Open waveguides are widely used in modern photonic devices, such as microstructured fiber filters and sensors. Their absorption and transmission spectra are the most important properties in determining the overall performance of the photonic devices. The imaginary parts of their eigenvalues have been commonly used to calculate the absorption and consequently the transmission spectra. Here we show that this formulism is generally incorrect and not consistent with the simulation results obtained by the beam propagation method. We revisit the fundamental theory for the absorption of open waveguides and present a general formulism. We found that parity-time-symmetry transitions, which have been conventionally ignored, play a critical role in the properties of the coupled waveguide. The absorption and transmission are highly dependent on the physical length of the system. On the basis of our findings, optimization criteria for designing photonic sensors and filters are presented.

The high-temperature sensitivity of the silicon material index limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependent-wavelength-shift microring resonator, designing a broadband athermal waveguide becomes a significant task. In this work, we propose a broadband athermal waveguide that shows a low effective thermo-optical coefficient of ±1×10 6/K from 1400 to 1700 nm. The proposed waveguide shows a low-loss performance and stable broadband athermal property when it is applied to ring resonators, and the bending loss of ring resonators with a radius of >30 μm is 0.02 dB/cm.

Optical coupling behavior and associated effects in two-dimensional implant-defined coherently coupled vertical-cavity surface-emitting laser (VCSEL) arrays are studied via both experiments and theoretical calculations. Experiments show that optical coupling between array elements can enhance the array’s output power. Additionally, optical coupling via leaky optical fields can provide extra optical gain for the array elements, which can then reduce the thresholds of these elements. Elements can even be pumped without current injection to emit light by receiving a strong leaky optical field from other array elements. Optical coupling can also cause unusual phenomena: the central elements in large-area coherently coupled VCSEL arrays that lase prior to the outer elements when the arrays are biased, or the average injection current required for each element to lase, which is much lower than the threshold for a single VCSEL. Theoretical calculations are performed to explain the experimental results.

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear- and circular-polarization incidences.

We investigate the diffraction of the guided modes of a dielectric slab waveguide on a simple integrated structure consisting of a single dielectric ridge on the surface of the waveguide. Numerical simulations based on aperiodic rigorous coupled-wave analysis demonstrate the existence of sharp resonant features and bound states in the continuum (BICs) in the reflectance and transmittance spectra occurring at the oblique incidence of a transverse-electric (TE)-polarized guided mode on the ridge. Using the effective index method, we explain the resonances by the excitation of cross-polarized modes of the ridge. Formation of the BICs are confirmed using a theoretical model based on coupled-wave theory. The model suggests that the BICs occur due to the coupling of quasi-TE and quasi-transverse-magnetic modes of the structure. Simple analytical expressions for the angle of incidence and the ridge width predicting the location of the BICs are obtained. The existence of high-Q resonances and BICs enables using the considered integrated structure for sensing, transformation of optical signals, and enhancing nonlinear light–matter interactions. Due to the Lorentzian line shape of the resonances near the BICs, the structure is also promising for filtering applications.

Short pulsed fiber lasers have been widely made using single-walled carbon nanotubes as a saturable absorber (SA). However, most of the currently used devices can only operate in one determined operation state with an unchangeable modulation SA depth in the cavity, which significantly limits their application in photonic devices. In this paper, well-aligned carbon nanotube arrays are synthesized using zeolite AlPO4-5 as a template, which features anisotropic optical absorption. The linear optical absorption of the as-synthesized carbon nanotube arrays can easily be tuned by adjusting a polarization controller, thus providing a tunable modulation depth for the carbon nanotube SA. By exploiting this SA in an erbium-doped fiber laser cavity, both Q-switched and mode-locked pulsed lasers are achieved by simply adjusting a polarization controller under a fixed pump power of 330 mW. In addition, the repetition rate of the Q-switching and pulse duration of the mode-locking can be tuned according to the variation of modulation depth. Moreover, soliton molecules can be obtained when the modulation depth of the SA is 4.5%.

Low-dimensional nanomaterials, owing to their unique and versatile properties, are very attractive for enormous electronic and optoelectronic applications. PbS quantum dots (QDs), characterized by a large Bohr radius and size-tunable bandgap, are especially interesting for photonic applications in the near-infrared region. Here, oleic acid capped colloidal PbS QDs as a saturable absorber are investigated for ultrashort-pulse generation. The PbS QDs exhibit outstanding nonlinear saturable absorption properties at 1550 nm: a modulation depth up to 44.5% and a thermal damage threshold larger than 30 mJ/cm2. By incorporating PbS QDs into a fiber laser, a transform-limited soliton pulse as short as 559 fs with a bandwidth of 4.78 nm is realized at 1563 nm. Numerous applications may benefit from the nonlinear saturable absorption properties of PbS QDs, such as near-infrared pulsed lasers and modulators.

We experimentally demonstrate for the first time, to the best of our knowledge, an all-fiber passively mode-locked laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for saturable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode fiber. Using the nonlinear multimode interference structure with 48.8 mm length step-index multimode fiber, the modulation depth has been measured to be ～5%. The passively mode-locked laser output pulses have a central wavelength of 1596.66 nm, bandwidth of 2.18 nm, pulsewidth of ～625 fs, and fundamental repetition rate of 8.726 MHz. Furthermore, the influence of total cavity dispersion on the optical spectrum, pulse width, and output power is investigated systematically by adding different lengths of single-mode fiber and dispersion compensation fiber in the laser cavity.

In past years, rare-earth-doped fluoride fiber lasers (FFLs) have developed rapidly in the mid-infrared (mid-IR) region. However, due to the lack of fiber optic devices and challenge of fluoride fiber splicing, most mid-IR FFLs have been demonstrated with free-space optic elements, limiting the advantages of all-fiber lasers for flexible delivery, stability, and compactness. Here, we report, to the best of our knowledge, the first pulsed all-fiber FFL in the mid-IR region. By taking advantage of the integration of black phosphorus flake, stable Q-switched and mode-locked pulses were obtained at 2.8 μm wavelength. We believe that this all-fiber design will promote the application of pulsed FFL in the mid-IR region.

The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at gigahertz and sub-terahertz frequencies. The frequency of the optically excited magnetization precession is easily tunable by the external magnetic field. On the other hand, the initial phase of the precession marginally depends on the magnetic field. Here we demonstrate an approach for the control of the precession phase by variation of the pump beam direction. In particular, we consider the case when the magnetization precession is excited by obliquely incident pump pulses in a magnetic dielectric film placed in the in-plane magnetic field. Theoretical consideration predicts that the initial phase should appear for a non-zero in-plane component of the pump wavevector orthogonal to the external magnetic field. Experimental studies confirm this conclusion and reveal that the phase grows with increase of the in-plane wavevector component. Variation of phase by 15 deg is demonstrated. Potentially, the phase could be changed even more pronouncedly by more than 90 deg. This work provides a simple way for additional manipulation with optically excited magnetization dynamics, which is of importance for different spintronic applications.

We have studied the two- and three-photon absorption (2PA and 3PA) properties of Mn-doped CsPbCl3 two-dimensional nanoplatelets (2D NPs) and cubic nanocrystals. Compared with their cubic counterparts, the Mn-doped 2D NPs exhibit stronger quantum confinement effects that can more efficiently enhance their dopant-carrier exchange interactions and multiphoton absorption. More specifically, the maximum volume-normalized 2PA and 3PA cross sections of the 2D NPs were 6.8 and 7.2 times greater than those of their cubic counterparts, respectively, reaching up to 1237 GM/nm3 in the visible light band and 2.24×10 78 cm6·s2·photon 2/nm3 in the second biological window, respectively.

As a kind of two-dimensional transition metal dichalcogenide material, tungsten diselenide (WSe2) has attracted increasing attention, owing to its gapped electronic structure, relatively high carrier mobility, and valley pseudospin, all of which show its valuable nonlinear optical properties. There are few studies on the nonlinear optical properties of WSe2 and correlation with its electronic structure. In this paper, the effects of spatial self-phase modulation (SSPM) and distortion influence of WSe2 ethanol suspensions are systematically studied, namely, the nonlinear refractive index and third-order nonlinear optical effect. We obtained the WSe2 dispersions SSPM distortion formation mechanism, and through it, we calculated the nonlinear refractive index n2, nonlinear susceptibility χ(3), and their wavelength dependence under the excitation of 457 nm, 532 nm, and 671 nm lasers. Moreover, by use of its strong and broadband nonlinear optical response, all-optical switching of two different laser beams due to spatial cross-phase modulation has been realized experimentally. Our results are useful for future optical devices, such as all-optical switching and all-optical information conversion.

We have developed a cost-effective and highly compact 100-Gb/s coarse wavelength division multiplexing (CWDM) transmitter optical subassembly (TOSA) using lens-free hybrid integration. To achieve large alignment tolerances, distributed feedback laser diodes (DFB-LDs) are butt-coupled to a four-channel silica-based planar lightwave circuit (PLC) arrayed waveguide grating, with the silicon sub-mounts and PLC adhesively bonded. Then, a flexible printed circuit is employed to connect the internal DFB-LDs and the exterior of the TOSA package for radiofrequency signal transmission, eliminating the expensive ceramic feed-through. The packaged CWDM TOSA, which is 15.8 mm×7.0 mm×6.0 mm in size, shows a side-mode suppression ratio of >40 dB, a 3 dB bandwidth of >18 GHz, and error-free transmission with an average optical output power of >0 dBm and dynamic extinction ratio of >4.0 dB at 25.78125 Gb/s over a 10 km single-mode fiber for all four lanes.

We experimentally demonstrate extraction of silicon waveguide geometry with subnanometer accuracy using optical measurements. Effective and group indices of silicon-on-insulator (SOI) waveguides are extracted from the optical measurements. An accurate model linking the geometry of an SOI waveguide to its effective and group indices is used to extract the linewidths and thicknesses within respective errors of 0.37 and 0.26 nm on a die fabricated by IMEC multiproject wafer services. A detailed analysis of the setting of the bounds for the effective and group indices is presented to get the right extraction with improved accuracy.

We report low-noise, high-performance single transverse mode 1.3 μm InAs/GaAs quantum dot lasers monolithically grown on silicon (Si) using molecular beam epitaxy. The fabricated narrow-ridge-waveguide Fabry–Perot (FP) lasers have achieved a room-temperature continuous-wave (CW) threshold current of 12.5 mA and high CW temperature tolerance up to 90°C. An ultra-low relative intensity noise of less than 150 dB/Hz is measured in the 4–16 GHz range. Using this low-noise Si-based laser, we then demonstrate 25.6 Gb/s data transmission over 13.5 km SMF-28. These low-cost FP laser devices are promising candidates to provide cost-effective solutions for use in uncooled Si photonics transmitters in inter/hyper data centers and metropolitan data links.

Raman spectroscopy is a versatile tool widely used for comprehensive probing of crystal information. However, generally when applied in narrow-band-gap van der Waals crystals, it is liable to form a “bug,” especially in transition-metal-dichalcogenides (TMDs). That is, several resonant Raman-scattering (RS) modes will inevitably appear in the Raman spectra with strong intensity, interfering with the desired signal of optical-phonon modes. Here, we propose cross-sectional polarized Raman scattering capable of regulating the intensity of RS modes in accordance with quasi-sinusoidal rules. Typically, for MoS2 and WS2, when the polarization vector of excited light is along the c axis of the crystal, all RS modes are nearly completely “expunged” from the Raman spectra. The mechanism is that the absorption of most TMDs with a space group of R3m for the light polarized along the c axis is infinitesimal, thus forming a small coupling intensity of electronic states excited optically and acoustic-phonon modes at point M, which in turn restrain the appearance of RS modes. The regulating strategy proposed can be applied to other van der Waals crystals so as to obtain a high signal-to-noise ratio Raman spectrum.