Excitation Of Whispering Gallery Modes Research Articles

Tunable intrinsic strong light-matter coupling in transition metal dichalcogenide nanoresonators.

Self-hybridizing structures based on transition metal dichalcogenides (TMDCs) are becoming promising candidates for the study of an intrinsic strong light-matter coupling because of the efficient mode overlap with much simplified geometries. However, realizing flexible tuning of intrinsic strong coupling in such TMDC-based structures is still challenging. Here, we propose a strategy for flexible tuning of the intrinsic strong light-matter coupling based on a bulk TMDC material. We report the first demonstration of the strong coupling of intrinsic excitons to whispering gallery modes (WGMs) supported by an all-TMDC nanocavity. Importantly, by simply controlling angles of incidence, a selective excitation of WGMs and an anapole can be realized, which enables a direct modulation of self-hybridized interactions from a bright WGM-exciton coupling to a dark anapole-exciton coupling. Our work is expected to provide unique opportunities for engineering a strong light-matter coupling and to open exciting avenues for highly integrated novel nanophotonic devices.

Carbon Dot-Decorated Polystyrene Microspheres for Whispering-Gallery Mode Biosensing

Whispering gallery mode (WGM) resonators doped with fluorescent materials find impressive applications in biological sensing. They do not require special conditions for the excitation of WGM inside that provide the basis for in vivo sensing. Currently, the problem of materials for in vivo WGM sensors are substantial since their fluorescence should have stable optical properties as well as they should be biocompatible. To address this we present WGM microresonators of 5–7 μm, where the dopant is made of carbon quantum dots (CDs). CDs are biocompatible since they are produced from carbon and demonstrate bright optical emission, which shows different bands depending on the excitation wavelength. The WGM sensors developed here were tested as label-free biosensors by detecting bovine serum albumin molecules. The results showed WGM frequency shifting, with the limit of detection down to 10−16 M level.

Enhanced upconversion luminescence of NaYF4:Er nanocrystals based on the synergy between whispering gallery mode excitation and förster resonant energy transfer

The upconversion luminescence at the visible band was excited by the high power density whispering gallery mode (WGM) on the surface of the microsphere. Under the WGM excitation, Er3+ of NaYF4:Er3+ directly absorbed three photons at 1525 nm, followed by the generation of upconversion luminescence. We used WS2 quantum dots (QDs) as donors and NaYF4:Er3+ upconversion nanoparticles (UCNPs) as acceptors to enhance upconversion luminescence by Förster resonant energy transfer (FRET). The physical mixture composed of monolayer WS2 and NaYF4:Er3+ nanoparticles was coated on SiO2 microsphere by immersion method. Excited by high power density WGM, three photons were absorbed by WS2-QDs based on the quantum size effect to generate excited electrons as donors and UCNPs as acceptors based on FRET, subsequently, the energy of the electrons were transferred to the luminescence center Er3+ to improve the upconversion luminescence efficiency. The mechanism model of upconversion luminescence enhancement based on FRET of WS2–NaYF4:Er3+ was proposed, and the enhancement effect was experimentally studied and demonstrated. To study the distance dependence of energy transfer between QD and UCNP pairs, we covered the surface of the silica microsphere with a controlled concentration of WS2 mixed with NaYF4:Er3+ nano-particles. When the mixed concentration of WS2-QDs was 0.15%, the upconversion luminescence reached the highest efficiency. The results showed that a twofold upconversion luminescence enhancement can be experimentally achieved. The proposed approach for enhancing the upconversion luminescence could benefit the improvement of imaging signal-to-noise ratio (SNR) using UCNPs for fluorescent labels in life sciences.

Turning whispering-gallery-mode responses through Fano interferences in coupled all-dielectric block-disk cavities.

Here, we theoretically demonstrate a strategy for efficiently turning whispering-gallery-mode (WGM) responses of a subwavelength dielectric disk through their near-field couplings with common low-order electromagnetic resonances of a dielectric block. Both simulations and an analytical coupled oscillator model show that the couplings are Fano interferences between dark high-quality WGMs and bright modes of the block. The responses of a WGM in the coupled system are highly dependent on the strengths and the relative phases of the block modes, the coupling strength, and the decay rate of the WGM. The WGM responses of coupled systems can exceed that of the individual disk. In addition, such a configuration will also facilitate the excitation of WGMs by a normal incident plane wave in experiments. These results could enable new applications for enhancing light-matter interactions.

Long-period D-fiber grating based robust and efficient bidirectional coupler for whispering gallery mode excitation.

A robust and efficient bidirectional coupler for whispering gallery mode (WGM) excitation based on a long-period grating (LPG) inscribed in D-fiber is theoretically and experimentally demonstrated. The LPG coupling the fundamental core mode to the forward propagating cladding modes according to the phase-matching condition not only enhances the evanescent field of the fiber but also selectively excites the WGM in a wavelength band of interest. Experimental results show that a maximum resonance contrast as high as 10.5 dB and a quality factor (Q-factor) on the order of 104 can be achieved in an LPG coupled spherical silica WGM resonator with a diameter of 242 µm, where the LPG with a pitch of 680 µm is fabricated by arc-discharging in a side-polished D-fiber with a maximum polishing depth of 56 µm. In addition to high robustness and efficiency, such an LPG-based WGM coupler also demonstrates bidirectionality, i.e., it is independent of the injection direction of the input light, which provides a reliable and flexible fiber coupler for the WGM resonator based practical applications.

Thermo-optical control of L-band lasing in Er-doped tellurite glass microsphere with blue laser diode.

Miniature lasers based on rare-earth ion-doped tellurite microsphere resonators with whispering gallery modes (WGMs) are promising devices for basic research and applications. However, the excitation of WGMs using an external pump is not a simple task requiring passive or active control. We propose and demonstrate the implementation of thermo-optical control of the L-band laser generation in an Er-doped in-band pumped tellurite glass microsphere using a cheap low-power blue laser diode and a constant-wavelength telecom laser as a pump. The proposed scheme ensures simplification and cost reduction of microlasers.

Proof-of-concept of a miniature pressure sensor based on coupled optical WGMs excited in a dielectric microsphere

Optical whispering-gallery mode (WGM) resonators retain increased interest as a key element in various high-resolution sensing and spectroscopic devices in modern optoelectronic, optofluidic and Lab-on-chip technologies. As a theoretical proof-of-concept, we present a new miniature pressure sensor based on coupled WGMs optically excited in a dielectric microsphere placed near a flexible reflective membrane which acts as an ambient pressure sensing element. WGMs are excited by free-space coupling of incoming optical radiation to a spherical microparticle. The distinctive feature of proposed sensor is double WGM excitation by forward and backward propagating radiation upon reflection from a membrane that causes WGMs interference in particle volume. Unlike known WGM-sensors, the proposed design utilize not the resonant frequency shift but the changes in optical intensity of resulting field established in the microsphere which carries information about the exact position of pressure-loaded membrane. The sensitivity of the proposed sensor strongly depends on the quality factor of the working resonance, as well as geometrical and mechanical parameters of flexible reflecting membrane. Important advantages of the proposed sensor are its miniature design and the absence of a mechanical contact of pressure-sensitive element with WGM resonator.

Whispering gallery mode excitation using exposed-core fiber.

Whispering gallery modes (WGMs) in micro-resonators are of interest due to their high Q-factors. Ultra-thin fiber tapers are widely deployed to couple light into micro-resonators but achieving stable and practical coupling for out-of-lab use remains challenging. Here, a new WGM coupling scheme using an exposed-core silica fiber (ECF) is proposed, which overcomes the challenge of using fragile fiber tapers. Microspheres are deposited onto the exposed channel for excitation via the evanescent field of the fiber's guided modes. The outer jacket of the ECF partially encapsulates the microspheres, protecting them from external physical disturbance. By varying the mode launching conditions in this few-mode ECF, in combination with a Fano resonance effect, we demonstrate a high degree of tunability in the reflection spectrum. Furthermore, we show multi-particle WGM excitation, which could be controlled to occur either simultaneously or separately through controlling the ECF mode launching conditions. This work can bring value towards applications such as optical switches and modulators, multiplexed/distributed biosensing, and multi-point lasing, integrated in a single optical fiber device that avoids fiber post-processing.

Highly stable in-fiber integrated silica microresonator

We demonstrate a compact in-fiber integrated microresonator based on a silica microsphere and microstructure fiber. Whispering-gallery-modes (WGMs) are effectively excited through evanescent field coupling between the silica microsphere and one of the input tube waveguides of negative curvature fiber. The WGM spectrum with the Q-factor of ∼2.56 × 103 and the extinction ratio of ∼15.2 dB is achieved. The transmission spectra evolution for 5 h reveals the long-term stability of WGM excitation. The experimental results indicate that the in-fiber integrated resonator has the advantages of high stability, high integration, robustness, alignment-free assembly structure, and low cost, motivating potential applications in the lab-in-fiber platform.

Chirality driven effects in multiphoton excited whispering gallery mode microresonators prepared by a self-assembly technique

Organic microresonators are highly attractive for numerous applications including biosensing, optical switching and metrology. Adding chiral materials to resonators can enhance their sensing properties due to the appearance of chiroptical effects. Here we study experimentally resonant multiphoton processes in spherical microresonators with the diameter of 2–12 m made of 1,1’-bi-2-naphthol derivatives by a self-assembly technique. Using nonlinear single-particle microscopy, we demonstrate that these microspheres provide excitation of whispering gallery modes (WGMs) in two- and three-photon excited luminescence with a Q-factor of up to 300. We also demonstrate that the nonlinear circular dichroism is enhanced due to the WGM excitation and reaches 7.7% and 11% in the two- and three-photon absorption, respectively. Pronounced modulation of the WGMs spectrum due to the mode’s splitting is observed in coupled binol-based microresonators, which opens a path for the control over their resonant response and makes them suitable for nonlinear optical sensing applications.

Fabrication and non-destructive characterization of tapered single-ring hollow-core photonic crystal fiber

We report on the properties of tapered single-ring hollow-core photonic-crystal fibers, with a particular emphasis on applications in nonlinear optics. The simplicity of these structures allows the use of non-invasive side-illumination to assess the quality of the tapering process, by observing the scattered far-field spectrum originating from excitation of whispering-gallery modes in the cladding capillaries. We investigate the conditions that ensure adiabatic propagation in the up- and down-tapers, and the scaling of loss-bands (created by anti-crossings between the core mode and modes in the capillary walls) with taper ratio. We also present an analytical model for the pressure profile along a tapered hollow fiber under differential pumping.

Hybrid nanostructure of SiO2@Si with Au-nanoparticles for surface enhanced Raman spectroscopy.

In this study, a structure of large-area orderly-arranged SiO2@Si core-shell nanoparticles decorated with Au nanoparticles was fabricated for surface-enhanced Raman spectroscopy (SERS). This hybrid structure features light confinement in the Si shells and a uniform distribution of localized electric hot spots. FDTD simulations were carried out to examine the near-field enhancement response of this structure. Results indicate that the strongly enhanced local electric field is attributed to the WGM-LSPR coupling, that is, the coupling of the whispering gallery mode (WGM) of Si nanoshells with the localized surface plasmon resonance (LSPR) of Au nanoparticles. The excitation of WGM comes primarily from the magnetic response of the Si shell with a minor modification by its electric response. The WGM-LSPR coupling of the structure is tunable through the change of geometric parameters of SiO2@Si particles. Raman scattering measurements were conducted on the samples fabricated, which agree well with the simulated results. The measured data gave a SERS G factor of ∼2 × 108 and showed highly sensitive and reproducible SERS signals of R6G with a high spatial uniformity on a 2 × 2 cm2 substrate consisting of an array of SiO2@Si (D = ∼220 nm/290 nm) particles whose outer surfaces were scattered with d = ∼20 nm Au particles.

Selective coupling of Whispering Gallery Modes in film coated micro-resonators.

Whispering Gallery Mode (WGM) micro-resonators like microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film coating is deposited. In order to exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded.

Surface-enhanced Raman scattering on dielectric microspheres with whispering gallery mode resonance

Conventionally, metallic nanostructures are used for surface-enhanced Raman spectroscopy (SERS), but recently there has been increasing interest in the enhancement of Raman scattering from dielectric substrates due to their improved stability and biocompatibility compared with metallic substrates. Here, we report the observation of enhanced Raman scattering from rhodamine 6G molecules coated on silica microspheres. We excite the whispering gallery modes (WGMs) supported in the microspheres with a tapered fiber coupler for efficient WGM excitation, and the Raman enhancement can be attributed to the WGM mechanism. Strong resonance enhancement in pump laser intensity and modified Raman emission from the Purcell effect in the microsphere resonator are observed from the experiment and compared with theoretical results. A total Raman enhancement factor of 1.4×104 is observed, with contribution mostly from the enhancement in pump laser intensity. Our results show that, with an efficient pumping scheme, dielectric microspheres are a viable alternative to metallic SERS substrates.

Whispering gallery modes in two-photon fluorescence from spherical DCM dye microresonators

Organic microstructures are well known for their resonator properties, which bring about whispering gallery mode (WGM) excitation. Here we report on experimental evidence of the WGM in the two-photon fluorescence (TPF) of DCM dye microspheres made using the self-assembly method. The WGM excitation accompanying the overall TPF in the spectral range from nm demonstrated a quality factor of approximately 102 for spheres that were several microns in diameter. The power dependence of the TPF intensity proved the second order nature of the interaction process involved.

Excitation of Whispering Gallery Modes in Silica Capillary Using SMF-MMF Joint With Lateral Offset

Whispering gallery modes (WGMs) have been effectively excited in a silica tube by employing a single-mode-fiber/multimode-fiber (SMF/MMF) joint with the lateral offset to enhance the evanescent field over the tapered MMF region. Experimental results indicate that thanks to the lateral offset at the SMF-MMF joint, the taper diameter is about one order of magnitude larger than that of a standard fiber taper coupler. The WGMs in the silica capillary have been theoretically analyzed by using the eigen equation to deal with WGMs in cylindrical microresonators. Our proposed WGM excitation scheme is more robust than the conventional WGM systems based on fiber tapers of a few micrometers in diameter, which is anticipated to serve as micro-fluidic-channel for biological and chemical sensing applications.

Enhanced nonlinear optical effects in organic frustum-shaped microresonators

We discuss photonic effects in arrays of frustum-shaped organic microstructures prepared from organic orange dye composed on a solid substrate by self-assembling technique. We demonstrate that such structures reveal strong amplification of second-order nonlinear optical response, including optical second harmonic generation (SHG) and two-photon fluorescence (TPF), as compared to that of a continuous film. This is associated with a strong light localization in microstructures composed of high refractive index material. The TPF and SHG power dependencies show that the observed effects are governed by high second-order nonlinearity of the dye damped by a strong absorption. FDTD calculations confirm that the mechanism of the light localization inside organic frustums can be in the form of whispering gallery modes excitation.

Stimulated lasing and self-excited stimulated Raman scattering of Nd3+ doped silica microsphere pumped by 808 nm laser

Self-stimulated Raman lasers have attracted more and more interest, because they have no need of additional Raman device, and they are compact in structure and also economical in cost. Self-stimulated Raman lasers are always emitted from crystalline mediums such as Nd3+:KGd(WO4)2, Nd3+:PbWO4 that are commonly used as laser host materials and proved to be available Raman-active mediums. The Nd3+ doped crystals possess high stimulated emission cross-section for laser emission and high Raman gain coefficients for Raman transitions, but the required pump powers (typically hundreds of milliwatts) are large in those experiments.The whispering-gallery mode (WGM) of silica microsphere cavity has achieved the highest Q factor (8×109) to date. The high Q factor and small mode volume make it possible to realize a resonant buildup of high circulating optical intensities, thereby drastically reducing the threshold powers for laser oscillation and stimulated nonlinear process. The coupler with optical fiber taper allows the excitation of WGMs with ultralow coupling loss, which significantly improves the overall efficiency to produce stimulated Raman laser. In this paper, we report the observation of ultralow threshold self-stimulated Raman laser operating in an Nd3+ doped silica microsphere, and the wavelength range can be extended to O-waveband 1143 nm.A high Q microsphere is fabricated with a thin Nd3+ doped silica layer covered by sol-gel method, in which smooth surface is formed by electrical arc-heating. An optical taper fiber is employed to couple the 808 nm laser into Nd3+ doped microsphere (NDSM) to form whispering gallery mode, which acts as the pump light. Based on 4f electron of neodymium ion transmission and optical oscillation in microsphere, the stimulated laser with a wavelength band of 1080 nm-1097 nm is excited. Due to high power density of the excited laser near the surface of orbit in microsphere, the first order self-stimulated Raman laser with a wavelength range of 1120-1143 nm is stimulated in the high Q microsphere. In a theoretical model, the formulas for calculating the output power and the threshold power of the oscillation laser and the self-stimulated Raman scattering are derived. In experiment, we succeed in getting single-mode and multi-mode laser oscillation due to the 4f layer electron transitions of Nd3+ ions, pumped by 808 nm laser. The results show that the NDSM emits a typical single-mode output laser at 1116.8 nm with a pump power of 8.33 dBm, also the relationship between the 1116.8 nm output power and the pump power with a threshold pump power of 3.5 mW. The multi-mode laser spectrum dependent on the microsphere morphology characteristics is observed, which varies by changing the couple position of the optical fiber taper with microsphere. The characteristics of the laser are discussed including the output power, threshold power, spectral line width, side-mode suppression ratio, etc. The NDSM will have many potential applications in new compact lasers. It is beneficial to wavelength converter and optical amplifier in O band.

Acoustooptic-Mode-Coupling-Based Whispering Gallery Mode Excitation in Silica-Capillary Microresonator

An acoustooptic-mode-coupling-based scheme is proposed for excitation of whispering gallery modes (WGMs) in a silica-capillary microresonator. By exploiting acoustooptic mode coupling in a cladding-etched optical fiber, the evanescent field is strengthened by the cladding modes coupled from the core mode through axial flexural wave modulation, leading to the significant enhancement of the mode coupling between evanescent field and WGMs in a silica-capillary microresonator. WGM resonance peaks have been experimentally acquired by employing a cladding-reduced optical fiber with a diameter of 20.68 μ m, which is more robust than the coupling fibers of a few microns in diameter used in conventional WGM excitation schemes. Our proposed WGM coupling approach has desirable merits such as compactness, ease of fabrication, improved WGM coupling efficiency, and robustness, which is anticipated to find potential applications in optical filtering, microresonator laser, fiber sensor, as well as signal processing. Additionally, by controlling the electric driving signals to apply acoustic modulation, it would be convenient to achieve flexible tuning of WGMs based on acoustooptic coupling approach.

Flexible beam-waist technique for whispering gallery modes excitation in polymeric 3D micro-resonators

The paper presents an original and simple optical excitation technique of 3D micro-resonators. The method is based on the laser beam-waist use without any additional excitation framework such as integrated waveguides or tapered fiber. This is particularly powerful for 3D resonators like spheres or microtoroids. Tests have been performed at visible (470nm) and infrared light (broad band source 800–880nm) on polymeric 3D micro-resonators elaborated by microfluidic techniques. By using adequate protocol for such flexible beam-waist technique, spectral analysis on resonant cavities of different sizes confirm the excitation of the expected whispering gallery modes, revealing free spectral range values close (around 5%) to the analytical ones.