Hybrid photonic crystal lasers composed of passive structural backbone and active colloidal quantum dots
능동형의 콜로이드 양자점과 수동형의 뼈대 구조로 이루어진 하이브리드 광자결정 레이저

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자연과학대학 물리·천문학부(물리학전공)
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서울대학교 대학원
학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2018. 8. 전헌수.
A photonic crystal (PhC) is a periodic structure in which two or more materials having different dielectric constants are arranged on a wavelength scale of an electromagnetic wave. The structure affects the motion of photons in the similar manner that ionic lattices affects the motion of electrons in the periodic potential. The electromagnetic waves passing through the PhC have an extraordinary photonic dispersion relation that cannot be found when the waves passing through a homogeneous medium. Photonic band-gap and group-velocity anomalies in dispersion are representative optical properties in PhC. The photonic band-gap is a frequency region where the states of the photon cannot exist. Also, photonic band-edge as an anomaly is a region in which the group velocity of photonic modes approaches zero. Two notions about photonic states can be used for confining the electromagnetic waves in strongly localized cavity structures or spatially extended standing-wave resonance forms and interaction between optical gain materials and photons can be remarkably increased.

PhC have great potential in that they can be fabricated in small areas and have low power consumption in generating coherent light source. Also, the PhC can be utilized for an efficient light source having designed modal properties for photonic integrated circuits (PICs). Hybrid PhC lasers, coherent light sources composed of a passive optical structure and external active gain materials, have a flexibility in selecting source wavelengths and waveguiding properties so that can be prominent coherent sources to realizing high density PICs. Especially, colloidal quantum dots (CQDs) are one of promising gain materials in future PICs picture. CQDs are efficiently light-emitting building blocks of nano-sizes synthesized by low-cost, solution-processed chemistry. Also, spectrally narrow emission wavelengths of CQDs are easily tunable by controlling the size of individual CQDs without deteriorating quantum efficiency.

In this thesis, in the first research topic, efficient platform for on-chip integration of CQD PhC laser and passive components is suggested. Silicon nitride is interesting material, which can be treated by CMOS-compatible process and also has relatively high refractive index and broad optical transparency from visible to near-infrared wavelength range. With combination of silicon nitride and CQDs gain materials, hybrid PhC lasers and passive devices are efficiently integrated in a way where silicon nitride waveguiding layer is shared by various components. A conceptual PIC is experimentally demonstrated, in which a PhC band-edge laser, a slab waveguide, and output couplers are all integrated on a single chip. Under excitation condition, coherent single-mode laser is emitted with well-defined in-plane emission directions, and subsequently coupled into the passive waveguide layer with a guaranteed high coupling efficiency. This configuration for active and passive photonic devices on a chip could be an alternative picture of conventional silicon-based PIC in near future.

In the second research topic, continuously tunable distributed feedback (DFB) laser device composed of CQD film on chirped grating is introduced. To make hybrid type tunable laser source composed of CQDs and fused silica, high-throughput laser interference lithography (LIL) method is employed instead of writing the entire nano-patterns by conventional electron beam lithography. To make efficiently line gratings chirped, Lloyd’s LIL configuration is modified by introducing cylindrical mirror parts. Also, CQD film is transferred onto wet-transfer method, which are intended for flat and dense CQD film to be positioned on the arbitrary substrate. Single-mode, surface-emitting DFB laser structures are numerically designed by finite-domain time-domain (FDTD) method. Also, laser wavelengths are observed to be progressively varied as optical excitation positions are translated in photoluminescence experiments. Continuously tunable coherent light source could be useful itself in a variety of applications. Moreover, the results demonstrate modified LIL method can produce high quality chirped gratings at less effort, which could be helpful for other sophisticated photonic devices.

In the appendant topic, bio-compatible silk DFB laser with physically transient characteristics are demonstrated. It is a kind of bio-compatible version of hybrid PhC laser composed of silk, fused silica, dyes. Silk fibroin is natural protein extracted from Bombyx mori caterpillar and draw public attention for its self-assembling property at optical wavelengths scale and optical transparency. The silk bio-ink, bio-compatible silk materials combined with sodium fluorescein dye commonly used for diagnostics in ophthalmology and optometry, is spin-casted on the robust fused silica grating substrate for DFB lasing. The organic device has limited lifetime due to the degradability of dye but silk active layer can be easily removed by dipping in water, and by re-coating the bio-ink new device can be obtained without difficulties. In addition, chemo-sensing capability for detecting hydrochloride vapor is also investigated.

In conclusion, in this these, I suggest hybrid type PhC laser of passive structural backbone and external gain material, especially CQDs. Efficient on-chip integrated coherent laser sources with passive structures are demonstrated, and tunable laser sources from efficient fabrication method are presented. These results are expected to contribute to the understanding of various optical devices and PIC, as well as to the efficient implementation.
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College of Natural Sciences (자연과학대학)Dept. of Physics and Astronomy (물리·천문학부)Physics (물리학전공)Theses (Ph.D. / Sc.D._물리학전공)
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