High Q-factor Resonances Research Articles

Coupling of high-quality-factor optical resonators

We improve theoretically and experimentally the problem of the coupling between a high Q-factor resonator and its external coupler. We have observed oscillations of ringing induced by the sweeping of the excitation frequency of an active microsphere. Thanks to this approach, the quality factor of an optical resonator was measured and we obtained Q = 5.8 × 108.

Localization of light on a cone: theoretical evidence and experimental demonstration for an optical fiber

The classical rays propagating along a conical surface are bounded on the narrower side of the cone and unbounded on its wider side. In contrast, it is shown here that a dielectric cone with a small half-angle γ can perform as a high Q-factor optical microresonator which completely confines light. The theory of the discovered localized conical states is confirmed by the experimental demonstration, providing a unique approach for accurate local characterization of optical fibers (which usually have γ ~ 10(-5) or less) and a new paradigm in the field of high Q-factor resonators.

Microwave Frequency Selective Surfaces with High Q-Factor Resonance and Polarization Insensitivity

Analysis of frequency selective surfaces with high Q-factor resonance and polarization insensitivity is the aim of this work. Two new array configurations are suggested, which support the trapped-mode regime. The arrays were optimized in order to obtain the highest Q-factor by varying some of their geometrical and physical parameters. Theoretical results are presented and discussed.

Temperature-stable and high Q-factor TiO2 Bragg reflector resonator

The highest Q-factor resonators employ whispering gallery modes in single-crystal sapphire but have poor temperature stability. Rutile was the first dielectric material used to construct a microwave dielectric resonator. However, its very high temperature coefficient of permittivity made it unsuitable for practical applications. This paper reports a high Q-factor (50 000) and temperature-stable spherical Bragg reflector resonator based on polycrystalline rutile operating at 29.9 GHz. Temperature stability is achieved by adjusting the electric filling factor of a spherical shell so that in combination with its highly negative temperature coefficient of permittivity, the effect of thermal expansion is exactly cancelled out.

Reconfigurable microfluidic photonic crystal slab cavities

We demonstrate the spectral and spatial reconfigurability of photonic crystal double-heterostructure cavities in silicon by microfluidic infiltration of selected air holes. The lengths of the microfluidic cavities are changed by adjusting the region of infiltrated holes in steps of several microns. We systematically investigate the spectral signature of these cavities, showing high Q-factor resonances for a broad range of cavity lengths. The fluid can be removed by immersing the device in toluene, offering complete reconfigurability. Our cavity writing technique allows for tolerances in the infiltration process and provides flexibility as it can be employed at any time after photonic crystal fabrication.

Microfiber Resonator in Polymer Matrix

We propose a simple technique to form miniature optical circuits using microfibers embedded into a low refractive index matrix. As an example we demonstrate a silica microfiber knot resonator embedded in a fluoroacrylate polymer. Fabrication issues and initial experimental results are reported. We also present simulations aimed at understanding the current limitations to the Q-factor and the role of the embedding polymer refractive index on the Q-factor of future resonators. It is anticipated that using commercially available polymers high Q-factor resonators with radii as small as 100 micrometers can be made by this technique.

Ultrasound detection using polymer microring optical resonator

Application of polymer waveguide microring resonators for high-frequency ultrasound detection is presented. The device consists of a microring optical resonator coupled to a straight optical waveguide which serves as input and output ports. Acoustic waves irradiating the ring waveguide induce strain modifying the waveguide cross section. As a consequence, the effective refractive index of optical waves propagating along the ring is modified. The sharp wavelength dependence of the high Q-factor resonator enhances the optical response to acoustic strain. High sensitivity is demonstrated experimentally in detecting broadband ultrasound pulses from a 10MHz transducer. Methods of extending the technique to form multi-element ultrasonic arrays for imaging applications are proposed.

Phase noise in self-injection-locked oscillators - Theory and experiment

Phase-noise analysis of the self-injection-locked oscillator is presented in this paper. The analysis is developed for different oscillator models and arbitrary self-injection feedback loops. The results are illustrated with specific cases of simple time-delay cable and a high-Q factor resonator. It is shown that the behavior of the phase noise is similar to an oscillator locked to an external low phase-noise source. The output phase noise can be reduced at the noise offset frequency near the carrier frequency, and returning to the free-running oscillator noise far from the carrier frequency for certain stable feedback delay ranges. The phase-noise reduction is affected by the self-injection signal strength and feedback transfer function for different oscillator equivalent-circuit models. The theory is verified by using a self-injection-locked GaAs MESFET oscillator operating at the X-band with delay cable loops. The self-injection-locked technique may be used to improve the phase noise of the existing oscillators.

Biochemical sensors based on polymer microrings with sharp asymmetrical resonance

Photonic microresonators have great potential in the application of highly sensitive sensors due to high Q-factor resonances and steep slopes between zero and unity transmission. A microring resonator with increased resonance slopes is proposed by introducing two partially reflecting elements implemented by waveguide offsets. This configuration produces a Fano-resonant line shape and can greatly enhance the sensitivity of the sensor. Polystyrene microring resonators were fabricated by the nanoimprinting technique, and the optical spectra were measured in glucose solutions of different concentrations. The shift in resonant wavelength and variation of the normalized transmitted intensity are linearly related to the concentration of the glucose solution.

Single-mode and multimode operation conditions in JINR-IAP millimeter-wave FEL-oscillator

The influence of the Q-factor of a two-mirror Bragg resonator on the efficiency and spectrum of radiation was studied both theoretically and experimentally for the JINR-IAP millimeter-wave free-electron laser (FEL)-oscillator driven by a linear induction accelerator (1 MeV/200 A/200 ns). In accordance with theoretical predictions, single-frequency operation in a low (optimal) Q-factor resonator and multifrequency operation in a high Q-factor resonator were observed. For an optimal resonator configuration at the frequency 31 GHz, single-mode generation with power of approximately 35 MW and efficiency of 26% was obtained. A significant drop in FEL efficiency was observed for multifrequency generation.

Theory of group synchronism regime in waveguide FEL driven by short electron bunches

A theory of group synchronism regime in a FEL driven by a train of short electron bunches is described. We suggest that translational velocity of particles coincides with group velocity of electromagnetic pulses and the period between neighboring bunches is equal to the period of electromagnetic pulse round trip through the resonator. The dispersion of electromagnetic pulse provides mutual synchronization of radiation inside an electron bunch. On the other hand, due to the guiding properties of electron bunch, the field is localized close to it. These two factors allow one to realize the stationary generation regime. Under the assumption of high Q-factor resonator a parabolic equation for the description of evolution of electromagnetic pulse profile has been used. The self-excitation conditions and the characteristics of stationary generation regime are determined.

Electromagnetic waves and oscillations in the layered semiconductor-dielectric-semiconductor structure

Theoretical and numerical studies of the electromagnetic properties of the layered semiconductor — dielectric — semiconductor (SDS) structure was carried out. It was shown that the weak damping guide and surface waves may exist in this structure and retardation of surface waves may be several times more than for the semiconductor — dielectric (SD) interface. If this structure contains reflections it leads to the formation of high Q-factor resonance oscillations. It was found that at the beginning of intermode coupling of two surface oscillations in the studied resonance structure, hybrid surface oscillations with near resonance frequencies are formed. Their electromagnetic field components along the wave propagation direction are orthogonal to each other and modulated along the SD interface by orthogonal envelopes.

Design of wide-range varactor-tuned microwave tunnel-diode oscillators

The paper describes the application of tunnel diodes to electronically tunable microwave oscillators. The properties of tunnel diodes are described, and oscillators with a wide tuning range are considered in detail. In order to achieve frequency stability, the tunnel diode should be weakly coupled to a relatively high-Q factor resonator, and this principle is applied to the design of electronically tuned oscillators using varactor diodes as tuning elements. Some comparisons are made with mechanically tuned oscillators designed according to similar principles. The form of construction is described and some results are discussed.