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Emitters, modulators and detectors for mid-infrared wavelengths are becoming increasingly valuable for free-space communication, interconnects, gas detection and other applications. Devices based on interband transitions require materials with very small band gaps for which the material technology is not as developed as for InP- and GaAs-based heterostructures. A promising alternative is using intersubband (intraband) transitions in low-dimensional heterostructures. So far most of the research for mid-infrared wavelengths has dealt with quantum cascade lasers and IR detectors. For electroabsorption modulators an essential advantage with intersubband (IS) transitions is that electron subbands run essentially in parallel so that the transition energy becomes independent of wave vector, thus enabling strong and narrow absorption peaks. Many-body effects give a further reduction of the IS absorption linewidths Γ [1], which is particularly beneficial for modulators, where the RC-limited modulation speed f3dB~ ¦£-3 [2]. In addition the rapid (~1 ps) LO-phonon-mediated IS relaxation renders IS absorption insensitve to saturation.
We here demonstrate efficient electroabsorption experimentally in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapour-phase epitaxy on an InP substrate. The multiple-quantum-well structure was heavily δ-doped in the barriers to achieve strong IS absorption. A modulation of 6 dB at λ = 6.0 ¦Ìm (ν = 50 THz) due to Stark shift of the IS absorption was achieved at an applied voltage swing as low as ¡À 0.5 V in a multipass waveguide structure. It is in quite good agreement with simulations [2]. Based on the experimental results it is estimated that an electroabsorption modulator with a low peak-to-peak voltage of Vpp=0.9 V and a modulation speed of f3dB ¡Ö 90 GHz can be realized with the present material by using a strongly confining surface plasmon waveguide of 30 μm length. This compares very favourably with interband modulation at similar wavelengths.
[1] R.J. Warburton, K. Weilhammer, C. Jabs, J.P. Kotthaus, M. Thomas and H. Kroemer, Physica E 7, 191 (2000).
[2] P. Holmström, IEEE J. Quantum Electron. 37, 1273 (2001).
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