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We demonstrate the use of laser diodes and multijunction photovoltaic power converters to efficiently deliver watts of electrical power for long-distance or cryogenic applications. Transmission through single-mode and multi-mode fibers at the wavelengths of 808 nm and 1470/1550 nm are studied. An electrical output power of ~0.1 W is obtained after a 5 km transmission through a standard single-mode SMF28 fiber fed with 0.25 W of optical power. An electrical output power of ~1 W is demonstrated after a 5 km transmission with a standard OM1 multi-mode fiber fed with ~2.5 W. Photovoltaic conversion efficiencies reaching Eff ~49% are obtained with an output voltage of ~5 V using commercial multijunction laser power converters. For low-temperature applications, an ultra-sensitive silicon photomultiplier (SiPM) is used to detect the residual light leaked from fibers as the temperature is decreased. Our study demonstrates that specific fiber types enable low-loss transmission compatible with cryogenic requirements and without light leakage triggering of the SiPM. A cryogenic power-over-fiber system at ~1470 nm is demonstrated with ~2 W of electrical power converted over a 10 m distance having a conversion efficiency of Eff > 65% at 77 K. [ABSTRACT FROM AUTHOR]

Optical wireless power transmission (OWPT) can be used for applications that cannot access traditional power using metal wires. Photovoltaic power-converting III-V semiconductor devices are the core components required for achieving such remote and galvanically isolated power deployments. The development of high-efficiency power converters has already propelled several sensors and probe applications. This growing applied physics field is leveraging the use of ubiquitous laser diode products, now commonly available at various wavelengths. Novel multijunction designs, based on the vertical epitaxial heterostructure architecture devices, have recently allowed fiber-based and free-space applications to quickly progress to higher electrical powers and to benefit from other laser wavelengths. Here, we discuss the perspectives of such multijunction power converters from the viewpoint of realizing additional OWPT deployments and for enabling more probe, sensor, or electronic subsystem power capabilities. The Perspective hence provides a roadmap for devices achieving not only higher conversion efficiency, but also elaborates on the practical aspects necessary to concurrently push the power converters to higher output powers. The photovoltaic multijunction power-converting device is particularly a game-changer for smartly increasing the output voltage and therefore maintaining practical optimal external loads at high laser input powers. Examples of conversion efficiencies above 60% for output powers up to 17.5 W are demonstrated at ∼808 nm in this study, and up to 22 W of output power is obtained with an efficiency of 48.9% at ∼980 nm. [ABSTRACT FROM AUTHOR]

Stable and reliable optical power converting devices are obtained using vertical multi-junction laser power converters. They are based on the GaAs and the InP material systems and are used for power-over-fiber or power-beaming applications. This study demonstrates that, in addition to providing the overall best conversion efficiencies with output voltages ideal for various applications, these semiconductor photovoltaic devices are very tolerant to beam non-uniformity, partial illumination, or beam displacement variations. Examples are given with two tight beams, each covering as little as ~7% of the cell area. An optical input power of 10 W was converted with still an efficiency of Eff ~59.4%. For an input power of 20 W, the illuminated area was set to ~22% without significantly affecting the conversion efficiency of Eff ~60%. Remarkably, for a beam diameter at ~65% of the chip length (i.e., covering ~35% of the chip area), a converted power of 29.5 W was obtained using a 12-junction GaAs device with a conversion efficiency of 61%. For a 10 junction InP-based device, an efficiency of Eff = 51.1% was obtained at an output voltage reaching as high as Voc = 5.954 V for an average optical intensity of 69 W/cm2 and an illumination area of ~57%. [ABSTRACT FROM AUTHOR]

Effects of luminescent coupling are observed in monolithic 5V, five-junction GaAs phototransducers. Power conversion efficiency was measured at 61.6%±3% under the continuous, monochromatic illumination for which they were designed. Modeling shows that photon recycling can account for up to 350mV of photovoltage in these devices. Drift-diffusion based simulations including a luminescent coupling term in the continuity equation show a broadening of the internal quantum efficiency curve which agrees well with experimental measurements. Luminescent coupling is shown to expand the spectral bandwidth of the phototransducer by a factor of at least 3.5 for devices with three or more junctions, even in cases where multiple absorption/emission events are required to transfer excess carriers into the limiting junction. We present a detailed description of the novel luminescent coupling modeling technique used to predict these performance enhancements. [ABSTRACT FROM AUTHOR]

The dissociative 193 nm photochemistry of both multilayer and submonolayer NH3 on Pt(111) has been studied. Upon irradiation with 193 nm light, the submonolayer covered Pt surface yields only a small amount of NH3 desorption. In contrast, the desorption products from the 193 nm irradiation of a multilayer NH3 covered Pt surface are both NH3 and atomic H. The NH3 photodesorption yield from the multilayer is much greater than from the submonolayer covered surface. A nearly field-free resonantly enhanced multiphoton ionization technique was used to carefully distinguish between H and NH3 photoproducts. From the multilayer, NH3 desorbs with a distinct bimodal hyperthermal velocity distribution. The atomic H velocity distribution exhibits both a hyperthermal component and, surprisingly, a ∼150 K thermalized component. Our multilayer data is consistent with adsorbate absorption, and we have proposed a model based on multiple collisions of desorbing species to explain our results. In this model, NH3 molecules at the surface photodissociate and directly eject H atoms into the gas phase with a hyperthermal kinetic energy. NH3 molecules buried within the multilayer also undergo photodissociation but their ejected H atoms suffer multiple collisions, losing kinetic energy and becoming thermalized prior to desorption into the gas phase. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polarized ultraviolet light from an excimer laser (193 nm) was used to photodesorb and photodissociate N2O adsorbed on a cold (80 K) Pt(111) surface. The photodesorbed species and their time of flight (TOF) were monitored by resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We have identified three major channels. The photodesorption of molecular N2 is observed only in the slowest channel where N2 produced by fragmenting the N2O is thermalized on the surface before desorbing. Evidence for this behavior includes both low (∼90 K) rotational and translational temperatures of the N2 fragments as well as a lack of correlation between rotational and translational energy. In the next fastest channel, hyperthermal N2O with a kinetic energy of 0.4±0.1 eV is seen to photodesorb. The photodesorbed hyperthermal N2O also has a substantial degree of internal vibrational excitation. The angular distribution of the N2O channel is peaked toward the surface normal. In the fastest channel, the release of ballistic oxygen atoms, a prompt axial recoil with no collisions with neighboring adsorbates, is seen along the tilted N2O molecular bond axis. The ballistic oxygen atoms leave the surface either in the ground state O(3P) or in the first electronically excited state O(1D). The kinetic energy of the O(3P) and of the O(1D) photoproducts is similar (0.5 eV) suggesting a common dissociative intermediate. In all of the channels observed, the dependence of the photoproducts yield on the polarization of the photodesorption laser indicates a hot carrier mediated mechanism at the surface. We propose a dissociative electron attachment model to explain the photochemistry of N2O/Pt(111) with 193 nm light. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The scattering of rotationally cold N2 from Ag(111) results in angular momentum alignment and orientation of the scattered molecules; measurement of the angular momentum polarization as a function of exit angle, final J state, and exit translation energy provides direct information on the dynamics of the collisions. In this paper, the orientation of the angular momentum vector of the scattered N2 molecules, A{1}1-(J) has been measured for slow, medium, and fast groups of molecules in single rotational states at fixed exit angles. With normal incidence scattering (θi=0°) and off-normal detection, for a given final J state, the ‘‘slow’’ molecules have a higher probability of tumbling backwards (‘‘back spin’’) than the ‘‘fast’’ molecules. Conversely, for glancing incidence scattering (θi=30°) with quasi-specular detection, the opposite trend is observed: the slow molecules have a higher probability of tumbling forwards (‘‘top spin’’) than the fast molecules. These experiments were simulated and analyzed using molecular dynamics trajectory calculations. The calculations show that the amount of gas kinetic energy transferred to the surface is sensitive to the narrow dispersion of impact sites and molecular orientations that lead to scattering into a given final rotational state at a given exit angle. The calculations demonstrate that for both incident angles, collisions near the top of a surface atom lead to slower final velocities than collisions with the hollow sites in analogy with the simple case of two colliding spheres. Therefore, the experimentally observed dependence of the angular momentum orientation on the exit velocity results from the correlation between the initial molecular bond angle and the impact site for scattering into a given J state and at a fixed exit angle. [ABSTRACT FROM AUTHOR]