Conformal coverage for two-dimensional arrays of microcavites with quasi-three dimensional confinement by distributed Bragg reflectors

Different from conventional three-dimensional confined microcavity fabrication method in which micropillar microcavities were obtained through the etching of planar semicoductor microcavities, we adopted the conformal coverage to fabricate two-dimensional arrays of quasi three-dimensional confined optical microcavities providing both vertical and lateral optical confinement by the distributed Bragg reflectors (DBRs). Our microcavity samples were directly deposited on the patterned substrates with two-dimensional arrays of air holes. The SEM and cross-section TEM images show that the periodicity of the patterned substrate was still kept after deposition while the growth of DBRs along the sidewalls occurred simultaneously, which provided the transverse optical confinement. In order to probe the optical modes of this kind of microcavities, room temperature photoluminescence signals from prepared microcavities were detected. Three resonant modes were presented and exhibited obvious angular dependence. We attributed these phenomena to quantization of the in-plane wave vector components confined by lateral DBRs.     

Far field emission of micropillar and planar microcavities lattice-matched to ZnTe

We investigate light emission from ZnTe-based microcavities containing CdTe quantum dots (QDs), with 2D (planar cavity) and 0D (pillar cavities) photonic confinement. The angular distribution from the planar cavity is presented as well as 2D cross-sections of the far field distribution of radiation from the micropillars. The efficient and desirable modification of the isotropic radiation of the QDs is shown for such structures. The diffraction observed is found to be inherent for such experiments with large numerical aperture of the lens and small diameters of the investigated pillars. This diffraction is successfully modeled.     

Multiwavelength ultralow-threshold lasing in quantum dot photonic crystal microcavities

We demonstrate multiwavelength lasing of resonant modes in linear (L3) microcavities in a triangular-lattice 2D photonic crystal (PC) slab. The broad spontaneous emission spectrum from coupled quantum dots, modified by the PC microcavity, is studied as a function of the intensity of incident optical excitation. We observe lasing with an ultralow-threshold power of approximately 600 nW and an output efficiency of approximately 3% at threshold. Two other resonant modes exhibit weaker turnon characteristics and thresholds of approximately 2.5 and 200 microW, respectively.     

Dot-in-a-dot: electronic and photonic confinement in all three dimensions

We have studied the optical properties of three-dimensionally confined photon states in a spherical microcavity (the photonic dots) resonantly excited by photons emitted from semiconductor nanocrystals (the quantum dots). Glass and polymer microspheres with sizes of 2<R<10 are characterized by spatially and temporally resolved micro-photoluminescence. The role of nanocrystal position and orientation is analyzed experimentally and theoretically. The emission spectra of single, bulk and hollow microspheres impregnated with CdSe quantum dots and quantum rods are investigated and the modification of the quantum dot radiative lifetime by the three-dimensional photon confinement is discussed. PACS 78.66.Hf; 61.46.+w; 42.60.Da     

New nonlinear and dispersion flattened photonic crystal fiber with low confinement loss

A new nonlinear dispersion flattened photonic crystal fiber with low confinement loss is proposed. This fiber has threefold symmetry core. The doped region in the core and the big air-holes in the 1st ring can make high nonlinearity in the PCF. And the small air-holes in the 1st ring and the radial increasing diameters air-holes rings in cladding can be used to achieve the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCFs structure parameters. A PCF with flattened dispersion is obtained. The dispersion is less than 0.8 ps/(nm km) and is larger than −0.7 ps/(nm km) from 1.515 μm to 1.622 μm. The nonlinear coefficient is about 12.6456 W⁻¹ km⁻¹, the fundamental mode area is about 10.2579 μm². The confinement loss is 0.30641 dB/km. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal fibers with high nonlinearities.     

Photoreflectance study of energy level structure of self-assembled InAs/GaAs quantum dots emitting at 1.3 μm

Photoreflectance and photoluminescence measurements were performed on the ensemble of self assembled InAs/GaAs quantum dots designed to emit at 1.3 μm. As many as six QDs-related optical transitions were observed in PR spectra, the energies of which were confirmed by high-excitation PL results. Numerical calculations allowed estimating the average size of the dots, which is larger than for standard InAs/GaAs QDs. This result is in agreement with structural data. Additionally, the energy level structure for such QDs was derived and compared with the electronic structure of standard InAs/GaAs dots. It was shown that the energy level structure of such large dots qualifies them for the active region of a laser emitting at 1.3 μm.     

GaAs pyramids on GaAs/AlAs Bragg reflectors as alternative microcavities

Pyramidal microcavities are a new class of optical resonators with potentially small mode volume and high quality factor. Our GaAs pyramids with embedded InGaAs quantum dots are placed on top of GaAs/AlAs distributed Bragg reflectors to increase the optical confinement at the base of the pyramids. The pyramidal shape is achieved by a wet-chemical etching process using an AlAs sacrificial layer. Temperature-dependent micro-photoluminescence measurements are used to verify optical modes.     

Evidence for confined tamm plasmon modes under metallic microdisks and application to the control of spontaneous optical emission

We demonstrate strong confinement of the optical field by depositing a micron sized metallic disk on a planar distributed Bragg reflector. Confined Tamm plasmon modes are evidenced both experimentally and theoretically, with a lateral confinement limited to the disk area and strong coupling to TE polarized fields. Single quantum dots controllably coupled to these modes are shown to experience acceleration of their spontaneous emission when spectrally resonant with the mode. For quantum dots spectrally detuned from the confined Tamm plasmon mode, an inhibition of spontaneous emission by a factor 40±4 is observed, a record value in the optical domain.     

Tunable resonant optical microcavities by self-assembled templating

Micrometer-scale optical cavities are produced by a combination of template sphere self-assembly and electrochemical growth. Transmission measurements of the tunable microcavities show sharp resonant modes with Q factors of >300 and 25-fold local enhancement of light intensity. The presence of transverse optical modes confirms the lateral confinement of photons. Calculations show that submicrometer mode volumes are feasible. The small mode volumes of these microcavities promise to lead to a wide range of applications. in microlasers, atom optics, quantum information, biophotonics, and single-molecule detection.     

Research on terahertz photonic crystal fiber characteristics with high birefringence

A new high birefringence photonic crystal fiber is proposed within the terahertz frequency region. It has two types of claddings, the inner is composed of six ellipse air holes arranged in a honeycomb array and the outer surrounded by circle holes. By using the full vector finite element method with anisotropic perfectly matched layers absorption boundary condition, the birefringence, chromatic dispersion and confinement loss of the fundamental mode are evaluated. The results show that the birefringence can achieve 10⁻³ when the wavelength increases from 600 μm to 900 μm. This structure will provide some reference value for the designing of high birefringence terahertz photonic crystal fiber.     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Alumina capped ZnO quantum dots multilayer grown by pulsed laser deposition

To extend the applicability of ZnO, with the bulk band gap of about 3.3 eV, into deep UV region, we have grown a multilayer of alumina capped ZnO quantum dots of mean in-plane sizes in the range of ∼1.8-3.6 nm at room temperature using alternate Pulsed Laser Deposition. Size dependent blue shift of the band gap of these dots up to ∼4.5 eV is observed in the optical absorbance spectra. The observed blue shift can be understood using the effective mass approximation in weak and strong confinement regimes.     

Loss-reduced highly birefringent selectively liquid-filled photonic crystal fibers

We have theoretically investigated the birefringence and loss properties of the selectively liquid-filled photonic crystal fibers with the liquid asymmetrically infiltrated into one-line air holes along x-axis. A high birefringence value B = 1.74 × 10⁻³ can be achieved at λ = 1.55 μm. By varying the index of the infiltrating liquid, the birefringence values are shown to be well tuned. In addition, the confinement losses can be efficiently reduced by diminishing the number of liquid holes, which is quite useful for optical devices.     

Planar defects in three-dimensional chalcogenide glass photonic crystals

Here we report on the direct laser writing fabrication of Fabry-Perot-type planar microcavities in a three-dimensional (3D) photonic crystal (PhC) embedded within a high-refractive nonlinear chalcogenide glass (ChG) film. The fabricated planar microcavities in a nonlinear ChG 3D PhC facilitate the observation of resonant modes inside the stop gap. The experimental results show that the length of the planar cavity can be well controlled by the fabrication power and thus be used to tune the defect modes. The tunability of the observed defect modes is confirmed by the theoretical prediction.     

Photonic crystal fiber with high birefringence and low confinement loss

In this paper, a highly birefringent index-guiding photonic crystal fiber with low confinement loss is proposed by enlarging the central row of air holes in the structure. By employing the multipole method, properties of this structure, including the effective index, birefringence and confinement loss, are investigated. Simulation results indicate that high birefringence of 1.65 × 10⁻³ can be reached at the wavelength of 1.55 μm, and a low confinement loss on the order of 10⁻⁶ dB/km can be achieved at the same wavelength. Moreover, the impacts of air hole sizes on birefringence and confinement loss are also analyzed in detail.     

Compact,wide bandwidth,multi-channel power dividers based on one-dimensional photonic crystal waveguides

We propose novel compact multiport power dividers based on one-dimensional photonic crystal omnidirectional reflective waveguides. The proposed power dividers have advantages of wide bandwidth, flexible extended output channel number and compact size. The power dividers are numerical simulations using finite-difference time-domain method. Near-complete transmission and uniform at every branch of output power of multi-port power dividers are observed within wide frequency range. For a 1D PhC 1 × 6 power divider, the six output port achieved nearly 16.5% transmission at each arm from 1545 nm to 1553 nm and the size of 1 × 6 power divider is 14.3 μm × 14.3 μm at 1.55 μm operation wavelength.     

Large negative dispersion in dual-concentric-core photonic crystal fiber with hybrid cladding structure based on complete leaky mode coupling

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm². The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.     

pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions

pH-dependent aggregation of thiol-capped CdTe quantum dots (QDs) in solutions was observed with a confocal microscope. The average size of the QD aggregates increased from 28 nm to 1.4 μm as the pH decreased from 12 to 3. The basic condition improved the dispersion of QDs while the acidic condition caused the detachment of surface ligands, leading to the aggregation of QDs. A PL lifetime of 80 ns was detected for QDs at pH from 12 to 7, while it was shortened to 57 and 34 ns at pH 5 and 3, respectively, due to the formation of surface defects.     

Photoluminescence study of ZnCdSe/ZnSe quantum dot systems

The optical properties of the populated ZnCdSe/ZnSe quantum dots have been studied by photoluminescence spectra measured with different laser excitation apertures at temperatures from 22 to 300 K. The differences of spectral features between small and large excitation spot suggest the existence of quantum dot size fluctuation in the system. The temperature evolution of photoluminescence spectral features revealed that two types of quantum dots with different densities and sizes coexist in ZnCdSe/ZnSe system. The energy spacings of the two kinds of quantum dot emissions are about 50 meV at various temperatures. The thermally activated lateral transfer processes of carriers populated in the two sorts of quantum dots are investigated by temperature dependences of spectral intensities.     

Investigations of optical coherence properties in an erbium-doped silicate fiber for quantum state storage

We studied optical coherence properties of the 1.53 μm telecommunication transition in an Er³⁺-doped silicate optical fiber through spectral holeburning and photon echoes. We find decoherence times of up to 3.8 μs at a magnetic field of 2.2 T and a temperature of 150 mK. A strong magnetic-field dependent optical dephasing was observed and is believed to arise from an interaction between the electronic Er³⁺ spin and the magnetic moment of tunneling systems in the glass. Furthermore, we observed fine-structure in the Erbium holeburning spectrum originating from superhyperfine interaction with ²⁷Al host nuclei. Our results show that Er³⁺-doped silicate fibers are promising material candidates for quantum state storage.