Academic literature on the topic 'Modulation transfer spectroscopy'

Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure. The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters. The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models. The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation. Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal. Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar. In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.

Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure. The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters. The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models. The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation. Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal. Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar. In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.

Diese Arbeit beschäftigt sich mit der Entwicklung und Untersuchung von optischen Absolutfrequenzreferenzen basierend auf rovibronischen Übergängen in molekularen Jod. Dabei werden Methoden der Doppler-freien Sättigungsspektroskopie angewendet, um einzelne Übergänge der Hyperfeinstruktur mit Linienbreiten unterhalb von 1 MHz im B-X System von molekularem Iod bei 532 nm, der zweiten harmonischen des Nd:YAG-Laser, aufzulösen. Elektronische Regelungstechniken ermöglichen eine präzise Stabilisierung der optischen Frequenz auf die Linienmitte der Übergänge mit einer Auflösung von Teilen in 10^5. Mit dem Ziel einer weltraumtauglichen Absolutfrequenzreferenz für zukünftige Weltraummissionen, wurden zwei Spektroskopiemodule konzipiert und in quasi-monolithischen Glaskeramik-Aufbauten, als sogenanntes elegant breadboard model und engineering model, realisiert. Diese Jodfrequenzreferenzen wurden im Detail in Bezug auf ihre Frequenzstabilität und Reproduzierbarkeit untersucht und Letzteres wurde für die angestrebte Weltraumqualifizierung ersten Umwelttests, sowohl vibrations- als auch thermischen Belastungstests, unterzogen. Für die Untersuchung der Frequenzstabilität dieser Jodreferenzen wurde ein auf einen optischen Resonator hoher Güte stabilisiertes Lasersystem für direkte Frequenzvergleiche bei 1064 nm realisiert. Die Analyse der Frequenzstabilität der Jod Referenzen zeigt eine Frequenzstabilität von 6x10^−15 bei 1 s, und weniger als 2x10^−15 bei 100 s Integrationszeit, was der bis heute besten veröffentlichten Frequenzstabilität entspricht die mit Jod Referenzen erreicht wurde. Mit der erreichten Frequenzstabilität ermöglichen diese Absolutfrequenzreferenzen präzise Lasersysteme für zukünftige Weltraummissionen wie z.B. zur Detektion von Gravitationswellen, zur Vermessung des Gravitationsfelds der Erde oder für Präzisionstest fundamentaler Theorien der Physik.<br>This thesis deals with the development and investigation of optical absolute frequency references based on rovibronic transitions in molecular iodine. Doppler-free saturation spectroscopy methods are employed to resolve individual transitions of the hyperfine structure with linewidths below 1 MHz in the B-X system of molecular iodine at 532 nm with the second harmonic of Nd:YAG lasers. Electronic feedback control systems are employed for laser frequency stabilization to the line center of the optical transitions with a line splitting of 10^5. With the goal of a space qualified optical absolute frequency reference for future laser-interferometric space missions, two spectroscopy setups were designed and realized in quasi-monolithic, glass-ceramic setups as so called elegant bread board model and engineering model. These iodine references were characterized in detail with respect to their frequency stability and reproducibility and the engineering model was subject to environmental tests, including vibrations and thermal cycling to verify its applicability in future space missions. For the investigation of the frequency instability of these iodine references, a frequency stabilized laser system was realized based on a temperature controlled high Finesse ULE cavity for direct frequency comparisons at 1064 nm. Analysis of the frequency stability of the iodine references revealed exceptionally low fractional frequency instability of 6x10^−15 at 1 s, averaging down to less than 2×10^−15 at 100 s integration time, constituting the best reported stability achieved with iodine references to date. With the demonstrated performance, these absolute frequency references enable precision laser systems required for future space missions that are dedicated to, e.g., the detection of gravitational waves, mapping of the Earth’s gravitational field or precision test of fundamental physics.

Advent of ultrashort lasers made it possible to probe various scattering phenomena in materials that occur in a time scale on the order of few femtoseconds to several tens of picoseconds. Nonlinear optical spectroscopy techniques, such as pump-probe, transient four wave mixing (TFWM), etc., are very common to study the carrier dynamics in various material systems. In time domain, the transient FWM uses several ultrashort pulses separated by time delays to obtain the information of dephasing and population relaxation times, which are very important parameters that govern the carrier dynamics of materials. A recently developed multidimensional nonlinear optical spectroscopy is an enhanced version of TFWM which keeps track of two time delays simultaneously and correlate them in the frequency domain with the aid of Fourier transform in a two dimensional map. Using this technique, the nonlinear complex signal field is characterized both in amplitude and phase. Furthermore, this technique allows us to identify the coupling between resonances which are rather difficult to interpret from time domain measurements. This work focuses on the study of the coherent response of a two dimensional electron gas formed in a modulation doped GaAs/AlGaAs quantum well both at zero and at high magnetic fields. In modulation doped quantum wells, the excitons are formed as a result of the inter- actions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the formation of Mahan excitons, which is also referred to as Fermi edge singularity (FES). Polarization and temperature dependent rephasing 2DFT spectra in combination with TI-FWM measurements, provides insight into the dephasing mechanism of the heavy hole (HH) Mahan exciton. In addition to that strong quantum coherence between the HH and LH Mahan excitons is observed, which is rather surprising at this high doping concentration. The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence be destroyed as a result of the screening and electron-electron interactions. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum 2DFT spectra. Theoretical simulations based on the optical Bloch Equations (OBE) where many-body effects are included phenomenologically, corroborate the experimental results. Time-dependent density functional theory (TD-DFT) calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system. Furthermore, in semiconductors under the application of magnetic field, the energy states in conduction and valence bands become quantized and Landau levels are formed. We observe optical excitation originating from different Landau levels in the absorption spectra in an undoped and a modulation doped quantum wells. 2DFT measurements in magnetic field up to 25 Tesla have been performed and the spectra reveal distinct difference in the line shapes in the two samples. In addition, strong coherent coupling between landau levels is observed in the undoped sample. In order to gain deeper understanding of the observations, the experimental results are further supported with TD-DFT calculation.

Des études préliminaires ont été menées sur un modulateur plasmonique pour des applications télécoms et sur un réseau de nanopointes recouvertes d'or pour des applications biomédicales. Nous proposons d'utiliser des modulateurs plasmoniques tout optique basés sur des nanoparticules métalliques déposées sur des films métalliques minces afin d'assurer la conversion des signaux électriques en signaux optiques par variation de l'intensité de la lumière transmise. L'originalité de ce projet provient de l'utilisation simultanée de plasmons de surface localisés et propagatifs pour produire la fonction optique. Cette géométrie offre de nombreux avantages par rapport aux systèmes actuellement proposés : réalisation simple, capacité d'intégration haute densité et efficacité de modulation. Une partie de ce travail de thèse a consisté à étudier ce type de modulateur en microscopie à fuites radiatives. L'étude de la réponse optique de nanoparticules d'or déposées sur des films métalliques minces a permis d'obtenir des amplifications de la transmission de lumière à travers ces films. Nous avons également développé une nouvelle technique de fluorescence couplant la spectroscopie de corrélation de fluorescence (FCS) avec des substrats plasmoniques nanostructurés afin de pouvoir caractériser des biomolécules in vivo en réduisant le volume d'observation. Les substrats plasmoniques utilisés sont des réseaux de pointes nanométriques recouvertes d'or. Des mesures de FCS sur des fluorophores diffusant à proximité de ces substrats ont été effectuées. Les résultats obtenus n'ont malheureusement pas été concluants : en réflexion (laser d'excitation réfléchi sur les substrats plasmoniques), le signal émis par les fluorophores n'a pas pu être discriminé de celui émis par les substrats. Récemment, des résultats prometteurs ont cependant été obtenus dans une autre configuration (lumière injectée et collectée par les fibres optiques des substrats)<br>Preliminary studies were carried on a plasmonic modulator for telecomunication applications and on a gold nanotips array for biomedical applications

Une nouvelle méthode de caractérisation des catalyseurs solides par spectroscopie infrarouge de molécules sondes a été mise au point. La pression d'équilibre de la molécule sonde à la surface du solide subit une perturbation de forme. Sinusoi͏̈dale et périodique (technique dite PMAS-Pressure Modulation of Adsorbed Species) ou prendre la forme d'un saut de pression (technique PJAS-Pressure Jump of Adsorbed Species). L'analyse des résultats par corrélation 2D ou par transformée de Fourier de la série en fonction du temps permet d'accéder à une spectroscopie bidimensionnelle IR 2D. Cette approche permet de décongestionner les spectres IR des mélanges d'espèces adsorbées ou d'une même molécule sonde sur plusieurs sites, d'identifier les bandes de faible intensité et de faciliter l'attribution des bandes. Les premiers essais de PMAS et PJAS ont été réalisés en phase gaz avec des acides carboxyliques (séparation des constituants) puis avec des solides (silice), et ont permis de passer à l'étude de l'adsorption d'acétonitrile sur mordénite. L'influence des conditions expérimentales de l'adsorption d'acétonitrile sur mordénite a d'abord été étudiée de manière classique, un comportement atypique des bandes (OH) en fonction de la température à été mis en évidence. La 2D-PJAS-IR a ensuite permis de relier les diverses formes d'acétonitrile adsorbé avec les divers sites d'adsorption possibles. Nous avons montré que la liaison-H particulièrement forte et la protonation à haute température ne se produisaient que dans les poches latérales de la structure microporeuse. Nous avons également obtenu la première mise en évidence directe par infrarouge de trois OH différents dans la mordénite.

Les lasers stabilisés à long terme sont utilisés dans de nombreux domaines en métrologie, leur incursion dans les applications spatiales se précise depuis quelques années, missions de physique fondamentale, géodésie... Ce travail concerne les lasers stables pour le projet LISA (détection spatiale des ondes de gravitation, mission prévue dans 10 ans); en vue de cette application spatiale, les montages doivent satisfaire plusieurs critères: compacité, stabilité mécanique, robustesse et fiabilité. Les références utilisables pour l'asservissement long terme (iode, chapitre 3) et court terme (cavité de Fabry-Pérot, chapitre 4) sont décrites, ainsi que leurs limitations principales. L'analyse et le choix de ces références de stabilisation seront couplés avec le choix des techniques de stabilisation (PDH, Tilt-Locking, Modulation Transfer). Dans les techniques de stabilisation, celle de Pound-Drever-Hall (PDH), un classique en matière de configuration, est comparée au Tilt-Locking, une technique continu et peu consommatrice en énergie, dans le cas d'une référence Fabry-Perot monolithique. Les calcules théorique pour chaque technique et chaque type de référence ammeneront à une description des sources de bruit et à une simulation des signaux d'erreur attendus. Les performances théoriques sur le long terme et les efficacités quantiques sont intercomparées. La deuxième partie, présentera les montages expérimentaux et les résultats obtenus, pour des lasers stabilisés sur Fabry-Perot et sur l'Iode moléculaire. Pour calibrer les dérives de fréquence du Fabry-Perot, sa fréquence de résonance sera mesurée par rapport à celle de la molécule, ce qui permettra de présenter des solutions obtenues analytiquement et numériquement pour l'asservissement en longueur de cette référence mécanique. En appendices, les détails de dimensionnement des bruits de LISA conduisant aux spécifications des lasers, le rappel théorique des ondes de gravitation ainsi que divers simulations de calculs effectués.

碩士<br>逢甲大學<br>光電物理研究所<br>92<br>In this thesis a 1.5-μm external cavity tunable diode laser (ECTDL) and hollow fiber with internal diameter of 700um was used to investigate the saturation absorption spectroscopy of acetylene. Using Modulation transfer technique (MT) to reduce Doppler background, and frequency modulation (FM) heterodyne technique to enhance the signal to noise ratio, we attend to observe the P(9) acetylene saturation absorption spectrum. With this scheme, the gas consumable was reduced, because hollow fiber has small volume.

Electron transfer is one of the fundamental process occurring in many chemical reactions. Electron transfer process has been under intensive study for many applications, for example artificial photosynthesis, where electrons from photo-excited chromophore molecules are harnessed to produce solar fuels in various forms. Transition metal complexes, such as ruthenium and rhenium complexes, play an important role in the continuing development of artificial photosynthetic devices. The electron transfer process in chromophores involving transition metal complexes often occurs on an ultrafast time scale from sub-ps to ns. To resolve such dynamics, ultrafast spectroscopic techniques are required. A variety of ultrafast techniques, such as time-resolved infrared spectroscopy and multi-pulse transient absorption spectroscopy, were used in this study to unravel the excited state electron transfer dynamics in a series of Re(I) complexes. Transition metal complexes often feature excited states that involve only partial electron transfer between the electron donating and accepting ligands, even for ligands with strong electron donating and accepting properties. It is often difficult to design a compact complex feature a full electron transfer excited state. Therefore, part of the work presented in this thesis was dedicated to the study of the electron transfer extent in the excited states of a series of [Re(N,N)(CO)3L]+ compounds, where N,N stands for electron accepting and L stands for electron donating ligands. By carefully designing the structure and redox properties of both the electron donor and acceptor, we demonstrated that essentially a full-electron charge transfer excited state can be prepared, while the designed Re(I) complex is still compact. To further extend the understanding of the electron transfer in transition metal complexes, modulation of the electron transfer rate in a compact Re(I) complex was studied. By perturbing the electron transfer process with a femtosecond mid-IR pulse, we showed that a 28% increase of the electron transfer rate was achieved. This study demonstrated the possibility of using a small energy mid-IR quanta to change the energy conversion process in a chromophore. Vibrational energy transfer in molecules is another important process in nature. Detailed understanding of the vibrational energy transfer on a molecule level is fundamentally important and essential for the development of molecular optical devices. It was recently discovered that the transport of vibrational energy in molecules can be fast and efficient due to its ballistic character. To understand the mechanism of the ballistic energy transport, experiments with several series of oligomers were performed using a relaxation-assisted two-dimensional infrared method. The energy transport speed was found to be dependent on transport initiation method and the transport pathways for different cases of initiation were identified. Detailed analysis on the chain band structure, group velocity and vibrational relaxation dynamics is presented.<br>acase@tulane.edu