Whispering gallery microresonators for second harmonic light generation from a low number of small molecules

Abstract

Unmarked sensitive detection of molecules is needed in environmental pollution monitoring, disease diagnosis, security screening systems and in many other situations in which a substance must be identified. When molecules are attached or adsorbed onto an interface, detecting their presence is possible using second harmonic light generation, because at interfaces the inversion symmetry is broken. However, such light generation usually requires either dense matter or a large number of molecules combined with high-power laser sources. Here we show that using high-Q spherical microresonators and low average power, between 50 and 100 small non-fluorescent molecules deposited on the outer surface of the microresonator can generate a detectable change in the second harmonic light. This generation requires phase matching in the whispering gallery modes, which we achieved using a new procedure to periodically pattern, with nanometric precision, a molecular surface monolayer.

High- Q Supercavity Modes in Subwavelength Dielectric Resonators

“…We reveal strong mode coupling and Fano resonances in high-index dielectric finite-length nanorods resulting in high-Q factors at the nanoscale. Thus, high-index dielectric resonators represent the simplest example of nanophotonic supercavities, expanding substantially the range of applications of all-dielectric resonant nanophotonics and meta-optics.Trapping of light in localized modes is extremely important for various applications in optics and photonics including lasing [1], sensing [2,3], harmonic generation [4,5], Raman scattering [6], and photovoltaics [7,8]. For many optical devices, it becomes critical to localize electromagnetic fields in small subwavelength volumes.…”

“…Trapping of light in localized modes is extremely important for various applications in optics and photonics including lasing [1], sensing [2,3], harmonic generation [4,5], Raman scattering [6], and photovoltaics [7,8]. For many optical devices, it becomes critical to localize electromagnetic fields in small subwavelength volumes.…”

Broadband light management using low-Q whispering gallery modes in spherical nanoshells

“…Whispering gallery mode (WGM) resonators operating at optical frequencies have been successfully used to build laser cavities, filters, sensors and wave mixers [22][23][24][25] . In these applications, resonators with a quality factor (Q) ranging from 10 5 to 10 9 or even higher are preferred, because high-Q resonators have very little energy leakage and high-frequency selectivity at the expense of a low coupling efficiency of light into the resonator.…”

Nonlinear optics in spheres: from second harmonic scattering to quasi‐phase matched generation in whispering gallery modes

“…The nonlinear material can then be deposited by simply dip-coating the surface of the sphere with a mono-molecular layer of nonlinear molecules. In the work reported in [63], an entire sphere together with its holding stem were immersed in a 1-propanol solution in which CV molecules were dissolved. Removing the stem and sphere from the solution at a constant speed of 1 mm/s produced a very homogeneous, one-molecule-thick, layer coating with a surface density that was proportional to the concentration of CV in the 1-propanol solution.…”

“…However, no linear diffraction was seen, indicating that the procedure followed did not induce a periodic modulation in the real part of the refractive index. This technique was applied to obtain a nonlinear periodic grating on a portion of the surface of a microsphere [63]. The pattern inscribed was designed from the domain distribution that was determined from the phase matching condition obtained from an equation similar to Eq.…”