Slow light optical modulators

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.

Mach–Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p–n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p–n diode. This length is shorter than commercial LiNbO3 modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index ng. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.

We report thorough investigations of photonic crystal waveguide properties in the slow light regime. The transmission and the group index near the cutoff wavelengths oscillate in phase in close analogy with the 1D photonic crystal behavior. The influence of having a finite number of periods in the photonic crystal waveguide is addressed to explain the spiky character of both the transmission and group index spectra. The profile of the slow-light modes is stretched out into the first and second rows of the holes closest to the waveguide channel. One of our strategies to ameliorate the design of photonic crystal devices is to engineer the radii of holes in these rows. A topology optimization approach is also utilized to make further improvements. The results of the numerical simulations and the optical characterization of fabricated devices such as straight waveguides with bends and couplers are presented. A nice match is found between theory and experiment.

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.

We present a compact integrated refractive index sensor using slow light in photonic crystal waveguides (PCWs). The PCWs are used in a Mach-Zahnder interferometer configuration and operated close to the edge of the mode-gap to achieve a compact design and high sensitivity.

The realization of mode-locked lasers capable of generating short optical pulses in the near-infrared is a key enabler for various applications including high data rate optical interconnects. The related technology mostly consists of bulky and stand-alone devices, which tends to hinder their widespread use. In addition, the integration of such devices onto chip-based platforms could bring advantages in terms of robustness and stability, while offering some prospects for the realization of advanced hybrid photonic-electronic architectures. There has been recently some progress regarding the miniaturization of fast pulsed lasers [1], but the underlying cavity typically remains longer than a few millimetres and their integration on a chip is still a challenge [2]. Here, we aim at realizing a compact integrated pulsed micro-laser using an innovative photonic crystal (PhC) cavity based on low dispersion slow light modes. Most of the characteristics of mode-locked lasers are tightly linked to the product of the cavity length L and the group index ng of the modes traveling in the cavity. While miniaturization thus tends to degrade the quality of the generated optical pulses, we counterbalance this effect through exploiting slow light (high ng) modes in PhC structures. Dispersion engineering techniques have been successfully developed in passive Si/air suspended PhC cavities for decreasing the otherwise typically high dispersion of these modes [3]. We adapted these techniques to our asymmetric InP/silica structures so as to provide a frequency-equidistant comb of modes - a pre-requisite to laser mode-locking. While our geometry well improves laser heat dissipation with respect to PhC cavities suspended in air, it poses additional constraints and could increase the cavity mode optical losses. Using 3D FDTD simulations, we demonstrated the validity of this approach and designed, as an example, a 30 μm long, linear dispersion PhC cavity, with a group index of 29, providing at least 9 equally-spaced modes over an 11.5 nm bandwidth, which can afford the generation of sub-picosecond optical pulses. Despite the structure asymmetry, quality factors were all above 10,000, i.e. sufficient for reaching laser operation. Compared with a standard strip-waveguide-based laser cavity, this design provides a length reduction factor of almost one order of magnitude, as given by the ratio of the group index of each structure. Various designs enabled us to achieve a range of low dispersion bands with different group index (15-40), offering a relevant trade-off between device miniaturization and short pulse duration. Based on these designs, PhC active structures including InAsP/ InP quantum wells or InAs/InP quantum dashes were fabricated using molecular beam epitaxy, III-V/silica bonding, e-beam lithography and reactive ion etching. Near-field optical microscopy was used to identify both the spectral and spatial signatures of the cavity modes, which were in good agreement with the simulations. We also probed our cavities by microphotoluminescence. These cavities provide a first step towards the realization of miniaturized integrated mode-locked lasers. Their robust hybrid InP/silica geometry offers the potential for integration with silicon photonic devices, while enabling the integration of a saturable absorber, such as graphene, for achieving mode-locked laser operation.

Broadly tunable and ultra-wideband slow light was theoretically demonstrated in a line defected photonic crystal waveguide (PCW) by infiltrating fluid with suitable refractive index into the first row of air holes adjacent to the waveguide. Simulation results showed that the working wavelength of the flat slow light in the optimized optofluidic PCW could be tuned from 1542.5 nm to 1569 nm with nearly constant group index of 80. Namely, broadly tunable slow light could be realized with the ultra-wide bandwidth over 26.5 nm and the group index up to 80. These achievements could not only open the possibility for achieving many excellent optical devices based on slow light in PCW, but also highlight the versatility, flexibility, and tunability offered by optofluidic infiltration schemes.

We report FWM (−9dB conversion) in a short (80μm) silicon dispersion engineered slow (c/30) light photonic crystal waveguide through launching a pulsed pump and a CW probe signal.

We present a simulation approach to estimate the effect of disorder induced backscattering in slow light photonic crystal line-defect waveguides. The backscattering leads to localization and thus limits the maximal length of such waveguides. Loss in passive waveguides in the localization regime reduces ripples in spectral transmission and group delay whereas gain increases these resonant multiple scattering effects.

An ultra-compact silicon Mach-Zehnder interferometer (MZI) modulator featuring p-i-n-diode-embedded photonic crystal waveguides has been fabricated. As carrier injection is the only practical option for optical modulation in silicon, a lower limit of current density (~10⁴A/cm²) exists for achieving gigahertz modulation in the widely employed p-i-n diode configuration. Electrical simulations have been performed to design and analyze the device. The device interaction length was reduced by one order of magnitude compared to the conventional waveguide based MZI modulators by taking advantage of the slow group velocity exhibited by photonic crystal waveguides (PCWs). A maximum modulation depth of 93% has been obtained under an injected current of 7.1 mA. High-speed optical modulation at 1 Gbit s^-1 in the 1.55 micron wavelength region was experimentally demonstrated. To our knowledge, this is the fastest speed ever achieved for a p-i-n diode based integrated silicon MZI modulator.

Observations of thermally driven transverse vibration of a photonic crystal waveguide (PCW) are reported. The PCW consists of two parallel nanobeams whose width is modulated symmetrically with a spatial period of 370 nm about a 240-nm vacuum gap between the beams. The resulting dielectric structure has a band gap (i.e., a photonic crystal stop band) with band edges in the near infrared that provide a regime for transduction of nanobeam motion to phase and amplitude modulation of an optical guided mode. This regime is in contrast to more conventional optomechanical coupling by way of moving end mirrors in resonant optical cavities. Models are developed and validated for this optomechanical mechanism in a PCW for probe frequencies far from and near to the dielectric band edge (i.e., stop band edge). The large optomechanical coupling strength predicted should make possible measurements with an imprecision below that at the standard quantum limit and well into the backaction-dominated regime. Since our PCW has been designed for near-field atom trapping, this research provides a foundation for evaluating possible deleterious effects of thermal motion on optical atomic traps near the surfaces of PCWs. Longer-term goals are to achieve strong atom-mediated links between individual phonons of vibration and single photons propagating in the guided modes (GMs) of the PCW, thereby enabling optomechanics at the quantum level with atoms, photons, and phonons. The experiments and models reported here provide a basis for assessing such goals.

Improving slow light effect in photonic crystal line defect waveguide by using eye-shaped scatterers

Reaching a regime of strong coupling between light and atoms is a long-sought goal in quantum optics, as it will enable non-linear optics at the photon level. Interfacing atoms with guided light in nanophotonic waveguides is a promising route to achieve such regimes. Experimental platforms include nanofibers [1] and photonic crystal waveguides (PCWs), owing to the large transverse confinement of their guided modes. With PCWs, tuning the dispersion relation gives rise to the additional benefit of a very low group velocity of the guided light [2] , allowing for strong coupling even in single pass. Moreover these systems can help explore waveguide-QED and probe exciting novel bandgap physics [3] . While encouraging values of couplings have been observed with first corrugated devices [4] , a photonic platform with trapped atoms via guided modes in the vicinity of a waveguide and lying deep in the strong coupling regime has yet to be demonstrated.

Recently, planar photonic crystals (PhCs) have emerged as one possible and flexible platform for integrating various optical components onto the same chip. Of the extraordinary dispersion properties that are exhibited by planar PhC waveguides lies the possibility of significantly reducing the group velocity at which light propagates [1]. Most significantly, combining slow light PhCs with a nonlinear medium enhances all kinds of optical nonlinear processes [2], with the potential for realising compact all-optical nonlinear devices that operate at low power. In addition to providing slow light propagation, PhC waveguides can be further engineered, so that slow light modes can present very low dispersion. A number of approaches have been used to achieve this, for example by slightly modifying the geometry of the PhC waveguide [3] or by using selective liquid infiltration [4]. The possibility of managing the dispersion in PhC waveguides is highly advantageous, as it provides a way to control higher order dispersion parameters in addition to the group velocity for optimizing processes that strongly depend on dispersive characteristics and nonlinearities. In our previous works, we have investigated both numerically [5] and experimentally [6] four-wave mixing (FWM) in short (80 µm) dispersion engineered slow light silicon photonic crystal waveguides. Compared with silicon PhC waveguides, chalcogenide waveguides do not suffer from two photon absorption (TPA) and free carrier absorption (FCA). Therefore, the high nonlinearity of chalcogenide glass and merging it with slow light in photonic crystals can potentially be used to make a very compact GVD and SPM compensation device. Mid-span phase conjugation is a technique that can compensate for both GVD and SPM, thereby allowing higher power transmission with fewer amplifiers.

The sensing of ambient gases is of growing interest for many applications as for example indoor air quality monitoring, gas leakage or fire detection. We present the fabrication of high aspect-ratio silicon (Si) pillar-based hybrid plasmonic-photonic crystal waveguides. This type of waveguide is suitable for ultra-sensitive gas sensing applications by providing enhanced light-matter interaction due to low group velocities (slow light) in photonic crystal waveguides and high evanescent field ratio. Eight-inch silicon substrate wafers are processed by means of lithography and deep reactive ion etch (DRIE, aka. Bosch etch), which took place in the industrial clean-room facilities of Infineon Technologies Austria AG in Villach. The fabrication processes were optimized to a level such that aspect ratios of around seven for pillars of 510 nm diameter could be reached. The discrepancy of measured diameters and diameters given by the design was found to be lower than 50 nm, with a standard deviation for the measured diameters lower than 20 nm.