Grating Layer Research Articles

SOI-based high performance multi-subpart profile grating mirror

In this study, a SOI-based high performance multi-subpart profile grating mirror (MPGM) is proposed and fabricated. It is shown that with the multi-subpart profile and strongly modulated thin grating layer, the MPGM experimentally demonstrated an ultra broadband reflection spectrum from 1.21 to1.62µm, very high reflectivity (R>97%), and low sensitivity to incident angle at the range of −12.3°<θ<+13.1°.

Thermoelectric Devices Incorporating Photonic Resonance Segments

Thermoelectric devices integrated with optical resonance absorbers are demonstrated. We design the absorbers with rigorous numerical methods and fashion experimental prototypes by thin-film deposition, patterning, and etching. A $\sim 2.5$ - $\mu \text{m}$ -thick p-type heavily doped polysilicon film on a $\sim 2$ - $\mu \text{m}$ layer of thermally grown SiO2 enables guided-mode resonance. The SiO2 layer additionally serves to thermally insulate the polysilicon layer from the Si substrate. A grating layer is etched into the polysilicon film to form the absorber. Thus, the polysilicon film works as functional material for both the absorber and the thermoelectric converter itself. Numerical simulations show that the resonance segment enhances absorption by $\sim 30$ % in the visible spectral range and by $\sim 40$ % in the infrared (IR) range relative to unpatterned devices. Moreover, the experimental results demonstrate significantly increased electrical output over reference devices. These simple devices can be applied as compact voltage generators and IR sensors.

Bistable 1060-nm High-Power Single-Mode DFB Laser Diode

We report a novel high-power bistable distributed feedback (DFB) semiconductor laser diode operating near 1060 nm, which is realized by inserting a high-bandgap electron barrier layer and a grating layer in a super large optical cavity laser design. Optical and electrical bistable characteristics are both observed for this device. An on-state optical output power of 98 mW and an on-off extinction ratio of 22 dB were demonstrated between the bistable states. A qualitative physical model based on carrier population profile shift is used to explain the switching effect. Optical flip-flop operation was also demonstrated.

Analysis of grating doublets for achromatic beam-splitting.

Achromatic beam-splitting grating doublets are designed for both continuous phase and binary phase gratings. By analyzing the sensitivity to lateral shifts between the two grating layers, it is shown that continuous-profile grating doublets are extremely difficult to fabricate. Achromatic grating doublets that have profiles with a constant first spatial derivative are significantly more resistant to lateral shifts between grating layers, where one design case showed a 17 times improvement in performance. Therefore, binary phase, multi-level phase, and blazed grating doublets perform significantly better than continuous phase grating doublets in the presence of a lateral shift between two grating layers. By studying the sensitivity to fabrication errors in the height of both grating layers, one grating layer height can be adjusted to maintain excellent performance over a large wavelength range if the other grating layer is fabricated incorrectly. It is shown in one design case that the performance of an achromatic Dammann grating doublet can be improved by a factor of 215 if the heights of the grating layers are chosen to minimize the performance change in the presence of fabrication errors.

Sign of the dominant charge carriers in photorefractive crystals determined by a phase-locked holographic technique

This work presents a holographic method based on active feedback techniques for determining the sign of the dominant charge carriers in photorefractive materials. A two-step procedure is proposed: first off a stationary phase-locked hologram is recorded; an electric field normal to the grating layers is then applied to the material, thus producing a running hologram. The sign of the charge carriers is determined by comparing the direction of the applied field with the direction of the hologram movement, which is known through the automatically attached light pattern. The method can be applied from highly photoconductive to highly insulating materials. Furthermore, no information on any material parameter is required. The method is validated by a set of holographic experiments using a Bi12TiO20 crystal that has electrons as the majority photocarriers.

Angle sensitive single photon avalanche diode

An ideal light sensor would provide exact information on intensity, timing, location, and angle of incoming photons. Single photon avalanche diodes (SPADs) provide such desired high (single photon) sensitivity with precise time information and can be implemented at a pixel-scale to form an array to extract spatial information. Furthermore, recent work has demonstrated photodiode-based structures (combined with micro-lenses or diffraction gratings) that are capable of encoding both spatial and angular information of incident light. In this letter, we describe the implementation of such a grating structure on SPADs to realize a pixel-scale angle-sensitive single photon avalanche diode (A-SPAD) built in a standard CMOS process. While the underlying SPAD structure provides high sensitivity, the time information of the two layers of diffraction gratings above offers angle-sensitivity. Such a unique combination of SPAD and diffraction gratings expands the sensing dimensions to pave a path towards lens-less 3-D imaging and light-field time-of-flight imaging.

Ultracompact resonator with high quality-factor based on a hybrid grating structure.

We numerically investigate the properties of a hybrid grating structure acting as a resonator with ultrahigh quality factor. This reveals that the physical mechanism responsible for the resonance is quite different from the conventional guided mode resonance (GMR). The hybrid grating consists of a subwavelength grating layer and an un-patterned high-refractive-index cap layer, being surrounded by low index materials. Since the cap layer may include a gain region, an ultracompact laser can be realized based on the hybrid grating resonator, featuring many advantages over high-contrast-grating resonator lasers. The effect of fabrication errors and finite size of the structure is investigated to understand the feasibility of fabricating the proposed resonator.

Frequency splitter based on the directional emission from surface modes in dielectric photonic crystal structures.

We demonstrate the numerical design and the experimental validation of frequency dependent directional emission from a dielectric photonic crystal structure. The wave propagates through a photonic crystal line-defect waveguide, while a surface layer at the termination of the photonic crystal enables the excitation of surface modes and a subsequent grating layer transforms the surface energy into outgoing propagating waves of the form of a directional beam. The angle of the beam is controlled by the frequency and the structure operates as a frequency splitter in the intermediate and far field region.

Dislocated Double-Layered Metal Gratings: Refractive Index Sensors with High Figure of Merit

We experimentally demonstrate an enhanced refractive index sensing using a dislocated double-layered metal grating (DDMG). The DDMG is capable of supporting a novel guided mode caused by the interaction between localized surface plasmon resonances (LSPRs) from the individual gold stripes and Wood’s anomaly in the dielectric grating layer between two gold gratings. We show that this guided mode can only be induced and mediated by the coupling of lattice plasmon resonances sustained by upper and lower gold grating through introducing a lateral displacement between the two gold gratings. The DDMG provides an experimental figure of merit (FOM) value up to 36 under normal incidence, which is superior to those of most of nanoplasmonic sensors relying on Fano resonances. Additionally, the DDMGs can be fabricated by a very simple and cost-effective method via a combination of two-beam interference lithography and metal deposition. Accompanied with high FOM and simple detection scheme, this sensor will be found in a wide range of applications in biomedical sensing.

Absorption Manipulation in a Narrowband Infrared Absorber Based on the Hybridization of Gap Plasmon and Fabry-Perot Resonance

We report a narrowband infrared absorber consisting of a metallic grating separated from a metallic substrate by a dielectric layer. In this absorber, the near-perfect absorption results from the hybridization of the gap plasmon mode and the Fabry-Perot resonance mode. The grating layer acts as an efficient absorbing material because of enhanced electric field in the grating slits, and the absorption spectrum can be manipulated by Fabry-Perot resonance. According to this spectrum manipulation mechanism, the resonance wavelength and bandwidth of absorption can be tuned by changing the dielectric layer thickness. Furthermore, the absorption resonance wavelength can be tuned by changing the incidence angle. This absorption manipulation mechanism can be applied in coherent thermal emitters.

Femtosecond laser writing of phase-tuned volume gratings for symmetry control in 3D photonic crystal holographic lithography

Volume diffractive optical elements promise widespread application in laser beam shaping, imaging and optical data storage by structuring refractive index modulation in the third axial dimension. Femtosecond laser direct writing has been exploited inside fused silica to assemble multiple 1D grating layers on Talbot planes and overcome the inherent weak diffraction efficiency otherwise found in low-contrast volume gratings. Here, we extend laser structuring from linear (1D) to orthogonally crossed (2D) gratings with the aim to write 3D photonic crystal templates in photoresist by 3D interference lithography. The formation of crossed grating structures present challenges in balancing the efficiency of diffraction orders owing to blazing and index overwriting effects requiring compensation by tuning the grating design and laser power exposure. In this way, six-layer grating designs have been fabricated and applied to exposure of thick photoresist, enabling the formation of 3D photonic crystal templates with bicontinuous structure. A systematic offsetting of orthogonal grating layers to establish phase offsets over 0 to π/2 range presents a precise means for controlling the photonic crystal structure symmetry between body centered tetragonal (BCT) and woodpile-like tetragonal (wTTR).

Further enhancement of light extraction efficiency from light emitting diode using triangular surface grating and thin interface layer.

Analysis has been done of improvement of forward directional light extraction efficiency of light emitting diodes (LEDs) by surface patterning of different types of one-dimensional profiles on indium-zinc-oxide films developed recently by our group using sol-gel technique. Finite-difference time-domain simulations by MEEP software have been used for this purpose. From the analysis, it is found that the patterned film is suitable for near-infrared LED. The optimized structure, which gives maximum improvement at around 1.040μm wavelength, is determined and fabricated using soft lithography. Further enhancement of the light output of the LED with the fabricated gratings is possible by introducing an interlayer within the top contact layer. The mathematical formulation of the coupling of light in structured/multilayered surfaces is also discussed.

Generation of individually modulated femtosecond pulse string by multilayer volume holographic gratings.

A scheme to generate individually modulated femtosecond pulse string by multilayer volume holographic grating (MVHG) is proposed. Based on Kogelnik's coupled-wave theory and matrix optics, temporal and spectral expressions of diffracted field are given when a femtosecond pulse is diffracted by a MVHG. It is shown that the number of diffracted sub-pulses in the pulse string equals to the number of grating layers of the MVHG, peak intensity and duration of each diffracted sub-pulse depend on thickness of the corresponding grating layer, whereas pulse interval between adjacent sub-pulses is related to thickness of the corresponding buffer layer. Thus by modulating parameters of the MVHG, individually modulated femtosecond pulse string can be acquired. Based on Bragg selectivity of the volume grating and phase shift provided by the buffer layers, we give an explanation on these phenomena. The result is useful to design MVHG-based devices employed in optical communications, pulse shaping and processing.

Extremely wide band Rayleigh resonant reflector with sharp angular spectra in near infrared wavelength band

In this paper, we propose an extremely broadband Rayleigh resonant reflector with sharp angular spectra operating in near infrared wavelength band, this device consisting of a single germanium resonant grating layer is designed and analyzed by using with the rigorous vector diffraction theory. At the Rayleigh angle, the first diffracted order can be appear from evanescent to a propagating one, thus, a very sharp angular spectrum characteristics can be presented in the device. Based on the guided mode resonant effect, high index material such as silicon and germanium can be designed as wide band reflector, beam splitter and polarizer in near infrared wavelength region. Through connecting Rayleigh phenomena and guided mode resonant effect, we can design a new kind of optical devices with versatile characteristics such as sharp angular spectra and extremely wide reflection band. In this paper, we present a Rayleigh resonant reflector with extremely high reflection (R>99.5%) for TE polarization light over ∼600nm wavelength range and sharp angular spectral distribution. In addition, it is shown from our calculations that the high-index nano-layer located adjacent to the substrate is seen to critically affect the resulting spectra of Rayleigh resonant reflector.

Numerical investigation of a grating and graphene-based multilayer surface plasmon resonance biosensor

A multilayer surface plasmon resonance biosensor (SPRB) incorporating a grating-graphene configuration is investigated for enhanced sensitivity. The numerical analysis of the impact of integrating a periodic array of subwavelength grating on top of a layer of graphene sheet for improving sensitivity is presented. The result of monitoring the biomolecular interactions of DNA hybridization is compared against the outcome of the conventional SPRB, a graphene-based multilayer SPRB, and a multilayer layer grating SPRB, and is mathematically validated. It is demonstrated that the inclusion of a grating and graphene layer on top of the gold thin film is an excellent candidate for a highly sensitive SPRB. To achieve further enhancement of sensitivity, the subwavelength grating is numerically optimized against its geometry including grating configurations (rectangular, sinusoidal, and triangular), grating depth, volume factor, and grating period.

Design and Analysis of Multicolor QDIP Based on Metallic Nanoslits Array

In this letter, we propose a new design for a multicolor InAs/GaAs quantum dot (QD) infrared photodetector based on metallic nanoslits array. The proposed device has an absorption coefficient of more than 50% for designed wavelengths in the infrared (IR) range. We analyze the absorption characteristics of the InAs/GaAs QDs affected by metallic nanoslits. Distributed Bragg reflector layers with a buried grating layer are used for enhancement of local field in active layer. The dielectric function of the InAs/GaAs QD layer is calculated by modified Maxwell-Garnett model considering homogeneous and inhomogeneous broadenings, which is then substituted into Maxwell’s equations. Results show absorption enhancement of ~100% for certain wavelengths in the IR range, compared with reference structure.

Pulse splitting by modulating the thickness of buffer layer of two-layer volume holographic grating

Based on Kogelnik's coupled-wave theory and matrix optics, generation of femtosecond double pulses by modulating thickness of the buffer layer of two-layer volume holographic grating (TL-VHG) is discussed. Expressions of diffraction field when a femtosecond pulse incidents on the TL-VHG are deduced. Simulation results show when thickness of the buffer layer increases from 6mm to 11 mm or even larger, one incident pulse splits into double femtosecond pulses with the same duration and peak intensity, and pulse interval is linearly proportional to the thickness. The reason of these phenomena is due to the interference of diffraction waves reconstructed from two gratings and phase shift resulting from the buffer layer thickness. Time-delay of diffracted double pulses is explained by group time delay of periodic media. It is shown that the slope of the pulse interval with respect to the thickness of buffer layer is 2 times of that of pulse time-delay. Furthermore, we demonstrate it is possible to control the output double pulses' duration and pulse interval by varying the grating thickness.

Transient grating formation in azo-doped polymer and its application to DNA-based tunable dye laser

In order to realize tunable lasing from DNA-complex thin films doped with laser dye, we made a two-layered device composed of a laser active film and a grating layer for wavelength selection. We characterized the dynamic response of azo dye doped polymer under excitation with a pulsed laser, finding that transient grating was available under adequate pumping conditions. By fabricating a bi-layered device with dye-doped DNA-lipid and azo-PMMA layers and exciting with three beams of pulsed laser, we demonstrated laser emission and indicated the possibility of practical dynamic tuning in red wavelength region.

An optical filter with angular selectivity of the transmittance

This paper presents an optical filter with thin-film grating layers on both surfaces of a glass sheet substrate. Macroscopic gratings are formed by alternating transmissive and absorbing bands of submillimeter width. Their relative placement on the surfaces of the filter provides angular selectivity of the transmittance—part of the radiation transmitted through the input macrograting is additionally blocked by the output macrograting, depending on the angle of incidence. The coefficients of directed transmittance of filters with different parameters are calculated by a graphoanalytical method. The mean deviations of the experimental data for these filter samples do not exceed 4% in the 0°–60° range of angles of incidence, and the maximum deviations are 5.6%. The possibility of using optical filters with angular selectivity of the transmission in architectural glazing is considered.

Wide band polarizer with suspended silicon grating

In this paper, we propose an ultra broadband polarizer operating in the telecommunication wavelength band; this device consisting a single silicon suspended resonant grating layer is designed with using the inverse mathematical method and rigorous vector diffraction theory. It is shown from our calculations that the device presents extremely high reflection (R>98%) for TE polarization light and high transmission (T>98%) for TM polarization over ∼330nm wavelength range; moreover, the extinction ratio is ∼100 in the central wavelength 1550nm. Furthermore, it is found with Rigorous Coupled Wave Analysis (RCWA) and near field distribution that the extremely wide band property for TE polarization is due to the excitation of strong modulation guided modes in the design wavelength range.