R. W. Dixon

Photoelastic Properties of Selected Materials and Their Relevance for Applications to Acoustic Light Modulators and Scanners

Measurements of the photoelastic tensor components and figures of merit of several materials selected for possible application in acoustic light modulators and scanners are presented. The most important results are that GaP is superior to fused quartz by nearly two orders of magnitude for modulation and scanning of visible light of wavelength longer than 0.58 μ, and that GaAs is similarly superior at wave‐lengths longer than about 1 μ.

Thermal resistance of heterostructure lasers

A GaAs−GaAlAs heterostructure laser is modeled as a stripe heat source embedded in a layered structure, and an analytic expression is given for the steady−state thermal resistance 〈R〉 of the model. Over the range of typical layer thicknesses and conductivities, and for heat generated uniformly in the active region, 〈R〉 varies between 14 and 31 K/W for a 12×375−μ active region. Four types of heat sinks are shown to contribute an additional 3 to 10 K/W. Design implications are drawn for various properties including layer thicknesses, heat−sink and bond parameters, and radiative heat transfer by spontaneous emission. In disagreement with the common tacit assumption of a unique active−region temperature, it is found that about 40% of the temperature drop within the laser occurs in the active region (center to edge).

Electrical characterization of heterostructure lasers

An improved phenomenological model of the electrical and optical characteristics of semiconductor heterostructure injection lasers is analyzed with emphasis on the nonlasing‐to‐lasing phase transition. The model includes both radiative and nonradiative carrier recombination, as well as electron‐photon coupling. Recently developed approximations of the Fermi‐Dirac integral are used, and they allow closed‐form solutions of the electron and photon rate equations to be obtained—even though the carrier populations are highly degenerate near lasing threshold. Because of the detailed information which harmonic‐voltage amplitudes provide, calculation of the functional forms of the voltage derivatives of the devices is emphasized, and expressions for the height and width of the second‐derivative peak, and for the slope at threshold of the first derivative, are obtained. The effects of carrier degeneracy and of nonradiative recombination are explicitly displayed, and both are found to be important in modifying the ...

Geometrical properties of random particles and the extraction of photons from electroluminescent diodes

A confined random‐particle flux (such as photons in an electroluminescent diode, integrating sphere, or room, music or other phonons in an auditorium, gas molecules in an imperfect vacuum chamber, neutrons in a reactor, etc.) is analyzed, and simple expressions are shown to exist for the mean path length, transit time, surface transmissivity, projected area, absorption rate, etc., that arise in its description. The results are applied to a theoretical and experimental analysis of the coupling of photons from their source within an indirect‐band‐gap electroluminescent diode (or within a sufficiently transparent heterostructure direct‐band‐gap diode) into the ambient or into a multimode optical fiber. We find that about two‐thirds of the internally generated photons will reach air from an encapsulated rough‐sawed red‐emitting GaP(Zn,O) electroluminescent diode of reasonably high purity and contact reflectivity, whereas in a comparable green‐emitting GaP(N) diode only about one‐fourth of the photons escape w...

Scanning Electron Microscope Characterization of GaP Red‐Emitting Diodes

Accurate measurements of the near‐junction properties of three different red‐emitting GaP p‐n junction structures have been made using the scanning electron microscope (SEM). The structures consisted of similar Zn, O‐doped p‐type liquid‐phase‐epitaxial (LPE) layers grown on different Te‐doped n‐type substrates, which include solution‐grown (SG) crystals, liquid‐encapsulated Czochralski (LEC) crystals, and LPE layers grown on LEC crystals. These junctions have yielded diodes with electroluminescent efficiencies as high as 6.0, 2.0, and 4.5%, respectively. These measurements, which include minority‐carrier diffusion lengths, the spatial distributions of luminescent efficiency, and the detailed properties of the space‐charge region, have provided a self‐consistent picture of the factors which determine electroluminescent efficiency. In this study, all of the junctions were determined to be n+‐p structures which exhibit uniform red luminescent efficiencies in the p region near the junction. The differences be...

Accelerated aging and a uniform mode of degradation in (Al,Ga)As double-heterostructure lasers

An investigation of certain of the degradation and accelerated aging characteristics of currently fabricated (Al,Ga)As lasers is described. Our observations of the properties of devices aged at 70 °C, both as light‐emitting diodes and as lasers, support two conclusions relevant to degradation: (1) The dominant degradation process in present devices is not lasing related in devices operated at power levels ⩽5 mW/face; (2) laser failure is strongly correlated with an approximately 50% reduction in the spontaneous light emission (at a given current below threshold) from these devices. These inferences are consistent with a spatially uniform mode of laser degradation not, for example, related to dark‐line defects. They also imply that an LED mode of laser testing could be effective in identifying and eliminating presently dominating degradation mechanisms, i.e., those limiting continuously operating laser lifetimes to median values of about five years at room temperature. A different degradation mechanism whi...

Luminescent Time Decay of Excitons Bound to Zn–O Complexes in Gap

Time‐decay measurements of the photoexcited red luminescence near room temperature in a series of GaP (Zn, O) samples with varying zinc concentrations and varying thermal histories (annealing conditions) are presented. A phenomenological analysis of the electron recombination processes in GaP (Zn, O) based on semiconductor statistics and a simple kinetic model is then discussed which takes the (accurately measurable) time decays as independent variables and predicts the values of less accurately known quantities. It is demonstrated that such elusive but important parameters as the internal quantum efficiency, and the relative strengths of radiative and nonradiative centers, can be quantitatively predicted using time‐decay and free‐hole‐concentration measurements. Curves of internal quantum efficiency are presented as functions of the experimentally measurable time decays and free‐hole concentrations; as is the ratio of the capture time of the nonradiative centers to the capture time of the Zn–O complexes....

Optical Investigation of Magnetically Induced Elastic‐Wave Dispersion in YIG

Optical beam probing techniques utilizing Bragg diffraction from elastic waves have been used to explore the phenomenon of elastic‐wave dispersion in yttrium iron garnet. Rotatory dispersion (acoustic Faraday effect) two orders of magnitude larger (approx. 220 rad/cm) than previously reported was obtained. Large group‐velocity dispersion has also been observed, (group velocities as slow as one‐half the normal value) and combined with the Faraday rotation measurements to permit the direct, independent, determination of values of the second magnetoelastic constant and the internal magnetic field. The dependence of the latter on position along the axis of the sample is obtained without computation or knowledge of the demagnetizing fields.

Fundamental and harmonic response voltages of a sinusoidally current‐modulated ideal semiconductor laser

It appears that derivative techniques will become quite useful in the characterization of semiconductor lasers. This paper clarifies the signal responses expected with these techniques by calculating the fundamental and second‐harmonic voltages which would be observed in the case of an ideal laser driven with a sinusoidally modulated current. The previously untreated nonanalytic lasing‐threshold region is explicitly included and the results are exact for any modulation amplitude. The harmonic voltages are then related to the first and second voltage derivatives and to the parameters characterizing (Al,Ga)As double‐heterostructure lasers.

Optical‐coupling efficiency of GaP : N green‐light‐emitting diodes

Optical‐coupling efficiencies of slurry‐diced GaP : N green‐light‐emitting electroluminescent diodes have been measured. Light losses due to absorption in such structures are complicated by the irregular diode geometry, the large refractive index, and the significant energy‐dependent optical absorption present in the green spectral region. We show that the optical‐coupling efficiency can be represented as a product of the red‐emission random‐path optical‐coupling efficiency and a ratio of the integrated external spectrum to the integrated internal spectrum. Optical‐coupling efficiencies of encapsulated green diodes in the range 25 ± 5% have been obtained from the measurements, i.e., three times more light is internally absorbed than is usefully transmitted to the ambient. Unencapsulated diodes have about one‐half of this optical‐coupling efficiency.