Mid-infrared dual-comb spectroscopy with electro-optic modulators

Ming Yan, Pei-Ling Luo, Kana Iwakuni, Guy Millot, Theodor W Hänsch, Nathalie Picqué


Abstract:Absorption spectroscopy of fundamental ro-vibrational transitions in the mid-infrared region provides a powerful tool for studying the structure and dynamics of molecules in the gas phase and for sensitive and quantitative gas sensing. Laser frequency combs permit novel approaches to perform broadband molecular spectroscopy. Multiplex dual-comb spectroscopy without moving parts can achieve particularly high speed, sensitivity and resolution. However, achieving Doppler-limited resolution in the mid-infrared still requires overcoming instrumental challenges. Here we demonstrate a new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators. The combs have a remarkably flat intensity distribution, and their positions and line spacings can be freely selected by simply dialing a knob. Using the proposed technique, we record, in the 3-μm region, Doppler-limited absorption spectra with resolved comb lines within milliseconds, and precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies of, for example, trace gases.

Enhanced second-harmonic generation from two-dimensional MoSe2 on a silicon waveguide

Haitao Chen, Vincent Corboliou, Alexander S Solntsev, Duk-Yong Choi, Maria A Vincenti, Domenico de Ceglia, Costantino de Angelis, Yuerui Lu, Dragomir N Neshev


Abstract:Two-dimensional transition-metal dichalcogenides (TMDCs) with intrinsically broken crystal inversion symmetry and large second-order nonlinear responses have shown great promise for future nonlinear light sources. However, the sub-nanometer monolayer thickness of such materials limits the length of their nonlinear interaction with light. Here, we experimentally demonstrate the enhancement of the second-harmonic generation from monolayer MoSe2 by its integration onto a 220-nm-thick silicon waveguide. Such on-chip integration allows for a marked increase in the interaction length between the MoSe2 and the waveguide mode, further enabling phase matching of the nonlinear process. The demonstrated TMDC–silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform, including efficient frequency conversion, parametric amplification and the generation of entangled photon pairs.

Phase-controlled propagation of surface plasmons

Basudeb Sain, Roy Kaner, Yehiam Prior


Abstract:Directional emission of electromagnetic radiation can be achieved using a properly shaped single antenna or a phased array of individual antennas. Control of the individual phases within an array enables scanning or other manipulations of the emission, and it is this property of phased arrays that makes them attractive in modern systems. Likewise, the propagation of surface plasmons at the interface between metal films and dielectric materials can be determined by shaping the individual surface nanostructures or via the phase control of individual elements in an array of such structures. Here, we demonstrate control of the propagation of surface plasmons within a linear array of nanostructures. The generic situation of plasmonic surface propagation that is different on both sides of a metal film provides a unique opportunity for such control: plasmons propagating on the slower side feed into the side with the faster propagation, creating a phased array of interfering antennas and thus controlling the directionality of the wake fields. We further show that by shaping the individual nanoantennas, we can generate an asymmetric propagation geometry.

Automated design of freeform imaging systems

Tong Yang, Guo-Fan Jin, Jun Zhu


Abstract:The automated design of imaging systems involving no or minimal human effort has always been the expectation of scientists, researchers and optical engineers. In addition, it is challenging to choose an appropriate starting point for an optical system design. In this paper, we present a novel design framework based on a point-by-point design process that can automatically obtain high-performance freeform systems. This framework only requires a combination of planes as the input based on the configuration requirements or the prior knowledge of designers. This point-by-point design framework is different from the decades-long tradition of optimizing surface coefficients. Compared with the traditional design method, whereby the selection of the starting point and the optimization process are independent of each other and require extensive amount of human effort, there are no obvious differences between these two processes in our design framework, and the entire design process is mostly automated. This automated design process significantly reduces the amount of human effort required and does not rely on advanced design skills and experience. To demonstrate the feasibility of the proposed design framework, we successfully designed two high-performance systems as examples. This point-by-point design framework opens up new possibilities for automated optical design and can be used to develop automated optical design in the areas of remote sensing, telescopy, microscopy, spectroscopy, virtual reality and augmented reality.

Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering

Fuxing Gu, Fuming Xie, Xing Lin, Shuangyi Linghu, Wei Fang, Heping Zeng, Limin Tong, Songlin Zhuang


Abstract:Single-mode lasing in whispering-gallery mode (WGM) microresonators is challenging to achieve. In bottle microresonators, the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability. In this work, by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. The pump intensity is engineered into an interference distribution on the bottle microresonator surface. By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection. Attractive advantages of the system, including high side-mode suppression factors >20 dB, large spectral tunability >8 nm, low-lasing threshold and reversible control, are presented. Our demonstrated approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics.

Long-lived electron emission reveals localized plasmon modes in disordered nanosponge antennas

Germann Hergert, Jan Vogelsang, Felix Schwarz, Dong Wang, Heiko Kollmann, Petra Groß, Christoph Lienau, Erich Runge, Peter Schaaf


Abstract:We report long-lived, highly spatially localized plasmon states on the surface of nanoporous gold nanoparticles—nanosponges—with high excitation efficiency. It is well known that disorder on the nanometer scale, particularly in two-dimensional systems, can lead to plasmon localization and large field enhancements, which can, in turn, be used to enhance nonlinear optical effects and to study and exploit quantum optical processes. Here, we introduce promising, three-dimensional model systems for light capture and plasmon localization as gold nanosponges that are formed by the dewetting of gold/silver bilayers and dealloying. We study light-induced electron emission from single nanosponges, a nonlinear process with exponents of n≈5...7, using ultrashort laser pulse excitation to achieve femtosecond time resolution. The long-lived electron emission process proves, in combination with optical extinction measurements and finite-difference time-domain calculations, the existence of localized modes with lifetimes of more than 20 fs. These electrons couple efficiently to the dipole antenna mode of each individual nanosponge, which in turn couples to the far-field. Thus, individual gold nanosponges are cheap and robust disordered nanoantennas with strong local resonances, and an ensemble of nanosponges constitutes a meta material with a strong polarization independent, nonlinear response over a wide frequency range.

Supercritical angle Raman microscopy: a surface-sensitive nanoscale technique without field enhancement

Diana Serrano, Stefan Seeger


Abstract:Raman scattering microscopy is a versatile tool for label-free imaging and molecular fingerprint analysis. Here, we provide the first demonstration that the selective collection of scattered signals exceeding the critical angle for total internal reflection enables surface-confined spontaneous Raman investigations at nanometre resolution. This high-axial selectivity leads to improved signal-to-background ratios, thus making this technique an excellent probe for surface-related molecular specimens. The richness of the spectroscopic information obtained through the supercritical angle Raman (SAR) collection path was proven by comparing its output with that of a parallel far-field collection path. Furthermore, we demonstrated that the proposed SAR technique is a versatile microscopy approach that can be used alone or in combination with amplified Raman modalities such as surface-enhanced resonance Raman scattering.

Enhanced optical performance of multifocal metalens with conic shapes

Yanjun Bao, Qiao Jiang, Yimin Kang, Xing Zhu, Zheyu Fang


Abstract:A multifocal metalens, which focuses incident light at multiple foci, has many applications in imaging systems and optical communications. However, the traditional design strategy of a multifocal metalens combines several lenses that have different focal points into a planar integrated unit, resulting in low imaging quality because of the high background noise. Here we show that the defects of the traditional method can be overcome by designing a metalens with conic shapes (the ellipse and the hyperbola); this approach could improve the imaging performance and substantially decrease the background noise of multifocal metalenses. These benefits arise from the intrinsic properties of the two conic curves, which can focus incident light constructively at all of the foci of the metalens. We further demonstrate that the proposed conic-shaped metalens can function well within a broadband operation wavelength that ranges from 600 to 900 nm with the dual polarity actively controlled by the incident circular polarized light. The great agreement between the experimental and simulation results demonstrates that our proposed metalens has significant potential for use in future integrated nanophotonic devices.

Continuous-wave laser operation of a dipole antenna terahertz microresonator

Luca Masini, Alessandro Pitanti, Lorenzo Baldacci, Miriam S Vitiello, Riccardo Degl'Innocenti, Harvey E Beere, David A Ritchie, Alessandro Tredicucci


Abstract:Resonators and the way they couple to external radiation rely on very different concepts if one considers devices belonging to the photonic and electronic worlds. The terahertz frequency range, however, provides intriguing possibilities for the development of hybrid technologies that merge ideas from both fields in novel functional designs. In this paper, we show that high-quality, subwavelength, whispering-gallery lasers can be combined to form a linear dipole antenna, which creates a very efficient, low-threshold laser emission in a collimated beam pattern. For this purpose, we employ a terahertz quantum-cascade active region patterned into two 19-μm-radius microdisks coupled by a suspended metallic bridge, which simultaneously acts as an inductive antenna and produces the dipole symmetry of the lasing mode. Continuous-wave vertical emission is demonstrated at approximately 3.5 THz in a very regular, low-divergence (±10°) beam, with a high slope efficiency of at least 160 mW A−1 and a mere 6 mA of threshold current, which is ensured by the ultra-small resonator size (VRES/λ3≈10−2). The extremely low power consumption and the superior beam brightness make this concept very promising for the development of miniaturized and portable THz sources to be used in the field for imaging and sensing applications as well as for exploring novel optomechanical intracavity effects.

Tailoring of electromagnetic field localizations by two-dimensional graphene nanostructures

Ze-Bo Zheng, Jun-Tao Li, Teng Ma, Han-Lin Fang, Wen-Cai Ren, Jun Chen, Jun-Cong She, Yu Zhang, Fei Liu, Huan-Jun Chen, Shao-Zhi Deng, Ning-Sheng Xu


Abstract:Graphene has great potential for enhancing light−matter interactions in a two-dimensional regime due to surface plasmons with low loss and strong light confinement. Further utilization of graphene in nanophotonics relies on the precise control of light localization properties. Here, we demonstrate the tailoring of electromagnetic field localizations in the mid-infrared region by precisely shaping the graphene into nanostructures with different geometries. We generalize the phenomenological cavity model and employ nanoimaging techniques to quantitatively calculate and experimentally visualize the two-dimensional electromagnetic field distributions within the nanostructures, which indicate that the electromagnetic field can be shaped into specific patterns depending on the shapes and sizes of the nanostructures. Furthermore, we show that the light localization performance can be further improved by reducing the sizes of the nanostructures, where a lateral confinement of λ0/180 of the incidence light can be achieved. The electromagnetic field localizations within a nanostructure with a specific geometry can also be modulated by chemical doping. Our strategies can, in principle, be generalized to other two-dimensional materials, therefore providing new degrees of freedom for designing nanophotonic components capable of tailoring two-dimensional light confinement over a broad wavelength range.