The invention relates to a WGM microresonator, applicable in sensing to the detection of physical or biological properties or as filters in optical fibre lines.

WGM waves or modes, a term derived as an abbreviation of "Whispering Gallery Modes"- i.e., grazing-incidence oblique modes, are a specific type of wave field resonance, for example acoustic or electromagnetic, inside a resonator cavity. As a result of total internal reflection, the wave propagates at the surface of the resonator. The resonance occurs for such wavelengths for which the optical path in one cycle is an integer multiple of the wavelength. In this case, constructive interference occurs. The resonant frequency depends on the geometry of the cavity, as well as on the dielectric constant of the medium in which the wave propagates and the dielectric constant of the surrounding. The resonator efficiency, the resonance line width and the intensity of the resonant modes inside the WGM cavity is described by the microresonator quality factor - the Q-factor. It is the ratio of the energy stored in the microresonator to the energy dissipated in one optical cycle and is given by the following formula :

where: cj _r - the resonance frequency, W - the energy stored in the resonator, P _d - the energy dissipated in one cycle, Dw _rnnHM - the half width of the resonance peak.

The WGM resonance modes can be generated in differently shaped resonators: spherical, toroidal, cylindrical, disc or ring shaped ones.

Electromagnetic WGM microresonators are characterised by a large value of the Q-factor, narrow resonance lines, large density of accumulated energy, multi-mode generation of resonance frequencies, as well as a small size. The electromagnetic WGM microresonators are widely used in practice, as reported by A. B. Matsko, V. S. Ilchenko, IEEE J. Sel. Top. Quant. Electron., 12 (2006) 15-32. The application possibilities include interferometry, spectroscopy, data storage devices, filters in optical fibre lines. A. M. Armani et al., Science, 317 (2007) 783-787 present also a wide range of their possible applications related to the technology of pressure sensors, temperature sensors, and sensors of other mechanical parameters, as well as biosensors, as the WGM resonators allow the analyte detection even at the single molecule level.

G. Lin. et al., Micro-Optics , 2010, 771622, doi:10.1117/12.853915, report the use of WGM resonators in laser technology, because they have a very low threshold for initiation of the laser action. The small size allow for easy integration and connection of many resonators within one fibre and, consequently, obtaining a tuneable laser.

The dielectric materials used to produce WGM resonators are often inorganic glasses, as they are transparent in the visible and infrared spectral ranges, allow doping with different methods, are resistant to thermal deformation in the range up to the softening temperature.

Rare earth ion-doped microresonators were reported by A. Rasoloniaina et al., Sci. Rep., 4 (2014) 4023, doi:10.1038/srep04319.

Organic dye-doped microresonators were reported by J. Yang, L. J. Guo, IEEE J. Sel. Top. Quant. Electron., 12 (2006) 143-147.

CdSe/ZnS quantum dot-embedded microresonators were reported by S. Pang, R. E. Beckham, K. E. Meissner, Appl. Phys. Lett., 92(22) (2008) 221108, doi: 10.1063/1.2937209.

The purpose of the present invention is to enhance the intensity of the resonance modes in a WGM resonator, while simultaneously narrowing the width of the resonance peak, and thus to increase the value of the Q-factor.

A WGM microresonator according to the invention is characterized in that in a dielectric matrix with positive real part of the electric permittivity Re(e)>0 in the UV/VIS/NIR wavelength range of electromagnetic radiation it comprises at least one type of plasmonic metal or semiconductor nanoparticles with negative real part of the electric permittivity Re(e)<0 and/or at least one type of luminescent semiconductor or perovskite quantum dots. As a dielectric matrix it preferably comprises a low-melting inorganic glass, most preferably a phosphate glass, and especially a glass with a formula Na3AI ₂ P30i2 _, in short NAP, or a glass with a formula Na5B ₂ P30i3, in short NBP, or a tellurium glass with composition: 80rnol%TeO ₂ -10rnol%ZnO-10mol%Na ₂ CC> _3, in short TZN.

As plasmonic nanoparticles in the matrix it preferably comprises metal or semiconductor nanoparticles with low optical losses, i.e. with imaginary part of the electrical permittivity Iiti(e)<30 F/m, and with melting temperature higher than the melting point of the matrix, most preferably silver nanoparticles, especially with diameter of 10-50 nm.

As quantum dots in the matrix it preferably comprises quantum dots of semiconductor material: CdTe or CdSe/ZnS, or of CsPbX3 perovskites, where X = Cl, Br, I. The diameter of the semiconductor quantum dots ranges preferably from 1 to 8 nm, and the perovskite quantum dots - from 1 to 15 nm.

A WGM microresonator according to the invention preferably comprises in the dielectric matrix 0.2 - 0.6 wt% quantum dots and 0.1 - 1 wt% metal nanoparticles. The WGM microresonator is preferably spherical in shape, and the diameter of microspheres is 1 - 300 pm.

A microresonator according to the invention is an active microresonator, in which internal amplification of the electromagnetic wave takes place, with simultaneous narrowing of the resonance peak width, which results in an increase of the Q-factor. The internal gain compensates for the losses resulting from the absorption of radiation. The amplification and the narrowing of the resonance peak are obtained by doping the dielectric matrix with nanoparticles with plasmonic properties and quantum dots. Metal or semiconductor nanoparticles with negative real part of the electric permittivity - Re(s)<0 located in a dielectric environment with positive real part of the electric permittivity Re(s)>0 show plasmonic properties. Under the influence of interaction with the electromagnetic wave, for a strictly defined frequency, dependent on the type and size of the nanoparticles, as well as their electric permittivities and the dielectric matrix, the wave field is coupled to the oscillations of the electron plasma in the nanoparticles. This results in amplification of the electromagnetic field with a frequency equal to the electron plasma frequency at the metal/semiconductor-dielectric interface. In turn, the interaction of the field induced by the plasmonic nanoparticles with the quantum dots results in enhanced quantum dot emission, shortening of the excited state lifetime and narrowing of the peak width of the quantum dot emission, which results in an increase in the Q-factor.

The frequency of the emission line from the quantum dots depends primarily on the chemical composition and the diameter of the dots. Simultaneous doping of the dielectric matrix with various types of quantum dots allows for obtaining microresonators with different resonance frequencies.

The admixture of quantum dots contained in the volume of the glass matrix in combination with plasmonic nanoparticles increase the Q-factor of the WGM microresonator.

The examples given below illustrate in more detail the WGM microresonator according to the invention in specific embodiments, without limiting the scope of its application, based on the drawings, wherein:

Fig.l shows photoluminescence as a function of wavelength for a WGM microresonator doped with CdTe quantum dots only, whereas

Fig.2 shows photoluminescence as a function of wavelength for a WGM microresonator appropriately co-doped with CdTe quantum dots and silver nanoparticles. A broad emission peak in the 500-800 nm range comes from the quantum dots. The co-doping results in appearance of a very narrow peak with a maximum at 510 nm. The existence of this peak is a result of excitonic emission enhanced due to the increase in radiative recombination of excitonic states in quantum dots as a result of the interaction with an induced electromagnetic field around the Ag nanoparticles.

Example 1

A WGM microresonator with a spherical shape and a diameter of 30 pm, wherein the matrix is a Na5B ₂ P30i3 glass doped with silver nanoparticles in an amount of 0.4 wt% and a diameter of 20 nm, and CdTe quantum dots with a diameter of 3 nm in an amount of 0.3 wt%. The luminescence spectrum of the WGM microresonator contains a broad band with a maximum at 590 nm, and a narrow band with a maximum at 510 nm. Example 2

A WGM microresonator with a spherical shape and a diameter of BO pm, wherein the matrix is a Na ₃ Al ₂ P ₃ 0i ₂ glass doped with CdTe quantum dots with a diameter of 3 nm in an amount of 0.3 wt%. The luminescence spectrum of the WGM microresonator contains a broad band with a maximum at 590 nm

Example 3

A WGM microresonator with a spherical shape and a diameter of 30 pm, wherein the matrix is a Na ₅ B ₂ P ₃ 0i ₃ glass doped with silver nanoparticles in an amount of 0.4 wt% and a diameter of 20 nm, and CdSe/ZnS quantum dots with a diameter of 3 nm and 0.3 wt% content. The luminescence spectrum of the WGM microresonator contains an emission band in the 600 - 700 nm range with a maximum at 662 nm.

Example 4

A WGM microresonator with a spherical shape and a diameter of 30 pm, wherein the matrix is a glass with composition: 80mol%TeC> ₂ -10mol%ZnO-10mol%Na ₂ C0 ₃ doped with silver nanoparticles in an amount of 0.4 wt% and a diameter of 20 nm, and CdTe quantum dots with a diameter of 3 nm in an amount of 0.3 wt%. The luminescence spectrum of the WGM microresonator contains a broad band with a maximum at 590 nm, and a narrow band with a maximum at 510 nm.

Example 5

A WGM microresonator with a spherical shape and a diameter of 30 pm, wherein the matrix is a glass with composition 80TeC> ₂ -10ZnO-10Na ₂ C0 ₃ doped with CsPbBr ₃ perovskite quantum dots with a diameter of 10 nm in an amount 0.3 wt%. The luminescence spectrum of the WGM microresonator contains a narrow band with a maximum at 513 nm.