Research Publications

Our scientific research and articles appear in peer-reviewed journals and other publications. The list includes research and development on our materials and contributions to device development efforts with academic and industrial collaborators. If you are interested in learning more about our research, please reach out.

Published scientific research and articles

Published Date: January 13, 2020

Ultrahigh Electro-Optic Coefficients, High Index of Refraction, and Long-Term Stability from Diels–Alder Cross-Linkable Binary Molecular Glasses

Abstract: The development of organic electro-optic (EO) materials that concurrently possess a high electro-optic coefficient (r33), high index of refraction, and long-term or high-temperature stability of chromophore alignment has been a crucial goal. To address this challenge, we developed a crosslinkable EO system consisting of two chromophores, HLD1 and HLD2, which can be electric field poled and then thermally crosslinked in situ to form a stable EO material. This approach avoids the necessity for nonlinear optically inactive materials such as polymers or small molecule cross-linkers, thus resulting in high chromophore density (>5 × 1020 molecules/cm3) and high index of refraction (n = 1.89 at 1310 nm) for HLD1/HLD2. Different ratios of HLD1 and HLD2 were evaluated to optimize poling efficiency and thermal stability of the poling-induced order. With 2:1 HLD1/HLD2 (wt/wt), a maximum r33 of 290 ± 30 pm/V was achieved in a cross-linked film. Thermal stability tests showed that after heating to 85 °C for 500 h, greater than 99% of the initial r33 value was maintained. This combination of large EO activity, high index of refraction, and long-term alignment stability is an important breakthrough in EO materials. HLD1/HLD2 can also be poled without the subsequent cross-linking step, and even larger maximum r33 (460 ± 30 pm/V) and n3r33 figure of merit (3100 ± 200 pm/V) were achieved. Hyperpolarizabilities of HLD and control molecules were analyzed by hyper-Rayleigh scattering and computational modeling with good agreement, and they help explain the high acentric order achieved during poling.

Citations:

Published Date: May 22, 2019

Molecular Engineering of Structurally Diverse Dendrimers with Large Electro-Optic Activities

Abstract: To boost electro-optic (EO) performance, a series of multichromophore dendrimers have been developed based on higher hyperpolarizability (CLD-type) chromophore cores that have been used previously (FTC-type dendrimers). The multichromophore dendrimers were molecularly engineered to have either three arms, two arms, or one arm; long or short linkers; and a fluorinated dendron (FD) or tert-butyldiphenylsilyl (TBDPS) shell. The EO performance obtained by FDSD (poling efficiency = 1.60 nm2 V–2), based on succinic diester linkers, was higher than the analogue with longer adipic diester linkers and higher than the analogs with fewer chromophore moieties. Due to the shorter succinic diester linker and improved site isolation, the dendrimer chromophore with TBDPS groups exhibited enhanced glass-transition temperature (Tg = 108 °C) and comparable poling efficiency (1.62 nm2 V–2) to the FD-containing version. These neat EO dendrimers have a higher index of refraction (n = 1.75–1.84 at 1310 nm) than guest–host polymeric EO materials (n ≈ 1.6, 1310 nm) and FTC-type EO dendrimers (n = 1.73, 1310 nm), which is important, because a key metric for Mach–Zehnder modulators is proportional to n3. In addition, binary chromophore organic glasses (BCOGs) were prepared by doping a secondary EO chromophore at 25 wt % into neat dendrimers. Enhancements of EO performance were found in all BCOG materials compared with neat dendrimers due to the effect of blending. As a result of increased chromophore density, the n values of the BCOGs improved to 1.81–1.92. One BOCG, in particular, displayed the highest poling efficiency (2.35 nm2 V–2) and largest EO coefficient (r33) value of 275 pm V–1 at 1310 nm, which represents a high n3r33 figure-of-merit of 1946 pm V–1. The high poling efficiencies and n3r33 figure-of-merit combined with excellent film forming confirm these neat dendrimers and BCOGs based on them as promising candidates for incorporation into photonic devices.

Citations:

Published Date: July 7, 2017

Effect of Rigid Bridge-Protection Units, Quadrupolar Interactions, and Blending in Organic Electro-Optic Chromophores

Abstract: A new organic electro-optic (EO) molecule was designed with two modifications aimed at increasing acentric order. The molecule is based on the well-known CLD donor-π bridge-acceptor template. The first structural modification introduces rigid aromatic fluorenyl and naphthyl site-isolation units (sterically bulky functional groups) to reduce aggregation. Site isolation units have been used in the past, but this is the first time that both the “front” and “back” of the CLD tetraene bridge have been modified with site-isolation units, and we had to introduce new synthetic methodology to do so. The second design element was the inclusion of cooperatively interacting aromatic dendron (HD) and fluoroaromatic dendron (FD) side groups to increase the acentric order. HD/FD units have previously been successfully used to increase EO performance, but we changed their location on the chromophore: they are attached to the donor and acceptor ends of the molecule to better match side chain ordering with the dipole moment of the molecule. Comparison chromophores were synthesized with alkyl (-MOM), hydroxyl (-OH), or HD units on the acceptor end of the molecule and either the traditional CLD bridge (T-bridge) or modified bridge (BB-bridge) for a family of eight chromophores. The HD/FD units increased glass transition temperature, Tg, by 4–21 °C, and the bulky bridge modification increased Tg by 27–44 °C, which is very beneficial as that results in extra thermal stability of the poling-induced acentric order. UV/vis absorbance spectroscopy shows that the site-isolation units reduce aggregation. Unfortunately, poor film formation of the neat materials precluded full chromophore evaluation in poling and r33 experiments. The EO performance obtained for HD-BB-FD and HD-BB-OH was lower than expected, with r33/Ep ≈ 1 nm2 V–2 at 1310 nm. We found that blending in 25 wt % YLD124 improved film-forming and poling efficiency. Due to the effect of blending and improved site isolation, r33/Ep improved to 2.1–2.3 nm2 V–2 for 3:1 HD-BB-FD:YLD124, HD-BB-OH:YLD124, and HD-BB-MOM:YLD124, and r33 as high as 351 pm V–1 was obtained with 3:1 HD-BB-MOM:YLD124. Chromophore blends were also evaluated in plasmonic organic hybrid (POH) phase modulators with slot lengths of 5–20 μm. In POH devices, r33 was as high as 325 pm V–1 at 1260 nm and 220 pm V–1 at 1520 nm. Overall, the increase in acentric order afforded by the HD/FD interactions was found to be small and resulted in no increase in r33 due to the reduced number density. Ultimately, the increase in r33/Ep afforded by the site isolation and blending resulted in a modest increase in r33/Ep relative to YLD124, but combined with the increased Tg, the chromophore system is a significant improvement and points to an important design strategy.

Citations:

  • Elder, D. L.; Haffner, C.; Heni, W.; Fedoryshyn, Y.; Garrett, K. E.; Johnson, L. E.; Campbell, R. A.; Avila, J. D.; Robinson, B. H.; Leuthold, J.; Dalton, L. R.. Chemistry of Materials. 2017, 29, 6457-6471. Doi: https://doi.org/10.1021/acs.chemmater.7b02020

Published Date: March 18, 2016

Structure–function relationship exploration for enhanced thermal stability and electro-optic activity in monolithic organic NLO chromophores

Abstract: We have developed a series of novel monolithic materials based on molecules previously explored as dopants in guest–host systems to study intrinsic structure–function relationships in organic electro-optic (EO) materials. In a library of EO molecules with varied bridge segments, molecular modification of the donor with bis(tert-butyldiphenylsilyl) groups led to improvement in formation of amorphous films and led to enhanced poling efficiency. Further modification to include a carbazole site-isolation group on the bridge effectively reduced intermolecular dipole–dipole interactions, led to a material with poling efficiency of approximately 3 (nm V−1)2, and an increased glass transition temperature to 20–40 °C higher than similar reported monolithic materials. This level of thermal stability is comparable to common guest/host systems, which incorporated poly(methyl methacrylate) (PMMA) as the host. Our research showed that π-bridge length and type impacted first molecular hyperpolarizability β of a chromophore, which is accordingly reflected in the EO response. These findings further promote the utility of monolithic materials for their increased EO behavior and improved thermal stability, making this material system a competitor of guest–host systems in commercial applications.

Citations:

  • Jin, W.; Johnston, P. V.; Elder, D. L.; Manner, K. T.; Garrett, K. E.; Kaminsky, W.; Xu, R.; Robinson, B. H.; Dalton, L. R.. Journal of Materials Chemistry C. 2016, 4, 3119-3124. Doi: https://doi.org/10.1039/C6TC00358C

Published Date: June 20, 2014

Benzocyclobutene barrier layer for suppressing conductance in nonlinear optical devices during electric field poling

Abstract: We measured the electro-optic (EO) coefficients (r33) of thin-film devices made from several monolithic, high number density organic EO chromophores with and without additional charge barrier layers. We found that a cross-linkable benzocyclobutene layer was very effective in suppressing unwanted, leakage current, keeping the effective poling voltage nearly identical to the applied voltage. This barrier layer proved to be superior to a titanium dioxide (TiO2) barrier layer. The suppression of the leakage current in combination with a new chromophore enabled the construction of EO devices that had r33 values in the range of 400–500 pm V−1 with poling fields ≥ 85 V μm−1.

Citations:

  • Jin, W.; Johnston, P. V.; Elder, D. L.; Tillack, A. F.; Olbricht, B. C.; Song, J.; Reid, P. J.; Xu, R.; Robinson, B. H.; Dalton, L. R.. Applied Physics Letters. 2014, 104, 243304. Doi: https://doi.org/10.1063/1.4884829

Published Date: January 3, 2014

Matrix-Assisted Poling of Monolithic Bridge-Disubstituted Organic NLO Chromophores

Abstract: Detailed synthesis, characterization, and electric field poling details are reported.

Citations:

Published Date: May 18, 2012

Nano-Engineering Lattice Dimensionality for a Soft Matter Organic Functional Material

Abstract: A high performing electro‐optic (EO) chromophore with covalently attached coumarin‐based pendant groups exhibits intermolecular correlation of coumarin units through molecular dynamics (MD) simulations. Unique, orthogonal molecular orientations of the chromophore and coumarin units are also evident when investigated optically. Such molecular orientation translates to reduced lattice dimensionality of the bulk C1 soft matter material system, leading to increased acentric order and EO activity. Results are corroborated by nanorheological experimental methods.

Citations:

  • Benight, S. J.; Knorr, D. B.; Johnson, L. E.; Sullivan, P. A.; Lao, D.; Sun, J.; Kocherlakota, L. S.; Elangovan, A.; Robinson, B. H.; Overney, R. M.; Dalton, L. R.. Advanced Materials. 2012, 24, 3263-3268. Doi: https://doi.org/10.1002/adma.201104949

Published Date: August 17, 2018

Optimization of Plasmonic-Organic Hybrid Electro-Optics

Abstract: Plasmonic-organic hybrid technology affords the potential for exceptional bandwidth, extremely small footprint, and very low drive voltages resulting in substantially improved energy efficiency for devices. Optical loss is a well-recognized problem for plasmonic technologies but is currently addressed with some notable success. Thereby, the optimization of electrically poled organic electro-optic (OEO) materials is most critical since a large electro-optical coefficient allows implementation of short active device structures that result in lower insertion losses and lower voltage-length products. Most importantly, short structures also guarantee largest bandwidths and best energy efficiencies. Yet, an efficient optimization of in-device performance of OEO materials requires the development of novel computational simulation methods, especially as waveguide width dimensions reach tens of nanometers in plasmonic waveguides and as electrode surface/material interfacial effects become more and more dominant. The focus of this communication is on novel multi-scale modeling methods, including coarse-grained Monte Carlo statistical mechanical simulations combined with quantum mechanical methods to simulate and analyze the linear and nonlinear optical properties for high chromophore number density solid-state OEO materials. New chromophores are developed with the assistance of theory and may lead to an order of magnitude improvement in device performance.

Citations:

  • Robinson, B. H.; Johnson, L. E.; Elder, D. L.; Kocherzhenko, A. A.; Isborn, C. M.; Haffner, C.; Heni, W.; Hoessbacher, C.; Fedoryshyn, Y.; Salamin, Y.; Baeuerle, B.; Josten, A.; Ayata, M.; Koch, U.; Leuthold, J.; Dalton, L. R.. Journal of Lightwave Technology. 2018, 36, 5036-5047. Doi: https://doi.org/10.1109/JLT.2018.2865882

Published Date: June 12, 2017

Silicon–Organic and Plasmonic–Organic Hybrid Photonics

Abstract: Chip-scale integration of electronics and photonics is recognized as important to the future of information technology, as is the exploitation of the best properties of electronics, photonics, and plasmonics to achieve this objective. However, significant challenges exist including matching the sizes of electronic and photonic circuits; achieving low-loss transition between electronics, photonics, and plasmonics; and developing and integrating new materials. This review focuses on a hybrid material approach illustrating the importance of both chemical and engineering concepts. Silicon–organic hybrid (SOH) and plasmonic–organic hybrid (POH) technologies have permitted dramatic improvements in electro-optic (EO) performance relevant to both digital and analog signal processing. For example, the voltage–length product of devices has been reduced to less than 40 Vμm, facilitating device footprints of <20 μm2 operating with digital voltage levels to frequencies above 170 GHz. Energy efficiency has been improved to around a femtojoule/bit. This improvement has been realized through exploitation of field enhancements permitted by new device architectures and through theory-guided improvements in organic electro-optic (OEO) materials. Multiscale theory efforts have permitted quantitative simulation of the dependence of OEO activity on chromophore structure and associated intermolecular interactions. This has led to new classes of OEO materials, including materials of reduced dimensionality and neat (pure) chromophore materials that can be electrically poled. Theoretical simulations have helped elucidate the observed dependence of device performance on nanoscopic waveguide dimensions, reflecting the importance of material interfaces. The demonstration and explanation of the dependence of in-device electro-optic activity, voltage–length product, and optical insertion loss on device architecture (e.g., slot width) suggest new paradigms for further dramatic improvement of performance.

Citations:

  • Heni, W.; Kutuvantavida, Y.; Haffner, C.; Zwickel, H.; Kieninger, C.; Wolf, S.; Lauermann, M.; Fedoryshyn, Y.; Tillack, A. F.; Johnson, L. E.; Elder, D. L.; Robinson, B. H.; Freude, W.; Koos, C.; Leuthold, J.; Dalton, L. R.. ACS Photonics. 2017, 4, 1576-1590. Doi: https://doi.org/10.1021/acsphotonics.7b00224

Published Date: November 11, 2015

Silicon-Organic Hybrid (SOH) and Plasmonic-Organic Hybrid (POH) Integration

Abstract: Silicon photonics offers tremendous potential for inexpensive high-yield photonic-electronic integration. Besides conventional dielectric waveguides, plasmonic structures can also be efficiently realized on the silicon photonic platform, reducing device footprint by more than an order of magnitude. However, neither silicon nor metals exhibit appreciable second-order optical nonlinearities, thereby making efficient electro-optic modulators challenging to realize. These deficiencies can be overcome by the concepts of silicon-organic hybrid (SOH) and plasmonic-organic hybrid integration, which combine SOI waveguides and plasmonic nanostructures with organic electro-optic cladding materials.

Citations:

  • Koos, C.; Leuthold, J.; Freude, W.; Kohl, M.; Dalton, L.; Bogaerts, W.; Giesecke, A. L.; Lauermann, M.; Melikyan, A.; Koeber, S.; Wolf, S.; Weimann, C.; Muehlbrandt, S.; Koehnle, K.; Pfeifle, J.; Hartmann, W.; Kutuvantavida, Y.; Ummethala, S.; Palmer, R.; Korn, D.; Alloatti, L.; Schindler, P. C.; Elder, D. L.; Wahlbrink, T.; Bolten, J.. Journal of Lightwave Technology. 2016, 34, 256-268. Doi: https://doi.org/10.1109/JLT.2015.2499763

Published Date: June 1, 2020

A monolithic bipolar CMOS electronic–plasmonic high-speed transmitter

Abstract: To address the challenge of increasing data rates, next-generation optical communication networks will require the co-integration of electronics and photonics. Heterogeneous integration of these technologies has shown promise, but will eventually become bandwidth-limited. Faster monolithic approaches will therefore be needed, but monolithic approaches using complementary metal–oxide–semiconductor (CMOS) electronics and silicon photonics are typically limited by their underlying electronic or photonic technologies. Here, we report a monolithically integrated electro-optical transmitter that can achieve symbol rates beyond 100 GBd. Our approach combines advanced bipolar CMOS with silicon plasmonics, and addresses key challenges in monolithic integration through co-design of the electronic and plasmonic layers, including thermal design, packaging and a nonlinear organic electro-optic material. To illustrate the potential of our technology, we develop two modulator concepts—an ultra-compact plasmonic modulator and a silicon-plasmonic modulator with photonic routing—both directly processed onto the bipolar CMOS electronics.

Citations:

  • Koch, U.; Uhl, C.; Hettrich, H.; Fedoryshyn, Y.; Hoessbacher, C.; Heni, W.; Baeuerle, B.; Bitachon, B. I.; Josten, A.; Ayata, M.; Xu, H.; Elder, D. L.; Dalton, L. R.; Mentovich, E.; Bakopoulos, P.; Lischke, S.; Krüger, A.; Zimmermann, L.; Tsiokos, D.; Pleros, N.; Möller, M.; Leuthold, J.. Nature Electronics. 2020, 3, 338–345. Doi: https://doi.org/10.1038/s41928-020-0417-9

Published Date: February 12, 2020

Ultra-High-Speed 2:1 Digital Selector and Plasmonic Modulator IM/DD Transmitter Operating at 222 GBaud for Intra-Datacenter Applications

Abstract: We demonstrate a 222 GBd on-off-keying transmitter in a short-reach intra-datacenter scenario with direct detection after 120 m of standard single mode fiber. The system operates at net-data rates of >200 Gb/s OOK for transmission distances of a few meters, and >177 Gb/s over 120 m, limited by chromatic dispersion in the standard single mode fiber. The high symbol rate transmitter is enabled by a high-bandwidth plasmonic-organic hybrid Mach-Zehnder modulator on the silicon photonic platform that is ribbon-bonded to an InP DHBT 2:1 digital multiplexing selector. Requiring no driving RF amplifiers, the selector directly drives the modulator with a differential output voltage of 622 mVpp measured across a 50 Ω resistor. The transmitter assembly occupies a footprint of less than 1.5 mm × 2.1 mm.

Citations:

  • Heni, W.; Baeuerle, B.; Mardoyan, H.; Jorge, F.; Estaran, J. M.; Konczykowska, A.; Riet, M.; Duval, B.; Nodjiadjim, V.; Goix, M.; Dupuy, J.-Y.; Destraz, M.; Hoessbacher, C.; Fedoryshyn, Y.; Xu, H.; Elder, D. L.; Dalton, L.; Renaudier, J.; Leuthold, J.. Journal of Lightwave Technology. 2020, 9 (1) , 2734-2739. Doi: https://doi.org/10.1109/JLT.2020.2972637

Published Date: April 12, 2019

Plasmonic IQ modulators with attojoule per bit electrical energy consumption

Abstract: Coherent optical communications provides the largest data transmission capacity with the highest spectral efficiency and therefore has a remarkable potential to satisfy today’s ever-growing bandwidth demands. It relies on so-called in-phase/quadrature (IQ) electro-optic modulators that encode information on both the amplitude and the phase of light. Ideally, such IQ modulators should offer energy-efficient operation and a most compact footprint, which would allow high-density integration and high spatial parallelism. Here, we present compact IQ modulators with an active section occupying a footprint of 4 × 25 µm × 3 µm, fabricated on the silicon platform and operated with sub-1-V driving electronics. The devices exhibit low electrical energy consumptions of only 0.07 fJ bit−1 at 50 Gbit s−1, 0.3 fJ bit−1 at 200 Gbit s−1, and 2 fJ bit−1 at 400 Gbit s−1. Such IQ modulators may pave the way for application of IQ modulators in long-haul and short-haul communications alike.

Citations:

  • Heni, W.; Fedoryshyn, Y.; Baeuerle, B.; Josten, A.; Hoessbacher, C. B.; Messner, A.; Haffner, C.; Watanabe, T.; Salamin, Y.; Koch, U.; Elder, D. L.; Dalton, L. R.; Leuthold, J.. Nature Communications. 2019, 10, 1694. Doi: https://doi.org/10.1038/s41467-019-09724-7

Published Date: December 5, 2019

Compact and ultra-efficient broadband plasmonic terahertz field detector

Abstract: Terahertz sources and detectors have enabled numerous new applications from medical to communications. Yet, most efficient terahertz detection schemes rely on complex free-space optics and typically require high-power lasers as local oscillators. Here, we demonstrate a fiber-coupled, monolithic plasmonic terahertz field detector on a silicon-photonics platform featuring a detection bandwidth of 2.5 THz with a 65 dB dynamical range. The terahertz wave is measured through its nonlinear mixing with an optical probe pulse with an average power of only 63 nW. The high efficiency of the scheme relies on the extreme confinement of the terahertz field to a small volume of 10−8THz/2)3. Additionally, on-chip guided plasmonic probe beams sample the terahertz signal efficiently in this volume. The approach results in an extremely short interaction length of only 5 μm, which eliminates the need for phase matching and shows the highest conversion efficiency per unit length up to date.

Citations:

Published Date: May 30, 2019

500 GHz plasmonic Mach-Zehnder modulator enabling sub-THz microwave photonics

Abstract: Broadband electro-optic intensity modulators are essential to convert electrical signals to the optical domain. The growing interest in terahertz wireless applications demands modulators with frequency responses to the sub-terahertz range, high power handling, and very low nonlinear distortions, simultaneously. However, a modulator with all those characteristics has not been demonstrated to date. Here, we experimentally demonstrate that plasmonic modulators do not trade-off any performance parameter, featuring—at the same time—a short length of tens of micrometers, record-high flat frequency response beyond 500 GHz, high power handling, and high linearity, and we use them to create a sub-terahertz radio-over-fiber analog optical link. These devices have the potential to become a new tool in the general field of microwave photonics, making the sub-terahertz range accessible to, e.g., 5G wireless communications, antenna remoting, Internet of Things, sensing, and more.

Citations:

  • Burla, M.; Hoessbacher, C.; Heni, W.; Haffner, C.; Fedoryshyn, Y.; Werner, D.; Watanabe, T.; Massler, H.; Elder, D. L.; Dalton, L. R.; Leuthold, J.. APL Photonics. 2019, 4, . Doi: https://doi.org/10.1063/1.5086868

Published Date: October 29, 2018

Microwave plasmonic mixer in a transparent fibre–wireless link

Abstract: To cope with the high bandwidth requirements of wireless applications1, carrier frequencies are shifting towards the millimetre-wave and terahertz bands. Conversely, data is normally transported to remote wireless antennas by optical fibres. Therefore, full transparency and flexibility to switch between optical and wireless domains would be desirable. Here, we demonstrate a direct wireless-to-optical receiver in a transparent optical link. We successfully transmit 20 and 10 Gbit s−1 over wireless distances of 1 and 5 m, respectively, at a carrier frequency of 60 GHz. Key to the breakthrough is a plasmonic mixer directly mapping the wireless information onto optical signals. The plasmonic scheme with its subwavelength feature and pronounced field confinement provides a built-in field enhancement of up to 90,000 over the incident field in an ultra-compact and complementary metal-oxide–semiconductor compatible structure. The plasmonic mixer is not limited by electronic speed and thus compatible with future terahertz technologies.

Citations:

  • Salamin, Y.; Baeuerle, B.; Heni, W.; Abrecht, F. C.; Josten, A.; Fedoryshyn, Y.; Haffner, C.; Bonjour, R.; Watanabe, T.; Burla, M.; Elder, D. L.; Dalton, L. R.; Leuthold, J.. Nature Photonics. 2018, 12, 749-753. Doi: https://doi.org/10.1038/s41566-018-0281-6

Published Date: October 10, 2018

Demonstration of long-term thermally stable silicon-organic hybrid modulators at 85 °C

Abstract: We report on the first demonstration of long-term thermally stable silicon-organic hybrid (SOH) modulators in accordance with Telcordia standards for high-temperature storage. The devices rely on an organic electro-optic sidechain polymer with a high glass transition temperature of 172 °C. In our high-temperature storage experiments at 85 °C, we find that the electro-optic activity converges to a constant long-term stable level after an initial decay. If we consider a burn-in time of 300 h, the π-voltage of the modulators increases on average by less than 15% if we store the devices for an additional 2400 h. The performance of the devices is demonstrated by generating high-quality 40 Gbit/s OOK signals both after the burn-in period and after extended high-temperature storage.

Citations:

  • Kieninger, C.; Kutuvantavida, Y.; Miura, H.; Kemal, J. N.; Zwickel, H.; Qiu, F.; Lauermann, M.; Freude, W.; Randel, S.; Yokoyama, S.; Koos, C.. Optics Express. 2018, 26, 27955-27964. Doi: https://doi.org/10.1364/OE.26.027955

Published Date: February 21, 2018

Three-Dimensional Phase Modulator at Telecom Wavelength Acting as a Terahertz Detector with an Electro-Optic Bandwidth of 1.25 Terahertz

Abstract: We report a thin film phase modulator employing organic nonlinear optical molecules, with an electro-optic bandwidth of 1.25 THz. The device acts as a polarization sensitive broadband Pockels medium for coherent electric field detection in a dual wavelength terahertz time-domain spectroscopy setup in the telecom band at 1550 nm. To increase the sensitivity, we combine a three-dimensional bow-tie antenna structure with strongly electro-optically active molecules JRD1 in poly(methyl methacrylate) supporting polymer. The antenna provides subwavelength field confinement of the terahertz wave with its waveguide gap with lateral dimensions of 2.2 μm × 5 μm × 4 μm. In the gap, the electric field is up to 150× stronger than in a diffraction limited space-time volume, such that an interaction length of only 4 μm suffices for the detection of fields below 10 V/m. This device is promising in the growing field of quantum optics in the terahertz, single photon terahertz detection, nonlinear imaging, and on-chip telecommunication.

Citations:

  • Benea-Chelmus, I.-C.; Zhu, T.; Settembrini, F. F.; Bonzon, C.; Mavrona, E.; Elder, D. L.; Heni, W.; Leuthold, J.; Dalton, L. R.; Faist, J.. ACS Photonics. 2018, 5 (4) , 1398-1403. Doi: https://doi.org/10.1021/acsphotonics.7b01407

Published Date: April 25, 2018

Low-loss plasmon-assisted electro-optic modulator

Abstract: For nearly two decades, researchers in the field of plasmonics—which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal—have sought to realize subwavelength optical devices for information technology, sensing, nonlinear optics, optical nanotweezers and biomedical applications. 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 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.

Citations:

  • Haffner, C.; Chelladurai, D.; Fedoryshyn, Y.; Josten, A.; Baeuerle, B.; Heni, W.; Watanabe, T.; Cui, T.; Cheng, B.; Saha, S.; Elder, D. L.; Dalton, L. R.; Boltasseva, A.; Shalaev, V. M.; Kinsey, N.; Leuthold, J.. Nature. 2018, 556, 483-486. Doi: https://doi.org/10.1038/s41586-018-0031-4

Published Date: November 3, 2017

Complete High-speed Plasmonic Modulator in a Single Metal Layer. Science

Abstract: Plasmonics provides a possible route to overcome both the speed limitations of electronics and the critical dimensions of photonics. We present an all-plasmonic 116–gigabits per second electro-optical modulator in which all the elements—the vertical grating couplers, splitters, polarization rotators, and active section with phase shifters—are included in a single metal layer. The device can be realized on any smooth substrate surface and operates with low energy consumption. Our results show that plasmonics is indeed a viable path to an ultracompact, highest-speed, and low-cost technology that might find many applications in a wide range of fields of sensing and communications because it is compatible with and can be placed on a wide variety of materials.

Citations:

Published Date: February 1, 2017

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

Abstract: 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.

Citations:

  • Heni, W.; Haffner, C.; Elder, D. L.; Tillack, A. F.; Fedoryshyn, Y.; Cottier, R.; Salamin, Y.; Hoessbacher, C.; Koch, U.; Cheng, B.; Robinson, B.; Dalton, L. R.; Leuthold, J.. Optics Express. 2017, 25 (3) , 2627-2653. Doi: https://doi.org/10.1364/OE.25.002627

Published Date: July 27, 2015

All-plasmonic Mach–Zehnder modulator enabling optical high-speed communication at the microscale

Abstract: Optical modulators encode electrical signals to the optical domain and thus constitute a key element in high-capacity communication links. Ideally, they should feature operation at the highest speed with the least power consumption on the smallest footprint, and at low cost. Unfortunately, current technologies fall short of these criteria. Recently, plasmonics has emerged as a solution offering compact and fast devices. Yet, practical implementations have turned out to be rather elusive. Here, we introduce a 70 GHz all-plasmonic Mach–Zehnder modulator that fits into a silicon waveguide of 10 μm length. This dramatic reduction in size by more than two orders of magnitude compared with photonic Mach–Zehnder modulators results in a low energy consumption of 25 fJ per bit up to the highest speeds. The technology suggests a cheap co-integration with electronics.

Citations:

  • Haffner, C.; Heni, W.; Fedoryshyn, Y.; Niegemann, J.; Melikyan, A.; Elder, D. L.; Baeuerle, B.; Salamin, Y.; Josten, A.; Koch, U.; Hoessbacher, C.; Ducry, F.; Juchli, L.; Emboras, A.; Hillerkuss, D.; Kohl, M.; Dalton, L. R.; Hafner, C.; Leuthold, J.. Nature Photonics. 2015, 9, 525-528. Doi: https://doi.org/10.1038/nphoton.2015.127

Published Date: November 16, 2015

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

Abstract: 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.

Citations:

  • Salamin, Y.; Heni, W.; Haffner, C.; Fedoryshyn, Y.; Hoessbacher, C.; Bonjour, R.; Zahner, M.; Hillerkuss, D.; Leuchtmann, P.; Elder, D. L.; Dalton, L. R.; Hafner, C.; Leuthold, J.. Nano Lett. 2015, 15 (12) , 8342–8346. Doi: https://doi.org/10.1021/acs.nanolett.5b04025

Published Date: February 27, 2019

Unraveling Excitonic Effects for the First Hyperpolarizabilities of Chromophore Aggregates

Abstract: Excitonic interactions often significantly affect the optoelectronic properties of molecular materials. However, their role in determining the nonlinear optical response of organic electro-optic materials remains poorly understood. In this paper, we explore the effects of excitonic interactions on the first hyperpolarizability for aggregates of donor–acceptor chromophores. We show that calculations of the first hyperpolarizabilty of chromophore aggregates based on a two-state model agree well with the more rigorous coupled perturbed Hartree–Fock method. We then use both time-dependent density functional theory calculations and the molecular exciton approximation to parametrize the two-state model. Use of the molecular exciton approximation to the two-state model (i) is appropriate for disordered aggregates (unlike band theory), (ii) is computationally efficient enough for calculating the first hyperpolarizability of materials that consist of thousands of interacting chromophores, and (iii) allows the unraveling of the effects of both excitonic interactions and electrostatic polarization of the chromophore electron density by its environment on the first hyperpolarizability of molecular materials. We find that use of the molecular exciton approximation to the two-state model does not introduce significant additional errors compared to those introduced by applying the two-state model alone. We determine that the absolute change to the first hyperpolarizability of chromophore aggregates due to excitonic interactions increases with the size of the aggregate. For all sizes of disordered aggregates of chromophores considered in this paper, the inclusion of excitonic interactions on average decreases the magnitude of the first hyperpolarizability by 12–14% compared to the case of non-interacting chromophores. Finally, we present a method for analytically calculating the first hyperpolarizability of a one-dimensional periodic array of chromophores within the molecular exciton approximation to the two-state model. This technique can be used to include an approximate correction for excitonic effects when simulating the electro-optic response of disordered and ordered organic materials.

Citations:

  • Kocherzhenko, A. A.; Shedge, S. V.; Sosa Vazquez, X.; Maat, J.; Wilmer, J.; Tillack, A. F.; Johnson, L. E.; Isborn, C. M.. The Journal of Physical Chemistry C. 2019, 123 (22) , 13818-13836. Doi: https://doi.org/10.1021/acs.jpcc.8b12445

Published Date: July 19, 2016

Systematic Generation of Anisotropic Coarse-Grained Lennard-Jones Potentials and Their Application to Ordered Soft Matter

Abstract: We have developed an approach to coarse-grained (CG) modeling of the van der Waals (vdW) type of interactions among molecules by representing groups of atoms within those molecules in terms of ellipsoids (rather than spheres). Our approach systematically translates an arbitrary underlying all-atom (AA) representation of a molecular system to a multisite ellipsoidal potential within the family of Gay–Berne type potentials. As the method enables arbitrary levels of coarse-graining, or even multiple levels of coarse-graining within a single simulation, we describe the method as a Level of Detail (LoD) model. The LoD model, as integrated into our group’s Metropolis Monte Carlo computational package, is also capable of reducing the complexity of the molecular electrostatics by means of a multipole expansion of charges obtained from an AA force field (or directly from electronic structure calculations) of the charges within each ellipsoid. Electronic polarizability may additionally be included. The present CG representation does not include transformation of bonded interactions; ellipsoids are connected at the fully atomistic bond sites by freely rotating links that are constrained to maintain a constant distance. The accuracy of the method is demonstrated for three distinct types of self-assembling or self-organizing molecular systems: (1) the interaction between benzene and perfluorobenzene (dispersion interactions), (2) linear hydrocarbon chains (a system with large conformational flexibility), and (3) the self-organization of ethylene carbonate (a highly polar liquid). Lastly, the method is applied to the interaction of large (∼100 atom) molecules, which are typical of organic nonlinear optical chromophores, to demonstrate the effect of different CG models on molecular assembly.

Citations:

Published Date: August 15, 2014

Optimum Exchange for Calculation of Excitation Energies and Hyperpolarizabilities of Organic Electro-optic Chromophores

Abstract: Organic electro-optic (OEO) materials integrated into silicon–organic hybrid devices afford significant improvements in size, weight, power, and bandwidth performance of integrated electronic/photonic systems critical for current and next generation telecommunication, computer, sensor, transportation, and defense technologies. Improvement in molecular first hyperpolarizability (β), and in turn electro-optic activity, is crucial to optimizing device performance. Common hybrid density functional theory (DFT) methods, while attractive due to their computational scaling, often perform poorly for optical properties in systems with substantial intramolecular charge-transfer character, such as OEO chromophores. This study evaluates the utility of the long-range corrected (LC) DFT methods for computation of the molecular second-order nonlinear optical response. We compare calculated results for a 14-molecule benchmark set of OEO chromophores with the corresponding experimentally measured β and one-photon absorption energy, λmax. We analyze the distance dependence of the fraction of exact exchange in LC-DFT methods for accurately computing these properties for OEO chromophores. We also examine systematic tuning of the range-separation parameter to enforce Koopmans’/ionization potential theorem. This tuning method improves prediction of excitation energies but is not reliable for predicting the hyperpolarizabilities of larger chromophores since the tuning parameter value can be too small, leading to instabilities in the computation of βHRS. Additionally, we find that the size dependence of the optimal tuning parameter for the ionization potential has the opposite size dependence of optimal tuning parameter for best agreement with the experimental λmax, suggesting the tuning for the ionization potential is unreliable for extended conjugated systems.

Citations:

  • Garrett, K.; Sosa Vazquez, X.; Egri, S. B.; Wilmer, J.; Johnson, L. E.; Robinson, B. H.; Isborn, C. M.. Journal of Chemical Theory and Computation. 2014, 10 (9) , 3821-3831. Doi: https://doi.org/10.1021/ct500528z

Published Date: June 26, 2014

Optimizing Calculations of Electronic Excitations and Relative Hyperpolarizabilities of Electro-optic Chromophores

Abstract: Organic glasses containing chromophores with large first hyperpolarizabilities (β) are promising for compact, high-bandwidth, and energy-efficient electro-optic devices. Systematic optimization of device performance requires development of materials with high acentric order and enhanced hyperpolarizability at operating wavelengths. One essential component of the design process is the accurate calculation of optical transition frequencies and hyperpolarizability. These properties can be computed with a wide range of electronic structure methods implemented within commercial and open-source software packages. A wide variety of methods, especially hybrid density-functional theory (DFT) variants have been used for this purpose. However, in order to provide predictions useful to chromophore designers, a method must be able to consistently predict the relative ordering of standard and novel materials. Moreover, it is important to distinguish between the resonant and nonresonant contribution to the hyperpolarizabiliy and be able to estimate the trade-off between improved β and unwanted absorbance (optical loss) at the target device’s operating wavelength.

Therefore, we have surveyed a large variety of common methods for computing the properties of modern high-performance chromophores and compared these results with prior experimental hyper-Rayleigh scattering (HRS) and absorbance data. We focused on hybrid DFT methods, supplemented by more computationally intensive Møller–Plesset (MP2) calculations, to determine the relative accuracy of these methods. Our work compares computed hyperpolarizabilities in chloroform relative to standard chromophore EZ-FTC against HRS data versus the same reference.

We categorized DFT methods used by the amount of Hartree–Fock (HF) exchange energy incorporated into each functional. Our results suggest that the relationship between percentage of long-range HF exchange and both βHRS and λmax is nearly linear, decreasing as the fraction of long-range HF exchange increases. Mild hybrid DFT methods are satisfactory for prediction of λmax. However, mild hybrid methods provided qualitatively incorrect predictions of the relative hyperpolarizabilities of three high-performance chromophores. DFT methods with approximately 50% HF exchange, and especially the Truhlar M062X functional, provide superior predictions of relative βHRS values but poorer predictions of λmax. The observed trends for these functionals, as well as range-separated hybrids, are similar to MP2, though predicting smaller absolute magnitudes for βHRS.

Frequency dependence for βHRS can be calculated using time-dependent DFT and HF methods. However, calculation quality is sensitive not only to a method’s ability to predict static hyperpolarizability but also to its prediction of optical resonances. Due to the apparent trade-off in accuracy of prediction of these two properties and the need to use static finite-field methods for MP2 and higher-level hyperpolarizability calculations in most codes, we suggest that composite methods could greatly improve the accuracy of calculations of β and λmax.

Citations:

Published Date: December 17, 2010

Measuring Order in Contact-Poled Organic Electrooptic Materials with Variable-Angle Polarization-Referenced Absorption Spectroscopy (VAPRAS)

Abstract: Organic nonlinear electrooptical (ONLO) chromophores must be acentrically ordered for the ONLO material to have electrooptic (EO) activity. The magnitude of the order is characterized by the acentric order parameter, ⟨cos3 β⟩, where β is the major Euler angle between the main axis of the chromophore and the poling field which imposes the acentric order. The acentric order parameter, which is difficult to measure directly, is related to the centrosymmetric order parameter, defined as ⟨P2⟩ = 1/2(3⟨cos2 β⟩ − 1), through the underlying statistical distribution. We have developed a method to determine centrosymmetric order of the ONLO chromophores when the order is low (i.e., ⟨P2⟩ < 0.1). We have extended the method (begun by Graf et al. J. Appl. Phys. 1994, 75, 3335.) based on the absorption of light to determine the centrosymmetric order parameter induced by a poling field on a thin film sample of ONLO material. We find that the order parameters, analyzed by two different methods, are similar and also consistent with theoretical estimates from modeling of the system using coarse-grained Monte Carlo statistical mechanical methods.

Citations:

  • Olbricht, B. C.; Sullivan, P. A.; Dennis, P. C.; Hurst, J. T.; Johnson, L. E.; Benight, S. J.; Davies, J. A.; Chen, A.; Eichinger, B. E.; Reid, P. J.; Dalton, L. R.; Robinson, B. H.. The Journal of Physical Chemistry B. 2011, 115 (2) , 231-241. Doi: https://doi.org/10.1021/jp107995t