Population inversion in optically pumped asymmetric quantum well terahertz lasers
Intersubband carrier lifetimes and population ratios are calculated for three- and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga1−xAlxAs quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation.
The evaluation of the matrix element for interband optical transitions in quantum wells using envelope functions
The expression for the matrix element of position for an interband transition in a wide deep quantum well is derived using the envelope function expansion. It is found that the matrix element is not determined by the intra-atomic-like matrix element evaluated using band-edge Bloch functions, the result one might be led to expect either on intuitive grounds or as the result of a supposedly approximate evaluation in which only the dominant term in the envelope function expansion of each wave function is used. To obtain the correct expression, terms in the envelope function expansion that become vanishingly small in the limit of wide wells must be retained. Usually the interband matrix element is comparable with that for allowed intersubband transitions. Some implications for the DC Stark effect for quantum wells and non-linear refraction in quantum dot structures are mentioned.
Magnetic plasma confinement for laser ion source.
A laser ion source (LIS) can easily provide a high current beam. However, it has been difficult to obtain a longer beam pulse while keeping a high current. On occasion, longer beam pulses are required by certain applications. For example, more than 10 micros of beam pulse is required for injecting highly charged beams to a large sized synchrotron. To extend beam pulse width, a solenoid field was applied at the drift space of the LIS at Brookhaven National Laboratory. The solenoid field suppressed the diverging angle of the expanding plasma and the beam pulse was widened. Also, it was observed that the plasma state was conserved after passing through a few hundred gauss of the 480 mm length solenoid field.