Query: Modeling of semiconductor saturable absorber mirrors using dynamic mode analysis

Candidate papers:

abstract: Semiconductor saturable absorber mirrors (SESAMs) are used to produce passively Q switched ultrashort pulsed lasers. Numerical modeling of physical effects of SESAM is required to effectively design this type of lasers. For this purpose, simulations are performed to study the dynamic behavior of Gauss modes, gain of modes and saturation of the saturable absorber mirror. The laser beam quality has to be good enough in order to avoid chaotic laser behavior. We extended our dynamic mode analysis (DMA) algorithm to calculate laser beam quality. This simulation technique is based on rate equations for a set of Gauss modes and population inversions. Gain of each mode can be calculated separately by solving the corresponding set of rate equations. We have assumed that the reflectivity of the mirror is spatially invariant in the SESAMs model. An additional rate equation is required to include the saturation of SESAM. This equation considers parameters such as modulation depth, saturation fluence and relaxation time. Simulation results show that our model can predict pulse energy and non chaotic behavior of the laser.
author_names: Christoph Pflaum

Zhabiz Rahimi
title: Modeling of semiconductor saturable absorber mirrors using dynamic mode analysis
venue: Photonics West Lasers and Applications in Science and Engineering
year: 2013

abstract: Experimental and theoretical results on the mode locking dynamics in vertical external cavity surface emitting lasers with semiconductor and graphene saturable absorber mirrors are reviewed with an emphasis on the role of nonequilibrium carrier effects. The systems are studied theoretically using a fully microscopic many body model for the carrier distributions and polarizations, coupled to Maxwell's equations for the field propagation. Pump probe measurements are performed with (sub 100 fs resolution. The analysis shows that the non equilibrium carrier dynamics in the gain quantum wells and saturable absorber medium significantly influences the system's response and the resulting mode locked pulses. The microscopic model is used to study the pulse build up from spontaneous emission noise and to determine the dependence of achievable pulse lengths and fluences on the amounts of saturable and non saturable losses and the optical gain. The change of the group delay dispersion (GDD) on the pump level is examined and the dependence of the pulse lengths on the total amount of GDD is demonstrated experimentally. Theory experiment comparisons are used to demonstrate the highly quantitative accuracy of the fully microscopic modeling.
author_names: Jorg Hader

Maik Scheller

Alexandre Laurain

Isak Ragnvald Kilen

Caleb W Baker

Jerome V Moloney

Stephan W Koch
title: Ultrafast non equilibrium carrier dynamics in semiconductor laser mode locking
year: 2017

abstract: This work presents an overview of a combined experimental and theoretical analysis on the manipulation of temporal localized structures (LSs) found in passively Vertical Cavity Surface Emitting Lasers coupled to resonant saturable absorber mirrors. We show that the pumping current is a convenient parameter for manipulating the temporal Localized Structures, also called localized pulses. While short electrical pulses can be used for writing and erasing individual LSs, we demonstrate that a current modulation introduces a temporally evolving parameter landscape allowing to control the position and the dynamics of LSs. We show that the localized pulses drifting speed in this landscape depends almost exclusively on the local parameter value instead of depending on the landscape gradient, as shown in quasi instantaneous media. This experimental observation is theoretically explained by the causal response time of the semiconductor carriers that occurs on an finite timescale and breaks the parity invariance along the cavity, thus leading to a new paradigm for temporal tweezing of localized pulses. Different modulation waveforms are applied for describing exhaustively this paradigm. Starting from a generic model of passive mode locking based upon delay differential equations, we deduce the effective equations of motion for these LSs in a time dependent current landscape
author_names: Julien Javaloyes

P Camelin

M Marconi

M Giudici
title: Electrical addressing and temporal tweezing of localized pulses in passively mode locked semiconductor lasers
venue: 2017 19th International Conference on Transparent Optical Networks (ICTON)
year: 2017

abstract: This work presents an overview of a combined experimental and theoretical analysis on the manipulation of temporal localized structures (LSs) found in passively Vertical Cavity Surface Emitting Lasers coupled to resonant saturable absorber mirrors. We show that the pumping current is a convenient parameter for manipulating the temporal Localized Structures, also called localized pulses. While short electrical pulses can be used for writing and erasing individual LSs, we demonstrate that a current modulation introduces a temporally evolving parameter landscape allowing to control the position and the dynamics of LSs. We show that the localized pulses drifting speed in this landscape depends almost exclusively on the local parameter value instead of depending on the landscape gradient, as shown in quasi instantaneous media. This experimental observation is theoretically explained by the causal response time of the semiconductor carriers that occurs on an finite timescale and breaks the parity invariance along the cavity, thus leading to a new paradigm for temporal tweezing of localized pulses. Different modulation waveforms are applied for describing exhaustively this paradigm. Starting from a generic model of passive mode locking based upon delay differential equations, we deduce the effective equations of motion for these LSs in a time dependent current landscape.
author_names: Julien Javaloyes

P Camelin

M Marconi

M Giudici
title: Electrical addressing and temporal tweezing of localized pulses in passively mode locked semiconductor lasers
venue: Applications of Optics and Photonics
year: 2017

abstract: In this study, time domain dynamic model of a mode locked two sections DBR laser was obtained. Sort duration and high power optical pulse generation from a semiconductor laser was examined by using obtained model. For this aim, while one of the laser sections was thought as reverse biased and so acted as a saturable absorber, other section thought as forward biased with a DC current and so acted as a gain section. A semiconductor laser biased this way can produce mode locked pulses by suitable adjusting of reverse bias and forward bias values and this method known as passive mode locking.
author_names: Caglar Duman

Bulent Cakmak
title: Time Domain Dynamic Analysis of 1550nm Monolithic Two Sections Mode Locked MQW Laser
year: 2015

abstract: Lowering the output noise of short pulse lasers has been a long standing effort for decades. Modeling the noise performance plays a crucial role in isolating the noise sources and reducing them. Modeling to date has either used analytical or semianalytical implementation of dynamical methods or Monte Carlo simulations. The former approach is too simplified to accurately assess the noise performance in real laser systems, while the latter approach is too computationally slow to optimize the performance as parameters vary over a wide range. Here, we describe a computational implementation of dynamical methods that allows us to determine the noise performance of a passively mode locked laser within minutes on a desktop computer and is faster than Monte Carlo methods by a factor on the order of 10. We apply this method to characterize a laser that is locked using a fast saturable absorber for example, a fiber based nonlinear polarization rotation device and a laser that is locked using a slow saturable absorber for example, a semiconductor saturable absorbing mirror. (c) 2018
author_names: C F

M R
title: Efficiently modeling the noise performance of short pulse lasers with a computational implementation of dynamical methods
year: 2019

abstract: Lowering the output noise of short pulse lasers has been a long standing effort for decades. Modeling the noise performance plays a crucial role in isolating the noise sources and reducing them. Modeling to date has either used analytical or semianalytical implementation of dynamical methods or Monte Carlo simulations. The former approach is too simplified to accurately assess the noise performance in real laser systems, while the latter approach is too computationally slow to optimize the performance as parameters vary over a wide range. Here, we describe a computational implementation of dynamical methods that allows us to determine the noise performance of a passively mode locked laser within minutes on a desktop computer and is faster than Monte Carlo methods by a factor on the order of 103. We apply this method to characterize a laser that is locked using a fast saturable absorber for example, a fiber based nonlinear polarization rotation device and a laser that is locked using a slow saturable absorber for example, a semiconductor saturable absorbing mirror.
author_names: Shaokang Wang

Thomas F Carruthers

Curtis R Menyuk
title: Efficiently modeling the noise performance of short pulse lasers with a computational implementation of dynamical methods
year: 2018

abstract: Vertical External Cavity Surface Emitting Lasers (VECSELs) have been used for mode locked operation by using a Semiconductor Saturable Absorber Mirror (SESAM) usually in the external cavity [1] but also integrated in the gain chip [2] Impressive progress has been achieved since the first demonstration of an SESAM mode locked VECSEL in terms of pulse duration, average optical power, and peak power. While the shortest pulses have been achieved with optically pumped active layers, electrically pumped structures are promising for a number of applications. A versatile and accurate, if fairly complex, delay differential theoretical model for mode locking in electrically pumping VECSELs based on the physical description of gain and SESAM chips has been presented [3] however only the linear cavity geometry has been studied. A somewhat different approach was taken in [4][5] where an experiment informed model representing the laser somewhat artificially as a sequence of gain, absorber, and dispersion elements in a unidirectional ring cavity (reminiscent of the classic Haus theory) was developed, and successfully reproduced the measured laser parameters in both pico and femtosecond regimes. In the current work, we present a somewhat simplified model based on the physical laser design as in [3] which however can accommodate realistic geometry, both linear and folded cavity. The model is applied to study different regimes of laser operation, including the possibility of colliding pulse mode locking at harmonics of the fundamental roundtrip frequency. As in [3] we base the derivation on the analysis of amplitudes of waves incident onto, and reflected from, elements of the laser cavity, using the amplitude of the field reflected from the "bulk" of the cavity as the dynamic variables for the description of light. Standard rate equations are used for carrier densities. In the case of a linear cavity the model is thus somewhat similar to that used in [3] though somewhat simplified. No matrices of the type used in [3] are needed to recalculate the dynamic variables in our simplified model. The use of phenomenological gain carrier density dependences and linewidth enhancement factors, while less rigorous than the simplified microscopic model of [3] enables relatively easy inclusion of polarization dependences, allowing double polarisation frequency comb generation to be simulated. In the case of a folded (Z type) cavity either the gain chip or the SESAM chip can be positioned "inside" the cavity. The dynamic variables in the case of the gain chip in the middle are the fields reflected from the bulk of the gain chip in the directions of the output mirror and the absorber chip, and from the bulk of the SESAM back to the gain chip In the case of the gain chip in the middle position, the difference in the dynamics of the saturation of the gain chip changes the pulse parameters and stability limits compared with the linear cavity, but there is no qualitative difference. In the case of the absorber section in the intracavity position, colliding pulse dynamics was simulated with a rational relation between the delay times. Further results will be presented at the conference. References [1] U. Keller, A. C. Tropper, Phys. Reports, 429, 67 (2006) [2] D.J.H.C. Maas et al. Appl Phys B, 88, 493 (2007) [3] J. Mulet and S. Balle, IEEE J. Quantum Electron. 41, 1148 (2005) [4] M. Hoffmann et al. Opt. Express 18, 10143 (2010) [5] O.Sieber et al. Appl.Phys.B. 113, 133 145 (2013)
author_names: Eugene A Avrutin

Krassimir Panajotov
title: Modelling dynamics of high bit rate mode locked VECSELs with different cavity geometries (Conference Presentation)
year: 2018

abstract: Laser self pulsing was a phenomenon exclusive to macroscopic lasers until recently, where self starting laser pulsation in a microscopic photonic crystal Fano laser was reported. In this paper a theoretical model is developed to describe the Fano laser, including descriptions of the highly dispersive Fano mirror, the laser frequency and the threshold gain. The model is based upon a combination of conventional laser rate equations and coupled mode theory. The dynamical model is used to demonstrate how the laser has two regimes of operation, continuous wave output and self pulsing, and these regimes are characterised using phase diagrams, establishing the regime of self pulsing numerically. Furthermore, the physics behind the self pulsing mechanism are explained in detail and it is demonstrated how cavity absorption makes the Fano mirror function as a saturable absorber, leading to Q switched pulse generation. A stability analysis is used to demonstrate how the dominant mechanism of instability is relaxation oscillations becoming un damped. Finally the effect of varying key self pulsing parameters is investigated by characterisation of the change in self pulsing regions.
author_names: T S Rasmussen

Yi Yu

Jesper Mork
title: Theory of Self pulsing in Photonic Crystal Fano Lasers
year: 2017

abstract: Diamonds approaching the nanoscale have the potential for use as probe materials as their optical properties can be sensitive to optical/electric fields, mechanical stress/ pressure, and the configuration of nuclear spins. The surface of nanodiamonds impacts their optical properties and sensing capabilities, and examining the nanodiamond surface through X ray absorption can give insights into molecular surface structures. Here, quantum dot models with varying amounts of surface carbon passivation are prepared, optimized, and compared. The loss of the diamond sp lattice is examined by investigating the bond length and tetrahedral character of the carbons comprising nanodiamonds for the appearance of aromatic sp surface domains. Electronic transitions in the carbon K edge region, using the energy specific time dependent density functional theory method, as well as vibrational spectra are computed from the optimized models. The surface reorganization is shown to affect the electronic characteristics of the nanodiamond. As a result, there is a distinct absorption peak in the carbon K edge region, along with stretching modes in the vibrational spectra, that can be correlated to the nature of the surface hybridization of the nanodiamond. INTRODUCTION Nanodiamonds are interesting materials due to their stability and potential for use in biological imaging, quantum computing, drug delivery, and sensing as a result of their lack of photobleaching, spin polarized photoluminescense, and long spin lifetimes when doped. Bulk, undoped diamonds are insulators with a band gap of approximately 5.5 eV. They are thus transparent to visible radiation; however, dopants such as the nitrogen vacancy and silicon split divacancy can lead to optical signatures in the visible region. These optical signatures can be tuned by altering the size of the nanocrystal. As the size of the nanocrystal decreases, discrete electronic states emerge at the band edges along with an energetic increment of the band gap as a result of the quantum confinement effect. In diamond, carbon is arranged into a highly symmetric, tetrahedral sp lattice. However, because of graphitic pre edge features that appear in the carbon K edge energy region of Xray and electron energy loss spectroscopy of nanodiamonds, it has been theorized that as diamond approaches the nanoscale the surface carbons arrange to form sp features. Earlier theories on the arrangement of the carbons predicted a diamond core with a graphitic shell around it. This model for describing the surface of the diamond still cannot describe an experimentally determined deviation from perfect lattice Bragg peaks. Thus, an improved model was proposed where nanodiamonds arrange themselves in a manner similar to that in Figure 1. In this model, there is an sp diamond core surrounded by a shell of sp carbon atoms where the diamond lattice deviates from its perfect tetrahedral arrangement, labeled S2 in the figure. Finally, this intermediate shell interacts with the graphitic, sp surface layer, labeled S1 in the figure. 25 Surface rearrangement, and thus the carbon hybridization, of nanodiamonds has been used to describe experimentally determined features on optical spectra, such as the carbon Xray absorption 285 eV pre edge. In the past few years, Received: January 12, 2018 Revised: March 8, 2018 Published: April 4, 2018 Figure 1. Proposed model for nanodiamond surface reorganization composed of a diamond lattice core, a layer of sp carbon where the diamond lattice deteriorates (S2) and the surface (S1) 1,25 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C 2018, 122, 8573 8580 (c) 2018 American Chemical Society 8573 DOI: 10.1021/acs.jpcc.8b00354 J. Phys. Chem. C 2018, 122, 8573 8580 D ow nl oa de d vi a U N IV O F W A SH IN G T O N o n Ju ly 2 7, 2 01 8 at 1 9: 14 :4 8 (U T C Se e ht tp s: /p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. the effects of surface passivation, symmetry, and functionalization on the diamond lattice reorganization have also been studied computationally. However, a detailed understanding of the interplay between the surface rearrangement (i.e. sp moieties, graphitic and sp strained layers) and the presence of localized defects in the lattice (i.e. dopants) on the pre edge features that appear in the carbon K edge energy region of the X ray absorbance spectrum has still not been fully accomplished. Thus, in this study the effects of different surface hybridizations on the Raman scattering vibrational spectroscopy and X ray absorption are examined using a finite cluster approach combined with harmonic treatment and energy specific timedependent density functional theory (TDDFT) respectively. This technique has shown very promising results for the theoretical characterization of both the excited electronic states of defect containing semiconductor clusters and light elements and the vibrational/dynamical properties of molecules. Rearrangement of the surface carbon hybridization is predicted through the structural response to different surface passivations. Hydrogen was chosen to passivate the surface carbon dangling bonds, given that hydrogen surface termination can be realized experimentally using a hydrogen plasma process. Other moieties such as those containing oxygen and nitrogen are commonly found on the surface as a byproduct of the manufacturing and purification methods including the detonation of high explosives, a high pressure, high temperature multianvil press, and, most recently, laserheated diamond anvil cells (LH DAC) The unique use of noble gas pressure media during LH DAC processing maximizes the probability of graphitic surface reconstruction. In this work, density functional theory calculations are used with a cluster approach to find the optimized molecular geometry for the structures of varying surface passivations, as shown in Figure 2. This level of theory has already been used effectively to study nitrogen doped nanodiamonds of similar dimensions. To validate the predicted structural reorganization, infrared and Raman spectra have been calculated on systems with various degrees of surface passivation and compared with the experiments. The origins of the carbon Kedge pre edge feature motivated the study of the X ray absorption spectroscopy (XAS) in this energy region. XAS is elementally specific with large separations between the responses for different elements and gives information about weakly bound states to which it excites. Therefore, information about the surface hybridization can be determined through XAS. Since the molecular origin of the X ray pre edge feature for nanodiamonds is not fully understood, this theoretical study is motivated by a desire to understand the relationship between this feature and the surface arrangement in nanodiamonds. METHODOLOGY Nanodiamonds were constructed to be nearly spherical with a bulk fcc lattice parameter of a 0.357 nm according to the procedure presented in ref 11. Hydrogen atoms were used to passivate the surface carbon atoms and to saturate surface dangling bonds. Although other moieties are generally present on the surface, chemical treatments of the surface can provide a more uniform passivation. Before hydrogen removal or optimization, the diamonds have C3v symmetry, and two sizes, ~1.4 nm (C182H142) and ~1.2 nm (C121H104) in diameter, have been used throughout the article. These systems are similar, but slightly smaller, than commercially available nanodiamonds from laser heated diamond anvil cell synthesis or detonation synthesis. Given that the Bohr exciton radius for diamond is ~1.6 nm, the electronic properties of these diamond clusters are expected to exhibit quantum confinement effects. Calculations were preformed using the Gaussian electronic structure software package. The ground state electronic structures were obtained by solving the Kohn Sham equation using the hybrid Becke, three parameter, Lee Yang Parr (B3LYP) functional with a 6 31g(d) basis. The theory level employed has been previously validated for the description of the electronic structure and optical properties of both pure and nitrogen vacancy doped diamonds of the same dimensions. To examine the effects of the surface passivation on the diamond lattice as the surface becomes more unsaturated, hydrogens were removed from the surface and the resulting structure was fully optimized. This surface reorganization is analogous to what has been hypothesized to occur when nanodiamonds are synthesized within a laser heated diamond anvil cell that uses a noble gas (i.e. neon or argon) as the nearly hydrostatic pressure medium. Hydrogens were removed in such a way as to preserve the symmetry of the unoptimized structures and to obtain a homogeneous hydrogen density across the surface of the nanodiamond (Figure 2) To verify the structures returned from the optimization were true minima and to compute the vibrational frequencies, the second derivatives of the energy with respect to the Cartesian nuclear coordinates were calculated. Given the number of atoms and possible arrangements of the systems studied, there are several structures with similar bonding structures and degenerate energies that can result from the optimizations. To ensure that the optimized structures represent physically valid models, XAS and Raman vibrational spectra were computed and compared to the experimental results. In addition, a detailed analysis of the surface reorganization is presented, providing a comparison with previous computational studies on diamonds of similar sizes. The average computed properties over each optimized nanodiamond structure are similar and representative of the experimental data, suggesting that the results presented herein are valid. The XAS was calculated using time dependent density functional theory (TDDFT) within the linear response framework and i
author_names: Ryan A Beck

Alessio Petrone

Joseph M Kasper

Matthew J Crane

Peter J Pauzauskie

Xiaosong Li
title: ect of Surface Passivation on Nanodiamond Crystallinity
year: 2018