Low loss hollow-core waveguide on a silicon substrate

Abstract

Optical-fi ber-based, hollow-core waveguides (HCWs) have opened up many new applications in laser surgery, gas sensors, and non-linear optics. Chip-scale HCWs are desirable because they are compact, light-weight and can be integrated with other devices into systems-on-a-chip. However, their progress has been hindered by the lack of a low loss waveguide architecture. Here, a completely new waveguiding concept is demonstrated using two planar, parallel, silicon-on-insulator wafers with high-contrast subwavelength gratings to refl ect light in-between. We report a record low optical loss of 0.37 dB/cm for a 9-µ m waveguide, mode-matched to a single mode fi ber. Two-dimensional light confi nement is experimentally realized without sidewalls in the HCWs, which is promising for ultrafast sensing response with nearly instantaneous fl ow of gases or fl uids. This unique waveguide geometry establishes an entirely new scheme for low-cost chip-scale sensor arrays and lab-on-a-chip applications.Keywords: hollow-core waveguide; high-contrast subwavelength grating; gas-sensing; silicon photonics.Conventional light guiding is achieved in a geometry where a high-refractive-index core is surrounded by a low-refractiveindex cladding. In the past decade, the opposite schemeguiding light through a low-index core surrounded by high-index cladding layers has emerged as a new tool for applications. In particular, hollow-core optical waveguides/fi bers are desirable for gas sensors and gas-based non-linear optics because of the increased lengths for light-matter interaction [1,2] , and for laser surgery to guide light in mid-to far-infrared wavelength regimes that lack low-absorption materials [3,4] . Chip-scale hollow-core waveguides (HCWs) are desirable because they enable cost-effective manufacturing of on-chip systems with the potential to monolithically integrate light sources, detectors and electronics. Chip-scale HCWs have been reported using metal [5] , distributed Bragg refl ectors [6,7] and anti-resonant refl ection layers [8,9] as the guiding refl ectors. However, their use is limited due to large optical losses because of insuffi cient refl ection.A hollow-core waveguide is best understood by the ray optics model, with an optical beam guided by zig-zag refl ections from the guiding walls [6,7,10] . The propagation loss is strongly dependent on the refl ectivity of the walls [6,7,10] due to the large number of refl ections for a given length (the number of refl ections is L λ /2 d 2 , where L is the length of the waveguide, d is the waveguide core height and λ is the wavelength of light used). Low losses can be obtained for HCW with core size in the tens of µ m [7] . However, a core of this size does not lend itself to a low bending loss or effi cient fi ber coupling. High-contrast subwavelength gratings (HCGs) have been found to offer very high refl ection for surface-normal incident light [11 -14] . Recently, we reported numerical simulation results of a one-dimensional (1D) waveguide guided by two parallel layers...

High-contrast gratings for integrated optoelectronics

“…In other words, lateral confinement is realized without any physical sidewalls! We experimentally demonstrated that its propagation loss is as low as 0.37 dB/cm [46], the lowest for HCW mode-matched to a single mode fiber. The second type of HCG HCW involves placing DBR on the side for lateral confinement.…”

“…One of the applications is hollow-core waveguide, where high reflection mirrors are required for light confinement. By placing two HCGs in parallel [45,46], or four HCGs in a cage-like scheme, hollow-core waveguides can be made to provide two-dimensional light confinement [45,47]. On this hollow-core waveguide platform, novel functionalities such as low-loss slow light [47,48], ultra-compact optical coupler, splitter [49] and switch [50] can be realized with special HCG designs.…”

Recent advances in high-contrast metastructures, metasurfaces, and photonic crystals

“…For practical application, lateral confinement is critical. Previously, we demonstrated a lateral confinement scheme by varying the HCG dimension laterally such that the effective index of the core is higher than that of the cladding [19]. This is a novel use of the effective index method in HCW.…”

“…Many extraordinary features were obtained with such simple structures. Notable accomplishments include HCG widely-wavelength-tunable VCSELs [11][12][13][14][15][16], HCG high-Q resonators [17,18], HCG hollow-core waveguides [19,20], HCG surface-normal couplers [21], and HCG high-NA planar lenses [22,23]. More recent advances include HCG phased arrays [24,25], surface-normal second harmonic emission from HCGs [26], flexible HCG metasurfaces [27], HCG VCSELs on silicon [28], 2D HCM tunable VCSELs [29], and metasurface holograms [30,31].…”

Mesoscale cavities in hollow-core waveguides for quantum optics with atomic ensembles

“…Since then ARROWs have found applications in lasers [30], single-molecule detection [31], and have been used to demonstrate electromagnetically induced transparency and slow light on a chip [6]. At the same time, an alternative approach to on-chip hollow-core waveguides, based on hollow core waveguides utilizing high-contrast subwavelength gratings (HCSWG), has been recently demonstrated by Yang et al [32]. While the reported propagation losses in these structures have been as low as 0.37 dB/cm, the waveguides have mode areas around 100 µm 2 for 9 × 11 µm sized cores, which is much larger than what has been achieved in ARROWs [33].…”