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The dielectric response of transparent conductive oxides near the bulk plasmon frequency is characterized by a refractive index less than vacuum. In analogy with x-ray optics, it is shown that this regime results in total external reflection and air-guiding of light. In addition, the strong reduction of the wavevector in the ITO below that of free space enables a new surface plasmon polariton mode which can be excited without requiring a prism or grating coupler. Ultrafast control of the surface plasmon polariton mode is achieved with a modulation amplitude reaching 20%.Epsilon-Near Zero (ENZ) materials are a class of optical materials characterized by a real part of the dielectric function close to zero. ENZ materials are of interest for a range of applications including tailoring of directional emission and radiation phase patterns, [1][2][3] air-guiding of electromagnetic waves, [4] and electromagnetic tunnelling devices. [5][6][7] While a lot of effort is aimed at achieving an ENZ response using artificial metamaterial resonators, some naturally occurring materials also show a strong reduction of the permittivity below that of vacuum. An example of naturally occurring low-index materials are noble metals where the optical permittivity ǫ is governed by the collective motions of the free electron gas known as bulk plasmons. According to the Drude model, the permittivity is given bywhere γ denotes the damping rate of the free electrons and the plasma frequency ω pl is given by ω pl = (N e 2 /ǫ 0 m) 1/2 . Around the (screened) bulk plasmon frequency ω bp ≡ ω pl / √ ǫ ∞ , the real part of the permittivity shows a transition from negative to positive values. Noble metals have carrier densities N exceeding 10 22 cm −3 , therefore their bulk plasmon plasma frequency is located in the UV region. In contrast, highly doped semiconductors typically have electron densities below 10 19 cm −3 and can be well described by the Drude model with a bulk plasmon plasma frequency in the THz range. Transparent conducting oxides (TCOs), with an electron density inbetween that of bulk metals and doped semiconductors, show a bulk plasmon frequency in the near-infrared. The resulting combination of a metal-like response in the infrared and a dielectric optical response in the visible region has stimulated application of TCOs as transparent electrical contacts and as heat reflecting windows. Recently, metal oxides such as indium-tin oxide (ITO) and aluminium-doped zinc oxide have received interest for their plasmonic response in relation to metamaterials and transformation optics. [8,9] The plasma frequency can be tuned by controlling the electron density using electrical or optical methods, opening up opportunities for nearinfrared electro-optic or optical modulators [7,10,11] and sensing devices. [12,13] Pioneering studies by Franzen and co-workers have investigated the plasmonic response of ITO and ITOgold hybrid structures in the metallic (negative epsilon) regime of ITO. [13] Next to a conventional surface plasmon polariton mode f...

Advances in silicon photonics have resulted in rapidly increasing complexity in integrated circuits. New methods that allow the direct characterization of individual optical components in situ, without the need for additional fabrication steps or test structures, are desirable. Here, we present a device-level method for the characterization of photonic chips based on a highly localized modulation in the device using pulsed laser excitation. Optical pumping perturbs the refractive index of silicon, providing a spatially and temporally localized modulation in the transmitted light, enabling time-and frequencyresolved imaging. We demonstrate the versatility of this all-optical modulation technique in imaging and in the quantitative characterization of a range of properties of silicon photonic devices, from group indices in waveguides, to quality factors of a ring resonator, and to the mode structure of a multimode interference device. Ultrafast photomodulation spectroscopy provides important information on devices of complex design, and is easily applicable for testing at the device level. I ntegrated silicon-based photonics has developed into a mature technology platform with a multitude of applications 1-4 , including telecommunications, healthcare diagnostics and optical sensors. As the technology progresses, device designs are becoming increasingly complex, with more functions integrated onto a single device 5 . The characterization of fabricated devices is an important step in the design cycle as it highlights differences between the intended design and the fabricated device, thus allowing the optimization of fabrication steps as well as of the entire design process. Established technologies, such as scanning electron microscopy (SEM), atomic force microscopy (AFM) and ellipsometry, are able to precisely measure device footprints, waveguide cross-sections, surface and sidewall roughness, film thickness and other geometric parameters.Direct access to the properties of light propagation in waveguide devices has proven more challenging, but numerous methods for the analysis of integrated optic elements have been proposed. Among them are reflectometry methods 6-9 , far-field scattering microscopy 10-13 , as well as the interrogation of structures with electron beams (cathodoluminescence) 14 , near-field optical probes 14-16 and AFM tips 17-23 . Near-field scanning optical microscopy (NSOM) is a powerful technique that gives direct access to light propagation, including phase information, and has high spatial resolution 15,16,24,25 . The drawbacks of scanning probe microscopy are its small field of view, slow scanning speeds and limited reproducibility and durability of the tips. Moreover, near-field techniques require direct access to the waveguide surface in order to interact with evanescent field components, limiting the analysis of devices covered with a top cladding for protection and stability.Here, we present a new approach for the characterization of silicon-on-insulator (SOI) waveguide elements at the devic...

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