Broadband Phase Research Articles

Highly efficient broadband second harmonic generation mediated by mode hybridization and nonlinearity patterning in compact fiber-integrated lithium niobate nano-waveguides

The inherent trade-off between efficiency and bandwidth of three-wave mixing processes in χ2 nonlinear waveguides is the major impediment for scaling down many well-established frequency conversion schemes onto the level of integrated photonic circuit. Here, we show that hybridization between modes of a silica microfiber and a LiNbO3 nanowaveguide, amalgamated with laminar χ2 patterning, offers an elegant approach for engineering broadband phase matching and high efficiency of three-wave mixing processes in an ultra-compact and natively fiber-integrated setup. We demonstrate exceptionally high normalized second harmonic generation (SHG) efficiency of up to ηnor ≈ 460% W−1 cm−2, combined with a large phase matching bandwidth of Δλ ≈ 100 nm (bandwidth-length product of Δλ · L ≈ 5 μm2) near the telecom bands, and extraordinary adjustment flexibility.

Ultrabroadband etalon-free detection of infrared transients by van-der-Waals contacted sub-10-µm GaSe detectors.

We demonstrate ultrabroadband electro-optic detection of multi-THz transients using mechanically exfoliated flakes of gallium selenide of a thickness of less than 10 µm, contacted to a diamond substrate by van-der-Waals bonding. While the low crystal thickness allows for extremely broadband phase matching, the excellent optical contact with the index-matched substrate suppresses multiple optical reflections. The high quality of our structure makes our scheme suitable for the undistorted and artifact-free observation of electromagnetic waveforms covering the entire THz spectral range up to the near-infrared regime without the need for correction for the electro-optic response function. With the current revolution of chemically inert quasi-two-dimensional layered materials, we anticipate that exfoliated van-der-Waals materials on index-matched substrates will open new flexible ways of ultrabroadband electro-optic detection at unprecedented frequencies.

Ultrathin Metal–Organic Framework: An Emerging Broadband Nonlinear Optical Material for Ultrafast Photonics

AbstractCrystalline porous metal–organic frameworks (MOFs) with nanometer‐sized void spaces, large surface areas and ordered reticular motifs have offered a platform for achieving disruptive successes in divisional fields. Great progress in exploring the linear and nonlinear optical features of MOFs has been achieved, yet third‐order optical nonlinearities in two‐dimensional (2D) MOFs have rarely been studied. Here, a broadband nonlinear optical amplitude modification and phase shift are demonstrated in a few‐layer nickel‐p‐benzenedicarboxylic acid MOF (Ni‐MOF). The calculated bandgap of Ni‐MOF decreases from 3.12 eV to 0.85 eV as the doping of Ni ions increases, indicating the ability of this material to be used for optical amplitude modulation from the visible to the near‐infrared region, which is experimentally confirmed via a Z‐scan technique. The determined third‐order optical nonlinearities resemble those of other low‐dimensional nonlinear optical materials, suggesting the wide potential of Ni‐MOF for application in optoelectronics. As an example, a Ni‐MOF‐based saturable absorber was implemented into fiber resonators to demonstrate its broadband mode‐locking operations. A femtosecond laser pulse was readily obtained in the telecommunication wavelength window in an integrated all‐fiber resonator. Considering the chemical compatibility and rich variability, these primary investigations pave the way towards advanced photonics based on multifeature MOF materials.

Digital correction method for realizing a phase-stable dual-comb interferometer

A phase-stable dual-comb interferometer measures materials' broadband optical response functions, including amplitude, frequency, and phase, making it a powerful tool for optical metrology. Normally, the phase-stable dual-comb interferometer is realized via tight phase-locking methods. This paper presents a post-correction algorithm that can compensate for carrier wave phase noise and interferogram timing jitter. The compensating signal is a beat between two combs using a free-running continuous wave laser as an optical intermediary. In our experiment, sub-hertz relative linewidth, ~1 ns relative timing jitter, and 0.2 rad precision in the carrier phase is demonstrated.

Electric-field induced second-harmonic generation of femtosecond pulses in atmospheric air

Electric-field induced second harmonic generation with a femtosecond Ti:sapphire pump laser in atmospheric air is investigated. Phase matching between fundamental and second harmonic waves is achieved via quasi-phase matching, employing periodic electrode arrangements on printed circuit boards with a total length of about 0.5 m. The weak chromatic dispersion of the atmospheric air enables broadband phase matching conditions and makes this method tailorable for frequency doubling broadband femtosecond laser pulses. A maximum energy of 0.13 nJ is observed in a 20 nm bandwidth around 395 nm.

Integrated‐Resonant Units: Integrated Resonant Unit of Metasurfaces for Broadband Efficiency and Phase Manipulation (Advanced Optical Materials 12/2018)

Integrated‐Resonant Units: Integrated Resonant Unit of Metasurfaces for Broadband Efficiency and Phase Manipulation (Advanced Optical Materials 12/2018)

Ultra-thin and -broadband microwave magnetic absorber enhanced by phase gradient metasurface incorporation

Based on the effect of anomalous reflection and refraction caused by the circularly cross-polarized phase gradient metasurface (PGM), an ultra-thin and -broadband composite absorber composed of metasurface and conventional magnetic absorbing film is proposed and demonstrated in this paper. In the case of keeping nearly the same thickness of absorbing layer, the equivalent thickness of magnetic absorbing film is enlarged by the effect of anomalous reflection and refraction, resulting in the expansion and improvement of the absorbing bandwidth and efficiency in low microwave frequency. A biarc metallic sub-cell for circularly crossed polarization is adopted to form a broadband phase gradient, by the means of rotating the Pancharatnam–Berry phases. As indicated in the experimental results, the fabricated 3.6 mm-thick absorber can averagely absorb microwave energy with the specular reflection below −10 dB in the frequency interval of 2–12 GHz, which shows a good match with simulated results. Due to ultra-thin thickness and ultra-wide operating bandwidth, the proposed application of PGM in absorbing can provide an alternative way to enhance the absorbing property of current absorbing materials.

A Wideband Class-AB Power Amplifier With 29–57-GHz AM–PM Compensation in 0.9-V 28-nm Bulk CMOS

A wideband amplitude to phase (AM–PM) compensated class-AB power amplifier (PA) suitable for highly integrated fifth-generation phased arrays is designed in 0.9-V 28-nm CMOS without RF ultra-thick top metal. Design techniques to realize broadband impedance transformation, power division/combining, and phase distortion linearization are discussed. Further, second-order effects due to practical layout constrains imposed by deep-scaled technologies are addressed and simple design solutions are proposed. The designed PA shows a measured $S_{21} -3$ dB bandwidth (BW) from 29 to 57 GHz (65%) with $|\text {AM}-\text {PM}| up to $P_{1~\text {dB}}$ . The measured $P_{\mathrm{ sat}}$ is 15.1 dBm over >56% $\text {BW}_{-1~\text {dB}}$ , with a peak power added efficiency of 24.2%. When a signal with wide modulation BW is applied, the realized PA enables up to 10.1-, 8.9-, and 5.9-dBm average $P_{\mathrm{ OUT}}$ while amplifying a 1.5-, 3-, and 6-Gb/s 64-quadratic-amplitude modulation, respectively, at 34 GHz. The in-band and out-of-band linearity measured in error vector magnitude and adjacent channel power ratio are always better than −25 dB and −30 dBc, respectively, without any digital pre-distortion.

Integrated Resonant Unit of Metasurfaces for Broadband Efficiency and Phase Manipulation

AbstractIntegrated‐resonant units (IRUs), incorporated with multiple resonators into one building block or one resonator with multiple modes, show a great capacity for achieving controllable smooth and linear phase dispersion as well as amplitude manipulation over a continuous and broad bandwidth. Based on an IRU library designed in the wavelength range of 400 to 667 nm, three achromatic deflectors showing constant steering angles of 9.5°, 19°, and 28°, respectively, are numerically validated. Achromatic metalenses with various numerical aperture (NA) values are further experimentally demonstrated, displaying an unvaried focal length throughout the bandwidth of 420–650 nm (≈50% bandwidth to the central wavelength). The focusing efficiency of the achromatic metalens with NA = 0.124 achieves 26.31%, 19.71%, and 20.37%, respectively, at wavelengths of 420, 550, and 650 nm. In addition, a multi‐nanorod IRU design is numerically optimized to achieve above 50% conversion efficiency from visible to near‐infrared (400–1400 nm). Such IRU design is then employed to construct a versatile polarization convertor, generating six different polarization states simultaneously upon one linear‐polarized illumination. The IRU approach with broadband control of amplitude and phase response provides an unprecedented platform in realizing multifunctional full‐color metadevices.

Laser Frequency Analysis Aided by Electronic Frequency Dividers

Conventional laser spectral analysis relies on direct measurement of power spectral density (PSD), which is not able to distinguish certain types of phase and frequency modulations without ambiguity. In this paper, we propose and demonstrate a modified scheme aimed at improving the specificity in laser frequency analysis. Our method is based on PSD measurement of the downshifted laser spectrum in the radio-frequency range but introduces an electronic frequency divider (EFD) prior to the spectrum analyzer. By monitoring the spectral changes caused by the EFD, we show, both theoretically and experimentally, that common scenarios of phase and frequency modulations, such as wideband frequency modulation (FM) and broadband phase modulation, can be differentiated without ambiguity. Moreover, the new method allows for quantitative assessment of wideband FM parameters such as modulation frequency, modulation index, and frequency deviation, which is not possible with any conventional spectral analysis methods. This technique may find potential applications in laser frequency metrology, microwave photonics, optical sensing, and radar/lidar technologies.

Dualfunction Dielectric Resonator as Antenna and Phase-Delay-Line Load: Designs of Compact Circularly Polarized/Differential Antennas

Dualfunction cylindrical dielectric resonator (DR) used as the load of a broadband phase delay line (PDL) and as a DR antenna (DRA) is presented for the first time. In the broadband PDL design, the cylindrical DR is used as the load to improve impedance match and phase shift. At the same time, the delay line is used to feed the same DR for the antenna part. By designing 90° and 180° PDLs, circularly polarized (CP) and differential DRAs can be obtained, respectively. As compared with those using a hybrid coupler, the new CP and differential DRAs can be made much more compact. Also, the external $50~\Omega $ load required by the hybrid coupler can now be avoided. For demonstration, two CP and differential cylindrical DRAs for 2.4 GHz WLAN applications are designed, fabricated, and tested. Good agreement between the measured and simulated results is observed. The CP design has an overlapping axial ratio and impedance passband of 24.2%. For the differential design, a broadside radiation mode with very low cross-polarization level is obtained.

Broadband mixing of {mathscr{P}}{mathscr{T}}-symmetric and {mathscr{P}}{mathscr{T}}-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer

Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enabling {mathscr{P}}{mathscr{T}}-symmetric and {mathscr{P}}{mathscr{T}}-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the {mathscr{P}}{mathscr{T}}-symmetric and {mathscr{P}}{mathscr{T}}-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to high-epsilon regime.

Scanning Enhanced Low-Profile Broadband Phased Array With Radiator-Sharing Approach and Defected Ground Structures

In this paper, a scanning enhanced low-profile broadband phased array is presented. This array is based on the radiator-sharing approach and the defected ground structures (DGSs). In this phased array, the radiator-sharing approach has been implemented by using metasurface antenna, and the scanning range has been remarkably enhanced by reducing element spacing. An attractive behavior in broadband impedance matching has still been obtained, in spite that closely spaced feedings are adopted. Moreover, the scanning performance is considerably improved by applying meander-line slots cut from the ground plane, forming DGS. In order to validate this proposed design, a nine-element linear phased array with DGSs has been fabricated and measured. Based on the uniform magnitude and the progressive phase, the capability of wide-angle scanning can be achieved over the measured bandwidth of 23.1% (4.6–5.8 GHz). At the center frequency of 5.2 GHz, this array scans up to 50° and the realized gain varies from 14.76 to 11.85 dBi.

Midinfrared wavelength conversion in hydrogenated amorphous silicon waveguides

Midinfrared (MIR) wavelength conversion based on degenerate four-wave mixing is theoretically investigated in hydrogenated amorphous silicon (a-Si:H) waveguides. The broadband phase mismatch is achieved in the normal group-velocity dispersion regime. The conversion bandwidth is extended to 900 nm, and conversion efficiency of up to −14 dB with a pump power of 70 mW in a 2-mm long a-Si:H rib waveguides is obtained. This low-power on-chip wavelength converter will have potential for application in a wide range of MIR nonlinear optic devices.

Single-Layer Broadband Phase Shifter Using Multimode Resonator and Shunt $\lambda $ /4 Stubs

This paper presents a new method for single-layer broadband phase shifter without needing any coupling structure. The phase shifter consists of a multimode resonator and two shunt $\lambda $ /4 stubs. Meanwhile, the bandwidth of proposed phase shifters can be achieved more than 100%. For prescribed return loss, phase shift value, and phase error, this paper develops a synthesis approach to determine all parameters of designed phase shifter. In addition, both the return-loss bandwidth and phase-error bandwidths can be considered simultaneously so as to determine the return loss and phase error for a desired bandwidth. The 45°, 90°, 135°, and 180° phase shifter are theoretically designed and presented. To validate the design method, a broadband 90° phase shifter is designed, fabricated, and measured. The synthesized, simulated, and measured results are found in good agreement with each other, and the experimental results show that the phase shifter achieves 90° ± 5°, $\vert S_{11}\vert dB, and insertion loss < 0.7 dB over the bandwidth from 1.43 to 4.45 GHz (102.7%).

Instantaneous voltage as an alternative to power- and phase-based interpretation of oscillatory brain activity

For decades, oscillatory brain activity has been characterized primarily by measurements of power and phase. While many studies have linked those measurements to cortical excitability, their relationship to each other and to the physiological underpinnings of excitability is unclear. The recently proposed Function-through-Biased-Oscillations (FBO) hypothesis (Schalk, 2015) addressed these issues by suggesting that the voltage potential at the cortical surface directly reflects the excitability of cortical populations, that this voltage is rhythmically driven away from a low resting potential (associated with depolarized cortical populations) towards positivity (associated with hyperpolarized cortical populations). This view explains how oscillatory power and phase together influence the instantaneous voltage potential that directly regulates cortical excitability. This implies that the alternative measurement of instantaneous voltage of oscillatory activity should better predict cortical excitability compared to either of the more traditional measurements of power or phase. Using electrocorticographic (ECoG) data from 28 human subjects, the results of our study confirm this prediction: compared to oscillatory power and phase, the instantaneous voltage explained 20% and 31% more of the variance in broadband gamma, respectively, and power and phase together did not produce better predictions than the instantaneous voltage. These results synthesize the previously separate power- and phase-based interpretations and associate oscillatory activity directly with a physiological interpretation of cortical excitability. This alternative view has implications for the interpretation of studies of oscillatory activity and for current theories of cortical information transmission.

Ultra-wideband RCS reduction using novel configured chessboard metasurface**Project supported by the National Natural Science Foundation of China (Grant Nos. 61372034 and 61501499).

A novel artificial magnetic conductor (AMC) metasurface is proposed with ultra-wideband 180° phase difference for radar cross section (RCS) reduction. It is composed of two dual-resonant AMC cells, which enable a broadband phase difference of 180°±30° from 7.9 GHz to 19.2 GHz to be achieved. A novel strategy is devised by dividing each rectangular grid in a chessboard configuration into four triangular grids, leading to a further reduction of peak bistatic RCS. Both full-wave simulation and measurement results show that the proposed metasurface presents a good RCS reduction property over an ultra-wideband frequency range.

Bandwidth analysis of non-collinear fourth and fifth harmonic generation in nonlinear uniaxial crystals

The optimal angle bandwidth and wavelength bandwidth of fourth-harmonic generation (FHG) and fifth-harmonic generation (FIFHG) of the 1064[Formula: see text]nm laser are analyzed based on the numerical calculation results of non-collinear type-I and type-II phase matching processes for general nonlinear uniaxial crystals with 1[Formula: see text]cm length. The non-collinear phase matching angles and effective nonlinear coefficients of FHG and FIFHG are calculated. The optimal angle bandwidth and wavelength bandwidth are obtained. The results are beneficial to broadband and efficient non-collinear phase matching FHG and FIFHG experiments and studies.

Fano-resonance-assisted metasurface for color routing

Controlling the phase of an electromagnetic field using plasmonic nanostructures provides a versatile way to manipulate light at the nanoscale. Broadband phase modulation has been demonstrated using inhomogeneous metasurfaces with different geometries; however, for many applications such as filtering, hyperspectral imaging and color holography, narrowband frequecy selectivity is a key functionality. In this work, we demonstrate, both theoretically and experimentally, a narrowband metasurface that relies on Fano resonances to control the propagation of light. By geometrically tuning the sub-radiant modes with respect to a fixed super-radiant resonance, we can create a phase modulation along the surface within a narrow spectral range. The resulting anomalous reflection measured for such a Fano-resonant metasurface exhibits a 100 nm bandwidth and a color routing efficiency of up to 81% at a central wavelength of λ=750 nm. The design flexibility provided by this Fano-assisted metasurface for color-selective light manipulation is further illustrated by demonstrating a highly directional color-routing effect between two channels, at λ=532 and 660 nm, without any crosstalk.

基于复合左右手传输线的移相器设计

本文利用复合左右手传输线特殊的非线性相位响应特性，提出一种宽带移相器的设计方法。宽带移相器的使用可以控制天线波束在宽带内实现较固定的倾角。该方法由主副两个通路组成四端口的差分移相，其中主通路由复合左右手传输线组成，副通路由传统的右手传输线构成。两通路在很宽的带宽内相位变化一致，能够实现宽带移相。通过HFSS软件仿真设计出90˚移相器，在8~15.9 GHz (66.4%)带宽内相位不平衡度为±3˚，带内回波损耗 < −12 dB，最大插入损耗为0.3 dB。该结构设计简单，加工方便。 A design of broadband phase-shifter is presented by utilizing the unique nonlinear phase response of the composite right-left handed transmission line. The use of wideband phase-shifter can control antenna beam to achieve a fixed angle in broadband. The four-port differential phase- shifter consists of two paths, one path using composite right-left handed transmission line while the other using conventional right hand transmission line. A stable phase difference can be obtained over a wide bandwidth, so that broadband phase shift can be achieved. Simulated results show that the 90˚, phase-shift with phase imbalance of ±3 can be achieved over a broad bandwidth from 8 GHz to15.9 GHz (66.4%), with the return losses more than 12 dB and the insertion losses less than 0.3 dB. Moreover, the proposed structure is easy to design and convenient to fabricate.