Lossy mode resonances in photonic crystal fibers

The interaction effect of the fundamental mode in a special photonic crystal fiber (PCF) with a thin-film absorbing coating deposited on a surface of a fiber cladding on the optical transmission of the PCF is theoretically studied. It is shown that the transmission has a multi-peak spectrum that is determined by the resonance capture of the fundamental PCF mode energy by the coating. In some cases, this capture is explained by a resonance coupling between the fundamental core mode and leaky modes of the coating, or between the fundamental PCF mode and cladding modes located between PCF air channels and the coating. Examples are presented of using this effect to develop fiber-optic sensors of refractive index or pressure, and to sense a nanoscale adsorption layer of ammonia molecules deposited on a coating surface contacting air.

Transformation of guided modes into bound states in the continuum

We study a silicon-based structure composed of parallel rods arranged in a periodic array. The structure supports guided modes which due to the deformation of structure in general become leaky modes. At the same time the modes split at the Brillouin zone surface by a pair with certain symmetry. Here we report on guided modes transformation into symmetry-protected BIC under deforming the structure, because of inherent field distribution governed by the Bragg-related mode hybridization.

Spectral tuning of diamond photonic crystal slabs by deposition of a thin layer with silicon vacancy centers

The controlled extraction of light from diamond optical color centers is essential for their practical prospective applications as single photon sources in quantum communications and as biomedical sensors in biosensing. Photonic crystal (PhC) structures can be employed to enhance the collection efficiency from these centers by directing the extracted light towards the detector. However, PhCs must be fabricated with nanoscale precision, which is extremely challenging to achieve for current materials and nanostructuring technologies. Imperfections inherently lead to spectral mismatch of the extraction (leaky) modes with color center emission lines. Here, we demonstrate a new and simple two-step method for fabricating diamond PhC slabs with leaky modes overlapping the emission line of the silicon vacancy (SiV) centers. In the first step, the PhC structure with leaky modes blue shifted from the SiV emission line is fabricated in a nanocrystalline diamond without SiV centers. A thin layer of SiV-rich diamond is then deposited over the PhC slab so that the spectral position of the PhC leaky modes is adjusted to the emission line of the SiV centers, thereby avoiding the need for nanoscale precision of the structuring method. An intensity enhancement of the zero-phonon line of the SiV centers by a factor of nine is achieved. The color centers in the thin surface layer are beneficial for sensing applications and their properties can also be further controlled by the diamond surface chemistry. The demonstrated PhC tuning method can also be easily adapted to other optical centers and photonic structures of different types in diamond and other materials.

Broadband Reflector Based on Double-Layer Subwavelength Gratings

A broadband reflector with a reflectance up to 97.8% over a 144 nm spectral range from 1476 to 1620 nm was proposed by comprising double-layer subwavelength gratings with one grating layer embedded in SiO2 layer. An analysis of the resonance leaky modes with overlapping gratings showed the physical mechanism of the wide broad high-reflectivity band. The structural parameter tolerance was analyzed and the variation of bandwidth was explained by combining the behaviors of the two guided mode resonances. The guided mode resonance and high refractive index contrast properties led to a good angular tolerance that exhibited an angular insensitivity (~ 11°) at 1550 nm. Overall, the broadband reflector may benefit the monolithic integration of optoelectronic devices.

Transmittance of Tapered Photonic Crystal Fibers With Absorbing Coatings

The interaction effect of the fundamental mode of a photonic crystal fiber (PCF) with a thin-film absorbing coating deposited on a surface of a fiber cladding on the optical transmission of the PCF is studied. It is shown that the transmission has a quasi-periodic spectrum that is determined either by a resonance coupling between the leaky fundamental PCF mode and leaky modes of the coating, or between the leaky fundamental PCF mode and cladding modes localized between PCF air channels and the coating. Examples are presented of using this effect for fiber-optic sensors of refractive index, pressure, and for sensing an adsorption layer of ammonia molecules deposited on a coating surface contacting with air.