Optical modes in 2-D imperfect square and triangular microcavities

Transformation of the whispering-gallery (WG)-modes in imperfect two-dimensional square and triangular microcavities with various degrees of deformation is studied by means of the Muller boundary integral equation technique. Continuous transformation of a circular microcavity, which supports double-degenerate WG-modes, toward a square or a triangle removes the degeneracy of certain WG-modes. The spectrum of the modes that split depends on the symmetry of the emerging microcavity: WG/sub 2m,n/-modes in case of a C/sub 4v/ symmetry (square) or WG/sub 3m,n/-modes in the case of C/sub 3v/ symmetry (triangle). In both cases, the modes with the highest Q-factors are nondegenerate modes with antisymmetrical field patterns. We estimate mode frequencies, quality factors and field distributions of practically achievable rather than "ideal" square and triangular microcavities, and compare the effect of various types of fabrication imperfections (corner sharpness, sidewall curvature and surface roughness) on their characteristics. Accurate study of the modal spectra enables us to confirm and explain previous observations, such as: 1) co-existence of the WG-like and volume modes in square microcavities; 2) the separation of the high-Q WG-like modes being twice that determined by the cavity length; and 3) much lower Q-factors of realistic concave-wall triangular microcavities than those of their ideal counterparts.     

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/spl acute/ approximation. In a free-standing nanowire cavity with dielectric constant /spl epsiv/=6.0 and a length of 5 /spl mu/m, quality factors of 130, 159, and 151 are obtained for the HE/sub 11/ 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 TE/sub 01/ and TM/sub 01/ modes, and the mode reflectivities are calculated from the mode quality factors.     

Modal analysis and engineering on InP-based two-dimensional photonic-crystal microlasers on a Si wafer

We report results on hexagonal-shaped microlasers formed from two-dimensional photonic crystals (PCs) using InP-based materials transferred and bonded onto SiO/sub 2// Si wafers. Two types of hexagonal cavities are investigated : single defect (one hole missing) cavities, so-called H1 cavities (1 /spl mu/m in diameter) and two holes missing per side H2 cavities (2 /spl mu/m in diameter). Their optical properties are analyzed using photoluminescence experiments, and plane wave method simulations have been performed for comparison. High Q modes (/spl sim/600/700) have been measured and they have been shown to enable laser effect at room temperature, under pulsed optical pumping (15% duty cycle and 25-ns pulsewidth). The study of these efficient mode characteristics gives guidance for further improvement of the operation conditions of PC lasers, such as the reduction of the threshold pumping power.     

Fabrication of high-packing-density vertical cavity surface-emitting laser arrays using selective oxidation

We describe the nearly-planar processing of two-dimensional vertical cavity laser arrays based on the selective conversion of AlAs to Al/sub x/O/sub y/. The individual lasers of 8/spl times/8 and 2/spl times/2 arrays are defined by native Al/sub x/O/sub y/ to achieve 4-/spl mu/m square active regions on 12-/spl mu/m center-to-center spacings. Interelement thermal coupling is characterized along with the optical mode structure.     

Lasing characteristics of GaInAsP-InP strained quantum-well microdisk injection lasers with diameter of 2-10 /spl mu/m

We have obtained pulsed lasing operation in 2-5-/spl mu/m diameter microdisk injection lasers using GaInAsP-InP compressively-strained multiple-quantum-well (MQW) wafers around room temperature. The effective cavity volume of the 2-/spl mu/m-diameter device is the smallest among those for any type of electrically-pumped lasers. The threshold current of this device was as low as 0.2 mA. Cavity modes in emission spectra observed under CW conditions coincide well with theoretically predicted whispering gallery modes. Further reduction of diameter to less than 1.5 /spl mu/m will realize the condition for spontaneous emission almost coupling into a single mode, which results in thresholdless lasing operation.     

Experimental analysis of the effect of metal thickness on the quality factor in integrated spiral inductors for RF ICs

The effect of metal thickness on the quality (Q-) factor of the integrated spiral inductor is investigated in this paper. The inductors with metal thicknesses of 5/spl sim/22.5 /spl mu/m were fabricated on the standard silicon substrate of 1/spl sim/30 /spl Omega//spl middot/cm in resistivity by using thick-metal surface micromachining technology. The fabricated inductors were measured at GHz ranges to extract their major parameters (Q-factor, inductance, and resistance). From the experimental analysis assisted by FEM simulation, we first reported that the metal thickness' effect on the Q-factor strongly depends on the innermost turn diameter of the spiral inductor, so that it is possible to improve Q-factors further by increasing the metal thickness beyond 10 /spl mu/m.     

"Anti" up the aperture [antiguided VCSEL structures]

Coupled vertical cavity surface-emitting laser (VCSEL) arrays are an attractive means to increase the coherent output power of VCSELs. Single-mode VCSELs, with output powers greater than 10 mW, would be useful as telecommunication transmitters /spl lambda/=1.3-1.55 /spl mu/m) or sources for optical interconnects. Commercially available single-mode VCSELs, even at shorter wavelengths /spl lambda/=0.85 /spl mu/m), are generally limited to a few milliwatts of output power. The conventional VCSEL structure incorporates a built-in positive-index waveguide, designed to support a single fundamental mode. Promising results in the 3-5 mW range (/spl lambda/=0.85 /spl mu/m) have been obtained from wet-oxidized, positive-index-guided VCSELs with small emission apertures (less than 3.5 /spl mu/m-dia). The small aperture size leads to a high electrical resistance and high current density, which can impact device reliability. By contrast, antiguided VCSEL structures have shown promise for achieving larger aperture single-mode operation. To obtain high single-mode powers with a larger emitting aperture, the use of a negative-index guide (antiguide) is beneficial. This paper discusses antiguided structures and some of their advantages when incorporated in 2-D VCSEL array structures.     

High-power highly reliable 1.02-1.06-/spl mu/m InGaAs strained-quantum-well laser diodes

By growing the InGaAs active layer at temperatures lower than in conventional growth, we extended the lasing wavelength and presented the high reliability in InGaAs strained-quantum-well laser diodes. Equivalent I-L characteristics were obtained for 1.02-, 1.05-, and 1.06-/spl mu/m laser diodes with a cavity length of 1200 /spl mu/m. Maximum output power as high as 800 mW and fundamental transverse mode operation at up to 400 mW were obtained at 1.06 /spl mu/m and an 1800-/spl mu/m cavity. Stable operation was observed for over 14 000 h under auto-power-control of 225 mW at 50/spl deg/C for the 1.02-, 1.05-, and 1.06-/spl mu/m lasers with a 900-/spl mu/m cavity.     

Characteristics of a photonic crystal defect waveguide-coupled quantum-dot photodiode

We have investigated the characteristics of an In/sub 0.4/Ga/sub 0.6/As self-organized quantum-dot (QD) resonant-cavity photodiode. The QD epitaxy and the design of the two-dimensional photonic crystal cavity are tailored for 1.3-/spl mu/m wavelength operation. The input excitation to the photodiode is provided with an in-plane defect waveguide designed with the same photonic crystal. The measured spectral photocurrent characteristics reflect mode coupling between the waveguide and detector and the resonant cavity effect due to total internal reflection and photonic bandgap confinement. The photocurrent response is explained with a model involving the circulating fields in the cavity. The characteristics are also dependent of cavity size. Enhancement and narrowing (/spl sim/ 10 nm) of the photoresponse at /spl lambda//spl sim/1.3 /spl mu/m are observed. A spectral dip, of /spl sim/ 10-nm width, also observed at 1.3 /spl mu/m is possibly due to the anticrossing mechanism, uniquely present in photonic crystal waveguides.     

High-power single-mode antiresonant reflecting optical waveguide-type vertical-cavity surface-emitting lasers

Antiresonant reflecting optical waveguide (ARROW) techniques are employed in vertical cavity surface emitting lasers (VCSELs) to achieve high-power single-mode emission. Using the effective-index method and fiber mode approximation, the cold-cavity lateral modal behavior for the circular shaped ARROW VCSEL demonstrates significant reduction of radiation loss from that of a single antiguide, while maintaining strong discrimination against high-order modes. The circular-waveguide is created by selective chemical etching and two-step metal-organic chemical vapor deposition growth, with proton implantation used to confine the current injection to the low-index core region. A single-mode CW power of 7.1 mW has been achieved from an 8 /spl mu/m diameter ARROW device (index step /spl Delta/n = 0.05, emission at /spl lambda//sub 0/ = 980 nm) with a far-field FWHM of 10/spl deg/. Larger aperture (12 /spl mu/m) devices exhibit multimode operation at lower drive currents with a maximum single-mode continuous-wave output power of 4.3 mW.     

New technique for quality factor measurements in undamped anddamped cavities

A new method for measuring quality factors in cavities is presented. This method is capable of measuring Q-factors in heavily damped as well as in undamped cavities. In addition, the possibility of separating overlapping modes and measuring their Q-factors is provided. Measurements on HOM (higher order mode) damped cavities for the DESY/THD linear collider project are presented     

Modes of multi-end-pumped nonplanar ring laser

A novel multi-end-pumped nonplanar ring Nd:YAG laser with two-mirror cavity was demonstrated. A model was developed for attaining all beam loop modes of the laser cavity and calculating the mirror separation of each mode. With single-diode laser pumping, the operations of different modes at 1.064 /spl mu/m were performed and the experimental results are in excellent agreement with the model. Using three laser diodes as the pumping sources led to over threefold increase in output power.     

Suppression of higher-order transverse modes in vertical-cavity lasers by impurity-induced disordering

Etched pillar, bottom emitting vertical cavity surface emitting lasers (VCSELs) employing a zinc diffused and disordered spatial mode filter are fabricated and tested. The filters successfully suppress lasing of higher order transverse modes in VCSELs with diameters greater than 11 /spl mu/m, while obtaining output powers of up to 1.5 mW with threshold currents as low as 2.7 mA. Emission is entirely single mode, with mode suppression ratio greater than 25 dB up to the highest output power levels.     

A new integrated optical-mode stripper configuration: Numerical analysis and design

A novel integrated-optical mode stripper configuration is proposed and analyzed numerically by the BPM. Its operation is based on the coupling of all higher order modes into regions with higher refractive index adjacent to the optical waveguide. It is shown that a high transmission of the fundamental mode and a high suppression of higher order modes can be realized at /spl lambda/=1.3 /spl mu/m and /spl lambda/=1.5 /spl mu/m.     

Room temperature operation of ultra-short quantum cascade lasers with deeply etched Bragg mirrors

Mid-infrared GaAs based bound-to-continuum quantum cascade microlasers with ridge waveguide geometry are fabricated by the monolithic integration of deeply etched semiconductor-air Bragg mirrors. Devices with ultra-short cavities of 50 and 150 /spl mu/m can be operated near room temperature (260 K) or at room temperature (300 K), respectively. 50 /spl mu/m-long devices show singlemode emission up to relatively high drive currents due to the large mode spacing of about 30 cm/sup -1/ (340 nm).     

1.4-/spl mu/m InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.     

Band-edge lasing in gold-clad photonic-crystal membranes

We investigate the possibility to achieve band-edge lasing in optically thick gold-clad photonic-crystal (PhC) membranes, with a dielectric thickness of around 1 /spl mu/m. We have performed a two-dimensional eigenmode-expansion analysis of band-edge resonators in one-dimensional PhCs. Material thresholds, quality factors, and emission efficiencies have been calculated for TE band-edge laser resonances on the second and third /spl Gamma/-point. The second /spl Gamma/-point sustains band-edge laser modes with quality factors above 2500 for a membrane thickness of 1 /spl mu/m and a cavity length of 20 periods, however, with a very poor surface-emission efficiency. Band-edge laser modes located on the third /spl Gamma/-point have lower quality factors but higher surface-emission efficiencies. In both cases, the PhC should be designed specifically to avoid coupling with lossy, higher order modes.     

Room-temperature 2.81-/spl mu/m continuous-wave operation of GaInAsSb-AlGaAsSb laser

GaInAsSb-AlGaAsSb multiple quantum-well (QW) lasers with an emission wavelength of 2.81 /spl mu/m are reported. The ridge waveguide lasers with highly strained QWs show continuous-wave laser emission up to 25/spl deg/C; in pulsed mode, the lasers operate up to 60/spl deg/C. For pulsed operation, a threshold current density of 360 A/cm/sup 2/ is found for devices with 30-/spl mu/m stripe width and 2-mm cavity length at room temperature. A low threshold current density at infinite length of 248 A/cm/sup 2/ is derived.     

Mode characteristics of semiconductor equilateral triangle microcavities with side length of 5-20 /spl mu/m

Semiconductor equilateral triangle microresonators (ETRs) with side length of 5, 10, and 20 /spl mu/m are fabricated by the two-step inductively coupled plasma (ICP) etching technique. The mode properties of fabricated InGaAsP ETRs are investigated experimentally by photoluminescence (PL) with the pumping source of a 980-nm semiconductor laser and distinct peaks are observed in the measured PL spectra. The wavelength spacings of the distinct peaks agree very well with the theoretical longitudinal mode intervals of the fundamental transverse modes in the ETRs, which verifies that the distinct peaks are corresponding to the enhancement of resonant modes. The mode quality factors are calculated from the width of the resonant peaks of the PL spectra, which are about 100 for the ETR with side length of 20 /spl mu/m.     

Comprehensive above-threshold analysis of antiresonant reflecting optical waveguide edge-emitting diode laser

A three-dimensional (3-D) above-threshold analysis has been performed for laterally antiguided laser structures of the antiresonant-reflecting-optical-waveguide type, of relatively large core width (/spl sim/ 10 /spl mu/m), for high-power, single-spatial-mode operation. A 3-D numerical code has been developed, which takes into account carrier diffusion in the quantum well as well as edge radiation losses. The laser characteristics are calculated as functions of the above-threshold drive level. Within the simulation, 3-5 higher order optical modes on a "frozen background" are computed by the Arnoldi algorithm. Because of the nonuniform gain saturation of the lasing mode, the modal gains for higher order modes increase with the drive current due to increasing overlap of their fields with the two-dimensional gain distribution. The onset of threshold for higher order modes puts an upper limit on the range for stable single-mode operation. The above-threshold analysis is done for various values of the width of the reflector region, below and above the lateral-antiresonance condition. It is found that the maximum intermodal discrimination, which in turn provides the maximum single-mode power, is obtained when the reflector-region width is /spl sim/25 % larger that its value at antiresonance. Then, for 10-/spl mu/m-core devices, stable, single-mode operation is found to occur to drive levels as high as 41 /spl times/ threshold, with single-mode output powers as high as 1.45 W.