Xiaoxu Deng

Independently tunable dual-band plasmonically induced transparency based on hybrid metal-graphene metamaterials at mid-infrared frequencies

A tunable dual-band plasmonically induced transparency (PIT) device based on hybrid metal-graphene nanostructures is proposed theoretically and numerically at mid-infrared frequencies, which is composed of two kinds of gold dolmen-like structures with different sizes placed on separate graphene interdigitated finger sets respectively. The coupled Lorentz oscillator model is used to explain the physical mechanism of the PIT effect at multiple frequency domains. The finite-difference time-domain (FDTD) solutions are employed to simulate the characteristics of the hybrid metal-graphene dual-band PIT device. The simulated spectral locations of multiple transparency peaks are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set, which is in good accordance with the theoretical analysis. Distinguished from the conventional metallic PIT devices, multiple PIT resonances in the hybrid metal-graphene PIT device are independently modulated by electrostatically changing bias voltages applied on corresponding graphene fingers, which can be widely applied in optical information processing as tunable sensors, switches, and filters.

Tunable multispectral plasmon induced transparency based on graphene metamaterials

A dynamically wavelength tunable multispectral plasmon induced transparency (PIT) device based on graphene metamaterials, which is composed of periodically patterned graphene double layers separated by a dielectric layer, is proposed theoretically and numerically in the terahertz frequency range. Considering the near-field coupling of different graphene layers and the bright-dark mode coupling in the same graphene layer, the coupled Lorentz oscillator model is adapted to explain the physical mechanism of multispectral EIT-like responses. The simulated transmission based on the finite-difference time-domain (FDTD) solutions indicates that the shifting and depth of the EIT resonances in multiple PIT windows are controlled by different geometrical parameters and Fermi energies distributions. A design scheme with graphene integration is employed, which allows independent tuning of resonance frequencies by electrostatically changing the Fermi energies of graphene double layer. Active control of the multispectral EIT-like responses enables the proposed device to be widely applied in optical information processing as tunable sensors, switches, and filters.

Wavelength Sensing With Subpicometer Resolution Using Ultrahigh Order Modes

The ultrahigh order modes of a symmetrical metal-cladding optical waveguide, which exhibit strong wavelength dispersion characteristics, are used to track small wavelength changes of the light output of laser diodes. In the case of free-space coupling, the wavelength shift is transformed into an intensity variation of the light reflected on a surface of the waveguide. The sensitivity considering the divergence of the incident light and the thickness of the upper cladding is analyzed. A wavelength resolution of 0.5 pm at a center wavelength of 859.8 nm is demonstrated experimentally

Picometer displacement sensing using the ultrahigh-order modes in a submillimeter scale optical waveguide.

An improved scheme for displacement measurement using the ultrahigh-order guided modes in a symmetrical metal-cladding optical waveguide is proposed. Based on this idea together with the lock-in amplification technique, a sensor with a stable displacement resolution of 3.3 pm is experimentally demonstrated without any complicated servo system.

An electro-optic polymer modulator based on the free-space coupling technique

An electro-optic (EO) polymer modulator employing a symmetrical metal-cladding waveguide structure is presented. Based on the free-space coupling technique, the energy of incident light on the metal-cladding is coupled directly from free space into the waveguide without using a prism, grating and other coupling components. An externally applied voltage modulates the reflected light intensity by changing the energy coupling efficiency of the incident light into the waveguide through the refractive index of the EO polymer. The fabricated modulator achieves an 8.2% modulation depth with 10 Vp−p driving voltage at 1 MHz. The device also provides the benefits of simple fabrication and compact size owing to the use of the free-space coupling technique.

Explicit expression of light reflection from inhomogeneous planar structures

An exact expression of reflection coefficient from inhomogeneous planar structures is derived in a simple and very explicit form by using the analytical transfer matrix method. It is revealed that the reflection coefficient is directly dependent on a single phase integral over the structure. Different from the WKBJ and the integral expansion methods, the presented phase integral is accumulated not only by the main waves, but also by the total subwaves, which are propagated in the structure with continuously varying index profile.

Device-quality second-order nonlinear optical poly(ester-imide) for electro-optic applications

Abstract A side-chain second-order nonlinear optical (NLO) poly(ester-imide) (PEI) possessing good organo-solubility, excellent film-forming property, high glass transition temperature (186 °C) and high thermal stability was synthesized. The NLO chromophore orientation in the polymer film achieved via corona poling showed high long-term stability at elevated temperatures up to 120 °C. A reflective electro-optic (EO) light modulator using this polymer as the EO material was fabricated and encoded electrical signals onto reflected light successfully. The EO coefficient γ33 of the PEI layer in the EO modulator was determined to be 11.5 pm/V at 650 nm by an improved attenuated-total-reflection method.

Polarization-independent and tunable comb filter based on a free-space coupling technique

A novel polarization-independent and tunable comb filter that consists of only one glass slab sandwiched between two gold films has been demonstrated. The filter has greater than 12 dB channel isolation, less than 0.2 dB insertion loss, and accurate 0.8 nm channel spacing.

Tunable polarization-independent plasmonically induced transparency based on metal-graphene metasurface

A tunable polarization-independent dual-band plasmonically induced transparency (PIT) device based on metal-graphene nanostructures is proposed theoretically and numerically at mid-infrared frequencies, which is composed of two kinds of center-symmetric metallic nanostructure array with different sizes and element numbers placed on separate graphene interdigitated finger sets, respectively. The coupled Lorentz oscillator model is used to explain the physical mechanism of PIT effect at multiple frequency domains. The finite-difference time-domain (FDTD) solutions are employed to simulate the characteristics of the polarization-independent metal-graphene PIT device, which is consistent with the theoretical analysis. The PIT peaks, obtained at two frequency domains, are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set without changing the geometrical parameter of the metallic nanostructure. By the carefully selected element numbers of nanostructure arrays, the resonance strength of the PIT peaks at two frequency domains are nearly close. And the PIT device has identical response to the various polarized incident field due to the center symmetry of the metallic nanostructure, which have advantages in practical applications with no polarization-dependent loss.

Nanoscale displacement measurement in a variable-air-gap optical waveguide

Instead of analyzing the fringe shift that was recently developed by us to detect slight displacement, an alternative approach by monitoring changes in the intensity of the light reflected from a variable-air-gap optical waveguide is presented in this work. Owing to the sensitive feature of the ultrahigh-order modes, a 1.7 nm resolution of displacement measurement is demonstrated without any complicated optical interference system and servotechniques.