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Field resolving spectrometer for mid-infrared molecular spectroscopy
Field resolving spectrometer for mid-infrared molecular spectroscopy
The interrogation of molecular samples with broadband mid-infrared (MIR) radiation results in highly specific “vibrational fingerprints,” containing a wealth of information on molecular structure and composition. This renders vibrational spectroscopy a powerful and versatile tool for applications ranging from fundamental science to the life sciences and to industrial applications. Conventional MIR spectroscopic techniques face severe limitations in detection sensitivity, in particular due to the poor coherence properties of common MIR sources as well as to the moderate detectivity and dynamic range of broadband MIR detectors. The research reported in this thesis has addressed the quest for novel routes towards tapping the potential of MIR spectral fingerprinting, harnessing modern, high-power femtosecond laser technology. The first part of the work reports the construction of octave-spanning, coherent femtosecond MIR sources, employing state-of-the-art 100-W-average-power-level thin-disk Yb:YAG modelocked oscillators. We demonstrated ultrabroadband coherent MIR sources with a brilliance exceeding that of MIR beamlines at 3rd-generation synchrotrons, and found that pulses emerging via intra-pulse difference frequency generation offer superior (and unparalleled) optical-waveform stability as compared to standard optical-parametric amplification. The temporal confinement of broadband MIR radiation to trains of sub-100-femtosecond pulses, together with field-resolved detection via electro-optic sampling (EOS) affords detection of the molecular fingerprint signal in the near-infrared region, where highly-efficient, high-dynamic-range detectors exist. Optimized EOS detection enabled a linear response over an intensity dynamic range of 150 dB at a central wavelength of 8.6 µm. This exceeds the previous state of the art by a large margin and has paved the way to high-signal-to-noise-ratio transmission measurements of aqueous biological samples like living cells and tissue. The waveform stability of the mid-infrared pulses plays a crucial role for real-life field-resolved spectroscopy measurements, and is of paramount importance for precision-metrological applications. In the second part of this thesis, high-quantum-efficiency EOS was employed for precision measurements of waveform jitter, evaluated for millions of pulses. This study demonstrated few-attosecond temporal jitter in the 1-Hz-to-0.625-MHz band, between the centre of mass of the driving near-infrared pulses, and individual field zero-crossings of the emerging, broadband mid-infrared field. This confirms the outstanding waveform stability achievable with second-order parametric processes with an order-of-magnitude improved accuracy compared to previous measurements. Furthermore, chirping the MIR pulse revealed attosecond-level optical-frequency-dependent waveform jitter, whose dynamics were quantitatively traced back to excessive intensity noise of the mode-locked oscillator. Thus, this study validated EOS as a broadband (both in the radio-frequency and in the optical domain), high-sensitivity measurement technique for the dynamics of optical waveforms beyond the standard, optical-spectrum-integrating carrier-envelope phase model. The instrument developed during this thesis was utilized for the first highly sensitive field-resolved measurements in the MIR molecular fingerprint region. It enabled the detection of molecular concentrations spanning 5 orders of magnitude down to 200-ng/mL in aqueous solutions and the examination of living biological systems with a thickness of up to 0.2 mm. Currently, the instrument is being used for the first large-scale studies on disease recognition based on vibrational fingerprinting of human blood serum. The implementation of intra-scan referencing, successfully carried out in the last weeks of this doctoral work, together with fast-scanning techniques and the extension of the MIR spectral bandwidth which are underway at our laboratory, promise to extend the technology pioneered in this thesis to new levels of sensitivity and reproducibility in vibrational spectroscopy. In addition to directly benefitting analytical applications, these developments are likely to afford novel insights into light-matter interactions.
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Hussain, Syed Ali
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Hussain, Syed Ali (2021): Field resolving spectrometer for mid-infrared molecular spectroscopy. Dissertation, LMU München: Faculty of Physics
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Abstract

The interrogation of molecular samples with broadband mid-infrared (MIR) radiation results in highly specific “vibrational fingerprints,” containing a wealth of information on molecular structure and composition. This renders vibrational spectroscopy a powerful and versatile tool for applications ranging from fundamental science to the life sciences and to industrial applications. Conventional MIR spectroscopic techniques face severe limitations in detection sensitivity, in particular due to the poor coherence properties of common MIR sources as well as to the moderate detectivity and dynamic range of broadband MIR detectors. The research reported in this thesis has addressed the quest for novel routes towards tapping the potential of MIR spectral fingerprinting, harnessing modern, high-power femtosecond laser technology. The first part of the work reports the construction of octave-spanning, coherent femtosecond MIR sources, employing state-of-the-art 100-W-average-power-level thin-disk Yb:YAG modelocked oscillators. We demonstrated ultrabroadband coherent MIR sources with a brilliance exceeding that of MIR beamlines at 3rd-generation synchrotrons, and found that pulses emerging via intra-pulse difference frequency generation offer superior (and unparalleled) optical-waveform stability as compared to standard optical-parametric amplification. The temporal confinement of broadband MIR radiation to trains of sub-100-femtosecond pulses, together with field-resolved detection via electro-optic sampling (EOS) affords detection of the molecular fingerprint signal in the near-infrared region, where highly-efficient, high-dynamic-range detectors exist. Optimized EOS detection enabled a linear response over an intensity dynamic range of 150 dB at a central wavelength of 8.6 µm. This exceeds the previous state of the art by a large margin and has paved the way to high-signal-to-noise-ratio transmission measurements of aqueous biological samples like living cells and tissue. The waveform stability of the mid-infrared pulses plays a crucial role for real-life field-resolved spectroscopy measurements, and is of paramount importance for precision-metrological applications. In the second part of this thesis, high-quantum-efficiency EOS was employed for precision measurements of waveform jitter, evaluated for millions of pulses. This study demonstrated few-attosecond temporal jitter in the 1-Hz-to-0.625-MHz band, between the centre of mass of the driving near-infrared pulses, and individual field zero-crossings of the emerging, broadband mid-infrared field. This confirms the outstanding waveform stability achievable with second-order parametric processes with an order-of-magnitude improved accuracy compared to previous measurements. Furthermore, chirping the MIR pulse revealed attosecond-level optical-frequency-dependent waveform jitter, whose dynamics were quantitatively traced back to excessive intensity noise of the mode-locked oscillator. Thus, this study validated EOS as a broadband (both in the radio-frequency and in the optical domain), high-sensitivity measurement technique for the dynamics of optical waveforms beyond the standard, optical-spectrum-integrating carrier-envelope phase model. The instrument developed during this thesis was utilized for the first highly sensitive field-resolved measurements in the MIR molecular fingerprint region. It enabled the detection of molecular concentrations spanning 5 orders of magnitude down to 200-ng/mL in aqueous solutions and the examination of living biological systems with a thickness of up to 0.2 mm. Currently, the instrument is being used for the first large-scale studies on disease recognition based on vibrational fingerprinting of human blood serum. The implementation of intra-scan referencing, successfully carried out in the last weeks of this doctoral work, together with fast-scanning techniques and the extension of the MIR spectral bandwidth which are underway at our laboratory, promise to extend the technology pioneered in this thesis to new levels of sensitivity and reproducibility in vibrational spectroscopy. In addition to directly benefitting analytical applications, these developments are likely to afford novel insights into light-matter interactions.