In recent years, whispering gallery mode (WGM) devices have extended their functionality across a number of research fields from photonics device development to sensing applications. Here, we will discuss some such recent applications using ultrahigh Q-factor hollow resonators fabricated from pretapered glass capillary. We will discuss device fabrication and different applications that can be pursued such as bandpass filtering, nanoparticle detection, and trapping. Finally, we will introduce our latest results on visible frequency comb generation.
In recent years, whispering gallery mode devices have extended their functionality across a number of research fields from photonics to sensing applications. Here, we will discuss environmental sensing applications, such as pressure, flow, and temperature using ultrahigh Q-factor microspheres fabricated from ultrathin optical fiber and microbubbles fabricated from pretapered glass capillary. We will discuss device fabrication and the different types of sensing that can be pursued using such systems. Finally, we will introduce the concept of using cavity ring-up spectroscopy to perform dispersive transient sensing, whereby a perturbation to the environment leads to a frequency mode shift, and dissipative transient sensing, which can lead to broadening of the mode, in a whispering gallery mode resonator.
In this work, hollow whispering gallery resonators with thin walls are filled with a water solution containing 500 nm
nanoparticles. The quasi-droplet modes of the hollow resonator create an optical scattering force which pushes the
particles around with velocities far exceeding 1.2 mm/s. The optical modes are observed to shift up to tens of GHz in the
presence of the nanoparticle. By using counter propagating modes, the position and direction of the particles are
controlled, this is the first time trapping and control of nanoparticles has been demonstrated in a quasi-droplet
In this research, we present a packaged add–drop filter composed of a silica microsphere resonator and a strongly coupled optical microfiber coupler. A one-step fabrication process using UV curable epoxy is shown to stabilize the microsphere resonator coupled to the microfiber coupler, which is used as add and drop ports. A high Q-factor of 3×107 is obtained at around 780 nm from the packaged microspheres coupled with the microfiber coupler in the add–drop configuration.
Flow sensing using the concept of a hot whispering gallery microlaser is presented. Silica microcapillaries or microbubbles, coated with a layer of erbium:ytterbium (Er:Yb) doped phosphate laser glass, result in a hollow, microbottle-shaped laser geometry. The Er:Yb doped glass outer layer is pumped at 980 nm via a tapered optical fiber and whispering gallery mode (WGM) lasing is recorded at 1535 nm. When gas passes through the capillary, the WGMs shift toward shorter wavelengths due to the cooling effect of the fluid flow. In this way, thermal tuning of the lasing modes over 70 GHz can be achieved. The output end of the capillary is connected to a mass flow sensor and the WGM shift rate as a function of flow rate and pump laser power is measured, with the results fitted using hot wire anemometry theory. Flow sensing can also be realized when the cavity is passively probed at 780 nm, with the estimated Q-factor of the WGMs being in excess of 105.
A thin-walled microbubble whispering gallery resonator was fabricated and filled with a polymer (Polydimethylsiloxane
: PDMS) to form a polymer quasi-droplet optical microcavity. The thermal shifting of the whispering gallery modes
(WGMs) was studied pre and post curing of the polymer. In both cases, large thermal shifts were observed. However,
the sign of the shift changed as the polymer hardened. The final state of the cavity showed a large red shift of the modes.
Complex mode mixing was also observed which results in EIT (electromagnetically induced transparency) and Fano-like
resonances. The time response of the polymer-filled bubble was investigated demonstrating regenerative self-oscillation
for a fixed laser detuning with a low threshold power.
The optical response of a silica microsphere pendulum evanescently coupled to a tapered optical fiber is studied. The
pendulum oscillation modulates the microsphere’s whispering gallery mode (WGM) resonance frequencies (dispersive
shift) and the external coupling rate (dissipative effect). These effects combine to give an observable modulation in the
transmitted optical power so that the tapered fiber also acts as the mechanical motion transducer. This unique mechanism
leads to an asymmetric response function of the transduction spectrum, i.e. the amplitude of the transmitted noise
depends on laser detuning. This phenomenon can be explained using coupled mode theory with a Fourier
transformation. The transduction of the mechanical motion and its relation to the external coupling gap was
experimentally investigated and showed good agreement with the theory.
The optical properties and sensing capabilities of fused silica microbubbles are studied numerically through a finite element method. Mode characteristics, such as the quality (Q) factor and the effective refractive index, can be determined for different bubble diameters and shell thicknesses. For sensing with whispering gallery modes, thinner shells lead to an improved sensitivity. However, the Q- factor decreases as the shell thickness reduces and this limits the final resolution. Here, we show that high resolution can be achieved when the microbubble acts as a quasi-droplet even for a water-filled cavity at the telecommunications C-band. Different sensing scenarios can be studied such as thermal sensing, pressure sensing, and nanoparticle detection. We investigate the onset of the quasi droplet regime for different modes in the microbubble.
It is experimentally shown that a large thermal blue shift of up to 100 GHz/K (0.2 nm/K at a wavelength of 775 nm) can be achieved with higher order radial modes in an ethanol-filled microbubble whispering gallery mode resonator (WGR). Q-factors for the most thermally sensitive modes are typically 105, equivalent to a measurement resolution of 8.5 mK. The thermal shift rate is determined for different modes when the core of the microbubble is filled with air, water, and ethanol. The measured shifts are compared against Finite Element Model (FEM) simulations. It is also shown that, if the microbubble is in the quasi-droplet regime, the fundamental TE mode in a bubble with a 500 nm wall is estimated to experience a shift of 35 GHz/K, while the effective index is still high enough to allow efficient coupling to a tapered optical fiber. Nonetheless, at a wall thickness of 1 μm, the most sensitive modes (n = 2) observed were still strongly coupled.
Optical modes of a microbubble are studied both theoretically and experimentally. It is shown that the thermal red shift of the whispering gallery mode can be counteracted by selecting a suitable wall thickness and core material. Furthermore, the temperature sensitivity can be enhanced by selecting the appropriate wall thickness and core fluid such that a significant blue shift up to 30GHz/K is possible. Calculated shift rates are compared to preliminary results.