Measurement of Vibrational Modes in Single ${\mathrm{SiO}}_2$ Nanoparticles Using a Tunable Metal Resonator with Optical Subwavelength Dimensions

Using a tunable optical subwavelength microcavity, we demonstrate controlled modification of the vibronic relaxation dynamics in a single ${\mathrm{SiO}}_2$ nanoparticle. By varying the distance between the cavity mirrors we change the electromagnetic field mode structure around a single nanoparticle and the radiative transition probability from the lowest vibronic level of the electronically excited state to the progression of phonon levels in the electronic ground state. We demonstrate redistribution of the photoluminescence spectrum between zero-phonon and phonon-assisted bands and modification of the excited state lifetime of the same individual ${\mathrm{SiO}}_2$ particle measured at different cavity lengths. By comparing the experimental data with a theoretical model, we extract the quantum yield of a single ${\mathrm{SiO}}_2$ nanoparticle.

Figure 1

Diagram of zero- ($k_0$), one- ($k_1$), and two- ($k_2$) phonon-assisted recombination on a defect state in the band gap of ${\mathrm{SiO}}_2$. Selecting those cavity modes, which are in resonance with a particular spectral band of the single ${\mathrm{SiO}}_2$ nanoparticle, we can tune the spectral emission of the particle due to enhancement and suppression of zero-phonon and phonon-assisted transitions.

Figure 2

(a)–(c): Measured (red curves; gray solid curves) and calculated (black curves) photoluminescence spectra of a single ${\mathrm{SiO}}_2$ nanoparticle placed inside the microresonator at different cavity lengths corresponding to resonance wavelengths of 535, 576, and 631 nm. The blue dotted curves are the calculated angular sensitivity functions. The dashed blue curves represent the Purcell factor. The grey shaded area shows a typical photoluminescence spectrum of a single ${\mathrm{SiO}}_2$ nanoparticle in free space, i.e., without cavity, shifted along the wavelength axis to fit the position of the cavity-controlled spectra. Inset in (b) shows irreversible photoluminescence bleaching of the nanoparticle (scanning direction up-down). The dashed circle represents the area where the nanoparticle is centered. (d)–(f): Transients of the same single ${\mathrm{SiO}}_2$ nanoparticle inside the microresonator as was used for recording the spectra. The decay curves (d)–(f) were acquired at the same cavity lengths, which were selected for measurements of single particle spectra shown in figures (a)–(c), respectively. All transients were fitted using a monoexponential decay function (red lines).

Figure 3

Values for the photoluminescence intensity, i.e., area under the spectral curve corrected according to the transmission or detection efficiency of the microscope (red circles) and measured decay rate (blue rectangles) of a single ${\mathrm{SiO}}_2$ nanoparticle placed inside the tunable microresonator tuned to three different resonances. Blue solid line shows the best fit to the measured decay rates giving values of 0.6 and 6.1 ns for the single ${\mathrm{SiO}}_2$ nanoparticle quantum yield and the free space lifetime, respectively. The calculations were done for the transition dipole moment of the particle oriented parallel to the cavity mirrors.