Second-harmonic generation efficiency in a waveguide-coupled photonic nanocavity

김흥준 성균관대학교 일반대학원 2016 국내석사

A photonic nanocavity with a small modal volume (V≈(λ/n)3) and high quality factor (Q≈103~107) can localize the intense light in tiny region. In the photonic nanocavity, because the strong interaction between light and matter of the cavity occurs, it is expected that nonlinear optical phenomena is enhanced. Second-harmonic generation (SHG), representative one of the nonlinear optical phenomena, has been investigated intensively in photonic crystal and plasmonic nanocavities. However, previous results of SHG in photonic nanocavities have remained as low SHG efficiency of 10-9~10-3%. This is due to no consideration of structure for efficient input coupling or no design of highly efficient conversion efficiency. In this paper, we theoretically investigate SHG efficiency in a waveguide-coupled photonic nanocavity because a waveguide is very useful for efficient input coupling and integration with other optical components. It is shown that there exists a maximal SHG efficiency at a critical input power and both the maximal efficiency and the critical power are determined mainly by two parameters such as Q factors and mode overlapping between fundamental and SHG modes. If the parameters are optimized, it can be obtained by maximal SHG efficiency of 50% in a waveguide-coupled cavity. Also, we propose a method of introducing a reflector at the waveguide exit for enhancement of the SHG efficiency. It is shown that the 100% efficiency can be achieved by adjusting the phase between the cavity and the reflector. This method is very useful for design of efficient nonlinear devices.

Second-harmonic generation efficiency in a waveguide-coupled photonic nanocavity

Kim, Heungjoon Sungkyunkwan university 2016 국내석사

나노임프린트 공정 기반 구조색 컬러필터 및 컬러필터-광전지 이중기능 소자 연구

장지윤 아주대학교 2017 국내석사

본 연구에서는 나노 구조 기반의 구조색 컬러필터와 컬러필터-광전지 이중기능 컬러필터를 구현하였다. 유기 염료나 화학적 안료를 기반의 기존 컬러필터 시스템은 지속적인 UV 조사, 고온, 습기에 민감하고 내구성이 약하며 확장성 측면에서 한계가 있다. 본 연구의 나노 임프린트 공정 기반 구조색 컬러필터는 이러한 기존 컬러필터의 단점들을 보완하여 총 세 가지 형태의 컬러필터를 제작 했다. 첫 번째는 TE 편광 (입사광의 전기장이 subwavelength 그레이팅 방향과 평행)에 대해 입사각 ±70° 까지 각도 의존성이 없는 CMY 반사형 컬러필터고 두 번째는 TE 편광에 대해 입사각 ±60°까지 각도 의존성이 없고 곡률 반경 10mm까지 일정한 광학 퍼포먼스를 보이는 플렉서블 RGB 투과형 컬러필터다. 마지막으로 세 번째는 컬러 생성 시 소비되는 빛 에너지를 전기 에너지로 전환 가능한 RGB 반사 컬러필터-광전지 이중기능 컬러필터다. RGB 각각의 컬러필터는 10.12%, 8.17%, 7.59% 의 광변환효율 (power conversion efficiency, PCE)을 갖는다. 본 연구의 기술들은 LCD 기술들, LEDs, 전자 종이 기술, 자가 전력 생산 디바이스, 플렉서블 전자기기, 에너지 절약 반사 디스플레이, BIPV 시스템과 같은 폭 넓은 분야에 적용 가능하다. In this work, angle invariant structual color filters employing strong resonance and plasmonic color filter-integrated perovskite solar cells based on localized surface plasmon resonance (LSPR) are demonstrated in ultrathin subwavelength grating. The previous color filtering systems based on organic dyes or chemical pigments are vulnerable to high temperature and moisture and have a practical limitation such as poor durabililty, complex fabrication process. However, three types of the proposed structural color filters can improve these weak points and are fabricated one-step simple nanoimprint lithography (NIL). First, the reflection type color filter is based on effective medium theory , resulting in angle independent with an incident angle up to ±70° under the TE-polarization (electric field of incident light is parallel to the direction of the subwavelength gratings). Furthermore, the proposed structual color filters are easily tuned by controlling the duty cycle of the nanostructured semiconductor grating fixed amorphous silicon (a-Si) semiconductor nanograting thickness. Secondly, the trasmission type color filter has a same concept to reflection type color filter. Also, it is observed that the structural color filters fabricated on a polyethylene terephthalate (PET) substrate has unchanged optical performance to 10mm bending radius condition. Lastly, high performance perovskite solar cells are integrated with ultrathin subwavelength plasmonic color filter exploiting LSPR and generate vivid color scheme for various practical application. 10.12%, 8.17%, 7.59% power conversion efficiency (PCE) are achieved in the individual RGB color filters, respectively. This technique presented in the work could be applied to a large number of applications, such as LCD technologies, LEDs, e-paper technologies, power-generating devices, flexible electronics, energy saving reflective displays and Building Integrated Photo Voltaic (BIPV) systems.

Numerical study of surface plasmon polaritons scattering at a planar metal-dielectric interface by an embedded dielectric nanocube

이서준 서울대학교 대학원 2017 국내석사

Surface plasmon polaritons (SPPs) are localized electromagnetic waves propagating along a planar metal−dielectric interface. SPPs have drawn much attention in a variety of applications, such as light-emitting diodes, solar cells, Raman scattering, chemical or biological sensors, and integrated plasmonic circuits since they are localized electromagnetic waves on a metallic surface with higher local field density. As they have been applied in many applications, the in- and out-coupling of SPPs become important to efficiently excite SPP modes (in-coupling of SPPs), convert SPP modes into far-field radiating modes, electromagnetic waves propagating away from the metal surface to which SPPs are confined (out-coupling of SPPs), or manipulate SPPs (reflection or transmission). Particularly, efficient scattering of SPPs into the far-field radiating modes or reflected or transmitted SPPs is demanded in areas such as thin-film spectroscopy, integrated plasmonic devices, or organic light-emitting diodes (OLEDs). In this numerical study, it is investigated that how SPPs at a planar metal−dielectric interface are scattered by a dielectric nanocube embedded in the metal layer. The scattering properties of the embedded nanocube in terms of cross sections of scattering and absorption, scattering patterns, reflection, and transmission are numerically analyzed employing three-dimensional finite element method based simulations. It is confirmed that this embedded nanocube structural system is capable of wavelength-selective SPPs scattering dependent on the size of the nanocube due to the plasmonic resonant modes that are excited in the nanocube. Moreover, the correlation between the scattering properties of the embedded nanocube and the characteristics of the plasmonic resonant modes found in the embedded nanocube is discussed, showing that a specific plasmonic mode, which similarly appears in each of different-sized nanocubes, is responsible for strong scattering of SPPs. Along with the strong outcoupling of SPPs, the strong reflection also occurs when resonant modes of the embedded nanocube are excited, while the transmission decreases. In addition, the scattering patterns of the scattered waves out-coupled by the embedded nanocube are also discussed. With further development, this study would contribute to efficient scattering of SPPs at a planar interface, which can increase the performance of integrated plasmonic devices and, especially, the efficiency of OLEDs by outcoupling of SPPs into the far field radiating modes by collection of embedded dielectric nanocubes with an appropriately chosen size distribution.

Efficient second-harmonic generation in ultrahigh-Q SiC photonic crystal nanocavities

Kim, Heungjoon Sungkyunkwan university 2021 국내박사

Nonlinear wavelength conversion occurred by light-matter interaction is important for a range of applications, such as visible laser light sources, narrowband single photon sources, and long-distance quantum information networks. Bulk nonlinear optical materials such as BBO and KTP are used for the wavelength conversion. However, since the optical interaction per length between light and the materials is too weak, it is difficult to miniaturize and integrate the wavelength-conversion devices with other optical components. For increase of the optical interaction and miniaturization of the devices, photonic nanostructures such as metasurfaces, ring resonators, photonic crystal waveguides, and nanocavities have been intensively used because the localization in the nanostructures can generate the strong nonlinear light-matter interaction. In particular, photonic crystal (PC) nanocavities have a great potential for realization of highly efficiency wavelength-conversion devices thanks to the wavelength-scale mode volumes and high quality (Q) factors of the cavities. Until now, second-harmonic generation (SHG), which is a representative nonlinear wavelength conversion in the PC nanocavities have been studied. However, the normalized and absolute SHG efficiencies have remained at low levels of 102%/W and 10-3%/W, respectively. This is due to the lack of a general design strategy to obtain high efficiency in a photonic nanocavity with an input waveguide and immature nanofabrication of high-Q PC nanocavities. In this dissertation, we study the SHG in a general photonic nanostructure: a waveguide-side-coupled resonator. First, we theoretically analyze the influence of the various parameters such as input power, Q factors, and nonlinear conversion coefficient between fundamental and SHG modes of the resonator on the SHG efficiency. We show that high Q factors and large nonlinear conversion coefficient are important to enhance the SHG efficiency. On the basis of the analysis of the SHG efficiency, we design a PC nanocavity. We calculate the Q factors and nonlinear conversion coefficient of the cavities and optimize the structural parameters for achievement of high SHG efficiency. Next, to realize highly efficient SHG in the nanocavity, we develop nanofabrication of the silicon carbide (SiC)-based PCs. SiC is one of the most promising wide-bandgap semiconductors for efficient SHG because of its excellent properties of large nonlinear optical response, ignorable linear and nonlinear absorption in wide wavelength range. By optimizing the nanofabrication process, we fabricate the SiC PC nanocavity with ultrahigh Q factor of 8.60×105, which is an order of magnitude higher than previous value reported in wide-bandgap semiconductor-based photonic nanocavities. Then, we investigate SHG characteristics. It is shown that normalized SHG efficiency of 7700%/W is obtained in the nanocavity, which is 19 times higher than previous highest efficiency reported in photonic nanocavities. Moreover, the dependency of the normalized efficiency on Q factor of the nanocavity is experimentally and theoretically shown. Lastly, we demonstrate the enhancement of absolute SHG efficiency in a waveguide-coupled nanocavity by introducing a reflector at the edge of the waveguide. We show experimentally the absolute SHG efficiency as high as 3.6×10-1%, which is 60 times as previous results. 빛과 물질 간의 상호 작용에 의해 발생하는 비선형 파장 변환은 가시광 레이저, 좁은 대역의 단일 광자 광원, 그리고 장거리 양자 정보 네트워크 등 여러가지 응용 분야에서 매우 중요한 역할을 한다. 파장 변환을 달성하기 위해, BBO와 KTP 같은 벌크 비선형 광학 물질이 주로 사용되고 있다. 하지만, 벌크 물질에서는 단위 길이 당 빛과 물질 간의 상호 작용이 약하기 때문에, 비선형 파장 변환 소자의 소형화와 집적화가 다소 어렵다. 소자의 소형화와 더불어 비선형 파장 변환의 향상을 위해, 메타표면, 링 공진기, 광자결정 도파로, 그리고 나노공진기와 같이 빛의 국재화를 통해 빛과 물질 간의 상호 작용을 크게 증대할 수 있는 광 나노구조체가 사용되고 있다. 특히, 파장 수준의 매우 작은 모드 볼륨과 높은 Q값을 갖는 광자결정 나노공진기가 고효율 파장 변환의 달성을 위한 구조체로서 주목받고 있다. 지금까지, 광자결정 나노공진기에서 비선형 파장 변환 중 대표적인 제2조화파 생성에 대한 연구가 많이 이루어졌다. 하지만, 제2조화파 생성의 규격화 및 절대 효율이 아직까지 102%/W 그리고 10-3% 수준에 머물러있다. 이러한 이유는 높은 파장 변환 효율을 얻기 위한 입력 도파로 결합형 나노공진기의 일반적인 설계 규칙이 없고, 높은 Q값을 갖는 광자결정 공진기의 제작이 아직까지 미성숙하기 때문이다. 본 논문에서는 도파로 측면 결합형 공진기 시스템에서의 제2조화파 생성에 대해서 연구한다. 먼저, 공진기의 입력 파워, Q값, 그리고 기본 모드와 제2조화파 모드 사이의 비선형 변환 계수 등 여러가지 변수가 제2조화파 생성 효율에 미치는 영향을 이론적으로 분석한다. 높은 Q값과 큰 비선형 변환 계수가 공진기에서의 제2조화파 생성 효율을 향상시키는데 중요함을 보인다. 이러한 제2조화파 생성 효율의 분석을 기반으로, 광자결정 나노공진기를 설계한다. 광자결정 나노공진기의 Q값과 비선형 변환 계수를 계산하고, 고효율 제2조화파 생성을 위해 공진기의 구조 변수를 최적화한다. 다음으로, 공진기에서 고효율 제2조화파 생성을 구현하기 위해, 실리콘카바이드 기반 광자결정의 나노 제작 기술을 개선한다. 실리콘카바이드는 넓은 전자 밴드갭 반도체로서, 큰 비선형 광학 계수와 광대역 저손실의 우수한 성질을 가지기 때문에, 고효율 제2조화파 생성에 매우 유용하다. 나노 제작 기술을 최적화하여, 고품질의 실리콘카바이드 광자결정 나노공진기를 제작하고, 기존의 넓은 전자 밴드갭 반도체 기반의 나노공진기에서 보고된 값보다 10배 이상 높은 8.60×105의 Q값을 달성한다. 이러한 높은 Q값의 실리콘카바이드 광자결정 나노공진기의 제2조화파 특성을 조사한다. 기존의 광 나노공진기에서 보고된 결과보다 19배 이상 큰 7700%/W의 규격화된 제2조화파 생성 효율이 달성됨을 보인다. 또한, 공진기의 Q값에 대한 규격화된 제2조화파 생성 효율의 의존성을 실험적, 그리고 이론적으로 보인다. 마지막으로, 도파로 가장자리에 반사기를 도입함으로써, 도파로 결합형 나노공진기의 제2조화파 생성 절대 효율이 향상됨을 입증한다. 기존의 결과보다 60배 이상 큰 3.6×10-1%의 절대 효율이 달성됨을 실험적으로 보인다.