Band-edge Effect Research Articles

Photoluminescence and Microstructure Investigation of SiO2@Y:Eu Photonic Bandgap Crystals

A photonic bandgap (PBG) crystal was established by self-assembly of monodispersed SiO2 submicrometer spheres. SiO2@Y2O5/or Y2SiO5:Eu3+ phosphors were then prepared by coating of Y3+ and Eu3+ at various concentrations on SiO2 PBG crystal templates. After calcination, red emission was obtained. In this study, the photoluminescent properties varied with different SiO2 sizes (periodicity of the PBG crystal), structure (ordered or random arrangement), and different calcination temperatures. Apparent enhancement of emission can be obtained in the phosphor with an ordered PBG crystal character because of the band-edge effect. Transmission electron microscopy cross-sectional images showed a core–shell structure of SiO2@Y2O3/or Y2SiO5:Eu3+ with a uniform 4–10 nm thickness of the crystalline shell layer.

High efficiency operation of butt joint line-defect-waveguide microlaser in two-dimensional photonic crystal slab

Butt joint line-defect-waveguide microlasers are demonstrated on photonic crystal slabs with airholes in a triangular lattice. Such microlaser is designed to increase the output power from the waveguide edge directly. The output power is remarkably enhanced to 214 times higher by introducing chirped structure in the output waveguide. The lasing mode operates in the linear dispersion region of the output waveguide so that the absorption loss due to the band-edge effect is reduced. The laser resonance is illustrated theoretically using the finite difference time domain method. A practical high power efficiency of 20% is obtained in this microlaser.

Lasing Characteristics of Ferroelectric Liquid Crystal in Dielectric Mirror Cavity

A laser action of ferroelectric liquid crystal (FLC) in a dielectric mirror cavity was investigated. The single-mode laser action was observed at the band edge of the FLCs. The lasing threshold was much lower than that of the simple FLCs without an external cavity because of the double optical confinement effects caused by the band-edge effect of the FLCs and a dielectric mirror cavity. The effect of the double optical confinement was confirmed by applying the electric field to unwound the FLC helix.

Local observation of modes from three-dimensional woodpile photonic crystals with near-field microspectroscopy under supercontinuum illumination

A near-field microscope coupled with a near-infrared (NIR) supercontinuum source is developed and applied to characterize optical modes in a three-dimensional (3D) woodpile photonic crystal (PC) possessing a NIR partial bandgap. Spatially resolved near-field intensity distributions under different illumination wavelengths demonstrate that the electric fields preferentially dwell in the polymer rods or in the gaps between rods, respectively, for frequencies below or above the stop gap, as predicted by the 3D finite-difference time-domain modeling. Near-field microspectroscopy further reveals that the position-dependent band-edge effect plays an important role in PC-based all-optical integrated devices.

GaN Photonic-Crystal Surface-Emitting Laser at Blue-Violet Wavelengths

Shorter-wavelength surface-emitting laser sources are important for a variety of fields, including photonics, information processing, and biology. We report on the creation of a current-driven blue-violet photonic-crystal surface-emitting laser. We have developed a fabrication method, named "air holes retained over growth," in order to construct a two-dimensional gallium nitride (GaN)/air photonic-crystal structure. The resulting periodic structure has a photonic-crystal band-edge effect sufficient for the successful operation of a current-injection surface-emitting laser. This represents an important step in the development of laser sources that could be focused to a size much less than the wavelength and be integrated two-dimensionally at such short wavelengths.

Photonic crystal templates for organic solid-state lasers

We report a solid-state laser composed of an Al2O3 photonic crystal and an organic gain medium. The photonic crystal was fabricated through the nanoimprinting technique and an electrochemical process, which enables transfer of a periodic pattern to the photonic crystal and control of its stop band. When the photonic crystal was infiltrated with the polymer medium containing laser dye, we obtained laser emission under optical excitation. The minimum laser threshold was about 2.4nJ∕pulse, where the linewidth of the laser emission was as narrow as 0.09 nm. We explain the laser action based on the band edge effect, while the precise control of the laser mode and oscillation wavelength depends on control of the stop band and small changes in the refractive index of the gain medium.

Spontaneous emission from a four-level atom embedded in photonic crystals via quantum interference

The spontaneous emission of a four-level atom embedded in a one-dimensional photonic crystal is investigated. The atom has two upper levels coupled by the same vacuum modes to a common lower level and is driven by a coherent field to an auxiliary level. Spontaneous emission can be suppressed significantly due to the combinational influences of the interference effect and the band edge effect. The radiation field emitted by the atom is also studied. Spontaneous emission can be modified by controlling the Rabi frequency of the driving.

Spontaneous emission properties of a two-level atom with an intense low-frequency field in a three-dimensional photonic crystal

We have investigated spontaneous emissions from a two-level atom embedded in a p hotonic crystal with three-dimensional dispersion relation as well as interacted with a coherent intense low-frequency field .Due to this field, additional deca y channels with exchange of one low-frequency photon during an atomic transition are created, resulting in the spontaneously induced quantum interference. The s pontaneous emission can be suppressed significantly because of the combinational influences of the quantum interference effect and the band edge effect. The pop ulation in the upper levels depends on the relative position of the upper level from the band edge. The frequency of the low-frequency field and the relative in tensity of the coupling strength for the transitions can affect spontaneous emis sion from the atom.

Band-edge photorefractive effect in semiconductors

We report here for the first time, two-beam coupling energy transfer using the Franz–Keldysh electrorefractive effect in photorefractive semiconductors. A large beam coupling gain coefficient (Γ=2.8 cm−1) exceeding the absorption coefficient (α=2.0 cm−1) is obtained in undoped semi-insulating GaAs using the new photorefractive process. The new photorefractive process differs from conventional photorefractivity in that the direction of energy transfer is dictated by the direction of an externally applied electric field. Using a moving grating technique and combining the electrorefractive grating with the conventional electro-optic grating, a very large gain Γ=16.3 cm−1 (α=3 cm−1, crystal length l=4mm) resulting in net amplification by more than a factor of 200 has been demonstrated.

Junction field-effect transistor single quantum well optical waveguide modulator employing the two-dimensional Moss–Burstein effect

A modulation-doped junction field-effect transistor incorporating an optical waveguide under the gate modulates light by the carrier band-filling effect (two-dimensional Moss–Burstein effect) in a single quantum well, achieving a 5:1 extinction ratio in a 250-μm-long waveguide for 4 V reverse gate-source bias Vgs swing and 0 V drain-source bias Vds. Similar performance is obtained over a 16 nm spectral range. A novel band-edge transparency effect is observed for Vds>0 allowing an extinction ratio of 10:1, corresponding to a change in absorption of 92 cm−1 to be obtained through band-gap dilation by hot electrons at biases of Vds =8 V. Below-band-gap refractive index modulation of 1.6×10−3 is obtained for a Vgs swing of 2.4 V. The novel junction field-effect transistor optical modulator also functions as a photovoltaic or photoconductive optical detector, a transistor, and a light-emitting diode.