Miniaturized guided-mode resonance laser based on a one-dimensional finite heterostructure cavity.

Lasers based on the resonant nanostructures have attracted much attention due to their low threshold and compact dimensions. Guided-mode resonance (GMR) structures have been studied in lasing configurations because of their optical field enhancement and convenient free space excitation. However, the GMR inherently requires a larger footprint and is not suitable for high-density packaging. Here, we present numerical evidence of a miniaturized laser implemented in a one-dimensional finite heterostructure cavity (FHC). A GMR resonator and distributed Bragg reflectors are integrated to create the FHC, which enables the efficient coupling and localization of the electric field. Numerical findings indicate that the threshold is approximately 22.5 µJ/cm2, while the emission region is confined within a length of 5.4 µm. In addition, by adjusting the coupling strength, it is capable to achieve controllable lasing emission. The proposed structure provides a compact source for high-capacity optical communications, sensing, and quantum information processing.

Miniaturized quasi-BICs based on a two-dimensional heterostructure to realize a low-threshold nanolaser.

Bound states in the continuum (BICs) have been demonstrated as an effective mechanism to achieve high quality (Q)-factor cavities for nanolasers. However, the development of a compact BIC laser with a low threshold has remained elusive. Here, we numerically report lasing action from symmetry-protected BICs in a two-dimensional heterostructure, which consists of compound gratings with finite cells surrounded by orthogonal distributed Bragg reflectors (DBRs). The compound grating is used to excite quasi-BIC resonance with a high Q-factor, and DBRs enable light confinement and localized electric fields to enhance light-matter interaction. The nanolaser with a threshold of 16.8 µJ/cm2 is achieved within a footprint as small as 3.35 × 3.35 µm2. By changing the phase adjusting gap or asymmetry degree, it is possible to control the lasing emission. This work reveals a new, to our knowledge, path toward compact BIC lasers with a simple scheme for applications that require a small footprint and low threshold.

Guided-mode resonance sensors with ultrahigh bulk sensitivity and figure of merit assisted by a metallic layer and structural symmetry-breaking.

We propose a refractive index sensor with both high bulk sensitivity and figure of merit (FOM) that engages the guided-mode resonance (GMR) effect with the assistance of a metallic layer and structural symmetry-breaking in the grating layer. Owing to the existence of the metallic layer, the electric field at resonance can be reflected to the sensing environment, and enhanced bulk sensitivity is realized. Meanwhile, the full width at half maximum of the GMR mode can be decreased by increasing the asymmetrical degree of the grating, thus obtaining a high FOM which benefits the sensing resolution. A bulk refractive index sensitivity of 1076.7 nm/RIU and an FOM up to 35889 RIU-1 are achieved simultaneously. Other structural parameters such as the refractive index and fill factor of the grating are systematically discussed to optimize the sensing performance. The proposed GMR sensor with both high bulk sensitivity and FOM value has potential uses in applications with more stringent sensing requirements.

Tunable nanolaser based on quasi-BIC in a slanted resonant waveguide grating.

Nanolasers based on quasi-bound states in the continuum (quasi-BIC) have attracted much attention owing to their unique optical properties providing strong light-matter interaction. Although various quasi-BIC lasers have been designed, so far, few efforts have been devoted to their tunability in wavelength. Here we propose an approach to employ quasi-BIC and guided mode in a slanted resonant waveguide grating. The proposed structure supports a specially designed eigenmode localized both in the grating and in the 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) layer, which allows it to obtain lasing emission as well as the ability to tune the wavelength. Numerical simulation results show that the threshold is approximately 7.75 µJ/cm2 with the tuning range being 28 nm. In addition, we show that the distribution of the lasing intensity between the transmission and reflection directions can be controlled by changing the parameters of the structure. This work shows good potential of combining quasi-BIC with guided mode to design tunable nanolaser.

Low-threshold lasing behavior based on quasi-bound states in the continuum in a slanted guided-mode resonance nanocavity.

In this study, hybrid resonance modes are obtained when symmetry-breaking is introduced into a guided-mode resonance (GMR) grating, which transforms bound states in the continuum (BICs) into quasi-BICs with a high-quality factor while retaining the intrinsic GMR mode. The structural parameters are modified such that GMR and quasi-BICs resonance occur at the pump and emission wavelengths of the gain medium, respectively. Resonant optical pumping and high-quality nanocavities are utilized simultaneously, and a low-threshold laser is realized. We theoretically demonstrate that the threshold can be reduced to 24.6 µJ/cm2, which is approximately 4 times lower than that of the laser based on GMR alone. The lasing action can be modulated by optimizing the asymmetry parameter and the electric field, and the threshold can be further reduced.

Enhanced lasing behavior enabled by guided-mode resonance structure embedded with double waveguide layers.

By doping a laser dye into two polyurethane layers in a guided-mode resonance (GMR) structure, we observed that the lasing emission can be enhanced by approximately fivefold. The structure comprises, from top to bottom, a grating layer, four alternating layers of polyurethane and Ta2O5, and a bottom substrate. Two different GMR wavelengths can be generated because of the two films of Ta2O5 serving as waveguide layers. The enhancement of the lasing emission is achieved by matching both the absorption and emission wavelength of the laser dye with the two GMR wavelengths. When the absorption wavelength matches the GMR wavelength, the formation of high intensity near the polyurethane layer serves to efficiently excite the laser dye. Additionally, as the emission wavelength overlaps with the GMR wavelength, the extraction of lasing intensity can be further increased in the preferred directions owing to the high reflection efficiency and directivity of the GMR. Moreover, we found that the linewidth is reduced to approximately 1.06 nm, and the estimated threshold is approximately 0.92mJ/cm2 when both excitation and extraction resonances occur in the waveguide structure.

Improving the sensitivity of guided-mode resonance sensors under oblique incidence condition.

We present an investigation on the use of oblique incidence condition to enhance the sensitivity of guided-mode resonance (GMR) sensors. By adjusting the incident angle, the enhancement of GMR sensitivity in non-subwavelength regime can be obtained. The measured results show that the bulk sensitivity of the GMR sensors with period of 809 nm climbs to 177% or 292% as the incident angle increases from 15° to 25° or 35°, respectively. The same trend is also obtained for the grating period of 994 nm. Simulations based on the rigorous coupled wave analysis (RCWA) method were performed, and we also built a new slab waveguide model to describe the relationship between bulk sensitivity and the incident angle. The present investigation demonstrates a new method for enhancing the bulk sensitivity of GMR sensor. Moreover, simple fabrication techniques can be utilized since a large grating period was used.

L-shaped ITO structures fabricated by oblique angle deposition technique for mid-infrared circular dichroism.

This paper proposes a mid-infrared chiral structure, which consists of L-shaped indium tin oxide (ITO) films formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions by oblique angle deposition technique. Experimental results demonstrate that the structure exhibit circular dichroism (CD) responses in the range of 2.5 - 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD response enhances. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. The experimental results also demonstrate that the CD signals are evidently stronger than those of the structure consisting of silver in the mid-infrared band. This work provides a new idea for the fabrication of mid-infrared chiral structures, which have potential applications in the polarization state control of mid-infrared lasers.

Non-homogeneous composite GMR structure to realize increased filtering range.

A non-homogeneous composite guided-mode resonant (GMR) filter structure is proposed that avoids the multi-mode resonance effect and increases resonant wavelength tuning range. The composite filter structure is engineered using a combination of a varied-line-spacing (VLS) grating layer with a wedge-shaped waveguide layer. The grating is fabricated by holographic interference lithography (IL), while the wedge-shaped layer is fabricated using masked ion beam etching (MIBE) technology. The resonant wavelength has been observed to vary as a function of the spatial position on the structure. In the fabricated structure, over a length of 30 mm, the grating period increment is measured to be 149.2 nm, whereas the increment of the waveguide film thickness is approximately 100 nm. Experimental results show that a primary reflectance peak is achieved spanning a wavelength range of 805.8-1119.0 nm. The device is designed using the rigorous coupled-wave analysis (RCWA) method, and the proposed device is toward the practical application of GMR filters.

Large-area broadband optical absorber fabricated by shadowing sphere lithography.

We report a large-area broadband optical absorber consisting of Ag/SiO2 stacked plasmonic layers fabricated on a self-assembly polystyrene sphere monolayer using the glancing angle deposition. Such an absorber can absorb more than 90% of light in the spectral range of 350 - 850 nm when the polystyrene spheres have a diameter of 750 nm. The broadband absorption is due to the overlap of localized plasmonic resonance wavelengths resulting from different patchy sizes and shapes of Ag coating on polystyrene spheres. Such a simple, flexible and large-area absorber has potential applications in light cloaking and energy conversion.

Growth and optical properties of Ag-Ti composite nanorods based on oblique angle co-deposition technique.

Ag-Ti composite nanorod structures with various Ag compositions were fabricated by the oblique angle co-deposition technique, and their optical transmission spectra are tuned by composition ratios of Ag and Ti, polarization directions, and deposition angles. Such tunable optical properties have potential applications in optoelectronics. Specially, for the Ag80 composite nanorod structures, there exists a wavelength, where it is isotropic. We also show that the transmission spectra of the Ag80 composite nanorod structure for the deposition angle of 87.5° are greater than 90%, while the transmission spectra for the 75° deposition angle are lower than 20%. Utilizing such a property, high or low transmission lenses can be designed.

Switchable reflection/transmission utilizing polarization on a plasmonic structure consisting of self-assembly polystyrene spheres with silver patches.

We report a plasmonic structure for switchable reflection and transmission by polarization. The structure is composed of a hexagonal-packed polystyrene sphere array with silver patches on them. Simulations and experiments demonstrated that the conversions between reflected beams and transmitted ones can be performed when the polarization directions of incident beams vary from 0° to 90°. A switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a patchy plasmonic structure serving as a switch between reflection and transmission have potential applications in photoelectric control devices.

Tunable guided-mode resonant filter with wedged waveguide layer fabricated by masked ion beam etching.

A compact, tunable guided-mode resonant (GMR) filter whose spectral reflectance wavelength varies as a function of the spatial position on the device is experimentally demonstrated. The filter incorporates a wedge-shaped waveguide layer that is fabricated using masked ion beam etching (MIBE) technology. A ceramic plate mask consisting of an isosceles triangular window is placed between the ion source and the sample to achieve different etching times at difference locations on the film. The increment in the magnitude of the film thickness is approximately 50 nm over a length of 33 mm, which results in a primary reflectance peak whose spectral location spans the range of 684.2-725.3 nm. The device is designed using the rigorous coupled-wave analysis (RCWA) method, and the proposed device is directed toward the practical application of GMR tunable filters.

Optical notch filter with tunable bandwidth based on guided-mode resonant polarization-sensitive spectral feature.

A novel bandwidth-tunable notch filter is proposed based on the guided-mode resonance effect. The notch is created due to the superposition spectra response of two guided-mode resonant filters. The compact, bandwidth tuning capability is realized by taking advantage the effect of spectra-to-polarization sensitivity in one-dimensional classical guided-mode resonance filter, and using a liquid crystal polarization rotator for precise and simple polarization control. The operation principle and the design of the device are presented, and we demonstrate it experimentally. The central wavelength is fixed at 766.4 nm with a relatively symmetric profile. The full width at half maximum bandwidth could be tuned from 8.6 nm to 18.2 nm by controlling the applied voltage in electrically-driving polarization rotator.

Electrically driving bandwidth tunable guided-mode resonance filter based on a twisted nematic liquid crystal polarization rotator.

A novel bandwidth-tunable filter is proposed based on nonpolarizing guided-mode resonance effect. The compact, electrically driving bandwidth-tunable optical filter is realized by taking advantage of the effect of bandwidth-to-polarization sensitivity and using a twisted nematic liquid crystal polarization rotator for simple and precise polarization control. The operation principle and the design of the device are presented. The center wavelength is fixed at 623.1 nm with a relatively symmetric line shape. The full-width at half-maximum bandwidth is tuned from 12 to 44.8 nm by controlling the voltage in the polarization rotator.