Numerical study of a high-resolution far-field scanning optical microscope via a surface plasmon-modulated light source

In this paper, we analyze light transmission through a single subwavelength slit surrounded by periodic grooves in layered films consisting of An and dielectric material. A subwavelength grating is scanned numerically by the finite difference time domain method in two dimensions. The results show that the transmission field can be confined to a spot with subwavelength width in the far field and can be useful in the application of a high-resolution far-field scanning optical microscope.

A proposal for low cross-talk square-lattice photonic crystal waveguide intersection utilizing the symmetry of waveguide modes

We propose an approach to construct waveguide intersections with broad bandwidth and low cross-talk for square-lattice photonic crystals. by utilizing a vanishing overlap of the propagation modes in the waveguides created by defects which support dipole-like defect modes. The finite-difference time-domain method is used to simulate the waveguide intersection created in the two-dimensional square-lattice photonic crystals. Over a bandwidth of 30 nm with the center wavelength at 1300 nm, transmission efficiency above 90% is obtained with cross-talk below -30 dB. Especially, we demonstrate the transmission of a 500-fs pulse at 1.3 Am through the intersection, and the pulse after transmission shows very little distortion while the cross-talk remains at low level meantime. (c) 2006 Elsevier B.V. All rights reserved.

Optical interleaver based on directional coupler in a 2D photonic crystal slab with triangular lattice of air holes

A small-size optical interleaver based on directional coupler in a 2D photonic crystal slab with triangular lattice of air holes is designed and theoretically simulated using plane wave expansion and finite-difference time-domain method. The interleaver is formed by two parallel and identical photonic crystal slab waveguides which are separated by three rows of air holes. The coupling region is designed below the light line to avoid vertical radiation. The simulated results show that the coupling coefficient is increased and the final length of the interleaver is decreased by enlarging the radius of the middle row of air holes. The transmission properties are analyzed after the interleaver's structure is optimized, and around 100 GHz channel spacing can be got when the length of the interleaver is chosen as 40.5 mu m. (c) 2006 Elsevier B.V. All rights reserved.

Effect of disorder on self-collimated beam in photonic crystal

We report a numerical analysis of various types of disorder effects on self-collimated beam in two-dimensional photonic crystal. Finite-difference time-domain (FDTD) method is used to simulate the process by using a pulse propagation technique. The position disorders along the directions parallel and perpendicular to the incidence are considered. We show that random disorder along the perpendicular direction will have a lesser effect on the performance of the dispersion waveguides than those along the parallel direction. Furthermore, the self-collimation waveguide (SCW) has new characteristics when compared with the photonic crystal line defect waveguide. (c) 2006 Elsevier B.V. All rights reserved.

Design of two-dimensional photonic crystal edge emitting laser for photonic integrated circuits

An edge emitting laser based on two-dimensional photonic crystal slabs is proposed. The device consists of a square lattice microcavity, which is composed of two structures with the same period but different radius of air-holes, and a waveguide. In the cavity, laser resonance in the inner structure benelits from not only the anomalous dispersion characteristic of the first band-edge at the M point in the first Brillouin-zone but also zero photon states in the outer structure. A line defect waveguide is introduced in the outer structure for extracting photons from the inner cavity. Three-dimensional finite-difference time-domain simulations apparently show the in-plane laser output from the waveguide. The microcavity has an effective mode volume of about 3.2(lambda/eta(slab))(3) for oscillation -mode and the quality factor of the device including line defect waveguide is estimated to be as high as 1300.

Dipole mode photonic crystal point defect laser on InGaAsP/InP

In this paper, we focus on the dipole mode of the two-dimensional (2D) photonic crystal (PC) single point defect cavity (SPDC) lasers and we report the fabrication and characterization of 2D PC SPDC lasers with the structure of adjusted innermost air holes. The photonic band and cavity Q factors are simulated by means of plane wave expansion (PWE) and finite-difference time-domain (FDTD), respectively. In order to improve the optical confinement of the SPDC, the diameter of the innermost holes was adjusted. Different lasing performances are observed experimentally. The experimental results agree with the theoretical prediction very well. (c) 2006 Elsevier B.V. All rights reserved.

WG-like modes selectivity in a square cavity with posts

We investigate the characteristics of Whispering-Gallery(WG)-like modes in a square cavity with posts by employing the two-dimentional (2D) finite-difference time-domain (FDTD) technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor (i.e. Q-factor) of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases with the mode spacing of square cavity twice in an optimized strucure.

Line defect splitters for self-collimated beams in photonic crystals

A one-to-two splitter for self-collimated beams in photonic crystal (PC) is designed by inserting one row of line defects. Finite-difference time-domain (FDTD) method is used to simulate the light propagation process. Our systematical studies show that the splitting ratio is a function of the airholes size of the line defect radius, and stays fairly constant as a function of frequency. Furthermore, it is shown the numerical results can be analyzed by coupled-mode theory. (C) 2005 Elsevier B.V. All rights reserved.

A differential recursive scheme for suppression of Peak to average power ratio for OFDM

A differential recursive scheme for suppression of Peak to average power ratio (PAPR) for Orthogonal frequency division multiplexing (OFDM) signal is proposed in this thesis. The pseudo-randomized modulating vector for the subcarrier series is differentially phase-encoded between successive components in frequency domain first, and recursion manipulates several samples of Inverse fast Fourier transformation (IFFT) output in time domain. Theoretical analysis and experimental result exhibit advantage of differential recursive scheme over direct output scheme in PAPR suppression. And the overall block diagram of the scheme is also given.

Mode characteristics for equilateral triangle optical resonators

Mode characteristics of equilateral triangle resonators (ETRs) are analyzed based on the symmetry operation of the point group C-3v. The results show that doubly degenerate eigenstates can be reduced to the A(1) and A(2) representations of C-3v, if the longitudinal mode number is a multiple of 6; otherwise, they form the E irreducible representation Of C-3v. And the one-period length for the mode light ray is half of the perimeter of the ETR. Mode Q-factors are calculated by the finite-difference time-domain (FDTD) technique and compared with those calculated from far-field emission based on the analytical near-field pattern for TE and TM modes. The results show that the far-field emission based on the analytical field distribution can be used to estimate the mode Q-factor, especially for TM modes. FDTD numerical results also show that Q-factor of TE modes reaches maximum value as the longitudinal mode number is a multiple of 7. In addition, photoluminescence spectra and measured Q-factors are presented for fabricated ETR with side lengths of 20 and 30 mu m, and the mode wavelength intervals are compared with the analytical results.

Analysis of mode characteristics for deformed square resonators by FDTD technique

The mode frequencies and quality factors (Q-factors) in two-dimensional (2-D) deformed square resonators are analyzed by finite-difference time-domain (FDTD) technique. The results show that the deformed square cavities with circular and cut corners have larger Q-factors than the perfect ones at certain conditions. For a square cavity with side length of 2 mu m and refractive index of 3.2, the mode Q-factor can increase 13 times as the perfect corners are replaced by a quarter of circle with radius of 0.3 pm. Furthermore the blue shift with the increasing deformations is found as a result of the reduction in effective resonator area. In square cavities with periodic roughness at sidewalls which maintains the symmetry of the square, the Q-factors of the whisperin gallery (WG)-like modes are still one order of magnitude larger that those of non-WG-like modes. However, the Q-tactors of these two types of modes are of the same order in the square cavity with random roughness. We also find that the rectangular and rhombic deformation largely reduce the Q-factors with the increasing offset and cause the splitting of the doubly degenerate modes due to the breaking of certain symmetry properties.

Analysis of mode quality factors and mode reflectivities for nanowire cavity by FDTD technique

The mode frequency and the quality factor of nanowire cavities are calculated from the intensity spectrum obtained by the finite-difference time-domain (FDTD) technique and the Pade approximation. In a free-standing nanowire cavity with dielectric constant epsilon = 6.0 and a length of 5 mu m, quality factors of 130, 159, and 151 are obtained for the HE11 modes with a wavelength around 375 nm, at cavity radius of 60, 75, and 90 nm, respectively. The corresponding quality factors reduce to 78, 94, and 86 for a nanowire cavity standing on a sapphire substrate with a refractive index of 1.8. The mode quality factors are also calculated for the TE01 and TM01 modes, and the mode reflectivities are calculated from the mode quality factors.

Analysis of mode quality factors for equilateral triangle semiconductor microlasers with rough sidewalls

The eigenmode characteristics for equilateral triangle resonator (ETR) semiconductor microlasers are analysed by the finite-difference time-domain technique and the Pade approximation. The random Gaussian correlation function and sinusoidal function are used to model the side roughness of the ETR. The numerical results show that the roughness can cause the split of the degenerative modes, but the confined modes can still have a high quality factor. For the ETR with a 3 mum side length and the sinusoidal fluctuation, we can have a quality factor of 800 for the fundamental mode in the wavelength of 1500 nm, as the amplitude of roughness is 75 mn.

Analysis of semiconductor microlasers with an equilateral triangle resonator by rate equations

Semiconductor microlasers with an equilateral triangle resonator (ETR) are analyzed by rate equations with the mode lifetimes calculated by the finite-difference time-domain technique and the Pade approximation. A gain spectrum based on the relation of the gain spectrum and the spontaneous emission spectrum is proposed for considering the mode selection in a wide wavelength span. For an ETR microlaser with the side length of about 5 mum, we find that single fundamental mode operation at about 1.55 mum can be obtained as the side length increases from 4.75 to 5.05 mum. The corresponding wavelength tuning range is 93 nm, and the threshold current is about 0.1 to 0.4 mA.

Computation of resonant frequencies and quality factors of cavities by FDTD technique and Pade approximation

The finite-difference time domain (FDTD) technique and the Pade approximation with Baker's algorithm are used to calculate the mode frequencies and quality factors of cavities. Comparing with the fast Fourier transformation/Pade method, we find that the Fade approximation and the Baker's algorithm can obtain exact resonant frequencies and quality factors based on a much shorter time record of the FDTD output.