TECHNOLOGY LASERS
It is possible to see how to improve the QCL by examining the operation of the bipolar junction transistor (BJT). When formed as a n-p-n structure, the electrons in this class of device are injected from a forward biased n-p emitter-base junction into the base, before they diffuse across to the collector and are swept away by the field in the reverse-biased p-n base- collector junction. This chain of events occurs as the carriers take a random walk from the quasi-neutral base into the collector junction field region. The ‘magic’ of the BJT stems from the use of a small hole current into the base contact, which controls a large electron current sourced by the emitter and captured by the collector.
Our team at the University of Illinois at Urbana-Champaign has combined the attributes of the BJT with those of the QCL to form an innovative device that provides independent control of field across, and current through, the quantum transition region. With this novel, hybrid design, which we refer to as the transistor-injected (TI) QCL, the electric field controls wavelength while the current controls the laser power.
The TI-QCL is similar in design to the HBT (see Figure 1). Key differences are the addition of layers on the top and bottom of the device to provide optical confinement, the inclusion of the cascade region, and some variations in layer thicknesses and doping levels.
It is possible to fabricate our novel laser on a range of substrates. It can be grown on an n-type conductive substrate, with the collector contact made to the back of the wafer (see Figure 2, which shows a simplified epitaxial layer structure); and it can be formed on an insulating substrate, with all contacts made to the top surface. What’s more, thanks to the similarity of the TI-QCL and the HBT, in terms of design and processing steps, it should be possible to produce this device in a commercial GaAs IC foundry.
The introduction of a third terminal offers plenty of freedom, in terms of device operation (see Figure 3 for a band structure diagram). Frequency modulation at fixed output power is possible by dithering the base-collector bias voltage while using a fixed emitter-
Figure 3. The band structure of the TI-QCL showing the emitter (E), base (B), and collector (C) contacts. In this n-p-n device, electrons are injected into the p-type base by the forward biased emitter-base junction and then diffuse across the base, before they are swept away by the field region in the reverse-biased base-collector junction. Within the field region of the base-collector junction, electrons travel through a staircase potential in discrete steps, emitting sub bandgap photons at a wavelength established by the design of the quantum wells and barriers as well as the applied electric field “ε” set by the base-collector bias.
base bias − adjustments to the voltage alter the slope of the bands, and thus the transition energy.
Varying frequency while maintaining a constant output power is extremely useful for identifying chemical species. By sweeping frequency back and forth across a molecular absorption line at a fixed output power, enhanced signal-to- noise performance is possible, enabling superior detection of low concentrations.
Communications applications may also be targeted with our TI-QCL, because frequency modulation has the potential to be very fast, as it is based upon the electric field change in the structure. Amplitude modulation is also possible, due to modulation of the emitter-base bias. In this case, modulation rates are limited by transit times across the base and through the quantum transition region, so are slower than those resulting from frequency modulation. In addition, frequency and amplitude can be modulated independently, by adopting a common base configuration that provides separate control of the emitter- base and base-collector junctions.
A further benefit of our TI-QCL is its low internal loss, which stems from the n-p-n
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structure and the fundamentally different method of current injection. Losses due to free carrier absorption are minimal, thanks to the absence of doping from the cascade region and much lighter doping levels near the cascade region. Further improvements to internal loss result from the creation of a depletion region around the QCL structure in the regions of high optical field intensity where absorption is greatest.
Due to these refinements, there is the promise of a lower laser threshold current, an increased differential quantum efficiency and a hike in wall plug efficiencies. An increase in the latter is highly valued, because this is critical for portability and energy-sensitive applications.
Modelling the device Device modelling is not trivial. To design a TI-QCL that works as intended requires modelling of the electron wavefunctions and energy levels under various electric fields, as well as modelling of optical modes and simplified modelling of the electrical transport characteristics.
Modelling of the electron wavefunctions and energy levels offers insights into laser properties, such as the gain characteristic.
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