QC lasers generate ps pulses in the mid-IR

Researchers at Bell Laboratories, Lucent Technologies (Murray Hill, NJ) have generated picosecond self-modelocked (SML) pulses from quantum-cascade (QC) lasers in the mid-infrared (IR), more specifically the 3- to 15-µm molecular fingerprint region. Given the lack of convenient, compact sources of ultrashort laser pulses within this wavelength band, applications ranging from time-resolved spectroscopy to coherent control could soon benefit from these developments.

Although picosecond and femtosecond pulses have become relatively easy to produce in several gas and solid-state laser media at wavelengths ranging from the ultraviolet to the near-IR, until now the molecular fingerprint region has been more or less "off limits" to picosecond pulses. This region is important, though, because of the many chemical and biological species that have telltale absorption features associated with molecular vibrations in the range.

The Bell Labs research team obtained evidence of picosecond SML pulses of mid-IR radiation in QC lasers that are based on intersubband electron transitions. These transitions involve quantized conduction-band states in semiconductor quantum wells and are characterized by some of the largest optical nonlinearities ever observed. In addition, intersubband carrier relaxation is controlled by a picosecond mechanism, namely scattering by optical phonons.

The experiments conducted at Bell Labs used several QC lasers that emitted at either 5 or 8 µm and were characterized by unusually long cleave lengths. Laser materials were grown by molecular beam epitaxy in an indium-gallium-arsenide/aluminum-indium-arsenide material system lattice matched to indium-phosphide structures.

Over a wide range of dc bias, devices were found to self-pulsate at their cavity roundtrip frequency, which was discovered using a fast quantum-well IR photodetector and a spectrum analyzer. The lasers emitted a multimode spectrum up to 1.5 THz, a range equal to about 50% of the gain bandwidth. This spectrum was characterized by a smooth multipeaked envelope, which the scientists say is consistent with pulsed emission with self-phase modulation, and cannot be explained in terms of unlocked multimode emission given the primarily homogeneously broadened narrow gain curves of the QC lasers used.

Although SML by Kerr-lensing has never previously been observed in semiconductor lasers, the project researchers claim self-modelocking in QC lasers can occur because of the ultrafast lifetime and large magnitude of the intrinsic nonlinearity of these lasers' active material. (see figure). They proposed the following mechanism: If the nonlinear index of the laser active material is positive, the center part of the beam transverse profile, where the intensity is higher, experiences a larger refractive index relative to the edges. The resulting nonlinear dielectric waveguide increases the beam confinement near its center, narrowing the beam diameter to a degree proportional to the optical power—essentially self-focusing or Kerr lensing—as was experimentally observed.

Since a key ingredient of SML is a decrease in optical losses with increasing intensity, in the presence of this saturable loss mechanism, it becomes favorable for lasers to emit ultrashort pulses because of their higher instantaneous intensity and lower losses relative to continuous-wave operations. The team reports that the SML-inducing mechanism differs fundamentally from prior demonstrations of SML, where the SML nonlinearity was provided by an external medium added inside the cavity or by some nonresonant transition in the laser host medium.

In conclusion, the research team at Bell Labs reports that all experimental findings were consistent with Kerr-lens SML. The group believes that the observation of self-focusing self-phase modulation confirmed the existence of a large index nonlinearity, a fundamental ingredient of Kerr-lens SML. In addition, the scientists obtained SML only in relatively long devices with thin dielectric blocking layers between the semiconductor material and the gold contact pads of the waveguide, where the saturable loss contribution resulting from self-focusing accounts for a substantial fraction of the overall losses.

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