Sotomitsu Ikeda

Asymmetric dual quantum well laser---wavelength switching controlled by injection current

We have performed the first experiment on a novel laser diode, the lasing wavelength of which can be switched with increasing current. Wavelength switching over 13 nm under continuous operation, as well as lasing at dual wavelengths, has been achieved for the first time.

Evidence of the wavelength switching caused by a blocked carrier transport in an asymmetric dual quantum well laser

The mechanism of wavelength switching in an asymmetric dual quantum well laser was investigated. Switching between 830 and 780 nm was achieved, with increasing injection current under cw as well as under pulsed operation. This is the widest switching range of monolithic LDs under cw operation ever reported. Time‐resolved spectra for the dual‐wavelength simultaneous lasing under short‐pulse operation show that emissions at the two wavelengths have a time difference. This provides evidence that holes are inhomogeneously injected into the two wells with a time difference and that their dynamics govern the lasing for each wavelength and the switching.

Wide‐range wavelength tuning of an asymmetric dual quantum well laser with inhomogeneous current injection

A new asymmetric dual quantum well laser diode with a segmented contact is proposed. With inhomogeneous current injection, the lasing wavelength can be changed quasi‐continuously over 22 nm, which is much wider than any tuning range previously reported. To achieve such wide‐range tuning, it is found to be essential that the length of the cavity and each segment (and the mirror reflectivity) of the laser diode should be within an optimized range. In the asymmetric dual quantum well laser diode structure used in the present experiment, in particular, the cavity length should be such that light of a shorter wavelength lases under homogeneous excitation.

Theory of asymmetric dual quantum well lasers

A basic theory of asymmetric dual quantum well lasers is presented. The lasers consist of two quantum wells of different energy gaps, which are separated by a high and/or thick barrier layer. The barrier layer blocks carrier transport between the wells so that the rate of the transport becomes comparable to the rate of the radiative recombination. It is shown that this leads to novel phenomena of dual‐wavelength lasing and wavelength switching with increasing injection current, in agreement with recent experimental results.

Polarization switching in AlGaAs/GaAs distributed feedback lasers between the stable single longitudinal modes

TE/TM polarization mode switching is demonstrated in an AlGaAs/GaAs distributed feedback laser diode. The device has two current injecting electrodes and as an active layer a conventional multiquantum well (MQW) structure. A small amplitude modulation of the current into one of the electrodes switches the polarization mode. Each mode corresponds to a stable single longitudinal mode of a distributed feedback laser. The lasing wavelength can be tuned continuously, without mode hopping, over 0.7 nm by controlling the total injection current. This new kind of polarization switching is promising for optical switching systems and optical frequency division multiplexing devices.

TWO-PHOTON-ABSORPTION SPECTRA OF QUANTUM-WELL EXCITONS IN STATIC ELECTRIC FIELDS

We have measured two-photon-absorption spectra of GaAs/Al0.4Ga0.6As quantum-well structures in a static electric field for photon energies near half the band gap energy, and found drastic field-induced-changes in the spectra. The two-photon-absorption peak at half the energy of the lowest light-hole exciton is induced by the static electric field normal to the quantum well layers, in agreement with a theory that takes account of quasi-two-dimensional exciton effects. With increasing the electric field, however, this peak grows more drastically than the theoretical prediction, and it approaches a large value predicted by another simplified theory based on a two-level model.