Optical Thin Films with Very Low Refractive Index and Their Application in Photonics Devices
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Optical Thin Films with Very Low Refractive Index and Their Application in Photonics Devices J.-Q. Xi1,2, Jong Kyu Kim1,3, Dexian Ye2, Jasbir S. Juneja4, T.-M. Lu2, Shawn-Yu Lin1,2 and E. F. Schubert1,2,3, 1 Future Chips Constellation 2 Department of Physics, Applied Physics, & Astronomy 3 Department of Electrical, Computer, & Systems Engineering 4 Department of Chemical & Biological Engineering Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA ABSTRACT The refractive index contrast in dielectric multilayer structures, optical resonators and photonic crystals is an important figure of merit, which creates a strong demand for high quality thin films with a very low refractive index. SiO2 nano-rod layers with low refractive indices n = 1.08, the lowest ever reported in thin-film materials, is grown by oblique-angle e-beam deposition of SiO2 with vapor incident angle θ = 85°. Scanning electron micrographs reveal a highly porous columnar structure of the low-refractive-index (low-n) film. The gap between the SiO2 nano-rods is ≤ 50 nm, i.e. much smaller than the wavelength of visible light, and thus sufficiently small to make scattering very small. Optical micrographs of the low-n film deposited on a Si substrate reveal a uniform specular film with no apparent scattering. The unprecedented low index of the SiO2 nano-rod layer is confirmed by both ellipsometry measurements and thin film interference measurements. A single-pair distributed Bragg reflector (DBR) employing the SiO2 nano-rod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-n films for applications in photonic structures and devices. INTRODUCTION In distributed Bragg reflectors (DBRs) [1], the refractive index contrast, which is the difference in refractive index between the two constituting materials, is directly related to the reflectivity, spectral width of stop band, and penetration depth. In optical micro-resonators [2], the effective cavity length, and hence the enhancement of spontaneous emission, is directly dependent on the index contrast. In photonic crystals [3], the photonic bandgap width is directly related to the index contrast. In semiconductor optoelectronics, dielectric material with very low refractive index is the key component for the optical parts in the devices. [4,5] This motivates the development of new “air-like” optical materials with a refractive index close to 1.0. Although multilayer structures with air-gaps have been demonstrated [1], the fabrication process requires under-etching, and hence is slow and costly. Moreover, air gaps completely lack structural stability, making them unsuitable for the majority of applications. MgF2, CaF2, and SiO2 are materials with refractive indices among the lowest available for conventional, dense optical coatings. However, their refractive indices, nMgF2 = 1.39, nCaF2 = 1.44, nSiO2 = 1.46, are much higher than the air’s index, 1.0. Nanoporous SiO2 thin film materials made from sol-gel processes [6,7] have low
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