Design of a robust photonic crystal mirror for MEMS VCSELs.

Wavelength tunable lasers with narrow dynamic linewidths are essential in many applications, such as optical coherence tomography and LiDAR. In this letter, we present a 2D mirror design that provides large optical bandwidth and high reflection while being stiffer than 1D mirrors. Specifically, we investigate the effect of rounded corners of rectangles as they are transferred from the CAD to the wafer by lithography and etching.

Active Fano resonance switch using dual-layer graphene in an embedded dielectric metasurface.

We propose an active optical Fano switch (OFS) based on an embedded dielectric metasurface (EDM) including dual-layer graphene (DLG). An EDM is a dielectric grating overlapped by two cladding layers, and it excites a Fano resonance. DLG is positioned inside the upper cladding layer to maximize light-graphene interaction. Thus, with a small change of the chemical potential (µc) of graphene, a resonance wavelength is tuned to switch the OFS on and off. First, a red-parity asymmetric Fano resonance is realized, and a sharp asymmetric lineshape is achieved by controlling the structural parameters of the EDM and the interaction between the Fano resonance and additional weak Fabry-Perot interference for efficient switching. The distance of a peak-to-dip wavelength (Δλp-d) and the change of chemical potential (Δµc) for switching is analyzed by varying the duty cycle (DC) and grating thickness (tg) of the EDM. Furthermore, switching contrast as a figure of merit (FoM) is analyzed. With DC of 0.5 and tg of 70 nm, the OFS requires Δλp-d of 7.3 nm and Δµc of 0.25 eV. The FoM of 0.97 is achieved. By adjusting the two parameters, the switching condition is tuned. In the case of a blue parity, the effect of the two parameters exhibits a similar trend to that of the red parity. The FoM, however, is lower due to the reversed parity.

Critically coupled Fabry-Perot cavity with high signal contrast for refractive index sensing.

Perfect absorption at a resonance wavelength and extremely low absorption at the wavelength range of off-resonance in a one-port optical cavity is required for refractive index (RI) sensing with high signal contrast. Here, we propose and analyze an absorption-enhanced Fabry-Perot (MAFP) cavity based on a critical coupling condition in a near-infrared wavelength range. For a one-port cavity, a thick bottom Au is used as a mirror and an absorber. To achieve the critical coupling condition, a top dielectric metasurface is employed and tailored to balance the radiation coupling and the absorption coupling rates, and the one-port cavity is theoretically analyzed using temporal coupled-mode theory. We investigate two types of MAFP structures for gas and liquid. The gas MAFP cavity shows a sensitivity of ~ 1388 nm/RIU and a full-width at half-maximum of less than 0.7 nm. This MAFP cavity resolves the RI change of 5 × 10-4 with a reflectance signal margin of 50% and achieves a signal contrast of ~ 100%. The liquid MAFP cavity shows a sensitivity of ~ 996 nm/RIU when RI of liquid changes from 1.30 to 1.38. With tailoring the period of the metasurface maintaining its thickness, a signal contrast of ~ 100% is achieved for each specific RI range.

Tunable dual-wavelength absorption switch with graphene based on an asymmetric guided-mode resonance structure.

We propose a tunable dual-wavelength absorption (TDWA) switch based on an asymmetric guided mode resonance (AGMR) structure. A TDWA switch consists of a graphene layer and an AGMR structure sandwiched by cap and slab layers on a buffer/silicon substrate. The AGMR structure adds a smaller grating unit cell next to a larger one, exciting a second resonance close to but distinct from the first resonance. For switching, the TDWA between an absorptive or reflective mode with each on-/off-state, the chemical potential of graphene is tuned from 0.0 eV to 0.6 eV. For the absorptive mode, two absorption peaks of ≥ 96.2% are separated by 23 nm, both having an on-off ratio of ∼15.52. For the reflective mode, two reflectance peaks of ≥ 93.8% are separated by 23 nm, having on-off ratios of 15.56 dB and 18.95 dB. The maximum on-off ratios of 39.98 dB and 34.55 dB are achieved near the reflectance peaks. Both the period of the AGMR and the cap thickness alters the two peak wavelengths linearly, while the grating width of the AGMR varies nonlinearly from 17 nm to 28 nm. The buffer excites a weak Fabry-Perot resonance, which interacts with the TDWA structure, the result of which is the two absorption peaks are varied. Finally, as the incidence angle of light increases up to 5.3°, the distance of the two peak wavelengths is tuned from ∼22 nm to ∼77 nm with ≥ 96% absorption or ≥ 93% reflectance in each mode.

Dual-guiding-layer resonance structure with an embedded metasurface for quasi-critical coupling without a perfect mirror.

We propose an all-dielectric quasi-one-port resonance structure that achieves near perfect absorption without the use of a back mirror. The structure mainly consists of a high-refractive-index silicon metasurface and surrounding high-refractive-index guiding layers. The dual-guiding-layer (DGL) structure has high background reflectance and is designed to have a ratio of two decay rates into the upper and lower regions within a wider range. When an absorbing material is introduced into a DGL system, it can be designed to achieve a near critical-coupling condition by reducing the constraints in the two decay rates. By using single-layer graphene as an absorbing material, the DGL resonance structure shows an absorption of ~ 97% and a phase change of ∼ 0.95π near the wavelength of 1550 nm, confirming quasi-critical coupling. The optimized DGL structure is relatively insensitive to potential fabrication imperfections, and consequently, the expected average peak wavelength and absorption are obtained as 1549.29 nm and 96.74%, respectively. Angle-dependent absorption confirms that maximum absorption occurs under normal incidence. The DGL absorber is also designed to cover the whole C-band region, in order to meet the quasi-critical-coupling condition. All mode profiles are similarly quasi-symmetric along the metasurface due to the same DGL resonance mechanism.

Ultrahigh-speed Si-integrated on-chip laser with tailored dynamic characteristics.

For on-chip interconnects, an ideal light source should have an ultralow energy consumption per bandwidth (operating en-ergy) as well as sufficient output power for error-free detection. Nanocavity lasers have been considered the most ideal for smaller operating energy. However, they have a challenge in obtaining a sufficient output power. Here, as an alternative, we propose an ultrahigh-speed microcavity laser structure, based on a vertical cavity with a high-contrast grating (HCG) mirror for transverse magnetic (TM) polarisation. By using the TM HCG, a very small mode volume and an un-pumped compact optical feedback structure can be realised, which together tailor the frequency response function for achieving a very high speed at low injection currents. Furthermore, light can be emitted laterally into a Si waveguide. From an 1.54-µm optically-pumped laser, a 3-dB frequency of 27 GHz was obtained at a pumping level corresponding to sub-mA. Using measured 3-dB frequen-cies and calculated equivalent currents, the modulation current efficiency factor (MCEF) is estimated to be 42.1 GHz/mA1/2, which is superior among microcavity lasers. This shows a high potential for a very high speed at low injection currents or avery small heat generation at high bitrates, which are highly desirable for both on-chip and off-chip applications.

Hybrid III-V/SOI resonant cavity enhanced photodetector.

A hybrid III-V/SOI resonant-cavity-enhanced photodetector (RCE-PD) structure comprising a high-contrast grating (HCG) reflector, a hybrid grating (HG) reflector, and an air cavity between them, has been proposed and investigated. In the proposed structure, a light absorbing material is integrated as part of the HG reflector, enabling a very compact vertical cavity. Numerical investigations show that a quantum efficiency close to 100 % and a detection linewidth of about 1 nm can be achieved, which are desirable for wavelength division multiplexing applications. Based on these results, a hybrid RCE-PD sample has been fabricated by heterogeneously integrating an InP-based material onto a silicon-on-insulator wafer and has been characterized, which shows a clear enhancement in photo-current at the designed wavelength. This indicates that the HG reflector provides a field enhancement sufficient for RCE-PD operation. In addition, a capability of feasibly selecting the detection wavelength during fabrication as well as a possibility of realizing silicon-integrated bidirectional transceivers are discussed.

Hybrid grating reflector with high reflectivity and broad bandwidth.

We suggest a new type of grating reflector denoted hybrid grating (HG) which shows large reflectivity in a broad wavelength range and has a structure suitable for realizing a vertical cavity laser with ultra-small modal volume. The properties of the grating reflector are investigated numerically and explained. The HG consists of an un-patterned III-V layer and a Si grating. The III-V layer has a thickness comparable to the grating layer, introduces more guided mode resonances and significantly increases the bandwidth of the reflector compared to the well-known high-index-contrast grating (HCG). By using an active III-V layer, a laser can be realized where the gain region is integrated into the mirror itself.

Antireflective grassy surface on glass substrates with self-masked dry etching.

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

Size-dependent optical behavior of disordered nanostructures on glass substrates.

We demonstrate the distinctive optical properties of disordered nanostructures on glass substrates in accordance with changes in the average size of the nanostructures. Dissimilar sizes of nanostructures were fabricated by using different thicknesses of thermally dewetted Ag nanoparticles as etch masks. Unlike a flat glass substrate, the nanostructured glasses (NSGs) show a changed optical characteristic. By increasing the size of the nanostructures, the wavelength of the peak transmittance of about 99% gradually moved from 730 to 2000 nm. To clearly discern the effect of the different sizes of nanostructures, the normalized angle-dependent transmittance spectra of the NSGs were analyzed. Only if the size becomes relatively larger than the wavelength of the incident light are the transmittance spectra more strongly affected by the incident angle as well as by the relative size, rather than by the Fresnel reflection.

Broadband antireflective glasses with subwavelength structures using randomly distributed Ag nanoparticles.

We demonstrate broadband antireflective glasses with subwavelength structures (SWSs) using randomly distributed Ag nanoparticles. Ag nanoparticles formed by a thermal dewetting process were used as an etch mask for dry etching to fabricate antireflective SWSs on the glass surface. The size and shape of Ag nanoparticles are changed by the different thickness of the Ag thin film. The morphology of SWSs fabricated by using the Ag thin films is well consistent with that of the Ag nanoparticles. The single-side SWS integrated glass exhibits improved transmittance of approximately 96% at 750 nm due to the graded refractive index profiles, while the transmittance is only approximately 92.5% for the flat surface. To reduce Fresnel reflection at the other side of the glass substrate, the SWSs with optimized Ag film thickness and dry etching conditions are formed on both sides of the glass. The dual-side SWS integrated glass show an average transmittance of approximately 97.5% in a wavelength range of 350-750 nm. Transmission band shrinkage effects of the SWS integrated glass are also observed with increased average size of the Ag nanoparticles.

Disordered antireflective subwavelength structures using Ag nanoparticles for GaN-based optical device applications.

This study reports disordered antireflective subwavelength structures (SWS) on GaN and indium tin oxide (ITO) surfaces fabricated using thermally dewetted Ag nanoparticles. It is shown that the average diameter of Ag nanoparticles, which determines the optical characteristics, can be simply controlled by changing the thickness of the Ag thin film and the annealing temperature. The fabricated GaN and ITO SWS with tapered profile exhibited very low reflectance compared to that of a flat surface over a wide wavelength range.

Multifunctional light escaping architecture inspired by compound eye surface structures: From understanding to experimental demonstration.

We present bioinspired artificial compound eye surface structures that consist of antireflective subwavelength structures (SWSs) on hexagonally patterned microstructures (MSs), for the purpose of efficient light escaping inside light-emitting materials/devices. Theoretical understanding and geometrical optimization of SWSs on MSs are described together with rigorous coupled-wave analysis. As a proof of this concept, AlGaInP red light-emitting diodes (LEDs) with SWS/MSs were fabricated, and a light output power enhancement of 72.47% was achieved as compared to that of conventional LEDs. The artificial compound eye structures are not limited to LEDs, and the fabrication process is compatible with most semiconductor device manufacturing processes; hence, this concept opens up new possibilities for improving the optical performance of various optoelectronic device applications.