Abstract Wico Hopman (eng)

This thesis describes the design, fabrication and characterization of one- and two-dimensional photonic crystals (PhCs). Two novel characterization methods have been proposed, the far-field scattering method to characterize slow-light phenomena in gratings and a near-field method for mapping the standing wave patterns in a photonic crystal resonator. The fabrication and characterization methods presented are highly useful for realizing a nanomechanical PhC switch, which is the main goal of the project in which the research was carried out.

Chapter 2: Photonic crystals & resonators

A brief review of the theory of one- and two-dimensional photonic crystals and resonators is presented. Some derivations of often used relations for the group velocity, the quality factor Q, and the free spectral range are given. Several design choices for waveguide gratings, photonic crystal resonators and microring resonators are also addressed.

Chapter 3: Fabrication

The strengths and weaknesses of the fabrication methods, as experienced in this work, are presented. This is followed by a detailed description of an example for the preparation of a stream file, which can be adopted for milling a pattern using a focused ion beam (FIB). A focused ion beam can be exploited for turning an ultrarough surface into a nanosmooth surface. An application can be, for example, the smoothening of the end facets of an optical chip, to increase the coupling efficiency. The FIB can also be used to generate submicrometer PhC holes. However, fabrication of submicrometer holes with perfectly vertical sidewalls, needed for low-loss propagation, is non-trivial in Si due to the strong redeposition effect. The scan strategy of the ion beam can be used as a parameter for sidewall-angle optimization, i.e. aiming to mill a submicrometer hole with perfect vertical walls. The optimization is performed for both isolated holes and arrays of holes. It is shown that the sidewall angles can be as small as 5 degrees in (bulk) Si and SOI if applying a relatively large dose, i.e. milling several micrometers deep. For the Si membranes (Si suspended in air), a minimum angle of only 1.5 degrees is obtained. It is concluded that the number of loops at a fixed dose per hole is the parameter that determines the sidewall angle and not the dwell time by itself.

Another fabrication method that has been exploited for the fabrication of photonic crystals is the combination of laser interference lithography (LIL) and conventional lithography and dry etching. The fabrication method, which allows for realizing apodization functions modulating both amplitude and phase of the grating, is demonstrated by fabricating a Si₃N₄ waveguide grating chirped by width-variation of the grated ridge waveguide. The presented method is suitable for fabricating high quality Si₃N₄ waveguide gratings and it has the advantage of being CMOS-compatible.

Chapter 4: Characterization & modeling results

The setups used for the experiments presented in this thesis are briefly summarized while providing the references to their detailed descriptions.

The fabrication and characterization of the refractometric and thermo-optical properties of a quasi-one-dimensional waveguide photonic crystal –a strong, 76-mm-long Bragg waveguide grating– are reported with transmission spectra around 660 nm. It is shown that the (steep) edge of the stopband can be used for detecting changes in the aqueous cladding refractive index of only 4´10^-⁴. A thermally induced spectral shift of approximately 7 pm/K was observed.

In section 4.3 it is experimentally shown that the fringes in the transmission spectrum near the stopband edge are associated with the occurrence of standing wave patterns in the waveguide grating. These patterns correspond well to the distributions found in the simulations of the waveguide grating. A novel and straightforward method for determining the group index and intensity enhancement over a wide wavelength range based on (Rayleigh) scattering observations is presented. This far-field scattering microscopy (FScM) method is compared with the so-called phase shift method and a method that uses the transmission spectrum to quantify the slow wave properties. The methods show a good agreement. A minimum group velocity of 0.04 c and a maximum intensity enhancement factor of approximately 14.5 for a 1000-period grating and a maximum group delay of about 80 ps for a 2000-period grating have been found. It is shown that the FScM method can be used for both displaying the intensity distribution of the Bloch resonances and for investigating out-of-plane losses. The gratings designed for infrared wavelengths are suggested for use as a slow-wave cladding-index sensor able to detect a minimum cladding index change of 10^–8, assuming a transmission detection limit of 10^–4.

The FScM characterization method described above can not be used to accurately map out the wave patterns in micrometer-sized resonators, because the size of the resonance is well below the diffraction limited resolution. Therefore, the remainder of chapter 4 is devoted to a novel measurement method, transmission scanning near-field microscopy (T-SNOM), which enables mapping out the intracavity intensity distribution of the standing wave in a cavity. It is shown by modeling of two orthogonal cross-sections of a PhC microcavity, that the resonance wavelength shift induced by a nanoprobe is approximately proportional to the local field intensity of the standing wave. In addition, the transmitted power can be used as a first approximation of the field intensity. This approximation becomes more accurate when the operation wavelength is set to the point where the slope of the Lorentzian shaped response is at maximum and the interaction levels are weak. It is concluded that a nanosized tip can be used to tune the resonance wavelength and quality factor.

The T-SNOM method was applied experimentally to characterize a silicon-on-insulator based PhC microcavity. The results showed that the standing wave patterns could be mapped out with high spatial accuracy (<100 nm) using an atomic force microscopy (AFM) tip in contact mode. Two AFM tips have been exploited for the nanomechanical interaction, a Si and Si₃N₄ tip. A detuning of the resonance wavelength by 2.3 nm was shown by expedient positioning of a Si AFM tip. With the same tip it was possible to set the Q between values of 615 and zero. Full on/off switching was found for displacements of less than 200 nm in the vertical direction, and for 500 nm in a lateral direction, at the strongest resonance antinode locations

In contact-mode operation, however, the tip wears out quickly. In other AFM applications, the well-known tapping mode operation has shown its value for probing delicate surfaces. This mode can also be used for the T-SNOM method. It is shown that a simple model of the tip dynamics in combination with a calibration curve is adequate to model the effect of the pre-set tapping height on the transmitted signal.

The simulations showed that the tip can efficiently reduce the out-of-plane scattering by positioning the tip in the vertical direction to a height of 0.5 mm (above a “hot-spot”) forming an anti-resonant cavity. A similar effect has been observed in the experiments where a small increase in transmission was detected when the tip was positioned before the cavity.

Chapter 5: Outlook

The on-going research related to the topics described in this thesis is presented. The challenges in FIB processing and the solutions and alternatives that are being investigated are described. It is suggested that the far-field scattering microscopy method can also be applied to slow-light PhC W1 waveguides, preliminary results are included. A preliminary result of the T-SNOM method on microring resonators is presented.

Finally, the application of the T-SNOM method and the nano-electro-mechanical system (NEMS) PhC switch is discussed.

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Home News Events Strategic Orientations research groups Publications PhD students Education MESA+ NanoLab Starting entrepreneurs Tech Park Industrial Partners Vacancies Links Sitemap Search	Abstract Wico Hopman (eng) This thesis describes the design, fabrication and characterization of one- and two-dimensional photonic crystals (PhCs). Two novel characterization methods have been proposed, the far-field scattering method to characterize slow-light phenomena in gratings and a near-field method for mapping the standing wave patterns in a photonic crystal resonator. The fabrication and characterization methods presented are highly useful for realizing a nanomechanical PhC switch, which is the main goal of the project in which the research was carried out. Chapter 2: Photonic crystals & resonators A brief review of the theory of one- and two-dimensional photonic crystals and resonators is presented. Some derivations of often used relations for the group velocity, the quality factor Q, and the free spectral range are given. Several design choices for waveguide gratings, photonic crystal resonators and microring resonators are also addressed. Chapter 3: Fabrication The strengths and weaknesses of the fabrication methods, as experienced in this work, are presented. This is followed by a detailed description of an example for the preparation of a stream file, which can be adopted for milling a pattern using a focused ion beam (FIB). A focused ion beam can be exploited for turning an ultrarough surface into a nanosmooth surface. An application can be, for example, the smoothening of the end facets of an optical chip, to increase the coupling efficiency. The FIB can also be used to generate submicrometer PhC holes. However, fabrication of submicrometer holes with perfectly vertical sidewalls, needed for low-loss propagation, is non-trivial in Si due to the strong redeposition effect. The scan strategy of the ion beam can be used as a parameter for sidewall-angle optimization, i.e. aiming to mill a submicrometer hole with perfect vertical walls. The optimization is performed for both isolated holes and arrays of holes. It is shown that the sidewall angles can be as small as 5 degrees in (bulk) Si and SOI if applying a relatively large dose, i.e. milling several micrometers deep. For the Si membranes (Si suspended in air), a minimum angle of only 1.5 degrees is obtained. It is concluded that the number of loops at a fixed dose per hole is the parameter that determines the sidewall angle and not the dwell time by itself. Another fabrication method that has been exploited for the fabrication of photonic crystals is the combination of laser interference lithography (LIL) and conventional lithography and dry etching. The fabrication method, which allows for realizing apodization functions modulating both amplitude and phase of the grating, is demonstrated by fabricating a Si₃N₄ waveguide grating chirped by width-variation of the grated ridge waveguide. The presented method is suitable for fabricating high quality Si₃N₄ waveguide gratings and it has the advantage of being CMOS-compatible. Chapter 4: Characterization & modeling results The setups used for the experiments presented in this thesis are briefly summarized while providing the references to their detailed descriptions. The fabrication and characterization of the refractometric and thermo-optical properties of a quasi-one-dimensional waveguide photonic crystal –a strong, 76-mm-long Bragg waveguide grating– are reported with transmission spectra around 660 nm. It is shown that the (steep) edge of the stopband can be used for detecting changes in the aqueous cladding refractive index of only 4´10^-⁴. A thermally induced spectral shift of approximately 7 pm/K was observed. In section 4.3 it is experimentally shown that the fringes in the transmission spectrum near the stopband edge are associated with the occurrence of standing wave patterns in the waveguide grating. These patterns correspond well to the distributions found in the simulations of the waveguide grating. A novel and straightforward method for determining the group index and intensity enhancement over a wide wavelength range based on (Rayleigh) scattering observations is presented. This far-field scattering microscopy (FScM) method is compared with the so-called phase shift method and a method that uses the transmission spectrum to quantify the slow wave properties. The methods show a good agreement. A minimum group velocity of 0.04 c and a maximum intensity enhancement factor of approximately 14.5 for a 1000-period grating and a maximum group delay of about 80 ps for a 2000-period grating have been found. It is shown that the FScM method can be used for both displaying the intensity distribution of the Bloch resonances and for investigating out-of-plane losses. The gratings designed for infrared wavelengths are suggested for use as a slow-wave cladding-index sensor able to detect a minimum cladding index change of 10^–8, assuming a transmission detection limit of 10^–4. The FScM characterization method described above can not be used to accurately map out the wave patterns in micrometer-sized resonators, because the size of the resonance is well below the diffraction limited resolution. Therefore, the remainder of chapter 4 is devoted to a novel measurement method, transmission scanning near-field microscopy (T-SNOM), which enables mapping out the intracavity intensity distribution of the standing wave in a cavity. It is shown by modeling of two orthogonal cross-sections of a PhC microcavity, that the resonance wavelength shift induced by a nanoprobe is approximately proportional to the local field intensity of the standing wave. In addition, the transmitted power can be used as a first approximation of the field intensity. This approximation becomes more accurate when the operation wavelength is set to the point where the slope of the Lorentzian shaped response is at maximum and the interaction levels are weak. It is concluded that a nanosized tip can be used to tune the resonance wavelength and quality factor. The T-SNOM method was applied experimentally to characterize a silicon-on-insulator based PhC microcavity. The results showed that the standing wave patterns could be mapped out with high spatial accuracy (<100 nm) using an atomic force microscopy (AFM) tip in contact mode. Two AFM tips have been exploited for the nanomechanical interaction, a Si and Si₃N₄ tip. A detuning of the resonance wavelength by 2.3 nm was shown by expedient positioning of a Si AFM tip. With the same tip it was possible to set the Q between values of 615 and zero. Full on/off switching was found for displacements of less than 200 nm in the vertical direction, and for 500 nm in a lateral direction, at the strongest resonance antinode locations In contact-mode operation, however, the tip wears out quickly. In other AFM applications, the well-known tapping mode operation has shown its value for probing delicate surfaces. This mode can also be used for the T-SNOM method. It is shown that a simple model of the tip dynamics in combination with a calibration curve is adequate to model the effect of the pre-set tapping height on the transmitted signal. The simulations showed that the tip can efficiently reduce the out-of-plane scattering by positioning the tip in the vertical direction to a height of 0.5 mm (above a “hot-spot”) forming an anti-resonant cavity. A similar effect has been observed in the experiments where a small increase in transmission was detected when the tip was positioned before the cavity. Chapter 5: Outlook The on-going research related to the topics described in this thesis is presented. The challenges in FIB processing and the solutions and alternatives that are being investigated are described. It is suggested that the far-field scattering microscopy method can also be applied to slow-light PhC W1 waveguides, preliminary results are included. A preliminary result of the T-SNOM method on microring resonators is presented. Finally, the application of the T-SNOM method and the nano-electro-mechanical system (NEMS) PhC switch is discussed.