Synthesis of Polycyclic Olefinic Monomers from Norbornadiene for Inverse Vulcanization: Structural and Mechanistic Consequences.

The preparation of high-sulfur content organosulfur polymers has generated considerable interest as an emerging area in polymer science that has been driven by advances in the inverse vulcanization polymerization of elemental sulfur with organic comonomers. While numerous new inverse vulcanized polysulfides have been made over the past decade, insights into the mechanism of inverse vulcanization and structural characterization of the high-sulfur-content copolymers remain limited in scope. Furthermore, the exploration of new molecular architectures for organic comonomer synthesis remains an important frontier to enhance the properties of these new polymeric materials. In the current report, the first detailed study on the synthesis and inverse vulcanization of polycyclic rigid comonomers derived from norbornadiene was conducted, affording a quantitative assessment of polymer microstructure for these organopolysulfides and insights into the inverse vulcanization polymerization mechanism for this class of monomers. In particular, a stereoselective synthesis of the endo-exo norbornadiene cyclopentadiene adduct (Stillene) was achieved, which enabled direct comparison with the known exo-exo norbornadiene dimer (NBD2) previously used for inverse vulcanization. Reductive degradation of these sulfur copolymers and detailed structural analysis of the recovered sulfurated organic fragments revealed that remarkable exo-stereospecificity was achieved in the inverse vulcanization of elemental sulfur with both these polycyclic dienyl comonomers, which correlated to the robust thermomechanical properties associated with organopolysulfides made from NBD2 previously. Melt processing and molding of these sulfur copolymers were conducted to fabricate free-standing plastic lenses for long-wave infrared thermal imaging.

Synthesis of Deuterated and Sulfurated Polymers by Inverse Vulcanization: Engineering Infrared Transparency via Deuteration.

The synthesis of deuterated, sulfurated, proton-free, glassy polymers offers a route to optical polymers for infrared (IR) optics, specifically for midwave IR (MWIR) photonic devices. Deuterated polymers have been utilized to enhance neutron cross-sectional contrast with proteo polymers for morphological neutron scattering measurements but have found limited utility for other applications. We report the synthesis of perdeuterated d14-(1,3-diisopropenylbenzene) with over 99% levels of deuteration and the preparation of proton-free, perdeuterated poly(sulfur-random-d14-(1,3-diisopropenylbenzene)) (poly(S-r-d14-DIB)) via inverse vulcanization with elemental sulfur. Detailed structural analysis and quantum computational calculations of these reactions demonstrate significant kinetic isotope effects, which alter mechanistic pathways to form different copolymer microstructures for deutero vs proteo poly(S-r-DIB). This design also allows for molecular engineering of MWIR transparency by shifting C-H bond vibrations around 3.3 µm/3000 cm-1 observed in proteo poly(S-r-DIB) to 4.2 µm/2200 cm-1. Furthermore, the fabrication of thin-film MWIR optical gratings made from molding of deuterated-sulfurated, proton-free poly(S-r-d14-DIB) is demonstrated; operation of these gratings at 3.39 µm is achieved successfully, while the proteo poly(S-r-DIB) gratings are opaque at these wavelengths, highlighting the promise of MWIR sensors and compact spectrometers from these materials.

Injection-locked highly Yb3+-doped uncoupled-61-core phosphate fiber laser.

Uncoupled multicore fibers are promising platforms for advanced optical communications, optical computing, and novel laser systems. In this paper, an injection-locked highly ytterbium (Yb3+)-doped uncoupled-61-core phosphate fiber laser at 1030 nm is reported. The 61-core fiber with a core-to-core pitch of 20 µm was fabricated with the stack-and-draw technique. Each core doped with 6-wt.% Yb3+ ions has a diameter of 3 µm and numerical aperture of 0.2. Linearly polarized single-frequency output of 9.1 W was obtained from the injection-locked cavity with a 10-cm-long gain fiber at a pump power of 23.6 W. The injection locking of all 61 cores was confirmed by inspecting the longitudinal modes of the individual lasers with a scanning Fabry-Perot interferometer. The performance of the injection-locked 61-core fiber laser was characterized and compared to that of the free-running operation in terms of optical spectrum, near- and far-field intensity profiles, and relative intensity noise.

Ultralow-quantum-defect single-frequency fiber laser.

A single-frequency distributed-Bragg-reflector fiber laser at 980 nm with a quantum defect of less than 0.6% was developed with a 1.5-cm 12 wt% ytterbium-doped phosphate fiber pumped by a 974.5-nm laser diode. Linearly polarized single-longitude-mode laser with a polarization extinction ratio (PER) of nearly 30 dB and spectral linewidth of less than 1.8 kHz was obtained. A maximum output power of 275 mW was measured at a launched pump power of 620 mW. The performance of the single-frequency fiber laser pumped at 909 nm and 976 nm was also characterized. This research demonstrated an approach to high-power single-frequency fiber laser oscillators with mitigated thermal effects.

Sulfenyl Chlorides: An Alternative Monomer Feedstock from Elemental Sulfur for Polymer Synthesis.

A polymerization methodology is reported using sulfur monochloride (S2Cl2) as an alternative feedstock for polymeric materials. S2Cl2 is an inexpensive petrochemical derived from elemental sulfur (S8) but has numerous advantages as a reactive monomer for polymerization vs S8. This new process, termed sulfenyl chloride inverse vulcanization, exploits the high reactivity and miscibility of S2Cl2 with a broad range of allylic monomers to prepare soluble, high molar-mass linear polymers, segmented block copolymers, and crosslinked thermosets with greater synthetic precision than achieved using classical inverse vulcanization. This step-growth addition polymerization also allows for preparation of a new class of thiol-free, inexpensive, highly optically transparent thermosets (α = 0.045 cm-1 at 1310 nm), which exhibit among the best optical transparency and low birefringence relative to commodity optical polymers, while possessing a higher refractive index (n > 1.6) in the visible and near-infrared spectra. The fabrication of large-sized optical components is also demonstrated.

Single-frequency fiber laser at 880†nm.

Single-frequency fiber lasers with extremely low noise and narrow spectral linewidth have found many scientific and practical applications. There is great interest in developing single-frequency fiber lasers at new wavelengths. In this paper, we report a single-frequency Nd3+-doped phosphate fiber laser operating at 880 nm, which is the shortest demonstrated wavelength for a single-frequency fiber laser thus far, to the best of our knowledge. An output power of 44.5 mW and a slope efficiency of 20.4% with respect to the absorbed pump power were obtained with a 2.5-cm-long 1 wt.% Nd3+-doped phosphate fiber. Our simulation results show that higher single-frequency laser output can be achieved with 1.5 wt.% or 2 wt.% Nd3+-doped phosphate fiber with mitigated ion clustering.

Diode-pumped 1.15 W linearly polarized single-frequency Yb3+-doped phosphate fiber laser.

Compact and robust high-power single-frequency laser oscillators are in great demand for some specific applications where narrow-linewidth lasers with extremely low noise are required. In this paper, we report a single-mode-diode-pumped watt-level single-frequency Yb3+-doped phosphate fiber laser at 1050 nm based on an all-fiber distributed Bragg reflector cavity. A maximum output power of 1.15 W with a slope efficiency of 66% was achieved with 18-mm-long 8 wt.% Yb3+-doped phosphate fiber. Stable, single-longitudinal-mode lasing with a spectral linewidth of 9.6 kHz and polarization extinction ratio of ∼30 dB was obtained.

Widely wavelength tunable Dy3+/Er3+ co-doped ZBLAN fiber lasers.

Wavelength tunable dysprosium-erbium (Dy3+/Er3+) co-doped ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber lasers pumped at 980 nm were developed with a bulk grating blazed at 3.1 µm in the Littrow configuration and their performances were investigated. A wavelength tunable range of 674.4 nm (2709.2 nm -3373.6 nm) was achieved with a 4.5-m 0.25 mol.% Dy3+/ 4 mol.% Er3+ co-doped ZBLAN fiber. Our experiments demonstrated that either Er3+ or Dy3+ can be lasing individually in a Dy3+/Er3+ co-doped ZBLAN fiber and a fiber laser with wavelength tunable range from 2.7 µm to 3.4 µm or longer wavelengths can be achieved with proper fiber and cavity design.

Cryogenic C-band wavelength division multiplexing system using an AIM Photonics Foundry process design kit.

Cryogenic environments make superconducting computing possible by reducing thermal noise, electrical resistance and heat dissipation. Heat generated by the electronics and thermal conductivity of electrical transmission lines to the outside world constitute two main sources of thermal load in such systems. As a result, higher data rates require additional transmission lines which come at an increasingly higher cooling power cost. Hybrid or monolithic integration of silicon photonics with the electronics can be the key to higher data rates and lower power costs in these systems. We present a 4-channel wavelength division multiplexing photonic integrated circuit (PIC) built from modulators in the AIM Photonics process development kit (PDK) that operate at 25 Gbps at room temperature and 10 Gbps at 40 K. We further demonstrate 2-channel operation for 20 Gbps aggregate data rate at 40 K using two different modulators/wavelengths, with the potential for higher aggregate bit rates by utilizing additional channels.

Magneto-optical properties of highly Dy3+ doped multicomponent glasses.

Due to their large effective magnetic moment, Dy3+-doped materials have attracted much interest for magneto-optical applications. In this paper, we report highly Dy3+ doped multicomponent glasses with concentrations from 40 wt.% to 75 wt.% and their magneto-optical properties. A Verdet constant of -7.4 rad/T/m at 1950 nm was measured with the 75 wt.% Dy3+-doped glass. This is the highest reported Verdet constant around 2 µm for a paramagnetic glass. Our experimental results show that highly Dy3+-doped glasses are promising isotropic magneto-optical materials for applications in the 2 µm wavelength region.

High sensitivity magnetometer using nanocomposite polymers with large magneto-optic response: retraction.

The referenced article [Opt. Lett.43, 4615 (2018)OPLEDP0146-959210.1364/OL.43.004615] has been retracted by the authors.

Thermal compensation of molded silicone optics.

Optical grade silicone has various properties that make it attractive for solar concentrators, such as excellent transmission across the solar spectrum and flexible moldability for freeform profiles. In this study, a glass-silicone lens structure is proposed to reduce the optothermal effect on the silicone lens. Experimental measurements and simulation modeling results demonstrate that the focal length sensitivity of the glass-silicone lens with respect to temperature can be reduced by a factor of 10 when compared to a silicone lens alone. This model has been extended to the simulation of a proposed two-stage silicone solar concentrator, consisting of an array of acylindrical lenslets and rows of waveguides that focus light onto microphotovoltaic cells. The optical efficiency of the solar concentration system showed a change of less than 10% compared to the efficiency at room temperature for temperature changes from -10∘C to 70°C.

Silicone optical elements for cost-effective freeform solar concentration.

The use of silicone optical elements is demonstrated for a concentrated photovoltaic system. These components show over 96% transmission through most of the solar spectrum and excellent temperature stability. Unique moldability enables the use of complex freeform designs. A light, compact, and cost-effective concentrated photovoltaic system based on silicone optics is proposed. In this system, air-plasma treatment is utilized to overcome the mechanical properties of silicone and difficulties with coating to reduce Fresnel loss. Lens arrays and waveguides are fabricated by injection molding following freeform optical design by LightTools. First-order characterizations are also performed.

Numerical investigation of GHz repetition rate fundamentally mode-locked all-fiber lasers.

GHz repetition rate fundamentally mode-locked lasers have attracted great interest for a variety of scientific and practical applications. A passively mode-locked laser in all-fiber format has the advantages of high stability, maintenance-free operation, super compactness, and reliability. In this paper, we present numerical investigation on passive mode-locking of all-fiber lasers operating at repetition rates of 1-20 GHz. Our calculations show that the reflectivity of the output coupler, the small signal gain of the doped fiber, the total net cavity dispersion, and the modulation depth of the saturable absorber are the key parameters for producing stable fundamentally mode-locked pulses at GHz repetition rates in very short all-fiber linear cavities. The instabilities of GHz repetition rate fundamentally mode-locked all-fiber lasers with different parameters were calculated and analyzed. Compared to a regular MHz repetition rate mode-locked all-fiber laser, the pump power range for the mode-locking of a GHz repetition rate all-fiber laser is much larger due to the several orders of magnitude lower accumulated nonlinearity in the fiber cavity. The presented numerical study provides valuable guidance for the design and development of highly stable mode-locked all-fiber lasers operating at GHz repetition rates.

Infrared Fingerprint Engineering: A Molecular-Design Approach to Long-Wave Infrared Transparency with Polymeric Materials.

Optical technologies in the long-wave infrared (LWIR) spectrum (7-14 µm) offer important advantages for high-resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR-transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so-called IR-fingerprint region. We report on a new molecular-design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co-monomers.

Optical and mechanical tolerances in hybrid concentrated thermal-PV solar trough.

Hybrid thermal-PV solar trough collectors combine concentrated photovoltaics and concentrated solar power technology to harvest and store solar energy. In this work, the optical and mechanical requirements for optimal efficiency are analyzed using non-sequential ray tracing techniques. The results are used to generate opto-mechanical tolerances that can be compared to those of traditional solar collectors. We also explore ideas on how to relieve tracking tolerances for single-axis solar collectors. The objective is to establish a basis for tolerances required for the fabrication and manufacturing of hybrid solar trough collectors.

High sensitivity magnetometer using nanocomposite polymers with large magneto-optic response.

Miniaturized magnetic field sensors are increasingly used in various applications, such as geophysical exploration for minerals and oil, volcanology, earthquake studies, and biomedical imaging. Existing magnetometers lack either the spatial or the temporal resolution or are restricted to costly shielded labs and cannot operate in an unshielded environment. Increasing spatio-temporal resolution would allow for real-time measurements of magnetic fluctuations with high resolution. Here we report on a new nanocomposite-based system for miniaturized magnetic field sensing. The sensor is based on Dy3+-doped magnetite and cobalt ferrite nanoparticles dispersed in a polymer matrix. Operation is feasible at room temperature and in an unshielded environment. A compact fiber-optic interferometer is used as the detection mechanism with 20 fT/√Hz sensitivity. We investigated the magnetic field response of the sensor and demonstrated the measurement of the human heartbeat as a potential application.

Athermal silicon optical add-drop multiplexers based on thermo-optic coefficient tuning of sol-gel material.

Silicon photonics has gained interest for its potential to provide higher efficiency, bandwidth and reduced power consumption compared to electrical interconnects in datacenters and high performance computing environments. However, it is well known that silicon photonic devices suffer from temperature fluctuations due to silicon's high thermo-optic coefficient and therefore, temperature control in many applications is required. Here we present an athermal optical add-drop multiplexer fabricated from ring resonators. We used a sol-gel inorganic-organic hybrid material as an alternative to previously used materials such as polymers and titanium dioxide. In this work we studied the thermal curing parameters of the sol-gel and their effect on thermal wavelength shift of the rings. With this method, we were able to demonstrate a thermal shift down to -6.8 pm/°C for transverse electric (TE) polarization in ring resonators with waveguide widths of 325 nm when the sol-gel was cured at 130°C for 10.5 hours. We also achieved thermal shifts below 1 pm/°C for transverse magnetic (TM) polarization in the C band under different curing conditions. Curing time compared to curing temperature shows to be the most important factor to control sol-gel's thermo-optic value in order to obtain an athermal device in a wide temperature range.

Widely-tunable high-repetition-rate terahertz generation in GaSe with a compact dual-wavelength KTP OPO around 2 µm.

A compact efficient high-repetition-rate doubly-resonant dual-wavelength KTP optical parametric oscillator (OPO), with output power up to 3.65 W and tuning ranges of 2.088-2.133 µm/2.171-2.122 µm for signal/idler waves, was deployed for terahertz (THz) generation in a GaSe crystal. Based on difference frequency generation (DFG), the THz wave was continuously tunable from 730.9 µm (0.41 THz) to 80.8 µm (3.71 THz), believed to be the first report of a compact high-repetition-rate widely-tunable THz source. The maximum THz average power reached 1.2 µW at 1.54 THz and the corresponding DFG efficiency was 7.8 × 10-7, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.

Designing spectrum-splitting dichroic filters to optimize current-matched photovoltaics.

We have developed an approach for designing a dichroic coating to optimize performance of current-matched multijunction photovoltaic cells while diverting unused light. By matching the spectral responses of the photovoltaic cells and current matching them, substantial improvement to system efficiencies is shown to be possible. A design for use in a concentrating hybrid solar collector was produced by this approach, and is presented. Materials selection, design methodology, and tilt behavior on a curved substrate are discussed.