On-chip low-loss all-optical MoSe2 modulator.

Monolayer transition metal dichalcogenides (TMDCs), like MoS2, MoSe2, WS2, and WSe2, feature direct bandgaps, strong spin-orbit coupling, and exciton-polariton interactions at the atomic scale, which could be harnessed for efficient light emission, valleytronics, and polaritonic lasing, respectively. Nevertheless, to build next-generation photonic devices that make use of these features, it is first essential to model the all-optical control mechanisms in TMDCs. Herein, a simple model is proposed to quantify the performance of a 35-µm-long Si3N4 waveguide-integrated all-optical MoSe2 modulator. Using this model, a switching energy of 14.6 pJ is obtained for a transverse-magnetic (TM) and transverse-electric (TE) polarized pump signals at λ = 480 nm. Moreover, maximal extinction ratios of 20.6 dB and 20.1 dB are achieved for a TM and TE polarized probe signal, respectively, at λ = 500 nm with an ultra-low insertion loss of <0.3 dB. Moreover, the device operates with an ultrafast recovery time of 50 ps, while maintaining a high extinction ratio for practical applications. These findings facilitate modeling and designing novel TMDC-based photonic devices.

Investigation of light-matter interaction in single vertical nanowires in ordered nanowire arrays.

Quasi one-dimensional semiconductor nanowires (NWs) in either arrays or single free-standing forms have shown unique optical properties (i.e., light absorption and emission) differently from their thin film or bulk counterparts, presenting new opportunities for achieving enhanced performance and/or functionalities for optoelectronic device applications. However, there is still a lack of understanding of the absorption properties of vertically standing single NWs within an array environment with light coupling from neighboring NWs within certain distances, due to the challenges in fabrication of such devices. In this article, we present a new approach to fabricate single vertically standing NW photodetectors from ordered InP NW arrays using the focused ion beam technique, to allow direct measurements of optical and electrical properties of single NWs standing in an array. The light-matter interaction and photodetector performance are investigated using both experimental and theoretical methods. The consistent photocurrent and simulated absorption mapping results reveal that the light absorption and thus photoresponse of single NWs are strongly affected by the NW array geometry and related light coupling from their surrounding dielectric environment, due to the large absorption cross section and/or strong light interaction. While the highest light concentration factor (∼19.64) was obtained from the NW in an array with a pitch of 1.5 µm, the higher responsivity per unit cell (equivalent to NW array responsivity) of a single vertical NW photodetector was achieved in an array with a pitch of 0.8 µm, highlighting the importance of array design for practical applications. The insight from our study can provide important guidance to evaluate and optimize the device design of NW arrays for a wide range of optoelectronic device applications.

Highly Efficient Near-Infrared Detector Based on Optically Resonant Dielectric Nanodisks.

Fast detection of near-infrared (NIR) photons with high responsivity remains a challenge for photodetectors. Germanium (Ge) photodetectors are widely used for near-infrared wavelengths but suffer from a trade-off between the speed of photodetection and quantum efficiency (or responsivity). To realize a high-speed detector with high quantum efficiency, a small-sized photodetector efficiently absorbing light is required. In this paper, we suggest a realization of a dielectric metasurface made of an array of subwavelength germanium PIN photodetectors. Due to the subwavelength size of each pixel, a high-speed photodetector with a bandwidth of 65 GHz has been achieved. At the same time, high quantum efficiency for near-infrared illumination can be obtained by the engineering of optical resonant modes to localize optical energy inside the intrinsic Ge disks. Furthermore, small junction capacitance and the possibility of zero/low bias operation have been shown. Our results show that all-dielectric metasurfaces can improve the performance of photodetectors.

Damage analysis of a perfect broadband absorber by a femtosecond laser.

Plasmonic metamaterial absorbers are particularly important in different applications such as photodetectors, microbolometers and solar cells. In this paper, we propose a tungsten boride (WB, a refractory ceramic) based broadband metamaterial absorber whose optical properties is numerically analyzed and experimentally characterized. We have also analyzed the damage characteristics of this absorber using a femtosecond laser and compared with an ordinary Au metamaterial absorber. We observe that WB has almost the double absorption bandwidth with absorption more than 90% over the spectral range of 950 to 1400 nm when compared with the Au counterpart. Furthermore, we show that Au metamaterial is damaged at the power of around 36.4 mW whereas WB metamaterial is not damaged at that power (WB has high Tammann temperature than Au)-however the atom of WB material was knocked off by the bombardment of a femtosecond laser.

High Fluence Chromium and Tungsten Bowtie Nano-antennas.

Nano-antennas are replicas of antennas that operate at radio-frequencies, but with considerably smaller dimensions when compared with their radio frequency counterparts. Noble metals based nano-antennas have the ability to enhance photoinduced phenomena such as localized electric fields, therefore-they have been used in various applications ranging from optical sensing and imaging to performance improvement of solar cells. However, such nano-structures can be damaged in high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics and nano-scale heat transfer systems. Having a small footprint, nano-antennas cannot handle high fluences (energy density per unit area) and are subject to being damaged at adequately high power (some antennas can handle just a few milliwatts). In addition, given that nano-antennas are passive devices driven by external light sources, the potential damage of the antennas limits their use with high power lasers: this liability can be overcome by employing materials with high melting points such as chromium (Cr) and tungsten (W). In this article, we fabricate chromium and tungsten nano-antennas and demonstrate that they can handle 110 and 300 times higher fluence than that of gold (Au) counterpart, while the electric field enhancement is not significantly reduced.

Optoelectronic figure of merit of a metal nanoparticle-quantum dot (MNP-QD) hybrid molecule for assessing its suitability for sensing applications.

Recently, many have studied various configurations of metal nanoparticle-quantum dot (MNP-QD) hybrid molecules based on different metals and tunable parameters. In this paper, we aim to incite the interest in using MNP-QD nanohybrids, which possess sensing capabilities superior to those of the individual constituents, for sensing applications that rely on scattered light. When assessing whether a given MNP-QD configuration is suited for an application, sometimes it is hard to assess the pros and cons of a given configuration against other candidates. Here we propose a simple, elegant relative figure of merit (RFoM), which focuses on maximizing the scattered intensity and the refractive index sensitivity of the nanohybrid, to rank the suitability of viable MNP-QD configurations for a particular sensing application. We use the proposed RFoM to analyse the optical spectra of noble, transition, post transition and alkali metal based MNP-QD nanohybrids using the representative metals Au, Ag, Cu, Al and Na, adopting a generalized nonlocal optical response (GNOR) method based cavity QED approach. Based on our observations, we suggest how the usage of MNP-QD nanohybrids could improve the conventionally studied tumour targeting applications. Moreover, we propose potential substitutes for noble metals conventionally considered for MNP-QD nanohybrids.

Magnesium diboride (MgB2) as a saturable absorber for a ytterbium-doped Q-switched fiber laser.

Magnesium diboride (MgB2) is a well-known superconductor at temperatures below 39 K. At higher temperatures, it behaves as a lossy material. In this paper, we examine the performance of MgB2 nano-particles as saturable absorber in a ytterbium-doped fiber ring laser at room temperature: we show that the nano-particles can produce pulses between 200 and 1700 ns. The dynamics of the saturable absorber are both examined as a stand-alone saturable absorber and in combination with an acousto-optic modulator. We believe, to the best of our knowledge, that this is the first time that MgB2 is used as a saturable absorber in a Q-switched laser.

Integration of bow-tie plasmonic nano-antennas on tapered fibers.

In this article, a new and flexible approach to control the electric field enhancement of bow-tie nano-antennas by integrating them on the lateral of a tapered optical fiber is proposed. The device is driven by a Q-switched laser and the performance of a fabricated nano-antenna in a quartz slide is tested by a Surface Enhanced Raman Scattering (SERS) experiment. A refractive index sensing experiment is also performed and a sensitivity of (240 ± 30) nm/RIU is found in the 1.33-1.35 index range.

Ytterbium-doped Q-switched fiber laser based upon manganese dioxide (MnO2) saturable absorber.

Manganese dioxide (MnO2) is an abundant material that is widely used in many devices, such as alkaline batteries. At infrared frequencies, MnO2 is lossy and strongly absorbs light. These characteristics make MnO2 a potential candidate as a low-cost saturable absorber in Q-switched lasers. In this paper, we examine the performance of MnO2 as a saturable absorber in an ytterbium-doped Q-switched fiber laser: we show that it can produce pulses with durations ranging from 300 to 1800 ns.

Absorption enhancement in graphene photonic crystal structures.

Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, is attracting significant interest because of its potential applications in electronic and optoelectronic devices. Although graphene exhibits almost uniform absorption within a large wavelength range, its interaction with light is weak. In this paper, the enhancement of the optical absorption in graphene photonic crystal structures is studied: the structure is modified by introducing scatterers and mirrors. It is shown that the absorption of the graphene photonic crystal structure can be enhanced about four times (nearly 40%) with respect to initial reference absorption of 9.8%. The study can be a useful tool for investigating graphene physics in different optical settings.

Finite-difference time-domain methods to analyze ytterbium-doped Q-switched fiber lasers.

Q-switched lasers are widely used in material processing, laser ranging, medicine, and nonlinear optics--in particular, Q-switched lasers in optical fibers are important since they cannot only generate high peak powers but can also concentrate high peak powers in small areas. In this paper, we present new finite-difference time-domain methods that analyze the dynamics of Q-switched fiber lasers, which are more flexible and robust than previous methods. We extend the method to analyze fiber ring lasers and compare the results with our experiments.

Polarizer based upon a plasmonic resonant thin layer on a squeezed photonic crystal fiber.

In this article, a polarizer based on surface plasmon resonance in a squeezed rectangular lattice is analyzed through a full-vector finite-element method solver. The device allows one state of polarization (e.g., y-polarized mode) to propagate through the fiber while the other state (x-polarized mode) is heavily attenuated: the modal losses for the x- and y-polarized modes are 1221 dB/cm and 1.6 dB/cm, respectively, at the wavelength of 1310 nm. Given the high differential attenuation between the two orthogonal polarization modes, the device could be used as a compact polarizer with potential applications in sensing, communications, and other areas.

Thick multilayered (silica/gold) dipole nano-antenna.

Nano-antennas are the optical equivalent of antennas that are used to transmit and receive information at radio frequencies. These antennas have been used in different applications in photonics such as optical imaging, particle manipulation, bio-sensing, and improvement of the performance of solar cells. In this article we study composite nano-antennas made of alternating layers of silica and gold. We show that a 50% filling factor leads to a 2.0 times increase in the electric-field enhancement factor when compared with a pure-gold antenna.

Offset semi-parabolic nanoantenna made of a photonic crystal parabolic mirror and a plasmonic bow-tie antenna.

In a parabolic mirror, light coming parallel to the antenna passes through its focal point. In this work, a waveguide feeds a semi-parabolic photonic crystal mirror and the emerging beam feeds a bow-tie antenna placed at the mirror's focal point-it is shown that the antenna system can not only feed a bow-tie antenna (producing a localized moderately high electric field) but also produces a directional radiation beam. The semi-parabolic mirror is also modified to reduce reflection back to the feeding waveguide.

Composite chromium and graphene oxide as saturable absorber in ytterbium-doped Q-switched fiber lasers.

In recent years, graphene and its compounds (e.g., oxides) have been used as saturable absorbers in passive Q-switched and mode-locked lasers, leading to the fabrication of compact pulsed fiber lasers. In this article, we study the operation of a Q-switched ytterbium-doped fiber ring laser based on a composite saturable absorber made of graphene oxide and chromium. We show that the addition of a thin layer of chromium can lead to pulse durations ranging from 3.5 to 9.4 µs and subsequently increasing the laser peak power.

Arrays of recycled power TM polarized nano-antennas.

In recent years, plasmonic nano-antennas have been used in a wide range of applications in sensing, particle detection, imaging and Surface Enhanced Raman Scattering (SERS) detection. Also, arrays of nano-antennas have been recently developed to produce more directional radiation beams or to operate over a wide range of wavelengths. In this article, it is shown that small arrays of nano-antennas can be created by recycling the power that flows through their antenna gaps.

Analysis of interference between two optical beams in a quasi-zero electric permittivity photonic crystal superlattice.

A quasi-zero-average-index photonic crystal structure has been recently demonstrated by using the concept of complementary media. It consists of dielectric photonic crystal superlattices with alternating layers of negative index photonic crystals and positive index dielectric media. This photonic crystal structure has unique optical properties, such as phase-invariant field and self-collimation of light. In particular, the nanofabricated superlattices can be used in chip-scale optical interconnects and interferometers with quasi-zero-average phase difference. However, in potential interconnect applications, crosstalk between neighboring signals needs to be avoided. In this article, we study simulations of the interference of propagating electromagnetic waves in a quasi-zero electric permittivity photonic crystal superlattice. The simulations here are restricted to TM modes, with the main electric field along the vertical direction.

Driving plasmonic nanoantennas with triangular lasers and slot waveguides.

Plasmonic nanoantennas can generate high-intensity electric fields in a very small area. However, being passive devices, they need to be excited by external laser sources. The excitation of nanoantennas by semiconductor lasers can be inefficient and a significant amount of light may return back to the laser source after being scattered by the nanoantenna. In this paper, it is shown that the amount of light being returned to the semiconductor laser can be reduced by using dielectric slot waveguides. These waveguides can transport the incident light to the nanoantennas, but the amount of nondirectional back-scattered light is reduced after propagation through the slot waveguide.

Improved transmission model for metal-dielectric-metal plasmonic waveguides with stub structure.

We present an improved analytical model describing transmittance of a metal-dielectric-metal (MDM) waveguide coupled to an arbitrary number of stubs. The model is built on the well-known analogy between MDM waveguides and microwave transmission lines. This analogy allows one to establish equivalent networks for different MDM-waveguide geometries and to calculate their optical transmission spectra using standard analytical tools of transmission-line theory. A substantial advantage of our model compared to earlier works is that it precisely incorporates the dissipation of surface plasmon polaritons resulting from ohmic losses inside any metal at optical frequencies. We derive analytical expressions for transmittance of MDM waveguides coupled to single and double stubs as well as to N identical stubs with a periodic arrangement. We show that certain phase-matching conditions must be satisfied to provide opt al filtering characteristics for such waveguides. To check the accuracy of our model, its results are compared with numerical data obtained from the full-blown finite-difference time-domain simulations. Close agreement between the two suggests that our analytical model is suitable for rapid design optimization of MDM-waveguide-based compact photonic devices.

Coupling of light from microdisk lasers into plasmonic nano-antennas.

An optical dipole nano-antenna can be constructed by placing a sub-wavelength dielectric (e.g., air) gap between two metallic regions. For typical applications using light in the infrared region, the gap width is generally in the range between 50 and 100 nm. Owing to the close proximity of the electrodes, these antennas can generate very intense electric fields that can be used to excite nonlinear effects. For example, it is possible to trigger surface Raman scattering on molecules placed in the vicinity of the nano-antenna, allowing the fabrication of biological sensors and imaging systems in the nanometric scale. However, since nano-antennas are passive devices, they need to receive light from external sources that are generally much larger than the antennas. In this article, we numerically study the coupling of light from microdisk lasers into plasmonic nano-antennas. We show that, by using micro-cavities, we can further enhance the electric fields inside the nano-antennas.