Optofluidic and photothermal control of InGaAsP photonic crystal nanocavities

A photonic crystal (PhC), which is an artificial material with a periodic modulation of the refractive index, provides an ultimate miniaturization of photonic devices since it can influence the flow of light on the optical wavelength scale. A particular useful device is the PhC cavity. It can be used to realize ultrasmall nanolasers, add/drop filters, and optical switches in the integrated circuits. But once photonic crystal cavities are fabricated, their spectral properties are fixed. Therefore, these structures are limited in their functionality due to the lack of active tuning of the resonances. Mechanisms providing an active tuning of the cavity resonances should be explored which is the main topic in this thesis. The combination of fluidics with photonics provides large tuning capabilities of the cavity resonances due to the change in the effective refractive index of the cavity after liquid infiltration. This combination also offers flexible, rewritable and reconfigurable photonic devices. A largely tunable photonic device can also be obtained by relying on thermal and photothermal effects, since the refractive index of the cavity is dependent on the temperature. Furthermore, the combination of the optofluidic and the (photo)thermal effect brings more control on the cavity resonances. In this thesis, we demonstrate and investigate several tuning mechanisms based on fluidic, thermal and photothermal effects. In chapter 2, the fabrication process and characterization method of the photonic crystal structures used for this thesis are described. 220 nm thick InGaAsP photonic crystal membranes having a hexagonal array of air holes were fabricated by using state of the art nanofabrication techniques. The membranes had epitaxially grown InAs quantum dots (QD's) incorporated for luminescence emission in the telecommunication wavelength range near 1.55 micron. The fabrication process parameters for the electron beam lithography, dry plasma etching and wet chemical etching steps were determined and optimized. Cavity designs in a hexagonal photonic crystal structure were obtained by finite difference time domain simulations. Different type of photonic crystal structures were fabricated by removing one (simple H1), three (L3), seven (H2) or a row (W1) of air holes. The smallest possible cavity is also obtained by modifying two adjacent air holes in their size and/or their position (H0). For the characterization of the fabricated structures, a custom-modified photoluminescence set-up was realized, which integrated conventional far-field and near-field-scanning optical microscopy (NSOM/SNOM), and Atomic Force Microscopy, and which allowed for in situ local liquid infiltration of the samples. Room temperature micro-photoluminescence (PL) measurements were conducted to investigate the optical properties of the nanocavities. The samples were mounted on an X-Y-Z stage. A continuous wave power tunable diode laser (¿ = 660 nm) is focused through high numerical aperture microscope objective (50x, N.A = 0.5). The laser spot has a spot size around 3 µm in diameter on the sample. Two different PL techniques were used to characterize the cavities. One is the conventional PL technique where the excitation of the cavities and the collection of PL emission are done by the same objective. For the second PL technique, only the excitation of the cavities is done by using the microscope objective. For the collection of the signal, an uncoated or a metal coated glass SNOM probe having an apex of 500 nm diameter is used. The collected PL was then dispersed in a monochromator and detected by a liquid nitrogen cooled InGaAs array. The use of a SNOM probe in this experiment also allows a near field imaging of the waveguide when an InGaAs avalanche photodiode detector (APD) is used. In chapter 3, the lithographic and photothermal control of the nanocavities are demonstrated. The lithographic tuning of the cavity modes is obtained by varying the geometric parameters of the photonic crystal such as lattice spacing and modified hole radius. A single cavity tuning is demonstrated by employing thermal and photothermal effects. The thermal tuning is obtained by varying the temperature of the cavities using a heating stage. The tuning rate is found to be 0.1 nm/°C. The photohermal tuning is obtained by varying the excitation laser intensity where we obtained a 15 nm redshift of the resonances which corresponds to a temperature increase of 150 °C. In chapter 4, the optofluidic control of the cavities is demonstrated by immersing the cavities into various liquids. The spatial intensity distribution of the resonant modes was studied experimentally by infiltration of the PhC holes with fluids of varying refractive index, consisting of water-sugar solutions. The shift of the resonance frequency with variation of the refractive index of the holes, is a direct measure of the overlap of the mode with the holes. By systematically varying the lithographically defined parameters of a given cavity type, the mode intensity distributions for different cavity types were obtained. These results can be applied for the design of PhC cavity sensors. A maximum sensitivity of 300 nm/RIU (refractive index unit) is observed which corresponds to ~ 25% mode overlap with the holes. Chapter 5 demonstrates the thermal and photothermal control of liquid crystal infiltrated nanocavities. A liquid crystal (LC) is a very suitable fluid for infiltration into the holes of the PhC to obtain tuning, since its refractive index can be tuned over a large range. The LC is a birefringent material, with an opposite temperature dependence of the extra-ordinary and ordinary refractive index, including an opposite jump at the LC phase transition temperature. We investigated the infiltration with the LC 5CB, which has a convenient clearing temperature Tc of 35 °C. Mode-dependent shifts of the resonant frequencies of the cavities were experimentally observed when the temperature was varied across the Tc. The genuine property of the birefringence of a LC was observed from the frequency shifts in opposite directions for differently polarized modes of the cavity. Apart from the applications, these observations were important for obtaining information on the orientation of the LC molecules. A tunable coupled cavity system is demonstrated in chapter 6. Two dissimilar photonic crystal cavities, an L3 and L60 type of cavities, are brought in a close proximity to investigate their coupling properties. Two coupling configurations, side and shoulder couplings, are analyzed by exciting the L3 cavity. A strong optical coupling is observed in the shoulder configuration for the fundamental mode of the L3 cavity. The coupling gives rise to hybridized modes in the spectral region of the L3 cavity mode. As the cavity is tuned by the photothermal effect, the hybrid modes are redshifted and level anticrossings are observed. Appendix A describes an exploratory research on tunable photonic crystal devices by local infiltration and removal of liquids. To this end, PhC waveguides of different widths were studied for implementing a selective infiltration process to create a fluidic cavity. Spatially resolved photoluminescence signatures of the waveguide cut-off frequencies revealed the properties of the infiltrated and empty parts