions in ethanol exhibiting a plasmon resonance at 521 nm is provided in Supplement 1, Fig. S1. To investigate the influence of the NP geometry, a colloidal suspension of Au nanobipyramids (NBPs) was prepared by seed-mediated growth method as previously described [19]. The Au NPBs were initially coated with cetyltrimethylammonium chloride (CTAC) surfactant, but it was exchanged by exposing the particles to a 0.1 mM solution of O-[2-(3-Mercaptopropionylamino)ethyl]-O'-methylpolyethylene glycol (PEG-SH) to help stabilize the particles in various solvents [20]. The colloidal suspension was centrifuged and redispersed into ethanol. Extinction spectrum is provided in Supplement 1, Fig. S1. Transverse and longitudinal plasmon resonances were located at 528 and 739 nm, respectively.

Emission PL spectra were collected using a confocal reflection microscope with a 20x objective lens and NA of 0.7 coupled to an optical fiber connected to a spectrometer (Acton SpectraPro 300i, Princeton Instruments) with a CCD camera. The trapping and excitation laser at 400 nm with a 130-fs pulse width and 1 kHz repetition rate was focused on the sample with spot size ∼5 µm. A neutral density filter was placed along the optical path to control the laser intensity. The fundamental laser beam at 800 nm was generated by a regenerative amplifier (Spectra-Physics, Spitfire) using Ti:sapphire (Spectra-Physics, Mai Tai) and Nd:YLF (Spectra-Physics, Empower) lasers. PL images were collected using the same lens with a fast camera (Mini AX, Photron) triggered by the laser excitation.

3. Results and discussions

Assembly of a WGM resonator via laser-induced cavitation is shown in Visualization 1. We observed the Au NP in solution using a high-speed camera and noticed bubbles were forming. Then, these transient bubbles coalesced and formed a stable and bigger bubble. Looking more closely we noticed that the NP was still moving suggesting that it was either trapped on the surface of the bubble and/or part of a droplet. To illustrate the self-assembly mechanism, a simple procedure is proposed and illustrated schematically in Fig. 1(a): (I) trapping a plasmonic NP by a focused laser beam; (II) bubble generation around the NP by laser-induced cavitation; (III) bubbles coalescence; (IV) bubble growth just below the cuvette surface; and (V) droplet formation containing a NP isolated by a vapor bubble. In step (I), plasmonic NPs dispersed in ethanol was loaded in a cuvette of 1-mm path length. Then, the sample was fixed on a confocal microscope with a 20x objective lens and NA of 0.7. In step (II), increasing the laser intensity to 580 MW/cm² triggered the formation of vapor bubbles around the trapped NPs. By the same objective lens, the bubbles generated around the NP were recorded using a high-speed camera synchronized with the laser. In steps (III-V), with the coalescence of a few bubbles, a stable bubble is shaped below the cuvette surface. A droplet containing a single NP is isolated from the liquid solution by the vapor bubble on the interface.

Right after the assembly of the droplet, we observed that these droplets generate white-light supercontinuum (SC) using a spectrometer synchronized with the laser. Spectral broadening is a nonlinear optical process that usually requires high pumping thresholds [21,22]. Here, the white-light SC generation was observed at relatively low pumping excitation which indicates strong light-matter interactions provided by the droplet. To better understand the phenomena, we propose an experiment using a laser dye. The solution was prepared with Coumarin 500 (C500) dye in 0.4 mM concentration and 20-nm-diameter Au NPs with a concentration of 2.7 × 10¹¹ mL⁻¹ dispersed in ethanol (details in Supplement 1, Fig. S1).

Several recent experimental and theoretical works have sought to describe the complex hydrodynamical processes of bubble formation via photoexcitation of a plasmonic NP in solution [23–25]. In our 20-nm diameter Au NPs excited at 400 nm, the bubble mechanism is primarily caused by energy deposited by the NP due to resistive losses in the non-radiative decay processes [25,26]. The vapor bubbles in the nanometer range generated by this process have a typical lifetime of a few nanoseconds. To achieve long lasting and larger bubbles of a few micrometers, mass diffusion of the gas into the bubbles may help to promote growth stability. Following the formation of a few bubbles, the interaction among them promotes either coalescence or dissolution [27]. These bubbles have been shown to be stable for days [25]. As for the temperature, the bubbles reach the spinodal temperature for 10-100 ps then rapidly reach thermal equilibrium with the solution [28]. During the stable bubble formation and expansion mechanism underneath the top surface of the cuvette, the movement of the NP creates a snow-plow effect, which shuttles the NP along the bubble interface [29]. In our experiments the NP moves to the interface between the microscopic bubble and the cuvette surface as shown in Visualization 1. It recruits solution from its surroundings to form the droplet isolated from the solution by the bubble.

Figure 2(a) shows room temperature photoluminescence (PL) emission spectra of a ∼4.8 µm diameter droplet of ethanol with C500 containing a single NP. The spectra with different colors are single shots above the lasing threshold obtained right-after the droplet self-assembly process. The PL spectra are composed of numerous sharp emission peaks with equal spacings, on top of broadband spontaneous emission characteristic of C500. The equidistant peaks in the spectra are strongly indicative of WGMs, which is further confirmed by the bright ring provided in Visualization 1. Note that the spectra and images were triggered to the laser excitation at 1 kHz. The shape of the isolated droplet slightly changes from shot to shot due to its liquid form. Thus, the WGM cavity is changed for each shot, leading to variation in the intensity and shape of the sharp peak emissions [30]. To demonstrate the stability of the WGM cavity, three spectra of the WGM emission were shown in the Fig. 2(a). The correspondence of the equidistant peaks in the three spectra suggests that the droplet remains with a constant size. We used the standard WGM model to determine its free spectral range (FSR) [9]

 

Fig. 2. (a) Emission spectra of 20-nm Au NP/Coumarin 500 in ethanol. The spectra with different colors are single shots above the lasing threshold. Peak positions and intensities vary for consecutive pulses due to the oscillation in shape of the cavity. (b) Enlarged emission spectrum of a WGM cavity with a diameter of 4.8 µm. (Scale bar:10 µm) (c) Plot of mode numbers derived from WGM model.

Download Full Size | PDF

To examine this phenomenon, we set up several control experiments. Initially we tried to form these bubble/droplet structures in neat ethanol + C500 but were unable to create these small ∼5.0 µm diameter droplets and hence could not observe PL sharp emission peaks (Supplement 1, Fig. S2). Next step, we dispersed SiO₂ particles in ethanol + dye and, similarly, were unable to observe PL sharp emission peaks (Supplement 1, Fig. S3).

Previously, S. Murai et al. [31] showed that amplified spontaneous emission (ASE) shifts with plasmon resonance, which in turn depends on the shape of the NP. To observe if NP shape affects the WGM, we generated these droplets using the Au nanobipyramids (NBPs). The NBPs are useful in this context because their transverse mode peak is roughly the same wavelength as the 20-nm diameter Au NP, whereas the longitudinal peak is ∼700 nm. Hence, it minimizes the effects of the plasmon resonance on dye emission. Figure 3 shows the room temperature PL spectra from the Au NBP/droplet structure with C500 dispersed in ethanol. The spectra with different colors are single shots above the lasing threshold obtained right-after the droplet self-assembly process. In comparison to the Au NP the sequential spectra have WGM peaks with slightly different wavelengths over time. This observation suggests that during the excitation sequence the cavity is oscillating in shape [30]. The dominant narrow peak emission shows an increase of the FWHM from 0.9 nm to 1.4 nm, indicating a Q-factor of 354 (Figs. 2(b) and 3). This suggests that the Au NBP geometry induces higher losses in the cavity compared to the Au NP.

 

Fig. 3. Emission spectra of Au nano bipyramid/Coumarin 500 in ethanol. The spectra with different colors are single shots above the lasing threshold. Peak positions and intensities vary for consecutive pulses due to the oscillation in shape of the cavity.

Download Full Size | PDF

To examine material-dependence, we generated these droplets using 30-nm diameter TiN NPs [32]. TiN is an interesting material because it supports a broad plasmon resonances. These NPs have a peak centered at 710-nm and spanning the visible range to NIR. And compared to noble metals such as Au, TiN has a much higher melting point and better chemical stability. Figure 4(a) shows the room temperature PL spectra of a single TiN NP inside a droplet structure containing Coumarin 500. The spectra with different colors are single shots above the lasing threshold obtained right-after the droplet self-assembly process. Again, the spectra are composed of numerous sharp emission peaks and broadband spontaneous emission. Compared to Au NPs, the sharp emission peaks are more closely spaced. Indeed, using n_cav = 1.36 and D = 9.7 µm, the theoretical FSR is 5.8 nm, which agrees well with the experimental observation (Fig. 4(b)). The cavity size was estimated by the PL images. The dominant narrow peak emission shows a FWHM of 0.9 nm with a Q factor of 540. By using the fundamental mode of the WGM model (r = 1, m = l) and ${n_{env}} = 1.0$, the observed emission peaks fit well with the 75–81 TM modes (Fig. 4(c)).

 

Fig. 4. (a) Emission spectra of TiN NP/Coumarin 500 in ethanol. The spectra with different colors are single shots above the lasing threshold. Peak positions and intensities vary for consecutive pulses due to the oscillation in shape of the cavity. (b) Emission spectrum of a single microbubble with a diameter of 9.7 µm. (c) Plot of mode numbers derived from WGM model.

Download Full Size | PDF

The plasmonic NP plays an important role in the formation and characteristics of our hybrid WGM microcavities. We observed that the sequential spectra of the droplet containing an anisotropic Au NBP have WGM peaks with slightly different wavelengths over time and lower Q-factor compared to Au NP. These results suggest that in the case of anisotropic NPs, the cavity is oscillating in shape during the excitation sequence and consequently decreasing the Q-factor. In addition, we demonstrated that the droplet containing TiN NP exhibited similar emission peaks wavelength range and Q-factor compared to Au NP. Although TiN is considered an alternative plasmonic material in the visible and near IR range, note that TiN NPs do not exhibit plasmonic properties at the laser excitation wavelength due to positive real permittivity at 400 nm. Therefore, these results suggest that the WGM emissions is weakly coupled to the plasmonic mode provided by the Au or TiN NPs. Another important aspect of the NP is that it not only is involved in the formation of the air bubble and isolated WGM droplet but also in promoting the optical coupling of the WGM cavity. Thus, combining the plasmonic NP to the optical cavity can provide a way of efficient free-space coupling of light in and out. When irradiated by the free-space 400-nm laser excitation, the NP acts as an antenna and confine the light into the droplet structure containing an organic dye, and then the emitted light from the gain medium is coupled to the NP to be scattered out.

4. Conclusion

In summary, a novel type of self-assembly hybrid WGM in solution was realized via laser-induced cavitation. The approach we have demonstrated in this study overcomes the challenges to realize free-standing and easy-to-fabricate hybrid plasmonic/photonic microcavities. These hybrid microcavities were demonstrated with two kinds of Au NPs in addition to TiN NP. The droplets containing C500 and different plasmonic NPs showed sharp emission peaks in the wavelength range of 475-515 nm. These sharp peaks were explained as WGM modes predicted by the size of the droplet and refractive indices of the solvent and surrounding environment. The same wavelength range of the emissions with different plasmonic NPs suggests that the lasing modes are not directly associated with the plasmon cavity but can arise from its photophysical processes. The plasmonic NP not only assists the formation of the droplet that acts as the WGM cavity, but also provide the coupling of light in and out. In particular, cavity diameter as small as 4.8 µm with Q-factor of 545 and FSR of 12 nm were observed in the droplet containing Au NP. Overall, we expect that our hybrid WGM serves as a route towards a self-assembly plasmonic/photonic microcavity in solution on demand for detection of molecular binding or single-particle detection.