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Between the visible light and radio waves, there is a particular family of electromagnetic waves, known as Terahertz waves. Among the various sources of THz waves, one of the most promising is the Quantum Cascade Laser (QCL). Unlike standard diode lasers, QCLs can be designed to emit electromagnetic radiation at different frequencies. Recently, they have been demonstrated to emit THz radiation with frequencies between 5 and 2 THz. QCLs are very compact sources, with dimensions of the order of a fraction of a millimeter, and are capable of emitting THz waves of very high power, higher than any other portable source of THz radiation. For these reasons they are likely to become the most widespread source of THz radiation in the years to come. In this project we want to demonstrate that THz QCLs can function in a very special way called mode-locking . When operated in this regime, lasers emit a regular series of very powerful and extremely short pulses. The repetition rate of the pulses, also called round-trip frequency is equal to one over the time needed for a photon to travel back and forth across the laser cavity. Semiconductor lasers can be forced into mode-locking by modulating the amplitude of the bias voltage at exactly the round trip frequency. This regime is called active mode-locking . Sometimes they can switch spontaneously from normal to mode-locked operation. In this case they are said to operate in a regime of passive mode-locking , which can occur for many different reasons, depending on the type of laser. During the last few years scientists have had some indications that QCLs may operate in a regime of mode-locking (passive and active), however they have not yet been able to prove it. In fact, the only way to distinguish between mode-locked operation, and what could be just a strong modulation of the output power at the round-trip frequency, is to measure the time duration of the output pulses. There are various techniques to perform this measurement, however QCLs emit at frequencies (mid-infrared and far-infrared) where most of these methods are very difficult to implement. In this project we propose to exploit a technique which is normally used to detect pulses of THz radiation in modern THz systems, and to apply this to THz QCLs. The technique, called photoconductive optical gating , uses the pulses generated by a visible mode-locked laser for probing those coming from the THz QCL. It is extremely powerful and versatile and will allow for the complete reconstruction of the temporal shape of the THz pulse. It has never beenapplied before to QCLs, and will give us the chance to find, without ambiguity, under which conditions these devices can be operated in a regime of mode-locking. Moreover, by understanding the fundamental physical processes, we will also be able to modify the design of QCLs to optimize mode-locking operation, for the production of ultra-short, ultra-high power THz pulses. Such type of pulses would be extremely useful for many applications where THz waves have a great potential. In particular they would be used in applications such as luggage scanning in airports, for the detection of explosives and non-metallic weapons. In fact, similarly to X-rays, THz waves can see through many everyday materials such as leather and most types of cloth. However, unlike X-rays, they can be used to recognize and distinguish an explosive from other harmless substances such as cheese or meat. In order to be able to penetrate thick layers, such as those forming ordinary luggage, they must be sufficiently powerful. By realizing mode-locked QCLs, we expect to produce THz pulses with peak powers in the order of 100 to 1000 times those generated with current THz systems.
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