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Dive into the research topics where J. V. Moloney is active.

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Featured researches published by J. V. Moloney.


Optics Communications | 2000

Ultraviolet filamentation in air

Jens Schwarz; Patrick Rambo; Jean-Claude Diels; E. M. Wright; J. V. Moloney

Ultrashort UV pulses are seen to self-focus in air and form meter length filaments. Various measurements (spectrum, size, conductivity) indicate that the balance of the processes that produce the filament are different in the UV compared to the IR.


Proceedings of SPIE, the International Society for Optical Engineering | 2006

Beyond the ABC: carrier recombination in semiconductor lasers

J. Hader; J. V. Moloney; S. W. Koch

It is demonstrated that fully microscopic many-body models are required for a correct description of the dominant carrier loss processes in semiconductor lasers, spontaneous emission and Auger recombination, and that they are able to quantitatively predict these losses. The density dependence of the losses assumed in semi-empirical approaches, J=AN+BN2+CN3, is shown to break down already near transparency. For the spontaneous emission it is shown to decrease from quadratic to linear (BN), Auger rates are shown to increase only quadratically (CN2) or even less.


Optics Express | 2014

Nonequilibrium and thermal effects in mode-locked VECSELs

J. V. Moloney; I. Kilen; A. Bäumner; Maik Scheller; S. W. Koch

Ultrafast femtosecond timescale dynamics in Vertical External Cavity Surface Emitting Lasers (VECSELs) have recently been employed to achieve record average power and duration mode-locked pulses by employing different types of saturable absorbers and Kerr Lens elements. Microscopic many-body dynamics are expected to dominate when attempting to push pulse durations below 100 fs. We present a preliminary microscopic simulation of ultrafast mode-locking in order to expose the role of hot carrier distributions in establishing ultrafast mode-locking.


Proceedings of SPIE | 2014

Microscopic many-body investigation of the efficiency droop in GaN based light emitting devices

J. Hader; J. V. Moloney; S. W. Koch

Fully microscopic models for the calculation of the carrier dynamics and resulting optical response are used to investigate the validity of various models that have been suggested as the cause for the efficiency droop in GaN-based devices. Models based on internal piezoelectric electric fields, carrier localization, Auger and density-activated defect recombination are analysed. In particular, the models are used to simulate aspects of a recent experiment in which green emitting quantum wells were pumped resonantly and emission from adjacent ultra-violet emitting wells was attributed to carrier redistributions due to Auger processes. It is shown that the UV emission can be explained as a direct result of the optical excitation without involving Auger processes.


Proceedings of SPIE | 2011

Power scaling of cw and pulsed IR and mid-IR OPSLs

J. V. Moloney; J. Hader; Tsuei Lian Wang; Yi. Ying; Yushi Kaneda; J. M. Yarborough; T. J. Rotter; Ganesh Balakrishnan; C. P. Hains; S. W. Koch; W. Stolz; B. Kunert; Robert Bedford

We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W) and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.


Optically based materials and optically based biological and chemical sensing for defence. Conference | 2005

Designing new classes of high power high brightness VECSELs

J. V. Moloney; Armis R. Zakharian; J. Hader; S. W. Koch

Optically-pumped vertical external cavity semiconductor lasers offer the exciting possibility of designing kW-class solid state lasers that provide significant advantages over their doped YAG, thin-disk YAG and fiber counterparts. The basic VECSEL/OPSL (optically-pumped semiconductor laser) structure consists of a very thin (approximately 6 micron thick) active mirror consisting of a DBR high-reflectivity stack followed by a multiple quantum well resonant periodic (RPG) structure. An external mirror (reflectivity typically between 94%-98%) provides conventional optical feedback to the active semiconductor mirror chip. The cold cavity needs to be designed to take into account the semiconductor sub-cavity resonance shift with temperature and, importantly, the more rapid shift of the semiconductor material gain peak with temperature. Thermal management proves critical in optimizing the device for serious power scaling. We will describe a closed-loop procedure that begins with a design of the semiconductor active epi structure. This feeds into the sub-cavity optimization, optical and thermal transport within the active structure and thermal transport though the various heat sinking elements. Novel schemes for power scaling beyond current record performances will be discussed.


Proceedings of SPIE | 2014

Systematic investigation of single- and multi-mode operation in vertical-external-cavity surface-emitting lasers

Matthias Wichmann; Mohammad Khaled Shakfa; Maik Scheller; Arash Rahimi-Iman; Bernd Heinen; J. V. Moloney; S. W. Koch; Martin Koch

We systematically study the single- and multi-mode emission of vertical-external-cavity surface-emitting lasers (VECSELs) using streak camera measurements and interferometric measurement techniques. In all experiments, the VECSEL chip is based on (GaIn)As multi-quantum wells as active medium designed for laser emission around 1010 nm. The emission is analyzed in dependence of the pump power, employing two resonator designs as well as different output couplers. We monitor the evolution of emission bandwidth and show that in our setups a stable two-color lasing –with both lasing intensities sharing the same gain region on the chip– is related to a sufficiently high number of longitudinal modes participating in the laser emission.


Proceedings of SPIE | 2013

Investigation of droop-causing mechanisms in GaN-based devices using fully microscopic many-body theory

J. Hader; J. V. Moloney; S. W. Koch

Fully microscopic many-body models are used to calculate the radiative losses in GaN-based light emitting devices. It is shown how simpler models under-estimate these losses significantly. Using the high accuracy of the models allows to eliminate the corresponding loss parameter (B) and its density- and temperature dependence from the space of parameters that are used to fit efficiency data. This allows to study the dependencies of the remaining processes with high accuracy. Using this model, it is show that many processes that have been proposed as causes for the efficiency droop either have wrong dependencies, magnitudes or require unreasonable assumptions to explain the phenomena in general. The most plausible droop model appears to be a combination of carrier delocalization at very low temperatures and pump powers, density- activated defect-recombination at low to medium pumping and injection/escape losses at strong pumping.


european quantum electronics conference | 2011

Power scaling and heat management in high-power VECSELs

S. Chatterjee; A. Chernikov; J. Herrmann; Maik Scheller; Martin Koch; B. Kunert; W. Stolz; S. W. Koch; Tsuei Lian Wang; Yushi Kaneda; J. M. Yarborough; J. Hader; J. V. Moloney

Many applications of vertical-external-cavity surface-emitting lasers (VECSELs) [1], such as intra-cavity frequency mixing rely on the high-power characteristics of the devices. Generally, overheating limits any lasers performance and, thus, efficient cooling concepts are crucial for the high-power output [2]. Here, we experimentally investigate the thermal properties of a high-power device, focusing on the generation, distribution and removal of excess heat under extreme pumping conditions. Different heat-spreading and heat-transfer approaches are analyzed. The performance of the device is optimized yielding a maximum emitted power beyond 70W from a single spot. Finally, the potential for power-scaling in VECSELs and its restrictions are examined. Details on the chip and the experimental setup are given in [3].


Proceedings of SPIE | 2010

Gaussian beam shaping based on multimode interference

X. Zhu; Axel Schülzgen; H. Li; J. V. Moloney; N. Peyghambarian

Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber beam transformers consisting of a short piece of MM fiber (~ 10 mm long) and a single-mode signal delivery fiber were carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The performance of this fiber device can be easily and widely manipulated through parameters including the ratio between the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated. Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and fiber delivery systems.

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S. W. Koch

École Normale Supérieure

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J. Hader

University of Arizona

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S. W. Koch

École Normale Supérieure

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B. Kunert

University of Marburg

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