External quantum efficiency of semiconductor photonic devices is directly measured by wavelengthdependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.External quantum efficiency (EQE or g ext ) is an important parameter that characterizes many photonic devices. It is widely used to evaluate light emitting diodes, photovoltaics, semiconductor lasers, and emerging technology such as laser-induced refrigeration of solids. 1 The essential idea is to have a single coefficient that accounts for both the efficiency of the photon-electron conversion process (internal quantum efficiency) and the efficiency of moving light into and/or out of the device (coupling efficiency). Internal quantum efficiency is deleteriously affected when electronic excitations lose energy through the production of heat. This nonradiative recombination can be mediated by phonons, interfaces, surfaces, dislocations, defects, and even other charge carriers. The light coupling efficiency is driven by Fresnel reflection and the condition of total internal reflection. This leads to photon recycling and the concomitant production of wasteful heat due to the presence of parasitic background absorption. It is often difficult to model or even anticipate how the various disparate mechanisms degrade performance. Instead, empirical data should guide device development. It is therefore crucial to have in place an experimental scheme for precision measurement of efficiency. This allows such problems to be identified and addressed systematically.There will be different descriptions of EQE and varying design goals depending on the application. A high performance solar cell, for example, must make maximum use of the available broadband spectrum and the resulting photoexcitations to produce an external current. A single color LED design, on the other hand, strives to couple the generated narrow-spectrum of light from the device. In this paper, we present a procedure for precision quantification of photoluminescence external quantum efficiency in bulk GaAs heterostructures. Device characterization is performed in the context of optical refrigeration (laser cooling in solids), where the demands on EQE are extreme. For this applica-tion, EQE is defined as the fraction of photo-excited electron-hole pairs which produce luminescence photons that escape the device into free-space. It has been established that laser-induced cooling of GaAs will occur only when EQE exceeds 99%, i.e., far greater than needed for useful operation of other semiconductor photonic devices. 2 In addition, it becomes p...
Abstract:We report a robust method of coherent detection of broadband THz pulses using terahertz induced second-harmonic (TISH) generation in a laser induced air plasma together with a controlled second harmonic optical bias. We discuss a role of the bias field and its phase in the process of coherent detection. Phase-matching considerations subject to plasma dispersion are also examined.
The design of doped n-p-n semiconductor heterostructures has a significant influence on the structures' nonradiative decay and can also affect their photoluminescence characteristics. Such structures have recently been explored in the context of semiconductor laser cooling. We present a theoretical analysis of optically excited n-p-n structures, focusing mainly on the influence of the layer thicknesses and doping concentrations on nonradiative interface recombination. We find that high levels of n-doping ͑10 19 cm −3 ͒ can reduce the minority-carrier density at the interface and increase the nonradiative lifetime. We calculate time-dependent luminescence decay and find them to be in good agreement with experiment for temperatures Ͼ120 K, which is the temperature range in which our model assumptions are expected to be valid. A theoretical analysis of the cooling characteristics of n-p-n structures elucidates the interplay of nonradiative, radiative, and Auger recombination processes. We show that at high optical excitation densities, which are necessary for cooling, the undesired nonradiative interface recombination rates for moderate ͑10 17 cm −3 ͒ n-doping concentrations are drastically increased, which may be a major hindrance in the observation of laser cooling of semiconductors. On the other hand, high n-doping concentrations are found to alleviate the problem of increased nonradiative rates at high excitation densities, and for the model parameters used in the calculation we find positive cooling efficiencies over a wide range of excitation densities.
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