Low Optical Loss Research Articles

Optimum core structural design of the polymer optical waveguides as edge couplers for co-packaging

We simulate the optical properties of polymer optical waveguides with different refractive index profiles in their cores as coupling components (edge couplers) between single-mode fiber and SiOx waveguides. In this paper, we focus on the single-mode operation of graded-index (GI) core polymer waveguides, for which we previously demonstrated low propagation loss under multimode operation. We design the optimum core structure (size and index contrast) for different refractive index profiles, and then demonstrate the unique optical properties of GI waveguides contributing to the low optical loss compared to the step-index counterparts, in particular, mode field diameter variation and taper angle tolerance.

Single-drive electro-optic frequency comb source on a photonic-wire-bonded thin-film lithium niobate platform.

Stable pulse and flat-top frequency comb generation are an indispensable component of many photonic applications, from ranging to communications. Lithium niobate on insulator is an excellent electro-optic (EO) platform, exhibiting high modulation efficiency and low optical loss, making it a fitting candidate for pulse generation through electro-optic modulation of continuous-wave (CW) light, a commonly utilized method for generating ultrashort pulses. Here, we demonstrate an on-chip electro-optic comb generation module on thin-film lithium niobate (TFLN) consisting of a Mach-Zehnder interferometer (MZI) amplitude modulator (AM) and a cascaded phase modulator (PM) system driven by a single-electrode drive. We show that when operated in the correct regime, the lithium niobate chips can generate frequency combs with excellent spectral power flatness. In addition, we optically package one of the pulse generator chips via photonic wire bonding. The pulses generated by the photonic-wire-bonded device are compressed to 840 fs pulse duration using an optical fiber and show extremely stable operation.

3D Laser Writing of Low-Loss Cross-Section-Variable Type-I Optical Waveguide Passive/Active Integrated Devices in Single Crystals.

Optical waveguides fabricated in single crystals offer crucial passive/active optical components for photonic integrated circuits. Single crystals possess inherent advantages over their amorphous counterpart, such as lower optical losses in visible-to-mid-infrared band, larger peak emission cross-section, higher doping concentration. However, the writing of Type-I positive refractive index modified waveguides in single crystals using femtosecond laser technology presents significant challenges. Herein, this work introduces a novel femtosecond laser direct writing technique that combines slit-shaping with an immersion oil objective to fabricate low-loss Type-I waveguides in single crystals. This approach allows for precise control of waveguide shape, size, mode-field, and refractive index distribution, with a spatial resolution as high as 700nm and a high positive refractive index variation on the order of 10-2, introducing new degrees of freedom to design and fabricate passive/active optical waveguide devices. As a proof-of-concept, this work successfully produces a 7mm-long circular-shaped gain waveguide (≈10µm in diameter) in an Er3+-doped YAG single crystal, exhibiting a propagation loss as low as 0.23dBcm-1, a net gain of ≈3dB and a polarization-insensitive character. The newly-developed technique is theoretically applicable to arbitrary single crystals, holding promising potential for various applications in integrated optics, optical communication, and photonic quantum circuits.

Rewritable Photonic Integrated Circuit Canvas Based on Low-Loss Phase Change Material and Nanosecond Pulsed Lasers.

Programmable photonic integrated circuits (PICs) are an increasingly important platform in optical science and engineering. However, current programmable PICs are mostly formed through subtractive fabrication techniques, which limits the reconfigurability of the device and makes prototyping costly and time-consuming. A rewritable PIC architecture can circumvent these drawbacks, where PICs are repeatedly written and erased on a single PIC canvas. We demonstrate such a rewritable PIC platform by selective laser writing a layer of wide-band-gap phase change material (PCM) Sb2S3 with a low-cost benchtop setup. We show arbitrary patterning with resolution up to 300 nm and write dielectric assisted waveguides with a low optical loss of 0.0172 dB/μm. We envision that using this inexpensive benchtop platform thousands of PIC designs can be written, tested, and erased on the same chip without the need for lithography/etching tools or a nanofabrication facility, thus reducing manufacturing cost and increasing accessibility.

Silicon‐Rich Nitride Refractive Index as a Degree of Freedom to Maximize Nonlinear Wave Mixing in Nanowaveguides

Silicon nitride is widely used in integrated photonics for optical nonlinear wave mixing due to its low optical losses combined with relatively high nonlinear optical properties and a wide‐range transparency window. It is known that a higher concentration of Si in silicon‐rich nitride (SRN) magnifies both the nonlinear response and optical losses, including nonlinear losses. To address the trade‐off, four‐wave mixing (FWM) is implemented in over a hundred SRN waveguides prepared by plasma‐enhanced chemical vapor deposition in a wide range of SRN refractive indices varying between 2.5 and 3.2 (measured in the C‐band). It is determined that SRN with a refractive index of about 3 maximizes the FWM efficiency for continuous‐wave operation, indicating that the refractive index of SRN is indeed a crucial optimization parameter for nonlinear optics applications. The FWM efficiency is limited by large nonlinear optical losses observed in SRN waveguides with indices larger than 2.7, which are not related to two‐photon absorption. Finally, the third‐order susceptibility and the nonlinear refractive index are estimated for multiple SRN refractive indices, and, specifically, the nonlinearities as large as and are estimated in a waveguide with an SRN refractive index of 3.2.

Dual-band high quality-factor perfect absorber based on uniaxial/biaxial anisotropic van der Waals polar crystals

Phonon polaritons (PhPs) in two-dimensional van der Waals crystals, such as hexagonal boron nitride (hBN) and α-phase molybdenum trioxide (α-MoO3), exhibit remarkable spatial confinement and extremely low optical losses, offering possibilities for the development of novel micro and nanophotonic devices at mid-infrared wavelengths. Here, we demonstrate the potential of the proposed Tamm structure to achieve dual-band high quality-factor perfect absorption within the mid-infrared range. This structure utilizes upper and lower spacer layers to isolate the anisotropic materials hBN and α-MoO3 from one-dimensional photonic crystals, where the Tamm phonon polaritons (TPhPs) are respectively excited at the interfaces, facilitating strong coupling with incident light and leading to significant dual-band perfect absorption phenomena. The results demonstrate that due to polariton resonances, the absorption peaks exhibit extremely narrow absorption bandwidths, with the maximum quality-factor reaching up to 297. In addition, tunable resonant wavelengths and polarization-dependent absorption properties are also demonstrated in the proposed Tamm structure. This study provides an alternative and straightforward approach for constructing dynamic and multifunctional optical devices such as optical modulators, tunable multi-band filters and biosensors.

Near-infrared mechanoluminescence sensor: A new method for on-site infrastructure detection.

Near-infrared mechanoluminescence is a phenomenon that produces high penetrating near-infrared light under external stimulation. Near-infrared light coincides with the biological window, lower optical loss, and the fact that the mechanoluminescence material is a medium that converts mechanical energy into light energy. The near-infrared mechanoluminescence material has potential application prospects in the fields of biological imaging, medical diagnosis, and monitoring of building materials. In this article, we report on a perovskite-type Sr3Sn2O7:Nd3+ near-infrared mechanoluminescence material, and its peaks locate in the first near-infrared window (800-1000 nm) and the second near-infrared window (1080, 1350 nm), respectively. Under the condition of pre-sintering with Li2CO3 as flux, the best sintering conditions are obtained, and the luminescence of material is in perfect agreement with the applied mechanical stress. In addition, a near-infrared mechanoluminescence sensor is proposed to solve the problem of building damage and timely maintenance.

Fabrication and Characterization of PDMS Waveguides for Flexible Optrodes.

With the growth of optogenetic research, the demand for optical probes tailored to specific applications is ever rising. Specifically, for applications like the coiled cochlea of the inner ear, where planar, stiff, and nonconformable probes can hardly be used, transitioning from commonly used stiff glass fibers to flexible probes is required, especially for long-term use. Following this demand, polydimethylsiloxane (PDMS) with its lower Young's modulus compared to glass fibers can serve as material of choice. Hence, the long-term usability of PDMS as a waveguide material with respect to variations in transmission and refractive index over time is investigated. Different manufacturing methods for PDMS-based flexible waveguides are established and compared with the aim to minimize optical losses and thus maximize optical output power. Finally, the waveguides with lowest optical losses (-4.8dB cm-1 ± 1.3dB cm-1 at 472nm) are successfully inserted into the optogenetically modified cochlea of a Mongolian gerbil (Meriones unguiculatus), where optical stimuli delivered by the waveguides evoked robust neuronal responses in the auditory pathway.

Electro-optic tuning in composite silicon photonics based on ferroionic 2D materials

Tunable optical materials are indispensable elements in modern optoelectronics, especially in integrated photonics circuits where precise control over the effective refractive index is essential for diverse applications. Two-dimensional materials like transition metal dichalcogenides (TMDs) and graphene exhibit remarkable optical responses to external stimuli. However, achieving distinctive modulation across short-wave infrared (SWIR) regions while enabling precise phase control at low signal loss within a compact footprint remains an ongoing challenge. In this work, we unveil the robust electro-refractive response of multilayer ferroionic two-dimensional CuCrP2S6 (CCPS) in the near-infrared wavelength range. By integrating CCPS into silicon photonics (SiPh) microring resonators (MRR), we enhance light-matter interaction and measurement sensitivity to minute phase and absorption variations. Results show that electrically driven Cu ions can tune the effective refractive index on the order of 2.8 × 10−3 RIU (refractive index unit) while preserving extinction ratios and resonance linewidth. Notably, these devices exhibit low optical losses and excellent modulation efficiency of 0.25 V.cm with a consistent blue shift in the resonance wavelengths among all devices for either polarity of the applied voltage. These results outperform earlier findings on phase shifters based on TMDs. Furthermore, our study demonstrates distinct variations in electro-optic tuning sensitivity when comparing transverse electric (TE) and transverse magnetic (TM) modes, revealing a polarization-dependent response that paves the way for diverse applications in light manipulation. The combined optoelectronic and ionotronic capabilities of two-terminal CCPS devices present extensive opportunities across several domains. Their potential applications range from phased arrays and optical switching to their use in environmental sensing and metrology, optical imaging systems, and neuromorphic systems in light-sensitive artificial synapses.

Optical Wireless Power Transmission under Deep Seawater Using GaInP Solar Cells

Optical wireless power transmission (OWPT) attracts attention because it enables wireless power transfer over longer distances than current wireless power transfer methods, irradiating laser light to a light-receiving element. In this study, an OWPT system was investigated under water and deep seawater using visible lasers with low optical absorption loss in water. Three laser beams (450 nm, 532 nm, and 635 nm) were transmitted through 30 cm, 60 cm, and 90 cm long tanks filled with tap water and deep seawater and were irradiated to 1.0 × 1.0 cm2 GaInP solar cells. The light reaching rate (ηop) of laser light and the system efficiency (ηsys) of the system (excluding the laser efficiency) were investigated. GaInP solar cells showed photo-electric conversion efficiencies of 30.6%, 40.3%, and 39.3% for 450 nm, 532 nm, and 635 nm irradiations, respectively. As a result, a 532 nm laser through a 90 cm water tank in tap water showed a 78.4% ηop and a 30.8% ηsys. Under deep seawater, a 532 nm laser through a 90 cm tank exhibited a 58.3% ηop and a 23.5% ηsys. A 532 nm green laser showed a higher efficiency than the other 450 nm and 635 nm lasers in this underwater system using GaInP solar cells.

Opto-electronic and thermophysical characteristics of A2TlAgF6 (A = Rb, Cs) for green technology applications.

Lead-free double perovskites are unique materials for transport and optoelectronic applications that use clean resources to generate energy. Using first-principle computations, this study thoroughly investigates the structural, thermoelectric, and optical attributes of A2TlAgF6 (A = Rb, Cs). Tolerance factor and formation energy estimates are used to verify that these materials exist in the cubic phase. Elastic constants with high melting temperature values are ductile when evaluated for mechanical stability using the Born stability criterion. The optical absorption band is adjusted from 2 to 4 eV via band gaps of 1.88 and 1.99 eV, as indicated by band structures. Analysis of optical properties reveals perfect absorption in the visible spectrum, whole polarization, and low optical loss. Furthermore, thermoelectric properties are assessed at 300, 500, and 700 K in the range of -0.5 to 3 eV for chemical potential (μ). The materials exhibit significant improvements in the Figure of Merit scale due to their elevated electrical conductivity, Seebeck coefficient, and extremely low thermal conductivity values.

Real-time characterizations of soliton explosions and pulsations in a Tm-doped fiber laser

Over the years, 2 μm mode-locked fiber laser has been widely developed as an important light source for various significant applications. Worth to note that, with the help of real-time spectroscopy technique, i.e., time-stretched dispersive Fourier transformation (TS-DFT), various complex nonlinear soliton dynamics have been revealed, such as soliton molecules, soliton explosions and soliton pulsations, enhancing our understanding of the ultrafast physics. However, due to the lack of appropriate dispersive components at 2 μm band with low optical transmission loss for TS-DFT measurement, the investigation of ultrafast soliton dynamical phenomena in the 2 μm mode-locked fiber lasers is still limited. In this work, we demonstrate the direct real-time observation of soliton explosion and pulsation phenomena at 2 μm band mode-locked fiber laser, for the first time as far as we know, by using specially designed linearly chirped fiber Bragg gratings as dispersive component for TS-DFT measurement. For explosions, we observe slight explosion and drastic explosion, respectively. Particularly, the soliton pulsating explosions are also characterized in real time where the explosions occur in the process of soliton pulsation. In addition, dual-wavelength soliton rains are also observed by carefully adjusting the cavity parameters. Our work provides supplements to the current investigation of 2 μm band ultrafast science, and might enrich our understanding of soliton dynamics.

Monolithic thin-film lithium niobate broadband spectrometer with one nanometre resolution

Miniaturised optical spectrometers are attractive due to their small footprint, low weight, robustness and stability even in harsh environments such as space or industrial facilities. We report on a stationary-wave integrated Fourier-transform spectrometer featuring a measured optical bandwidth of 325 nm and a theoretical spectral resolution of 1.2 nm. We fabricate and test on lithium niobate-on-insulator to take full advantage of the platform, namely electro-optic modulation, broad transparency range and the low optical loss achieved thanks to matured fabrication techniques. We use the electro-optic effect and develop innovative layouts to overcome the undersampling limitations and improve the spectral resolution, thus providing a framework to enhance the performance of all devices sharing the same working principle. With our work, we add another important element to the portfolio of integrated lithium-niobate optical devices as our spectrometer can be combined with multiple other building blocks to realise functional, monolithic and compact photonic integrated circuits.

Enhanced terahertz high-harmonic generation from high-Q quasi-bound states in the continuum empowered by permittivity-broken metasurface

The extraordinary emergence of all-dielectric resonant meta-photonics underpinned by high refractive index and low optical loss materials promises a standout platform for unprecedented manipulation and subwavelength control of light. Recent significant breakthroughs in meta-photonics have to do with the exploration of the non-radiative eigenmodes that lie inside the light cone, called the bound states in the continuum (BICs), which have demonstrated considerable potential to achieve high quality (Q) factors but require ultra-small structure asymmetry, complicating fabrication and hindering precise control of Q factors. Here, we propose a scheme to excite quasi-BICs by breaking symmetry in the permittivity of the comprising meta-atoms where small permittivity perturbation renders the same effect in the conventional geometrically asymmetric design. Empowered by the insignificant permittivity perturbation introduced from chemical doping and carrier injection, arbitrarily small permittivity asymmetry, that is, infinitely high-Q factor, can be precisely and dynamically tailored. As an example of application, we numerically show the THz range, exceptionally high conversion efficiency for both third-harmonic and fifth-harmonic generation from the permittivity-broken metasurface (MS) benefiting from the extreme field confinement at high-Q quasi-BICs resonance. Our results present a feasible and efficient strategy to mitigate the shortcomings of previous photonic platform and shall shed light on the advances of nonlinear all-dielectric THz-MS.

Broadband transmissive polarization rotator by gradiently twisted α -MoO3

Polarization engineering has been proven to enhance the capabilities of light manipulation and thus facilitate the development of integrated photonic devices. In this study, we introduce a polarization rotator based on a gradiently twisted α-MoO3 thin film, which works in the mid-infrared range and functions in a transmission mode. To be specific, the proposed device is constructed by gradiently twisted α-MoO3 multilayers with a subwavelength thickness of only 5 μm, namely, one-third of the working wavelength. Our analytical calculation demonstrates the efficacy of this subwavelength thin film rotator in converting a linearly polarized wave into its orthogonal counterpart, thanks to its chiral nature. The twisted α-MoO3 multilayers exhibit the capability to significantly manipulate dispersion characteristics while maintaining low optical losses, thereby enabling a wide bandwidth exceeding 2.5 THz with a polarization ratio surpassing 17 dB. Moreover, the operational frequency can be adjusted across a 3.4 THz range by altering the incident angle of the incident waves. This adaptable design, characterized by its polarization versatility, can be customized to suit practical applications within wireless communication, radar systems, optical switching, and imaging technologies.

Power-efficient programmable integrated multiport photonic interferometer in CMOS-compatible silicon nitride

Silicon nitride (SiN x ) is an appealing waveguide material choice for large-scale, high-performance photonic integrated circuits (PICs) due to its low optical loss. However, SiN x PICs require high electric power to realize optical reconfiguration via the weak thermo-optic effect, which limits their scalability in terms of device density and chip power dissipation. We report a 6-mode programmable interferometer PIC operating at the wavelength of 1550 nm on a CMOS-compatible low-temperature inductance coupled plasma chemical vapor deposition (ICP-CVD) silicon nitride platform. By employing suspended thermo-optic phase shifters, the PIC achieves 2× improvement in compactness and 10× enhancement in power efficiency compared to conventional devices. Reconfigurable 6-dimensional linear transformations are demonstrated including cyclic transformations and arbitrary unitary matrices. This work demonstrates the feasibility of fabricating power-efficient large-scale reconfigurable PICs on the low-temperature ICP-CVD silicon nitride platform.

Effect of hydrostatic pressure on opto-electronic, elastic and thermoelectric properties of the double perovskites Rb<sub>2</sub>SeX<sub>6</sub>(X=Cl,Br): a DFT study

Double perovskites find applications across a diverse range of situations and varying pressure conditions. In this work, Quantum ESPRESSO code with a plane wave basis set was used to study the opto-electronic, elastic, and thermoelectric properties of Rb2SeX6 (X=Cl, Br) double perovskites under hydrostatic pressure (0 - 8 GPa). Perdew-Burke-Ernzerhof for Solids (PBESol) with generalized gradient approximation (GGA) was used as exchange-correlation functional. The band gap values of the materials decrease under hydrostatic pressure. Rb2SeCl6 has a band gap value of 2.44 eV at 0 GPa, 2.21 eV at 2 GPa. Above 2 GPa, the material has a metallic nature. Rb2SeBr6 has a band gap value of 1.56 eV at 0 GPa, but has a metallic nature under hydrostatic pressure (2 GPa to 8 GPa). The optical properties results indicate that the materials exhibit maximum absorption, high reflectivity, low optical loss in the visible and ultraviolet regions, good optical conductivity, and a refractive index suitable for use in opto-electronic applications. The materials are confirmed to be mechanically stable under all the hydrostatic pressure values studied. Electrical conductivity, thermal conductivity, and Seebeck coefficient (S ) values of the studied materials increase with an increase in hydrostatic pressure and temperature. The maximum value of S for Rb2SeBr6 is 0.248 x 103 (m V/k), while for Rb2SeCl6, maximum S = 0.175 x 103 (m V/k). The positive values of S suggest that the predominant charge carriers of Rb2SeCl6/Br6 are holes. Also, Rb2SeBr6 has a figure of merit (ZT) value of 3.44, while for Rb2SeCl6, ZT = 1.07. Since the values of ZT are greater than unity, the two double perovskite materials have good ZT values for thermoelectric device engineering. The results also suggest that Rb2SeBr6 is a better thermoelectric material than Rb2SeCl6.

Perspectives and progress on wurtzite ferroelectrics: Synthesis, characterization, theory, and device applications

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.

Crafting Near‐Infrared Photonics via the Programmable Assembly of Organic Heterostructures at Multiscale Level

AbstractOrganic single crystals with near‐infrared (NIR) emission demonstrate their excellent optical communications from well photonic confinement and low optical waveguide loss, which are considered as competitive candidates toward advanced optoelectronics. However, the increasingly diverse and sophisticated application demands result in the complicated design of NIR devices, which is hardly realized solely by the intrinsic properties of individual crystals. Herein, a programmable assembly strategy is presented to fabricate organic heterostructures. Triphenylene (TP), pyrene (Py) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) are primary selected to prepared organic cocrystals with narrow band gap and near‐infrared emission. Importantly, the charge‐transfer alloy with tunable emission from 700 nm to 850 nm and branched heterostructures with multichannel characteristics are prepared from these organic cocrystals by following the growth kinetics process at molecular level and lattice matching principle at structural level, respectively. Theses prepared heterostructures exhibit optical logic operation capabilities, which can serve as optical modulators. This work provides new insights into the manufacturing of organic NIR heterostructures applied in advanced optoelectronics.

Multilayer dielectric reflector using low-index nanolattices.

Dielectric mirrors based on Bragg reflection and photonic crystals have broad application in controlling light reflection with low optical losses. One key parameter in the design of these optical multilayers is the refractive index contrast, which controls the reflector performance. This work reports the demonstration of a high-reflectivity multilayer photonic reflector that consists of alternating layers of TiO2 films and nanolattices with low refractive index. The use of nanolattices enables high-index contrast between the high- and low-index layers, allowing high reflectivity with fewer layers. The broadband reflectance of the nanolattice reflectors with one to three layers has been characterized with peak reflectance of 91.9% at 527 nm and agrees well with theoretical optical models. The high-index contrast induced by the nanolattice layer enables a normalize reflectance band of Δλ/λo of 43.6%, the broadest demonstrated to date. The proposed nanolattice reflectors can find applications in nanophotonics, radiative cooling, and thermal insulation.