|Title:||Random laser in polymer optical fiber and other disorder optical systems|
|Advisors:||Tam, Hwa-yaw (EE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Lasers -- Testing
|Department:||Department of Electrical Engineering|
|Pages:||xi, 202 pages : color illustrations|
|Abstract:||Random lasers (RLs) are unconventional (mirror-less) lasers whose lasing feedbacks are synergistically achieved by multiple light scattering and optical gain amplification. Due to their easy fabrication, unique properties, and promising applications, much attention has been devoted to RLs over the past two decades. In this thesis, several different RLs are demonstrated in various optical systems. Their lasing dynamics are studied by means of experimental investigation and theoretical simulation. Utilizing these RLs, sensing and imaging applications are carried out, showing the potential of RLs in wide field of research and application. In the first work, I demonstrated a one-dimensional random fiber laser based on the polymer optical fiber (POF). The core of the POF was doped with TiO2 nanoparticles and Rh640 perchlorate dyes. A multimode and coherent random lasing was successfully obtained. The waveguide effect provided by the POF greatly increased the multi-scattering events for light propagation and resulted in laser cavity with much shorter length than the scattering mean free path. As an illustration of potential applications, a clear speckle-free imaging was demonstrated using the POFRL as the illumination source. Furthermore, a four-level Monte Carlo method-based numerical model is proposed to describe the lasing dynamic in the POFRL. Second work presented a RL cytometer demonstrated in an optofluidic device filled with gain medium and human breast normal/cancerous cells. The multiple light scattering induced by the micro-scale human cells promoted the random lasing and influenced the lasing properties in terms of laser modes, spectral wavelengths, and lasing thresholds. A sensing strategy has been proposed based on analyzing the lasing properties. Based on this manner, the relationship between the lasing properties and both the whole-cell and the subcellular biophysical properties has been clarified. Furthermore, the malignant alterations of the cell suspensions are successfully detected.|
In third work, I reported a hybrid two-photon RL device comprising double-resonant gold nanorods (GNRs) randomly embedded in an all-inorganic perovskite quantum dot (PQD) thin film. With increasing the GNR distribution density, the hybrid device exhibited an intriguing four-stage transition from amplified spontaneous emission (ASE), incoherent to coherent random lasing and back to ASE. By spectrally matching the longitudinal and transverse localized surface plasmon resonances of the GNRs with the two-photon absorption and emission wavelengths of the PQDs, the threshold power density monotonically decreased throughout the whole four-step evolution due to the double-plasmon-resonance-boosted population inversion in the PQDs. Using the hybrid laser device as the illumination source, the speckle-free two-photon imaging can be achieved. In the fourth work, I reported the first experimental evidence of the glassy behavior in a RL with more complex energy level structures. This novel RL was demonstrated based on the electrospun polymer fibers with the assistant of Forster resonance energy resonance energy transfer (FRET). The electropun technology employed in the experiment promised high-volume production of RL devices with multiple types of the laser dyes, enabling the comprehensive investigation of lasing properties in multi-energy level RL system. Clear paramagnetic phase and spin-glass phase have been observed in the FRET-assisted RL under different pump energy. The replica symmetry breaking (RSB) phase transition was verified to be occurred at the laser threshold, which is robust among the RLs with different donor-acceptor ratio.
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