Condensed Matter Seminar

Friday, August 23, 2013

3:30 PM, Room 203 Physics

(Refreshments 3:00 PM, Room 242)

Professor Paul Harrison

University of Leeds United Kingdom

School of Electronic & Electrical Engineering,

University of Leeds, United Kingdom

Surface Acoustic Wave Modulation of Quantum Cascade Lasers

Quantum cascade lasers are n-type unipolar semiconductor heterostructure lasers fabricated many repeats of an active region unit cell that is itself comprised of several quantum wells.  The electron energy levels and lifetimes within an active region are engineered to create a population inversion between two levels which when coupled with a resonant cavity or waveguide can lead to gain (amplification).  GaAs-based devices give quantum wells that are one or two hundred meV deep and hence transitions between electron states are typically in the mid- or far-infrared (Terahertz) regions of the spectrum.  These wavelengths have already been shown to be useful for chemical and biological sensing. It is then of interest to achieve precise and continuous dynamical tuning of the laser wavelength, certainly within limits set by the active transition linewidth. One possibility for this is to employ the distributed feedback (DFB) lasers, rather than the conventional end-mirror resonator lasers, where the distributed feedback would be provided by gain and refractive index modulation caused by a surface acoustic wave.  The latter are generated by applying alternating voltages to interdigitated metallic fingers deposited on the surface of the “transducer” part of the device, which produces a mechanical wave through the piezoelectric effect.  This wave in turn modulates the electron density within the active region of the laser, hence the gain and the refractive index, providing an optical feedback.  The operating frequency is tuned by changing the surface acoustic wave frequency, i.e. the DFB grating period.  The laser intensity modulation is also possible via the acoustic wave modulation depth, i.e. its power.  We report on simulations of this effect and the initial experimental measurements.