Large Spectral Range Research Articles

On-Chip Optical Microresonators With High Electro-Optic Tuning Efficiency

Electro-optic tuning by using lithium niobate (LiNbO3) possesses the unique features of sub-picosecond response time, low power consumption, reduced thermal noise, and combination of nonlinear-optic applications. Here, we demonstrate a new device design and fabrication to realize on-chip electro-optically tunable LiNbO3 microresonators with high performance. This device design solves the problem of cancel-out of effective index variation induced by electro-optic effect in the microresonators made of anisotropic materials. Besides, it enables the device functions of high quality factor, polarization-selective electro-optic tuning, and large free spectral range simultaneously with few high-order modes. The microdisk structure with the air-bridge wiring produced by the standard semiconductor process facilitates the realization of integrated photonic circuits. The presented device achieves the electro-optic tuning rate up to 29.2 pm/V and the large free spectral range of 17.1 nm. The proposed device design offers highly-efficient platforms for nonlinear optics, cavity optomechanics, Kerr frequency comb generation, and cavity quantum electrodynamics.

Selective tuning of optical modes in a silicon comb-like photonic crystal cavity

Abstract Realizing multiply resonant photonic crystal cavities with large free spectral range is key to achieve integrated devices with highly efficient nonlinear response, such as frequency conversion, four-wave mixing, and parametric oscillation. This task is typically difficult owing to the cavity modes’ sensitivity to fabrication disorder, which makes it hard to reliably achieve a comb-like spectrum of equally spaced modes even when a perfect matching is theoretically predicted. Here we show that a comb-like spectrum of up to eight modes with very high quality factor and diffraction limited volumes can be engineered in the bichromatic-type potential of a two-dimensional photonic crystal cavity fabricated in a thin silicon membrane. To cope with the tight tolerance in terms of frequency spacings and resonance linewidths, we develop a permanent post-processing technique that allows the selective tuning of individual confined modes, thus achieving an almost perfect frequency matching of high Q resonances with record finesse in silicon microresonators. Our experimental results are extremely promising in view of ultra-low power nonlinear photonics in silicon.

Demonstration of daytime wind measurement by using mobile Rayleigh Doppler Lidar incorporating cascaded Fabry-Perot etalons.

A well-designed filter assembly is incorporated to an earlier mobile Rayleigh Doppler Lidar developed at University of Science and Technology of China (USTC) for wind measurement round the clock. The filter assembly consists of two cascaded Fabry-Perot Etalons (FPEs) and a narrow-band interference filter (IF), which are optimized to filter out strong solar background radiation during daytime. The high resolution FPE is mainly used to compress the whole bandwidth of the filter assembly, whereas the low resolution FPE with relatively large free spectral range (FSR) is primarily used to block the unwanted periodic transmission peaks of high resolution FPE arising within the narrow-band IF passband. Some test experiments are carried out and demonstrate that the filter assembly have an overall peak transmission of 33.32% with a bandwidth of 2.41 pm at 355 nm. When applying it to the USTC mobile Rayleigh Doppler Lidar, the daytime background is only 3% or less than before. Consequently, the detectable altitude during daytime increases to ∼51 km with wind velocity accuracy of ±7.6 m/s.

Broadband continuous single-mode tuning of a short-cavity quantum-cascade VECSEL

Changing the length of a laser cavity is a simple technique for continuously tuning the wavelength of a laser but is rarely used for broad fractional tuning, with a notable exception of the vertical-cavity surface-emitting laser (VCSEL)1,2. This is because, to avoid mode hopping, the cavity must be kept optically short to ensure a large free spectral range compared to the gain bandwidth of the amplifying material. Terahertz quantum-cascade lasers are ideal candidates for such a short cavity scheme as they demonstrate exceptional gain bandwidths (up to octave spanning)3 and can be integrated with broadband amplifying metasurfaces4. We present such a quantum-cascade metasurface-based vertical-external-cavity surface-emitting laser (VECSEL) that exhibits over 20% continuous fractional tuning of a single laser mode. Such tuning is possible because the metasurface has subwavelength thickness, which allows lasing on low-order Fabry–Perot cavity modes. Good beam quality and high output power are simultaneously obtained. Terahertz quantum-cascade lasers with over 20% continuous fractional tuning of a single laser mode are now possible thanks to metasurfaces.

Demonstration of opposing thermal sensitivities in hollow-core fibers with open and sealed ends.

The output phase and propagation time of an optical signal propagating through a hollow-core optical fiber (HCF) drift with changes in environmental temperature significantly less than in conventional optical fibers. In all earlier experimental studies, however, the simplifying assumption was made that the thermo-optic effect of air was negligible. In this Letter, we present, to the best of our knowledge, the first experimental demonstration that the air inside a HCF core can make an appreciable contribution to the fiber's thermal sensitivity with the performance depending on whether the fiber is open to the atmosphere or sealed at both ends (e.g., spliced to solid fiber pigtails). We measure both the sensitivity of the accumulated phase as well as the signal propagation time for both open and sealed HCF and show that these are opposite in sign. Most importantly, we show that the thermal sensitivity contribution from the air inside an open HCF has the sign opposite to the effect of fiber elongation (which is otherwise the dominant effect responsible for the overall thermal sensitivity of HCF). We then go on to show that these two effects can be used to balance each other out in order to achieve zero thermal sensitivity for both accumulated phase and propagation time. We demonstrate this property experimentally over a large spectral range.

Tunable large free spectral range microring resonators in lithium niobate on insulator

Microring resonators are critical photonic components used in filtering, sensing and nonlinear applications. To date, the development of high performance microring resonators in LNOI has been limited by the sidewall angle, roughness and etch depth of fabricated rib waveguides. We present large free spectral range microring resonators patterned via electron beam lithography in high-index contrast Z-cut LNOI. Our microring resonators achieve an FSR greater than 5 nm for ring radius of 30 μm and a large 3 dB resonance bandwidth. We demonstrate 3 pm/V electro-optic tuning of a 70 μm-radius ring. This work will enable efficient on-chip filtering in LNOI and precede future, more complex, microring resonator networks and nonlinear field enhancement applications.

A remote sensing adapted approach for soil organic carbon prediction based on the spectrally clustered LUCAS soil database

The estimation of the soil organic carbon (SOC) content plays an important role for carbon sequestration in the context of climate change, food security and soil degradation. Reflectance spectroscopy has proven to be a promising technique for SOC quantification in the laboratory and increasingly from air- and spaceborne platforms, where hyperspectral imagery provides great potential for mapping SOC on larger scales with regular updates. When applied on larger scales, soil prediction accuracy decreases due to the inhomogeneity of samples. In this paper, we examined if spectral clustering of the LUCAS EU-wide topsoil database is successful without using other covariates than the spectral database and can improve SOC model performance compared to a reference model that was calibrated on the whole database without clustering. Different clustering methodologies were tested, including a k-means clustering based on principal component analysis or based on spectral feature variables, combined with partial least squares regression (PLSR) models, and a clustering based on a local PLSR approach which builds a different multivariate model for each sample to be predicted. Furthermore, in order to allow for subsequent application to hyperspectral remote sensing data, atmospheric water wavelengths were removed from the analyses. The local PLSR approach achieved best results and was additionally applied to LUCAS spectra resampled to the upcoming hyperspectral EnMAP sensor which led to good results: R2 = 0.66, RMSEP = 5.78 g kg−1 and RPIQ = 1.93. The k-means clustering approach showed slightly better results than the reference model. Overall, our results showed similar performances for SOC prediction models compared to other approaches using PLSR with a larger spectral range and other soil parameters as covariates. This study shows that (i) it is possible to transfer the local PLSR approach onto a wavelengths reduced spectral library and to predict estimations of SOC at low-cost with reasonable accuracy based on large scale soil databases; and (ii) that the local regression approach is a valuable tool for SOC prediction models based solely on spectral data without the use of other soil covariates.

Optical properties of plasmonic components based on a nanoscale three-dimensional plasmonic waveguide

In this paper, the properties of five kinds of passive nanoscale optical devices (including directional coupler, waveguide ring resonator, add-drop filter, nano light focusing device, and Mach–Zehnder interferometer, etc.) based on a new nanoscale three-dimensional plasmonic waveguide the authors proposed previously are presented. For the proposed nanoscale optical directional coupler, the coupling length does not show monotonic increase with the increase of edge-to-edge distance between two waveguides, and thus there exists an optimal edge-to-edge distance of around 0.1 μm with a minimum coupling length of about 2.7 μm and high coupling efficiency of 93.4%. And the waveguide ring resonator with a very small ring radius of 1.1 μm and a separate gap of 80 nm features an extinction ratio as small as 2.9%, with large free spectral range of 194 nm, and a bandwidth of 24 nm. Moreover, an add-drop filter with the same ring radius of 1.1 μm and a separate gap of 80 nm features a 24.4% extinction ratio. The optical properties of the devices are numerically investigated by the finite-difference time-domain simulation method. These excellent properties make them have great potential in nanophotonic integrated circuits.

Fingerprint-to-CH stretch continuously tunable high spectral resolution stimulated Raman scattering microscope.

Stimulated Raman scattering (SRS) microscopy is a label-free method generating images based on chemical contrast within samples, and has already shown its great potential for high-sensitivity and fast imaging of biological specimens. The capability of SRS to collect molecular vibrational signatures in bio-samples, coupled with the availability of powerful statistical analysis methods, allows quantitative chemical imaging of live cells with sub-cellular resolution. This application has substantially driven the development of new SRS microscopy platforms. Indeed, in recent years, there has been a constant effort on devising configurations able to rapidly collect Raman spectra from samples over a wide vibrational spectral range, as needed for quantitative analysis by using chemometric methods. In this paper, an SRS microscope which exploits spectral shaping by a narrowband and rapidly tunable acousto-optical tunable filter (AOTF) is presented. This microscope enables spectral scanning from the Raman fingerprint region to the Carbon-Hydrogen (CH)-stretch region without any modification of the optical setup. Moreover, it features also a high enough spectral resolution to allow resolving Raman peaks in the crowded fingerprint region. Finally, application of the developed SRS microscope to broadband hyperspectral imaging of biological samples over a large spectral range from 800 to 3600 cm-1 , is demonstrated.

Photoluminescence of Ag+ and Agn+m in co-doped Pr3+/Yb3+ fluorophosphate glasses: tuning visible emission and energy transfer to Pr3+/Yb3+ ions through excitation in different silver species

Here, we report the luminescence properties of Yb3+/Pr3+ dual-doped fluorophosphate glasses containing silver clusters. Through the solubility of silver into the glass system as function of its initial concentration, charged species characterized as Ag+ and Ag m n+ were formed and identified according to their absorption and emission spectral responses. By changing the transition metal concentration, multi-energy transfer mechanisms were observed between silver and Yb3+/Pr3+ co-doped glasses. The stabilization of silver species seems to be detrimental to the radiative processes of the rare earth ions under excitation at 444 nm (Pr3+3H4 → 3P0 absorption line). In other words, the main visible emissions of Pr3+ and the NIR emission of Yb3+ decrease with increment of Ag concentration. However, energy transfer occurs for large spectral range by the excitation of the silver species. In consequence, the visible and NIR emission could be activate in a broadband from ultraviolet to the blue part of the spectral region. These two observations highlight the competitive absorption processes and probabilities between the cationic elements and the interest to use the transition metal species as sensitizers to excite the rare earth elements.

Compact and Broadband Optical Add-Drop De-Multiplexer With Cascaded Elliptical Micro-Rings on SOI

We demonstrate a four-channel de-multiplexer based on third-order cascaded elliptical micro-rings on silicon-on-insulator. By applying cascaded elliptical micro-rings structure, the fabrication tolerance was released. The spectral responses of four drop-ports over the S, C, and L bands (1450–1630 nm) were measured, which showed an enhanced performance. The four-channel de-multiplexer has box-like transmission spectra with an ultra-wide 3 dB bandwidth of about 2.6 nm, high out-of-band rejection ratio of more than 40 dB, low-channel cross talk of about −38 dB, small footprint of about 0.0012 mm2, and large operational spectral range of more than 180 nm. The de-multiplexer can support high-data-rate signal transmission capacity.

An Integrated Low-Voltage Broadband Lithium Niobate Phase Modulator

Electro-optic phase modulators are critical components in modern communication, microwave photonic, and quantum photonic systems. Important for these applications is to achieve modulators with low half-wave voltage at high frequencies. Here we demonstrate an integrated phase modulator, based on a thin-film lithium niobate platform, that simultaneously features small on-chip loss (~ 1 dB) and low half-wave voltage over a large spectral range (3.5 - 4.5 V at 5 - 40 GHz). By driving the modulator with a strong 30-GHz microwave signal corresponding to around four half-wave voltages, we generate an optical frequency comb consisting of over 40 sidebands spanning 10 nm in the telecom L-band. The high electro-optic performance combined with the high RF power-handling ability (3.1 W) of our integrated phase modulator are crucial for future photonics and microwave systems.

Multipole Resonances in Transdimensional Lattices of Plasmonic and Silicon Nanoparticles

ABSTRACTTransdimensional photonics has emerged as a new field of science and engineering that explores the optical properties of materials and nanostructures in the translational regime between two and three dimensions. In the present work, we study an example of such transdimensional lattice consisting of nanoparticle array, and we aim at a direct comparison of lattice resonances excited in the periodic lattices of either plasmonic (gold) or silicon nanoparticles of the same size and interparticle spacing. We numerically analyze extinction cross-sections and reflection from the array, and we include electric and magnetic dipoles and electric quadrupoles into consideration. Lattice resonances are excited at the wavelength close to Rayleigh anomaly which is defined by the array periodicity, and different multipoles respond to one or another period of rectangular array depending on incident light polarization. We show that lattice resonances originating from dipole moments are extended to the larger spectral range than electric-quadrupole lattice resonances. Overlap of resonances causes a decrease in reflection (generalized Kerker effect) and, in the case of electric quadrupole and magnetic dipole moments, the coupling of the multipoles is enabled by the lattice.

Broadband TE Optical Isolators and Circulators in Silicon Photonics Through Ce:YIG Bonding

Optical isolators and circulators are fundamental building block in photonic integrated circuits to block undesired reflections and routing light according to a prescribed direction. In silicon photonics, heterogeneous integration of magneto-optic garnet bonded on a pre-patterned silicon layer has been demonstrated to be an effective solution for manufacturing optical isolators and circulators for TM polarized light. However, most integrated semiconductor lasers emit TE polarized light, which indicates the need to find a reliable solution for this polarization. In this paper, we demonstrated broadband optical isolators and circulators for TE polarized light based on heterogeneous bonding on the silicon photonics platform. To achieve this goal, an integrated adiabatic coupler and a broadband polarization rotator are designed and optimized. The nonreciprocal behavior is induced through an energy-efficient integrated electromagnet with a minimum power consumption of 3 mW. Two isolators/circulators are fabricated with small and large free spectral range, respectively. In the former case, an optical isolation ratio as large as 30 dB is measured at 1555 nm with an insertion loss of 18 dB, while for the broadband circulator, an optical isolation larger than 15 dB is guaranteed over more than 14 nm (1.75 THz) for all port combinations with an insertion loss between 14 and 18 dB at 1560 nm. Finally, it has been theoretically shown that the insertion loss can be reduced below 6 dB with design and fabrication improvements. To the best of the authors’ knowledge, the proposed integrated TE optical circulator is the first experimental demonstration of this device in silicon photonics.

Exploiting supramolecular assemblies for filterless ultra-narrowband organic photodetectors with inkjet fabrication capability

Strongly coupled dye molecules are known to produce narrowband absorption in a large spectral range. Here we exploit this feature to achieve organic photodetectors with ultra-narrow full-width at half-maximum response at low bias voltage.

Real-Time Temperature Sensor Based on In-Fiber Fabry–Perot Interferometer Embedded in a Resin

A simple to fabricate, cost-effective, robust, and highly accurate fiber optic temperature sensor is proposed and experimentally demonstrated. The sensor head consists of an in-fiber Fabry–Perot interferometer (FFPI) embedded in a mold made of a polyester resin. FFPI, which in general present good strain sensitivity and low-temperature sensitivity, was assembled by splicing a microsegment of capillary fiber of 14.66 μ m between two single-mode fibers. The reflected spectrum of the FFPI exhibited a single interference dip within a span of 80 nm. The dimensions of the resin mold can be modified by changing the ambient temperature, which in turn changes the length of the FFPI air-cavity. The dip of the reflected spectrum is shifted toward shorter (larger) wavelengths when external temperature is decreased (increased). The large free spectral range (FSR) allowed to monitor the temperature changes by either a wavelength or an intensity interrogation scheme. Using a laser and an optical power meter it was possible to demonstrate a real-time temperature sensor with a sensitivity of −60.79 nW/°C and a resolution of 0.5 °C, in the range of 10–30 °C. We believe that the practicality of this real-time sensor can be very attractive for environmental sensing application.

Plasmonic quantum size effects in silver nanoparticles are dominated by interfaces and local environments

The physical properties of metals change when their dimensions are reduced to the nano-scale and new phenomena such as the localized surface-plasmon resonance (LSPR) appear. This collective electronic excitation can be tuned over a large spectral range by adapting the material, size and shape. The existing literature is as rich as it is controversial—for example, size-dependent spectral shifts of the LSPR in small metal nanoparticles, induced by quantum effects, are reported to the red, to the blue or entirely absent. Here we report how complementary experiments on size-selected small silver nanoparticles embedded in silica can yield inconsistent results on the same system: whereas optical absorption shows no size effect in the range between only a few atoms and ~10 nm, a clear spectral shift is observed in single-particle electron spectroscopy. Our quantitative interpretation, based on a mixed classical/quantum model, resolves the apparent contradictions, not only within our experimental data, but also in the literature. Our comprehensive model describes how the local environment is the crucial parameter controlling the manifestation or absence of quantum size effects. The origin of size-dependent shifts of surface plasmon resonances in metal nanoparticles has been controversial for decades. A combined experimental and theoretical study on silver samples and their environments now provides a quantitative picture.

Double injection resonator

A single silicon double injection resonator provides flexible response shapes, large free spectral range and tolerance to temperature deviations and fabrication defects, paving the way for high-performance integrated photonics.

Ultrasmall in-plane demultiplexer enabled by an arrayed one-dimensional photonic crystal nanobeam cavity

We propose a design for silicon-on-chip integrated eight-channel wavelength division multiplexing (WDM) demultiplexer, which consists of parallel-arrayed one-dimensional (1-D) photonic crystal nanobeam cavities (PCNCs) with high-Q over 105 and large free spectral range of ∼200 nm. To the best of our knowledge, this is for the first time that a 1-D PCNC-based demultiplexer is presented. The performance of the device is investigated theoretically by using three-dimensional finite-difference time-domain method. To enable eight-channel parallel arrayed 1-D PCNCs to be coupled to on-chip optical networks for higher integration and multiplex application, an 1 × 8 taper-type equal optical power splitter is used to connect all channels simultaneously. The total device footprint is as small as 12 μm × 15 μm (width × length), which is decreased by five times compared to that per channel in the recent two-dimensional (2-D) PC-based demultiplexer. Moreover, the average channel spacing smaller than 115 GHz is achieved, which is more than two times smaller than that of 2-D PC nanocavity devices, demonstrating that the arrayed nanocavities have the potential for developing ultracompact 100-GHz spaced filters in a dense WDM system. Thus, we believe that the results demonstrated in this work is promising for the future on-chip photonics integrated circuits and optical communication systems.

A large free spectral range of 74.92 GHz in comb peaks generated by SU-8 polymer micro-ring resonators: simulation and experiment

The feasibility of integrated SU-8 polymeric optical waveguides on a silicon platform was demonstrated experimentally and supported by simulation. Micro-ring resonator (MRR) configurations were designed and fabricated using SU-8 polymer. Simulation is carried out using the finite-difference time-domain method to improve the design. Both the simulation and experimental results confirm that SU-8 can be successfully used in the optical MRR. Passive MMI-coupled SU-8 single coupler and double coupler with a free spectral range of 74.92 GHz is achieved practically and corresponds with the simulation result. Moreover, other important parameters such as the full width at half maximum, Q factor and finesse are calculated from the simulation and practical results to support the feasibility of the SU-8 polymer MRR.