Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Applied Physics | en_US |
| dc.contributor.advisor | Yu, Siu Fung (AP) | en_US |
| dc.creator | Dong, Xuezhe | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/14477 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Non-steady state regulation of laser emission: applications to lanthanide-doped glasses and carbon quantum dots | en_US |
| dcterms.abstract | The precise control of laser emission properties in diverse gain media is a central challenge in modern photonics, underpinning applications ranging from optical communication and sensing to biomedical imaging and quantum information processing. Lanthanide-doped glasses and carbon quantum dots (CQDs) represent two distinct yet complementary classes of materials. Lanthanide-doped glasses offer high photochemical stability, sharp emission lines, and broad spectral coverage from ultraviolet (UV) to near-infrared (NIR) through multi-step energy transfer (ET) mechanisms, but their steady-state performance tuning is inherently constrained by fixed crystal-field environments and equilibrium population distributions. Conversely, CQDs offer broadband tunability, facile solution-processability, and strong environmental sensitivity due to their surface functionalization capabilities, yet suffer from lower optical gain coefficients and higher scattering losses compared to inorganic crystalline counterparts. The overarching theme of this dissertation is to develop, implement, and validate a non-steady state regulation framework—combining spatial and temporal control of energy transfer dynamics—to construct a universal method for regulating the laser performance of various gain materials and overcome their respective limitations. | en_US |
| dcterms.abstract | Efficient near-infrared (NIR) to ultraviolet (UV) multiphoton upconversion is critical for applications in photocatalysis, sterilization, biomedicine, and compact lasers. However, inefficiencies from energy losses in intermediate states, including radiative transitions, cross-relaxation (CR), nonradiative decay, and quenching, limit UV emission intensity. This thesis presents a novel approach to enhance NIR-to-UV upconversion in lanthanide (Ln³⁺)-doped fluoroaluminate glass-ceramics (GCs) through spatial and temporal control of energy transfer (ET), achieving significant UV emission enhancement and low-threshold UV random lasing. Using Förster-Dexter theory, we identified that ET rates depend on donor-acceptor distances, which are large in amorphous glasses, leading to low ET efficiency and defect quenching. Molecular dynamics (MD) simulations guided the design of glass compositions [ (30+x) AlF3-(54-x)(Ca, Sr, Ba)F₂-16YbF₃, x = 0 -12 mol%] to precipitate nanocrystals with tunable (Ca, Sr, Ba)/Yb ratios. Higher AlF₃ content forms compact networks, reducing alkali-earth ion surroundings around [YbFₓ] polyhedra, enabling hexagonal Yb1-y(Ca, Sr, Ba)yF3-y or cubic (Ca, Sr, Ba)1-xYbₓF2+x nanocrystals via heat treatment. These nanocrystals enrich Ln³⁺ ions, shortening inter-ion distances (e.g., Tm³⁺-Tm³⁺ from 42.7 Å in glass to 19.0 Å in hexagonal nanocrystals), thus increasing ET rates. Yb³⁺/Tm³⁺/Gd³⁺-tri-doped GCs were synthesized with optimized doping (10 mol% Yb³⁺, 0.07 mol% Tm³⁺, 5 mol% Gd³⁺) to maximize ET upconversion (ETU) and CR while minimizing quenching. In GC-39Al (hexagonal Yb1-y(Ca, Sr, Ba)yF3-y:Tm,Gd), UV emission at 311 nm (Gd³⁺: 6Pj → 8S7/2) increased 267-fold compared to precursor glass, driven by enhanced ET rates (e.g., Yb³⁺-Tm³⁺ ET from 352 s⁻¹ to 950 s⁻¹). Power-dependent spectra confirmed efficient population of high-lying states (Tm3+: ¹D₂, ¹I₆) via CR and ET to Gd³⁺, enabling six- to seven-photon upconversion, including the first 205 nm emission in bulk materials. Temporal control further enhanced UV emission by modulating excitation pulse widths (40 ms to 0.5 ms) at constant power (2.55×10⁵ mJ·cm⁻²). Python-simulated rate equations showed that shorter pulses increase peak power, accelerating high-energy state population while suppressing mid-level accumulation and back ET. The UV emission ratio rose from 1.61% to 38.81%, with Gd³⁺’s long lifetimes reserving UV photons. Room-temperature UV random lasing at 311 nm, 275 nm, and 253 nm was achieved in GC-39Al under 980 nm ns pulses, with a low threshold (15 mJ·cm⁻²) and narrow linewidths (0.14–0.25 nm), outperforming nanocrystal systems. This thesis establishes a framework for efficient NIR-to-UV upconversion via MD-guided Ln³⁺ enrichment and pulse-compression kinetics, paving the way for compact UV lasers with applications in medical and industrial fields. | en_US |
| dcterms.abstract | Carbon quantum dots (CQDs) are promising solution-processable nanomaterials for next-generation photonic devices due to their tunable emission from ultraviolet to near-infrared, enabled by surface functionalization and environmental sensitivity. However, their application in high-coherence lasers is limited by scattering losses and lower gain coefficients compared to colloidal quantum dots. This thesis introduces a novel framework for achieving tunable-wavelength broadband liquid-state CQD lasers, leveraging concentration-dependent Förster resonance energy transfer (FRET) and a gain-guided waveguide in a cuvette-based cavity, spanning 641–710 nm. O-phenylenediamine-based tunable CQDs (TCQDs) were synthesized via a solvothermal method, exhibiting uniform 4 nm spherical morphology with a 0.21 nm lattice spacing. Concentration-dependent photoluminescence (PL) and absorption spectra (2⁻⁵ to 1 g·L⁻¹ in DMF) revealed FRET-induced redshifted emission (585–700 nm), deconvoluted into stable luminescent centers at ~600, ~630, and ~700 nm. Time-resolved PL confirmed reduced fluorescence lifetimes (e.g., 3.05 to 1.74 ns at 700 nm) with increasing concentration, indicating efficient photon redistribution to lower energy states. This enabled tunable random lasing with thresholds rising from 1.91 to 27.85 mJ·cm⁻² as concentration decreased, with Fourier transform analysis showing cavity lengths below 100 μm. A gain-guided waveguide was engineered by matching the solvent's refractive index (e.g., DMF, n=1.450) to the quartz cuvette, confining emission in a 200×500 μm channel. An external aluminum mirror facilitated optical feedback, enabling transitions from random lasing to Fabry–Pérot cavity multimode and single-mode lasing. Mode spacing increased from 0.71 to 4.66 nm as mirror spacing decreased from 485.23 to 38.04 μm, fitting the Fabry–Pérot model. The optical gain (5.85–50.17 cm⁻¹) exceeded cavity loss (4.773 cm⁻¹), supporting lasing emission. Refractive index mismatch in solvents like ethanol (n = 1.38) prevented lasing, highlighting index matching's role in coherence. Transient absorption spectroscopy revealed carrier dynamics, with ground-state bleaching at 515 nm and excited-state absorption at 600 nm within 1.09 – 6.14 ps, confirming rapid energy redistribution critical for population inversion. This work establishes a versatile platform for spectrally tunable liquid-state lasers, integrating FRET-optimized CQDs, index-matched cavities, and external feedback for modal control. The approach, applicable to other colloidal quantum dots, advances compact, low-cost photonic devices for integrated systems, with future improvements targeting precise cavity geometries for broader single-mode lasing. | en_US |
| dcterms.extent | xlvii, 118 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2026 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.accessRights | open access | en_US |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/14477