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Study of carbon-nano material-based optical device and its application to fiber pulse laser system

  • 민태열 (TAEYOL MIN, 아주대학교 일반대학원)

초록/요약

Carbon nanomaterials—notably graphene and carbon nanotubes—exhibit broadband spectral response covering from near infrared to teraherz, pronounced third-order optical nonlinearities, and field-effect-based optical tunability, making them ideal platforms for advanced photonic and nonlinear-optic devices. Especially, carbon-nanotube (CNT) network films form an optically homogeneous, randomly interconnected network structures of one-dimensional CNTs, whose thickness can be precisely controlled from tens to hundreds of nanometers by adjusting the concentration of CNT dispersions during vacuum filtration. Despite their random percolation, these networks faithfully retain both semiconducting and metallic CNT characteristics, exhibiting strong, tunable optical transitions across the near-infrared due to the ensemble of various chiralities. In particular, the first excitonic (S₁₁) absorption band can be significantly modulated under low-voltage electrolytic gating, making CNT films an ideal platform for broadband, tunable electro-optic devices. Leveraging these properties, this dissertation advances two major works: a tunable flat diffractive optic device and a polymer-free CNT saturable absorber enabling dual-axis control of Q-switched fiber-laser pulses. In Chapter 2, we demonstrate a diffractive optical element fabricated in a CNT network film as a type of Fresnel zone plate (FZP). Porous CNT films—composed of arc-made SWCNTs dispersed, vacuum-filtered, and transferred to fused-silica substrates—were prepared with thicknesses from 50 nm to 450 nm by adjusting dispersion concentration. These optically homogeneous, randomly connected network films preserve both semiconducting and metallic characteristics and exhibit uniform transmission for all polarizations. When integrated into an ion–gel gating device, CNT films show thickness-enhanced modulation of the first excitonic (S₁₁) absorption at 1.55 µm under ±1.8 V ion-gel gating, transmittance changes increase from 8.5 % (50 nm) to 21.3 % (200 nm) and 28.9 % (450 nm). Considering a target wavelength of 1550 nm, we patterned bridged-Fresnel zone plates (𝑁𝑁𝑁𝑁 = 0.60) composed of 21 Fresnel zones electronically interconnected in the CNT network film via direct laser writing without substrate damage. Zero-bias measurements yield a first-order diffraction efficiency of 5.6% (versus 7.1% predicted by simulation), with diffraction-limited focal spots exhibiting a lateral FWHM of 0.95 λ (simulated 0.90 λ) and an axial depth-of-focus of 7.10 λ (simulated 5.80 λ), in good agreement with theoretical expectations. The bridged‐FZP’s electrical tunability originates from ion– gel gating–induced modulation of the absorption contrast within the intact, even‐ numbered Fresnel zones. By sweeping the gate voltage from –1.8 V to +1.8 V, the carrier population in the percolating CNT network is modulated—holes are depleted under negative bias and accumulated under positive bias—thereby tuning the S₁₁ absorption at 1.55 µm. At +1.8 V, hole reduction within the intact Fresnel zones strengthens the absorption contrast between alternating rings, producing a pronounced first-order focus with subwavelength lateral confinement and maximal intensity. Conversely, at –1.8 V, electron injection into the even-numbered zones reduces CNT absorption by approximately 29 %, diminishing the binary contrast and suppressing first-order focus intensity by 72 %. Furthermore, the second‐order focus exhibits a 40 % modulation depth, following the same trend. Such high‐depth, reversible tuning of multiple diffraction orders, without mechanical or phase‐change elements, highlights the unique dynamic beam‐shaping capability of porous CNT network films. In Chapter 3, we demonstrate an all-fiber, polymer-free carbon-nanotube (CNT) saturable absorber (SA) on a side-polished fiber (SPF) platform and its application to passively Q-switched erbium-doped fiber lasers with unprecedented dual-axis control. Freestanding CNT network films were fabricated by vacuum filtration of optimized dispersions and transferred directly onto the SPF, forming a clean, percolating nanotube network with smooth, monotonic broadband absorption across the near-infrared band. Nonlinear transmission measurements under 1550 nm yield saturable losses of 23.7 % for transverse-electric (TE) polarization and 8.2 % for transverse-magnetic (TM) polarization, indicating sufficient modulation depth to support passive Q-switching for all intracavity polarization states. When incorporated into an all-fiber ring cavity comprising a 976 nm pump diode, erbium-doped fiber gain section, optical isolator, polarization controller and 10/90 output coupler—the CNT-SA reliably initiates passive Q-switching. With the polarization controller fixed at an arbitrary position, increasing pump power from 44.0 mW to 93.7 mW raises the small-signal gain and accelerates energy extraction. The Q-switched repetition rate therefore climbs from 7.6 kHz to 16.0 kHz, the pulse full-width at half-maximum contracts from 16.2 µs to 7.6 µs, and the single-pulse energy grows from 63.4 nJ to 215.6 nJ. Conversely, at a constant pump power of 63.6 mW, continuous rotation of the intra-cavity polarization from TE to TM reduces the effective modulation depth. This polarization-axis tuning reverses the conventional behavior: pulse durations broaden from 9.7 µs to 15.9 µs, repetition rates increase from 2.0 kHz to 12.0 kHz, and pulse energies decrease from 287 nJ to 68 nJ—all without disrupting the Q-switching operation. Numerical integration of the coupled power–gain–loss rate equations, using a fourth- order Runge–Kutta solver with absorber parameters linearly interpolated between TE and TM polarizations, faithfully reproduces the polarization-axis behavior: as the effective modulation depth decreases from 23.7 % toward 8.2 %, the simulated intracavity power builds up more slowly to a lower peak, leading to broader Q- switched envelopes and reduced pulse energy, while the reduced total loss accelerates gain recovery between pulses. Finally, by adjusting pump power and polarization simultaneously as dual-axes control, we maintain one pulse parameter constant while varying the other—such as compressing the pulse width from 14.4 µs to 8.2 µs at a fixed 7.0 kHz or tuning repetition rate from 2.6 kHz to 15.2 kHz at a fixed 9.7 µs—without ever interrupting the Q-switching process. This first all-fiber implementation of agile, pulse shaping via dual-axis control offers a versatile, alignment-free platform for real-time pulse profiling.

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목차

Chapter 1. Introduction 1
Chapter 2. Tunable diffractive optical element (DOE) based on CNT network film 4
2.1 Diffractive optical elements employing two-dimensional materials 4
2.2 Tunable diffractive optical elements 5
2.3 Preparation of optically homogeneous CNT network film 7
2.4 Thickness-enhanced gate tunability in porous CNT network films 8
2.5 Direct laser writing system using femtosecond pulses 10
2.6 Polarization-dependent ablation in CNT network films 11
2.7 Gate-tunable focusing in FZPs based on CNT network film 12
2.8 Numerical and Experimental Validation of Focusing Performance. 14
2.9 Gate-controlled multi-order diffraction and high-depth focusing modulation in CNT–FZPs 16
Chapter 3. Q-switched fiber lasers based on CNT saturable absorber self-adapting all intra-cavity polarizations 20
3.1 Low-dimensional materials integrated in various optical fiber platforms 20
3.2 All-fiber saturable absorber based on low-dimensional materials for pulsed fiber laser 21
3.3 CNT network films as saturable absorbers: key advantages. 23
3.4 Toward polymer-free carbon nanotube saturable absorbers 24
3.5 Polymer-free CNT network film: fabrication and optical characterization 26
3.6 Polymer‐free CNT network film onto side‐polished fiber: exceptional high-modulation for all-intra cavity polarizations 28
3.7 All-fiber Erbium-doped Q-switched laser employing polymer-free CNT saturable absorber 31
3.8 Tunable Q-switched output via pump power adjustment. 31
3.9 Continuously tunable Q‐switching operation by intra-cavity polarization adjustments 33
3.10 Opposite trends in Q‐switched pulse dynamics under pump and polarization tuning 35
3.11 Numerical Simulation of Dual‐Axis Q‐Switching Dynamics 37
3.12 Versatile dual-axes pulse shaping via simultaneous pump and polarization control 40
Conclusion 42
References 44
국문요약 51

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