Abstract
The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.
Highlights
The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology
Perturbations to the electron transport in the quantum-cascade laser (QCL) were monitored via the device voltage, with the THz emission being simultaneously monitored on a Schottkydiode detector
We have reported the modulation of a QCL using optically generated bulk acoustic-phonon pulses propagating through the QCL heterostructure
Summary
The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. QCLs exhibit ~ps carrier lifetimes, potentially allowing ultrafast amplitude or frequency modulation[5], a key requirement for frequency shifting, high data-bit transmission[6], active modelocking[7] and frequency comb synthesis[8], as well as amplitude, frequency[9] and phase stabilisation[10] As such, these sources are ideally suited for applications in THz metrology[11], high-resolution spectroscopy[12,13] and ultra-fast wireless communications[6,14]. >100 MHz modulation is achievable with the latter approach, the bandwidth is still fundamentally limited by the parasitic capacitance of the modulating element To overcome this limit, one must exploit alternative physical processes to control the QCL gain or cavity losses. Direct laser photoexcitation of carriers in the QCL allows modulation of the free-carrier absorption and THz power[23], the bandwidth is limited by carrier recombination (~700 ns) and thermal (~μs) effects[24]
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