Towards compact and integrated mode-locked lasers (Conference Presentation)

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.

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.

A novel surface emitting laser diode consisting of a three-dimensional (3D) photonic band-gap crystal cavity is proposed. Spontaneous emission is controlled so as to radiate in the lasing direction with a narrow radiation angle by introducing a plane phase shift region into the cavity. The radiation pattern of the localized phase shift mode in the photonic band gap is analyzed with a plane-wave method by using a two-dimensional (2D) model. We show that the radiation angle of spontaneous emission in the photonic crystal cavity is as narrow as that of the stimulated emission of conventional surface emitting laser. Therefore, the photonic band-gap crystal cavity laser operates as a light source without threshold and spatial-emission-noise, and therefore is very attractive for use as the light source in spatially integrated optical circuits.

We proposed a novel design of a current-driven photonic crystal (PC) cavity laser using circular resonators. Resonant modes can be varied by adjusting the diameter of the resonator [1]. This feature enables us to realize wavelength-division multiplexing (WDM) on a PC structure without using the conventional optical multiplexer. Then we find the requirement of applying the proposed PC cavity lasers to WDM in the 1.3-µm range by calculating the photonic band-gap (PBG) of a GaAs based 2-dimensional (2D) PC structure. According to the simulation result, the filling ratio expressed as r/a (r is the radius of the air holes and a is the lattice constant of the PC structure) should be in a certain range. Furthermore, a regular PC structure with a=340nm meeting the requirement was also successfully fabricated. The simulation and experimental results show the possibility of realizing WDM using our proposed PC cavity lasers.

Slight disorder effects in two dimensional photonic crystal structures

Summary form only given. Photonic crystal (PC) cavity is employed in high-order-mode gyrotron using the concept of photonic band gap (PBG), to overcome mode competition and the limited beam propagation in harmonic operation devices. It has been an important issue to couple the cavity mode out to the external structure effectively. In this study, the coupling of the photnic crystal cavity with high order mode, TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">130</sub> mode, is studied both numerically and experimentally. The diameter and period of alumina rod, which has dielectric constant of 9.85 and loss tangent of 0.0001, are 1.15mm and 3.75mm, respectively. Twenty seven rods (3x9) are removed for rectangular PC cavity to support TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">130</sub> mode at about 32GHz, which has smooth variation of field strength through lateral direction. By removing one line of rods and connecting to WR-28, the coupled wave is measured. Sensitive coupling depending on the coupling position is obtained both numerically and experimentally. Using MAGIC2D code, TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">130</sub> photonic crystal cavity gyrotron is simulated. Effective coupling of high order mode photonic crystal cavity (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">130</sub> mode) is achieved. The measured coupling is in a good agreement with simulated one. The details of design and experiment of high order mode gyrotron using photonic crystal cavity will be presented along with coupling of high order mode photonic crystal cavity.

We exploit slow light (high ng) modes in planar photonic crystals in order to design a compact cavity, which provides an attractive path towards the miniaturization of near-infrared integrated fast pulsed lasers. By applying dispersion engineering techniques, we can design structures with a low dispersion, as needed by mode-locking operation. Our basic InP SiO2 heterostructure is robust and well suited to integrated laser applications. We show that an optimized 30 μm long cavity design yields 9 frequency-equidistant modes with a FSR of 178 GHz within a 11.5 nm bandwidth, which could potentially sustain the generation of optical pulses shorter than 700 fs. In addition, the numerically calculated quality factors of these modes are all above 10,000, making them suitable for reaching laser operation. Thanks to the use of a high group index (28), this cavity design is almost one order of magnitude shorter than standard rib-waveguide based mode-locked lasers. The use of slow light modes in planar photonic crystal based cavities thus relaxes the usual constraints that tightly link the device size and the quality (peak power, repetition rate) of the pulsed laser signal.

We demonstrate the fabrication of photonic crystal (PhC) cavities on optical nanofibers using femtosecond laser ablation. PhC cavities with cavity lengths varying from 0.54 to 3.43 mm are fabricated by controlling the profile of the nanocrater array formed on the nanofiber. Such PhC cavities show high transmission of 87% for a finesse of 39. For higher finesse values from 150 to 500, the transmission can still be maintained at 20%-25%. Due to the strong confinement of the field and the efficient coupling to single-mode optical fibers, such nanofiber-based PhC cavities may become an interface between quantum and classical networks.

: The goal of this project was to demonstrate the feasibility of a multi-spectral infrared (IR) photodetector using hybrid nanomaterials in photonic crystal (PC) cavities for enhanced absorption at selected wavelengths. The simultaneous sensing of multiple IR wavelengths, particularly in the near-IR and mid-IR is important because it enables intelligent surveillance, target recognition, and chem-bio detection. This project investigated InAs/GaAs epitaxial quanttun dots (EQDs) and colloidal quantum dots (CQDs). By incorporating these nanomaterials in one-dimensional (1D) and two-dimensional (2D) PC cavities, the possibility of multi-spectral configurability and enhanced absorption was explored. It was demonstrated that integrating EQDs with 2D PC cavities (etched air holes) is challenging due to difficulty in matching photonic bandgap defect modes with InAs/GaAs EQD absorption peaks. Nonetheless, enhanced absorption was demonstrated in a specific defect mode at 8 um at low temperatures. In contrast, it was straightforward to enhance absorption at specific wavelengths at room temperature by integrating CQDs with 1D PC cavities (polymer distributed Bragg reflectors). An enabling technology was matrix-assisted pulsed laser evaporation.

When a metamaterial (MM) is embedded in a one-dimensional photonic crystal (PC) cavity, the ultra-strong coupling between the MM plasmons and the photons in the PC cavity gives rise to two new polariton modes with high quality factor. Here, we investigate by simulations whether such a strongly coupled system working in the terahertz (THz) frequency range has the potential to be a better sensor than a MM (or a PC cavity) alone. Somewhat surprisingly, one finds that the shift of the resonance frequency induced by an analyte applied to the MM is smaller in the case of the dual resonator (MM and cavity) than that obtained with the MM alone. However, the phase sensitivity of the dual resonator can be larger than that of the MM alone. With the dielectric perturbation theory - well established in the microwave community - one can show that the size of the mode volume plays a decisive role for the obtainable frequency shift. The larger frequency shift of the MM alone is explained by its smaller mode volume as compared with the MM-loaded cavity. Two main conclusions can be drawn from our investigations. First, that the dielectric perturbation theory can be used to guide and optimize the designs of MM-based sensors. And second, that the enhanced phase sensitivity of the dual resonator may open a new route for the realization of improved THz sensors.

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10−3 W−1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation

We present the design of an ultra-low power consumption tunable photonic crystal (PhC) nanobeam cavity. The simulated temperature dependence of the PhC cavity is about 79 pm/K, due to the high thermo-optic coefficient of silicon. And the calculated tuning efficiency of the tunable PhC cavity is 45.8 pm/μW, which is the highest efficiency ever reported.

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.

A novel photonic crystal (PC) all-optical switch based on the shift of defect mode is demonstrated. Two photonic crystal waveguides are connected by a photonic crystal cavity. The point defect of the photonic crystal cavity is filled with phenylacetylene liquid cyrstals intermingled with azobenzene. The output of the optical switch is controlled by adjusting the rotation angle of the liquid crystals' orientation vector. And the optical switch is numerically investigated by using the finite difference time domain (FDTD) method and plane wave expansion method. Numerical simulation shows that the optical switch based on the PC cavity can be changed by adjusting the applied linearly polarized light. Results show that the switch has low threshold power density, great extinction ratio, and small size.

We investigate the optical coupling of excitons confined in site-controlled semiconductor quantum dots (QDs) with the modes of photonic crystal (PhC) cavities membrane cavities and waveguides. The structures are fabricated by organometallic vapor phase epitaxy of InGaAs/GaAs heterostructures on patterned (111)B GaAs substrates combined with electron-beam-based nanolithography. We elucidate the role of pure dephasing and phonon interaction on the weak exciton-cavity mode coupling, and demonstrate the simultaneous coupling of several QDs to the same optical mode of various PhC cavity configurations. In addition, coupling of QD systems to PhC waveguides for implementing single photon transport in the face of optical disorder is studied. Implications on the potential of constructing integrated quantum photonic circuits for on-chip manipulation of quantum light will be discussed.