1. The refractive index of mpSi and μpSi structures has some anisotropy.
a) True
b) False
Explanation: The refractive index of mpSi μpSi structures and presents certain anisotropy along the specific crystallographic axes. This anisotropy, which can range from 1 to 10 %, is associated with the elongated pore shape along the growth direction.
2. The porosity of mpSi and μpSi relies mainly on the ______________
a) thickness of the material
b) current density
c) etching agent
d) temperature
Explanation: The porosity of mpSi and μpSi relies mainly on the current density applied during the etching process. For this reason, multi-layered mpSi and μpSi structures, which consist of stacks of porous silicon layers featuring different levels of porosity, can be produced from top to bottom by modifying the current density in the course of the etching process.
3. μpSi and mpSi structures are widely used to produce optical structure.
a) True
b) False
Explanation: As a result of their geometric and optoelectronic properties, these structures have been extensively used to produce a variety of optical structures such as micro-cavities, distributed Bragg reflectors, waveguides, omnidirectional mirrors and rugate filters.
4. The effective refractive index of pSi can be varied with the current density.
a) True
b) False
Explanation: The effective refractive index of each layer is directly related with the porosity level. So, the effective refractive index of pSi can be engineered in depth by alternating the current density during the etching process.
5.Thickness of layers in the mpSi and μpSi structures can be controlled by_____
a) current density
b) etching agent
c) etching time
d) porosity
Explanation: The thickness of each layer can be precisely controlled by the etching time as the former is directly proportional to the latter. Therefore, the interaction between incident light and matter in mpSi and μpSi structures can be engineered to produce optical nanostructures for a broad range of applications.
6. mpSi and μpSi structures are used to develop distributed Bragg reflectors.
a) True
b) False
Explanation: Multilayered mpSi and μpSi structures can be used to develop distributed Bragg reflectors (DBR). Basically, a pSi-based DBR structure consists of multiple layers of pSi with different porosity levels and thicknesses
7. The structure of the mpSi- or μpSi-based DBR affects their refractive indices.
a) True
b) False
Explanation: The interfaces between two consecutive layers of dielectric material reflect incident light due to the effective refractive index contrast. Therefore, constructive light interference can be engineered by designing the structure of the mpSi- or μpSi-based DBR (i.e. by designing the porosity and thickness of each layer).
8. DBR structures based on_____ can be implemented into Fabry–Pérot filters.
a) mpSi
b) pSi
c) SiO2
d) graphite
Explanation: DBR structures based on mpSi and μpSi can be implemented into Fabry–Pérot filters, also called optical microcavities, which are composed of two parallel DBRs with a space layer of different effective refractive index in between.
9. The effective optical thickness of the space layer is taken equal to _____
a) frequency
b) wavelength
c) amplitude
d) product of refractive index and density of the material
Explanation: The effective optical thickness of the space layer (i.e. product between the physical thickness and the effective refractive index of the layer) is typically engineered as the wavelength of the incident light or half of it.
10. Microcavities based on mpSi and μpSi can be used to develop optical pass-band filters.
a) True
b) False
Explanation: The reflectivity spectrum of microcavities based on mpSi and μpSi shows a narrow pass-band centred at the wavelength where the light is reflected with the highest intensity. Therefore, the structural design of microcavities based on mpSi and μpSi makes it possible to develop optical pass-band filters and sensors.