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TECHNOLOGY LASERS


this, sources must be independently modulated in amplitude and frequency at high data rates.


For all the applications outlined above, the ideal source is compact and highly efficient, so that a battery can power it. Operation at typical ambient temperatures, without the need for cooling is also essential for widespread uptake in portable handheld systems, or in systems that are field deployed for environmental monitoring.


Figure 1. The transistor-injected quantum cascade laser is a three-terminal device that is designed to obtain coherent radiation from mid-infrared wavelengths through terahertz frequencies. This device utilizes the transistor effect and minority carrier injection to enhance the performance of the quantum cascade laser. Gold arrows show the electron flow through the structure, which is controlled by the grey hole flow into the p-type base.


QCLs: Pros and cons A great breakthrough that has paved the way to the realisation of practical sources in the mid-infrared and beyond came in 1994, with the development of the first quantum cascade laser (QCL). This two-terminal unipolar n+


-i-n device


can generate photons with desired wavelengths if layer thicknesses are carefully engineered, and the applied voltage adjusted. Changing the voltage modifies the separation between the energy states, so it determines the emission wavelength and influences the rate at which these transitions occur, because the electron wavefunction in the cascade region is field dependent.


Thanks to the wavelength of the QCL being only loosely constrained by material parameters – and governed by the engineering of the layer thicknesses for a specific bias voltage – this class of laser has demonstrated emission over a very broad wavelength range.


Figure 2. An example of a typical epitaxial structure for the TI-QCL. The collector contains low- refractive-index layers for optical-mode confinement. Within the base-collector junction is the quantum cascade gain region. Above the quantum transition region is a p-type, lightly doped base. Graded doping that increases near the emitter-base junction minimises free carrier absorption. The emitter itself is formed from InGaP to ensure a high emitter injection efficiency, and within the overall emitter structure are low refractive index layers to provide optical mode confinement. The entire structure is capped with a heavily doped n-type contact layer.


Tuning emission through adjustments in voltage is an attractive feature of the QCL, but it comes at a price: changes in voltage also influence the lifetime of the photons. This means that when there are adjustments to the electric field across the cascade region, this changes both the shape of the electron wavefunction and the state energy, and because the QCL is two-terminal device, changes in output power follow. So in short, adjusting the applied voltage changes both the emission wavelength and the output power. Clearly this is undesirable, because in many applications it is preferable to fix the output power while adjusting the wavelength.


Copyright Compound Semiconductor October 2014 www.compoundsemiconductor.net 67


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