Cavity mode manipulated by single gold nanoparticles

Microlasers, which are microscale coherent light sources, have attracted worldwide interest due to their intriguing prospects in on-chip optical communication, sensing, information processing, and so on.^1–9 Since the first microlaser was realized by InGaAs quantum wells in the 1980s,¹⁰ the pace of laser miniaturization has never stopped. As is known, gain material and microcavity are two key elements to establish microlasers. In the past few decades, diverse gain media and state-of-the-art nano-fabrication techniques serve as an impetus in the advancement of microlaser performance.^{2,5,8,11–13}

The ability to manipulate microlaser performance is highly desirable so as to promote on-chip classical and quantum information-processing technology.^12,14 Aside from diverse gain media used, output features of the microlaser, including wavelength, lasing mode, linewidth, output power, polarization or vortex, temporal property, and nearfield or farfield spatial properties, are carefully investigated with various strategies and technologies.^12,15 In particular, cavity mode manipulation is dispensable so as to achieve either single mode operation or information processing.^16–19 

Recently, it is noted that bottle microresonators have attracted much attention with various applications in optical delay line, optomechanics, frequency conversion, microlaser, and so on.^20–26 Theoretically, whispering gallery modes (WGMs) exist in bottle microresonators with features such as high Q-factor, small mode volume, especially favorable tunability stemming from their bottle resonator geometry, and spatial structure of the cavity modes.²⁷ By taking advantage of large fractions of evanescent fields and spatial distribution of cavity modes, efforts have been taken in order to achieve a single mode operation of lasing.^28–30 

Meanwhile, we have noticed that a combination of plasmonics and microlasers has been thriving.^31–34 Plasmonic microlasers usually exhibit advantages of ultrasmall mode volume beyond diffraction limitation, low-threshold, and enhanced exciton–polariton interaction, which are feasible for improving the performance of lasing and sensing applications.

Here, we have successfully demonstrated reconfigurable and tunable cavity mode manipulations of bottle microlasers with single gold nanoparticles (Au NPs). Single Au NPs were precisely transferred onto the surface of bottle microresonators with the aid of fiber tapers. By taking advantage of large fractions of evanescent fields and spatial distribution of cavity modes, such deposited single Au NPs could effectively quench the lasing mode of a bottle resonator in a reconfigurable and selective manner. Furthermore, single mode lasing, with a high side mode suppression factor ∼13 dB, was achieved by using precisely positioned Au NPs.

Bottle cavities composed of Rhodamine 6G (R6G) and epoxy resin were fabricated (see the supplementary material for more details). In addition, single Au NPs, with the dimension of 84 × 40 nm and scattering around 620 nm (see Fig. S1), were transferred onto the surface of the selected bottle resonator by using a fiber taper (see Fig. S2). In order to exclude the influence of the fiber taper, the bottle microresonator was first contacted with a clean fiber taper, and spectral signals were simultaneously collected before and after the fiber taper contact. We found that the collected spectra remained the same, which confirmed that the bottle microresonator was not disturbed by the fiber taper contact.

To investigate the effect of Au NP deposition on lasing, we first deposited the Au NP onto the surface of a 5.5-μm-diameter bottle microresonator, as shown in Figs. 1(a) and 1(b). A 532 nm pulsed laser with a 20 Hz repetition rate and a 5 ns pulse width was used to excite the microresonators. The generated photoluminescence (PL) signals were collected by using a microscope objective (50×, NA = 0.6) and then were delivered to a spectrometer (Princeton instruments iso 160). The as-deposited Au NP can be easily found under a microscope with a 100× objective [Fig. 1(b)]. As seen from Fig. 1(c), the red curve shows lasing spectra collected before Au NP deposition with a dominant peak located at 572 nm under an excitation of 75 µW/cm². Surprisingly, all the lasing peaks almost disappeared after Au NP deposition under the same excitations. Lasing peaks reappeared by increasing the excitation power to 105 µW/cm², while the dominant lasing peaks shifted to 567 nm. Evolutions of power dependent emission intensity, before and after Au NP deposition, are shown in Fig. 1(d). From the superlinear curves, we found that the lasing threshold increased from 45 µW/cm² to 80 µW/cm² after Au NP deposition.

Meanwhile, we did not observe any enhanced emission from the Au NP under the microscope, which is an indication that there was no strong coupling between the Au NP and microresonators.³⁴ Considering the two times increased lasing threshold, it can be inferred that the Au NP deposited here contributed as an extra optical loss of cavities and did not interact with the active whispering gallery mode in the strong coupling regime.

To further verify this point, we performed experiments by depositing the Au NP onto a single mode bottle microresonator, as shown in Fig. 2. A bottle microresonator with a diameter of 3 µm was selected for the experiment. As can be seen, under an excitation of 100 µW/cm², such a small microresonator was initially single mode lasing at a peak wavelength of 573 nm. However, after Au NP deposition in the center region of the microresonator, lasing disappeared no matter how high we increased the pump power; meanwhile, no strong scattering could be observed from the Au NP. Again, the Au NP here seems to play a role in bringing more optical loss and quenching the lasing mode accordingly. By using the shining intensive laser upon the gold nanoparticle, the nanoparticle was then blasted off, and the lasing behavior reappeared, which verified the reconfigurable manner of mode manipulation here.

Numerical simulations were performed in order to investigate the effect of a single Au NP on such small microresonators. Formal theory of the WGM in the bottle microresonator shows that there are three independent characteristic freedoms to determine a single cavity mode.²⁶ For simplicity, here, we only consider the equatorial whispering gallery mode of the bottle microresonator. A two-dimensional finite-difference time-domain method was employed for simulations with our own codes in this paper. A 4-μm-diameter microresonator was simulated with and without the Au NP of 80 nm diameter, as shown in Fig. 3. For TM polarized excitations,³⁵  Fig. 3(b) shows spectral resonances of whispering gallery modes with and without the Au NP, respectively (the full spectrum can be found in Fig. S3 of the supplementary material). It is clear that field intensities of the cavity mode resonance decreased significantly, and the calculated FWHM was changed from 1.069 nm to 1.294 nm at a resonance wavelength of 595 nm, which is consistent with the corresponding temporal analyses, with the quality factor decreased about two times due to Au NP deposition. Furthermore, during our simulation, no matter how we change the shape, size, and orientation of nanogold or the size of the microresonator in order to modify the plasmonic resonance accordingly, it always manifests that the Au NP brought in a scattering loss of such a small cavity, while no strong coupling was observed. From the simulation, we could also conclude that whispering gallery modes in the shorter wavelength range experienced less scattering losses originated from the Au NP, with a slight decrease in either intensity or quality factor. This is easy to understand because whispering gallery modes of short wavelengths (e.g., 574 nm) usually have smaller fractions of the evanescent wave outside the cavity; thus, the Au NP introduced fewer scattering losses correspondingly. This theoretical outcome was in good agreement with experimental results, as shown in Fig. 1(c), where the dominant lasing wavelength (573 nm) shifted to a short wavelength (567 nm) after Au NP deposition.

It is straightforward to selectively manipulate cavity modes by different locations of Au NP deposition on the bottle microresonators. On the one hand, from the numerical simulation and previous experimental results, one can conclude that Au NPs could efficiently affect cavity modes of such small microresonators by means of introducing a significant optical loss of cavity. On the other hand, considering the small size (<6 µm) of bottle resonators investigated here, the free spectral range (FSR) between modes regarding the azimuthal quantum number differing by one was estimated to be around 15 nm, while mode spacing regarding the axial quantum number was approximately 4 nm.²⁷ As long as relatively small size (e.g., <6 µm) bottle resonators were chosen, one could expect that only the cavity mode with different axial quantum numbers could exist, in accordance with previous experimental observations.²⁹ To be noted, cavity modes with different axial quantum numbers in bottle microresonators have distinct spatial distributions along the axial dimensions.²⁷ Therefore, by carefully adjusting the position of Au NPs on the bottle microresonators, we could possibly manipulate cavity modes in a selective manner.

To demonstrate this principle, a small microresonator with a diameter of 4.3 µm was used, as shown in Fig. 4. Before Au NP deposition, multimode lasing can be clearly seen from the optical image [see Fig. 4(c)] and the associated spectra [Fig. 4(e)]. We then identified the modes with their azimuthal and axial quantum numbers, which confirmed that the existing modes had distinct spatial distributions with each other, in good agreement with the fluorescent image [Fig. 4(c)]. Then, an Au NP was carefully transferred with sub-micrometer distance away from the equator of the bottle microresonator. Under the same pump excitation, one could easily find that the distribution of red lasing emission now shrinks to a much smaller region along the equator, with intensities of neighboring lasing modes almost invisible [see Fig. 4(d)]. The spectral data show that only the dominant peak centered at 577.4 nm was reserved, with other lasing wavelengths disappeared. In addition, it was found that the intensity of the emission peak (577.4 nm) decreased by 85% under the same pump excitation after Au NP deposition. According to the previous discussion, this is easy to understand that all the non-equatorial modes experienced appreciable optical losses after Au NP deposition, while the equatorial mode did not. For non-equatorial modes, introduced optical losses were fairly large compared with optical gain obtained, resulting in the prevention of lasing action; while for equatorial modes, introduced optical losses were less and lasing could be maintained. Notably, such obtained single mode lasing shows a high side mode suppression factor ∼13 dB.

It is important to point out here that experiments performed using bottle microresonators with small diameters (<6 µm), which are not shown here, behaved similarly. However, this is not true for bottle microresonators with large diameters (>10 µm). Following the above discussion, the deposited Au NP could bring in a significant scattering loss, which is comparable to the optical gain obtained by the corresponding cavity mode. Since the Au NP is quite small with respect to the lasing wavelength, one could thus expect that such an introduced optical loss is very small. For bottle microresonators with small diameters (<6 µm), the intrinsic quality factor of the cavity is usually very small (∼1000), and lasing modes usually have different axial quantum numbers with distinct spatial distributions. Therefore, the lasing action of such a cavity mode would be quite sensitive to surrounding changes, including the introduced optical loss, which makes the lasing action efficiently prevented. For bottle microresonators with large diameters (>10 µm), the intrinsic quality factor of the cavity could be considerably large (∼10 000) compared with the introduced loss, resulting in inefficient cavity mode manipulation. It was noted that no signs of spectral splitting or linewidth reductions were observed either theoretically or experimentally, which otherwise indicates strong coupling phenomena.^36,37 All these are in good agreement with our numerical results. Taking a close look at the previous studies, we found that high-Q is essential for the successful demonstration of strong coupling phenomena.^36,37 However, in this paper, the poor quality factor of such a small cavity prevented the observation of strong coupling. In addition, it should be pointed out here that reabsorption of the emission by R6G, which can be inferred that lasing wavelengths were at the tail of the absorption band (the same as in the microdroplets³⁸), was another source of optical losses of cavity, which prevented strong coupling from occurring. Furthermore, it is noted that the laser emission now is far from the plasmonic resonance. For plasmonic resonance matching with the lasing wavelengths, it is intuitive to expect that the local field in the vicinity of the gold nanoparticle could be enhanced significantly, resulting in stronger interaction between the cavity and the gold nanoparticle. However, for such a small cavity with a poor quality factor, Au NP deposition could introduce a significant scattering loss so as to quench the lasing action, as shown in Fig. 2. From this aspect, there was no big difference for lasing wavelength on or off resonance since all the local modes were quenched with the deposited gold nanoparticles.

To summarize, we have demonstrated a simple method for manipulating cavity modes of bottle microresonators. The mechanism of introducing extra optical loss via depositing single Au NPs was both theoretically and experimentally revealed. Further experiment showed that such mode manipulations were efficient, and single mode lasing was successfully achieved with a high side mode suppression factor ∼13 dB.