Yanhui Zhao

Directional excitation of surface plasmons with subwavelength slits

We propose a method to manipulate the excitation direction of surface plasmons using subwavelength slits fabricated in a metallic film. By designing the specific effective index for each slit, the relative phase of plasmonics generated at the slit exit aperture can be tailored. Therefore, the electromagnetic field intensity along one direction on the metal surface can be enhanced or suppressed by surface plasmon interference. Numerical calculations performed by the finite-difference time-domain method illustrate our theoretical design.

Subwavelength imaging with anisotropic structure comprising alternately layered metal and dielectric films

Subwavelength imaging can be obtained with alternately layered metallodielectric films structure, even when the permittivity of metal and dielectric are not matched. This occurs as the effective transversal permittivity tends to be zero or the vertical one approaches infinity, depending on the permittivity value of the utilized dielectric and metal material. Evanescent waves can be amplified through the structure, but not in a manner of fully compensating the exponentially decaying property in dielectric. Numerical illustration of subwavelength imaging is presented for variant configuration of anisotropic permittivity with finite layer number of metallodielectric films.

Sub-diffraction-limited interference photolithography with metamaterials

We present that an interference lithography technique beyond the diffraction limit can be theoretically achieved by positing an anisotropic metamaterial under the conventional lithographic mask. Based on the special dispersion characteristics of the metamaterial, only the enhanced evanescent waves with high spatial frequencies can transmit through the metamaterial and contribute to the lithography process. Rigorous coupled wave analysis shows that with 442nm exposure light, one-dimensional periodical structures with 40nm features can be patterned. This technique provides an alternative method to fabricate large-area nanostructures.

Ultrafast optical switching using photonic molecules in photonic crystal waveguides.

We study the coupling between photonic molecules and waveguides in photonic crystal slab structures using finite-difference time-domain method and coupled mode theory. In a photonic molecule with two cavities, the coupling of cavity modes results in two super-modes with symmetric and anti-symmetric field distributions. When two super-modes are excited simultaneously, the energy of electric field oscillates between the two cavities. To excite and probe the energy oscillation, we integrate photonic molecule with two photonic crystal waveguides. In coupled structure, we find that the quality factors of two super-modes might be different because of different field distributions of super-modes. After optimizing the radii of air holes between two cavities of photonic molecule, nearly equal quality factors of two super-modes are achieved, and coupling strengths between the waveguide modes and two super-modes are almost the same. In this case, complete energy oscillations between two cavities can be obtained with a pumping source in one waveguide, which can be read out by another waveguide. Finally, we demonstrate that the designed structure can be used for ultrafast optical switching with a time scale of a few picoseconds.

Super resolution imaging by compensating oblique lens with metallodielectric films.

In this letter we propose a compensating oblique lens which can realize super resolution imaging. The imaging structure consists of two parts constructed by metallodielectric films but positioned in different orientation. Super resolution optical imaging can be obtained with uniform light intensity and tunable magnification by changing parameters of the structure. Design principles and examples are given and illustrated with numerical simulation.

Longitudinal wave function control in single quantum dots with an applied magnetic field

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.

Resolving exciton diffusion in InGaAs quantum wells using micro-photoluminescence mapping with a lateral excitation

We report a spatially resolved photoluminescence mapping of InGaAs quantum wells. The photoluminescence was collected on top of the quantum well, with a HeNe laser pumping horizontally or vertically. In the horizontal configuration, at temperature of 68 K, the spectral linewidth narrows from 2.8 to 2.2 meV with the peak shifting from 1.4425 to 1.4415 eV, while at 3.8 K these changes were not observed. This demonstrates that photo-generated carriers can diffuse away from the laser spot and relax to the lower energy states in the case when the charge carriers are thermally activated. The spectra narrowing in the vertical configuration, which could not be observed, is due to the fact that the emitted light was always collected from the same spot of the pumping laser without diffusion.

Refractive index of a single ZnO microwire at high temperatures

We report a study of refractive index of a wurtzite ZnO single crystal microwire at a temperature range from room temperature to about 400 K using optical cavity modes. The photoluminescence (PL) spectra of the ZnO microwire at different temperatures were performed using a confocal micro-photoluminescence setup. The whispering gallery modes observed in the PL spectra show a redshift both in the ultraviolet and the visible range as the temperature rises. The redshift is used to extract the refractive index of the ZnO microwire. The dispersion relations are deduced at different temperatures, and the results show that the refractive index increases with raising temperature for both transverse electric and transverse magnetic modes. The refractive index increases faster at a shorter wavelength, which is due to the fact that the shorter wavelength is closer to the resonance frequencies of ZnO microwire according to the Lorentz oscillator model.

Demagnifing super resolution imaging based on surface plasmon structures

An imaging mechanism for demagnifing features above wavelength into desired images beyond the diffraction limit is proposed in this letter. The super resolution ability (about two times and even more that of diffraction limit) arises from the surface plasmon wave excitation and amplification associated with metallic grating structure. Two specifically designed masks are projected to the grating surface from both sides, at one of which the superimposed field forms the desired images. Conceptually formalism of the imaging process is presented using spatial Fourier analysis and illustrated with numerical simulations.

Recombination processes in CuInS2/ZnS nanocrystals during steady-state photoluminescence

We report on a temperature- and excitation-power-dependent photoluminescence (PL) study of CuInS2/ZnS nanocrystals dispersed on a SiO2/Si substrate with a confocal micro-PL system. With increasing the excitation power at 22 K and room temperature, the PL spectra are blue-shifted because of the state filling. At low temperature, a small peak is observed at the low energy side of the spectrum, which could be due to the Forster resonance energy transfer between different nanocrystals. The integrated PL intensity increases sublinearly as a function of excitation power with a power factor of around 2/3, which demonstrates the Auger recombination dominated process in the nanocrystals, especially under the high excitation power.