ABSTRACT
Optoelectronics is based on electronic devices, which are used for emitting, modulating, transmitting, or
sensing light. At a very fundamental stage, these devices require interaction of light with an electronic
current, effectively converting photons into electrons or vice versa. The temporal response of an
optoelectronic emitter, for example, is therefore always limited by the fastest available rise time of a
current-pulse generator. Similarly, detection of light in a photo-detector can only be accomplished
directly with a few picosecond temporal resolution [75]. On the electronics side, additional constraints
can be imposed by parasitic inductances or capacitances in the electronic circuitry and high-frequency
attenuation mechanisms in microwave cables. Streak cameras [71], which merge the generation of
photoelectrons and their temporal resolution into one device, overcome some limitations. Nevertheless,
even these fastest direct optoelectronic detection devices are typically limited to a response time of the
order of 1 ps. The examples discussed so far rely on a direct interaction of light with an electronic
current. In the following, we will describe ways to circumvent the electronic bandwidth problem. The
fundamental idea behind ultrafast optoelectronics becomes clear from the latest developments for
optical communication networks. In early fibre optic data links, light was converted back into an
electronic current prior to any processing. This has been replaced by all-optical means of processing
photonic data streams. A major improvement in terms of data capacity has been achieved by the method
of wavelength-division multiplexing, which allows for terabit/second rates by simultaneously trans-
mitting many channels at different wavelengths through one and the same fibre. In this chapter, we will
introduce methods to provide the fundamental optoelectronic functions of emitting, modulating,
transmitting, and sensing light, all with a temporal response or resolution of a few femtoseconds. These
methods are ultimately limited only by the duration of the optical cycle itself. We will refer to these
schemes as ultrafast optoelectronic devices, even though the individual optoelectronic components,
mediating between photons and electrons, can be inherently slow. The methods described split the
optoelectronic process into two steps, an ultrafast all-optical step, which ensures sufficient bandwidth,
and a second slow electronic step to allow for efficient conversion between photons and electrons at a
strongly reduced bandwidth.
