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Time-resolved terahertz (THz) spectroscopy has been shown as a powerful technique to non-invasively determine the charge carrier properties in photoexcited semiconductors. However, the long wavelengths of terahertz radiation reduce the applicability of this technique to large samples. Using THz near-field microscopy, we show THz measurements of the lifetime of 2D single exfoliated microcrystals of transition metal dichalcogenides (WS2). The increased spatial resolution of THz near-field microscopy allows spatial mapping of the evolution of the carrier lifetime, revealing Auger assisted surface defect recombination as the dominant recombination channel. THz near-field microscopy allows for the non-invasive and high-resolution investigation of material properties of 2D semiconductors relevant for nanoelectronic and optoelectronic applications.

Time-resolved terahertz (THz) spectroscopy is a powerful technique for the determination of charge transport properties in photoexcited semiconductors. However, the relatively long wavelengths of THz radiation and the diffraction limit imposed by optical imaging systems reduce the applicability of THz spectroscopy to large samples with dimensions in the millimeter to centimeter range. Exploiting THz near-field spectroscopy, we present the first time-resolved THz measurements on a single exfoliated 2D nanolayered crystal of a transition metal dichalcogenide (WS2). The high spatial resolution of THz near-field spectroscopy enables mapping of the sheet conductance for an increasing number of atomic layers. The single-crystalline structure of the nanolayered crystal allows for the direct observation of low-energy phonon modes, which are present in all thicknesses, coupling with free carriers. Density functional theory calculations show that the phonon mode corresponds to the breathing mode between atomic layers in the weakly bonded van der Waals layers, which can be strongly influenced by substrate-induced strain. The non-invasive and high-resolution mapping technique of carrier dynamics in nanolayered crystals by time-resolved THz time domain spectroscopy enables possibilities for the investigation of the relation between phonons and charge transport in nanoscale semiconductors for applications in two-dimensional nanodevices.

Exciton-polaritons (EPs) are hybrid light–matter quasi-particles with bosonic character formed by the strong coupling between excitons in matter and photons in optical cavities. Their hybrid character offers promising prospects for the realization of non-equilibrium Bose–Einstein condensates (BECs), and room-temperature BECs are possible with organic materials. However, the thresholds required to create BECs of organic EPs remain still high to allow condensation with electrical injection of carriers. One of the factors behind these high thresholds is the very short cavity lifetimes, leading to a fast EP decay and the need to inject higher exciton densities in the reservoir to form the condensate. Here a BEC of EPs in organic dyes and all-dielectric metasurfaces at room temperature is demonstrated. By using dielectric metasurfaces that exhibit very low losses it is possible to achieve cavity lifetimes long enough to allow an efficient population of EP states via vibrational relaxation and radiative pumping. It is shown how polariton lasing or non-equilibrium Bose–Einstein condensation is achieved in several cavities, and one of the lowest reported thresholds for BECs in organic materials is observed.

This research argues that using an electron beam with high kinetic energy to pump perovskite quantum dots can significantly boost the efficiency of the low-frequency photon radiation conversion. Firstly, we measure the random lasing threshold and luminescence threshold of CsPbX 3 films pumped by an electron beam. Then, we simulate the spatial distribution of the electron beams in CsPbX 3 films. Combined with the above data, a low-frequency photon radiation conversion model based on the electron pumped perovskite quantum dots is presented. This could be a way to create a terahertz source with a high-power output or to multiply the terahertz power.

Electromagnetic field confinement on sub-wavelength scales is possible at the expense of increasing optical losses. Examples are surface plasmon polaritons and resonant structures, which suffer from absorption losses and radiative outcoupling, respectively. Here, the first experimental evidence of strong electromagnetic field confinement along the lateral (out-of-plane) direction in a near lossless metasurface that supports a symmetry-protected Bound State in the Continuum is shown, which fully suppresses the radiative losses of resonant scatterers. This experimental demonstration, supported by coupled dipole calculations, is measured by a double near-field probe technique to locally excite and detect the time evolution and confinement of the electric field onto the surface, achieving an amplitude decay length from the metasurface of λ/38 at 0.39 THz.

Antenna-coupled photomixers, serving as emitters and receivers of terahertz (THz) radiation, are the central active components of coherent optoelectronic THz systems. Here, we focus on the continuous-wave modality, which finds ample use for spectroscopy, sensing, and ranging and plays a major role in upcoming ultrahigh-frequency telecommunication applications. We demonstrate with planar receivers based on low-temperature-grown GaAs that the use of interdigital graphene electrodes instead of metal fingers enhances the responsivity by more than one order of magnitude. The increase in responsivity leads to a strongly enlarged dynamic range and doubles the frequency range covered in the detection process. We show by simulations that these improvements are a consequence of the transparency of the finger electrodes for visible/near-infrared laser radiation, which also activates the area under the fingers for the mixing process, not only their edges as in conventional devices with metal electrodes.

We present an experimental investigation of vibrational strong coupling of C=O bonds in poly(methyl methacrylate) to surface lattice resonances (SLRs) on arrays of gold particles in infrared and Raman spectra. SLRs are generated from the enhanced radiative coupling of localized resonances in single particles by diffraction in the array. Compared to previous studies in Fabry-Perot cavities, particle arrays provide a fully open system that easily couples with external radiation while having large field confinement close to the array. We control the coupling by tuning the period of the array, as evidenced by the splitting of the C=O vibration resonance in the lower and upper vibro-polaritons of the IR extinction spectra. Despite clear evidence of vibrational strong coupling in IR transmission spectra, both Raman spectroscopy and micro-Raman mapping do not show any polariton signatures. Our results suggest that the search for vibrational strong coupling in Raman spectra may need alternative cavity designs or a different experimental approach.

The use of millimeter-wave (mm-wave) frequencies is required in order to support the increasing number of connected devices expected from the fifth generation (5G) of mobile communications. Subsequently, the generation of radio-frequency (RF) carriers ranging from 10 GHz to 300 GHz and their transport through optical distribution network (ODN) is a key element of the future 5G fronthaul. Optically assisted RF carrier generation is one of the most promising solutions to tackle this issue, allowing a wide use of analog radio-over-fiber (ARoF) architectures. However the main limitation of these optical methods is related to the finite coherence of lasers sources, which can dramatically degrade data transmission in analog formats. To mitigate its impact, the use of orthogonal frequency-division multiplexing (OFDM) as the 5G standard allows employing efficient phase noise compensation algorithms. Therefore, in this study, we present an experimental demonstration of a mm-wave generation technique based on an optical phase-locked loop (OPLL) that fulfills the frequency specifications for 5G. Then, an algorithm is introduced that improves data recovery at reception and reduces the impact of a possible high phase noise carrier. Finally, a back-to-back data transmission experiment is performed, demonstrating the efficiency of the algorithm to reach the 5G requirements. These results emphasize the use of OPLLs as a viable solution to generate mm-wave carriers for 5G and beyond.

Although we don’t know exactly how 6G networks will look like beyond 2030, we may assume that we will most likely communicate using holograms, digital twins, and that the merge of real and virtual reality had happen in an hyper-reality internet. Such new ways of communications will demand large capacity and ultra-low latency with increased demands for quantum safe data transmission and storage. Developing such technologies such as Terahertz frequency, terabit data wireless links, seems a futuristic endeavor. This paper considers the role of photonics in tackling the challenge to realize systems to generate, detect, multiplex and process terabit volumes of data in 6G systems and networks.

With a fully functional point-to-point quantum key distribution link, we demonstrate secret key retrieval by a pair of encryptors and investigate how their addition impacts key network performance indicators on a 10 Gbit/s data channel.

In Phase-Modulated Continuous Wave (PMCW) radar the phase symbols can be approximated by constellation points from a quadrature transmitter. However, this leads to mapping errors w.r.t. the ideal symbols. This paper evaluates the effects of the mapping errors in P4 polyphase codes on the performance of PMCW radars based on quadrature transmitters. Two methods are analysed to reduce these errors. Firstly, rotating the phase symbols in the constellation before mapping to the constellation diagram is analysed. Secondly, the effect of applying dither is analysed. System-level simulation results validate both methods and show improvements of the relative sidelobe level exceeding 20 dB.

Fifth-generation mobile networks (5G) is the established solution to satisfy the highly demanding key performance indicators such as traffic volumes, bit-rate, latency, and power consumption, among others of the future telecommunication infrastructure. The already saturated sub-6 GHz spectral band does not accommodate such requirements and forces the move towards higher frequencies, with the millimeter-wave (mm-wave) domain being an adequate band to operate. However, the exploitation of mm-wave signals in the mobile cells implies the deployment of an enormous quantity of small cells with associated equipment, footprint, and control. Thus, analog radio-over-fiber (ARoF) emerges as a suitable technology because of their attractive benefits such as low latency, low hardware complexity, and reduced power consumption. However, through investigation of experimental ARoF systems adhering to the 5G standard is scarce. Therefore, in this work, a novel and efficient bidirectional ARoF scheme based on multicore fiber (MCF) and oriented to 5G mm-wave communications is proposed and experimentally validated. The setup configurations are according to the 5G standard, enabling a wireless link at 26 GHz (n258, K-band) and time division duplex (TDD) communication. The proposed scheme is thoroughly evaluated under all the 5G numerologies and with different bandwidth settings. Moreover, key design considerations of the experimental testbed are explained and discussed to optimize the final yields of the system. The experimental results of both transmission directions are compared and analyzed, and prove the viability of the proposed bidirectional ARoF system as an excellent solution to be part of the future 5G mm-wave network.

The network slicing concept divides physical networks into logical networks and abstracts the network resources. With the help of virtualization technologies, these abstracted network resources can be allocated to service providers and resources can dynamically be added to these slices based on users' demands. The infrastructure sharing model with slicing makes it possible for services to lease the resources of the infrastructure provider. This study considers optical network resource allocation from a profit generation perspective with a game, in which service providers bid to lease C-RAN fronthaul paths via auctioning with Vickrey-Clarke-Groves outcomes. The game aims to distribute fronthaul resources with a social-welfare maximizing outcome. Service providers maximize their revenue by predicting user demand and requesting bandwidth resources from the infrastructure provider by bidding in the auction. Users have the option to change their association and switch between the service providers to maximize their utility. The results display that a balanced profit and social welfare trade-off can be achieved in converged optical and mmWave radio networks infrastructure sharing scenario with Vickrey-Dutch auctioning and distributed decision-making.

We present a wideband watt-level power amplifier (PA) for the Ka -band designed and implemented in the 0.25- μ m SiGe:C BiCMOS technology. The core of the design is a chip with multiple custom PA unit cells (PA-cells), which are interfaced with a power combiner placed on a laminate. The power combiner is based on a principle of the recently proposed multichannel transition with spatial power combining functionality, where an array of strongly coupled microstrip lines (MLs) interface a single substrate integrated waveguide (SIW). The realized watt-level PA combining four differential cascode PA-cells achieves a saturated output power ( Psat ) of 30.8 dBm with 26.7% power-added efficiency (PAE). The 64-QAM modulation tests confirm the competitive PA performance on multi-Gb/s communication signals. The obtained combination of the high PAE and high Psat over a wide frequency band (30%) is an advantageous property of the proposed solution with respect to the previously published designs. This high performance is the result of using the proposed architecture with low-loss (0.6 dB) and wideband (54%) parallel spatial power combiner. Moreover, the presented joint EM-circuit-thermal optimization allows achieving optimal system-level performance by taking into account various critical multiphysics effects occurring in the combined PA. This article describes the design and performance of the whole integrated structure and its individual components.

Confining electromagnetic fields as tightly as possible has driven research in optics and photonics. Conventionally, extreme sub-wavelength confinement is possible at the cost of absorption losses in resonant structures. Here, we experimentally show that it is possible to get an extreme confinement of the electromagnetic field in a resonant system without losses. We exploit symmetry protected bound states in the continuum which fully suppress any radiative losses. Using our unique local excitation and detection terahertz microscope we can measure the confinement of the electric field at the surface of our array, revealing an amplitude decay of λ/38 at 0.39 THz.

We report on a novel and comprehensive method to determine leaf wetness by using machine learning algorithms applied on transmission THz time-domain data of a vast set of moisture patterns. More generally, our approach opens ways to determine properties of materials with a complex structure for which a model-based signal processing approach is not feasible.

Despite the large range of promising applications for Terahertz technology, its real deployment has been limited by bulky equipment, mechanical disturbance, high power consumption, and low flexibility systems. Here, we present the design of a THz platform based on photonic integrated devices and photoconductive antennas that can reduce the footprint of all discrete optics in THz spectroscopy systems by two orders of magnitude. The integrated platform aims to contribute to the technological bridge that will take the THz spectrum from bulky systems to on-chip platforms.

Terahertz (THz) photomixer devices are becoming attractive components for coherent optoelectronic THz systems due to their ability to operate as THz emitters and receivers. Enhanced THz photomixer emitters based on graphene flakes and silver nanowires nanoelectrodes overcome the performance of classical interdigitated photomixers with metal electrodes. Here, we performed a simulation of the hybrid nanoelectrodes structure based on an array of graphene flakes and silver nanowires (AgNWs) to comprehend the plasmonic motion of the generated carriers in the active area of the device. The simulation results show that the conductivity of Ag-NWs was improved by the conductivity of graphene flakes. Where the graphene flakes create a spatial electric field close to their edges. Due to the confined electric field, the carriers will be transported between graphene flakes and Ag-NWs by a spatial plasmonic field, and subsequently making the graphene flakes act as connected rather than isolated. Therefore, graphene and Ag-NWs allow simultaneous carrier transport in the hybrid nanostructure.

The generation and transmission of millimeter-wave signals for 5G applications require the use of broadband and high output power photodetectors to bridge from the optical and electronic domains. Therefore, the deep knowledge on the equivalent circuit characteristics of these devices is vital. This study reviews and analyzes de-embedding techniques contributing to the characterization of the physical aspects within the active region of uni-traveling carrier photodiodes. De-embedding methods analytically remove the parasitic effects of the electrical transmission lines connected to their active area allowing the extraction of their series resistance and junction capacitance toward the synthesis of an equivalent circuit with lumped elements. The open-short technique is examined and a systematic error introduced by this process underlines the vulnerability of the method on removing parasitics with higher complexity. This error is quantified leading to the implementation of a corrected equation converging with the characteristic features of an $S$-parameter-based de-embedding. These characteristics are also analyzed through simulation approaches showing minimal equivalent inaccuracies on eliminating more complex symmetrical parasitics. A thorough comparison between these three methods is conducted through the calculation of lumped components corresponding to the active region of diodes with different sizes.

A combined non-orthogonal multiple access (NOMA) and multiband carrierless amplitude and phase modulation (multiCAP) scheme is proposed for flexible resource provisioning in coherent passive optical networks (PONs). While the proposed combination increases the data-rate and the number of users through the inclusion of NOMA and multiCAP, the coherent reception increases the range and splitting factor of the network through optical power budget enhancement. The proposed system is experimentally evaluated providing a 20 Gb/s aggregated data-rate per wavelength with 10G optoelectronics and applied in two main PON scenarios with four spectral configurations from full-band CAP to four-band multiCAP. The first PON scenario consists of an existing PON that the network operator requires to increase the number of users; NOMA with full-band CAP or multiCAP can be introduced replacing one or more users of the existing PON by one or several new sub-networks and multiplexing the sub-networks by NOMA. In the other PON scenario, several PONs can be nested sharing an initial splitter and achieving a larger number of users by the combination of NOMA and multiCAP. The proposed techniques are fully compatible with other multiplexing techniques such as dense wavelength division multiplexing, which should be considered to achieve higher numbers of users. Gobierno de Aragón (T20_20R); Ministerio de Economía y Competitividad (TEC2017-85752-R, TEC2017-90034-C2-2-R); Electronic Components and Systems for European Leadership (H2020/ECSEL BRAINE - 876967).

A novel and compact millimeter-wave (mm-Wave) spatial power combiner is developed integrating a silicon-based integrated circuit (IC) in a metal waveguide (WG). As an initial step toward integrating a silicon-based active IC in a WG, a passive back-to-back (B2B) transition incorporating a four-way spatial power splitter and combiner is realized at $E$ -band (71–86 GHz). In contrast to existing solutions, the proposed design considers power splitting and combining using a low-loss wireless transition between the IC and the WG. The proposed B2B structure comprises an IC implemented using the Institute for High Performance Microelectronics (IHP’s) 0.13- $\mu \text{m}$ SiGe BiCMOS technology integrated into the $H$ -plane of a WG. The IC is postprocessed and assembled in the WG prototype. The measured prototype shows a return loss better than 13 dB, an average insertion loss of 1.7 dB for a single transition, and a fractional bandwidth of 26.4% (69–90 GHz).

This article presents the experimental demonstration of a fully photonics-based heterodyne subterahertz (sub-THz) system for wireless communications. A p-i-n photodiode is used as a broadband transmitter to upconvert the signal to the sub-THz domain and a photoconductive antenna downconverts the received wave to an intermediate frequency around 3.7 GHz. The optical signals used for photomixing are extracted from two independent optical frequency combs with different repetition rates. The optical phase locking reduces the phase noise of the sub-THz signal, greatly improving the performance of the system when phase modulation formats are transmitted. The sub-THz carrier is tuned between 80 and 320 GHz in 40-GHz steps, showing a power variation of 21.8 dB. The phase noise at both ends of the communication link is analyzed and compared with the phase noise of the received signal with different wireless carriers. As a proof-of-concept, a 100-Mbit/s binary-phase-shift-keying signal is successfully transmitted over 80-, 120-, and 160-GHz carriers, achieving a bit error rate below 10 −5 in the first two cases. These results show the great potential of THz communications driven by photonics to cover an extensive portion of the THz range without relying on electronic components that limit the operating range of the system to a concrete frequency band.

Exciton transport in most organic materials is based on an incoherent hopping process between neighboring molecules. This process is very slow, setting a limit to the performance of organic optoelectronic devices. In this Article, we overcome the incoherent exciton transport by strongly coupling localized singlet excitations in a tetracene crystal to confined light modes in an array of plasmonic nanoparticles. We image the transport of the resulting exciton–polaritons in Fourier space at various distances from the excitation to directly probe their propagation length as a function of the exciton to photon fraction. Exciton–polaritons with an exciton fraction of 50% show a propagation length of 4.4 μm, which is an increase by 2 orders of magnitude compared to the singlet exciton diffusion length. This remarkable increase has been qualitatively confirmed with both finite-difference time-domain simulations and atomistic multiscale molecular dynamics simulations. Furthermore, we observe that the propagation length is modified when the dipole moment of the exciton transition is either parallel or perpendicular to the cavity field, which opens a new avenue for controlling the anisotropy of the exciton flow in organic crystals. The enhanced exciton–polariton transport reported here may contribute to the development of organic devices with lower recombination losses and improved performance. peerReviewed

Analog radio-over-fiber (ARoF) technology has proven to be a promising solution to be part of the future millimeter-wave (mm-wave) 5G/6G architecture due to its attractive benefits such as simplified remote antenna units (RAUs), low-power consumption, and low cost. However, ARoF channels present hefty drawbacks, such as phase noise and nonlinear effects, that need to be addressed. The probabilistic amplitude shaping (PAS) technique is able to reduce the impact of such drawbacks, allowing a fine optimization of channel capacity usage. In particular, enumerative sphere shaping (ESS) implementation stands out as an excellent PAS approach because of its energy-efficiency and low complexity for short blocklengths. In this work, for the first time to the best of our knowledge, an ESS scheme is evaluated in an experimental bidirectional mm-wave ARoF setup oriented towards 5G communications. Furthermore, a novel soft ESS demapping algorithm is proposed and explained. The experimental results confirm the ESS technique, together with the proposed algorithm, as a convenient solution to enhance the channel capacity use of mm-wave ARoF systems for the 5G/6G fronthaul.

A V-band Vivaldi antenna design for beyond-5G base station applications operating within V-band is demonstrated. The device can be used for interfacing with radio-over-fibre links and optoelectronic components through a coplanar waveguide to slotline transition. Simulations show an end-fire radiation with a 3 dB gain bandwidth ranging between 46.9-77.4 GHz combined with the reflection coefficients remaining below-10dB for the same frequency range. A comb structure inspired by optical grating couplers is added providing an increased maximum gain of 12.6 dBi at 68.5 GHz. Measurement results validate the promising characteristics of the planar antenna.

This paper studied the constraint mechanism for power device design based on perovskite quantum dots pumped by an electron beam. Combined with device designing, an experimental system of self-saturation luminescence and aging failure was designed for CsPbBr3 films. On this basis, we further completed the self-saturation luminescence and aging failure experiment and constructed a model of self-saturation luminescence and aging failure for CsPbBr3 device designing. Three constraints were proposed after analyzing and discussing the experimental data. Firstly, too high of a pumping current density makes it difficult to effectively promote the enhancement of luminescence efficiency. Secondly, radiation decomposition and aging failure of CsPbBr3 films are mainly related to the polarized degree of CsPbBr3 nanocrystals. Thirdly, by increasing the pumping electric field, the pumping energy can be effectively and widely delivered to the three-dimensional quantum dots film layer space, and there is a nonlinear relationship between the attenuation of the pumping energy density and the increment of the pumping electric field, which will effectively avoid the local high-energy density of instantaneous optical pumping.

[EN] The sixth generation (6G) mobile systems will create new markets, services, and industries making possible a plethora of new opportunities and solutions. Commercially successful rollouts will involve scaling enabling technologies, such as cloud radio access networks, virtualization, and artificial intelligence. This paper addresses the principal technologies in the transition towards next generation mobile networks. The convergence of 6G key-performance indicators along with evaluation methodologies and use cases are also addressed. Free-space optics, Terahertz systems, photonic integrated circuits, softwarization, massive multiple-input multiple-output signaling, and multi-core fibers, are among the technologies identified and discussed. Finally, some of these technologies are showcased in an experimental demonstration of a mobile fronthaul system based on millimeter 5G NR OFDM signaling compliant with 3GPP Rel. 15. The signals are generated by a bespoke 5G baseband unit and transmitted through both a 10 km prototype multi-core fiber and 4 m wireless V-band link using a pair of directional 60 GHz antennas with 10 degrees beamwidth. Results shown that the 5G and beyond fronthaul system can successfully transmit signals with both wide bandwidth (up to 800 MHz) and fully centralized signal processing. As a result, this system can support large capacity and accommodate several simultaneous users as a key candidate for next generation mobile networks. Thus, these technologies will be needed for fully integrated, heterogeneous solutions to benefit from hardware commoditization and softwarization. They will ensure the ultimate user experience, while also anticipating the quality-of-service demands that future applications and services will put on 6G networks., This work was partially funded by the blueSPACE and 5G-PHOS 5G-PPP phase 2 projects, which have received funding from the European Union's Horizon 2020 programme under Grant Agreements Number 762055 and 761989. D. PerezGalacho acknowledges the funding of the Spanish Science Ministry through the Juan de la Cierva programme.

Fifth-generation mobile networks (5G) are the solution for the demanding mobile traffic requirements, providing technologies that fulfill the requisites of different type of services. The utilization of the millimeter-wave (mm-wave) band is the straightforward technique to achieve high bit rates. Moreover, analog radio-over-fiber (ARoF) brings outstanding benefits such as low cost, low power consumption, and high spectral efficiency, among others. Thereby, mm-wave ARoF is a strong candidate to pave the way for common public radio interface (CPRI) in the fronthaul for the future 5G architecture. As orthogonal frequency-division multiplexing (OFDM) is the adopted waveform in the 5G standard, it should be also utilized in mm-wave ARoF systems for 5G. However, phase noise is one of the most degrading factors in mm-wave OFDM ARoF systems. Therefore, in this work, an analysis of the phase noise is carried out through an experimental setup up. The configuration of this setup enables to gradually modify the final phase noise level of the system. Furthermore, an original and novel algorithm to compensate the phase noise in OFDM receivers is proposed. The performance of this algorithm is experimentally evaluated through the setup for different phase noise levels and different subcarrier spacings. The obtained results show the effectiveness of the proposed algorithm under those conditions, highlighting the viability of mm-wave OFDM ARoF for 5G and beyond.

This article presents an experimental demonstration of a high-capacity millimeter-wave 5G NR signal transmission with analog radio-over-fiber (ARoF) fronthaul over multi-core fiber and full real-time processing. The demonstration validates the core of the blueSPACE fronthaul architecture which combines ARoF fronthaul with space division multiplexing in the optical distribution network to alleviate the fronthaul capacity bottleneck and maintain a centralized radio access network with fully centralized signal processing. The introduction of optical beamforming in the blueSPACE architecture brings true multi-beam transmission and enables full spatial control over the RF signal. The proposed ARoF architecture features a transmitter that generates the ARoF signal and an optical signal carrying a reference local oscillator employed for downconversion at the remote unit from a single RF reference at the central office. A space division multiplexing based radio access network with multi-core fibre allows parallel transport of the uplink ARoF signal and reference local oscillator at the same wavelength over separate cores. A complete description of the real-time signal processing and experimental setup is provided and system performance is evaluated. Transmission of an 800 MHz wide extended 5G NR fronthaul signal over a 7-core fibre is shown with full real-time signal processing, achieving 1.4 Gbit/s with a bit error rate $$ < 3.8 × 10 - 3 and thus below the limit for hard-decision forward error correction with 7% overhead.

Using diffraction-limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single-crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high-density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited-state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet–triplet annihilation (TTA). We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.

In this paper we report the implementation and experimental evaluation of a proposed hybrid communication ecosystem for CCAM applications such as cooperative adaptive cruise control (CACC) and smart intersections. Three wireless technologies have been suggested for communications between vehicles and intelligent traffic lights and are evaluated in this work: ITS-G5 based on IEEE 802.11p and LTE sidelink with PC5 air interface for direct short range links, and regular mobile LTE with LTE Uu air interface for long range or indirect links. The applications used are independent of the communication channel, to enable a comparison on the application level of the different communication technologies. Field experiments were carried out with two CACC-equipped vehicles and three intelligent traffic lights in two field test locations under ideal, i.e., no-traffic, conditions and with real traffic. Experimental results related to CACC show that the best performance in terms of latency is achieved by the ITS-G5 system, while LTE PC5 and LTE Uu links show a penalty of 20 and 50 ms respectively. However, experimental results show that all three communication technologies were still able to guarantee string stable performance of the vehicle platoon. Regarding the smart intersections, an analysis based on field measurements and comparison between long- and short-range solutions is proposed; the analysis includes the impact of each channel on the applications such as speed advisory and green light prediction. The reported experimental evaluation shows the potential of current mobile technologies for CCAM use cases and highlights the way for further CCAM applications based on 5G and beyond mobile networks.

In this work we have demonstrated a waveguide integrated uni-traveling carrier photodiode on an InP-membrane-on-silicon platform with 3 dB bandwidth beyond 110 GHz. With design optimization and an improved process, devices as small as $ \text{3}\times \text{2}\;\mu \text{m}^2$ are successfully realized. An electrical equivalent circuit model based on measured S-parameters revealed ultra-small series resistance and junction capacitance as low as 6.5 $\Omega$ and 4.4 fF, respectively, in the diodes. The model also provided insight in the photocurrent dependent characteristics in the bandwidth and resonsivity of the devices. Finally, data transmission measurements are demonstrated, showcasing the high speed telecommunication abilities of the UTC-PD.

Over the past decades, Terahertz (THz) systems development has taken a major step forward together with laser-based technologies for the generation and detection of THz signals. Optoelectronic generation of continuous-wave THz signals relies on mixing two optical signals oscillating in a high-speed photoconductive antenna, for which the photocurrent depends on the incident optical power. The capability to operate THz fields in passive devices can be combined smoothly with photonic integrated circuit (PIC) technologies to enable photonic chips with enhanced THz efficiency. There are still many open questions about implementation and improvement to reduce equipment size, noise, alignment efforts, electrical and optical power consumption, and to increase the system flexibility. In this manuscript, we introduce a new THz system platform based on photonic integrated circuits and micro-structured photoconductive antennas that will increase the integration by reducing the footprint of THz spectrometers by more than 3 orders of magnitude. Additionally, a new integrated lens-antenna consisting of a Fresnel zone plate is designed and implemented for a nanocontact based THz photomixer. The new design replaces the standard conventional bulky silicon lens, which normally no THz photomixer can avoid. THz measurements showed a comparable behavior with the Fresnel zone plate to that of the conventional bulky silicon lens, demonstrating its readiness for photonic integrated circuits-based THz systems. This integration platform will represent a technological jump from currently bulky equipment and devices to extremely flexible, portable, and energy efficient THz systems-on-chip, at much lower costs.

We report on the design, fabrication, and analysis of a tunable device combining nanoparticle arrays that support collective surface lattice resonances (SLRs) with liquid crystals (LCs). The optoelectronic tunability of the nematic LC and the dependency of sharp SLRs on the refractive index of the environment are exploited to achieve spectral tunability. This tunability is electrically controlled by switching between planar and homeotropic states in the LC, which allows for a rapid and reversible tuning of the SLR wavelength with a large degree of control. This device also offers the possibility to switch “on” and “off” the presence of a quasi-guided mode in the indium tin oxide electrode. The manipulation of these resonances with an external parameter can be used to expand the functionalities of plasmonic metasurface devices.

Three convergent processes are likely to shape the future of the internet beyond-5G: The convergence of optical and millimeter wave radio networks to boost mobile internet capacity, the convergence of machine learning solutions and communication technologies, and the convergence of virtualized and programmable network management mechanisms towards fully integrated autonomic network resource management. The integration of network virtualization technologies creates the incentive to customize and dynamically manage the resources of a network, making network functions, and storage capabilities at the edge key resources similar to the available bandwidth in network communication channels. Aiming to understand the relationship between resource management, virtualization, and the dense 5G access and fronthaul with an emphasis on converged radio and optical communications, this article presents a review of how resource management solutions have dealt with optimizing millimeter wave radio and optical resources from an autonomic network management perspective. A research agenda is also proposed by identifying current state-of-the-art solutions and the need to shift all the convergent issues towards building an advanced resource management mechanism for beyond-5G.

QKD networks are costly infrastructures. This paper introduces the concept of time-sharing of QKD resources, namely receivers, transmitters and quantum links. A number of strategies for resource sharing in QKD are described. The approach is valid for any point-to-point QKD system that implements end-to-end key exchange via one-time-pad-based trusted relay. A quick modelling and analysis of one of these strategies on a sample network proves the potential for QKD CAPEX saving. Coordinated smart scheduling of slices via SDN controllers is required.

5G and beyond-5G networks need to support all the traffic pressure by extensive Internet of Things networks. In this document, we review how to improve the flexibility of 5G networks by conducting slicing. We describe the four main pillars for this transition: mass adoption of Software Defined Networking, collocation of aggregators at Multi-access Edge Computing platforms, virtualization of infrastructure and removal of Application Functions through distributed responsibilities over the network. We will present an overview of the efforts being conducted for each of these points and the vision for beyond-5G networks.

Networking is enabling a continuum comprising cloud, edge and last-mile systems. This continuum allows services and applications to join the fabric at any point and have access to processing power independently of the location of entrance. To enable this paradigm, we need to make sure we have a networking fabric capable of transporting and access network resources seamlessly and communication channels that are securitized, ensuring confidentiality and integrity of data. A challenge in securitization is that it takes computing resources away, and hence it may reduce the overall network performance. Hardware accelerations or offloads enable to reduce the burden on processing resources of securitization processes; in this contribution, we present how IPsec and TLS protocols provide a tool for a high level of securitization of the communication channels and approaches to accelerate/offload those protocols.

In this work we present, analyze, and demonstrate an efficient bidirectional ARoF system for K-band 5G fronthaul based on multicore fiber. As the 5G standard establishes, the experimental testbed is configured regarding the time division duplex scheduling and tested by transmitting OFDM signals. Furthermore, the configuration of the setup enables the reusability of several components for both directions, allowing the reduction of the power consumption, complexity, and cost of the system. The bidirectional testbed is experimentally evaluated under different bandwidth configurations and power levels, achieving a maximum throughput of 4Gbit/s over 9m of wireless link. The experimental results prove and validate the proposed bidirectional ARoF scheme as a promising solution for the future mm-Wave 5G fronthaul.

Fifth generation (5G) is the emerging mobile communications platform that aims to meet the market requirements in terms of enhanced broadband connectivity based on harnessing small cell and mmWave technology. These two in synergy will provide high capacity gain not only through the hyperdense deployment of small cell but also through accessing large swathes of untapped spectrum at mmWave frequencies. The envisaged architecture entails an integrated optical wireless network architecture, where optical technology will complement radio in order to handle the new demands on capacity over the backhaul and fronthaul network, leading to the notion of analog radio over fiber (ARoF). The goal of this chapter is to provide novel approaches to optimize the performances of mmWave ARoF systems that includes developing enabling technology from a digital to signal processing (DSP) and device perspective.

This chapter elaborates on the beneficial aspects and hardware implementations of incorporating ultradense WDM-PONs (UDWDM-PONs) with hybrid optical-wireless fronthaul links and fiber to the home applications. Simulation results on the synthesis of a low-cost and low-energy consumption optoelectronic unit within the future 5G base stations (BS) are presented. In addition, an advanced neural network is investigated capable of compensating for the linear and nonlinear effects induced by semiconductor optical amplifiers (SOA).

This paper presents a beyond 5G fronthaul network with dynamic beamforming and -steering. The proposed fronthaul solution deploys optical beamforming (OBF) by combining space division multiplexing (SDM), analogue radio-over-fiber (ARoF), and the novel optical beam forming network (OBFN) technologies. From the service management and orchestration (MANO) point of view, the proposed fronthaul solution also deploys an advanced software defined networking (SDN) and Network Function Virtualization (NFV) control and orchestration architecture developed with the goal to optimally manage and reconfigure the physical layer resources (i.e., optical and radio) at the central office and cell sites (i.e., pool of baseband units (BBUs), remote radio heads (RRHs), ARoF transceivers and OBFNs). The proposed beyond 5G fronthaul architecture is primarily oriented to deploy massive machine-type communication (mMTC) services with high-bandwidth requirements, such as for industry 4.0. In this paper we experimentally validate the novel OBFN system, and the dynamic SDN/NFV MANO of the transport connectivity and network services for optical beamforming. The obtained experimental results show that the overall delay for the provisioning and removal of an OBF service, considering the contribution of the involved optical and radio systems and the SDN/NFV MANO layer, is 134s and 18s respectively. The reconfiguration of the OBF service to add or remove a beam can be performed in the range of 65–87s.

This article presents the experimental demonstration of synchronization of two integrated semiconductor distributed Bragg reflector lasers, fabricated with a generic multiproject wafer platform, by means of injection locking. Substantial linewidth reduction and frequency stabilization of the lasers were shown during locking of the lasers to an optical frequency comb. Phase noise was measured for different injected powers and different laser cavities. For a generation of millimeter-wave signals up to 80 GHz, two lasers were simultaneously locked to the comb. Fine-tuning was performed by tuning the repetition rate of the comb and coarse-tuning was carried out by switching to another comb line. A suppression ratio of 37 dB was achieved for unwanted comb lines. The achieved signal purity, phase noise, and suppression of unwanted components demonstrate the viability of injection locking for the generation of high-quality signals at sub-THz and THz frequencies and with substantial tunability.

Bound states in the Continuum (BICs) represent a new paradigm for resonant photonics due to their infinite lifetimes associated with the full suppression of radiation losses. This property makes it impossible to directly investigate BICs with standard far-field spectroscopy. In this contribution, we demonstrate the local excitation and the direct measurement of the near-field of BICs in arrays of dimer resonators and report their extremely long lifetimes due to the out-of-phase field oscillations in the resonators.

Terahertz time-domain spectroscopy enables the contact-free determination of charge carrier properties in metals and semiconductors. However, the applicability of THz spectroscopy is severely reduced by the low spatial resolution imposed by the diffraction limit. We have developed a time-resolved THz near-field microscope to circumvent this limitation, allowing the determination of the carrier dynamics with a resolution of a few microns. We use this microscope to measure a single exfoliated flake of WS2 with domains of different thickness. The THz spectra reveal phonon modes in the sheet conductance. We also observe a biexponential carrier recombination associated to surface-assisted Auger scattering.

7 pags., 7 figs., Bound states in the continuum (BICs) represent a new paradigm in photonics due to the full suppression of radiation losses. However, this suppression has also hampered the direct observation of them. By using a double terahertz (THz) near-field technique that allows the local excitation and detection of the THz amplitude, we are able to map for the first time the electromagnetic field amplitude and phase of BICs over extended areas, unveiling the field-symmetry protection that suppresses the far-field radiation. This investigation, done for metasurfaces of dimer scatterers, reveals the in-plane extension and formation of BICs with antisymmetric phases, in agreement with coupled-dipole calculations. By displacing the scatterers, we show experimentally that a mirror symmetry is not a necessary condition for a BIC formation. Only π-rotation symmetry is required, making BICs exceptionally robust to structural changes. This work makes the local field of BICs experimentally accessible, which is crucial for the engineering of cavities with infinite lifetimes., The authors thank K. de Mare for help in the development of the setup and Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) (Vici 680-47-628); Spanish Ministerio de Ciencia e Innovación (MICIU/AEI/FEDER, UE) through the grants MELODIA (PGC2018-095777−B-C21) and NANOTOPO (FIS2017-91413-EXP), and Ministerio de Educación, Cultura y Deporte through a PhD Fellowship (FPU15/03566) for funding.

This work studies and experimentally evaluates the performance enhancement by using PS-OFDM in a 5G mm-Wave ARoF system at 25 GHz for all 5G numerologies and with different phase noise levels.

A real-time millimeter wave radio-over-fiber link is experimentally demonstrated transmitting up to 1.4 Gbit/s 5G OFDM signals through a multi-core fiber to an 8 × 4 antenna array employing analog beamforming to radially track a moving receiver.