Ultra-narrow Bandwidth Research Articles

420-nm Faraday Optical Filter With 2.7-MHz Ultranarrow Bandwidth Based on Laser Cooled 87Rb Atoms

Using laser cooling technology, we demonstrate an ultranarrow-bandwidth Faraday anomalous dispersion optical filter (FADOF) which operates on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5^{2}\text{S}_{1/2}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F = 2$ </tex-math></inline-formula> ) to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6^{2}\text{P}_{3/2}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F' = 3$ </tex-math></inline-formula> ) transition at 420 nm based on cold 87Rb atoms. The filter achieves a single 2.7(2) MHz passband at a peak transmission of 3.2%. It is the narrowest bandwidth FADOF known at present, as the influence of Doppler broadening on the bandwidth narrowing of FADOF is effectively overcome by cold atoms. This ultranarrow-bandwidth FADOF has a figure of merit (FOM) value of 11.85 GHz−1, which is seven times larger than the largest FOM achieved by thermal vapor atomic optical filter published to date. Furthermore, the scheme can be extended to almost all kinds of atomic optical filters and may find applications in active Faraday optical frequency standard and self-stabilizing laser.

Narrowband mid-infrared thermal emitters based on the Fabry-Perot type of bound states in the continuum.

The development of narrow-band thermal emitters operating at mid-infrared (MIR) wavelengths is vital in numerous research fields. However, the previously reported results obtained with metallic metamaterials were not successful in achieving narrow bandwidths in the MIR region, which suggests low temporal coherence of the obtained thermal emissions. In this work, we demonstrate a new design strategy to realize this target by employing the bound state in the continuum (BIC) modes of the Fabry-Perot (FP) type. When a disk array of high-index dielectric supporting Mie resonances is separated from a highly reflective substrate by a low refractive index spacer layer with appropriate thickness, the destructive interference between the disk array and its mirror with respect to the substrate leads to the formation of FP-type BIC. Quasi-BIC resonances with ultra-high Q-factor (>103) are achievable by engineering the thickness of the buffer layer. This strategy is exemplified by an efficient thermal emitter operating at a wavelength of 4.587 µm with the on-resonance emissivity of near-unity and the full-width at half-maximum (FWHM) less than 5 nm even along with consideration of metal substrate dissipation. The new thermal radiation source proposed in this work offers ultra-narrow bandwidth and high temporal coherence along with the economic advantages required for practical applications, compared to those infrared sources made from III-V semiconductors.

Strong field enhancement and hot spot manipulation based on anapole state in Si disk-ring metasurface

In this work, a metasurface consisting of disk-ring Si nanostructures on a SiO2 substrate is proposed and numerically simulated. The Si disk-ring structure can excite and manipulate anapole state, and periodic array supports lattice resonance simultaneously, which can improve the near-field enhancement in the gap region. The maximum E-field enhancement can be up to 346 times, and a transmission dip with an ultra-narrow bandwidth of 0.34 nm and a Q-factor of 4460 is shown. When the central disk has an offset, the near-field enhancement can be affected. In the case of y-direction offset, the maximum E-field enhancement can be up to 468 times. The near field energy is concentrated in the gap region, easy to interact with probing molecule in surrounding environment. Meanwhile, the designed metasurface shows a sensing performance of 387 nm/RIU. The strong energy confinement and high Q-factor of the proposed structure give an opportunity to enhance the interaction between light and matter, which may have potential applications in nonlinear optics, optical sensors and surface enhanced Raman scattering.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Design an all-optical 10-channel demultiplexer using ultra-narrow bandwidth filters based on nano-ring resonators in square lattice index

Design an all-optical 10-channel demultiplexer using ultra-narrow bandwidth filters based on nano-ring resonators in square lattice index

High-resolution silicon photonic sensor based on a narrowband microwave photonic filter

Microwave photonic sensors are promising for improving sensing resolution and speed of optical sensors. In this paper, a high-sensitivity, high-resolution temperature sensor based on microwave photonic filter (MPF) is proposed and demonstrated. A micro-ring resonator (MRR) based on silicon-on-insulator is used as the sensing probe to convert the wavelength shift caused by temperature change to microwave frequency variation via the MPF system. By analyzing the frequency shift with high-speed and high-resolution monitors, the temperature change can be detected. The MRR is designed with multi-mode ridge waveguides to reduce propagation loss and achieves an ultra-high Q factor of 1.01 × 106. The proposed MPF has a single passband with a narrow bandwidth of 192 MHz. With clear peak-frequency shift, the sensitivity of the MPF-based temperature sensor is measured to be 10.22 GHz/°C. Due to higher sensitivity and ultra-narrow bandwidth of the MPF, the sensing resolution of the proposed temperature sensor is as high as 0.019 °C.Graphical

Ultra-narrow bandwidth mid-infrared thermal emitters achieved with all-dielectric metasurfaces

Narrow bandwidth and highly efficient mid-infrared (MIR) thermal emitters with low manufacturing cost and simple geometries have been continuously pursued by scientists. Unfortunately, conventional MIR thermal emitters based on metallic metamaterials suffer from large bandwidths mainly due to dissipations losses. In this work, we present our experimental demonstration of MIR thermal emitters with ultra-narrow bandwidth by exploiting the quasi bound-state-in-the-continuum mode supported by an all-dielectric metasurface structure. The whole structure is composed of a zigzag array of germanium elliptical disks, separated from the gold conductive substrate by a dielectric spacer layer made from the refractory material of Al2O3. Our experimental results demonstrate that even with an amorphous form of evaporated materials for both dielectrics, an emission bandwidth around 80 nm in the MIR can be obtained, one order of magnitude narrower than those achieved with metallic metamaterials. The bandwidth can in principle be further narrowed by a large extent when dielectrics of better quality are used. We further demonstrate with numerical results that this thermal emitter features additional advantages including high directionality, linear polarization, and large spectral tunability. Our results represent a significant step towards the realization of high-performance and low-cost MIR thermal emitters.

Ultra-Narrow Bandwidth Fabry-Perot Optical Filter Illuminated by a Gaussian Beam

We implement an ultra-narrow bandwidth (UNB) Fabry-Perot (FP) optical filter that has two flat mirrors illuminated by a normally incident Gaussian beam from an optical fiber. Owing to the finite waist radius of the Gaussian beam, the UNB FP optical filter’s transmittance curve deviates significantly from that of the plane wave incidence case. Applying an amplified spontaneous emission to the UNB FP optical filter, we find the waist radius of the Gaussian beam from the beat noise spectrum of the UNB FP optical filter output. Then, the transmittance of the UNB FP optical filter is evaluated by expanding the Gaussian beam as infinitely many plane waves. With this measurement method, we implement the UNB FP optical filter having its optical bandwidth down to 270 MHz.

Single Longitudinal Mode Parity-Time Symmetric Brillouin Fiber Laser Based on Lithium Niobate Phase Modulator Sagnac Loop

A single longitudinal mode (SLM) parity-time (PT) symmetric Brillouin fiber laser (BFL) based on z-cut lithium niobate phase modulator (LN-PM) Sagnac loop with Hz level linewidth is proposed and experimentally verified. A 10 km single-mode fiber provide SBS gain, while an LN-PM Sagnac loop consisting of an LN-PM and two polarization controllers (PCs) is used to achieve PT symmetry. Two mutually linked feedback loops supporting orthogonally polarized light are created by utilizing inherent birefringence of LN waveguide from LN-PM without electrical signal modulation. One of the feedback loops experiences gain in a clockwise direction and the other loss in a counterclockwise direction. The polarization state of the stokes injected into the LN-PM is controlled by adjusting the PC, and SLM BFL can be achieved when the gain and loss are well matched and exceed the coupling coefficient to break the PT symmetry. Compared with existing BFL studies, this design does not call for frequency matching of several composite cavity structures or precise control of ultra-narrow bandwidth bandpass filters. In addition, a frequency-locking system based on Pound-Drever-Hall technology is employed to lessen the instability and external disturbances brought on by the ring cavity. In the experiment, the linewidth of PT symmetry BFL based on LN-PM Sagnac loop is 3.85 Hz, according to the measured linewidth of 77 Hz at the −20 dB power point. A 65 dB optical signal-to-noise ratio and a 43 dB maximum side mode suppression ratio are measured. Furthermore, the PT symmetry BFL's wavelength is tuned between 1550 and 1550.41 nm.

One-pot synthesis of multicolor carbon quantum dots: One as pH sensor, one with ultra-narrow emission as fluorescent sensor for uric acid

In this work, tricolor carbon quantum dots (CQDs) including blue-, green-, and orange-CQDs were prepared in one-pot. Impressively, the full width at half maximum of blue-CQDs narrowed greatly to 8 nm, serving as the narrowest bandwidth emissive CQDs so far, to our knowledge, which even compared to colloidal semiconductor quantum dots. Based on the blue-CQDs exhibited highly selective and sensitive ‘on-off’ response towards Fe3+ and the outstanding antioxidant properties of uric acid, we constructed ‘on-off-on’ switching sensor to achieve quantitative measurement of uric acid. The green-CQDs served as simple pH sensor qualitatively judging acid rain based on ultrasensitive spectral redshift property at pH < 5.6. The sensors based on CQDs withstood actual sample matrix effects, pH and temperature change, and long-term storage. The ultra-narrow bandwidth emissive CQDs and fluorescent sensors based on distinctive optical properties of CQDs are meaningful and attractive research directions.

Polarization-selective narrow band dual-toroidal-dipole resonances in a symmetry-broken dielectric tetramer metamaterial.

Here we propose a metasurface consisting of symmetry-broken dielectric tetramer arrays, which can generate polarization-selective dual-band toroidal dipole resonances (TDR) with ultra-narrow linewidth in the near-infrared region. We found, by breaking the C4v symmetry of the tetramer arrays, two narrow-band TDRs can be created with the linewidth reaching ∼ 1.5 nm. Multipolar decomposition of scattering power and electromagnetic field distribution calculations confirm the nature of TDRs. A 100% modulation depth in light absorption and selective field confinement has been demonstrated theoretically by simply changing the polarization orientation of the exciting light. Intriguingly, it is also found that absorption responses of TDRs on polarization angle follow the equation of Malus' law in this metasurface. Furthermore, the dual-band toroidal resonances are proposed to sense the birefringence of an anisotropic medium. Such polarization-tunable dual toroidal dipole resonances with ultra-narrow bandwidth offered by this structure may find potential applications in optical switching, storage, polarization detection, and light emitting devices.

Plasmon resonances of graphene-assisted core-bishell nanoparticles

We study the Localized Surface Plasmon Resonance (LSPR) in graphene-assisted core-bishell nanoparticles which consist of a graphene layer (outer shell) wrapped around a metal shell and either a dielectric or a metal core. Small nanoparticles with a size much smaller than the wavelength of incident light are assumed, and the quasi-static approximation is applied to develop analytic equations to describe the absorption, scattering, and extinction efficiencies . The proposed nanostructures exhibit two LSPRs; one is in the visible range and corresponds to a plasmon mode of the core-inner shell composite, while the second lies in the near infrared (NIR) and is induced by the graphene plasmons excited at the outer shell. Interestingly, the LSPR of graphene has an ultra-narrow bandwidth and can be tuned in the NIR by altering the physical parameters of graphene, such as the Fermi energy and the number of graphene layers. Therefore, the LSPR peak of graphene is promising for medical applications. In addition, the LSPR of graphene can be tuned to the visible range near the position of the first LSPR, resulting in two narrow linewidth peaks. These resonance peaks could be beneficial for highly sensitive LSPR-based sensors.

Contra-Directional Coupling Phase-Shifted Grating with High Q-Factor and Ultra-Narrow Bandwidth

Contra-Directional Coupling Phase-Shifted Grating with High Q-Factor and Ultra-Narrow Bandwidth

An ultra-narrow-band optical filter based on zero refractive index metamaterial

Owing to the photonic band gap effect and defect state effect, photonic metamaterials have received much attention in the design of narrow bandpass filters, which are the key devices of optical communication equipment such as wavelength division multiplexing devices. In this work, based on zero-index metamaterial (ZIM), a compact filter with both high peak transmission coefficient and ultra-narrow bandwidth is proposed. The photonic metamaterial with conical dispersion and Dirac-like point is achieved by optimizing the structure and material component parameters of dielectric rods with square lattice in air. It is demonstrated that a triply degenerate state can be realized at the Dirac-like point, which relates this metamaterial to a zero-index medium with effective permittivity and permeability equal to zero simultaneously. Electromagnetic (EM) wave can propagate without any phase delay at this frequency, and strong dispersion occurs in the adjacent frequency cone, leading to dramatic changes in optical properties. We introduce a ZIM into photonic metamaterial defect filter to compress the bandwidth to the realization of ultra-narrow bandpass filter. The ZIM is embedded into the resonant cavity of line defect filter, which is also composed of dielectric rods with square lattice in air. In order to increase the sensitivity of the phase change with frequency, the Dirac-like frequency is adjusted to match the resonant frequency of the filter. We study the transmission spectrum of the structure through the COMSOL Multiphysics simulation software, and find that the peak width at half-maximum of the filter decreases as the thickness of ZIM increases, and the peak transmittance is still high when bandwidth is greatly compressed. The zero phase delay inside the slab can be observed. Through field distribution analysis, the zero-phase delay and strong coupling characteristics of electromagnetic field are observed at peak frequency. The comparison with conventional photonic metamaterials filter is discussed. We believe that this work is helpful in investigating the realization of ultra-narrow bandpass filters.

Single-frequency linearly polarized Q-switched fiber laser based on Nb2GeTe4 saturable absorber

We report a single-frequency linearly polarized Q-switched fiber laser based on an Nb2GeTe4 saturable absorber (SA). The Nb2GeTe4 SA triggers passive Q-switching of the laser, and an un-pumped Yb-doped fiber together with a 0.08-nm-bandwidth polarization-maintaining fiber Bragg grating (FBG) acts as an ultra-narrow bandwidth filter to realize single-longitudinal-mode (SLM) oscillation. The devices used in the laser are all kept polarized, so as to ensure linearly polarized laser output. Stable SLM linearly polarized Q-switching operation at 1064.6 nm is successfully achieved, producing a laser with a shortest pulse width of 1.36 μs, a linewidth of 28.4 MHz, a repetition rate of 28.3 kHz–95.9 kHz, and a polarization extinction ratio of about 30 dB. It is believed that the single-frequency linearly polarized pulsed fiber laser studied in this paper has great application value in gravitational wave detection, beam combining, nonlinear frequency conversion, and other fields.

Cold-atom optical filtering enhanced by optical pumping

Atomic optical filters such as Faraday anomalous dispersion optical filters (FADOFs) or similar technologies can achieve very narrow optical bandwidth close to the scale of atomic linewidth, which can be greatly reduced in cold atoms. However, limited by the number of cold atoms and the size of the cold atomic cloud, the number of atoms interacting with the laser is reduced, and the transmission remains as low as 2%. In this work, we introduce the optical pumping into the cold atomic optical filter to solve this problem. Circular polarized optical pumping can produce polarization of the atomic ensemble and induce dichromatic as well as the Faraday rotation. We demonstrate a cold-atom optical filter which operates on the 87Rb 52S1/2 (F=2) to 52P3/2 (F′=2) transition at 780 nm. The filter achieves an ultranarrow bandwidth of 6.6(4) MHz, and its peak transmission is 15.6%, which is nearly 14 times higher than that of the cold-atom optical filter realized by Faraday magneto-optic effect. This scheme can be extended to almost all kinds of atomic optical filters and may find applications in self-stabilizing laser and active optical clock.

Narrow linewidth parity-time symmetric Brillouin fiber laser based on a dual-polarization cavity with a single micro-ring resonator.

A narrow linewidth parity-time (PT) symmetric Brillouin fiber laser (BFL) based on dual-polarization cavity (DPC) with single micro-ring resonator (MRR) is proposed and experimentally investigated. A 10 km single-mode fiber provides SBS gain, while a DPC consisting of optical coupler, polarization beam combiner and a MRR, is used to achieve PT symmetry. Due to the reciprocity of light propagation in the MRR, the PT symmetry BFL based on DPC implements two identical feedback loops that are connected to one another, one with a Brillouin gain coefficient and the other with a loss coefficient of the same magnitude, to break a PT symmetric. Compared with existing BFL studies, this design does not call for frequency matching of compound cavities structures or without ultra-narrow bandwidth bandpass filters. In the experiment, the 3-dB linewidth of PT symmetry BFL based on DPC with single MRR is 11.95 Hz with the threshold input power of 2.5 mW, according to the measured linewidth of 239 Hz at the -20 dB power point. And a 40 dB maximum mode suppression ratio are measured. Furthermore, the PT symmetry BFL's wavelength is tuned between 1549.60 and 1550.73 nm. This design with single longitudinal mode output can be applied to high coherent communication systems.

All-Dielectric Nanostructured Metasurfaces with Ultrahigh Color Purity and Selectivity for Refractive Index Sensing Applications

In this study, two kinds of nanostructured metasurfaces are presented and compared to exhibit ultrahigh color purity and selectivity in the visible wavelength range. The proposed nanostructured metasurfaces are composed of all-dielectric materials, which are Si3N4 fabricated on quartz substrates to form one-dimensional (1D) and two-dimensional (2D) nanogratings (NGs). These nanostructured metasurfaces exhibit over 98 % reflection intensity with ultra-narrow bandwidths less than 20 nm to perform vivid red, green, and blue colors, implying the potentials in high-saturation display. Among nanostructured metasurfaces, 2D-NGs allow a wider range of incident angle than 1D-NGs and show polarization-independent characteristic owing to the symmetric arrangement in nanoscale in the incident plane. Furthermore, the numerical simulation results prove that 1D-NGs and 2D-NGs exhibit single- and dual-resonance when exposed on the ambient environment with different refractive indexes. Such excellent optical performances verify their potentials not only in high-resolution display but also in chemical sensing applications.

Ultra-narrow bandwidth and large tuning range single-passband microwave photonic filter based on Brillouin fiber laser

A scheme to perform ultra-narrow filter bandwidth and high frequency selectivity microwave photonic filter (MPF) with wide tuning range is proposed and experimentally demonstrated. The ultra-narrow bandwidth of the MPF is implemented by a Brillouin laser resonator, which is composed of a cascaded ring Fabry–Pérot (CR-FP) resonator formed by a main cavity with a cavity length of 100 m and a secondary cavity with a cavity length of 10 m. A Brillouin laser resonator is formed by the main cavity to obtain an extremely narrow comb-shaped Brillouin gain spectrum. The Vernier effect of the double ring cavity can effectively suppress the side modes, thereby realizing the narrow linewidth Brillouin laser filtering. Besides, two identical tunable lasers provide the stimulated Brillouin scattering (SBS) pump light and optical carrier signal for this device respectively. After the interaction between the Brillouin gain spectrum and the optical modulation signal, the filter passband can be stably tuned by simply changing the wavelength of SBS pump light. The experimental results show that the microwave photonic filter can be stably tuned in the frequency range of 0–20 GHz, the out-of-band rejection ratio is about 20 dB, and the minimum 3 dB bandwidth is about 114 Hz.

Silver mirror for enhancing the magnetic plasmon resonance and sensing performance in plasmonic metasurface

We report a novel method for enhancing magnetic plasmon resonances (MPRs) and sensing performance of metasurface consisting of a 1D Ag nanogroove array by using an opaque Ag mirror. The Ag mirror can block the transmission channel of light, so the radiative damping of MPRs excited in Ag nanogrooves is strongly reduced, and therefore the linewidth of MPRs is noticeably decreased. Because of ultra-narrow bandwidth and great magnetic field enhancement at MPRs, the metasurface shows very high sensitivity (S = 700 nm RIU−1, S* = 70 RIU−1) and figure of merit (FOM = 100, FOM* = 628), which holds great potential in the label-free biomedical sensing.