Bloch surface waves at the telecommunication wavelength with lithium niobate as the top layer for integrated optics

Photonic Crystal Surface Modes for Trapping and Waveguiding of Ultracold Atoms

The design of a photonic system for the trapping and waveguiding of ultracold atoms far above a dielectric surface is proposed and analyzed. The system consists of an optical rib waveguide deposited on a planar one-dimensional photonic crystal, which sustains two wavelengths of photonic crystal surface modes tuned in the red and blue sides relative to the atomic transition of the neutral atom. The addition of a third blue-tuned wavelength to the system allows the neutral atoms to be stabilized in the lateral dimension above the rib waveguide. Trapping atoms at relatively large distances, more than 600 nm above the dielectric surface, allows to reduce the influence of Casimir-Polder forces in this system. The detailed design methodology and specifications of the photonic system are provided. The presented design can be employed in atomic chips and quantum sensors.

Pulse modulation by Bloch surface wave excitation

Considering dielectric multilayers with N identical bilayers and an additional terminating layer, we address the effect of Bloch surface wave excitation on the temporal characteristics of short optical pulses. When such a resonant excitation occurs within the spectrum of the incident pulse, the reflected pulse splits into leading and trailing parts, the latter having an exponentially decaying tail. The role of the number of bilayers and the level of absorption in the multilayer stack is illustrated.

Near-Zero-Index Slabs on Bloch Surface Wave Platform for Long-Range Directional Couplers and Optical Logic Gates

Near-zero-index materials and structures, with their extraordinary optical behaviors of phase-free propagation resulting in directional radiation, provide a possible approach for directional coupling and optical logic gates in photonic integrated circuits. However, the radiation from the near-zero-index structures is limited to a short range of a few hundreds of nanometers. A Bloch surface wave (BSW), an electromagnetic surface wave that can be excited at the interface between an all-dielectric multilayer and a dielectric medium with a low-loss optical mode, provides a solution to increase the propagation length. In this work, we present a nanostructured near-zero-index slab integrated on the all-dielectric metal-free BSW platform for long-range surface wave radiation. By employing the long-range directional surface-wave radiation, a directional coupler and optical logic gates based on the BSW near-zero-index slabs are realized. The proposed directional couplers achieve long coupling distances (the electric-field magnitude ratio between the input slab and output slab is 0.22 with a 50 μm coupling distance), which is 2 orders of magnitude longer than that of conventional directional couplers based on evanescent wave coupling. By controlling the interference pattern of the BSW between the slabs, the XOR logic gate is experimentally demonstrated with a significant extinction ratio of 27.9 dB at telecommunications wavelengths. The BSW near-zero-index logic gates and the directional coupler with long-range light propagation provide an approach to the development of photonic integrated circuits and metal-free surface wave-based applications.

Efficient Optical Sensing Based on Phase Shift of Waves Supported by a One-Dimensional Photonic Crystal

Interferometric methods of optical sensing based on the phase shift of the Bloch surface waves (BSWs) and guided waves (GWs) supported by a one-dimensional photonic crystal are presented. The photonic crystal, composed of six SiO₂/TiO₂ bilayers with a termination layer of TiO₂, is employed in the Kretschmann configuration. Under resonance condition, an abrupt phase change is revealed, and the corresponding phase shift is measured by interferometric techniques applied in both the spectral and spatial domains. The spectral interferometric technique employing a birefringent quartz crystal is used to obtain interference of projections of p- and s-polarized light waves reflected from the photonic crystal. The phase shifts are retrieved by processing the spectral interferograms recorded for various values of relative humidity (RH) of air, giving the sensitivity to the RH as high as 0.029 rad/%RH and 0.012 rad/%RH for the BSW and GW, respectively. The spatial interferometric technique employs a Wollaston prism and an analyzer to generate an interference pattern, which is processed to retrieve the phase difference, and results are in good agreement with those obtained by sensing the phase shift in the spectral domain. In addition, from the derivative of the spectral phase shifts, the peak positions are obtained, and their changes with the RH give the sensitivities of 0.094 nm/%RH and 0.061 nm/%RH for the BSW and GW, respectively. These experimental results demonstrate an efficient optical sensing with a lot of applications in various research areas.

Sensing based on Bloch surface wave and self-referenced guided mode resonances employing a one-dimensional photonic crystal

Sensing abilities of a one-dimensional photonic crystal (1DPhC) represented by a multilayer dielectric structure are analyzed theoretically and experimentally, using a new wavelength interrogation interference method. The structure comprising a glass substrate and six bilayers of TiO₂/SiO₂ with a termination layer of TiO₂ is employed in both gas sensing based on the Bloch surface wave (BSW) resonance and liquid analyte sensing based on a self-referenced guide-mode resonance (GMR). We model the spectral interference reflectance responses in the Kretschmann configuration with a coupling prism made of BK7 glass and show that a sharp dip with maximum depth associated with the BSW excitation is red-shifted as the refractive index (RI) changes in a range of 1-1.005. Thus, a sensitivity of 1456 nm per RI unit (RIU) and figure of merit (FOM) of 91 RIU^-1 are reached. Similarly, we model the responses for aqueous solutions of ethanol to show that dips of maximum depth are associated with the GMRs, and the highest sensitivity and FOM reached are 751 nm/RIU and 25 RIU^-1, respectively. Moreover, we show that one of the dips is with the smallest shift as the RI changes, and hence it can be used as a reference. The theoretical results are confirmed by the experimental ones when the BSW resonance is used in sensing of humid air with a sensitivity of 0.027 nm/%relative humidity (RH) and FOM of 1.4×10^-3 %RH^-1. Similarly, the GMR is used in sensing of aqueous solutions of ethanol, and the highest sensitivity and FOM reached 682 nm/RIU and 23 RIU^-1, respectively. The reference dip is also resolved and this self-reference makes the measurement more accurate and repeatable, and less sensitive to optomechanical drifts.

Bloch Surface Wave Resonance Based Sensors as an Alternative to Surface Plasmon Resonance Sensors

We report on a highly sensitive measurement of the relative humidity (RH) of moist air using both the surface plasmon resonance (SPR) and Bloch surface wave resonance (BSWR). Both resonances are resolved in the Kretschmann configuration when the wavelength interrogation method is utilized. The SPR is revealed for a multilayer plasmonic structure of SF10/Cr/Au, while the BSWR is resolved for a multilayer dielectric structure (MDS) comprising four bilayers of TiO2/SiO2 with a rough termination layer of TiO2. The SPR effect is manifested by a dip in the reflectance of a p-polarized wave, and a shift of the dip with the change in the RH, or equivalently with the change in the refractive index of moist air is revealed, giving a sensitivity in a range of 0.042-0.072 nm/%RH. The BSWR effect is manifested by a dip in the reflectance of the spectral interference of s- and p-polarized waves, which represents an effective approach in resolving the resonance with maximum depth. For the MDS under study, the BSWRs were resolved within two band gaps, and for moist air we obtained sensitivities of 0.021-0.038 nm/%RH and 0.046-0.065 nm/%RH, respectively. We also revealed that the SPR based RH measurement is with the figure of merit (FOM) up to 4.7 × 10-4 %RH-1, while BSWR based measurements have FOMs as high as 3.0 × 10-3 %RH-1 and 1.1 × 10-3 %RH-1, respectively. The obtained spectral interferometry based results demonstrate that the BSWR based sensor employing the available MDS has a similar sensitivity as the SPR based sensor, but outperforms it in the FOM. BSW based sensors employing dielectrics thus represent an effective alternative with a number of advantages, including better mechanical and chemical stability than metal films used in SPR sensing.

Multimode Interference of Bloch Surface Electromagnetic Waves

Integrated photonics aims at on-chip controlling light in the micro- and nanoscale ranges utilizing the waveguide circuits, which include such basic elements as splitters, multiplexers, and phase shifters. Several photonic platforms, including the well-developed silicon-on-insulator and surface-plasmon polaritons ones, operate well mostly in the IR region. However, operating in the visible region is challenging because of the drawbacks originating from absorption or sophisticated fabrication technology. Recently, a new promising all-dielectric platform based on Bloch surface electromagnetic waves (BSWs) in multilayer structures and functioning in the visible range has emerged finding a lot of applications primarily in sensing. Here, we show the effect of multimode interference (MMI) of BSWs and propose a method for implementing the advanced integrated photonic devices on the BSW platform. We determine the main parameters of MMI effect and demonstrate the operation of Mach-Zehnder interferometers with a predefined phase shift proving the principle of MMI BSW-based photonics in the visible spectrum. Our research will be useful for further developing a versatile toolbox of the BSW platform devices which can be essential in integrated photonics, lab-on-chip, and sensing applications.

Sensing concept based on Bloch surface waves and wavelength interrogation

We report on a new, to the best of our knowledge, sensing concept based on Bloch surface waves (BSWs) and wavelength interrogation that utilizes the interference of $ s $s- and $ p $p-polarized waves from a one-dimensional photonic crystal (1DPhC), represented by a multilayer structure comprising a glass substrate and four bilayers of $ {{\rm TiO}_2}/{{\rm SiO}_2} $TiO₂/SiO₂ with a termination layer of $ {{\rm TiO}_2} $TiO₂. We show that when a standard approach based on measurement of the reflectance of a $ p $p- or $ s $s-polarized wave in the Kretschmann configuration fails to confirm the excitation of the BSW, a new approach is successful. We demonstrate that the BSW excitation shows up as a dip with maximum depth, and resonance thus obtained is comparable in magnitude with resonance commonly exhibited by surface plasmon resonance (SPR). The new sensing concept is verified experimentally for ethanol vapors. The BSW resonances are resolved within two band gaps of the 1DPhC with sensitivities of 3272 nm/RIU and 1403 nm/RIU, and figures of merit of $ 43.7 \;{{\rm RIU}^{ - 1}} $43.7RIU^-1 and $ 173.2 \;{{\rm RIU}^{ - 1}} $173.2RIU^-1, respectively. This research, to the best of the authors' knowledge, is the first demonstration of a new SPR-like response that can be utilized in a wide range of sensing applications.

Polarization-Modulated, Goos⁻Hanchen Shift Sensing for Common Mode Drift Suppression

A polarization-modulation-based Goos–Hanchen (GH) sensing scheme leveraging the polarization-dependence of the Bloch surface wave enhanced GH shift is proposed and experimentally demonstrated. Based on a simple setup utilizing a liquid crystal modulator to switch the polarization state of the input beam periodically, the alternating positions of the reflected beam for both polarizations are monitored by a lock-in amplifier to handily retrieve the GH shift signal. The conventional direct measurement of the beam position for the target state of polarization is vulnerable to instabilities in the optomechanical setup and alignment. Our proposed scheme provides a sensitive yet robust GH shift-sensing setup where the common mode drift and noise could be suppressed to ensure better system stability.