Theses supervised by Doç. Dr. Fatih Ömer İlday
13 theses · İhsan Doğramacı Bilkent University
Doğrusal olmayan sıcaklık profilinin lazer materyal işleme için düzenlenmesi
Ablation cooled material removal opened up great opportunities for understanding nonlinear processes. Especially using lasers as a tool to tailor nonlinear temperature gradient of material and engineering them to achieve effects such as high ablation efficiency, speed, and low collateral damage. Numerical simulations showed such engineering of the temperature gradient of material is possible for any repetition rate falling inside the ablation cooling regime. Two temperature model is used to investigate the effects of repetition rate, pulse energy, and burst duration. Simulations suggest ablation can continue indefinitely as burst duration increases. They also suggest there is an optimum pulse energy for any repetition rate in terms of efficiency of ablation regarding the material. The results of the simulations are confirmed by experiments using lasers with 1.6 GHz, 1.46 GHz, and 13 GHz repetition rate on biological and technical materials. The ablation threshold for a single pulse is lowered ~100 times compared to our previous publication. Finally, related studies that can build upon the shown results are presented. A new thin-disk laser oscillator scheme is proposed that implements mode-locking regimes already established in fiber lasers. Dissipative soliton and similariton simulation results are promising for further studies. They can achieve high energy pulses with the help of nonlinear effects instead of limited by it. Then, a new computer generated hologram algorithm is explained where hundreds of layers can be generated from a single hologram. The algorithm utilizes diffusion as a tool to increase the degree of freedom which in turn decreases the cross-talk between layers.
Hiyerarşik olarak çoklu darbe mod-kilit dinamiği
Passive mode-locking is the self-assembly of optical energy in a laser cavity towards narrow pulses. Often, the energy available in the cavity goes into multiple coexisting pulses with little control on their number, their energies, or their temporal positions. This phenomenon is vaguely known as pulse energy quantization and has been anecdotally linked to pulse splitting induced by optical nonlinearity. Research has focussed mainly on avoiding pulse energy quantization and any complex behavior associated with it while driving the pulses to higher and higher energies. The complex multi-pulsing behavior is often regarded as an output of a black box filled with complex nonlinear dynamics with little hope to control it. There's a want for a clear and workable understanding of these dynamics. The central point of this thesis is that standard, few-dimensional, nearly deterministic nonlinear dynamics offers this understanding; while mode-locking is indeed the result of noisy interactions of thousands of optical modes shaping the optical pulses, due to fast pulse-shaping processes involved in mode-locking, the pulse shapes tend to be slaved to one or few order parameters, mainly the pulse energy. Then, the complex behavior is understood as the result of a much simpler nonlinear dynamical system. This understanding is supported by our experiments on a multi-pulsing Mamyshev oscillator, and in return, it guides us towards reliably controlling it. With this control, multiple pulsation, which has been little more than a scientific curiosity, becomes technologically valuable for applications such as ablation-cooled material removal and frequency metrology. First, we address the issue of multiple pulsation. We define an energy map which describes the evolution of each pulse. Using the energy map, we show that stable coexistence of multiple pulses is permitted despite their competition on the gain if the growth of a pulse is self-limiting, i.e., if the energy map features a stable fixed point even at a constant gain. The energy map similarly explains the intimately related phenomena of period-doubling, non-identical pulses, and response to perturbations. The physical processes leading to these phenomena in our laser and others are discussed in parallel, and analogies to other systems are drawn. Many attractors are permitted by the energy map, highlighting the effect of bifurcations, hysteresis, and kinetics of pulse formation. Accordingly, we present guidelines for the control of multi-pulsing lasers and a procedure to control the number of pulses in a Mamyshev oscillator. Having controlled the number of pulses, we turn our attention to the temporal organization they take. Pulses coexisting in a laser cavity tend to evolve towards stable patterns due to long-range interactions between them. Several interaction mechanisms have been proposed in the literature, but the pulse interactions are still poorly understood. This is due partially to the multitude of possible interaction mechanisms and partially to the focus of their discussion on the physical processes that allow the pulses to interact without analyzing the dynamics that result from these interactions. We argue that the temporal organization dynamics is slaved and present the form of the dynamical system for all long-ranged interaction mechanisms and use it to derive for the first time the stability criterion for harmonic mode-locking. A comparison between the interaction mechanisms suggests the dominance of acoustically mediated interactions in our oscillator. We show theoretically that the acoustic effect is coupled to the single-pulse evolution dynamics and influences the individual pulse energies, which in turn, slave their speeds. This is a distinguishing feature of pulse interactions in our oscillator. We show experimentally that these interactions permit multiple stable fixed points for a given number of pulses and demonstrate noise-induced transitions as well as bifurcation based on parameters of single-pulse mode-lock dynamics, confirming our interaction theory. Lastly, we demonstrate drastic manipulation of the acoustic interactions using a novel secondary loop, allowing richer pulse patterns, and further supporting our interactions theory.
Ayrık biriktirme modellerinin simülasyonunda gelişmeler
The main purpose of this work is to describe the surface growth phenomena. Initially, we give a brief introduction to the analytical studies, in particular, done by differential equations such as Edward-Wilkinson and Kardar-Parisi-Zhang equations, and then introduce the discrete surface growth models as a means to describe these phenomena with a restricted number of rules. The universality properties of these models help us categorize them in three different classes, namely Gaussian, Edward-Wilkinson, and Kardar-Parisi-Zhang universality classes. Five different numerical growth models are explained, and the statistical properties of the growing interfaces in 1D systems are examined via computational studies. Growth properties of 1D structures are viewed in two different ways, one being the scaling exponent of roughness changing with respect to time, and the other the distribution of height fluctuations. In this research, we show that our numerical simulation findings are in accord with the theoretical and analytical predictions. In addition, for the ballistic deposition and restricted solid-on-solid models, we introduce a binning method [1] which improves our numerical results. Moreover, we propose a novel modification on the ballistic deposition model by detecting the thin-deep wells on the interface and averaging them out which enables us to report an improvement in our numerical results with a noteworthy development in the physical properties of the system. Furthermore, we investigate the effect of the change of the deposition rate on the universal behavior of the system, as a result, we introduce critical values for the deposition rate with respect to the size of the system. With these modifications, we are one step closer to defining the complex behavior of the growth phenomena.
Ultra-düşük gürültülü fiber lazer sistemleri ve uygulamaları
Fiber laser systems are intensely studied for and already utilized in a wide range of scientific, biomedical and industrial applications. Scientifically, fiber lasers are widely used for spectroscopy, laser-matter interactions, nonlinear and quantum optics experiments, among others. The industrial applications range from the well-established, such as laser-material processing, laser marking, and various forms of optical sensing to niche or upcoming applications such as high- speed circuit testing, inspection of packaged foods, additive manufacturing. In all applications outside the research laboratory, long-term stability of the lasers operation is of paramount importance. Fiber lasers are clearly advantageous in this respect, as the optical fibers provide isolated paths for light propagation, minimizing the impact of environmental effects, and generally render the laser system nearly or completely free from mechanical misalignment. In addition to long-term stability of the laser operation, short-term (typically less than 1 second) stability, or fluctuations of the laser output is of crucial importance as in many situations, it effectively determines the signal-to-noise ratio, sets the resolution or otherwise limits the quality of the measurement. Fluctuations or noise impact both the intensity and phase of the laser output. As part of this thesis, first, the intensity noise of mode-locked fiber lasers is characterized systematically for the major mode-locking regimes over a wide range of parameters. It is found that equally low-noise performance can be obtained in all regimes. Losses in the cavity influence noise strongly without a clear trace in the pulse characteristics. Noise level is found to be virtually independent of pulse energy below a threshold for the onset of nonlinearly induced instabilities. Instabilities that occur at high pulse energies are characterized. It is found that continuous-wave peak formation and multiple pulsing influence noise performance moderately. However, at high pulse energies, an abrupt increase of the intensity noise is encountered, corresponding to up to 2 orders of magnitude increase in noise. These results effectively constitute guidelines for minimization of the laser noise in mode-locked fiber lasers. For the high-power laser systems that utilize external amplification in fiber amplifiers, the added noise due to amplification is usually predominantly determined by the pump source, assuming that the amplifier design is correctly made and amplified spontaneous emission (ASE) is minimized. Many high-power amplifiers utilized multi-mode pump diodes, which have much higher noise levels. A high-power fiber laser system where the amplifiers are seeded by low intensity noise pulses is analyzed in detail. When operating at its maximum power level (10 W), the amplified output exhibits an integrated (from 3 Hz to 250 kHz) intensity noise of 0.2%, whereas the seed signals intensity noise is less than 0.03%. The origins of the added noise is analyzed systematically using modulation transfer functions to ascertain contributions of the pump source. The transfer of the noise in the seed signal is also analyzed, as well as contributions of ASE, which can be significant. Prediction of intensity noise by modulation transfer functions supplies a lower limit for the intensity noise of fiber lasers and amplifiers. The second part of the thesis applies the know-how on low-noise fiber lasers that was developed in the first part to a scientific problem. As part of a col- laboration with researchers from Ruhr-University at Bochum, Germany, we have developed a custom, low-noise laser system for spectroscopy of micro-plasma dis- charges. Absorption spectroscopy is a commonly used technique to determine the presence of a particular substance or to quantify the amount of substance present in the plasma discharge. However, the absorbance is usually small, at the level of one part in a thousand or less. Therefore, low-noise laser signals are required to detect such low differences. We developed a low-noise fiber laser system for the absorption spectroscopy studies of reactive species in a micro-plasma discharge. The laser setup also produces high-energy picosecond pulses, which are powerful enough to trigger the plasma ignition and transition into other transient states of plasma. Since both pulses are generated from the same mode-locked oscilla- tor, they have excellent mutual synchronization. We demonstrate the possibility for pump-probe experiments by initiating breakdown on a picosecond time scale (pump) with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum (probe). The third part of this thesis the laser-noise know-how to address a technolog- ical problem, namely the development custom, low-noise fiber lasers for LADAR applications. Two different fiber laser systems are constructed as transmitter sources of direct detection and coherent detection LADAR systems and tested for realistic scenarios. Both LADAR systems succeeded to detect 1 cm-diameter wire from a distance of 1 km in a measurement time shorter than 100 s, which is comparable to the best performing commercial LADAR systems.
Ultrahızlı fiber lazerlerin biyofotonik uygulamaları: Biyomalzeme yüzey modifikasyonundan hücre-altı nanocerrahiye
Just a year after the invention of the LASER in 1960, it was demonstratedthat lasers could be used for the treatment of certain skin abnormalities. At present, lasers are extensively used in a broad range of medical treatments. After the development of femtosecond pulse lasers in the 1980s, even more exciting possibilities in a diverse range of fields have been realized. Accordingly, ultrashort pulse lasers are widely used in biological applications in recent years. In parallel to these, fiber laser systems have increasingly been utilized in a wide range of scientific and biomedical applications, since they are highly compatible systems for being employed for industrial and biomedical applications. Consequently, the aim of this Ph.D. thesis proposal is to develop compact, simpler to operate, and cost-efficient ultrafast fiber lasers with different repetition rates and pulse energies. By using such systems, we demonstrate the biophotonic applications of these lasers on two different biological research fields. As a part of this thesis study, we develop ultrafast fiber lasers and apply them in biomaterial surface modification. We demonstrate that different surfaces with micro- and nano-scale topographies can be generated at high speed, precision and repeatability. The outcomes of biomaterial surface modification with different laser parameters are compared in terms of topographical uniformity and repeatability. Additionally, a variety of topographical modifications are assessed with respect to the efficiency on cell attachment and proliferation on metal implants. As the second part of this thesis, we develop a custom-built ultrafast fiberlaser-integrated microscope system for nanosurgery and tissue ablation experiments. Subsequently, we employ this system in order to make high-precision cutsonto different biological specimens ranging from the tissue level to sub-cellular level, such as a part of an axon or a single organelle. Finally, we improve this integrated system in a way that it becomes capable of generating optical pulses inany desired sequence possible. This is achieved by using acousto-optic modulators(AOM) and custom-developed field-programmable gate arrays (FPGA).
Transparan malzeme işleme, görüntüleme ve spektroskopi amaçlı ultra kısa ve kısa atımlı fiber lazer geliştirilmesi
Since the invention of the laser in the 60s, the main advances in laser technology were done in two directions; shorter pulses and higher powers. In order to achieve this purpose, many laser types are developed and always replaced with simpler, smaller, cheaper alternatives that can deliver the same or better parameters. In the past 20 years, ber lasers have become an important alternative that can match and even enhance the performance of currently used lasers while reducing the complexity, costs and instability. Optical bers, which are the main components of ber lasers, were rst de- veloped just as a substitute for conventional cables since they offer much less attenuation in carrying signals over long ranges. So, most of the studies were focused on making the bers better for communication channels. After realizing that ber lasers offer better beam qualities, which is also a vital parameter for many laser applications, researches started nding ways to use bers for lasers and they achieved this in 80s by the rst ever utilization of low-attenuation ac- tive bers. After the invention of double-clad bers, utilization of diode lasers for pumping and development of efficient rare-earth doped bers, ber lasers became more than just a research topic in the laboratory and began to nd use in many applications. The utilization of ber lasers for short (nanoseconds) and ultrashort (picosec- onds, femtoseconds) pulse generation was a difficult task for researchers. The biggest challenge to overcome was nonlinear effects caused by the con nement of the beam into small volumes. By using smart designs like chirped pulse ampli - cation and highly doped lasers, pulse energies and peak powers close to solid-state ultrafast lasers can be achieved. These nonlinear effects were not just problems in the power scalability of ber lasers, on the other hand, they were an opportunity for new possible applications. For example, using these nonlinear effects inside bers, supercontinuum generation was demonstrated and found usage in many areas like spectroscopy, imaging and metrology. Today, more than 50 worldwide companies sell short-pulse ber lasers for ap- plications as diverse as ophthalmology, micromachining, medical imaging and precision metrology. Especially, ber-laser-based micromachining is routinely im- plemented in the fabrication processes for widely used consumer products. New applications of ber lasers are being continuously developed. Consequently, in this Ph.D. thesis study, new application areas of ber lasers are investigated. Ultrashort and short pulsed ber lasers are developed and uti- lized for biological and transparent material processing, spectroscopy and imag- ing. In the rst part of thesis study, we have demonstrated the use of a custom- built ber laser-based microscope system for nanosurgery and tissue ablation experiments. Through the use of custom FPGA electronics acting through ber- coupled AOMs, we are able to generate custom pulse sequence. Using this system, we have made photodisruption experiments in tissue level, cellular level and sub- cellular level. In the second part of this thesis study, in collaboration with Bogazici Univer- sity, we have developed a nanosecond ber laser system that is able to generate wavelength components of 600 nm to 1300 nm, developed speci cally for pho- toacoustic excitation. Using this system, we have made photoacoustic signal excitation in a ceramic sample and prepared the system for further experiments to generate photoacoustic images from biological specimens. In the third part of thesis study, in collaboration with ODTU, the development of a THz-TDS system driven by a novel Yb-doped ber laser whose repetition rate can be tuned, speci cally for fast scan THz measurements, is realized. Char- acterization of the built laser system is done considering the necessities for the OSCAT technique as an alternative method for fast scan THz measurements. Stability of the oscillator is examined in terms of power, spectrum and pulse duration with the changing repetition rate of the laser. Using this system THz waveforms are generated at different wavelengths and the system is prepared for further research in spectroscopy. In the last part of the thesis study, a high-pulse energy femtosecond laser system is developed and utilized for transparent material processing. The laser output is coupled to a fast galvo-scanner system, and a synchronized translation stage such that very wide areas (10 cm x 10 cm) are able to be processed with very high speed (2 m/s). Using this system, glass samples are cut, engraved and photodarkened.Keywords: Ultrashort Pulse Laser, Ultrafast Laser, Fiber Laser, Fiber Ampli er, High Pulse Energy Laser, Ultrafast Material Processing, Nanosurgery, Tissue Ablation, Multiphoton Ablation, Photoacoustic Imaging, Supercontinuum Gen- eration, Photothermal Effects, THz Spectroscopy, Photodarkening, Glass Pro- cessing
Optik uygulamalar için silikonun üç boyutlu işlenmesi
Micromachining of silicon with lasers is being investigated for the last three decades. Until now, interior silicon modification without inscribing the surface has not resulted in success. Such an ability could enable disruptive technologies in nanophotonics by paving the way for producing monolithic optoelectronic chips. Here, we report a maskless, one step photo-induced method to generate subsurface modifications in silicon with pulsed infrared lasers for indefinitely large areas. We demonstrate continuous, highly controllable structures buried in the bulk of silicon wafers and investigate the underlying mechanism. Further, we utilize the method for spatial information encoding and fabrication of optical components in the infrared regime in silicon. This silicon processing technology can be useful in various applications, including multilayer silicon chips, solar cells and optofluidics.
Ablasyon soğutmanın katarakt ameliyatı için küme-modlu fiber lazer ile uygulaması
Cataract is the most common cause of preventable blindness in the world. Each year more than 19 million operations are performed to treat cataract. While history of cataract surgery goes as far as 2000 BC, beginning with the last quarter of the 20th century, cataract surgery procedures has benefited from advancements in ophthalmology such as phaco-emulsification, intra-ocular lens (IOL) and precision tools. There was another transformation of special interest to this thesis: In 2009, the cataract surgery was performed on a human being using a femtosecond laser for the first. Use of lasers has since gathered much attention for their high precision and repeatability in cataract surgery, but there are still challenges that their prevent wide-spread adaptation. The leading challenges can roughly be group into technical and fundamental. The technical challenges include the high cost, complexity and bulk of the associated laser technology. The more fundamental challenges relates to the interaction of ultrafast pulses with tissue. Namely, there is always interest in further reducing collateral damage and post-operation complications, which can potentially be done by reducing required pulse energies or average powers. Such an advance would also simplify the required laser technology indirectly. The principle aim of thesis has been to help overcome these challenges. In this thesis, a Yb-doped fibre laser system generating femtosecond pulses was designed to exploit recently discovered ablation-cooled laser-material removal technique. The laser system was then integrated with optical coherence tomography (OCT) for in-situ imaging during cataract surgery. In order to keep required average powers low, ablation-cooled regime is accessed through burst-mode operation of the laser. This custom-built system aims to enhance further the procedure with lower collateral tissue damage, cleaner, efficient cuts with a compact and robust structure. Preliminary experiments have been conducted on blood agar, plexiglas and extracted bovine eyes comparing the new ablation-cooled regime with the traditional regime. All experiments indicate that this regime achieves ablation with smaller pulse energies and reduced thermal effects to nearby tissue. Specifically, the pulse fluency required for corneal incision is decreased by a factor of ~15 compared to previous publications. The system has been developed into a transportable laboratory prototype, ready to be used by medical doctors through custom-developed computer control software.
Femtosaniye kızılötesi fiber lazerin çok-foton silikon mikro işleme'ye geliştirilmesi
Femtosecond laser is widely used in material processing. Application of ultrashort lasers makes it possible to process with higher precision compared to picosecond and nanosecond lasers. Moreover, a major challenge in picosecond and nanosecond laser processing is providing enough power for ablation. In the femtosecond regime, the peak power required for ablation can be achieved at lower pulse energies compared to picosecond and nanosecond pulses. Additionally, high peak intensity of femtosecond laser allows 3D material processing through multiphoton absorption by focusing the laser beam inside the bulk of material, for which the linear absorption is low (The bandgap of the material is wider than the photon energy). The same approach can be used for multiphoton surface processing, which would increase the processing precision. Such lasers could be useful for both surface and subsurface processing depending on where we focus the beam. For the past 50 years, silicon has been one of the most widely used materials in electronics technology including micro- and nanoelectronics, solar cell technology, telecommunications, etc. To the best of our knowledge, there is no existing technology up to now, which allows both surface and subsurface processing of silicon with the same laser. Er-doped fiber laser is operating at 1.55 µm wavelength, where the photon energy of the laser is less than the silicon bandgap energy. We designed and built an Er-doped all-fiber-integrated pulsed laser for multiphoton surface processing of silicon. The pulse duration of the compressed pulse is 390 fs. The laser system is capable of supplying up to 1.3 W output power at 905 kHz repetition rate, namely 1.5 µJ energy per pulse. The output beam is nearly diffraction limited with high beam quality. The laser beam is applied to process the silicon surface at different pulse energies. The depth of the trenches generated by the laser beam at various power levels is measured to investigate how the ablation depth varies with power. Subsurface silicon processing with the same laser will be investigated in our future work.
Soğuk ablasyonlu malzeme kaldırma uygulamaları için ultra hızlı fiber lazer tasarımına yeni yaklaşımlar
Applications of ultrafast laser material processing have become extremely diverse, yet ultrafast material processing is still extremely complex, costly and quite slow in terms of material removal, which is particularly taxing for biological tissue removal, rendering ultrafast lasers uncompetitive compared to mechanical techniques. This thesis represents a series of work about developing fiber laser systems which address this technological problem. The motivation of this thesis is to develop fiber laser systems for applying the ablation cooled laser material removal idea which has recently proposed by our group \cite{kerse} for tissue and material processing. Ablation cooling becomes significant above a certain repetition rate, which depends on the thermal diffusivity of the target material. Besides, the speed with which the laser beam can be repositioned over a target is limited. As a remedy, burst-mode operation, also proposed by our group \cite{hamit} has been implemented, where the laser produces groups of high repetition rate pulses, which are, in turn, repeated with a lower frequency. Consequently, the burst-mode fiber laser system operating at 1 µm was demonstrated with an all-fiber architecture and we scaled it to 100 MHz intra-burst repetition rate and 1 MHz burst repetition rate with the average power of 150 W for high power applications. Additionally, a detailed investigation on the limits of continuously-pumped all-fiber burst mode laser system was reported. Besides all the practical advantages of the ablation cooling idea compared to other laser-material interactions, laser ablation depends on laser operating wavelength because materials have wavelength dependent absorption and scattering values. In terms of underlying laser technology, ultrafast tissue ablation experiments require a laser system operating around 2 µm where laser tissue interaction is much stronger due to the local peak of water absorption for achieving a high ablation efficiency. Therefore, this thesis also focuses on transferring know-how on burst-mode operation to the Tm/Ho doped fiber system, operating around 2 µm, which addresses requirements for an efficient tissue ablation process without any collateral damage. The physics of the laser-material interaction assisted by ablation cooling idea is also valid for tissue ablation, so the repetition rates of several GHz are necessary for fully exploiting this effect. Toward this goal, we developed core technologies, which were constituted by three different stages: (i) starting from a novel mode-locked oscillator with a repetition rate in the GHz range, (ii) followed by the construction of a Tm-doped pump source based on the WDM cascade architecture and (iii) finally the amplification of the Ho-doped fiber with a dual wavelength pumping concept.
Fiber salıngaçlarda optik darbelerin doğrusal olmayan ve dengeden uzak dinamikleri
Fundamentals of mode locking of lasers have been extensively studied and well established for the last three decades. However, it continues to be an intensely studied field. The continued interest is, in part, due to the scientific and technological applications enabled by the generation of ultrashort pulses of light using mode-locking. There is also a deeper reason for the interest. Despite decades of effort, there is still no encompassing theory of mode-locking that applies to the broad range of dynamics displayed by modern mode-locked lasers, in particular, fiber lasers. Mode-locking is a collective phenomenon that arises from the nonlinear interactions between thousands of optical modes supported by a laser cavity, which is typically initiated from laser noise in the cavity. In addition to many unanswered questions from a nonlinear dynamics perspective, there has been limited progress from the point of the thermodynamics, even though mode-locking corresponds to a far-from-equilibrium steady state of a laser. The central premise of this thesis is that mode-locked lasers are invaluable as experimental platforms not only for nonlinear phenomena, but also for far-from-equilibrium dynamics of nonlinear systems, where there is a particular shortage of convenient platforms for experimentation, in addition to the practical interest in development of technically superior lasers. After introductory discussions, we report the direct generation of sub-hundred femtosecond pulses through the interaction of third order dispersion (TOD) and self-phase modulation (SPM) by using two dispersion delay lines (DDLs) inside a laser cavity. Moreover, we report dynamics that are consistent with an effective negative nonlinearity, which is explained through an interplay between self-phase modulation (SPM) and second order dispersion (GVD) for a chirped pulse. Despite numerous studies on their nonlinear dynamics, relatively little is known about the thermodynamics and fluctuations-induced dynamics of mode-locking. We investigate transitions from CW to single pulsing, and then to multipulsing states in the presence of nonlinearity, feedback mechanisms, laser noise (as a source of fluctuations) and the laser's response to externally injected modulations or fluctuations. Near critical points (instability attractors), dissipative soliton (DS) states are observed to interact between themselves and with their environment which is often followed by random transitions among different pulsing states. This critical behavior appears to be caused by soliton-soliton or soliton-generated dispersive wave interactions in addition to periodic breathing, due to the periodic boundary conditions of the cavity, leading to bifurcations and the onset of chaos. Irrespective of specifics parameters of states, measured noise level ({\em i.e.}, the strength of fluctuations) of the laser usually starts at a low value, and then slightly reduced as the DS's energy is increased. Further increases in power (nonlinearity) drive it towards a noisy critical state, where random creation or annihilation of pulses occur just before a new steady state is formed. These noise-induced transitions between steady states far from equilibrium could conceivably shed light on the thermodynamics of other far-from-equilibrium systems. Finally, we demonstrate direct electronic control over mode-locking states using spectral amplitude and phase modulation by incorporating a spatial light modulator (SLM) at a Fourier plane inside the cavity. The modulation enables us to halt and restart mode locking, suppress instabilities, induce controlled reversible and irreversible transitions between mode-locking states, and perform advanced pulse shaping inside a cavity. We also introduce a simple method to manipulate femtosecond optical pulses by directly applying dynamic periodic phase modulation mask on the optical spectrum inside oscillator. With the application of such dynamic periodic linear spectral phase mask we can control the pulse dynamics, demonstrating the capability to tune the pulse-to-pulse separation time, pulse tweezing, blue- and red-shifting of spectral components and pulse splitting. This technique, which is introduced for the first time to our knowledge, may be used in a range of applications such as coherent quantum control, nonlinear spectroscopy, microscopy, in data storage, in the switching of optical and magnetic properties of materials, as well as studies on the fundamentals of oscillator dynamics and other self-organized phenomena in spatiotemporally extended systems.
Büyüyen arayüzlerin uzaysal ve zamansal salınımlarında Tracy-Widom dağılımı sergileyen bir sürekli ortam denklemi
A wide variety of surface growth phenomena involves random processes that result in correlated stochastic dynamics. Such dynamics is most succinctly described by a nonlinear equation known as the Kardar-Parisi-Zhang (KPZ) equation. It is of particular interest that the random fluctuations observed along a growing interface described by the KPZ equation turn out to be correlated, with statistics that match the so-called Tracy-Widom distribution. The correlated fluctuations pertain only to the space dimension, namely, along the growing interface. The fluctuations of any given point along the interface over time remain uncorrelated, thus exhibiting Gaussian fluctuations. This is to be expected since the KPZ equation and the experimental systems where the Tracy-Widom statistics have been observed lack mechanisms to induce temporal correlations. Recently, a new mechanism of dissipative self-assembly has been reported, where the self-assembly process is driven by an intrinsic feedback mechanism that is expected to induce temporal correlations. Indeed, such correlations have been experimentally observed with statistics that match the Tracy-Widom probability distribution. Here, we explore the theory of the emergence of correlated temporal fluctuations in such a system when a simplified feedback mechanism is introduced. We develop a highly simplified model, which formally constitutes a modified KPZ equation. We, then, show that this modified equation exhibits temporal fluctuations that are well described by Tracy-Widom fluctuations, up to at least the eight moment, in excellent agreement with the experimental results.
Nonlinear lazer litografide uzamsal ve zamansel simetri kırılması
Symmetry breaking is ubiquitous in nonlinear systems. This is also the case for Nonlinear Laser Lithography (NLL), in which an ultrafast laser beam incident on a material surface causes the infinite fold rotational symmetry of the material surface to be broken. In the case of linear polarization, line like structures are obtained that possess 2-fold rotational symmetry. We discuss two types of NLL, one due to the formation of oxide structures (Oxidation NLL) and the other due to material ablation (Ablation NLL). The existence of both types of structures is known for many years, however, although the regularity of oxidative structures has been significantly improved by our group earlier, the same was not true for ablative structures. Here, using the technique for Oxidation NLL and the parameters for ablative structures, we were able to achieve highly regular ablative structures which we call Ablation NLL. We demonstrate the coexistence of these two NLL structures on the same surface and how a plane can be tiled using them. Furthermore, we explore the phase space of NLL and determine the regions of the phase space occupied by the two NLL structures. We also demonstrate the versatility of NLL by obtaining Oxidation and Ablation NLL structures on several metals as well as on Silicon. We also discuss temporal symmetry breaking in NLL. If the laser beam is not incident normal to the surface and is tilted towards or away from the scanning direction, it can cause the period of the NLL structures to decrease or increase respectively. One can thus discern if a video of the beam creating a pattern while scanning over the surface along a line is run forward or backward. This dependence on the scanning direction leads to temporal symmetry breaking and is reminiscent of the Doppler effect. These symmetry breakings can be important for future research in this field along with possible commercial applications, some of which we have discussed here.