Enhanced terahertz emission from thin film semiconductor/metal interfaces

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Ramakrishnan, G.
Terahertz light is electromagnetic radiation, similar to visible light. The photons that the terahertz light is comprised of carry a much smaller amount of energy compared to the visible light photons. Unlike visible light, terahertz light can pass through materials like plastic, cardboards, wood etc.; a very useful property which enables it to replace harmful X-rays in many security applications. However, it is not possible to see the terahertz photons with our naked eyes, and it requires special detectors to observe them.
A lot of attention has been drawn to terahertz radiation recently because of its potential use in various applications in national security (as mentioned before), and in the biomedical and the semiconductor industries. Essential to any terahertz device is a suitable terahertz source. There are different methods to generate this type of radiation. After the advent of ultrafast lasers, an optical technique was developed which became very popular afterwards. In very simple terms, this technique can be considered as producing an extremely quick disturbance in a suitable material using an extremely quick flash of laser light. Here the phrase `extremely quick' refers to femtosecond time scales where one femtosecond is one millionth of one billionth of a second. The quick electromagnetic disturbance can lead to the emission of a pulse of electromagnetic radiation of a different frequency: terahertz light. Certainly, this process depends on the material in which the disturbance is created, which we will see in a bit more detail below. It is this method of terahertz generation we focus on in this thesis.
Let us now have a closer look at this. Only certain materials have this property of converting a flash of laser light efficiently into a flash of terahertz light, for example, some semiconductors. What type of a disturbance can a flash of laser light, (a laser pulse), create in such a material? In the case of semiconductors, the incident light pulse can lead to the excitation of mobile conduction electrons by providing them with the required energy. The semiconductor becomes momentarily a conductor. If it was initially kept under an external voltage bias, a momentary current is thus induced by the light pulse. A time-varying current can act as a source of electromagnetic waves. The emitted flash of light in this case is a terahertz pulse. Similar momentary disturbances can also be produced in certain nonlinear crystals without really exciting electrons from their bound states, but by causing an ultrafast displacement of the bound charges. In both these cases, the emitted light pulse carries information about the material's response to the femtosecond flash of light, which in fact is information about the material per se. For example, we see that the illumination of graphite with femtosecond laser pulses results in the emission of terahertz light pulses. The properties of the emitted terahertz pulse are suggestive of a transient photocurrent produced in the material. Graphite consists of stacks of atomic planes of carbon which are loosely attached to each other. Electric conductivity along a direction perpendicular to these planes is known to be very low as in this case electrons have to jump from one plane to the other. However, in our experiments the emitted terahertz pulses indicate a resultant photocurrent flowing in that direction.
Oxidized copper surfaces are known to act as a semiconductor-diode. A semiconductor diode is a device which restricts the electric current to flow through it in only one direction. In the case of oxidized copper surfaces, this is possible by a potential barrier formed at the interface between copper and cuprous oxide. When a femtosecond light pulse excites electrons at such an interface, and frees electrons in it, a quick pulse of current flows across the interface. This transient current emits a terahertz pulse.
The same idea can also be applied to different other semiconductor-metal interfaces. We have shown that terahertz pulses can be produced by exciting thin films of germanium and silicon deposited on a gold substrate. If the thin films of these semiconductors prepared on a glass substrate are illuminated with femtosecond light pulses, the emitted terahertz pulses are very feeble. When the thin films of the semiconductors are on a gold substrate, a surprising enhancement of the generated terahertz light from such thin films is observed. The later part of the thesis concentrates on the different possible ways in which the gold substrate can contribute to the enhancement of terahertz radiation from thin films.
When coherent laser light is incident on an extremely thin film of a semiconductor material deposited on a metal surface, light reflected from the top and the bottom of the film can result in a complete or partial reduction of the reflected light. It is equivalent to trapping the light inside the film, which leads to enhanced absorption in the thin film. This is sometimes called `coherent optical absorption'. Very strong absorption of the pump light can be achieved in thin films as compared to bulk materials, as a result of this. When light is strongly absorbed by the semiconductor, more electrons will be freed and a stronger transient current can be produced which can result in a stronger terahertz emission. This leads to the counter-intuitive result that less material emits more terahertz light.
The concentration of laser light inside the terahertz generation material can also be done by making use of surface plasmon excitation. Surface plasmons are light waves bound to the interface between a metal and a dielectric. In our case, since the terahertz generation takes place at the interface between a metal and a semiconductor, surface plasmons can play a role in the process. As surface plasmons are bound to the interface, they can enhance the local intensity of the pump light at the interface where the generation of terahertz radiation takes place. Using this method, we demonstrate the enhancement of terahertz emission from a layer as thin as a single molecular layer of a nonlinear optical material called hemicyanine. We also go on to show that enhanced terahertz emission can be achieved from semiconductors deposited on a nanostructured metal surface, where again surface plasmons are excited. Concentration of the laser light intensity using plasmonics also leads to terahertz emission from the metal surfaces itself, i.e., without any semiconductor on top.
In short, this thesis discusses the possibilities of terahertz generation from ultrathin semiconductor layers, metals and their interfaces, and on different optical techniques to enhance the terahertz emission. These techniques not only help in the study of the properties of ultrathin layers of materials, but can also help in miniaturizing terahertz sources for various applications.