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We report the design and characterization of a high performance integrated arbitrary filter from 1450 nm to 1640 nm. The filter's target spectrum is chosen to suppress the night-sky OH emission lines, which is critical for ground-based astronomical telescopes. This type of filter is featured by its large spectral range, high rejection ratio and narrow notch width. Traditionally it is only successfully accomplished with fiber Bragg gratings. The technique we demonstrate here is proven to be very efficient for on-chip platforms, which can bring many benefits for device footprint, performance and cost. For the design part, two inverse scattering algorithms are compared, the frequency domain discrete layer-peeling (f-DLP) and the time domain discrete layer-peeling (t-DLP). f-DLP is found to be superior for the grating reconstruction in terms of accuracy and robustness. A method is proposed to resolve the non-uniformity issue caused by the non-zero layer size in the DLP algorithm. The designed 55-notch filter is 50-mm-long and implemented on a compact Si3N4/SiO2 spiral waveguide with a total length of 63 mm. Experimentally, we demonstrate that the device has a insertion loss as low as 2.5 dB, and that the waveguide propagation loss is as low as 0.10 dB/cm. We are also able to achieve uniform notch depths and 3-dB widths of about 28 dB and 0.22 nm, respectively., Comment: 8 pages, 8 figures, accepted for publication in Journal of Lightwave Technology

Optical microcavity based four wave mixing (FWM) and its various applications, such as entangled photon pair generation, have been intensively studied in the past few years. Compared to the micro-r...

An ultra-high intrinsic quality factor (Q int ) of 13 million for a Fabry-Perot Bragg grating resonator is demonstrated. The loss of the Si3N4 waveguide platform is evaluated to be 0.018 dB/cm.

Perovskite solar cells have been extensively studied and have recently demonstrated a power conversion efficiency above 23%. In our present approach, we select poly(triaryl amine) (PTAA) as the hole-transport material and use an inverted solar cell planar structure. We propose a sequential method to dope the PTAA thin film in order to reduce the solar cell's series resistance. A modified inter-diffusion method is adopted to improve the perovskite film. We optimize the solvent-assisted annealing condition to maximize the perovskite grain size and reduce the pin-hole density. The impact of PTAA's doping concentration on the solar cell's PV performance is investigated. Compared to conventional PEDOT:PSS-based perovskite solar cell, we observe an improvement in both the open-circuit voltage and the short-circuit current, resulting in a power conversion efficiency enhancement from 12.7% to 15.3%.

We present the design of a broadband Si3N4 polarization beam splitter based on asymmetric directional couplers. A 20 dB polarization extinction ratio has been achieved over a bandwidth of 103 nm experimentally.

Holographic displays and computer-generated holography offer a unique opportunity in improving optical resolutions and depth characteristics of near-eye displays. The thermally-modulated Nanopho-tonic Phased Array (NPA), a new type of holographic display, affords several advantages, including integrated light source and higher refresh rates, over other holographic display technologies. However, the thermal phase modulation of the NPA makes it susceptible to the thermal proximity effect where heating one pixel affects the temperature of nearby pixels. Proximity effect correction (PEC) methods have been proposed for 2D Fourier holograms in the far field but not for Fresnel holograms at user-specified depths. Here we extend an existing PEC method for the NPA to Fresnel holograms with phase-only hologram optimization and validate it through computational simulations. Our method is not only effective in correcting the proximity effect for the Fresnel holograms of 2D images at desired depths but can also leverage the fast refresh rate of the NPA todisplay 3D scenes with time-division multiplexing.

We show theoretically and experimentally that incorporating structured slow-light waveguides inside nanobeam cavities leads to a nonlinear dependence of the Q factor on the cavity length. We employ this effect to demonstrate Q-factor enhancement in silicon nitride nanobeam cavities despite their low refractive index contrast.

We present a detailed study of a parasitic effect connected to integrated Bragg gratings. An appreciable transmission drop is observed experimentally and originates from the backward cladding-mode coupling.

We design and fabricate an integrated arbitrary filter near 1550 nm. The 50 mm long 55-notch filter is implemented on a low-loss SisNVSiOi spiral waveguide. The notches have uniform depths/widths of about 28 dB/0.22 nm.

We present the design and preliminary experimental results of a novel silicon nitride polarization beam splitter. Measured data show that a 33 dB polarization extinction ratio is achieved over a bandwidth of 18 nm.

We report the optimization of reconstruction algorithm and experiment for an integrated arbitrary filter. A 43-notch filter near 1550 nm is implemented with an ultra-low-loss Si3N4/SiO2 spiral waveguide. All notches have uniform depths/widths of about 20 dB/0.2 nm.

A Fabry-Perot Bragg grating cavity enhanced four-wave mixing process with a conversion efficiency of -37.7 dB is demonstrated experimentally on a Si3N4 chip. A thermal-tuning based phase matching approach is also demonstrated.

We show that the Purcell and Q factors of cavities incorporating structured slow-light media increase with the square and cube of the cavity length.

We report the generation of a Fabry-Perot Bragg grating cavity enhanced four-wave mixing signal with a conversion efficiency of -37.7 dB experimentally. A thermal-tuning based dispersion compensation technique is also demonstrated for zero dispersion implementation.

Here we use an electron beam lithography system to pattern an ${\rm{S}}{{\rm{i}}_{3}}{{\rm{N}}_{4}}{\rm{/Si}}{{\rm{O}}_{2}}$ echelle grating using silver as a reflector on the grating grooves. The grating in this letter achieves 1.39 dB on-chip loss, a spectral resolution of ∼1300, a 1.2 dB channel nonuniformity, and less than −30 dB adjacent channel crosstalk for the transverse electric field polarization. We establish that stitching errors can lead to appreciable adjacent channel crosstalk if proper precaution is not taken to minimize them.

A novel approach is developed for measuring small losses in highly transparent Si3N4/SiO 2 waveguides on a silicon chip. The approach is particularly applicable to waveguides written by high-resolution patterning techniques, such as e-beam lithography, whose lengths cannot be easily increased beyond several centimeters. This method is based on measuring the transmission of an optical cavity formed by two highly reflective ( R at least 0.999) simple Bragg gratings and a uniform waveguide between the two gratings whose length can be varied to increase the loss fitting accuracy. A theoretical model based on an ABCD matrix method is developed and used for the final loss value fitting. Experimentally, a cavity with extinction ratio over –70 dB and quality factor Q = 1.02×106 is realized. The fitting results show a waveguide loss of 0.24 ± 0.01 dB/cm and a grating loss of 0.31 ± 0.01 dB/cm. These results are obtained with relatively high index contrast ( Δn > 0.001) gratings with 0.1-pm wavelength scanning resolution. It is expected that with better design and wavelength scanning technique, this approach is applicable more generally to measure waveguide loss coefficients as low as 0.001 dB/cm.

In this work, we present a CuInSe2 (CIS) thin-film solar cell with 14.7% efficiency and 40.2-mA/cm 2 short-circuit current. The samples were fabricated by the selenization of stacked elemental layers under selenium vapor in atmospheric ambient within a simple graphite box. Some samples were treated with a postdeposition treatment (PDT) with potassium fluoride (KF), whereas some were not, thus allowing for useful comparison. The use of the KF PDT led to two interesting improvements on our samples: formation of a hole-blocking layer between CdS/CIS interface, which reduces the tunneling recombination and significantly increases the open-circuit voltage. In addition, the KF PDT clearly reduces the symmetry of the nonohmic shunt leakage at low forward bias and reverse bias, which can be explained by a space-charge-limited current model, indicative of metal–semiconductor–metal (MSM) shunts. Discussion is presented to understand possible defects introduced during our processing steps and the physical origin of the shunt paths and why they might be of the MSM type. New directions to improve the performance of CIS solar cells are presented for future work using this very simple fabrication technique.

The temperature dependence of the photoluminescence from InAs/GaAs quantum dot solar cell (QDSC) is studied. A modified on-resonance z-scan measurement technique is used to extract the quantum dot saturation intensity from room temperature to 170K. The saturation intensity appreciably decreases when the temperature is decreased but is still comparable to the sun intensity at maximum concentration at 170 K. This implies that the energy conversion efficiency of this QDSC, even at 170 K, cannot be enhanced by invoking the concept of an intermediate band solar cell.

Astrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs., Review Paper. 18 pages, 6 figures. Accepted and published in MDPI: Applied Sciences

Proximity effect can affect thermal-modulated nanophotonic phased array holographic displays. The impact of proximity effect on holographic imagery is investigated and the improvement using proximity effect correction methods is demonstrated.

We present the design and experimental results of a novel polarization beam splitter (PBS) with a high polarization extinction ratio (PER) made on a Si3N4 platform. The PBS is composed of two identical polarization-independent multi-mode interferometers and two identical apodized Bragg gratings. The operating principle of this device is based on the fact that the TE and TM stopbands of the grating are centered at different wavelengths. The reflected and transmitted light from the gratings are routed to separate output ports by the two-MMI configuration. The experimental results show that a PER of > 30 dB is achieved over a bandwidth of 22 nm, with an insertion loss of ∼ 1.1 dB. The total length of the device is ∼ 820 µm.

An easy-to-fabricate but very efficient fiber-to-waveguide coupler is theoretically analyzed and experimentally demonstrated. In this design, light from a single-mode fiber can be butt coupled into a single-mode high-index-contrast Si $_{3}$ N $_{4}$ /SiO $_{2}$ waveguide with a measured coupling efficiency of 96% at the wavelength of 1550 nm and $>$ 90% in the spectral range from 1450 to 1650 nm. Large horizontal and vertical alignment tolerances of 3.8 $\mu \text{m}$ and 3.6 $\mu \text{m}$ , respectively, are obtained between the fiber and the waveguide coupler. All these experimental results agree well with simulations. The waveguide coupler also features ease of end-facet cleaving and can be used in ultrabroadband high coupling efficiency applications.

An easy-to-fabricate but very efficient fiber-to-waveguide coupler is theoretically analyzed and experimentally demonstrated. In this design, light from a single-mode fiber can be butt coupled into a single-mode high-index-contrast Si3N4/SiO2 waveguide with a measured coupling efficiency of 96% at the wavelength of 1550 nm and >90% in the spectral range from 1450 to 1650 nm. Large horizontal and vertical alignment tolerances of 3.8 μm and 3.6 μm, respectively, are obtained between the fiber and the waveguide coupler. All these experimental results agree well with simulations. The waveguide coupler also features ease of end-facet cleaving and can be used in ultrabroadband high coupling efficiency applications.

We present an e-beam written Si 3 N 4 /SiO 2 Echelle Grating (EG) with a silver coated reflector. The EG exhibits a 1.4 dB on-chip loss, a spectral resolution power of ∼1300, and more than 30dB signal-to-noise ratio. Crosstalk mechanism is also analyzed in this paper.

We have developed an innovative loss characterization approach suited for record-high Q (> 1×106) Bragg grating cavities. The demonstration sample’s loss is evaluated to be 0.24 dB/cm, with a resolution limit of 0.001 dB/cm.

Variation of leakage current in interband cascade lasers (ICLs) with different carrier concentrations achieved by coating the facet with antireflections (AR) coating was experimentally studied. Single-layer Al2O3 and double-layer ZnS-YF3, ZnS-SiO2, and Ti2O5-SiO2 AR coatings are applied to midinfrared (mid-IR) ICL devices to achieve reflectance ranging from 0.15 to 7 × 10-4. By monitoring the laser performance before and after coating, it was observed that, for lower reflectivity coatings, the leakage current appreciably rises with increasing carrier concentration, thus diminishing the slope efficiency improvements and, in fact, degrading the slope efficiency at very low reflectance. It was observed that the ratio of leakage to threshold current could increase by 17% for high carrier concentrations at very low AR coating values. The results also allow for experimental optimization of AR coatings for increased slope efficiency in mid-IR ICLs. Therefore, in this paper, we propose an optimal value for the AR coatings in order to maximize the power in ICLs.

One promising application of photonics to astronomical instrumentation is the miniaturization of near-infrared (NIR) spectrometers for large ground- and space-based astronomical telescopes. Here we present new results from our effort to fabricate arrayed waveguide grating (AWG) spectrometers for astronomical applications entirely in-house. Our latest devices have a peak overall throughput of ~23%, a spectral resolving power (${\lambda}/{\delta}{\lambda}$) of ~1300, and cover the entire H band (1450-1650 nm) for Transverse Electric (TE) polarization. These AWGs use a silica-on-silicon platform with a very thin layer of Si3N4 as the core of the waveguides. They have a free spectral range of ~10 nm at a wavelength of ~1600 nm and a contrast ratio or crosstalk of about 2% (-17 dB). Various practical aspects of implementing AWGs as astronomical spectrographs are discussed, including the coupling of the light between the fibers and AWGs, high-temperature annealing to improve the throughput of the devices at ~1500 nm, cleaving at the output focal plane of the AWG to provide continuous wavelength coverage, and a novel algorithm to make the devices polarization insensitive over a broad band. These milestones will guide the development of the next generation of AWGs with wider free spectral range and higher resolving power and throughput., Comment: Accepted at Optics Express, Special Issue on Astrophotonics (18 pages, 10 figures)

A CuInSe2 (CIS) thin film solar cell (without Ga) with 14.7% efficiency is demonstrated based on the use of a potassium fluoride (KF) post-deposition treatment (PDT). By comparison, a CIS solar cell with 10.3 % efficiency is obtained without KF PDT. No anti-reflection (AR) coating was used. KF PDT helps to form a hole-blocking layer between CdS/CIS interface and reduce the symmetry of the non-ohmic shunt leakage at low forward bias and reverse bias. The hole-blocking layer reduces the tunneling recombination and significantly increases the open circuit voltage. A space charge limited (SCL) current model is used to explain the symmetry feature of the current vs voltage, which is indicative of metal-semiconductor-metal (MSM) shunts. To understand possible defects and shunt path, some discussion is presented.

A new approach to derive the below-bandgap absorption in InAs/GaAs self-assembled quantum dot (QD) devices using room temperature external quantum efficiency measurement results is presented. The significance of incorporating an extended Urbach tail absorption in analyzing QD devices is demonstrated. This tail is used to evaluate the improvement in the photo-generated current. The wetting layer and QD absorption contributions are separated from the tail absorption. Several absorption peaks due to QD excited states and potentially different size QDs are observed. An inhomogeneous broadening of 25meV arising from the variance in the size of QDs is derived. Copyright © 2014 John Wiley & Sons, Ltd.

We present experimental and simulation results of sidewall Bragg gratings (BGs) implemented on a silicon nitride waveguide platform, with emphasis on the appreciable drop of transmission on the high-frequency side of the grating stopband due to the coupling into backward cladding/radiative modes. We differentiate this from the coupling into backward slab modes as observed in BGs implemented in silicon-on-insulator ridge waveguides. The dimension of the device top cladding affects the shape of the drop pattern. Different approaches for circumventing this drop pattern are discussed. A better understanding of this physical phenomenon will help guide the design of BGs on integrated photonic platforms.

We present results on the selective area growth (SAG) of Gallium Nitride (GaN) nanowires on Si substrate without any buffer layer by radio frequency plasma-assisted molecular beam epitaxy. Full selectivity was achieved with a thin Ti metal film used as a mask. The mask was fabricated with a lift-off method to avoid damage to the mask or Si substrate. The growth on the Ti film was totally suppressed at a low substrate temperature of 840 °C which is by far the lowest critical temperature for SAG. An InGaN thin layer was embedded in the GaN nanowires to realize a site controllable single photon source. Transmission electron microscopy (TEM) indicates single crystalline quality of defect-free GaN nanowires and the embedded InGaN quantum dot structure.