Turn-key Kerr soliton generation and tunable microwave synthesizer in dual-mode Si3N4 microresonators.

This study investigates the thermal compensation mechanism in dual-mode Si3N4 microresonators that demonstrates the ease of generation of single-solitons with nearly octave-wide spectral bandwidth. The deterministic creation of soliton frequency combs is achieved by merely switching the wavelength of a tunable laser or a semiconductor diode laser in a single step. The pump frequency detuning range that can sustain the soliton state is 30 gigahertz (GHz), which is approximately 100 times the resonance linewidth. Interestingly, these dual-mode resonators also support the coexistence of primary combs and solitons, enabling their utilization as functional microwave synthesizers. Furthermore, these resonators readily facilitate the generation of diverse multi-solitons and soliton crystals. This work presents a simplified system to access high-performance and versatile Kerr solitons, with wide-ranging applications in optical metrology, microwave photonics, and LiDAR.

Self-heating allows athermal laser diode wavelength control using a thermally insulating sub-mount over a 70 °C range.

The wavelength of a single frequency quantum dot distributed feedback (DFB) laser operating in the O-band is athermalised over a 74 °C ambient temperature range. Two techniques are presented, one utilising the laser self-heating for tuning control, the other using a resistive heater. Both techniques show greatly improved power efficiency over conventional wavelength control schemes, and both demonstrate wavelength stability of better than 0.1 nm (17.5 GHz) without mode hops over the entire temperature range. The use of a high operating temperature quantum dot laser together with an innovative submount design to increase the thermal impedance of the device enables the improved use of the laser self-heating for wavelength tuning. The submount design entails the laser being suspended over an air gap with the use of glass supports, preventing heat from escaping from the diode.

Versatile octave-spanning soliton crystals with high conversion efficiency in a Si3N4 microresonator.

Microresonator-based soliton crystals are a key recent advancement in the study of the rich nonlinear dynamics of soliton states. The soliton crystals are self-organized temporal pulses filling the microresonator cavity and have strong comb lines with wide spacing making them of great interest in many potential applications such as communication and meteorology. However, achieving a broad spectrum, tunable repetition rates, and high conversion efficiency are still a challenge. Here, we report the deterministic generation of versatile octave-spanning soliton crystals with various repetition rates via avoided mode crossings. In addition, we investigate the conversion efficiency of the obtained soliton crystals and achieved above ∼50% in one of the devices with a suitable coupling. Our results pave the way for accessing coherent broad and tunable on-chip soliton crystals, thus requiring a rigorous and viable microcavity design to engineer the desired mode coupling position.

Monolayer Capping Provides Close to Optimal Resistance to Laser Dewetting of Au Films.

Next-generation heat-assisted magnetic recording (HAMR) relies on fast, localized heating of the magnetic medium during the write process. Au plasmonic near-field transducers are an attractive solution to this challenge, but increased thermal stability of Au films is required to improve long-term reliability. This work compares the effect of nanoscale Al, AlOx, and Ta capping films on Au thin films with Ti or Ta adhesion layers for use in HAMR and other high-temperature plasmonic applications. Thermal stability is investigated using a bespoke laser dewetting system, and SEM and AFM are extensively used to interrogate the resulting dewet areas. The most effective capping layers are found to be 0.5-1 nm of Al or AlOx, which can eliminate dewetting under certain conditions. Even one monolayer of AlOx is shown to be highly effective in reducing dewetting. In the case of thicker capping layers of Ta and AlOx, the Au film can easily dewet underneath, leaving an intact capping layer. It is concluded that thinner capping layers are most effective against dewetting as the Au cannot dewet without breaking them and pulling them apart during the dewetting process. A simple model based on energetics considerations is developed, which explains how thinner capping layers can more effectively protect the metal from pore or fissure creation. The model provides some convenient guidelines for choosing both the substrate and capping layer, for a given metal, to maximize the resistance to laser-induced damage.

Wide-waveguide high-power low-RIN single-mode distributed feedback laser diodes for optical communication.

We present an 8-µm-wide 800-µm-long high-power, single-mode and low RIN DFB laser using a dual-waveguide structure. The introduced passive lower waveguide has weakenes the lateral optical confinement for the ridge waveguide, and thus reduces losses caused by the p-doped layers and maintains single mode stability of the laser. The fabricated laser exhibited an output power higher than 170 mW and a relative intensity noise (RIN) below -157 dB/Hz along with a side-mode suppression-ratio (SMSR) over 55 dB. The temperature tuning from -10°C to 60°C allows an 8.6 nm wavelength tunability with a variation coefficient of 0.12 nm/K. The relaxation oscillation frequency is around 8 GHz, and the linewidth is about 250 kHz at 100 mW output power for the fabricated laser. The characteristics of the proposed high-power laser, including high slope efficiency, single mode stability and low noise, make it a suitable candidate for optical communication.

Near-octave-spanning breathing soliton crystal in an AlN microresonator.

The soliton crystal (SC) was recently discovered as an extraordinary Kerr soliton state with regularly distributed soliton pulses and enhanced comb line power spaced by multiples of the cavity free spectral ranges (FSRs), which will significantly extend the application potential of microcombs in optical clock, signal processing, and terahertz wave systems. However, the reported SC spectra are generally narrow. In this Letter, we demonstrate the generation of a breathing SC in an aluminum nitride (AlN) microresonator (FSR ∼374GHz), featuring a near-octave-spanning (1150-2200 nm) spectral range and a terahertz repetition rate of ∼1.87THz. The measured 60 fs short pulses and low intensity-noise characteristics confirm the high coherence of the breathing SC. Broadband microcombs with various repetition rates of ∼0.75, ∼1.12, and ∼1.5THz were also realized in different microresonators of the same size. The proposed scheme shows a reliable design strategy for broadband soliton generation with versatile dynamic control over the comb line spacing.

Wide range thermal and athermal operation of slotted surface grating lasers.

Athermalisation is a procedure in which the wavelength of a semiconductor laser remains unchanged even as the temperature is altered. This is achieved by altering the currents that flow through the laser so as to maintain the wavelength and avoid mode hops. In this study, we demonstrate that lasers operating with a large red-shift with respect to the gain peak yield the best performance in terms of the highest temperature operation and also in terms of the widest athermal operating range. In particular, a device with red detuning of approximately 25 nm yields the best results. This device is athermalised continuously (without mode hops) from 5 to 106 oC, and discontinuously to 115 oC while maintaining wavelength stability of $\pm$0.4 GHz/0.003 nm and side mode suppression ratio of above 40 dB in most of the continuous range and above 30 dB in the discontinuous regime. Operating in this manner will enable semiconductor lasers to be used without a thermoelectric cooler in applications where the temperature changes substantially.

1.3 µm wavelength tunable single-mode laser arrays based on slots.

Two twelve-channel arrays based on surface-etched slot gratings, one with non-uniformly spaced slots and another with uniformly spaced slots are presented for laser operation in the O-band. A wavelength tuning range greater than 40 nm, with a side-mode suppression ratio (SMSR) > 40 dB over much of this range and output power greater than 20 mW, was obtained for the array with non-uniform slots over a temperature range of 15 °C - 60 °C. The introduction of multiple slot periods, chosen such that there is minimal overlap among the side reflection peaks, is employed to suppress modes lasing one free spectral range (FSR) from the intended wavelength. The tuning range of the array with uniformly spaced slots, on the other hand, was found to be discontinuous due to mode-hopping to modes one FSR away from the intended lasing mode which are not adequately suppressed. Spectral linewidth was found to vary across devices with the lowest measured linewidths in the range of 2 MHz to 4 MHz.

Octave-spanning Kerr frequency comb generation with stimulated Raman scattering in an AlN microresonator.

Octave-spanning optical frequency combs (OFCs) are essential for various applications, such as precision metrology and astrophysical spectrometer calibration. In this Letter, we demonstrate, for the first time to our knowledge, the generation of octave-spanning Kerr frequency combs ranging from 1150 to 2400 nm in aluminum nitride (AlN) microring resonators, by pumping the TM00 modes at 250 mW on-chip power. By simply adjusting the pump detuning, we observe the transition and coexistence of Kerr OFC and stimulated Raman scattering. For the TE00 mode in the same device, a broadband Raman-assisted frequency comb is demonstrated by adjusting the pump power and tuning. These results indicate a crucial development for the fundamentals of nonlinear dynamics and comb applications in AlN.

Photolithography allows high-Q AlN microresonators for near octave-spanning frequency comb and harmonic generation.

Single-crystal aluminum nitride (AlN) possessing both strong Pockels and Kerr nonlinear optical effects as well as a very large band gap is a fascinating optical platform for integrated nonlinear optics. In this work, fully etched AlN-on-sapphire microresonators with a high-Q of 2.1 × 106 for the TE00 mode are firstly demonstrated with the standard photolithography technique. A near octave-spanning Kerr frequency comb ranging from 1100 to 2150 nm is generated at an on-chip power of 406 mW for the TM00 mode. Due to the high confinement, the TE10 mode also excites a Kerr comb from 1270 to 1850nm at 316 mW. In addition, frequency conversion to visible light is observed during the frequency comb generation. Our work will lead to a large-scale, low-cost, integrated nonlinear platform based on AlN.

Controlled Cavity-Free, Single-Photon Emission and Bipartite Entanglement of Near-Field-Excited Quantum Emitters.

We report theoretical statistics of 1- and 2-qubit (bipartite) systems, namely, photon antibunching and entanglement, of near-field excited quantum emitters. The sub-diffraction focusing of a plasmonic waveguide is shown to generate enough power over a sufficiently small region (<50 × 50 nm2) to strongly drive quantum emitters. This enables ultrafast (10-14 s) single-photon emission as well as creates entangled states between two emitters when performing a controlled-NOT operation. A comparative analysis of silicon and near-zero index materials demonstrates advantages and uncovers challenges of embedding quantum emitters for single-photon emission and for bipartite entanglement. The use of a movable plasmonic waveguide, in lieu of stationary nanostructures, allows high-speed rasterization between sets of qubits and enables spatially flexible data storage and quantum information processing. Furthermore, the sub-diffraction focusing of the waveguide is shown to achieve cavity-free dynamic entanglement. This greatly reduces fabrication constraints and increases the speed and scalability of nanophotonic quantum devices.

Genetic algorithm optimization of high order surface etched grating tunable laser array.

A genetic algorithm is developed with a view to optimizing surface-etched grating tunable lasers over a large optimization space comprised of several variables. Using this approach, a new iteration of slotted lasers arrays are optimized showing significant improvements over previous designs. Output power, lower grating order, fabrication tolerance and performance at high temperatures are among key parameters improved. The new designs feature a much lower grating order (24-29) than used previously (37). The biggest improvement is a near doubling to slope efficiency to 0.1-0.13 mW/mA, with wavelengths from the array covering the C-band . The designs show a reduced sensitivity to etch depth variations. Designs with linewidths down to 100 kHz are also simulated. This algorithm can be readily applied to different wafer materials to efficiently generate slotted lasers designs at new wavelengths.

Comparison of Metal Adhesion Layers for Au Films in Thermoplasmonic Applications.

If thermoplasmonic applications such as heat-assisted magnetic recording are to be commercially viable, it is necessary to optimize both thermal stability and plasmonic performance of the devices involved. In this work, a variety of different adhesion layers were investigated for their ability to reduce dewetting of sputtered 50 nm Au films on SiO2 substrates. Traditional adhesion layer metals Ti and Cr were compared with alternative materials of Al, Ta, and W. Film dewetting was shown to increase when the adhesion material diffuses through the Au layer. An adhesion layer thickness of 0.5 nm resulted in superior thermomechanical stability for all adhesion metals, with an enhancement factor of up to 200× over 5 nm thick analogues. The metals were ranked by their effectiveness in inhibiting dewetting, starting with the most effective, in the order Ta > Ti > W > Cr > Al. Finally, the Au surface-plasmon polariton response was compared for each adhesion layer, and it was found that 0.5 nm adhesion layers produced the best response, with W being the optimal adhesion layer material for plasmonic performance.

850†nm GaAs/AlGaAs DFB lasers with shallow surface gratings and oxide aperture.

We present and experimentally demonstrate a novel oxide-confined ridge-waveguide distributed feedback (DFB) laser with the first-order surface grating using only a single growth step. The metal contacts are laterally offset from the ridge waveguide to inject current thus avoiding unwanted light absorption from the electrodes. The oxide aperture is defined by selective wet oxidation of aluminium-rich material, which confines the injection current from the electrodes to the active layer under the ridge waveguide. This allows that a thin ridge layer can be used with relatively higher refractive index compared to the active layer and thus the grating can be shallowly etched but provides a strong coupling effect. The fabricated 150 µm-long DFB laser exhibited a relatively low threshold current of 8 mA and a side mode suppression ratio (SMSR) up to 50 dB at the injected current of 32 mA around 4 times threshold at 20 °C. Stable single mode operation has been observed for the fabricated DFB laser over the temperature range from 10 to 50 °C. The variation of wavelength with temperature Δλ/ΔT was 0.06 nm/°C. The proposed laser may have advantages combined both DFB lasers and vertical-cavity surface-emitting lasers (VCSELs), such as single mode, stabilized polarization, potentially narrow linewidth and low power consumption. In addition, the laser is regrowth free, thus has advantages of low cost and high reliability.

High-performance InP-based Mach-Zehnder polarization beam splitter with a 19 dB extinction ratio across C-band.

A high-performance InP-based polarization beam splitter (PBS) using a symmetrical Mach-Zehnder interferometer is experimentally demonstrated. The waveguides are aligned along the [011] direction, which results in a small reverse bias required and easy adjustment to realize the PBS. The experimental results indicate that the polarization extinction ratio (PER) is over 19 dB in the wavelength range from 1525 to 1570 nm with one arm injected with a 4.32 mA current and the other arm reversed biased at 5.14 V simultaneously. The PER is measured to remain above 18 dB in the same wavelength range even when the width varies by ±200 nm.

The Search for Consumers of Web-Based Raw DNA Interpretation Services: Using Social Media to Target Hard-to-Reach Populations.

BACKGROUND: In recent years, there has been a proliferation of third-party Web-based services available to consumers to interpret raw DNA from direct-to-consumer genetic testing companies. Little is known about who uses these services and the downstream health implications. Identifying this hard-to-reach population of consumers for research raised questions about the most effective recruitment methods to undertake. Past studies have found that Web-based social media survey distribution can be cost-effective for targeting hard-to-reach populations, yet comparative efficacy information across platforms is limited. OBJECTIVE: The aim of this study was to identify the most effective Web-based strategies to identify and recruit the target population of direct-to-consumer genetic testing users who also made use of third-party interpretation services to analyze their raw genetic data. Web-based survey recruitment methods varying by social media platform and advertising method were compared in terms of cost-effectiveness and demographics of survey respondents. METHODS: A total of 5 Web-based survey distribution conditions were examined: 4 paid advertising services and 1 unpaid service. For the paid services, a 2x2 quasi-experimental design compared social media platforms (Facebook vs Twitter) and advertising tracking metrics (by click vs by conversion). The fifth unpaid comparison method consisted of study postings on the social media platform, Reddit, without any paid advertising. Links to identical Web-based versions of the study questionnaire were posted for 10 to 14 days for each of the distribution conditions, which allowed tracking the number of respondents that entered and completed the questionnaire by distribution condition. RESULTS: In total, 438 individuals were recruited to the study through all conditions. A nearly equivalent number of participants were recruited from paid campaigns on Facebook (n=159) and Twitter (n=167), with a smaller sample recruited on Reddit (n=112). Significantly more participants were recruited through conversion-tracking (n=222) than through click-tracking campaigns (n=104; Z=6.5, P<.001). Response rates were found to be partially driven by organic sharing of recruitment materials among social media users. Conversion tracking was more cost-effective than click tracking across paid social media platforms. Significant differences in terms of gender and age distributions were noted between the platforms and between the tracking metrics. CONCLUSIONS: Web-based recruitment methods were effective at recruiting participants from a hard-to-reach population in a short time frame. There were significant differences in the effectiveness of various paid advertising techniques. Recruitment through Web-based communities also appeared to perform adequately, yet it may be limited by the number of users accessible in open community groups. Future research should evaluate the impact of organic sharing of recruitment materials because this appeared to play a substantial role in the observed effectiveness of different methods.

Synthesis of centimeter-size free-standing perovskite nanosheets from single-crystal lead bromide for optoelectronic devices.

Considerable attention has been drawn to the lead halide perovskites (LHPs) because of their outstanding optoelectronic characteristics. LHP nanosheets (NSs) grown from single crystalline lead halide possess advantages in device applications as they provide the possibility for control over morphology, composition, and crystallinity. Here, free-standing lead bromide (PbBr2) single-crystalline NSs with sizes up to one centimeter are synthesized from solution. These NSs can be converted to LHP while maintaining the NS morphology. We demonstrate that these perovskite NSs can be processed directly for fabrication of photodetector and laser arrays on a large scale. This strategy will allow high-yield synthesis of large-size perovskite NSs for functional devices in an integrated photonics platform.

Correction: The Search for Consumers of Web-Based Raw DNA Interpretation Services: Using Social Media to Target Hard-to-Reach Populations.

[This corrects the article DOI: 10.2196/12980.].

Less is More: Improved Thermal Stability and Plasmonic Response in Au Films via the Use of SubNanometer Ti Adhesion Layers.

The use of a metallic adhesion layer is known to increase the thermo-mechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heat-assisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0 to 5 nm were fabricated, and their thermal stability, electrical resistivity, and plasmonic response were measured. Subnanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200 °C), compared to more commonly used adhesion layer thicknesses that are in the range of 2-5 nm. For adhesion layers thicker than 1 nm, the diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 and 5 nm adhesion layer samples. The superiority of subnanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger and spectrally sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.

Non-resonant light scattering in dispersions of 2D nanosheets.

Extinction spectra of nanomaterial suspensions can be dominated by light scattering, hampering quantitative spectral analysis. No simple models exist for the wavelength-dependence of the scattering coefficients in suspensions of arbitrary-sized, high-aspect-ratio nanoparticles. Here, suspensions of BN, talc, GaS, Ni(OH)2, Mg(OH)2 and Cu(OH)2 nanosheets are used to explore non-resonant scattering in wide-bandgap 2D nanomaterials. Using an integrating sphere, scattering coefficient (σ) spectra were measured for a number of size-selected fractions for each nanosheet type. Generally, σ scales as a power-law with wavelength in the non-resonant regime: σ(λ)∝[λ/〈L〉]-m, where 〈L〉 is the mean nanosheet length. For all materials, the scattering exponent, m, forms a master-curve, transitioning from m = 4 to m = 2, as the characteristic nanosheet area increases, indicating a transition from Rayleigh to van der Hulst scattering. In addition, once material density and refractive index are factored out, the proportionality constant relating σ to [λ/〈L〉]-m, also forms a master-curve when plotted versus 〈L〉.