Surface Photonics Group
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The progress in nanoscale growth of semiconductors has led to the first successful demonstration of nanodevices. These advances have generated an intense interest in the physical, chemical and material science communities for developing new materials, novel nanoscale characterization tools and new functionalities. Semiconductor nanowires have received increasing attention in recent years as one-dimensional structures and building blocks for nanodevices. Semiconductor nanowires with diameters ranging from a few to several hundred nanometers can be grown on a substrate using the vapor-liquid-solid (VLS) method. By modifying the growth conditions, lateral and longitudinal control over the nanowire size, composition and doping can be achieved. Depending on the geometry, these nanowires may exhibit quantum confinement, light emission and lasing. Optimized light-emitting semiconductor nanowires are interesting for the development of light-emitting diodes (LEDs) provided they can be electrically excited. Due to the low dimensionality of nanowires, the out-coupling of light generated in nanowire based LEDs could be very efficient. Given their large surface-to-volume area, semiconductor nanowires can be used as sensitive optical (bio-)sensors.
In the group Surface Photonics, we focus on the investigation of the optical properties of individual semiconductor nanowires and their ensembles with different parameters (wire radius, length, semiconductor filling fraction). The aim of this research is to understand better these nanostructures and describe properties important for photovoltaic applications, e.g., antireflection and light absorption. We also tackle the reverse problem, i.e., light emission by individual and ensembles of nanowires.
Light travels as a straight beam in homogeneous media, but its direction can change when it arrives at an interface between two different media. A fraction of light is reflected into the first medium and the rest is refracted into the second medium. Nearly 100% of the light is reflected at large angles of incidence at an interface between air and a solid material such as a semiconductor. Interfaces which reduce this reflection exist in nature. For example, the eyes of moths are covered with tapered nanostructures, which increase the animal’s eyesight by allowing more light to enter the eyes. Inspired by these biostructures, we have developed a method which drastically reduces the reflection between air and a semiconducting substrate. This method consists of the growth of nanowires with different lengths or with the same length, but conically shaped. In this way a gradual change from air to semiconductor is achieved, which leads to an efficient light coupling into the semiconductor and minimizes the reflection over a broad range of colors and angles of incidence. The reduction of the reflection can increase the sensitivity of light detectors as well as increase the efficiency of solar cells and improve the outcoupling of light from LEDs.
Arrays of semiconductor nanowires absorb light very efficiently even though their volume density is very low (typically a few percent). This low density of the material allows the incident light to see only a minor contrast in refractive index between air and nanowire layer which minimizes losses due to reflection. 3um long InP nanowires grown on InP substrate cause 98% of incident light to be absorbed in the sample. However, this is not the antireflection mechanism described in the previous paragraph. Here, low refractive index contrast is merely an overture to a more sophisticated phenomenon. Light can couple into the guided modes in individual wires. As the diameter of the nanowires increases, longer wavelengths of light are coupled into guided modes. For visible light, this process causes an increased absorption as light gets coupled into a mode. On the other hand, when the wavelength of light is longer than the electronic band gap wavelength, i.e. the material does not absorb, the very same mechanism offers antireflection properties effectively guiding the light into underlying substrate. Those combined absorptive and antireflection properties are very promising for photovoltaic applications as they could trigger new design of nanowire solar cells.
Besides of the antireflective and absorptive properties of arrays of nanowires they also offer the possibility to influence the direction of light they emit. A periodic arrangement of nanowires causes the particular wavelengths of light to propagate only in certain directions in the form of so called Bloch modes. The light emitted from nanowires can couple to these modes and propagate in the structure. Due to the finite length of the nanowires, light will ‘leak out’ from the sample in a specific way giving rise to directional light emission. We were able to observe this phenomenon by using arrays of heterostructured nanowires which could emit light at different wavelengths in the infrared. The position of the light source, i.e. the heterostructure, inside nanowires also has an impact on the directional properties of emission. This research gives aims to improve the design of novel, nanowire-based light emitting diodes.
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