Waveguide and Surface Plasmon Coupled Infrared Devices Using Semiconductor Quantum Wells

In this paper we propose and present detailed calculations on a new method for using the intersubband transiiion in modulation doped semiconductor quantum wells gown from Alpa l ,k and GaAs in infra red devices (modulators and detectors) working at 10 um wavelength. The quantum wells are embedded near the the surface of a thick AlAs layer (1-9 microns) and capped by a metal electrode and coupling to the intersubband transition is mediated by either a "leaky" guided mode or a surface plasmon. Our calculations show that coupling of radiation to intersubband transitions can be strong under such circumstances. The fabrication of optoelectronic devices such as detectors or modulators operating at 10 pm using the intersubband intransition in a 111-V semiconductor system such as GaAs/AlxGal-,As would take advantage of the highly developed technologies of these materials and potentially allow the integration of such devices with high speed electronics and optoelectronic devices operating at other wavelengths. The subband energy spacing can be tuned to the energy of 10 pm wavelength photons by choosing a suitable well width, typically 82 A. Dipole selection rules only allow the transition for an electric field oscillating perpendicular to the quantum well. Typical 111-V semiconductors tend to have large refractive indices. These will refract incident light so that it propagates with only a small component of its electric field oscillating in the plane of the the well. The light then only couples weakly to the intersubband transition. The spectroscopic studies of the intersubband transition in the literature (1-3) have used light incident at the Brewster angle in order to maximise the component of the electric field oscillating perpendicular to the quantum well. Typically, a peak absorption of 3-5% is obtained from a stack of 50 quantum wells. The weakness of this coupling means that some additional method must be found to increase the strength of interaction with the intersubband transition in order to make a useful optoelectronic device. Lyon and Goosen (4) have cohsidered the use of the resonant modes of a diffraction grating to provide this enhancement. In this paper we propose and present detailed theoretical modelling of, an alternative structure for enhancing the intersubband absorption so that a large proportion of the incident light is absorbed in a small number of quantum wells, typically 4. The carrier density in the quantum wells can now be controlled by a Schottky gate and the absorption modulated. The initial application we propose for this structure is as a reflective modulator. The structure (see figure 1) consists of a thick, 1-9 microns, layer of AlAs grown on a semi-insulating GaAs substrate. The quantum wells (modulation doped) are grown on top of the AlAs layer and the whole structure is capped with a metal top layer. A GaAs prism with anti-reflection coated surfaces is mounted on the substrate and is used to couple The structure has two different operational modes according to the angle of incidence. When light is incident at an angle slightly less than the critical angle for total internal reflection at the AlAsIGaAs interface a guided wave is launched into the AlAs with its electric field oscillating almost perpendicular to the quantum wells. The second mode of operation involves the excitation of a surface plasmon at the interface between the metal and the semiconductor. The electric field associated with the surface plasmon has a large component perpendicular to the plane of the interface and thus couples strongly to the intersubband Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875117