Micro-Ring Resonator Assisted Photothermal Spectroscopy of Water Vapor.

We demonstrated, for the first time, micro-ring resonator assisted photothermal spectroscopy measurement of a gas phase sample. The experiment used a telecoms wavelength probe laser that was coupled to a silicon nitride photonic integrated circuit using a fibre array. We excited the photothermal effect in the water vapor above the micro-ring using a 1395 nm diode laser. We measured the 1f and 2f wavelength modulation response versus excitation laser wavelength and verified the power scaling behaviour of the signal.

Enhanced Fano resonances in a silicon nitride photonic crystal nanobeam-assisted micro ring resonator for dual telecom band operation.

Silicon-based Micro Ring Resonators (MRR) are a powerful tool for the realization of label free optical biosensors. The sharp edge of a Fano resonance in a Silicon Nitride (Si3N4) platform can boost photonic sensing applications based on MRRs. In this work, we demonstrate enhanced Fano resonance features for a Si3N4 Micro Ring Resonator assisted by a Photonic Crystal Nanobeam (PhCN-MRR) operating in the TM-like mode at the O-band wavelengths. Our findings show that the fabricated PhCN-MRR results in increased asymmetric resonances for TM-like mode compared with TE-like mode operation in the C-band. As a result, a versatile and flexible design to realize Fano resonance with polarization dependent asymmetry in the C and O telecom bands is presented.

Mid-Infrared Photothermal Spectroscopy for the Detection of Caffeine in Beverages.

Caffeine is the most widely consumed stimulant and is the subject of significant ongoing research and discussions due to its impact on human health. The industry's need to comply with country-specific food and beverage regulations underscores the importance of monitoring caffeine levels in commercial products. In this study, we propose an alternative technique for caffeine analysis that relies on mid-infrared laser-based photothermal spectroscopy (PTS). PTS exploits the high-power output of the quantum cascade laser (QCL) sources to enhance the sensitivity of the mid-IR measurement. The laser-induced thermal gradient in the sample scales with the analytes' absorption coefficient and concentration, thus allowing for both qualitative and quantitative assessment. We evaluated the performance of our experimental PTS spectrometer, incorporating a tunable QCL and a Mach-Zehnder interferometer, for detecting caffeine in coffee, black tea, and an energy drink. We calibrated the setup with caffeine standards (0.1-2.5 mg mL-1) and we benchmarked the setup's capabilities against gas chromatography (GC) and Fourier-transform infrared (FTIR) spectroscopy. Quantitative results aligned with GC analysis, and limits of detection matched the research-grade FTIR spectrometer, indicating an excellent performance of our custom-made instrument. This method offers an alternative to established techniques, providing a platform for fast, sensitive, and non-destructive analysis without consumables as well as with high potential for miniaturization.

Compact angled multimode interference duplexers for multi-gas sensing applications.

A compact, low-loss 2 × 1 angled-multi-mode-interference-based duplexer is proposed as an optical component for integrating several wavelengths with high coupling efficiency. The self-imaging principle in multimode waveguides is exploited to combine two target wavelengths, corresponding to distinctive absorption lines of important trace gases. The device performance has been numerically enhanced by engineering the geometrical parameters, offering trade-offs in coupling efficiency ratios. The proposed designs are used as versatile duplexers for detecting gas combinations such as ammonia-methane, ammonia-ethane, and ammonia-carbon dioxide, enabling customization for specific sensing applications. The duplexers designed are then fabricated and characterized, with a special focus on assessing the impact of the different target wavelengths on coupling efficiency.

Highly efficient and selective integrated directional couplers for multigas sensing applications.

The design and fabrication of a compact, low-loss, broadband directional coupler (DC) based duplexer operating in the near-infrared (NIR) region are demonstrated. The duplexer exhibits high selectivity and coupling efficiency (CE), for target wavelengths of 1530 nm and 1653.7 nm, making it applicable in systems for the multi-gas detection of ammonia and methane. The measured CE for the duplexer is 73% and 76% at 1530 nm and 1653.7 nm respectively. These results demonstrate the effectiveness of the duplexer as a broadband and scalable power source for highly sensitive sensing techniques, like quartz-enhanced photoacoustic spectroscopy (QEPAS). Its compact size and low-loss characteristics make it highly portable and well-suited for drone-based multi-gas detection applications.

Unlocking the monolithic integration scenario: optical coupling between GaSb diode lasers epitaxially grown on patterned Si substrates and passive SiN waveguides.

Silicon (Si) photonics has recently emerged as a key enabling technology in many application fields thanks to the mature Si process technology, the large silicon wafer size, and promising Si optical properties. The monolithic integration by direct epitaxy of III-V lasers and Si photonic devices on the same Si substrate has been considered for decades as the main obstacle to the realization of dense photonics chips. Despite considerable progress in the last decade, only discrete III-V lasers grown on bare Si wafers have been reported, whatever the wavelength and laser technology. Here we demonstrate the first semiconductor laser grown on a patterned Si photonics platform with light coupled into a waveguide. A mid-IR GaSb-based diode laser was directly grown on a pre-patterned Si photonics wafer equipped with SiN waveguides clad by SiO2. Growth and device fabrication challenges, arising from the template architecture, were overcome to demonstrate more than 10 mW outpower of emitted light in continuous wave operation at room temperature. In addition, around 10% of the light was coupled into the SiN waveguides, in good agreement with theoretical calculations for this butt-coupling configuration. This work lift an important building block and it paves the way for future low-cost, large-scale, fully integrated photonic chips.

High-Q asymmetrically cladded silicon nitride 1D photonic crystals cavities and hybrid external cavity lasers for sensing in air and liquids.

In this paper we show a novel design of high Q-factor silicon nitride (SiN) 1D photonic crystal (PhC) cavities side-coupled to curved waveguides, operating with both silica and air cladding. The engineering of the etched 1D PhC cavity sidewalls angle allows for high Q-factors over a wide range of upper cladding compositions, and the achievement of the highest calculated Q-factor for non-suspended asymmetric SiN PhC structures. We show the employment of these type of SiN PhC cavities in hybrid external cavity laser (HECL) configuration, with mode-hop free single mode laser operation over a broad range of injected currents (from 25 mA to 65 mA), milliwatts of power output (up to 9 mW) and side-mode suppression ratios in the range of 40 dB. We demonstrate the operation of these devices as compact and energy efficient optical sensors that respond to refractive index changes in the surrounding medium the measurement of sodium chloride (from 0% to 25%) and sucrose (from 0% to 25%) in aqueous solution. In HECL configuration, the RI sensor exhibits a 2 orders of magnitude improvement in detection limit compared to the passive microcavity. We also discuss the possibility for applying these devices as novel transducers for refractive index changes that are induced by analyte specific absorption of infrared radiation by the target analytes present in gas or liquid phase.

Long cavity photonic crystal laser in FDML operation using an akinetic reflective filter.

A novel configuration of a Fourier domain mode locked (FDML) laser based on silicon photonics platform is presented in this work that exploits the narrowband reflection spectrum of a photonic crystal (PhC) cavity resonator. Configured as a linear Fabry-Perot laser, forward biasing of a p-n junction on the PhC cavity allowed for thermal tuning of the spectrum. The modulation frequency applied to the reflector equalled the inverse roundtrip time of the long cavity resulting in stable FDML operation over the swept wavelength range. An interferometric phase measurement measured the sweeping instantaneous frequency of the laser. The silicon photonics platform has potential for very compact implementation, and the electro-optic modulation method opens the possibility of modulation speeds far beyond those of mechanical filters.

ODX: A Fitness Tracker-Based Device for Continuous Bacterial Growth Monitoring.

Continuous monitoring of bacterial growth in aqueous media is a crucial process in academic research as well as in the biotechnology industry. Bacterial growth is usually monitored by measuring the optical density of bacteria in liquid media, using benchtop spectrophotometers. Due to the large form factor of the existing spectrophotometers, they cannot be used for live monitoring of the bacteria inside bacterial incubation chambers. Additionally, the use of benchtop spectrometers for continuous monitoring requires multiple samplings and is labor intensive. To overcome these challenges, we have developed an optical density measuring device (ODX) by modifying a generic fitness tracker. The resulting ODX device is an ultraportable and low-cost device that can be used inside bacterial incubators for real-time monitoring even while shaking is in progress. We evaluated the performance of ODX with different bacterial types and growth conditions and compared it with a commercial benchtop spectrophotometer. In all cases, ODX showed comparable performance to that of the standard benchtop spectrophotometer. Finally, we demonstrate a simple and useful smartphone application whereby the user is notified when the bacterial concentration reaches the targeted value. Due to its potential for automation and mass production, we believe that the ODX has a wide range of applications in biotechnology research and industry.

Realization of a flat-band superprism on-chip from parallelogram lattice photonic crystals.

By optimizing the dispersion curve of a parallelogram-based 2D photonic crystal superprism for constant angular group velocity dispersion over a broad bandwidth, we designed a device capable of experimentally demonstrating linear dispersion from 1500 to 1600 nm with clear separation of as many as eight channels, while maintaining a compact footprint.

Author Correction: Reflection from a free carrier front via an intraband indirect photonic transition.

The original version of this article contained an error in first sentence of the Acknowledgements, which incorrectly read 'M.A.G, D.J., A.Y.P. and M.E. acknowledge the support of the German Research Foundation under grant no. EI 391/13-2, and appreciate the support of CST, Darmstadt, Germany, with their Microwave Studio Software.' The correct version states '261759120' in place of 'EI 391/13-2'. This has been corrected in both the PDF and HTML versions of the article.

Optimizing band-edge slow light in silicon-on-insulator waveguide gratings.

A systematic analysis of photonic bands and group index in silicon grating waveguides is performed, in order to optimize band-edge slow-light behavior in integrated structures with low losses. A combination of numerical methods and perturbation theory is adopted. It is shown that a substantial increase of slow light bandwidth is achieved when decreasing the internal width of the waveguide and the silicon thickness in the cladding region. It is also observed that a reduction of the internal width does not undermine the performance of an adiabatic taper.

Reflection from a free carrier front via an intraband indirect photonic transition.

The reflection of light from moving boundaries is of interest both fundamentally and for applications in frequency conversion, but typically requires high pump power. By using a dispersion-engineered silicon photonic crystal waveguide, we are able to achieve a propagating free carrier front with only a moderate on-chip peak power of 6 W in a 6 ps-long pump pulse. We employ an intraband indirect photonic transition of a co-propagating probe, whereby the probe practically escapes from the front in the forward direction. This forward reflection has up to 35% efficiency and it is accompanied by a strong frequency upshift, which significantly exceeds that expected from the refractive index change and which is a function of group velocity, waveguide dispersion and pump power. Pump, probe and shifted probe all are around 1.5 µm wavelength which opens new possibilities for "on-chip" frequency manipulation and all-optical switching in optical telecommunications.

Wavelength stability in a hybrid photonic crystal laser through controlled nonlinear absorptive heating in the reflector.

The need for miniaturized, fully integrated semiconductor lasers has stimulated significant research efforts into realizing unconventional configurations that can meet the performance requirements of a large spectrum of applications, ranging from communication systems to sensing. We demonstrate a hybrid, silicon  photonics-compatible photonic crystal (PhC) laser architecture that can be used to implement cost-effective, high-capacity light sources, with high side-mode suppression ratio and milliwatt output output powers. The emitted wavelength is set and controlled by a silicon PhC cavity-based reflective filter with the gain provided by a III-V-based reflective semiconductor optical amplifier (RSOA). The high power density in the laser cavity results in a significant enhancement of the nonlinear absorption in silicon in the high Q-factor PhC resonator. The heat generated in this manner creates a tuning effect in the wavelength-selective element, which can be used to offset external temperature fluctuations without the use of active cooling. Our approach is fully compatible with existing fabrication and integration technologies, providing a practical route to integrated lasing in wavelength-sensitive schemes.

Photonic crystal slow light waveguides in a kagome lattice.

Slow light photonic crystal waveguides tightly compress propagating light and increase interaction times, showing immense potential for all-optical delay and enhanced light-matter interactions. Yet, their practical application has largely been limited to moderate group index values (<100), due to a lack of waveguides that reliably demonstrate slower light. This limitation persists because nearly all such research has focused on a single photonic crystal lattice type: the triangular lattice. Here, we present waveguides based on the kagome lattice that demonstrate an intrinsically high group index and exhibit slow and stopped light. We experimentally demonstrate group index values of >150, limited by our measurement resolution. The kagome-lattice waveguides are an excellent starting point for further slow light engineering in photonic crystal waveguides.

Lithographic wavelength control of an external cavity laser with a silicon photonic crystal cavity-based resonant reflector.

We report the experimental demonstration of a new design for external cavity hybrid lasers consisting of a III-V semiconductor optical amplifier (SOA) with fiber reflector and a photonic crystal (PhC)-based resonant reflector on SOI. The silicon reflector is composed of an SU8 polymer bus waveguide vertically coupled to a PhC cavity and provides a wavelength-selective optical feedback to the laser cavity. This device exhibits milliwatt-level output power and side-mode suppression ratios of more than 25 dB.

Tight focus of light using micropolarizer and microlens.

Using a binary microlens of diameter 14 µm and focal length 532 nm (NA=0.997) in resist, we focus a 633 nm laser beam into a near-circular focal spot with dimensions (0.35 ± 0.02)λ and (0.38 ± 0.02)λ (λ is incident wavelength) at full width half-maximum intensity. The area of the focal spot is 0.105λ(2). The incident light is a mixture of linearly and radially polarized beams generated by reflecting a linearly polarized Gaussian beam at a 100 µm × 100 µm four-sector subwavelength diffractive optical microelement with a gold coating. The focusing of a linearly polarized laser beam (the other conditions being the same) is found to produce an elliptical focal spot measuring (0.40 ± 0.02)λ and (0.50 ± 0.02)λ. To our knowledge, this is the first implementation of subwavelength focusing of light using a pair of micro-optic elements (a binary microlens and a micropolarizer).

Extraction of group index of lossy photonic crystal waveguides.

We present a numerical approach to extract group index in photonic crystal (PhC) waveguides using two- and three-dimensional finite-difference time-domain methods and make a quantitative study of the effects of loss on slow light propagation in PhC waveguides. PhC waveguides are simulated with varying material loss and varying PhC waveguide length. Finally, we validate our method by comparing three-dimensional simulation results with experimental results.

Experimental demonstration of original optical filter based on multiply coupled waveguides.

We present an experimental demonstration of an optical filter based on multiply coupled waveguides that has previously been demonstrated only numerically. The experimental results show a good match to numerical modeling using a 2D finite-difference time-domain method that utilizes a modified effective index method (MEIM) approximation. The MEIM correctly describes both the phase and the group indices of 3D silicon wire, providing the means to study complicated and large photonic structures with moderate computer resources and simulation time. An optical filter with a set of 12 directional couplers, constructed on a SOITEC silicon-on-insulator wafer with 220 nm of silicon on a 2-µm-thick buried oxide layer, provides at optical wavelength 1.59 µm a free spectral range about 16 nm and 3 dB linewidth about 1.6 nm. These parameters are limited by the radius of curvature used (5 µm) and the small structure sizes available (40 µm×200 µm). Fabrication imperfections, such as sidewall roughness, cause moderate variations in the waveguide width, approximately 2 nm, which results in parasitic responses in the transmission spectrum of the drop spate. This effect could be decreased through improvements in technology and by decreasing the length of the connecting waveguides. Our results prove that proposed filter can be manufactured by modern CMOS compatible technology and is promising for a range of applications in photonics.

Optically induced indirect photonic transitions in a slow light photonic crystal waveguide.

We demonstrate indirect photonic transitions in a silicon slow light photonic crystal waveguide. The transitions are driven by an optically generated refractive index front that moves along the waveguide and interacts with a signal pulse copropagating in the structure. We experimentally confirm a theoretical model which indicates that the ratio of the frequency and wave vector shifts associated with the indirect photonic transition is identical to the propagation velocity of the refractive index front. The physical origin of the transitions achieved here is fundamentally different than in previously proposed refractive index modulation concepts with fixed temporal and spatial modulation frequencies; as here, the interaction with the refractive index front results in a Doppler-like signal frequency and wave vector shift. Consequently, the bandwidth over which perfect mode frequency and wave vector matching is achieved is not intrinsically limited by the shape of the photonic bands, and tuning of the indirect photonic transitions is possible without any need for geometrical modifications of the structure. Our device is genuinely nonreciprocal, as it provides different frequency shifts for co- and counterpropagating signal and index fronts.