MONDAY MORNING / CLEO’YY / 27

*Also with Alpes Lasers, rue Charnprtvqres 2, CH-2008 Neuchdtel, Switzerland **Swiss Institute of Technology, CH-1015 Lau- sanne, Switzerland; E-mail: Ursula.Oersterle@ epfl. ch 1. J. Faist, C. Sirtori, F. Capasso, D.L. Sivco,

J.N. Baillargeon, A.L. Hutchinson, A.Y. Cho, Photon. Technol. Lett. 10, 1100 (1998).

2. J. Faist, F. Capasso, C. Sirtori, D.L. Sivco, A.L. Hutchinson, and A. Y. Cho, Nature 387,777 (1997).

JMA4 9:30 am

Superlattice quantum cascade lasers operating at very long wavelengths (A - 17 pm)

Alessandro Tredicucci, Federico Capasso, Claire Gmachl, Deborah L. Sivco, Albert L. Hutchinson, Alfred Y. Cho, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974 USA

Quantum cascade (QC) lasers operating on intersubband transitions between conduction band states in InGaAs/AlInAs heterostruc- tures have proven so far to be extremely versa- tile, covering the range of wavelengths of the two atmospheric windows (3.4 pm-13 pm).’ Their extension to even longer wavelengths is, however, problematic, due to the increasingly smaller radiative efficiency of intersubband

band free-carrier absorption, which are roughly proportional to h2.

On the other hand, QC lasers based on in- terminiband transitions in semiconductor su- perlattices (SL) present the advantage of large oscillator strengths and intrinsic population inversion and can be driven with very large current densities2 A new design of the active region in which the applied electric field is compensated with an appropriate variation of the SL period and duty-cycle has recently shown record performances at h - 8 ~ m . ~

Here we present the realization of a QC laser employing these so-called “chirped” su- perlattices and operating at A - 17 bm, which represents the first demonstration of a semi- conductor injection laser based on intra-band transitions at wavelengths beyond the atmo- spheric windows.

A schematic band diagram of the structure, which has been grown by molecular beam epi- taxy (MBE) in the InGaAs/InAlAs material system lattice matched to InP substrate, is shown in Figure 1. The energy levels in each active SL are grouped together in two well defined minibands, with the intermediate in- jector region bridging them together across the cascade stages. Laser action takes place at the edge of the minigap ( E 70 meV) between minibands I1 and I, where well delocalized wavefunctions result in a large transition di- pole matrix element z = 4.0 nm.

Figure 2 shows the laser emission spectrum of a ridge-waveguide device as recorded in pulsed operation at a temperature of 140 K. The wavelength is approximately 17 p m and the laser oscillates on several longitudinal

transitions and to the optical losses from intra-

JMA4 Fig. 1. Conduction band profile of two chirped SL active regions with the connecting injector under an applied electric field of 2.05 X lo4 V/cm. The moduli squared of the wavefunc- tions and the two minibands into which the states are grouped are also shown. The lifetimes are determined by optical phonon emission and for the lowest state of miniband I1 we compute at cryogenic temperature -rII = 0.5 ps, while for the uppermost state of miniband I we obtain -rI < 0.3 ps, with a relaxation time between them Of TII,I = 3.6 PS.

Wavelength (pm) 18 17 16

c s d U

.e 0 2

g

2

3 H

.3 v)

w 560 580 600 620

Wavenumber (cm-1)

JMA4 Fig. 2. Emission spectrum of a laser 0.72 mm long and 22.5 km wide at a temperature of 140 K, driven in pulsed mode with a current of 4.7 A. The pulse width was 50 ns and the repeti- tion rate 82.4 kHz. In the inset we report the electroluminescence spectrum as measured from a round mesa of 125 prn diameter operated with current pulses of 2 A (400 ns width, 82.4 kHz repetition rate) at 5 K.

modes as is typical for Fabry-Perot resonators. The electroluminescence signal was measured from another piece of the wafer processed in round mesas. A single emission peak is ob- tained, centered approximately at 70 meV and with a full-width-at-half-maximum of 5.7 meV (see inset).

The peak output power as a function of driving current is reported in Figure 3 for vari- ous temperatures. More than 12 mW are achieved at 5 K, with still a few mW at 120 K. Another device with a larger ridge width of 34 p m showed a maximum operating tempera- ture of 175 K. As for the voltage-current char- acteristic, the low current conductivity corre- sponds to a transport regime dominated by tunneling between minibands I of consecutive stages; after an intermediate situation in which miniband I is aligned with the minigap of the following stage and the curve shows a large

Current Density (kA/cm2) 10 20 30

Current (A)

JMA4 Fig. 3. Pulsed light-current character- istics of the same device of Figure 2. They have been measured with f/l KBr optics (50% collec- tion efficiency) from a single laser facet using a calibrated pyroelectric detector. The laser was driven with 50 ns current pulses at a 1% duty- cycle in bursts of 2000 pulses. The signals were measured employing a boxcar averager with an adjustable gate. The voltage-current response curve at 5 K is also shown.

differential resistance, a sudden increase in the conductivity is observed, since injection from miniband I into miniband I1 of the next stage takes place.

This material is based upon work supported in part by DARPA/US Army Research Office under Contracts DAAH04-96-C-0026 and

1. F. Capasso, J. Faist, C. Sirtori, andA.Y. Cho, Solid State Commun. 102, 231 (1997); C. Gmachl, F. Capasso, A. Tredicucci, D.L. Sivco, A.L. Hutchinson, and A.Y. Cho, Electron Lett. 34, 1103 (1998); J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, S.G. Chu, and A.Y. Cho, Appl. Phys. Lett. 72, 680 (1998).

2. G. Scamarcio, F. Capasso, C. Sirtori, J. Faist, A.L. Hutchinson, D.L. Sivco, and A.Y. Cho, Science 276, 773 (1997); A. Tredicucci, F. Capasso, C. Gmachl, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, J. Faist, and G. Sca- marcio, Appl. Phys. Lett. 72,2388 (1998).

3. A. Tredicucci, F. Capasso, C. Gmachl, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, Appl. Phys. Lett. 73,2101 (1998).

DAAG55-98-C-0050.

JMA5 9:45 am

Non-cascaded intersubband Injection lasers and scaling with the number of stages in quantum cascade lasers

Claire Gmachl, Federico Capasso, Alessandro Tredicucci, Deborah L. Sivco, Albert L. Hutchinson, Alfred Y. Cho, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974 USA One key feature of quantum cascade (QC) la- sersl is the cascading scheme: typically N - 25 periods of alternated active regions and elec- tron injectors form a common active waveguide core which increases the optical confinement and therefore the modal gain suf- ficiently to overcome the increased waveguide losses at mid-infrared (3-13 pm) wavelengths. Consequently, the cascading scheme results in