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Title: Nanofiber based light-matter interaction for high performance laser spectroscopy
Authors: Qi, Yun
Degree: Ph.D.
Issue Date: 2020
Abstract: Trace gas detection is critical for industrial safety, environmental monitoring and public security. Among the various kinds of techniques, the molecular spectroscopy may be the most all-round method for trace gas detection, in consideration of its high selectivity and sensitivity. Optical nanofiber is widely recognized as an excellent platform for light-matter interaction, due to its guided-wave property, high intensity of the evanescent field and the immunity from mode noise. The nanofiber based molecular spectroscopy is then expected to achieve promising performance in trace gas sensing, which is the research target of this thesis. Photothermal interferometry (PTI) is generally a more sensitive method for trace gas detection compared with the direct absorption spectroscopy (DAS) for the same optical absorption length, because the power of pump beam in PTI spectroscopy can be set sufficiently high to achieve larger phase modulation without the issue of power saturation for the probe beam. Both the traditional free space and the newly-developed hollow-core photonic bandgap (HC-PBG) fiber based PTI rely on the absorption induced temperature and thus refractive index (RI) change of the air, which, however, has a relatively low thermo-optic coefficient compared with solid materials. An intriguing advantage of the nanofiber based PTI is that the generated heat due to evanescent wave absorption can be transferred to the nanofiber. Due to the orders of magnitude higher thermo-optic coefficient of the core material than the air, the phase modulation amplitude and thus the photothermal signal in nanofiber based PTI spectroscopy can be much improved. With a piece of 12 mm long and 800 nm-diameter silica nanofiber, the trace acetylene detection limit of 600 parts per billion (ppb) have been achieved. Numerical simulations predict that for the nanofiber/waveguide made of materials with much larger thermo-optic coefficients, such as silicon, further tremendous enhancement in photothermal phase modulation efficiency can be achieved. The nanofiber based PTI method is only applicable to the molecules with strong absorption lines and fails for the homodiatomic ones such as hydrogen. However, the high-intensity evanescent field associated with the nanofiber makes the stimulated Raman scattering (SRS) process for Raman-active molecules much more efficient compared with the free space and the HC-PBG fiber counterparts. The nanofiber enhanced SRS for high-sensitivity and fast-response hydrogen detection is demonstrated. The high sensitivity results from the large SRS gain and the single-mode operation of the nanofiber. The trace hydrogen detection limit of 3 ppm with response time less than 10s has been achieved using a piece of 48-mm-long and 700-nm-diameter silica nanofiber. The change of polarization states in both the pump and probe beam should be the primary cause for the long term signal drift, which is also addressed.
The evanescent field is tightly constrained around the nanofiber, so the light-matter interaction time is limited due to the inevitable thermal motion of the molecules, which results in additional transit time broadening. The shorter the interaction time, the broader the linewidth and thus the lower the peak absorption coefficient of the gas molecules. To which extent the transient time effect contributes to the overall linewidth, is explored by numerical simulation using the density matrix method as well as verified by experiments. It is concluded that for the absorption lines of common molecules in the near infrared band at the atmospheric pressure, the transit time broadening is negligible compared with the dominant collisional broadening. However, for the nanofiber-based SRS of hydrogen, the situation is much different. Due to the lighter molecular weight and thus higher thermal velocity, the transit time effect of hydrogen is the dominant broadening factor over both the collisional and Doppler broadening at the atmospheric pressure. Besides the spectroscopic applications of nanofiber, the potential ability of light manipulation by nanofiber is also explored. It is found that the non-circularly symmetric optical field of the linearly polarized fundamental modes can produce a transient temperature distribution with a two-fold rotational symmetry in the surrounding air, which results in a transient birefringence of the nanofiber. This transient photothermal phenomenon can then be exploited for all-optical polarization switching, which may find application in optical communication. With a piece of 15-mm-long and 600 nm-diameter nanofiber in pure acetylene of 4 bar and a 15 ns pulsed pump with peak power of 30W, polarization switching with efficiency up to 90 %, on and off time of 11 and 37 ns have been achieved.
Subjects: Photothermal spectroscopy
Laser spectroscopy
Hong Kong Polytechnic University -- Dissertations
Pages: xx, 127 pages : color illustrations
Appears in Collections:Thesis

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