Chun Zeng

Design of ultrathin plasmonic quarter-wave plate based on period coupling

Based on an analysis of the surface admittance of a plasmonic film with a substrate, we propose an ultrathin quarter-wave plate consisting of a periodic plane array of symmetrical L-shaped plasmonic antennas. The period, which determines the coupling among L-shaped antennas, is an important parameter for optimizing the performance of the structure. Numerical simulation results show that an Au quarter-wave plate designed in this Letter can efficiently convert a linearly polarized light at normal incidence into circularly polarized light, whose ellipticity is 0.994 at an operating wavelength of 1550 nm. The thickness is only 30 nm, which is nearly 1/50 of the wavelength of incident light.

Wave propagation in deep-subwavelength mode waveguides

This Letter proposes a dielectric waveguide with deep-subwavelength mode sizes. Results of both frequency domain and time domain analysis show that the effective mode area is below λ(0)(2)/400 and can even reach λ(0)(2)/1000 (λ(0) is the wavelength in vacuum). The effective electrical mode area can be comparable to that of a hybrid plasmonic subwavelength confinement waveguide, with reduced optical absorption. In contrast to slot waveguides, which guide light in low-index materials, the proposed structure guides light in high-index materials. Results obtained in this Letter show that the losses are sensitive to the surface roughness on the tens of nanometers scale. The structure can be used to design ring resonators with a quality factor comparable to that of a diffraction-limited dielectric ring resonator with the same standing wave numbers. The property can be applied in nonlinear effect enhancement or laser design with ultralow threshold.

Unidirectional transmission realized by two nonparallel gratings made of isotropic media

We realize a unidirectional transmission by cascading two nonparallel gratings (NPGs) made of isotropic, lossless, and linear media. For a pair of orthogonal linear polarizations, one of the gratings is designed as a polarizer, which is a reflector for one polarization and a transmitter for the other; another grating is designed as a polarization converter, which converts most of one polarized incident wave into another polarized transmitted wave. It is demonstrated by numerical calculation that more than 85% of the incident light energy can be transmitted with less than 1% transmission in the opposite direction for linearly polarized light at normal incidence, and the relative bandwidth of the unidirectional transmission is nearly 9%. The maximum transmission contrast ratio between the two directions is 62 dB. Unlike one-way diffraction grating, the transmitted light of the NPGs is collinear with the incident light, but their polarizations are orthogonal.

Small-signal analysis of bidirectional operating characteristics in a Raman ring laser with external optical injections

Employing the small-signal analysis, we study the bidirectional operating characteristics in a Raman ring laser. Bidirectional operation is found not stable in a silicon ring because of the existence of two-photon absorption. Using polar crystals which have different forward and backward Raman gain or introducing external optical injections helps to establish stable bidirectional operation in a Raman ring laser.

Brillouin/Raman compensation of the Kerr-effect-induced bias in a nonlinear ring laser gyroscope

In this Letter, the beat frequency at rest of a ring laser gyroscope with nonlinear effects is discussed in detail. Even without an additional intensity-stabilizing system, the random nullshift bias induced by the Kerr effect is compensated by the phase shift associated with the stimulated Brillouin/Raman scattering. And the nonlinear stimulated scattering also serves as the gain mechanism of the gyroscope. And thus the influence of the fluctuation of the injected pump intensity on the beat frequency is eliminated.

Improving the room-temperature confinement of light by miniaturizing mode sizes into a deep subwavelength scale using dielectric spheres in metal cavities.

The confinement of light within nanometer-scale regions may result in the significant enhancement of light-matter interactions. However, light confinement to nanometers is hindered by the diffraction limit of a dielectric material. For a dielectric cavity, if the material loss is negligible, reducing the cavity size usually causes a significantly increase in radiation loss. Surface plasmons show great promise for potential subwavelength light confinement. However, in most circumstances, light confinement by dissipative metallic materials can cause ohmic losses at optical frequencies. In such cases, the realization of light confinement with deep subwavelength mode sizes results in great losses and thus has low quality factors. In the present study, a three-dimensional light confinement with deep subwavelength mode sizes is achieved using dielectric spheres in metal cavities. Contrary to other mechanisms for subwavelength light confinement that are based on the use of dielectric or metal cavities, the nanometer-scale regions ensure that most of the light energy is confined away from the metal-dielectric interfaces, thereby decreasing light absorption in the metal cavity. In turn, the metal cavity decreases the radiation loss of light. Thus, high quality factors ranging from 2×10(2) to 6×10(2) can be obtained at room temperature. An effective electrical mode volume ranging from 7×10(-5)λ(0)(3) to 2×10(-4)λ(0)(3) (where λ(0) is the resonant wavelength in a vacuum) can be achieved. Therefore, this method of three-dimensional light confinement with deep subwavelength mode sizes using dielectric spheres in metal cavities may have potential applications in the design of nanolasers, nanophoton detectors, nonlinear optical switches, and so on.

Complete optical on-chip isolation based on asymmetric stimulated Raman gain/loss

In this Letter, we suggest an on-chip integrated isolator based on an add-drop filter with unidirectional Raman-induced gain (or loss). For the steady state, complete one-way propagation for monochromatic signals is realized in the bus waveguide. For transient transmission, the burr is suppressed. And the power consumption is reduced by enhancing the Q factor of the resonator at the control frequency.

Great optical buffering capacity for optical delay line and extraordinary optical reflection and mode conversion with extremely weak dielectric perturbations based on circular Bragg resonators

Circular Bragg resonators (CBRs) are analyzed in both the frequency domain and the time domain based on the scattering matrix method and the numerical model. The CBR with the same size as a dielectric ring can be designed to have denser resonant mode distributions in the frequency domain, and the expansion of the slow light band is imposed by the combination of multiresonant modes. Thus the expansion is independent of group velocity and is not limited by the delay-bandwidth product constraint in static photonic structures, which is deduced for a single resonant mode. Hence, the CBR can store more bits than a dielectric ring. For certain parameters, clockwise (CW) and counterclockwise (CCW) modes in the CBR are quite sensitive to dielectric perturbations, which are weak enough that they have little effect on the CW mode and CCW mode in a dielectric ring. When light propagates along a line waveguide coupled with the CBR, and if there are weak dielectric perturbations in the CBRs, extraordinary reflections could be produced and there exists strong coupling and conversion between CW and CCW modes in the CBR. The optical property indicates that extremely weak dielectric perturbations in the CBR play an important role in mode conversion. These unique properties of CBRs may find applications in the design of practical optical delay line buffers, and they also provide a new method to achieve light control by mode conversion in passive optical resonators.

Dispersion compensation based on the combination of coupled ring resonator and photonic crystal structures

An optical delay line of coupled resonator optical waveguide (CROW) compensated by photonic crystal waveguide (PhCW) is proposed. In the structure, etching the periodic holes around the waveguide of the ring resonator waveguide does not increase the size of the CROW. Theoretical studies and numerical models indicate that through careful design, CROW and PhCW exhibit different group velocity dispersion (GVD) properties at a certain frequency range. Optical signal can not only be compensated in terms of GVD, but can also be delayed with longer time period. Due to the propagation mode mismatch of the two structures, optical loss becomes inevitable.