Modes In Gratings Research Articles

Influence of material and geometry parameters on resonance linewidths of plasmonic modes in gratings made from highly doped Ge1− x Sn x

Highly doped group IV semiconductors such as Ge or GeSn are promising candidates for plasmonic mid infrared applications. The lower effective mass of GeSn alloys in comparison to pure Ge can result in lower plasma wavelengths and extend the application wavelength range. Devices made from doped GeSn alloys, therefore, are one interesting route towards plasmonic applications in the mid-infrared wavelength range, possibly extending to the NIR. Here, we specifically explore how spectrally narrow plasmonic resonances can be obtained in comb-like grating antennas by combining aspects of material growth with geometry optimization. We investigate both in simulation and in experiment how the interplay of localised surface plasmon resonances and Rayleigh anomalies can be tuned to achieve narrow extinction peaks originating from the resulting surface lattice resonances generated in our antennas made from highly doped Ge1−x Sn x .

All-fiber mode exchange of high-speed DP-QPSK signals using mechanical long-period fiber grating

Mode exchange (ME) can effectively improve the utilization of network resources in short-distance mode division multiplexing (MDM) communication systems. In this paper, a mechanical long-period fiber grating (LPFG) mode converter is used to realize the channel exchange of LP01- and LP11-mode dual-polarization (DP) signals, with the channel crosstalk of −14 dB or less. We experimentally tested the receiver sensitivities of two DP channels, which are dependent on channel crosstalk and polarization-dependent loss (PDL), and demonstrated the practicability of the real-time data transmission and exchange for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) in multiple-input multiple-output free (MIMO-free) MDM system. The experimental results show that the ME system can provide the optical power margin of about 25 dB, capable of effectively supporting the metropolitan transmission and exchange between the data centers.

Enhancement of quantum cascade laser intersubband transitions via coupling to resonant discrete photonic modes of subwavelength gratings.

We present an optical spectroscopic study of InGaAs/AlInAs active region of quantum cascade lasers grown by low pressure metal organic vapor phase epitaxy combined with subwavelength gratings fabricated by reactive ion etching. Fourier-transformed photoluminescence measurements were used to compare the emission properties of structures before and after processing the gratings. Our results demonstrate a significant increase of the photoluminescence intensity related to intersubband transitions in the mid-infrared, which is attributed to coupling with the grating modes via so called photonic Fano resonances. Our findings demonstrate a promising method for enhancing the emission in optoelectronic devices operating in a broad range of application-relevant infrared.

Sharp MIR plasmonic modes in gratings made of heavily doped pulsed laser-melted Ge1-xSnx

Plasmonic structures made out of highly doped group-IV semiconductor materials are of large interest for the realization of fully integrated mid-infrared (MIR) devices. Utilizing highly doped Ge1-xSnx alloys grown on Si substrates is one promising route to enable device operation at near-infrared (NIR) wavelengths. Due to the lower effective mass of electrons in Sn compared to Ge, the incorporation of Sn can potentially lower the plasma wavelength of Ge1-xSnx alloys compared to that of pure Ge. However, defects introduced by the large lattice mismatch to Si substrates as well as the introduction of alloy scattering limit device applications in practice. Here, we investigate pulsed laser melting as one strategy to increase material quality in highly doped Ge1-xSnx alloys. We show that a pulsed laser melting treatment of our Ge1-xSnx films not only serves to lower the material’s plasma frequency but also leads to an increase in active dopant concentration. We demonstrate the application of this material in plasmonic gratings with sharp optical extinction peaks at MIR wavelengths.

Airflow vibration of diaphragmatic breathing: model and demonstration using optical biosensor

Optical fiber is increasingly popular and appreciated as a modern sensor technology in various sectors, one of which is for medical functions. This study was conducted to detect human diaphragmatic breathing flow using theoretical and experimental approaches. Initially, the lung model was formed using the finite element method and the Navier-Stokes equation by applying the principles of momentum and continuity. Furthermore, fiber Bragg grating (FBG) and single mode fiber (SMF) were experimentally designed with sinusoidal patterned macro-scale bending as a stretch sensor in a breathing belt applied to the diaphragm. The simulation model shows the airflow velocity increases up to 4 m/s when it flows into smaller branches. While the experimental results show that the largest power loss occurs at a buffer diameter of 0.8 cm. The power loss detected in SMF is a maximum of -0.18 dBm during inhalation and a minimum of -0.28 dBm during expiration. However, FBG bending is superior with high sensitivity.

High stability microfiber Sagnac loop liquid refractive index sensor with FBG mode selecting

As a high-resolution sensor, it is very important to improve the spectral stability of microfiber Sagnac loop interferometer. A fiber Bragg grating (FBG) mode selecting method is proposed and demonstrated. The method can filter out some cladding modes by FBG in the Sagnac loop and make working modes energy more concentrated, which shows that the interference spectrum becomes more uniform. At the same time, the sensitivity and linearity of the sensor can be improved slightly by this method. In our experiment, the resolution of sensor can get 1.41 × 10-5 ∼ 7.49 × 10-6 in the liquid refractive index range of 1.3330∼1.38481. Moreover, this method can effectively improve the spectral stability of the interferometer, which shows that the dip wavelength absolute error of time fluctuation is improved from ±0.358 nm to 0.021 nm and from ±0.116 nm to 0.000 nm at 1536.6 nm and 1559.9 nm initial dip wavelength, respectively. This method also has advantages of compact structure, good mechanical properties, and easy preparation. On this basis, the FBG also can be replaced by various modes filters to develop more multifunctional high stability interferometers according to the detection needs.

High-power fiber laser with real-time mode switchability

A hundred-watt-level spatial mode switchable all-fiber laser is demonstrated based on a master oscillator power amplifier scheme. The performance of the amplifier with two seed lasers, i.e., with the acoustically induced fiber grating (AIFG) mode converter inside and outside the seed laser cavity, is investigated. Real-time mode switching with millisecond scale switching time between the LP01 and LP11 modes while operating in full power (>100 W) is realized through an AIFG driven by radio frequency modulation. This work could provide a good reference for realizing high-power agile mode switchable fiber lasers for practical applications.

Wavelength-Tunable, Ultra-Broadband, Biconical, Long-Period Fiber Grating Mode Converter Based on the Dual-Resonance Effect.

We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO2 laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP01 to LP11, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength.

Analysis of Dielectric Waveguide Grating and Fabry-Perot Modes in Elastic Grating in Optical Detection of Ultrasound.

In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures.

Band shaping and emission control via waveguide plasmon polaritons

Photonic nanostructures can improve the efficiency and functionality of optoelectronic devices by controlling propagation and extraction of light in waveguides. The paradigmatic example is the extraction of light by matching the grating and waveguide mode momenta. A higher functionality can be achieved, however, via strong coupling between waveguide and plasmon modes. Here, we show that such hybridized systems provide a convenient strategy to tailor the dispersion band of the waveguide. By controlling the metallic grating height, the bands evolve from a Dirac cone to a flatband and a parabolic profile. We show an application of this system to light emitting waveguides, where a flatband leads to a sharply defined emission cone, with a 1.8 fold gain in emission efficiency and a Purcell factor of 2.4. We show that 72.2% of all radiated power is contained within the flatband region and is eight times larger than the total emission by the dipole in a homogenous medium. The system thus combines efficiency gain with the opportunity of beam shaping.

Few-mode tilted long period fiber grating mode filter based on dual-peak resonance near phase-matching turning point

This paper presents a novel few-mode tilted long period fiber grating (FM-TLPFG) mode filter based on the dual-peak resonance near phase-matching turning point (PMTP). On the basis of the coupled mode theory, the dual-peak resonance and the mode coupling in the FM-TLPFG are analyzed. The influences of grating period, tilt angle and grating length on the filter characteristics are analyzed. As the results show, the coupling characteristics between the core modes (LP01 and LP11) and the cladding modes will affect the bandwidth and the core mode loss of mode filters. As the efficiency of the resonant peak LP01-HE1,19 achieves ∼100%, the bandwidth can reach 70 nm. And the maximum coupling between the core mode LP01 and the cladding mode HE1,19 occurs in the tilt angle range of 80-82°, and the grating length range of 34–46 mm. Compared with the reported fiber mode filter, the performance of the FM-TLPFG mode filter is better. The bandwidth of FM-TLPFG mode filter is about 2.7 times than the reported filter, and the mode loss of LP01 is higher.

Opto-Electronic Refractometric Sensor Based on Surface Plasmon Resonances and the Bolometric Effect

The bolometric effect allows us to electrically monitor spectral characteristics of plasmonic sensors; it provides a lower cost and simpler sample characterization compared with angular and spectral signal retrieval techniques. In our device, a monochromatic light source illuminates a spectrally selective plasmonic nanostructure. This arrangement is formed by a dielectric low-order diffraction grating that combines two materials with a high-contrast in the index of refraction. Light interacts with this structure and reaches a thin metallic layer, that is also exposed to the analyte. The narrow absorption generated by surface plasmon resonances hybridized with low-order grating modes, heats the metal layer where plasmons are excited. The temperature change caused by this absorption modifies the resistance of a metallic layer through the bolometric effect. Therefore, a refractometric change in the analyte varies the electric resistivity under resonant excitation. We monitor the change in resistance by an external electric circuit. This optoelectronic feature must be included in the definition of the sensitivity and figure of merit (FOM) parameters. Besides the competitive value of the FOM (around 400 RIU − 1 , where RIU means refractive index unit), the proposed system is fully based on opto-electronic measurements. The device is modeled, simulated and analyzed considering fabrication and experimental constrains. The proposed refractometer behaves linearly within a range centered around the index of refraction of aqueous media, n ≃ 1.33 , and can be applied to the sensing for research in bio-physics, biology, and environmental sciences.

Ultra-Narrow Spectral Response of a Hybrid Plasmonic-Grating Sensor

We configure and analyze a nanostructured device that hybridizes grating modes and surface plasmon resonances. The model uses an effective index of refraction that considers the volume fraction of the involved materials, and the propagation depth of the plasmon through the structure. Our geometry is an extruded low-order diffraction grating made of dielectric nano-triangles. Surface plasmon resonances are excited at a metal/dielectric interface, which is separated from the analyte by a high-index dielectric layer. The optical performance of the refractometric sensor is highly competitive in sensitivity and figure of merit (FOM) because of the the ultra-narrow spectral response (below 0.1 nm). Moreover, it is operative within a wide range of the index of refraction (from 1.3 till 1.56), and also works under normal incidence conditions.

A self-calibration method with high-accuracy and low-cost for large-area motion stage based on digital grating projection mode

We report a self-calibration method with high-accuracy and low-cost for large-area motion stage. A programmable digital grating projection mode based on digital micromirror device was proposed, which can effectively avoid defocusing problem of calibration system and complete a stable high-accuracy calibration. Instead of adopting the high-cost laser interferometer measurement system, a self-calibration method based on reverse strategy was developed, which can easily obtain an optimal positional accuracy. This presented method can be expected to provide a new route for developing various applications such as laser direct writing equipment, automated optical inspection equipment.

LP01-LP11a mode converters based on long-period fiber gratings in a two-mode polarization-maintaining photonic crystal fiber

A long-period fiber grating (LPFG) mode converter based on a two-mode polarization-maintaining photonic crystal fiber (PM-PCF) is proposed and demonstrated. The mode converters realize conversions between the LP01 modes and LP11a modes with parallel polarization directions. Different from typical conventional mode converters, the PM-PCF-LPFG mode converters most notably can separate out two linearly polarized LP11a modes at different wavelengths. The highest mode-conversion efficiency is more than 99%. In addition, the bandwidth of the mode converter is adjustable by changing the grating number of the LPFG.

Enhanced transmission via cavity modes in gratings formed by subwavelength metallic cylinders

In the last few decades, special attention has been paid to the optical response of structures composed of periodic subwavelength slits in thin metallic sheets. Extraordinary transmission and evanescent-to-propagating conversion are two of the main effects that have been most investigated in these systems. In this paper, we present an alternative way of enhancing the intensity diffracted by a grating through morphological electromagnetic resonances. Unlike nanoslit arrays, in this system the cavities are formed by periodically distributed chains of metallic subwavelength cylinders, which behave like walls that confine the fields and thus show resonant behavior. The set of cavities is illuminated by an evanescent wave generated by total internal reflection. We show that resonant coupling of cavity modes excited by the inhomogeneous wave produces enhanced transmission of up to 92% and also that the system has the ability of steering most of the transmitted intensity in a particular direction given by the diffraction orders. The results are compared with those obtained from a similar structure formed by periodically distributed perfectly conducting solid walls.

Reverse design for diffraction gratings by modal method

A novel method for reverse design of the rectangular-groove dielectric surface-relief grating by modal method is presented. The principle of the reverse design method is the excitation and coupling between the propagating modes in grating. When the modes 0 and 1 are transmission through the grating, the phase difference is accumulated. By adjusting the cumulative phase difference between the modes 0 and 1, the energy can be reallocated between the − 1st and 0th order diffraction light. Thereby the diffraction efficiency of the grating − 1st and 0th order are adjusted. By using these reverse design method, we analyze the design of the beam splitter and blazed grating. As an example, a − 1st transmitted order blazed grating has been designed by reverse design method. The comparative results have shown that the geometrical parameters of the grating calculated by modal method agree well with the results simulated by the rigorous couple-wave analysis (RCWA).

Dark plasmonic modes in diatomic gratings for plasmoelectronics

AbstractOwing to the unique ability of nanostructured metals to confine and enhance light waves along metal‐dielectric interfaces, plasmonics has enabled unprecedented flexibility in manipulating light at the deep‐subwavelength scale. With regard to the spectral behavior of plasmonic resonances, the spectral location of a resonance can be tailored with relative ease while the control over the spectral linewidth represents a more daunting task. In this paper, we present sharp resonance features by introducing dark plasmonic modes in diatomic gratings. The induced asymmetry in the metallic structure facilitates the generation of a dark mode with significantly suppressed radiative loss leading to an ultra‐sharp spectral feature ∼5 nm wide. We further use this metallic structure as an optoelectronic platform for the transduction of light waves to electrical signals via a plasmoelectric effect. The light concentrating ability of dark plasmonic modes, in conjunction with the ultra‐sharp resonance feature at a relatively low loss offers a novel route to enhanced light‐matter interactions with high spectral sensitivity for diverse applications. image

In Depth Investigation of Lattice Plasmon Modes in Substrate-Supported Gratings of Metal Monomers and Dimers

We study theoretically and numerically bidimensional square gratings of monomers and dimers of gold nanocylinders supported on a dielectric substrate, under plane wave illumination as a function of the angle of incidence and of the polarization. The number of parameters investigated makes that system a rich platform for the investigation of how grating coupling, and in particular edge diffraction which corresponds to the grazing propagation of a particular diffracted order, influence the surface plasmons response of nanoparticles. In particular, the considered periods are comparable to the range of incident wavelength, which makes the interpretation of the observed phenomena complex due to the large number of diffraction orders coming into play. In order to analyze those systems, we perform exact numerical simulations using Green’s tensor method, and compare them to a simplified approach based on the coupled-dipole approximation. The systematic identification of the grazing diffracted orders, combined wit...

Matching long-period grating modes and localized plasmon resonances: effect on the sensitivity of the grating to the surrounding refractive index.

The sensitivity and dynamical range of an optical-fiber transducer consisting of a long-period grating coated with gold nanoparticles is investigated. For a grating with an 80μm spatial periodicity, the resonances close to the turning point lie within the 450-900nm spectral range. Employing a bottom-up production route, the localized surface plasmon resonance of gold nanoparticles is matched to the grating resonances; it is shown that this results in an increase in the refractive index sensitivity of the device. The device also shows increased dynamic range and enhanced refractive index sensitivity in water.