Quantitative Optical and Structural Comparison of 3D and (2+1)D Colloidal Photonic Crystals.

Colloidal crystals are excellent model systems to study self-assembly and structural coloration because their periodicities coincide with the wavelength range of visible light. Different assembly methods inherently introduce characteristic defects and irregularities, even with nearly monodisperse colloidal particles. Here, we investigate how these imperfections influence the structural coloration by comparing two techniques to obtain colloidal crystals. 3D colloidal crystals produced by convective assembly are well-ordered and periodically arranged but show microscopic cracks. (2+1)D colloidal crystals fabricated by stacking individual monolayers show a decreased hexagonal order and limited crystal registration between single monolayers in the z-direction. We investigate the optical properties of both systems by comparing identical numbers of layers using correlative microspectroscopy. These measurements show that the less ordered (2+1)D colloidal crystals exhibit higher reflected light intensities. Macroscopic reflection integrating all angles shows that the reflected light intensity levels out with an increasing number of layers, whereas incoherent scattering increases. Although both types of colloidal crystal show similar angle-dependent color shifts in specular reflection, the less-ordered structure of the (2+1)D colloidal crystal scatters light within a larger angular range under diffusive illumination. Our results suggest that structural coloration is surprisingly robust toward local defects and irregularities.

Parity-time synthetic photonic lattices.

The development of new artificial structures and materials is today one of the major research challenges in optics. In most studies so far, the design of such structures has been based on the judicious manipulation of their refractive index properties. Recently, the prospect of simultaneously using gain and loss was suggested as a new way of achieving optical behaviour that is at present unattainable with standard arrangements. What facilitated these quests is the recently developed notion of 'parity-time symmetry' in optical systems, which allows a controlled interplay between gain and loss. Here we report the experimental observation of light transport in large-scale temporal lattices that are parity-time symmetric. In addition, we demonstrate that periodic structures respecting this symmetry can act as unidirectional invisible media when operated near their exceptional points. Our experimental results represent a step in the application of concepts from parity-time symmetry to a new generation of multifunctional optical devices and networks.

Cascaded phase-preserving multilevel amplitude regeneration.

The performance of cascaded in-line phase-preserving amplitude regeneration using nonlinear amplifying loop mirrors has been studied in numerical simulations. As an example of a spectrally efficient modulation format with two amplitude states and multiple phase states, the regeneration performance of a star-16QAM format, basically an 8PSK format with two amplitude levels, was evaluated. An increased robustness against amplified spontaneous emission and nonlinear phase noise was observed resulting in a significantly increased transmission distance.

All-optical phase-preserving multilevel amplitude regeneration.

The possibility of all-optical phase-preserving amplitude regeneration for star-8QAM is demonstrated using a modified nonlinear optical loop mirror. Experiments show a reduction in amplitude noise on both amplitude levels simultaneously, considering two different types of signal distortions: deterministic low-frequency amplitude modulation and broadband amplitude noise. Furthermore, using this amplitude regeneration, the robustness against nonlinear phase noise from fiber nonlinearity in a transmission line is increased. The scheme suppresses the conversion of amplitude noise to nonlinear phase noise. This is shown for simultaneous amplitude regeneration of the two amplitude states as well as for amplitude regeneration of the high-power states only. If the transmission is limited by nonlinear phase noise, single-level operation at the more critical higher-power state will benefit because of the wider plateau region. Numerical simulations confirm the experimental results.

Development of an advanced multiwavelength emission detector for the analytical ultracentrifuge.

An advanced design of the analytical ultracentrifuge with multiwavelength emission detection (MWE-AUC) is presented which offers outstanding performance concerning the spectral resolution and range flexibility as well as the quality of the data acquired. The excitation by a 520 nm laser is complemented with a 405 nm laser. An external spectrograph with three switchable tunable gratings permits optimisation of the spectral resolution in an order of magnitude range while keeping the spectral region broad. The new system design leads also to a significant reduction of systematic signal noise and allows the assessment and control of inner filter effects. Details regarding the very large signal dynamic range are presented, an important aspect when studying samples in a broad concentration range of up to five orders of magnitude. Our system is validated by complementary studies on two biological systems, fluorescent BSA and GFP, using the commercial Optima AUC with absorbance detection for comparison. Finally, we demonstrate the capabilities of our second generation MWE-AUC with respect to multiwavelength characterisation of gold nanoclusters, which exhibit specific fluorescence depending on their structure. Overall, this work depicts an important stepping stone for the concept of multiwavelength emission detection in AUC. The MWE-AUC developed, being to our knowledge the first and sole one of its kind, has reached the development level suitable for the future in-depth studies of size-, shape- and composition-dependent emission properties of colloids.

Observation of defect states in PT-symmetric optical lattices.

We provide the first experimental demonstration of defect states in parity-time (PT) symmetric mesh-periodic potentials. Our results indicate that these localized modes can undergo an abrupt phase transition in spite of the fact that they remain localized in a PT-symmetric periodic environment. Even more intriguing is the possibility of observing a linearly growing radiation emission from such defects provided their eigenvalue is associated with an exceptional point that resides within the continuum part of the spectrum. Localized complex modes existing outside the band-gap regions are also reported along with their evolution dynamics.

Optical gap solitons and truncated nonlinear Bloch waves in temporal lattices.

We experimentally demonstrate the formation and stable propagation of various types of discrete temporal solitons in an optical fiber system. Pulses interacting with a time-periodic potential and defocusing nonlinearity are shown to form gap solitons and nonlinear truncated Bloch waves. Multi-pulse solitons with defects, as well as novel structures composed of a strong soliton riding on a weaker truncated nonlinear Bloch wave are shown to propagate over up to eleven coupling lengths. The nonlinear dynamics of all pulse structures is monitored over the full propagation distance which provides detailed insight into the soliton dynamics.

Photon propagation in a discrete fiber network: an interplay of coherence and losses.

We study light propagation in a photonic system that shows stepwise evolution in a discretized environment. It resembles a discrete-time version of photonic waveguide arrays or quantum walks. By introducing controlled photon losses to our experimental setup, we observe unexpected effects like subexponential energy decay and formation of complex fractal patterns. This demonstrates that the interplay of linear losses, discreteness and energy gradients leads to genuinely new coherent phenomena in classical and quantum optical experiments. Moreover, the influence of decoherence is investigated.

A multiwavelength emission detector for analytical ultracentrifugation.

In this study, a new detector for multiwavelength emission analytical ultracentrifugation (MWE-AUC) is presented, which allows measuring size- or composition-dependent fluorescence properties of nanoparticle ensembles. Validation of the new setup is carried out via comparison to a benchtop photoluminescence spectrometer and the established extinction-based multiwavelength analytical ultracentrifuge (MWL-AUC). The results on fluorescent proteins and silica particles demonstrate that the new device not only correctly reproduces sedimentation and diffusion coefficients of the particles but provides also meaningful fluorescence spectra. As an application example for a sample exhibiting a broad particle size distribution, spectra and size of graphene oxide nanoplatelets are extracted simultaneously. Narrowly distributed CdSe/ZnS quantum dots showing size- and structure-dependent shifts of their fluorescence spectra are analyzed as well. The combination of MWE- and MWL-AUC provides a comprehensive framework for the optical characterization for nanoparticles and macromolecules in terms of their extinction and emission properties.

Anderson localization in synthetic photonic lattices.

Synthetic photonic lattices provide unique capabilities to realize theoretical concepts emerging in different fields of wave physics via the utilization of powerful photonic technologies. Here we observe experimentally Anderson localization for optical pulses in time domain, using a photonic mesh lattice composed of coupled fiber loops. We introduce a random potential through programmed electro-optic pulse phase modulation, and identify the localization features associated with varying degree of disorder. Furthermore, we present a practical approach to control the band-gap width in photonic lattices by varying the coupling between the fiber loops, and reveal that the strongest degree of localization is limited and increases in lattices with wider band-gaps. Importantly, this opens a possibility to enhance or reduce the effect of disorder and associated localization of optical pulses.

Experimental observation of spectral Bloch oscillations.

We report on the first, to our knowledge, experimental observation of spectral Bloch oscillations in an optical fiber employing the interaction between a probe signal and a traveling-wave periodic potential. The spectrum of weak probe pulses is shown to oscillate on account of their group-velocity mismatch to the periodic field. The behavior of a cw probe spectrum reveals the actual discrete nature of the effect. Recurrences of the spectrum after one and two Bloch periods are demonstrated.