Passive Characterisation Reflection

Passive characterisation is performed by looking at the reflection spectrum of the photonic crystal structures and by comparing the results with those expected from simulation.

The design and properties of the reflection measurement setup are explained in detail in Section 2.6. In principle, the measurement system for the reflection and the emission characterisation is the same. For the case of the reflection measurement, the light source is external and is imaged directly onto the sample. The reflected light is collected and coupled into the spectrum analyser. For the thermal emission, the light source is the chip itself and the same light collection principles are applied. The light source is a broadband tungsten-halogen source with a wavelength range of 300 - 2600 nm.

The setup has two different methods of light illumination and collection. The first of these two modes is referred to as setup-A (Fig. 2.9). For reflection, the light source is focused down onto the sample and the reflected light is collected and coupled into the multimode fibre connected to the optical spectrum analyser (OSA). This reflected light is collected over a cone extended around the normal to the sample. The collected light from the sample is imaged with a 1:1 mag- nification ratio onto the facet of the multimode fibre. Therefore, the core size of the fibre and its numerical aperture (NA) determine the size of the cone of collected light and over what area the light is collected from. In our case, the multimode fibre has a core diameter of 105µm and an NA of 0.22 corresponding to a collection cone with an angle of≈12◦ to normal.

The second method, setup-B (Fig. 2.10), introduces two extra lenses to the optical setup. This system illuminates the sample with near-plane waves of light. This is in contrast to setup-A where the sample is illuminated with focused wavefronts. For light collection, either reflection or emission, the system only images the plane wave emission or reflection from the sample onto the facet of the fibre. The nice feature of setup-B over setup-A is that the plane wave illumination provides a very accurate representation of the plane wave illumination in the COMSOL 3D simulations, therefore allowing a good comparison between the simulated behaviour and the real device behaviour. The disadvantage, however, is that the intensity is strongly reduced, by approximately a factor of 10.

Firstly, to calibrate the system and the OSA, the reflection spectrum of an unpatterned and undoped SOI sample is measured. The emission spectrum of the light source is taken into account prior to measuring. The SOI contains two layers, 220 nm silicon layer on top of a 2 µm oxide layer. The refection spectrum, measured using both setup configurations, is shown in Fig. 3.22a and b. Due to the multilayer nature of the sample, the reflection spectrum is not a simple Fabry-P´erot response but a combination of the interference between the two layers. The simulated normal reflection response of the SOI is shown in both panels of Fig. 3.22 in red. The refractive index of silicon was taken from [70] to include the correct dispersion of silicon over the wavelength range. The refractive index of the oxide layer was taken as 1.47. Very good agreement between the measured and simulated spectrum is achieved for both measurement setups. The poor signal to noise ratio of setup-B compared to setup-A is obvious, due to the lower intensity. For the simulation, the thickness of the SOI was reduced to 215 nm to obtain the very good agreement. This thickness is within the wafer

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Wavelength (µm) Reflection Measured − Setup A Simulation 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Wavelength (µm) Reflection Measured − Setup B Simulation (a) (b)

Figure 3.22: Reflection measurements from an unpatterened and undoped SOI sam- ple. (a) Measured reflection spectrum using measurement setup-A and (b) using mea- surement setup-B. The simulated normal reflection spectrum in each panel is the same and is computed using the 3D COMSOL model using a silicon slab thickness of 215 nm.

manufacturers tolerance on the silicon device layer thickness of ±5 nm.

Since a very good agreement between the simulated and measured refection spectrum of the SOI sample was achieved, this became the reference measurement for the setup. Often it is suggested to use metallic mirrors for calibration and reference, however their performance over a broad wavelength range may not be flat and consistent. With the SOI sample we accurately know all the optical properties and thicknesses of the the three layers so it is a much better reference structure.

We examine the optical behaviour of the square hole array photonic crystal slab by measuring the reflection spectrum. To measure the reflection spectrum, ideally one would like to have the slab perfectly flat and completely suspended in air with no defects or rows of holes missing along the crystal. Then, using plane wave illumination, obtain the reflection spectrum from the structure. This method would allow one to compare the measured results with the expected re- flection spectrum as shown in Fig. 3.5a. Unfortunately, getting a perfectly flat photonic crystal suspended completely in air is quite difficult. As shown previ- ously, even to get a relatively large piece of photonic crystal suspended across a SOI wafer is not possible. To solve these issues and make the characterisation of the slabs possible, the crystal pattern was etched into the silicon layer and the oxide layer was not removed, hence measuring the reflection spectrum of the photonic crystal on top of an oxide layer. This way, all the parameters are known and the crystal is completely flat and without defects or missing rows of holes.

Figure 3.23a shows the measured reflection spectrum for the crystal on ox- ide, taken with setup-A measurement configuration and Fig. 3.23b is taken with setup-B configuration. The simulated spectrum in both plots is the same and is

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Wavelength (µm) Reflection Measured − Setup A Simulation 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Wavelength (µm) Measured − Setup B Simulation (a) (b) Reflection

Figure 3.23: Reflection measurements for a square hole array undoped photonic

crystal slab; period 600 nm, hole radius 120 nm, on 2 µm oxide layer on a silicon

substrate. (a) Measured reflection spectrum using measurement setup-A and (b) using measurement setup-B. The simulated normal reflection spectrum in each panel is the same and is computed using the 3D COMSOL model using a silicon slab thickness of 215 nm.

the normal reflection spectrum for the square hole array photonic crystal slab, period 600 nm, hole radius 120 nm and silicon thickness 215 nm on 2 µm oxide on a substrate of silicon calculated using the 3D COMSOL model. Very good agreement between the measured and simulated reflection results is achieved for both measurement configurations. However, a closer examination of the mea- sured reflection spectrum from setup-A reveals a second peak at 1.325µm. This peak is not present in the spectrum measured with setup-B. This peak is present because the light in setup-A is collected over a cone of angles compared to the plane wave collection of setup-B. Therefore, setup-B is the more accurate method and achieves better agreement to the simulation results. The only disadvantage is that the signal level is much lower and the noise is a lot more prevalent in the measurements for setup-B.

The above characterisation is with a large crystal lattice and no rows of missing holes. Very good agreement with the expected reflection behaviour is achieved. However, looking at the real thermal emitter, what effect does remov-

ing 4 rows of holes every 25 µm have on the crystal performance? Maybe the

real question is, how the 25µm crystal span compare to the propagation distance of the photonic crystal resonances. To do full 3D simulations to examine and investigate these properties is very difficult, because the computational domain would be too large and is simply not feasible. In fact, it is easier to fabricate the structures and to measure the reflection spectrum. The experiment will not give us all the information about the propagation distances or the modes, but it will give us a very accurate indication of the effect it has on the reflection spectrum.

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Reflection Uniform Crystal Support Period 5µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Uniform Crystal Support Period 10µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Reflection Uniform Crystal Support Period 15µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Uniform Crystal Support Period 20µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Reflection Uniform Crystal Support Period 25µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Uniform Crystal Support Period 30µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Reflection Uniform Crystal Support Period 35µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Uniform Crystal Support Period 40µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0 0.2 0.4 0.6 0.8 1.0 Wavelength (µm) Reflection Uniform Crystal Support Period 45µm 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (µm) Uniform Crystal Support Period 50µm (d) (e) (f) (g) (h) (i) (j) (a) (b) (c)

Figure 3.24: Examining the effect of four missing rows of holes on the square array photonic crystal slab. The period between the missing rows of holes is varied from 5

µm to 50 µm, (a) - (j) The reflection spectrum (measured using setup A), each for a

different period, along with reflection spectrum for the uniform crystal are plotted.

A series of crystal patterns were fabricated on undoped SOI and the oxide was not removed. These lattices contained strips of four missing rows of holes. The period between these strips was varied from 5 µm up to 50 µm. For each the reflection spectrum was measured using setup-A (light collected over a cone of angles around the normal). The results are presented in Fig. 3.24 where each reflection spectrum is plotted along with the uniform crystal reflection spectrum (in red) of Fig. 3.23b.

The results show that there is indeed a deviation in the reflection spectrum for smaller lattice sizes. For the larger periods of 45µm to 50µm, the spectra almost completely agree with the uniform lattice. As the period between the missing rows of holes increases from 5 µm, the resonance peaks slowly take shape. The peak at 1.5µm seems to be most affected by the missing rows of holes. Firstly the position of the peak has shifted to longer wavelengths suggesting the resonance sees the extra unpatterened silicon as this increases the effective index and so shifts the resonance to longer wavelengths. On the other hand, the resonance around 1.3 µm stays in the same location with no real wavelength shift.