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High resolution spectroscopy using a CW stabilized dyelaser

Felix Schuda

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

The subject of this thesis is the construction of a stabilized cw dye laser and its use with a sodium atomic beam to gain information about the range of validity of certain atomic radiation theories. By lengthening the laser cavity, cooling the dye solution and adding many other refinements the bandwidth of a single mode cw dye laser was reduced to 3 MHz over a period of ten seconds. The laser was designed so that by the insertion of three uncoated glass etalons single mode tunable operation was achieved. The purpose of stabilizing the dye laser was to be able to perform experiments on the hyperfine structure of sodium at high enough resolution and intensity to see any possible differences in the theoretical predictions of Quantum Electrodynamics, the optical Bloch equations, and Neoclassical Radiation Theory. The traditional method of eliminating Doppler shifting was used, that is a well-collimated thermal atomic beam of sodium was constructed. By crossing the atomic and laser beams at right angles the natural linewidth of 10 MHz was observable with only one or two MHz residual broadening. Two types of experiments were carried out: those studying the absorption spectrum, and those studying the emission spectrum. In the first experiments Lorentzian lineshapes were observed for all applied field strengths. This proved that if the Lamb shift of Neoclassical Theory exists at all it is small compared with 10 MHz for the sodium D lines. Further, the sharply peaked spectrum predicted by Neoclassical Theory for a certain field strength was not observed. The width of the absorption at high field strengths was not in good agreement with either theory. In the second type of experiment the observed emission spectra were in qualitative, but not good quantitative, agreement with Quantum Electrodynamics when the effects of nearby levels were included. The results are in disagreement with a two-level neoclassical treatment, and we have been unable thus far to obtain better agreement in more sophisticated treatments. Undoubtedly a better multi-level calculation is required in all theories since our system is not really two-level. In addition, we have found that a factor of 2 increase in the critical intensity calibration would bring QED into reasonable agreement with most of the experiments. However, we have been unable to establish the origin of such a factor. While these results were not sufficient to finally and absolutely settle the controversy regarding Quantum Electrodynamics and Neoclassical Theory they did provide a great deal of new information. For the first time the dynamic Stark effect was observed near resonance in the optical region with data that could be compared to theory. Also, the measurements of the absorption spectrum were the most complete and among the highest resolution measurements yet obtained by direct optical techniques. And finally, a number of of optical pumping phenomena were observed.

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Absorption spectra of pulse-train-excited sodium two-level atoms

E. Krüger

The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.

Journal of the Optical Society of America B•Jan 1, 1995•11 citations

Stark-effect study of excited states in sodium using two-photon spectroscopy

Roberta Hawkins, W. T. Hill, Frank V. Kowalski +2 more

We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the $6^{2}S$, $7^{2}S$, $8^{2}S$, $5^{2}D$, and $6^{2}D$ states in sodium. Beams from a fixed-frequency, single-mode ${\mathrm{Ar}}^{+}$ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants $a$ for the $S$ states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the $D$ states were obtained.

Physical Review A•Mar 1, 1977•31 citations

Observation of the resonant Stark effect at optical frequencies

Felix Schuda, Carlos R. Stroud, Michael Hercher

The spectrum of resonantly scattered light at right angles to a sodium atomic beam is reported. The light source was a CW dye laser tuned to resonance with a hyperfine component of the D 2 line, and incident at right angles to the atomic beam. The spectrum, with the Stark effect sidebands, was recorded as a function of both the laser intensity and its detuning from resonance. The overall resolution is better than 20 MHz.

Journal of Physics B Atomic and Molecular Physics•May 11, 1974•414 citations

Coherent transient spectroscopy with quantum cascade lasers

James M. R. Kirkbride

This thesis is concerned with coherent effects in high resolution mid-infrared gas phase spectroscopy using quantum cascade lasers (QCLs). An introductory chapter explains the importance of QCLs as radiation sources in the mid-infrared region of the spectrum and goes on to detail their development and structure. A discussion of coherent effects in spectroscopy follows, leading into the second chapter which discusses the theories relevant to the experimental sections of the thesis. In chapter 2 the theory underpinning direct and velocity selective, Doppler-free spectroscopy is discussed and a density matrix formalism is followed to derive the equations of motion that govern coherent excitation effects in two-level systems. In the final part of the chapter this treatment is extended to three-level systems. The equations derived in this chapter form the basis of quantitative interpretations of the phenomena observed in experimental data and presented in the remainder of the thesis. In chapter 3 the characterisation of a high power, narrow linewidth QCL is carried out. This laser is then used to perform direct and sub-Doppler resolution spectroscopy on NO, demonstrating non-linear absorption at high laser intensities and providing a measurement of the laser linewidth in the limit of slow frequency tuning. As the slow tuning rate increases, evidence of coherent transient effects is presented and density matrix theory used to model this behaviour. The data presented include the first observations of asymmetric Lamb dips and the onset of rapid passage oscillations from a Lamb dip. Pump-probe experiments on NO, utilising two cw QCLs are presented in chapter 4. The high level of velocity selection afforded by QCL excitation leads to coherent transient signals at far lower probe scan rates than previously reported. The effect of altering both the scan rate and the gas pressure and the importance of hyperfine structure are presented. A radio frequency noise source applied to one of the lasers is shown to broaden the laser linewidth, leading to rapid dephasing. A two-colour polarisation spectroscopy experiment is also presented which allows the measurement of both the absorption and the Doppler-free dispersion signals and the three-level density matrix formalism presented at the end of chapter 2 used to model the non-linear response of the system. The final chapter details the use of an acousto-optic modulator to create a pulse of mid-IR light using a cw QCL and the application of this to time resolved pump-probe spectroscopy. This capability suggests the prospect of achieving coherent population transfer by stimulated Raman adiabatic passage (STIRAP) using two such pulses. Simulations based on a simple three-level model and including Zeeman coherences are presented, which take the measured properties of the lasers used in this thesis as inputs to predict the potential population transfer achievable in NO as well as providing useful information about the angular momentum polarisation of the excited molecules. An experimental realisation of STIRAP would require the lasers to be stabilised, and so the final part of the chapter details experimental attempts to achieve stabilisation of an external cavity QCL, and suggests future avenues for improved implementation.

•Jan 1, 2014•0 citations

Frequency correlated laser beam generation in sodium vapor: sodium-density and pump-intensity dependence

Prem Kumar, Bernard Matthew Poelker

When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.

Annual Meeting Optical Society of America•Jan 1, 1988•0 citations

Coherent optical transient measurements of absolute and differential Stark shifts in atomic sodium

Peter Hannaford, W. R. MacGillivray, M. C. Standage

A coherent optical transient technique has been used for the first time to study the Stark effect in atoms. This technique involves the recording of transients in the intensity of single-mode laser radiation transmitted by an atomic vapour following the sudden application of an electric-field pulse. It allows both absolute and differential Stark shifts to be measured to a precision which is limited only by the power-broadened natural linewidth of the transition. The feasibility of this technique is demonstrated by measuring the scalar ( alpha0) and tensor ( alpha2) polarisabilities associated with the sodium 32P1/2and 32P3/2levels of sodium. The resolution is such that the effect of the hyperfine structure on the Stark shifts has to be included in the analysis. Our values for the polarisabilities are alpha0=0+or-3.3 kHz (kV cm-1)-2and alpha2=28.1+or-3.9 KHz (kV cm-1)-2. These results are compared with experimental and theoretical values obtained by other authors.

Journal of Physics B Atomic and Molecular Physics•Dec 28, 1979•9 citations
The subject of this thesis is the construction of a
stabilized cw dye laser and its use with a sodium atomic beam to
gain information about the range of validity of certain atomic
radiation theories. By lengthening the
laser cavity, cooling the dye solution and adding many other
refinements the bandwidth of a single mode cw dye laser was reduced
to 3 MHz over a period of ten seconds. The laser was designed so
that by the insertion of three uncoated glass etalons single mode
tunable operation was achieved. The purpose of stabilizing the dye
laser was to be able to perform experiments on the hyperfine
structure of sodium at high enough resolution and intensity to see
any possible differences in the theoretical predictions of Quantum
Electrodynamics, the optical Bloch equations, and Neoclassical
Radiation Theory.
The traditional method of
eliminating Doppler shifting was used, that is a well-collimated
thermal atomic beam of sodium was constructed. By crossing the
atomic and laser beams at right angles the natural linewidth of 10
MHz was observable with only one or two MHz residual broadening.
Two types of experiments were carried out:
those studying the absorption spectrum, and those studying the
emission spectrum. In the first experiments Lorentzian lineshapes
were observed for all applied field strengths. This proved that if
the Lamb shift of Neoclassical Theory exists at all it is
small compared with 10 MHz for the sodium D lines. Further, the
sharply peaked spectrum predicted by Neoclassical Theory for a
certain field strength was not observed. The width of the
absorption at high field strengths was not in good agreement with
either theory. In the second type of experiment the observed
emission spectra were in qualitative, but not good quantitative,
agreement with Quantum Electrodynamics when the effects of nearby
levels were included. The results are in disagreement with a
two-level neoclassical treatment, and we have been unable thus far
to obtain better agreement in more sophisticated treatments.
Undoubtedly a better multi-level calculation is required in all
theories since our system is not really two-level. In addition, we
have found that a factor of 2 increase in the critical intensity
calibration would bring QED into reasonable agreement with most of
the experiments. However, we have been unable to establish the
origin of such a factor.
While these
results were not sufficient to finally and absolutely settle the
controversy regarding Quantum Electrodynamics and Neoclassical
Theory they did provide a great deal of new information. For the
first
time the dynamic Stark effect was observed near resonance in
the optical region with data that could be compared to theory.
Also, the measurements of the absorption spectrum were the most
complete and among the highest resolution measurements yet obtained
by direct optical techniques. And finally, a number of of optical
pumping phenomena were observed.
The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.
We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the 62S6^{2}S62S, 72S7^{2}S72S, 82S8^{2}S82S, 52D5^{2}D52D, and 62D6^{2}D62D states in sodium. Beams from a fixed-frequency, single-mode Ar+{\mathrm{Ar}}^{+}Ar+ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants aaa for the SSS states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the DDD states were obtained.
This thesis is concerned with coherent effects in high resolution mid-infrared gas phase spectroscopy using quantum cascade lasers (QCLs). An introductory chapter explains the importance of QCLs as radiation sources in the mid-infrared region of the spectrum and goes on to detail their development and structure. A discussion of coherent effects in spectroscopy follows, leading into the second chapter which discusses the theories relevant to the experimental sections of the thesis. In chapter 2 the theory underpinning direct and velocity selective, Doppler-free spectroscopy is discussed and a density matrix formalism is followed to derive the equations of motion that govern coherent excitation effects in two-level systems. In the final part of the chapter this treatment is extended to three-level systems. The equations derived in this chapter form the basis of quantitative interpretations of the phenomena observed in experimental data and presented in the remainder of the thesis. In chapter 3 the characterisation of a high power, narrow linewidth QCL is carried out. This laser is then used to perform direct and sub-Doppler resolution spectroscopy on NO, demonstrating non-linear absorption at high laser intensities and providing a measurement of the laser linewidth in the limit of slow frequency tuning. As the slow tuning rate increases, evidence of coherent transient effects is presented and density matrix theory used to model this behaviour. The data presented include the first observations of asymmetric Lamb dips and the onset of rapid passage oscillations from a Lamb dip. Pump-probe experiments on NO, utilising two cw QCLs are presented in chapter 4. The high level of velocity selection afforded by QCL excitation leads to coherent transient signals at far lower probe scan rates than previously reported. The effect of altering both the scan rate and the gas pressure and the importance of hyperfine structure are presented. A radio frequency noise source applied to one of the lasers is shown to broaden the laser linewidth, leading to rapid dephasing. A two-colour polarisation spectroscopy experiment is also presented which allows the measurement of both the absorption and the Doppler-free dispersion signals and the three-level density matrix formalism presented at the end of chapter 2 used to model the non-linear response of the system. The final chapter details the use of an acousto-optic modulator to create a pulse of mid-IR light using a cw QCL and the application of this to time resolved pump-probe spectroscopy. This capability suggests the prospect of achieving coherent population transfer by stimulated Raman adiabatic passage (STIRAP) using two such pulses. Simulations based on a simple three-level model and including Zeeman coherences are presented, which take the measured properties of the lasers used in this thesis as inputs to predict the potential population transfer achievable in NO as well as providing useful information about the angular momentum polarisation of the excited molecules. An experimental realisation of STIRAP would require the lasers to be stabilised, and so the final part of the chapter details experimental attempts to achieve stabilisation of an external cavity QCL, and suggests future avenues for improved implementation.
When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.
A coherent optical transient technique has been used for the first time to study the Stark effect in atoms. This technique involves the recording of transients in the intensity of single-mode laser radiation transmitted by an atomic vapour following the sudden application of an electric-field pulse. It allows both absolute and differential Stark shifts to be measured to a precision which is limited only by the power-broadened natural linewidth of the transition. The feasibility of this technique is demonstrated by measuring the scalar ( alpha0) and tensor ( alpha2) polarisabilities associated with the sodium 32P1/2and 32P3/2levels of sodium. The resolution is such that the effect of the hyperfine structure on the Stark shifts has to be included in the analysis. Our values for the polarisabilities are alpha0=0+or-3.3 kHz (kV cm-1)-2and alpha2=28.1+or-3.9 KHz (kV cm-1)-2. These results are compared with experimental and theoretical values obtained by other authors.