Quantum well infrared photodetectors are intersubband devices, built up by a stack of quantum wells
and designed to absorb radiation in the mid-IR region. The basic working principle of a QWIP is a
single quantum well, which is placed between two barriers. Well width and barrier height are adjusted
in a way that the ground state is confined inside the well and the first excited state aligns with the
top of the barrier. The ground state is filled with electrons, which can be excited by light of the
appropriate energy. The excited electrons are extracted by applying an external voltage and are measured
as a photocurrent. In contrast to conventional mid-IR detectors, QWIPs have a very fast response which
makes them suitable for data transmission. They are realized in the robust GaAs/AlGaAs material system
which shows advantages in terms of fabrication ease, yield and homogeneity. All this makes them highly
interesting for pixel arrays in thermal imaging.
Conduction band diagram of a QWIP structure
Related active research topics:
Photonic crystals (PhCs) are built up by a strong, periodic modification of the refractive index, that
affects light propagation in very much the same way as a solid state crystal effects the transport properties
of electrons. The strong light-matter interaction leads to peculiar effects like slow-light propagation, photonic
band gaps or even negative refractive indices. Just like in semiconductors, PhCs have a band structure which
characterizes their photonic properties and therefore presents the basis for all further applications.
By fabricating a quantum well infrared photodetector (QWIP) as a PhC, incident light gets resonantly
coupled into the device whenever a photonic state is present. Each PhC-mode therefore appears as peak
in the spectral photocurrent. In this way we created new and direct characterization procedure for photonic
crystals. Broad-band light is polarization filtered and shined at different angles onto the device in order
to map the photonic band structure including its polarization and symmetry properties as well as the coupling
efficiency of different PhC modes.
Mapping of responsivity peaks to photonic crystal modes
Due to quantum mechanical selection rules QWIPs are only sensitive to light with the electric field
polarized perpendicular to the surface and therefore cannot detect surface incident light.
One of the main application fields though is thermal imaging, which calls for surface-sensitive
photodetectors. In our group we fabricate GaAs/AlGaAs QWIPs as 2D photonic crystals (PhCs).
They can diffract incident light and make these detectors sensitive to surface incident light -
a necessity for the realization of pixel arrays. As the in-coupling is highly resonant it is
possible improve signal-to-noise performance and rise operating temperature. Additionally the
unique symmetry/polarization properties of PhCs are believed to increase functionality of the devices.
SEM image of a photonic crystal QWIP device
We demonstrate monolithic integrated quantum cascade detectors (QCDs) enhanced by plasmonic lenses.
Surface normal incident mid-infrared radiation is coupled to surface plasmon polaritons (SPP) guided to and
detected by the active region of the detector. The lens extends the optical e
ective active area of the device
up to a 5 times larger area than for standard mesa detectors or pixel devices while the electrical active region
stays the same. The extended optical area increases the absorption eciency of the presented device as well
as the room temperature performance while it o
ers a exible platform for various detector geometries. A
photocurrent response increase at room temperature up to a factor of 6 was observed.
Scanning electron microscope image of a plasmonic lens quantum cascade detector with circular gold SPP guiding region.