Broadband Tunable Infrared Light Emission from Metal-Oxide-Semiconductor Tunnel Junctions in Silicon Photonics.

Broadband near-infrared light emitting tunnel junctions are demonstrated with efficient coupling to a silicon photonic waveguide. The metal oxide semiconductor devices show long hybrid photonic-plasmonic mode propagation lengths of approximately 10 µm and thus can be integrated into an overcoupled resonant cavity with quality factor Q ≈ 49, allowing for tens of picowatt near-infrared light emission coupled directly into a waveguide. The electron inelastic tunneling transition rate and the cavity mode density are modeled, and the transverse magnetic (TM) hybrid mode excitation rate is derived. The results coincide well with polarization resolved experiments. Additionally, current-stressed devices are shown to emit unpolarized light due to radiative recombination inside the silicon electrode.

Dual sideband receiver for radio-over-fiber.

A dual sideband reception scheme for radio-over fiber (RoF) links is introduced. It is shown that the new receiver can increase the performance of noise-limited systems by up to 3 dB (2.97 dB in a lab back-to-back experiment). The receiver scheme exploits the fact that current RoF links do not realize their full potential. This is because in typical RoF receivers, the radio-frequency (RF) signals are mapped back to the optical domain by means of electro-optical modulator. In this process energy typically is lost as only one of the two generated sidebands is subsequently used. The suggested receiver exploits the signal of both sidebands. The receiver scheme was subsequently tested in a full optical-RF-optical transmission link at RF carrier frequencies of 228 GHz over a free-space channel spanning distances of 1400 m for symbol rates of up to 48 Gbaud 4 QAM. Here, we could achieve SNR improvements of up to 2.6 dB.

Measuring dielectric and electro-optic responses of thin films using plasmonic devices.

This paper introduces a simple method for the measurement of the relative permittivity and the Pockels coefficient of electro-optic (EO) materials in a waveguide up to sub-THz frequencies. By miniaturizing the device and making use of plasmonics, the complexities of traditional methods are mitigated. This work elaborates the fabrication tolerance and simplicity of the method, and highlights its applicability to various materials, substrates and configurations. The method is showcased using drop-casted perovskite barium titanate (BaTiO3, BTO) nano-particle thin-films and it has previously been used to measure epitaxial thin film BTO. In this work we show the effective relative permittivity of drop casted BTO to be εeff ∼ 30 at 200 MHz, dropping to ∼ 18 at 67 GHz and similarly, the effective Pockels coefficient was found to be reff ∼ 16 at 350 MHz and ∼ 8 at 70 GHz. These values are a factor > 50 below the values found for thin film BTO. Yet, the fact that the method can be applied to such different samples and Pockels strengths gives testimony to its versatility and sensitivity.

Low-loss plasmon-assisted electro-optic modulator.

For nearly two decades, researchers in the field of plasmonics 1 -which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal 2 -have sought to realize subwavelength optical devices for information technology3-6, sensing7,8, nonlinear optics9,10, optical nanotweezers 11 and biomedical applications 12 . However, the electron motion generates heat through ohmic losses. Although this heat is desirable for some applications such as photo-thermal therapy, it is a disadvantage in plasmonic devices for sensing and information technology 13 and has led to a widespread view that plasmonics is too lossy to be practical. Here we demonstrate that the ohmic losses can be bypassed by using 'resonant switching'. In the proposed approach, light is coupled to the lossy surface plasmon polaritons only in the device's off state (in resonance) in which attenuation is desired, to ensure large extinction ratios between the on and off states and allow subpicosecond switching. In the on state (out of resonance), destructive interference prevents the light from coupling to the lossy plasmonic section of a device. To validate the approach, we fabricated a plasmonic electro-optic ring modulator. The experiments confirm that low on-chip optical losses, operation at over 100 gigahertz, good energy efficiency, low thermal drift and a compact footprint can be combined in a single device. Our result illustrates that plasmonics has the potential to enable fast, compact on-chip sensing and communications technologies.

Broadband Metallic Fiber-to-Chip Couplers and a Low-Complexity Integrated Plasmonic Platform.

We present a plasmonic platform featuring efficient, broadband metallic fiber-to-chip couplers that directly interface plasmonic slot waveguides, such as compact and high-speed electro-optic modulators. The metallic gratings exhibit an experimental fiber-to-slot coupling efficiency of -2.7 dB with -1.4 dB in simulations with the same coupling principle. Further, they offer a huge spectral window with a 3 dB passband of 350 nm. The technology relies on a vertically arranged layer stack, metal-insulator-metal waveguides, and fiber-to-slot couplers and is formed in only one lithography step with a minimum feature size of 250 nm. As an application example, we fabricate new modulator devices with an electro-optic organic material in the slot waveguide and reach 50 and 100 Gbit/s data modulation in the O- and C-bands within the same device. The devices' broad spectral bandwidth and their relaxed fabrication may render them suitable for experiments and applications in the scope of sensing, nonlinear optics, or telecommunications.

Compact Mid-Infrared Gas Sensing Enabled by an All-Metamaterial Design.

The miniaturization of mid-infrared optical gas sensors has great potential to make the "fingerprint region" between 2 and 10 µm accessible to a variety of cost-sensitive applications ranging from medical technology to atmospheric sensing. Here we demonstrate a gas sensor concept that achieves a 30-fold reduction in absorption volume compared to conventional gas sensors by using plasmonic metamaterials as on-chip optical filters. Integrating metamaterials into both the emitter and the detector cascades their individual filter functions, yielding a narrowband spectral response tailored to the absorption band of interest, here CO2. Simultaneously, the metamaterials' angle-independence is maintained, enabling an optically efficient, millimeter-scale cavity. With a CO2 sensitivity of 22.4 ± 0.5 ppm·Hz-0.5, the electrically driven prototype already performs at par with much larger commercial devices while consuming 80% less energy per measurement. The all-metamaterial sensing concept offers a path toward more compact and energy-efficient mid-infrared gas sensors without trade-offs in sensitivity or robustness.

Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon.

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r42 = 923 pm V-1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s-1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies.

Coherent few mode demultiplexer realized as a 2D grating coupler array in silicon.

We demonstrate an on-chip coherent mode scrambling demultiplexer for polarization multiplexed few mode signals. The device has been fabricated in the standard silicon-on-insulator platform. The mode demultiplexer consists of an array of 2D grating couplers for dual polarization few mode fiber-to-chip coupling and optical hybrids realized by 4×4 MMIs. The array of perfect vertical 2D grating couplers allows us an efficient fiber-to-chip coupling with experimental peak coupling efficiencies of -5.2 dB and -9.0 dB at 1570 nm for LP01 and LP11 modes, respectively, while simulated coupling efficiencies at 1550 nm are -3.6 dB and -3.3 dB for LP01 and LP11, respectively. We successfully performed a back-to-back three LP modes division multiplexing transmission experiment with single polarization 32 Gbaud QPSK signals using the fabricated mode demultiplexer relying on offline MIMO DSP techniques.

Deep learning based digital backpropagation demonstrating SNR gain at low complexity in a 1200†km transmission link.

A deep learning (DL) based digital backpropagation (DBP) method with a 1 dB SNR gain over a conventional 1 step per span DBP is demonstrated in a 32 GBd 16QAM transmission across 1200 km. The new DL-DPB is shown to require 6 times less computational power over the conventional DBP scheme. The achievement is possible due to a novel training method in which the DL-DBP is blind to timing error, state of polarization rotation, frequency offset and phase offset. An analysis of the underlying mechanism is given. The applied method first undoes the dispersion, compensates for nonlinear effects in a distributed fashion and reduces the out of band nonlinear modulation due to compensation of the nonlinearities by having a low pass characteristic. We also show that it is sufficient to update the elements of the DL network using a signal with high nonlinearity when dispersion or nonlinearity conditions changes. Lastly, simulation results indicate that the proposed scheme is suitable to deal with impairments from transmission over longer distances.

100 GBd IM/DD transmission over 14†km SMF in the C-band enabled by a plasmonic SSB MZM.

100 Gb/s NRZ-OOK transmission over 14 km standard single mode fiber in the C-band is demonstrated with a simple intensity modulation and direct detection scheme. The transmission concept utilizes single sideband modulation and comprises a single differential digital-to-analog converter with adjustable phase offset, a new dual electrode plasmonic Mach-Zehnder modulator, a laser at 1537.5 nm, standard single mode fibers, a photodiode, an analog-to-digital converter, and linear offline digital signal processing. The presented SSB concept requires no DSP and complex signaling at the transmitter. The demonstrated SSB transmitter increased the possible transmission distance by a factor of 4.6 compared to a DSB transmitter. We also investigated the equalization requirements. A T/2-spaced feedforward equalizer requires 27 taps to achieve transmission over 10 km with a BER below the HD-FEC limit. In comparison to a DSB transmitter, the SSB transmitter reduced the receiver DSP complexity by a factor of 13.7.

Compact, ultra-broadband plasmonic grating couplers.

We demonstrate all-metallic grating couplers that enable vertical, compact and broadband fiber-coupling. The grating couplers are based on a metal layer and directly convert a vertical fiber mode into surface plasmon polaritons (SPPs). In combination with a focusing arrangement, the grating couplers require only a small footprint of 13.5 × 12 µm2. We characterize the grating couplers with both periodic and apodized gratings and experimentally show a 1-dB bandwidth of 115 nm with a coupling efficiency of 2.9 dB.

Low-loss hybrid plasmonic coupler.

We demonstrate a low-loss coupling scheme between a silicon photonic waveguide and a hybrid-plasmonic waveguide. Measured coupling efficiencies reach up to 94% or -0.27 dB. The metal-insulator-semiconductor structure is fabrication-tolerant and adaptable to a wide range of materials including those used in CMOS processes. The coupler is a promising building block for low-loss active plasmonic devices.

120 GBd plasmonic Mach-Zehnder modulator with a novel differential electrode design operated at a peak-to-peak drive voltage of 178 mV.

A new plasmonic Mach-Zehnder modulator is demonstrated at a bit rate of 120 Gb/s NRZ-OOK with low peak-to-peak driving voltages of 178 mVpp below the HD-FEC limit. Such record low driving voltage requirements potentially translate into an electrical drive power consumption of 862 aJ/bit. The low drive voltages have been made possible by a new differential Mach-Zehnder modulator electrode design. The differential electrode design is optimized for the balanced driving circuitry and reduces the effectively required driving voltage by a factor of four (Vπ/4). The potential of the transmitter scheme is further demonstrated by a transmission experiment over 500 m of single mode fiber at the C-band with a BER performance below the KP4 FEC limit.

Spectral signature of nonlinear effects in semiconductor optical amplifiers.

Optical spectra of signals at the output of semiconductor optical amplifiers (SOA) provide useful insight into amplifier nonlinearities. In this work, we determine the parameters of an analytical SOA model with a pump-probe experiment by evaluating the measured spectra of the pump and probe pulses at the SOA output. The analytical lumped SOA model considers carrier depletion, carrier recovery, spectral hole burning, two-photon absorption, and we include an additional effect termed 'two-photon induced free-carrier absorption', that is responsible for creating an identifiable blue-shifted component in the spectra. We are able to relate the underlying physical nonlinear effects to the spectral peculiarities of the output pump and probe spectra, and give guidelines for the exploitation of these nonlinear effects for optical signal processing. In addition, with a much-simplified SOA model and by replacing the pump pulse with modulated data we show that the output spectrum is altered in a manner consistent with phase patterning effects.

Constellation modulation - an approach to increase spectral efficiency.

Constellation modulation (CM) is introduced as a new degree of freedom to increase the spectral efficiency and to further approach the Shannon limit. Constellation modulation is the art of encoding information not only in the symbols within a constellation but also by encoding information by selecting a constellation from a set of constellations that are switched from time to time. The set of constellations is not limited to sets of partitions from a given constellation but can e.g., be obtained from an existing constellation by applying geometrical transformations such as rotations, translations, scaling, or even more abstract transformations. The architecture of the transmitter and the receiver allows for constellation modulation to be used on top of existing modulations with little penalties on the bit-error ratio (BER) or on the required signal-to-noise ratio (SNR). The spectral bandwidth used by this modulation scheme is identical to the original modulation. Simulations demonstrate a particular advantage of the scheme for low SNR situations. So, for instance, it is demonstrated by simulation that a spectral efficiency increases by up to 33% and 20% can be obtained at a BER of 10-3 and 2×10-2 for a regular BPSK modulation format, respectively. Applying constellation modulation, we derive a most power efficient 4D-CM-BPSK modulation format that provides a spectral efficiency of 0.7 bit/s/Hz for an SNR of 0.2 dB at a BER of 2 × 10-2.

Nonlinearities of organic electro-optic materials in nanoscale slots and implications for the optimum modulator design.

The performance of highly nonlinear organic electro-optic (EO) materials incorporated into nanoscale slots is examined. It is shown that EO coefficients as large as 190 pm/V can be obtained in 150 nm wide plasmonic slot waveguides but that the coefficients decrease for narrower slots. Possible mechanism that lead to such a decrease are discussed. Monte-Carlo computer simulations are performed, confirming that chromophore-surface interactions are one important factor influencing the EO coefficient in narrow plasmonic slots. These highly nonlinear materials are of particular interest for applications in optical modulators. However, in modulators the key parameters are the voltage-length product UπL and the insertion loss rather than the linear EO coefficients. We show record-low voltage-length products of 70 Vµm and 50 Vµm for slot widths in the order of 50 nm for the materials JRD1 and DLD164, respectively. This is because the nonlinear interaction is enhanced in narrow slot and thereby compensates for the reduced EO coefficient. Likewise, it is found that lowest insertion losses are observed for slot widths in the range 60 to 100 nm.

Atomic Scale Plasmonic Switch.

The atom sets an ultimate scaling limit to Moore's law in the electronics industry. While electronics research already explores atomic scales devices, photonics research still deals with devices at the micrometer scale. Here we demonstrate that photonic scaling, similar to electronics, is only limited by the atom. More precisely, we introduce an electrically controlled plasmonic switch operating at the atomic scale. The switch allows for fast and reproducible switching by means of the relocation of an individual or, at most, a few atoms in a plasmonic cavity. Depending on the location of the atom either of two distinct plasmonic cavity resonance states are supported. Experimental results show reversible digital optical switching with an extinction ratio of 9.2 dB and operation at room temperature up to MHz with femtojoule (fJ) power consumption for a single switch operation. This demonstration of an integrated quantum device allowing to control photons at the atomic level opens intriguing perspectives for a fully integrated and highly scalable chip platform, a platform where optics, electronics, and memory may be controlled at the single-atom level.

Multi-format carrier recovery for coherent real-time reception with processing in polar coordinates.

A blind frequency and phase search algorithm for joint frequency and phase recovery is introduced. The algorithm achieves low complexity due to processing in polar coordinates, which reduces the amount of multiplications. We show an implementation for real-time processing at 32 GBd on FPGA hardware. The hardware design allows for dynamic multi-format operation, where the format can be switched flexibly after each clock cycle (250 MHz, 128 Symbols) between 4QAM, 8QAM, and 16QAM. The performance of the algorithm is evaluated with respect to laser phase noise, carrier frequency offset, and carrier frequency offset drift. The effect of working with limited hardware resources is investigated. An FPGA implementation shows the feasibility of our carrier recovery algorithm with a negligible penalty when compared to a floating point simulation.

Direct Conversion of Free Space Millimeter Waves to Optical Domain by Plasmonic Modulator Antenna.

A scheme for the direct conversion of millimeter and THz waves to optical signals is introduced. The compact device consists of a plasmonic phase modulator that is seamlessly cointegrated with an antenna. Neither high-speed electronics nor electronic amplification is required to drive the modulator. A built-in enhancement of the electric field by a factor of 35,000 enables the direct conversion of millimeter-wave signals to the optical domain. This high enhancement is obtained via a resonant antenna that is directly coupled to an optical field by means of a plasmonic modulator. The suggested concept provides a simple and cost-efficient alternative solution to conventional schemes where millimeter-wave signals are first converted to the electrical domain before being up-converted to the optical domain.