Doctor of Philosophy / Department of Physics / Artem Rudenko / Carlos Trallero-Herrero / Ultrafast laser systems (pulse durations 10-100 femtoseconds) allow for the practical production of intense fields (≥ 10¹⁴ W/cm²) in a table-top, laboratory setup. The development of this technology has opened the door to studying the interaction of intense laser fields with atoms, molecules, and solid media. These experiments revealed a wealth of dynamics and interplay between the field, ion, and the freed electron, which has led to the production of first attosecond pulses and opened the field of attosecond science. The dynamics of the electron in an intense laser field are fundamental to strong- field phenomena such as higher-order harmonic generation, high energy above threshold ionization, and non-sequential double ionization. As the electron can be strongly accelerated by the instantaneous field, the dynamics depend on both the field's amplitude and wavelength. The latter dependence comes from the fact that the period of the field increases with wavelength. Thus, the electron is accelerated for a longer time and the energy gained is proportional to the wavelength squared. Recent evidence supports the claim that the electron- field interaction at longer wavelengths must include the contribution of the magnetic field and/or the radiation pressure of the field, adding to the wealth of effects associated with strong- field interactions. This thesis explores several routes towards fulfilling gaps in our understanding of the wavelength-scaling of strong- field interactions. I first demonstrate several important developments that reduce the complexity of generating non-sinusoidal, light transient waveforms in the near-infrared, opening the ability to tailor waveforms for more control on strong- field interactions. Next, I demonstrate the development of a strong- field, femtosecond source at wavelengths from 5 micrometers to 9 micrometers. To date, this is the first few-cycle, strong- field (≥ 10¹⁴ W/cm²) source in the long-wave infrared. An important advantage to this design is the wavelength tunability, which provides a control knob for understanding strong- field interactions across a broad wavelength range. Afterwards, I present applications of wavelength tunable sources in strong- field absorption in semiconductors. Specifically, I measure the absorbance of a strong laser field in gallium arsenide as a function of laser polarization, which varies the density of states available to the electron. This is performed for four laser wavelengths spanning 1.2 micrometers to 2.4 micrometers. With these absorbance measurements, we can compare the dependence of the photoexcitation rate on several parameters and compare it to theory. We find that the change in absorbance with density of states deviates from theoretical predictions as the photon order for the photoexcitation increases from two to three. This could be attributed to the field modifying the energy-momentum relationship of the conduction band. To conclude the thesis, I present simulations on a recent experimentally demonstrated technique for amplifying few-cycle electric fields. Due to the difficulty in making these sources, the model I developed includes many of the parameters involved in designing the system. This simulation can be used to plan design criteria, such as nonlinear crystal thickness, for peak performance of the amplification process.

Made available in DSpace on 2014-10-09T12:27:54Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:48Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Submitted by CARLA CONSTANZA MANGANELLI (carlacm.89@gmail.com) on 2017-12-01T15:28:42Z No. of bitstreams: 1 DISSERTAÇÃO DEFINITIVA - online - Repositorio.pdf: 996196 bytes, checksum: 7399044c17373c4a4a6d22f0e53e9bbf (MD5) / Approved for entry into archive by LUCIA SILVA PARRA null (lucia@ia.unesp.br) on 2017-12-04T12:14:50Z (GMT) No. of bitstreams: 1 manganelli_cc_me_jabo.pdf: 941849 bytes, checksum: 9248a7466ffb33e1a145b960af79cd2d (MD5) / Made available in DSpace on 2017-12-04T12:14:50Z (GMT). No. of bitstreams: 1 manganelli_cc_me_jabo.pdf: 941849 bytes, checksum: 9248a7466ffb33e1a145b960af79cd2d (MD5) Previous issue date: 2017-10-10 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / As coberturas plásticas têm diferentes pr opriedades térmicas e de radiação, e podem afetar o rendimento e a qualidade do tomate. Neste estudo, compararam - se a influência das coberturas plásticas coloridas (vermelho, azul, e cinza/preto) e a densidade de plantio no crescimento do tomateiro, o rend imento e a qualidade dos frutos com o da cobertura preta convencional e o solo descoberto (controle) no desenvolvimento do tomateiro ‘Micro - Tom’. Além disso, determinou - se a acumulação de pigmentos, a temperatura do solo e a água total aplicada. Os resulta dos indicaram que o peso seco e a área foliar das plantas cultivadas com cobertura azul, vermelha e cinza/preta foram significativamente maiores do que o tratamento controle em parcelas de alta densidade. Em comparação com o tratamento controle, o rendimen to total, o peso do fruto e o peso total de frutos por planta foram consequentemente melhorados nas plantas crescidas com cobertura vermelha e cinza/preta em altas densidades. Contudo, o número de frutos não foi afetado. Além disso, o tamanho do fruto aume ntou com cobertura vermelha em parcelas de baixa densidade. Por outro lado, as temperaturas do solo registradas sob a cobertura vermelha foram as mais adequadas para o tomateiro nessas condições climáticas. Em conclusão, os tratamentos com cobertura colori da afetaram positivamente o crescimento e o rendimento do tomateiro em parcelas de alta densidade, e a cobertura vermelha em particular poderia ser usada para melhorar o rendimento e o tamanho dos frutos de tomate ‘Micro - Tom’, devido à otimização da temper atura e o ambiente de luz. / Plastic mulches have different thermal and radiation properties, and they can affect tomato fruit yield and quality. In this study, the influence of coloured plastic mulches (red, blue and gray on black) and planting density on tomato growth, fruit yield and quality was compared with that of conventional black mulch and uncovered soil (control) in developing ‘Micro Tom’ tomatoe s. In addition, the pigment accumulation, soil temperature and total applied water were determined. The results indicated that the dry weight and leaf area of plants grown over blue, red and gray/black mulches were significantly higher than the control tre atment in high - density plots. Compared to the control treatment, the total tomato yield, fruit weight and total fruit weight per plant were consequently improved in plants grown over red and gray/black mulches at high densities. However, the fruit number w as not affected. In addition, the fruit size increased over red plastic mulc h when planted at a low density . In addition, the soil temperatures registered under the red mulch were the most suitable for the tomatoes in these climatic conditions. In conclusi on, treatments employing coloured mulch positively affected the growth and yield of tomatoes in high - density plots, and red mulch in particular could be used to improve the yield and fruit size of Micro - Tom tomatoes because of the optimization of the therm al and light environment. / 134597/2015-0

Um Luminescent Solar Concentrators (LSC) é basicamente uma base plástica transparente ou vidro contendo centros luminescentes.Enquanto o recente avanço dos conversores de comprimento de onda tenha aberto novas possibilidades para o desenvolvimento de células solares mais eficientes, estes também abriram novas frentes de pesquisa.Uma destas frentes envolve os problemas encontrados com o uso de novos materiais. Como por exemplo, transferência de energia em polímeros. Surgem propriedades interessantes a respeito dos processos luminescentes ao longo destes sistemas planares.Por estes motivos esses sistemas podem ser aplicados como dispositivos optoeletrônicos, sensores e também em conversores de comprimento de onda. Este estudo descreve a fabricação de guias de onda planares multipoliméricos baseados em uma matrix acrílica (PMMA) dopada com polímeros emissores de luz. A função desta blenda polimérica é absorver um largo espectro de luz e re-emitir ao longo do guia de onda. Os dispositivos foram estudados em termos de suas características de absorção, emissão e excitação.Os polímeros emissores utilizados neste trabalho foram: MEH-PPV, Super Yellow, ADS-108GE, ADS-329BE, e LAPS-16. Estes materiais foram dissolvidos em tolueno e depositados pela técnica casting sobre substratos de vidro contendo filmes metálicos de alumínio.Usando esta configuração, a luz de excitação incidente é acoplada ao guia de onda por absorção e re-emissão através dos polímeros luminescentes. O estudo dos LSCs é realizado por medidas das emissões laterais e normais dos guias. Estes guias apresentam um espectro de emissão com características típicas de uma cavidade do tipo Fabri-Perot, com linhas extremamente estreitas (3 nm) e altamente polarizadas na direção paralela (modo TE) e perpendicular (TM) ao plano do guia de onda.A emissão destes guias é fortemente dependente de sua geometria e do índice de refração efetivo. Os mesmos filmes poliméricos, porém mais espessos (30 m), foram utilizados como conversores de comprimento de onda visando aplicação em conversores solares luminescentes (Luminescence Solar Converters, ou LSCs). O uso de baixas concentrações das moléculas utilizadas foi necessário para evitar possíveis formações de agregados quando altas concentrações de PMMA são utilizadas na preparação dos filmes. A metalização de uma das faces planares possibilitou o aumento da eficiência de conversão dos comprimentos de onda nos LSCs. A luminescência total dos polímeros é completamente convertida em luz de alto comprimento de onda na emissão lateral.Este processo não depende do comprimento de onda de excitação. A metalização de uma das faces planares possibilitou o aumento da eficiência de conversão dos LSCs. Neste sistema, a luz absorvida é convertida em luz de alto comprimento de onda e alta pureza espectral através de processos de reabsorção, relaxação interna e re-emissão que ocorrem ao longo do plano do LSC. A eficiência de conversão e as perdas em função da concentração polimérica na matriz foram estudadas para os LSCs.O escape dos fótons nas interfaces para ângulos menores que o ângulo crítico (c=arcsen(1/n)) e processos de transferência de energia em LSCs com alta concentração polimérica devem ser otimizados para sua aplicação como dispositivos. / Luminescent Solar Concentrators (LSC) are basically composed of a transparent plastic or glass substrate containing luminescent centers.While the recent advance in wavelength converters has opened up many new possibilities for development of more efficient photovoltaic cells, they have also raised new issues. A number of these issues involve problems of dealing with new materials and the understanding of physical process like energy transfer in polymers. Interesting features arise from the luminescent process along these planar systems. Therefore this system can be applied as optoelectronics devices, sensors and also in wavelength converters. This study describes the fabrication of planar multi-polymeric optical waveguides based on acrylic matrix (PMMA) doped with a light emitting polymers. The function of thispolymeric blendis to absorb a broad spectrum of light and re-emit it along the waveguide. The devices were studied in terms of its absorption, emission and excitation characteristics.The emitting polymers used in this research are: poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), Super Yellow, Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene}] called ADS108GE, Poly(9,9-dioctylfluorenyl-2,7-diyl) called ADS239BE, and poli-(9,9-n-dihexil-2,7-fluorenodilvinilene-alt-1,4-fenilenovinileno) or Laps16. Films of these materials dissolved in toluene were deposited by casting the polymethacrylate solution on top of a glass substrate containing a reflective aluminum layer. Using this configuration, the incident excitation light is coupled to the wave guide by its absorption and then re-emitted by the luminescent polymers. The LSCs characterization was carried out by measuring the lateral and the normal emissions of the guide. These waveguides show a light emission spectrum with characteristics similar to the Fabri-Perot cavity, where very narrow peaks (3 nm) and highly polarized emission in both TE and TM modes were observed. In addition, the emission is strongly dependent on its geometry and the effective refraction index. In other hand, thicker polymeric guides (30 m) were used as luminescence solar converters (LSCs). The optimal concentration of conjugated polymers solutions has to be very low in order to avoid the formation of aggregates since high PMMA concentrations have to be used in the LSC preparation.The total polymeric luminescence is fully converted to light of high wavelength for the case of lateral emission. This process does not depend on the excitation wavelength. The conversion efficiency was increased by evaporating metals on one planar face of the LSC. In this structure, light is absorbed and then converted to a higher wavelength with high spectral pureness through re-absorption, relaxation and re-emission processes occurring along the LSC plane. The conversion efficiency and the losses along the converter were studied as a function of the polymeric concentration, in order to optimize these structures. A better understanding of the process of energy transfer and photon leakage through the interfaces for angles smaller than the critical angle (c=arcsen(1/n)) is necessary for using LSCs as applied devices.

Made available in DSpace on 2014-10-09T12:27:54Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:48Z (GMT). No. of bitstreams: 0 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Lasers de estado sólido sintonizáveis com uma estreita largura de linha de emissão na região do vermelho são uma alternativa conveniente aos lasers de corante para aplicações em espectroscopia de alta resolução. Nesse trabalho, foram investigados lasers contínuos de Nd:YLiF4 e Nd:YVO4 operando em frequência única na região de 1,32 - 1,34μm, assim como a geração de segundo harmônico (GSH) desses lasers usando cristais de BiB3O6 (BiBO) e KTiOPO4 com inversão periódica de domínios (ppKTP), para a obtenção da emissão no vermelho (0,65 - 0,67μm). Utilizando um laser de Nd:YVO4 operando em frequência única em uma configuração em anel com um cristal não linear BiBO do tipo I, demonstrou-se o recorde de 680mW no vermelho em 671,1nm, sem a utilizaçãde nenhum elemento seletivo. Uma intonia em todo o ganho (~4 nm) foi obtida atravésda inserção de um etalon com filme refletor (R = 40%) e com 100μm de espessura, o que reduziu a potência de saída no vermelho para 380mW no comprimento de onda de maior ganho (671,15 nm). Em 1342nm foi demonstrada uma potência de saída de 1,5W em frequência única quando utilizado um espelho de saída com transmissão de 2%. Foi demonstrado ainda uma ótima eficiência de conversão de segundo harmônico em um laser em anel de Nd:YLF na polarização π (λ = 1321,5nm) quando usando um cristal de ppKTP. Este laser forneceu 1,4W em frequência única no vermelho em 660,5nm. Essa potência é a máxima que pode ser extraída desse laser no segundo harmônico e no fundamental quando utilizado um espelho com transmissão ótima. Utilizando um etalon com filme refletor (R = 25%), o laser pôde ser sintonizado em Δλ ~ 1,6nm. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP / FAPESP: 06/57945-3

Transmissores agnósticos em comprimento de onda colorless [1], isto é, transmissores que não estão restritos à operação em um único comprimento de onda, são considerados como um dos pré-requisitos para os projetos de redes ópticas passivas baseadas na multiplexação por divisão em comprimento de onda Wavelength Division Multiplexing Passive Optical Network. Esta configuração reduz consideravelmente os custos operacionais e de instalação da rede WDM-PON, uma vez que um único tipo de transmissor pode ser usado em qualquer terminal de assinante, independentemente dos comprimentos de onda pré-estabelecidos para os usuários em questão. Entretanto, a possibilidade de aumento do número de usuários e da capacidade de transmissão agregada, com poucas modificações na estrutura da rede WDM-PON, garantindo uma melhor relação custo-benefício, são questões também essenciais no projeto de sistemas multi-comprimento de onda.Tendo em vista o panoramo exposto, neste trabalho é proposta uma nova topologia de reuso de portadora óptica para redes WDM-PON, a qual combina sinais autoalimentados e remodulação óptica no domínio da intensidade, visando o atendimento das premissas tecnológicas para cenários de nova geração. Como metodologia, investigações experimentais são conduzidas em termos de caracterizações ópticas em função de vários parâmetros e avaliações sistêmicas em taxas simétricas de 1,25 Gb/s, no âmbito de redes de acesso e acesso estendido. Utilizam-se transmissores colorless do tipo amplificadores ópticos semicondutores refletivos e fontes ópticas banda larga baseadas em emissão espontânea amplificada, de forma a garantir a simplificação dos terminais dos usuários. Adicionalmente, são realizadas análises comparativas entre essa proposta frente aos trabalhos encontrados na literatura técnica, quando possível, em termos de custos operacionais, desempenho sistêmico e viabilidade de expansão para cenários a taxas simétricas de 10 Gb/s. De acordo com os resultados obtidos, acredita-se que essa topologia possa ser expandida para cenários de até 10 Gb/s com o uso de técnicas apropriadas de apagamento de portadora, mantendo a melhor relação custo-beneficio para sistemas de remodulação no domínio da intensidade. / Wavelength agnostic (colorless) transmitters [1], i.e, transmitters which are not fixed to a specific wavelength operation, are regarded as one of the essential keys for the designing of wavelength division multiplexing passive optical networks (WDM-PONs). The use of colorless transceiver leads to a reduction in the mainentence cost of the network, once the same basic optoelectronic hardware can be employed to provide any wavelength required by a specific optical network termination (ONT). However, the possibility of a graceful increase on the number of ONTs and transmission capacity offered by the network, with no disruptive changes in the WDM-PON infra-structure, while keeping a cost-effective topology, are also attractive features in the design of future access networks. In this context, a novel wavelength-reuse topology for WDMPON is proposed in this work. The wavelength-sharing technique is based on combined self-seeded and remodulated signals in the intensity domain, in order to comply with the technology requierements for next generation networks. Experimental analysis are carried out in respect to optical characterization and system evaluation on symmetric rates of 1.25 Gb/s, aiming both short reach and extended reach scenarios. Reflective semicondutor optical amplifiers and broadband light sources based on amplified spontaneous emission are used as colorless transceivers in order to guarantee a costeffective ONT configuration. Also, comparative analyses between this proposal and works found in the technical literatural are established in terms of maintenance costs, system performance and feasibility of expansion to symmetric 10 Gb/s scenario. According to the results, we believe that network could be expanded to symmetric 10 Gb/s scenario with the development of proper erasure technique, while keeping the best cost-effective configuration of intensity-remodulated systems.

D.Ing. / Optical fibre-based devices (e.g. fibre gratings) play an important role in the optical com- munications and sensing industry. One type of fibre grating, the long-period grating (LPG), is becoming more and more popular as a simple and versatile component for a multitude of applications in optical engineering. The sensitivity of LPGs to various external perturbations and their ability to manipulate selectively light propagating in optical fibres make them well- suited to creating fibre-based devices. LPGs can be used in various applications, for example as gain equalisers for erbium-doped fibre amplifiers, as channel routers in optical add-drop multiplexers and as sensors. LPGs are typically fabricated by exposing photosensitive optical fibre to ultraviolet light. However, a variety of other techniques can be used to fabricate LPGs, including exposure to carbon dioxide (CO2) laser light. Over the years, it has become evident that CO2 laser- induced LPGs exhibit unique properties and features that can be harnessed to develop devices for important applications. It is necessary to understand the physical properties and optical characteristics of CO2 laser-induced LPGs to harness, manipulate and enhance their features. Research has been conducted on the development of an automated fabrication system that produces axially symmetric LPGs in single-mode fibre with a CO2 laser. A detailed study was undertaken on the design of uniform and non-uniform LPGs for wavelength-division multiplexing networks, as well as the characterisation of the CO2 laser beam propagating in the LPG fabrication system. These LPGs have been designed, using either analysis or syn- thesis techniques. The polarisation-dependent loss of the LPGs has also been investigated. Wavelength-selective couplers (WSCs) have been constructed using CO2 laser-induced LPGs and it was shown that these couplers were e®ective but not efficient in routing power to the output port of the tapping fibre. The physical properties, optical transmission characteris- tics, applications and other related issues of CO2 laser-induced LPGs have been investigated and satisfactory experimental results have been obtained. Areas for potential future research concerning CO2 laser-induced LPGs have been identified and discussed.

Active galaxies have been in the forefront of astronomic research since their first discovery, at least 50 years ago (e.g. Schmidt, 1963; Matthews & Sandage, 1963). The putative supermassive black hole (SMBH) at their center characterizes their properties and regulates the evolution of these objects. In this thesis, I study the 'demographics' and 'ecology' of active galactic nuclei (AGN) in the context of their evolution and the interaction with their environments (mainly their host galaxy). The number density of AGN has been found to peak at 1 < z < 3 (e.g. Ueda et al., 2003; Hasinger et al., 2005; Richards et al., 2005; Aird et al., 2010), similar to the star formation history (e.g. Silverman et al., 2008a; Aird et al., 2010). However, when taking into account obscuration, faint AGN are found to peak at lower redshift (z ≤ 2) than that of bright AGN (z ≈ 2 - 3; e.g. Hasinger et al., 2005; Hopkins et al., 2007; Xue et al., 2011). This qualitative behaviour is also broadly seen in star-forming galaxies (e.g. Cowie et al., 1996) and is often referred to as 'cosmic downsizing', although this term has developed a number of usages with respect to galaxies (e.g. Bundy et al., 2006; Cimatti et al., 2006; Faber et al., 2007; Fontanot et al., 2009). Though this behaviour is well established up to z ≈ 3, the nature of how and when the initial seed of these AGNs were formed remains an open question. For this study, I use Chandra surveys to study some of the most distant AGN in the Universe (z > 3). The combination of two different size and depth Chandra surveys (Chandra-COSMOS and ChaMP) provides me with the largest to-date z > 3 AGN sample, over a wide range of rest-frame 2-10 keV luminosities [log (Lₓ/erg s⁻¹) = 43.3-46.0] and obscuration (NH = 10²⁰ - 10²³ cm⁻²). I find strong evidence about a strong decline in number density of X-ray AGN above z ≈ 3, and also the association of this decline with a luminosity-dependent density evolution (LDDE; e.g. Gilli et al., 2007). Especially at high redshifts, the different evolution models predict quite different numbers of AGNs. The large size and the wide X-ray luminosity range of this sample reduces the uncertainties of previous studies at similar redshifts making it possible to distinguish between the different models and suggest that observations appear to favour the LDDE model. The observed AGN downsizing behaviour seen via the measured X-ray luminosity function (XLF) could arise due to changes in the mass of the typical active SMBH and/or changes in the typical accretion rate. But how does the growth of SMBHs over cosmic time influence its environment? A powerful way to address this question is to compare the host galaxy properties over a wide range of AGN and accretion rate types. Radio-jets are one of the most prominent constituents of AGN as they can interact directly with the host galaxy. Although AGN with radio jets are rare (they make up to 10 per cent of the total AGN population) radio galaxies make up over 30 per cent of the massive galaxy population and it is likely that all massive galaxies go through a radio-loud phase, as the activity is expected to be cyclical (e.g Best et al., 2005). It is therefore, important to investigate the impact of radio jets on the host galaxy and particularly the star formation. The method I follow focuses on the comparison of the host galaxy properties between optically selected quasar samples, with and without strong radio emission associated with powerful radio-jets, matched in AGN luminosity. Herschel far-infrared observations are used to trace the star formation in the host galaxy, providing minimal AGN contamination. In my first approach, I have constructed a sample of radio-loud and radio-quiet quasars from the Faint Images Radio Sky at Twenty-one centimetres (FIRST) and the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), over the H-ATLAS Phase 1 Area (9h, 12h and 14.5h). The main result of this work is that RLQs at lower AGN luminosities tend to have on average higher FIR and 250-μm luminosity with respect to RQQs matched in AGN luminosity and redshift. However, evolution effects could be strong as the quasars in this sample cover a wide range of redshifts (0.4 < z < 5). Therefore, I follow a second approach with the advantage of a QSO sample selection at a single redshift epoch, decomposing the evolution effects from the AGN/star-formation study. The results indicate that radio-jets in powerful QSOs can both suppress and enhance the star formation in their host galaxies. These fundings are consistent with a galaxy mass and jet-power dependence model. Then we expect more massive galaxies to have more star-formation for a given jet-power because their star-formation is more enhanced by the jet. Although radio-jets are the best candidates for a direct AGN impact to the host galaxy, many models refer to an AGN feedback associated with energetic AGN winds and outflows which are expected to suppress the star formation in powerful AGN when compared to the overall galaxy population. My results do not suggest star formation is suppressed in the hosts of optically selected QSOs at z ≈ 1, with more than 30 per cent of them being associated with strong star formation rates (SFR ≈ 350 M⊙ yr⁻¹). Although different interpretations are possible, this result can be explained through periods of enhanced AGN activity and star-forming bursts, possibly through major mergers. However, optical QSOs comprise only a small fraction of the total AGN population. Even if the 'unified model' predicts that the host galaxy properties should not be affected by the viewing angle (type-1 vs. type-2 AGN), several studies have shown results supporting a scenario departing from the basic model. Investigating star formation in the hosts of 24 μm selected type-1&2 AGN, I found that the type-2 AGNs display on average higher star-formation rate than type-1 AGNs. This result is in agreement with previous studies suggesting an undergoing transition between a hidden growth phase and an unobscured AGN phase.

Processing of Si in the semiconductor and solar cell industry has been dominated by the Diode Pumped Solid State (DPSS) Ultraviolet (UV) laser. Recent advances in laser source technology have produced fibre lasers with Master Oscillator Power Amplifier (MOPA) architectures that offer high repetition rates, high operational efficiencies, and pulse modulation controls exceeding those of typical Q-switched DPSS lasers. The aim of this research is to investigate 1 μm fibre laser machining of Si with a view to identifying the influential laser parameters for optimum processing of high quality, high efficiency micro drilling and surface texturing applications. A secondary aim is to develop a greater understanding of the laser material interactions and material removal mechanism when using fast rise-time nanosecond laser pulse envelopes. The IR fibre laser was able to perform percussion drilling and single pulse machining on the polished Si over a range of intensities up to 1.22 GW/cm2. With the optimum parameters, the micro-sized holes generated by the IR laser have a well defined edge, no heavy recast and no cracks. With a pulse shape of fast rise time (<7.5 ns for a 10-90% rise in signal), a high front peak power zone (approaching 14 kW) and an energetic long tail (40-180 ns), the absorption coefficient of Si at IR wavelength increased dramatically with time and temperature due to the fact that the liquid Si has a metal like absorption behavior. As a result, Si was quickly melted and the rest of pulse energy was able to remove the liquid Si effectively. The machining process left a limited amount of resolidified melt droplets and vapor condensates, which could be washed off ultrasonically. The drilling process was energy efficient when melt expulsion dominated the machining mechanism (0.08-0.2 mJ pulse energy depending on the pulse durations). The low energy pulse (~0.2 mJ) can achieve similar depth as the high energy pulse (~0.7 mJ), so high repetition rates of 100 kHz can be used to instead of 25 kHz, resulted in high processing speed. In addition, by comparing the single pulse machining with the state of the art UV laser, the IR fibre laser machined deeper features and better surface finish in the pulse energy region of >0.07 mJ. With the pulse shaping capability, the material properties can be varied and the wavelength factor can be minimized. The results suggest that applications like microvia drilling can now be carried out with the more flexible and low cost IR fibre laser. The increased repetition rates of fibre laser can increase production speed to satisfy the needs of drilling ~10 thousands holes per second, required by the modern semiconductor and solar cell production. The shortened optical penetration length of 1 μm wavelength laser on Si with increasing temperature and sufficient thermal diffusion length resulted from the asymmetrical fibre laser pulse and the dynamic properties of Si produced a thick liquid layer. A one-dimensional heat conduction model based on the surface heating source predicted that this superheated liquid layer was able to stay above 4706 K (0.905 times the thermal critical temperature 5200 K of Si) for longer than 70 ns to induce explosive boiling. This proposed material removal mechanism was also confirmed by the shadowgraph images, showing particulates ejection lasting up to ten microseconds after the laser pulse. The estimated hole depth based on the explosive boiling alone were different from the measured ones at varying peak power densities (<1.22 GW/cm2) but fixed pulse duration (200 ns), since Si was removed by a mixture of mechanisms. With varying pulse durations (40-200 ns) but fixed peak power density (~0.63 GW/cm2), the estimated depth based on the explosive boiling was in close agreement with the measured ones (6% difference on average). The SEM images at this power density showed a micron- /submicron-sized debris field, which was also observed with the explosive boiling in the past. Although the improved quality of Si machining was demonstrated with the 1 μm MOPA based fibre laser, the setup of this system was only applicable to surface texturing, blind holes and through holes of less than 100 μm in depth. Further research is required to demonstrate the capability of more energetic pulse with higher peak power and large pulse duration range to explore more machining options.

No description available.