Miniaturized Whispering Gallery Mode (WGM) temperature sensor has great potentials of high resolution and great
on-chip integration capability. This study focuses on the development of this kind of sensor based on the shifting
wavelength of the optical resonance due to thermal expansion and thermo-optic effects of a silica microsphere. Excellent
linear dependence of the wavelength shift versus temperature rise is observed for three sizes of microspheres (D=90μm,
145μm and 313μm) in small temperature ranges (≤17K) at very low temperatures (113±1K to 173K). By comparing this
observation with the results of similar sizes of microspheres of our previous study in near room temperature as well as
with a theoretical analysis, a conclusion is drawn that thermal expansion and thermal optic coefficients need to be further
studied for microscale silica materials. Ultra high resolution sensing capability as well as potentials of integrated &
miniaturized applications of the WGM temperature sensor is discussed. A method is designed to initially characterize the
WGM temperature measurement noise level due to self-heating effect of the WGM resonance.
Whispering-gallery (WG) modes in photonic microdevices made of dielectric circularly planar resonators are analyzed. The wave equation is solved by using the method of separation of variables based on the eigenvalue technique. The resonant frequency at an azimuthal mode is decided by iteration using the bisection method based on the continuity conditions at the resonator peripheral boundary. The radial mode is determined by the critical points of the field intensity profile in the radial direction via the first derivative test. The resonance frequencies as well as the E-field distributions in two exemplary small resonators are presented for a variety of modes. Comparison with numerical predictions is conducted, and a good agreement is found. The geometric optics method is found inappropriate for small resonators.
KEYWORDS: Resonators, Waveguides, Optical microcavities, Systems modeling, Energy coupling, Finite element methods, Energy efficiency, Chemical elements, Scattering, Wave propagation
Photon tunneling between an optical resonator and a light-delivery coupler is strongly dependent on the gap dimension which can vary from zero to size of an optical wavelength involved. In this systematic report, we investigate the gap effects of whispering-gallery modes in two modeling systems: a waveguide-coupling resonator of 2μm and 10μm in diameter, respectively. Maxwell's equations which govern the EM wave propagation and photon tunneling in the microsystems are solved using the finite element method. The simulation accuracy and sensitivity is examined. It is found that when the maximum element size in the computationally sensitive regions is below 1/8 of the wavelength involved, the calculations are accurate. An optimal gap exists for maximum energy coupling and is a strong function of the wavelength of the resonant mode. The Q factor increases exponentially with increasing gap and saturates as the gap approaches the optical wavelength. An optimum gap can be defined at the half maximum energy coupling where both the Q factor and coupling efficiency are high. We also calculate the effects of gap width on the resonance shift. We find that the resonance wavelength is increased (decreased) with decreasing gap width for the 10μm (2μm) diameter resonator with narrow gap widths.
In this paper, we present the design, fabrication and characterization of the whispering-gallery mode (WGM) miniature sensors for potential use in biosensing at the nanometer scale. In order to understand and investigate the characteristics of WGM resonances, we designed and fabricated a number of sensors with different dimensions. Each sensor is a micro/nano-structure consisted of a microdisk as the resonating cavity and a micro waveguide for light delivery and collection. In addition to the waveguides having uniform cross-section dimensions, tapered waveguide was also considered in our studies. A simulation model was employed to characterize the EM field and radiation energy density of the designed sensors. The gap effects on WGM resonance in terms of quality factor and full width at half maximum (FWHM) were evaluated. Following the design and characterization, the sensors were fabricated in 1.3μm-thick Si3N4 film using 248nm optical lithography and conventional silicon IC processing. Top and down SEM measurements of the fabricated sensors were conducted and the data for the sensors in one device are given.
This report characterizes the whispering-gallery mode (WGM) resonators with the design of waveguide and microdisk coupling microstructure. In order to understand and optimize the design, studies over a broad range of resonator configuration parameters including the microdisk size, the gap separating the microdisk and waveguide, and the waveguide width are numerically conducted. The finite element method is used for solving the Maxwell's equations which govern the propagation of electromagnetic (EM) field and the radiation energy transport in the micro/nano-structured WGM systems. The EM field and the radiation energy distributions in the WGM resonator are obtained and compared between the on-resonance and off-resonance cases. A very brilliant ring with strong EM field and high radiation intensity is found inward the peripheral surface of the microdisk under the first-order resonance. While under the second-order resonance, there are two bright rings; and the outer ring inward the peripheral surface is thin and weaker than the internal ring. The microdisk size affects significantly the resonant frequencies and their intervals. The gap also has a slight effect on the resonant frequencies. The effect of waveguide width on the resonant frequencies is negligible. However, the gap as well as the waveguide width does obviously influence the qualify factor and the finesse of the resonant modes.
KEYWORDS: Tumors, Tissues, Absorption, Signal detection, Natural surfaces, Monte Carlo methods, Tissue optics, Optical properties, Near infrared, Cancer
An optical temporal log-slope difference mapping approach is proposed for cancerous tumor detection; in which target tissues are illuminated by near-infrared ultrashort laser pulses, and backscattered time-resolved light signals are collected. By analyzing the log-slopes of the temporally decaying signals, a log-slope distribution on the detection surface is obtained. After administration of absorption contrast agents, the presence of cancerous tumors increases the decaying steepness of the transient signals. The mapping of log-slope difference between native tissue and absorption-enhanced cancerous tissue indicates the location and projection of tumors on the detection surface. In this paper, we examine this method in the detection of tumor inside a model tissue through Monte Carlo simulation. The tissue model has a spherical tumor of different sizes embedded at the tissue center. It is found that tumors with size not less than 4 mm in diameter in the tissue model can be accurately projected on the detection surface by the proposed log-slope difference mapping method. The image processing is very fast and does not require any inverse optimization in image reconstruction. Parametric studies are conducted to examine to the influences of absorption contrast, tumor size and depth.
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