Dirk Jalas

Comment on "Nonreciprocal light propagation in a silicon photonic circuit".

We show that the structure demonstrated by Feng et al. (Reports, 5 August 2011, p. 729) cannot enable optical isolation because it possesses a symmetric scattering matrix. Moreover, one cannot construct an optical isolator by incorporating this structure into any system as long as the system is linear and time-independent and is described by materials with a scalar dielectric function.

Resonance splitting in gyrotropic ring resonators

We present the theoretical concept of an optical isolator based on resonance splitting in a silicon ring resonator covered with a magneto-optical polymer cladding. For this task, a perturbation method is derived for the modes in the cylindrical coordinate system. A polymer magneto-optical cladding causing a 0.01 amplitude of the off-diagonal element of the dielectric tensor is assumed. It is shown that the derived resonance splitting of the clockwise and counterclockwise modes increases for smaller ring radii. For the ring with a radius of approximately 1.5μm, a 29GHz splitting is demonstrated. An integrated optical isolator with a 10μm geometrical footprint is proposed based on a critically coupled ring resonator.

Optical three-port circulators made with ring resonators

We propose a circulator consisting of a ring resonator coupled to three waveguides with Bragg reflectors at one end of each waveguide. A magneto-optically active material placed inside the ring resonator causes the two counter-propagating modes to split in resonance frequency, which can be exploited for perfect circulation by properly adjusting the coupling between the three waveguides and the ring. Such a device features a transmission spectrum that is similar to three-port photonic crystal circulators but is much simpler to build as it only contains elements that have already been experimentally realized.

Electrochemical tuning of the optical properties of nanoporous gold

Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.

Effective medium model for the spectral properties of nanoporous gold in the visible

The spectral properties of nanoporous gold are distinguished by two peaks in the transmission spectrum. Unlike earlier works, we do not attribute the peaks in the transmission to two separate localized plasmon resonances. Instead we show that the spectral shape can be understood as that of diluted gold with a spectrally narrow dip in transmission that arises from the averaged electric field approaching zero. Thus, the transmission characteristics are rather featured by a dip in one broad transmission curve than by two distinct peaks. Nanoporous gold is approximated by the effective medium model of a cubic grid of gold wires.

Pulse compression and broadening by reflection from a moving front of a photonic crystal

Previously, the effect of pulse bandwidth compression or broadening was observed in reflection from a moving front together with the Doppler shift. In this letter, an approach is presented, which alters pulse bandwidth without change in the central frequency. It occurs when light is reflected from a moving front of an otherwise stationary photonic crystal. This means that the photonic crystal lattice as such is stationary and only its boundary to the environment is moving, thus extruding (or shortening) the photonic crystal medium. The compression (broadening) factor depends on the front velocity and is the same as for the conventional Doppler shift. Complete reflection and transformation of the pulse can be achieved even with weak refractive index contrast, what makes the approach experimentally viable.

Single mode thermal emission.

We report on the properties of a thermal emitter which radiates into a single mode waveguide. We show that the maximal power of thermal radiation into a propagating single mode is limited only by the temperature of the thermal emitter and does not depend on other parameters of the waveguide. Furthermore, we show that the power of the thermal emitter cannot be increased by resonant coupling. For a given temperature, the enhancement of the total emitted power is only possible if the number of excited modes is increased. Either a narrowband or a broadband thermal excitation of the mode is possible, depending on the properties of the emitter. We finally discuss an example system, namely a thermal source for silicon photonics.

Mechanism that governs the electro-optic response of second-order nonlinear polymers on silicon substrates

We use a modified Teng-Man technique to investigate the poling induced electro-optic activity of chromophore-doped organic polymers poled on silicon substrate in a thin film sample configuration. We reveal a fundamental difference between the poling processes on silicon substrate and ITO substrate. The electro-optic activity for polymers poled on silicon substrate is reduced which we ascribe to space charge formation at the silicon - organic interface that distorts the field distribution in the polymer film during high field poling, and therefore limits the effective induced polar order. We demonstrate that the electro-optic activity on silicon substrate can be improved by inserting a 5 nm thin dielectric layer of Al2O3 between the silicon substrate and the polymer which reduces the leak-through current during poling, thereby allowing for higher applicable poling voltages.

Nonreciprocal silicon waveguides and ring resonators with gyrotropic cladding

Concepts are demonstrated for nonreciprocal Mach-Zehnder interferometers (MZIs) and ring resonators with polymer gyrotropic cladding. The isolation with −20 dB suppression is theoretically demonstrated in a 2µm radius ring resonator.

Nonreciprocal silicon-organic nanophotonic structures

We present a setup to measure the nonreciprocal magneto-optical phase shift in air and polymer cladded silicon on insulator waveguides. A high sensitivity could be achieved for the setup sufficient to determine the effect produced by silicon and silica. A silicon waveguide covered with a Fe3O4 nanoparticle containing polymer shows an amplitude modulation resulting from Faraday ellipticity which is of the same order of magnitude as the Faraday effect in silicon. We further present the theoretical concept of an optical isolator based on resonance splitting in a silicon ring resonator covered with a magneto-optical polymer cladding. A polymer magneto-optical cladding causing a 0.01 amplitude of the offdiagonal element of the dielectric tensor is assumed. It is shown that the derived resonance splitting of the clockwise and counterclockwise modes increases for smaller ring radii. For the ring with a radius of approximately 1.5 μm, a 29 GHz splitting is demonstrated. An integrated optical isolator with a 10μm geometrical footprint is proposed based on a critically coupled ring resonator.