Fabry-Perot Cavity Modes Research Articles

An enhanced model for the modulated reflectance spectra of vertical‐cavity surface‐emitting laser structures

AbstractModulation spectroscopy has proved to be unique in being able to probe the vital active‐layer quantum well (QW) transitions in vertical‐cavity surface‐emitting lasers (VCSELs). We have been characterising an all‐AlGaAs‐based VCSEL structure designed to operate at high temperatures, using photo‐modulated reflectance (PR) as a function of temperature up to 180 °C. We previously developed an empirical model to fit VCSEL PR spectra when the features from the QW transitions and the Fabry–Perot cavity mode (CM) overlap and interact, and this fitting function has, so far, proved to be very successful. However, the present VCSEL has a particularly high finesse cavity and our established model is unable to fit the unusually detailed interacting CM/QW PR spectra. Here, we perform new reflectivity calculations for this VCSEL structure. These provide new insight into the Seraphin coefficients of the CM, and their inter‐relationship, when they lie near the QW transitions. This deepens our understanding of VCSEL PR lineshapes, yielding an enhanced model which we successfully fit to the temperature‐dependent PR spectra of the present VCSEL. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lasing from Thick Anisotropic Layer Sandwiched between Polymeric Cholesteric Liquid Crystal Films

We have demonstrated a new type of lasing mode in a dye-doped 100-µm-thick nematic liquid crystal layer sandwiched between two polymeric cholesteric liquid crystal films functioning as a photonic crystal. The fabricated cell exhibits several characteristic dips in the transmittance spectrum in addition to fine fringes originating from a Fabry–Perot cavity mode. These dips are due to the phase retardation between optical eigenmodes in the birefringent medium, which is not realized in an isotropic layer. The cell shows multimode lasing at wavelengths corresponding to transmittance maxima within the stopband region when the nematic layer is doped with dyes.

Waveguide analysis of organic light-emitting diodes fabricated on surfaces with wavelength-scale periodic gratings

Numerical techniques for the analysis of multilayer waveguide structures were used to study the modes that exist in organic light-emitting diode (OLED) devices. The analysis revealed that waveguide modes of the OLED structure could be grouped, according to the behavior of modal-field profiles in the air cover and the glass substrate, into one of four different "families": (i) bound mode, (ii) semibound modes, (iii) leaky modes, and (iv) nonphysical modes. Four different OLED samples were fabricated on glass substrates on which photoresist gratings had been previously formed. The theory was used to compute the angles at which light from these devices should exit into the air. Theory and data agreed well for the semibound modes for all samples; however, they did not agree so well for the leaky modes. Further investigation revealed that better agreement between theory and data could be obtained with these modes being analyzed as Fabry-Perot cavity modes. The theoretical relation between leaky waveguide modes and Fabry-Perot cavity modes is discussed.

Ultrafast photoinduced shift of cavity modes in semiconductor microcavity under femtosecond excitations

Ultrafast photoinduced changes of interference (cavity) modes frequencies in ZnSe films on metal substrates were investigated by femtosecond pump-supercontinuum probe spectroscopy. For the pumping energy below the energy gap of ZnSe the laser pulse excites mainly electrons of the metal, i.e. the boundary of the cavity. Nonequilibrium carriers of the metal penetrate into ZnSe film through the Schottky electron barrier and change dielectric function of the semiconductor microcavity. Photoinduced changes in the boundary condition and in optical thickness of the microcavity lead to observable ultrafast shifts of cavity modes with time less the time for establishment of the Fabry-Perot cavity modes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self‐Assembled Silica Photonic Crystal as a Liquid‐Crystal Alignment Layer and its Electro‐optic Applications in Fabry–Perot Cavity Structures

Microgrooves on self-assembled photonic-crystal (PC) films align liquid crystals (LCs) without any additional rubbing or surface treatment. Nematic (see Figure and inside cover) or twisted-nematic LCs can be formed, depending on the relative orientation of the microgrooved films. A LC Fabry–Perot (FP) cavity was fabricated using these PC films, and electro-optic tuning of FP cavity modes is demonstrated.

Analysis of parametric oscillatory instability in power recycled LIGO interferometer

We present the analysis of a nonlinear effect of parametric oscillatory instability in power recycled LIGO interferometer with the Fabry–Perot (FP) cavities in the arms. The basis for this effect is the excitation of the additional (Stokes) optical mode with frequency ω 1 and the mirror elastic mode with frequency ω m, when the optical energy stored in the main FP cavity mode with frequency ω 0 exceeds the certain threshold and the frequencies are related as ω 0≃ ω 1+ ω m. The presence of anti-Stokes modes (with frequency ω 1a≃ ω 0+ ω m) can depress parametric instability. However, it is very likely that the anti-Stokes modes will not compensate the parametric instability completely.

Accurate methods for study of light emission from quantum wells confined in a microcavity

Photomodulated reflectance (PR) and conventional reflectance studies have been performed on an InGaP/AlGaInP/Al/sub x1/Ga/sub y1/As/Al/sub x2/Ga/sub y2/As resonant-cavity light emitting diode structure in the red spectral region. The PR spectra show prominent signals from the Fabry-Perot cavity mode and the quantum-well (QW) ground state excitonic transition. This high-precision technique, and its variations as functions of incidence angle and temperature, as reported in this article, allow one to investigate light emission from the QW confined in a microcavity with relation to the Fabry-Perot mode, and is the only known nonconductive, nondestructive method of doing so.

Temperature dependence of the reflectivity in absorbing Bragg reflectors

The reflectivity of absorbing Bragg reflectors consisting of a GaAs/AlAs Bragg mirror and a InGaAs/InGaAsP multiple-quantum-well cavity layer was studied as a function of temperature. An absorption dip in the stop band due to the optical confinement of the Fabry-Perot resonance was observed in the reflectivity spectra. The absorption intensity of the dip increased with temperature and was explained by the resonant coincidence of the Fabry-Perot cavity mode and the quantum-well absorption. The temperature-dependent reflectivity spectra were successfully reproduced using the transfer matrix method and the linear dependence of the refractive index on temperature.

Photoreflectance line shape symmetry and quantum-well ground-state exciton energy in vertical-cavity surface-emitting laser structures

We report photoreflectance studies on the coupling between the Fabry–Perot cavity mode (CM) and the quantum well (QW) ground-state excitonic feature in vertical-cavity surface-emitting laser structures. Changes in the symmetry of the CM-QW spectral feature occur when the angle of incidence of the probe beam is altered. Using detailed simulations, we explain how this is related to an unusual reversal of the roles of the Seraphin coefficients and QW dielectric function, in determining the line shape. Our study suggests a way to find the exciton energy, in situations where a distinct QW feature is not seen because of large broadening of the QW dielectric function combined with high reflectivity of Bragg mirrors and relatively narrow CM width.

Growth characterisation of InxGa1−xAs/GaAs/AlAs vertical-cavity surface-emitting laser structure using photomodulated reflectance

Of crucial importance for vertical-cavity surface-emitting lasers (VCSELs) is the energy separation of the Fabry–Perot cavity mode and peak of the gain spectrum of the quantum-well (QW) active region. Since this is affected by growth variations, nondestructive characterisation of VCSEL wafers is usually required prior to full processing, to check that structures have been grown to specification. It is shown that photomodulated reflectance (PR) spectroscopy is useful for prefabrication testing of VCSEL wafers, for the example of a near-infrared InxGa1−xAs/GaAs/AlAs VCSEL. By varying either the probe position on the wafer or the temperature, PR was used to study the interaction between the cavity mode and both the ground-state QW exciton and all the higher-order QW transitions. Firstly, a growth-induced shift across the wafer was exploited to tune the cavity-mode wavelength through resonance with all the QW excitonic transitions at room temperature. Secondly, by cooling from room to liquid-helium temperatures, each QW transition was blue-shifted into resonance with the cavity mode. In both methods, the PR signal was enhanced at such resonances, which could provide a sensitive way of locating `sweet spots' on a nonuniform VCSEL wafer (where operating devices might be fabricated). Since PR can detect all the QW transitions, it is able, by comparing the measured and predicted transition energies, to provide much information about the QW cavity and growth.

Tuning the Pore Size and Surface Chemistry of Porous Silicon for Immunoassays

To use porous silicon as an optical interferometric biosensor, the pores must be sufficiently large to allow easy ingress of reagents and the layer must also display Fabry-Perot optical cavity modes. Here the detection antibody is rabbit IgG and the analyte is α-rabbit IgG conjugated to horseradish peroxidase (HRP). For this model system, the pores should be >50 nm in diameter. Such diameters have been obtained in 0.05 Ω cm n-type silicon using anodisation followed by chemical etching in ethanolic KOH and also by anodising 0.005 Ω cm p-type material. The latter also displays optical cavity modes. The silicon surface is oxidised in ozone, silanised using aminopropylmethoxysilanes with one, two or three methoxy groups, and cross linked to IgG using glutaraldehyde. High specific binding is found for mono-, di- and tri-methoxy silanes, but the lowest non-specific binding is found for silanisation with the tri-methoxy silane.

Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films

Hexagonally shaped ZnO nanocrystal thin films were fabricated on sapphire(0001) substrates by laser molecular beam epitaxy. Nanocrystal structure was investigated by atomic force microscopy and transmission electron microscopy. Epitaxial growth of ZnO nanocrystal thin films on sapphire substrates was found to occur in a spiral and grain growth mode. The grain growth mode was interpreted by taking higher order epitaxial relationship of oxygen sublattice units between ZnO and sapphire into account. Nanocrystal size could be tuned from 50 to 200 nm controlling film thickness, growth conditions and stoichiometry of the target. The films having small nanocrystal size of about 50 nm showed excitonic stimulated emission having peak energy of 3.2 eV at room temperature with a very low threshold (24 kW cm −2). Mode transition from excitonic stimulated emission to electron hole plasma appeared above another threshold (50 kW cm −2). Well defined Fabry–Perot cavity mode was observed in the emission spectra measured from side edge of the film. It was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.

Real time in-situ thickness control of Fabry—Perot cavities in MBE by 44 and 88 wavelength ellipsometry

We have demonstrated how to use the 44 wavelength ellipsometer for in-situ closed-loop feedback control of a molecular beam epitaxy in order to grow reproducible AlAs GaAs Fabry—Perot cavities. Sample-to-sample reproducibility of the Fabry—Perot cavity mode position measured using normal incident reflection was better than 0.2% among six samples grown at different times and different growth conditions. This shows that a reproducible control of AlAs GaAs layer thickness is achievable using the 44 wavelength ellipsometer. The new 88 wavelength ellipsometer includes UV wavelengths from 2770 to 4150 Å. The closed-loop in-situ control of Fabry—Perot cavity growth will be explained and the results from the two types of ellipsometers will be compared.