Epsilon-Near-Zero Al-Doped ZnO for Ultrafast Switching at Telecom Wavelengths: Outpacing the Traditional Amplitude-Bandwidth Trade-Off

Transparent conducting oxides have recently gained great attention as CMOS-compatible materials for applications in nanophotonics due to their low optical loss, metal-like behavior, versatile/tailorable optical properties, and established fabrication procedures. In particular, aluminum doped zinc oxide (AZO) is very attractive because its dielectric permittivity can be engineered over a broad range in the near infrared and infrared. However, despite all these beneficial features, the slow (> 100 ps) electron-hole recombination time typical of these compounds still represents a fundamental limitation impeding ultrafast optical modulation. Here we report the first epsilon-near-zero AZO thin films which simultaneously exhibit ultra-fast carrier dynamics (excitation and recombination time below 1 ps) and an outstanding reflectance modulation up to 40% for very low pump fluence levels (< 4 mJ/cm2) at the telecom wavelength of 1.3 {\mu}m. The unique properties of the demonstrated AZO thin films are the result of a low temperature fabrication procedure promoting oxygen vacancies and an ultra-high carrier concentration. As a proof-of-concept, an all-optical AZO-based plasmonic modulator achieving 3 dB modulation in 7.5 {\mu}m and operating at THz frequencies is numerically demonstrated. Our results overcome the traditional "modulation depth vs. speed" trade-off by at least an order of magnitude, placing AZO among the most promising compounds for tunable/switchable nanophotonics.Comment: 14 pages, 9 figures, 1 tabl

Optical rectification in semiconductor waveguides

In this thesis, we study optical to microwave conversion and generation of ultrashort electrical pulses by the use of optical rectification at telecommunication wavelengths, λ = 1550 nm. By using optical rectification, an electromagnetic pulse is generated in a completely passive semiconductor waveguide. This pulse is coupled in a microwave transmission line with periodically loaded ground electrodes to create a velocity-matched structure. The optical waveguide and the microwave transmission line form the optical rectification device. Although in theory, the width of the electrical pulse in a travelling wave structure is limited only by the duration of the optical excitation pulse, imperfections in the velocity matching will attenuate and disperse most of the electrical pulse. The calculated effective optical refractive index of the rectification devices, nopt - 3.30, matches the measured effective microwave index in one of our structures namely DevO68 (nmw = 3.30). If the structure is slightly velocity-mismatched, losses as high as 14 dB/mm at frequencies of 1 THz will affect the propagation of the electrical pulse. The optical rectification device was fabricated using conventional photolithography techniques and e-beam lithography techniques. The advantages of e-beam lithography are: better pattern definition, perfect alignment and easier lift-off process. The only disadvantage is the cost associated with running the e-beam writer and maybe the time it takes to complete a pattern. The semiconductor material system of choice for the rectification devices is GaAs / AlGaAs due to its well-known large nonlinear coefficient. The use of GaAs/AlGaAs with light at λ = 1550 nm, presents serious absorption effects. The absorption effects mask the pure optical rectification signal and therefore must be minimised. The most significant absorption effect at λ = 1550 nm is two-photon absorption (TPA), which in more than one experiment gave us pulses of a few nanosecons duration. Our rectification device is engineered to minimise TPA, and this is the perhaps the hardest challenge in the design of the device. This also maybe the reason why there is not rectification devices such as ours reported in the literature working at λ = 1550 nm. The reason why we wanted to work with GaAs/AlGaAs is the potential integration of the rectification device in optoelectronic systems. In the final rectification device, we could observe a clear polarization dependence of the generated signal indicating optical rectification. The signal detected was small in magnitude, ~75 dBm and on top of an offset signal which is believed to be TPA. Nevertheless, we proved that an optical rectification signal could be generated and detected by experimental means. Finally, Q-switched diode lasers in Al-quaternary material were fabricated and evaluated as possible sources for the rectification devices. The lasers produced a pulse train ranging from 1 GHz to 2 GHz depending on the bias current. We reckon that our measurement set-up is not ideal to characterize the rectification signal but is the simplest set-up capable of giving us an indicative result. The time domain observation of the optical rectification signal has still to be done and the integration of a photoconductive switch to the optical rectification device seems to be the most obvious solution to achieve this

Thin-film Lithium Niobate Photonics for Electro-optics, Nonlinear Optics, and Quantum Optics on Silicon

Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on lithium niobate. These thin-film LN waveguides offer over an order of magnitude improvement in optical confinement, and about two orders of magnitude reduction in waveguide bending radius, compared to conventional LN waveguides. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices are demonstrated in this work. First, two types of compact electrooptic modulators are presented – microring modulators, and Mach-Zehnder modulators. Next, two distinct approaches to nonlinear optical frequency converters are implemented – periodically poled lithium niobate, and mode shape modulation (grating assisted quasi-phase matching). Following this, stochastic variations are added to the mode shape modulation approach to demonstrate random quasi-phase matching. Afterward, broadband photon-pair generation is demonstrated in the miniaturized periodically poled lithium niobate, and spectral correlations of the biphoton spectrum are reported. Finally, extensions of the aforementioned results suitable for future work are discussed

On-Chip Nanoscale Plasmonic Optical Modulators

In this thesis work, techniques for downsizing Optical modulators to nanoscale for the purpose of utilization in on chip communication and sensing applications are explored. Nanoscale optical interconnects can solve the electronics speed limiting transmission lines, in addition to decrease the electronic chips heat dissipation. A major obstacle in the path of achieving this goal is to build optical modulators, which transforms data from the electrical form to the optical form, in a size comparable to the size of the electronics components, while also having low insertion loss, high extinction ratio and bandwidth. Also, lap-on-chip applications used for fast diagnostics, and which is based on photonic sensors and photonic circuitry, is in need for similar modulator specifications, while it loosens the spec on the modulator’s size. Silicon photonics is the most convenient photonics technology available for optical interconnects application, owing to its compatibility with the mature and cheap CMOS manufacturing process. Hence, building modulators which is exclusively compatible with this technology is a must, although, Plasmonics could be the right technology for downsizing the optical components, owing to its capability in squeezing light in subwavelength dimensions. Hence, our major goal is to build plasmonic modulators, that can be coupled directly to silicon waveguides. A Plasmonic Mach-Zehnder modulator was built, based on the orthogonal junction coupling technique. The footprint of the modulator is decreased to 0.6 4.7, extinction ratio of 15.8 dB and insertion loss of 3.38 dB at 10 volts was achieved in the 3D simulations. The voltage length product for the modulator is 47 V. The orthogonal junction coupler technique minimized the modulator’s footprint. On the other hand, photonic sensors favorably work in the mid-infrared region, owing to the presence of a lot of molecules absorption peaks in this region. Hence, III-V semiconductor media is used for this type of applications, owing to the availability of laser sources built of III-V media, and to the lower losses that these materials have in mid-infrared region. Hybrid plasmonic waveguide, formed of doped InAs, AlAs and GaAs is studied extensively. Based on this waveguide an electro-absorption modulator is built. The device showed an extinction ratio of 27 dB at 40 length, and 1.2 dB of insertion loss. The small device footprint predicts a much lower energy consumption

Photonic platform and the impact of optical nonlinearity on communication devices

It is important to understand properties of different materials and the impact they have on devices used in communication networks. This paper is an overview of optical nonlinearities in Silicon and Gallium Nitride and how these nonlinearities can be used in the realization of optical ultra-fast devices targeting the next generation integrated optics. Research results related to optical lasing, optical switching, data modulation, optical signal amplification and photo-detection using Gallium Nitride devices based on waveguides are examined. Attention is also paid to hybrid and monolithic integration approaches towards the development of advanced photonic chips

Heterogeneous integration on silicon photonics

To enhance the functionality of the standard silicon photonics platform and to overcome its limitations, in particular for light emission, ultrafast modulation, and nonlinear applications, integration with novel materials is being investigated by several groups. In this paper, we will discuss, among others, the integration of silicon waveguides with ferroelectric materials such as lead zirconate titanate (PZT) and barium titanate (BTO), with electro-optically active polymers, with 2-D materials such as graphene and with III-V semiconductors through epitaxy. We discuss both the technology and design aspects