Large Spectral Range Research Articles

Probing coherence properties of shallow implanted NV snsembles under different oxygen terminations

Abstract Nitrogen vacancy (NV) color centers in diamond have shown great potential for various applications in quantum technology due to their long coherence times, high sensitivity to magnetic fields and atomic scale resolution. However, one major challenge in utilizing near surface NV centers is the decoherence caused by spins and charges fluctuating on the surface, which affects the spin properties of the sensors. To reduce the induced noise, various oxygen surface treatments such as low power oxygen plasma treatment and annealing under oxygen atmosphere have been explored to terminate the diamond surface and reduce its impact on NV coherence. We showed that the NV center's coherence time can be enhanced up to a factor of 3 over a large spectral range of noise. Double electron electron resonance (DEER) measurements revealed an extra source of decoherence, scaling similarly as the P1 spin bath. The improvement in coherence times is accompanied with an increase in measured ketone/ether content and reduction of sp2 signal in X-ray photoelectron spectroscopy (XPS) measurements. Finally we compared the performance of different NV ensembles and surface treatments for sensing external proton spins. The oxygen annealing is an effective procedure of enhancing the spin coherence times and reducing broad band spin noise experienced by shallow implanted ensemble NV centers in diamond.

Composition‐Graded Perovskite Microwire Toward Broad Wavelength Tunable Single‐Mode Lasing

AbstractContinuously manipulating the resonant wavelength of lasing modes within a large spectral range is of great significance for expanding device functionality. Here, to synthesize a single CsPbClxBr3‐x perovskite microwire with the energy bandgap gradient spanning from 2.33 to 2.83 eV along the length direction defined as the axis by using the vapor‐phase anion exchange method is proposed. A high‐quality (≈103), wide range (≈60 nm), and continuously tunable single‐mode laser is achieved in as‐prepared perovskite microwire alloy, which provides both gain media and microresonator. Simultaneously, the exciton recombination dynamics and atomic‐scale interdiffusion mechanisms at different components are clarified through time‐resolved photoluminescence (PL) spectra and theoretical calculations. The vacancy defects have a significant impact on the interdiffusion of halogen anions, excitonic recombination lifetime, and fluorescence quantum efficiency. The work provides a new strategy for the construction of new‐type broadband tunable lasers and high‐precision microspectrometers.

Toward Gigahertz Photodetection with Transition Metal Dichalcogenides.

ConspectusTransition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical-chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Echelon grating refractive index sensor.

There are few reports on optical refractive index sensors that have both high resonant-wavelength resolution (RWR) and high refractive index sensitivity (RIS). Herein, based on an echelon grating, we design a refractive index sensor that combines the two advantages together. The principal fringe of echelon grating has a small full width at half maximum and a good signal-to-noise ratio, leading to a high RWR. The wavefront splitting interference makes the sensor have high RIS. The large free spectral range (FSR) of the principal fringes expands the dynamic range of the sensor. The experimentally realized RWR, RIS, and FSR are 2 × 10-2 nm, 1.14 × 104 nm/RIU (RIU: refractive index unit), and 130 nm, respectively. The detection limit of refractive index is 1.59 × 10-6 RIU. The dynamic range of the sensor is 1.14 × 10-2 RIU. In addition, there are schemes to improve RWR and RIS, which can further reduce the detection limit of refractive index. The echelon grating refractive index sensor features low detection limit, low cost, high stability, and good robustness.

Sharp bend and large FSR ring resonator based on the free-form curves on a thin-film lithium niobate platform.

Sharp bends are crucial for large-scale and high-density photonics integration on thin-film lithium niobate platform. In this study, we demonstrate low-loss (<0.05 dB) and sharp bends (Reff = 30 µm) using free-form curves with a 200-nm-thick slab and a rib height of 200 nm on x-cut lithium niobate. Employing the same design method, we successfully realize a compact fully-etched ring resonator with a remarkably large free spectral range of 10.36 nm experimentally. Notably, the equivalent radius of the ring resonator is a mere 15 µm, with a loaded Q factor reaching 2.2 × 104.

Design of ultrahigh-Q silicon microring resonators based on free-form curves.

A design method for ultrahigh-Q microring resonators (MRRs) based on Bezier free-form curves was proposed and demonstrated. An MRR consisting of a specially designed 180° waveguide bend, a directional coupler, and two low-loss multi-mode strip waveguides was designed. The free-form curves were used to increase the degree of freedom in the design, shaping the waveguide bend with a gradient width and curvature. This design effectively reduced the propagation loss caused by the roughness of waveguide sidewalls and the mode mismatch loss caused by the excitation of high order modes. The small effective radius of only 20µm enabled the MRR to have a large free spectral range (FSR) and a compact and flexible structure. The MRR was manufactured using a standard process provided by foundry and measured to have an ultrahigh loaded Q factor of 1.86 × 106 and a FSR of about 1 nm.

Generation of infrared photon pairs by spontaneous four-wave mixing in a CS2-filled microstructured optical fiber

We experimentally demonstrate frequency non-degenerate photon-pair generation via spontaneous four-wave mixing from a novel CS2-filled microstructured optical fiber. CS2 has high nonlinearity, narrow Raman lines, a broad transmission spectrum, and also has a large index contrast with the microstructured silica fiber. We can achieve phase matching over a large spectral range by tuning the pump wavelength, allowing the generation of idler photons in the infrared region, which is suitable for applications in quantum spectroscopy. Moreover, we demonstrate a coincidence-to-accidental ratio of larger than 90 and a pair generation efficiency of about 10-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{-2}$$\\end{document} per pump pulse, which shows the viability of this fiber-based platform as a photon-pair source for quantum technology applications.

Design and analysis of dual‐band plasmonic band pass filter using meandering step impedance resonator

AbstractIn this article, we demonstrated the minimum significant dielectric thickness of silicon dioxide (SiO2) below the diffraction limit for the confinement in the metal–insulator–metal (MIM) waveguide and utilizing this idea, we further exhibited the detail design analysis of dual‐band plasmonic meandering step impedance resonator (MSIR), which can achieve a high quality‐factor compared to a traditional step impedance resonator (SIR). MSIRs have sharper wavelength selectivity compared to SIRs, making them more suitable for applications that require precise wavelength tuning. The tunability of reflection and transmission characteristics with respect to design parameters has been reported. The designed MSIR has demonstrated a good passband response at wavelengths λ = 1414‐nm (E‐band) and 1577‐nm (L‐band) as well as turnability with design parameters. This filter achieved a large frequency spectral range of 139 nm and a high Q factor of ~3 × 104. This work enhances the capability of on‐chip filtering in the plasmonic platform.

Tunable four-wave mixing enabled by a self-phase modulation of chirped pulses.

We experimentally demonstrate how a concatenation of the standard and microstructure fiber segments permits adjusting the four-wave mixing sideband position over a large spectral range by varying the chirp of an input pulsed pump at a fixed wavelength in the presence of a self-phase modulation. The blue- and redshifted sidebands can stand aside over ∼200 nm and ∼450 nm from the pump, respectively, which agrees well with the numerical simulations. We validate our approach by showing the feasibility of CARS imaging.

Normal mode analysis in multi-coupled non-Hermitian optical nanocavities

Coupled optical cavities are an attractive on-chip optical platform for realizing quantum mechanical concepts in electrodynamics and further developing non-Hermitian photonics. In such systems, an intercavity interaction is often considered as a key parameter to understand the system’s behaviors but its estimation/calculation is typically limited for some simplified systems owing to extended complexities. For example, multi-coupled photonic crystal (PhC) nanocavities exhibiting strong resonances with a large free spectral range can serve as an excellent test-bed to study non-Hermitian optical properties when spatially non-uniform gain is introduced. However, the detailed quantitative analysis such as spectral tracing of cavity normal modes is often limited in commercially available numerical tools because of the required massive computation resources. Herein, we report on a concept of spatial overlap integrals (SOIs) between the eigenmodes in non-coupled PhC nanocavities and utilize them to obtain the intercavity interactions in passively coupled PhC nanocavity systems. With the help of coupling strength factors calculated from SOIs, we were able to fully exploit the coupled mode theory (CMT) and readily trace the detailed spectral behaviors of normal modes in various multi-coupled PhC nanocavities. Full-wave numerical simulation results verified the proposed method, revealing that the characteristics of original eigenmodes from non-coupled PhC nanocavities can act as key building blocks for analyzing the normal modes of multi-coupled PhC nanocavities. We further applied this SOI method to various multi-coupled PhC nanocavities with non-symmetric optical gain/loss distributions and successfully observed the unusual spectral evolution of normal modes and the correspondingly occurring unique non-Hermitian behaviors.

A graphene-metal hybrid biosensor for SEIRA spectroscopy applications

Detecting the building blocks of all living organisms plays a vital role in medical diagnosis and biological research. Infrared (IR) spectroscopy is one of the techniques for detecting biomolecules, such as DNA, RNA, and lipids, through their vibrational fingerprints. IR spectroscopy provides biosensing by directly measuring changes in absorption spectra; however, nanometric-size molecules reveal a weak signal-to-noise ratio with IR light, which is a disadvantage for sensitive bio-detection. Surface-Enhanced Infrared Spectroscopy (SEIRA) based on different structures and materials has been widely used to overcome this problem. In this study, a SEIRA platform based on a hybrid graphene-metal nanoantenna array is designed by combining the enhanced sensitivity of graphene with its electrical tunability and strong-field enhancement with metallic structures. The plasmonic resonance of graphene nano-slit is dynamically tuned to detect fingerprints of protein monolayer at different frequencies. The same detection behavior has been statically demonstrated by metallic antennas. Our results indicate that the sensitivity of the SEIRA sensor platform is quite high due to the combination of graphene and metal hybrid structure compared to the metal resonator alone because of the strong light confinement in the graphene layer. The proposed biosensor can detect the Amid-I and Amid-II bonds, which create relatively strong vibrational fingerprints, as well as the Amide-III bond, which is a very weak bond of a monolayer protein-goat-mouse Immunoglobulin (IgG) protein. It can also be a very useful characterization tool for identifying the molecular fingerprints of unknown chemicals and molecules with its capability to span a large spectral range either dynamically or statically. The strong light absorption via metallic structures compensates for the large areal coverage of graphene sheets, which is quite a challenge for dynamic tuning in real applications.

Nested micro-ring refractive index sensor based on a subwavelength grating waveguide and theVernier effect.

In this paper, a nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect is proposed. In this scheme, the nested micro-ring structure is combined with a subwavelength grating structure to enhance the contact area between the optical field and the analyte, and the wavelength offset is doubled through the Vernier effect. The proposed sensor can effectively increase sensing sensitivity, taking into account the improvement of the free spectral range. This structure enables the device to reach a sensitivity of 8030nm/RIU near 1550nm wavelength in a deionized water environment, with a detection limit of 5.659×10-5RIU and free spectral range of 41.956nm. The device suggested in this study has a greater reduced footprint than the conventional micro-ring resonant sensor, measuring just 35µm×25µm. Due to its high integration, high sensitivity, and large free spectral range compared to conventional micro-ring resonant sensors, such structures are of great value in biosensing and environmental monitoring.

Tunable Optical Activity in Twisted Anisotropic Two-Dimensional Materials.

Twisted van der Waals structures exhibit a variety of unusual electrical and optical phenomena and could provide a powerful means for designing nanodevices with tunable chiral properties. However, programming intrinsic chiral properties of the film on the atomic scale remains a great challenge due to the limitations of fabrication and measurement techniques. Here, we report a highly tunable large optical activity of twisted anisotropic two-dimensional (2D) materials, including black phosphorus (BP), ReS2, PdSe2, and α-MoO3, by varying the twist angle between the stacked layers. The chirality can be deliberately tailored through the engineering of the symmetry, band structure, and anisotropy of 2D materials, demonstrating the high tunability of the chirality. The results show the highest thickness-normalized ellipticity value (13.8 deg μm-1, twisted ReS2) and ellipticity value (1581 mdeg, twisted BP) among the systems based on 2D materials. It is also shown that the chiroptical response exists in an extremely large spectral range from the visible to the infrared. Furthermore, the twisted ReS2 enabled spin-selective control of the information transformation. These results show that highly controllable chirality in twisted 2D anisotropic materials has considerable potential in on-chip polarization optics, nano-optoelectronics, and biology.

A silicon micro-ring resonator with unprecedented large free spectral range via double injection

A silicon-on-insulator submicron multimode micro-ring resonator symmetrically coupled to two bus waveguides via bent asymmetrical directional coupler to achieve unprecedented-large FSR is proposed based on double-injection configuration. An analytical model based on the transfer matrix method is developed to analyze the proposed structure and the physical mechanism for the FSR enlargement is thoroughly analyzed. In addition, the coupling angle is cleverly selected to improve the spectral shape and the extinction ratio at the resonant wavelengths. Finite difference time domain simulations show that ultra large FSRs of 180 nm and 206 nm can be obtained.

Broadband microwave photonic channelizer with high optical spectrum efficiency based on odd- and even-order local optical frequency combs

Microwave photonic channelizers assisted by coherent dual optical frequency combs (OFCs) have been widely considered for wideband radio frequency (RF) spectrum analysis. However, it requires dual OFCs with a large free spectral range (FSR) and multiple comb lines to support a wide RF coverage, leading to a complicated system structure and low optical spectrum efficiency. To this end, we propose a simplified, high optical spectrum-efficiency microwave photonic channelizer, wherein the odd-order and even-order OFCs with a double frequency spacing are generated and isolated to directly slice two frequency-shifted modulated optical spectra into a series of subbands. N comb lines can support 4 × N channels but take up a significantly reduced optical spectrum range. Partial experiments and simulations are presented to demonstrate the scheme. By exploiting a four-lines odd-order OFC and three-lines even OFC, a wideband RF spectrum range of 0–56 GHz is divided into 28 channels with a channel bandwidth of 2 GHz, and various wideband signals are spectrally analyzed with a high measurement accuracy and large image rejection suppression higher than 30 dB. Moreover, a 16-quadrature amplitude modulation (16QAM) vector signal with a center frequency of 7 GHz and a bit rate of 1 Gbps is channelized and successfully recovered with a good constellation diagram.

Metamaterial graphene photodetector with bandwidth exceeding 500 gigahertz.

Although graphene has met many of its initially predicted optoelectronic, thermal, and mechanical properties, photodetectors with large spectral bandwidths and extremely high frequency responses remain outstanding. In this work, we demonstrate a >500 gigahertz, flat-frequency response, graphene-based photodetector that operates under ambient conditions across a 200-nanometer-wide spectral band with center wavelengths adaptable from <1400 to >4200 nanometers. Our detector combines graphene with metamaterial perfect absorbers with direct illumination from a single-mode fiber, which breaks with the conventional miniaturization of photodetectors on an integrated photonic platform. This design allows for much higher optical powers while still allowing record-high bandwidths and data rates. Our results demonstrate that graphene photodetectors can outperform conventional technologies in terms of speed, bandwidth, and operation across a large spectral range.

Tunable holography with independent transflective information channels enabled by interleaved soft materials

Holography is promising to fully record and reconstruct various fundamental properties of light. However, the simultaneous realization of multifunctional holographic channels, prompt tunability and broadband working spectrum for holography remains challenging. Here, we propose and experimentally demonstrate a transflective holography scheme based on the interleaved soft materials of achiral and chiral liquid crystals (LCs). The desired phase profile is programmed into the tailored surface alignment of the achiral and chiral LCs by photopatterning, while the coexistence of such soft materials is configured by a micro-regional patterned polymer template. Correspondingly, a pair of independent holographic images is achieved along the transmission and reflection channels, which is also characterized by the distinctive response to the polarization of incidence. Benefitting from the excellent electrical responsiveness, the transmission holography can be conveniently tuned among a large spectral range under a mild voltage, and meanwhile the broadband working spectrum of reflection holography is well maintained. We expect this work to enrich the functionalities of LC holography and inspire more advances for the higher integration level, greater information capacity of holography.

Research on Optical Spectrum Processing for Photonic-Assisted Broadband RF Cross- Eye Jamming System

Cross-eye jamming technology has been used in electronic warfare, which attempts to protect a military platform from monopulse tracking radars. The cross-eye jamming technology has the ability of inducing angular errors to the monopulse tracking radars by transmitting two jamming signals with equal amplitudes and opposite phases. At present, high operation frequency and broadband cross-eye jamming system has been rarely demonstrated. Therefore, the present cross-eye jamming systems are hard to jam frequency-agile radar or multi-band radar, whose carrier frequency covers a large spectral range. In this paper, a photonic-assisted broadband radio frequency (RF) cross-eye jamming system is proposed and experimentally demonstrated. To achieve effective jamming effect, the intercepted radar signal is modulated to optical carriers and the phase shift is realized by optical spectrum processing. The relationship between system parameter tolerances and jamming effects are also simulated. Furthermore, the RF transfer function of X-band, Ku-band and K-band has been obtained in the experiments. The experimental results show that the amplitude and phase mismatch are below 2.15 degrees and 0.4 dB, respectively. Calculated cross-eye gain is 39dB.

Silicon Microspheres for Super‐Planckian Light Sources in the Mid Infrared

AbstractSilicon microspheres with a diameter in the range of 2–3 micrometers constitute photonic nanocavities that emit light through their Mie resonances when heated at high temperatures. At 500–600 °C these microresonators show a particular mid‐infrared (MIR) emission dominated by the lowest order modes. Such resonances feature a large free spectral range, about 600 cm−1, and a high proximity to the critical coupling condition. In fact, resonances with high‐quality factor, around 160 are found. It corresponds to the limit of detection of their measuring setup, being 600 the theoretical value. Most importantly, several modes emit light above the calculated black body limit because they feature an optical absorption cross‐section larger than their geometric one. All these characteristics set silicon microspheres as very promising zero‐dimensional materials for developing micrometric and sub‐wavelength light sources in the MIR.

Femtosecond laser line-by-line tilted Bragg grating inscription in single-mode step-index TOPAS/ZEONEX polymer optical fiber.

In this Letter, we demonstrate 8°-tilted fiber Bragg grating (TFBG) inscription in single-mode step-index TOPAS/ZEONEX polymer optical fibers (POFs) using a 520 nm femtosecond laser and the line-by-line (LbL) writing technique. As a result of the tilt angle and the fiber refractive index, a large spectral range of cladding mode resonances covering 147 nm is obtained. The evolution of the transmitted spectrum is analyzed as a function of the surrounding refractive index (SRI) in a large range from 1.30 to 1.50. The cutoff cladding mode shows a refractive index sensitivity of 507 nm/RIU (refractive index unit). For single-resonance tracking near the cutoff mode, the sensitivity is at least 6 nm/RIU, depending on the exact wavelength position of the cladding modes. The main originality of our work is that it produces, for the first time, to the best of our knowledge, a TFBG in POF that operates in the refractive index range of aqueous solutions. The sensing capability for a large range of refractive index values is also relevant for (bio)chemical sensing in different media.