Spotlight on “Broadband on-chip near-infrared spectroscopy based on plasmonic grating filter array”

Broadband high-resolution spectroscopy: plasmonics makes it cheap on a chip. The development of portable, cost-effective solutions for on-site sensing requires spectroscopy systems that use neither expensive nor bulky instruments. On-chip spectrometers are good candidates due to their inherent low cost, but they typically suffer from a limited spectral resolution or operating bandwidth, or they need sophisticated optical coupling methods to couple light into sub-micron-scale waveguides. In this Optics Letters article, R. Li and coworkers effectively exploit the combination of subwavelength gratings with plasmonic nanostructures to realize an ultra-compact, broadband on-chip spectrometer with a very high resolution. The device integrates 28 individual subwavelength plasmonic grating filters in a footprint of less than 1.5 mm2, providing a spectral resolution as high as 10 nm over a near-infrared (IR) operation bandwidth of 270 nm. An accuracy comparable to that of conventional Fourier Transform IR spectroscopy is achieved thanks to a post-processing numerical method compensating for spurious side peaks in the transmission spectrum of each plasmonic filter. The optical transmission pattern of the entire filter array can be also acquired through a CCD camera, enabling the monitoring of all optical wavelengths simultaneously. Neither moving elements nor critical optical alignment systems are employed, thus improving the system reliability and simplifying measurement operations. The result is a spectrometer that is ultracompact, cost-effective, broad-band, high-resolution, reliable, robust and easy to use… definitely promising for future portable IR spectroscopy systems

Spotlight on “Broadband Mid-Infrared Frequency Comb Generation in a Si3N4 Microresonator”

No more than four years ago, a broadband integrated frequency comb generator operating in the near-infrared range was highlighted in Spotlight on Optics. (https://www.osapublishing.org/spotlight/summary.cfm?id=220223). At that time, the realization of an entire device on a photonic chip set a first milestone towards what we called the “comb generator dream” of extremely compact, cheap and robust optical comb generators. A promising step forward compared to traditional technologies exploiting ultrafast mode-locked lasers, which definitely work very well, but suffer from bulkiness, cost and limited line-separation issues. Since then, optical comb generators have become even more attractive for many applications, such as optical clocks, precision frequency metrology, high-speed communications systems and optical waveform synthesizers. One of the next frontiers is now moving deeply into the mid-infrared range, where the advent of optical comb generators is expected to bring new tools for advanced precision spectroscopy, molecular structure investigation and gas sensing. Four years later, we are here to comment on another milestone in the optical comb generator route. The same material as before, namely silicon nitride (Si3N4). And the same research group, joining the teams of M. Lipson and A. Gaeta from Cornell University. Yet, much longer wavelengths now, well above 2 μm. And a completely new story begins. Unfortunately for people working in integrated optics, moving from a wavelength range to another is not a copy-and-paste process, even when dealing with passive devices. Not only can material properties change dramatically, but the behaviour of materials commonly used at certain wavelengths can be almost unknown at others, because of the lack of characterization instruments or even light sources. And this is the case of Si3N4, whose optical properties have been thoroughly studied at telecom wavelengths and below, but not in the mid-infrared range. For a reliable description of the optical properties of Si3N4 in the mid-infrared range, K. Luke and co-workers first derived a wavelength extended version of the Sellmeier equation. They achieved this by characterizing the refractive index and the absorption coefficient of the material over an ultra-broad wavelength range, spanning from the ultraviolet (193 nm) up to the far infrared (33 μm). Their results demonstrate that Si3N4 can provide strong enough anomalous dispersion to generate wide spectral combs in the near-infrared range, yet requiring an optical waveguide with a height (about 1 μm) well beyond the thicknesses typically limited by the intrinsic film stress. To overcome the mechanical stress limit, a technique previously developed by the same group was employed, which is based on crack isolation trenches realized before Si3N4 film deposition. Further, to prevent stress-induced wafer bowing, both sides of the wafers were processed. However, this was not enough. Loss was actually the key issue to address, because efficient resonator-based comb generators require very high Q-factors. In order to minimize absorption losses in the film, multiple annealing steps were performed during film deposition. These techniques allowed to increase the Q-factor of a microring resonator from a value of 55,000, that was measured on a reference single anneal device, to a record Q value of 1.0 × 106 at a wavelength of 2.6 μm, this being the highest Q ever achieved for on-chip resonators operating in this wavelength range. The authors claim also that the Q factor could be even improved with optimized etching process and improved anneal cycling during Si3N4 deposition. After reading this paper, one can’t help but to think about the next episode of the integrated comb generator saga. It is difficult to predict how the story will evolve, if even longer wavelengths will be covered with Si3N4 comb sources, or other materials will come into play. Let’s hope to have news soon, hopefully before the next four years

Spotlight on “Accurate interchannel pitch control in graded index circular-core polymer parallel optical waveguide using the Mosquito method”

The successful story of fiber optics has demonstrated the superiority of optical communications over competing technologies in long-haul data transmissions. Now, short-range high-capacity data interconnects are envisioned as the next field where optics and photonics technologies are likely to play a revolutionary role. In this contest, energy consumption per bit is the key number, and photon transmission is potentially much less power hungry than electronic transmission. This is the reason why, in a near future, massive data exchanges between and inside supercomputers are expected to be carried by photons. A part of this new story has already become reality. Board-to-board interconnects in supercomputers are already performed though multimode fiber links, providing high-bandwidth-density wirings with lower power consumption than that required by electric cables. But we can definitely do more; we can try to bring photons closer and closer to the brain of supercomputers. The new frontier is chip-to-chip optical interconnects, that is photonics links providing high-capacity and energy saving communications between electronic chips. Although this research field has been under the lens for several years, a winner photonic technology has not emerged yet. And the race is getting faster and faster. Silicon photonics is considered one of the most promising technology candidates, because it enables the integration of fast modulators, wide-band routing and switching architectures, and photodetectors onto the same chip. In other words, all we need for realizing end-to-end optical links on a chip. Yet, nothing is for free, and silicon photonics has still to solve challenging issues related to strong sensitivity to fabrication tolerances, and to temperature and environmental fluctuations. Another option is given by the use of multimode polymer waveguide arrays, which are drawing much attention because they offer high-density wirings with very low-propagation loss, low interchannel crosstalk, high bandwidth, and high coupling efficiency with multimode fiber, VCSELs and photo detectors (PDs). The main issue with this technology is the development of a simple, low-cost and reliable process to fabricate cm-scale long waveguide arrays with inter-waveguide pitches of a few tens of μm. A very original and surprisingly effective solution has been proposed by Kinoshita and co-workers, who have developed the so-called Mosquito method. In this technique, UV curable silicone resins are employed for fabricating the core and the cladding of polymer optical waveguides. The core resin is dispensed into the cladding resin, deposited in a liquid phase on a substrate from a needle mounted on a syringe, whose position is controlled by a horizontally scanning robot. After UV curing and baking of the core and cladding resins, a circularly-shaped core waveguide with a graded-index profile is magically obtained. This method allows the simplification of the fabrication of polymer optical waveguides, since photomasks, large-scale UV exposure apparatus, and chemical etching processes are no longer needed. Further, graded index core waveguides exhibit better performance than step-index waveguides in terms of loss and mutual optical crosstalk. By optimizing the fabrication process, waveguide arrays with a circular core of 50 μm and a pitch as small as 62.5-μm have been realized in this work, with an impressive control of waveguide size, spacing and circularity. The suitability of the Mosquito method for high-bandwidth-density on-board and board-to-board optical interconnects is confirmed by the demonstration of 12 x 11.3 Gbps signal transmissions in an array of 12 waveguides without any signal deterioration. This successful result is a clear demonstration that technological processes must not be stuck in conventional schemes, because different ways of thinking are the primary key to the most significant advances. It is difficult to say at this point if the Mosquito method has the potential to compete with more consolidated photonic technologies, but it is always good to have alternative routes to follow. Let’s wait for more news from Kinoshita and co-workers

Breakthroughs in Photonics 2013: Toward Feedback-Controlled Integrated Photonics

We present an overview of the main achievements obtained in 2013 on the monitoring, stabilization, and feedback loop control of passive and active photonic integrated circuits. Key advances contributed to the evolution of photonic technologies from the current device level toward complex, adaptive, and reconfigurable integrated circuits

Low loss, high contrast optical waveguides based on CMOS compatible LPCVD processing: technology and experimental results

A new class of integrated optical waveguide structures is presented, based on low cost CMOS compatible LPCVD processing. This technology allows for medium and high index contrast waveguides with very low channel attenuation. The geometry is basically formed by a rectangular cross-section silicon nitride (Si3N4) filled with and encapsulated by silicon dioxide (SiO2). The birefringence and minimal bend radius of the waveguide is completely controlled by the geometry of the waveguide layer structures. Experiments on typical geometries will be presented, showing excellent characteristics (channel attenuation ≤ 0.1 dB/cm, IL ≤ 1.5 dB, PDL ≤ 0.2 dB, Bg ≤ 1×10-4, bend radius « 1 mm)

Spotlight on “Ultralow crosstalk Nanosecond-scale Nested 2x2 Mach-Zehnder Silicon Photonic Switch”

Fast switching in silicon photonics gets record performance. High-performance supercomputing and datacentre networks yearns for energy-efficient solutions for fast and high-capacity data switching in high-port-count nodes. Electronics is the current technology, but it is bound to resort soon to heavy parallelization and power-hungry multi-chip architectures; optical technologies already offer 3D-MEMS switches, which are however too slow for applications that require data-packet reconfiguration. Silicon photonics has the right stuff to become a key technology for low-power switch fabrics operating at nanosecond-scale. Yet, to date, high loss and high optical crosstalk limit the port-count of silicon photonic switches to a handful of I/Os. In this Optics Letters article, N. Dupuis and coworkers present something that is likely to become a game-changing silicon photonic building block. The idea is rather simple indeed, essentially consisting of 2 × 2 nested-Mach-Zehnder switch where the conventional straight-line phase-shifter integrated in one arm is replaced by a Mach-Zehnder phase shifter. This design enables to fully exploit the energy-efficient and fast switching provided by free-carrier plasma dispersion effect in silicon waveguides, without paying the price of the inherent loss associated with free-carrier absorption. A record crosstalk value of -34.5 dB is achieved, with only 2 dB loss and a remarkably small switching time of 4 ns. Everything we need is monolithically integrated onto a small silicon chip, hosting the photonic switch, the CMOS driver and interface logic. Another brick in the silicon photonic route towards energy efficient computing networks has been added

Spotlight on “Diffractive Waveplate Arrays”

Manipulation of light beams traditionally has been performed by using lenses, prisms, waveplates, and other discrete components. With such bulky elements, we have a rather limited ability to implement complex optical systems, where light beams can be switched, steered, (de)focused, and reconfigured arbitrarily. Much higher flexibility is gained by utilizing photonic metasurfaces that can be engineered at the nanoscale. Among these, diffractive waveplates can be potentially designed to realize all types of optical beam transformation. With respect to conventional devices, such as Fresnel lenses or phase plates, the optical functionality of diffractive waveplates is related to the molecular orientation pattern of their structure (and not to structure discontinuities), offering the possibility to realize much more complex geometries. In this work, S.V. Serak and coworkers gather the most recent advances in the development of new-generation diffractive waveplate arrays. Advanced functionalities, like high-resolution beam shapers and vector vortex waveplates, are demonstrated by designing highly complex architectures made of several overlapped waveplate arrays. High tunability of these devices is achieved by using low-voltage electrical signals to control the orientation of a liquid crystal layer of only a few microns in thickness, thus achieving extremely compact beam steering devices, tunable lenses, and controllable polarization filters. Furthermore, large-area diffractive waveplate arrays are realized on arbitrary (even flexible) substrates by fast and low-cost fabrication processes. It seems that diffractive waveplate arrays have the right stuff to revolutionize electro-optical devices in many fields of application, such as displays, smart windows, optical communications, beam shaping, and optical communications. A new avenue is open

Spotlight on “Experimental observations of thermo-optical bistability and self-pulsation in silicon microring resonators”

Even though the origins of optical resonators date back to the pioneering work by C. Fabry and A. Pérot over a century ago, research on optical resonators is still surprisingly vivid. Optical resonators are a kind of magic box from which unexpected phenomena keep on coming out, especially when different physical mechanisms simultaneously take place and mutually interact. In particular, nonlinear dynamics in optical resonators is a never-ending source of discoveries. Let us consider, for instance, bistability and self-pulsation effects, which can be originated in a resonator at a sufficiently high optical power level. Bistability is related to the existence of two possible stable states, and is of particular interest, for example, in the realization of optical memories, flip-flops and logic units. Self-pulsation, typically occurring at higher power levels, is instead associated with a periodic swinging of the cavity between two non-stable states, which results in the breaking of an input continuous wave into a pulse-train-like output waveform. Here, applications in the field of on-chip clock distribution are envisioned. While nowadays it is not unusual to hear about bistability and self-pulsation, if we were to think we know everything about the nonlinear dynamics of optical resonators, we are probably in for some big surprises. In fact the physics underneath nonlinear processes is much richer than one would expect at first, especially in semiconductor cavities such as silicon microring resonators. In silicon waveguides, free carriers generated by nonlinearity play a twofold and self-counteracting role in terms of refractive index change: a decrease in the refractive index due to free-carrier dispersion (FCD) and an increase in the refractive index due to the thermooptical (TO) effect, that is waveguide heating associated with interband and intraband carrier relaxation and phonon excitation. Depending on the time response of these two effects, the refractive index change can be balanced or not, resulting in a strongly different nonlinear dynamics of the resonator. In a passive silicon microring, the free-carrier lifetime and the thermal decay time are both constants, so that the only way to trigger and control self-pulsation is through the power and the wavelength of the input field. Having a way to control at least one of these characteristic times would provide a powerful knob to arbitrarily switch the state of the resonator across different nonlinear regimes, giving an additional degree of freedom to control bistability to self-pulsation mechanisms. The work by the group of S. Chen L. Zhang et al. proves that this is possible indeed. They investigated the nonlinear dynamics of a silicon microring resonator with an embedded PN junction. When no voltage is applied to the junction, free carriers (that in the 1550 nm wavelength range are mainly induced by two photon absorption) are naturally swept out of the waveguide by the built-in field of the PN junction and recombine at the Si-SiO2 interface in a time scale of about 3 ns. In these conditions, FCD and TO effects are effectively balanced and self-pulsation is experimentally observed for an input power as low as a few milliwatts. By detuning the input wavelength with respect to the microring resonance, the frequency of self-pulsation can be continuously tuned from 10 to 20 MHz, and the duty ratio itself of the output waveform can be varied almost from 1 to 0. If the PN junction is reversed biased (-1 V), free carriers are swept out of the optical waveguide much more quickly, so that the free carrier lifetime reduces by two orders of magnitude (about 0.03 ns). In this regime, the balance between the free-carrier dispersion and thermooptical effect is broken and self-pulsation is inhibited at any input power and wavelength. This means that the nonlinear dynamics of a resonator can be dramatically modified in a fully controllable way simply by means of an external electric control. These results open new horizons in the application of integrated optical resonators, which turn out to be extremely flexible devices, both in the linear and nonlinear regime, as well as key building blocks for the next generation of reconfigurable photonic integrated circuits. For those people interested in fundamentals science, this work teaches us that the way towards a full understanding of optical resonators is still long and, who knows, maybe the most exciting part of the story is yet to come

Spotlight on “Hybrid metal-dielectric nanocavity for enhanced light-matter interactions”

Hybrid metal-dielectric nanocavities: in medio stat virtus Strong confinement of the light in a very small volume is the key towards efficient broadband cavity quantum electrodynamics (CQED) systems. Scientists are rushing to push light confinement to the limit, deeply in the sub-wavelength regime, playing on either the geometry or the material of optical cavities. Dielectric and metallic cavities have been proved to be both viable approaches, but… what if we try to go right in the middle? This is actually where Y. A. Kelaita and coworkers have gone in their research. In this work, they have demonstrated that a hybrid metal-dielectric nanocavity can provide more than one order of magnitude reduction in mode volume compared to state of the art photonic crystal CQED systems. The proposed cavity consists of a dielectric cylindrical nano-pillar, realized on a conventional InAs/GaAs quantum photonics platform, which is laterally coated with silver. InAs quantum dots provide large internal quantum efficiency and short radiative lifetime; the metallic confinement leads to strong and broadband spontaneous emission rate. The fabrication of such a nanocavity is challenging because a conformal metal coating is required to sustain efficiently the cavity mode. To this aim a novel metal evaporation technique has been developed that leaves the top of the cavity free of metal, enabling surface-emission and good collection efficiency. Strong enhancement of broadband spontaneous emission is demonstrated at 10 K, but the authors believe that the proposed platform can be easily extended to emerging room-temperature quantum systems. They also think that their fabrication strategy would enable the realization of more sophisticated structures with even smaller mode volumes. Nanocavities go hybrid, quantum electrodynamics goes better

Spotlight on “Development of integrated mode reformatting components for diffraction-limited spectroscopy”

Photonic dicers bring the light everywhere on a chip and light gets reformatted. This is indeed what we can do with the laser writing technique developed by D. MacLachlan and co-workers, enabling to build optical waveguides of any size and connect them in a 3D space. In other words we can bring the light wherever we want across a photonic chip. An example? We can take the light out of a large multimode waveguide and dice it in a 3D array of stacked single mode waveguides; then we can deliver the waveguides through the photonic chip or even rearrange all the light-paths in a diffraction limited pseudo-slit waveguide. And if we like, we can also go back to a multimode waveguide, and so forth. Well beyond the well-known concept of a photonic lantern, here optical modes can be reformatted arbitrarily and many times, because every waveguide transition is adiabatic and low loss. Applications are not limited to astrophotonics and advanced spectrography, but can be envisioned in all the fields where mode manipulation is required, such as in high-capacity optical transmission systems exploiting spatial mode multiplexing