R.J.P. Engelen

Flatband slow light in photonic crystals featuring spatial pulse compression and terahertz bandwidth.

Paradoxically, slow light promises to increase the speed of telecommunications in novel photonic structures, such as coupled resonators [1] and photonic crystals [2,3]. Apart from signal delays, the key consequence of slowing light down is the enhancement of light-matter interactions. Linear effects such as refractive index modulation scale linearly with slowdown in photonic crystals [3], and nonlinear effects are expected to scale with its square [4]. By directly observing the spatial compression of an optical pulse, by factor 25, we confirm the mechanism underlying this square scaling law. The key advantage of photonic structures over other slow light concepts is the potentially large bandwidth, which is crucial for telecommunications [5]. Nevertheless, the slow light previously observed in photonic crystals [2,3,6,7] has been very dispersive and featured narrow bandwidth. We demonstrate slow light with a bandwidth of 2.5 THz and a delay-bandwidth product of 30, which is an order of magnitude larger than any reported so far.

The effect of higher-order dispersion on slow light propagation in photonic crystal waveguides

We have studied the dispersion of ultrafast pulses in a photonic crystal waveguide as a function of optical frequency, in both experiment and theory. With phase-sensitive and time-resolved near-field microscopy, the light was probed inside the waveguide in a non-invasive manner. The effect of dispersion on the shape of the pulses was determined. As the optical frequency decreased, the group velocity decreased. Simultaneously, the measured pulses were broadened during propagation, due to an increase in group velocity dispersion. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. The asymmetry increased as the group velocity decreased. The asymmetry of the pulses is caused by a strong increase of higher-order dispersion. As the group velocity was reduced to 0.116(9)·c, we found group velocity dispersion of -1.1(3)·106 ps2/km and third order dispersion of up to 1.1(4)·105 ps3/km. We have modelled our interferometric measurements and included the full dispersion of the photonic crystal waveguide. Our mathematical model and the experimental findings showed a good correspondence. Our findings show that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion.

Local probing of Bloch mode dispersion in a photonic crystal waveguide

The local dispersion relation of a photonic crystal waveguide is directly determined by phase-sensitive near-field microscopy. We readily demonstrate the propagation of Bloch waves by probing the band diagram also beyond the first Brillouin zone. Both TE and TM polarized modes were distinguished in the experimental band diagram. Only the TE polarized defect mode has a distinctive Bloch wave character. The anomalous dispersion of this defect guided mode is demonstrated by local measurements of the group velocity. The measured dispersion relation and measured group velocities are both in good agreement with theoretical calculations.

Novel instrument for surface plasmon polariton tracking in space and time

We describe the realization of a phase-sensitive and ultrafast near-field microscope, optimized for investigation of surface plasmon polariton propagation. The apparatus consists of a homebuilt near-field microscope that is incorporated in Mach-Zehnder-type interferometer which enables heterodyne detection. We show that this microscope is able to measure dynamical properties of both photonic and plasmonic systems with phase sensitivity.

Flatband Slow Light in Photonic Crystal Waveguides

A photonic crystal waveguide that features slow light without noticeable dispersion is demonstrated using a higher order even mode in a W2 waveguide on a SOI platform.

The structure of light in photonic crystal waveguides

With a phase-sensitive near-field microscope we measure the propagating of light through a 2D photonic crystal waveguide. We study how the different Bloch harmonics of the propagating light evanescently decay into the air above the waveguide. Furthermore, exploiting the phase sensitivity of our microscope, we are able to reconstruct the electric vector field distribution with subwavelength resolution. In the complex field we observe both time-dependent and time-independent polarization singularities and determine the topology of the surrounding electric field.

Subwavelength structure of the evanescent field of an optical Bloch wave

We investigated the evanescent field above a photonic crystal waveguide. In such a waveguide, light is confined in the in-plane direction by a photonic bandgap, and in the out-of-plane direction by total internal reflection. We show that the evanescent decay of the field above the waveguide is non-trivial due to the Bloch nature of the mode. As a result complex intensity patterns appear at the nanoscale.

Eigenstates of Photonic Crystal Structures Visualized in Real Space and in k-Space

Photonic crystal structures allow an unprecedented control of light on length scales equivalent to the wavelength. The intricate interaction of light and the periodic lattice can lead to phenomena like localization, negative refraction and slow light. In order to understand the optical behaviour of such novel structures, an investigation of the underlying photonic eigenstates is crucial, since the propagation of light through them is governed by their photonic eigenstates and the coupling between these states. Here we investigate the propagation of light pulses through a complex photonic crystal device in real-time. Analysis of the photonic eigenstates in k-space allows different states to be identified. By tracking the evolution of the eigenstates in both k-space and time, we uncover the dynamics of the eigenstates and their mutual coupling directly on femtosecond time-scales.

Broadband and low loss slow light in SOI photonic crystal waveguides

The phenomenon of slow light in photonic crystal waveguides is discussed. Rather than maximising the slowdown factor, we believe that slow light is only useful for all-optical data processing if there is sufficient bandwidth, hence a slowdown factor of order 10-100 is more favourable, given that it enables bandwidths of order 1 THz or more to be realised. As a specific example, we demonstrate a slowdown factor of 12 (group index of 25) over a bandwidth of 2.5 THz in a W2 photonic crystal waveguide. Furthermore, slow light can only be useful if it is not compromised by losses. Due to recent improvements in our technology, we can now achieve losses of order 4 dB/cm, which is amongst the best reported for W1 photonic crystal waveguides.

The effect of higher order dispersion on slow light propagation in photonic crystal waveguides

We have studied the dispersion of femtosecond pulses in a photonic crystal waveguide. We found that slow propagating pulses were asymmetrically broadened, due to higher order dispersion. With decreasing group velocity, the asymmetry increased.