2 / QELS'99 / MONDAY MORNING

J,,[kA~rn'~]

2 3

J,,[kA~rn'~]

2 3 I [AI

JMA3 Fig. 1. Optical power versus injected current for various temperatures, as indicated. Inset: Threshold current density as a function of temperature.

35 periods active region embedded in an opti- cal waveguide.

Figure 1 shows the optical power versus drive current for various temperatures be- tween T = -7OC and 50C from a single facet. The slope eficiency decreases from dP/dI = 43 mW/A at T = -70C to dP/dI = 16 mW/A at T = 50C, with a maximum power of 10 mW at this temperature. In the inset of Figure 1, the threshold current density Jth is plotted as a function of temperature. It has a value of only 6kA/cm2 at 30C. The data between --70C and 50C can be described by the usual expo- nential behavior J - exp(T/T,) with an aver- age To - 200 K. This extremely weak depen- dence of the threshold current on temperature is typical of laser based on intersubband tran- sitions, especially those operating at long wavelengths. The output power and range of operating temperatures of this device are thus perfectly suitable for room temperature spec- troscopy.

The top contact metallization consists of two segments of equal length, allowing us to inject different current densities J, and J2 (Fig- ure 2). The laser photon energy can now be electrically tuned towards larger energies, ap- proximately tracking the peak ofthe gain spec- trum of the section with the larger injected current as it was done in an earlier work on photon-assisted tunneling transition devices.2 This is possible because the optical transition in our three well active region structure is also diagonal in real space.

As shown in Figure 2, the measured photon energy and pulsed (100ns) current at threshold in the second section is plotted as a function of the current in the first section. The tuning range shown in Figure 2 for this laser (10.2- 10.5pm, corresponding a tuning of27cm-') is significant for applications such as gas moni- toring for room temperature detection of am- monia. This electrical tuning range is wider than the one obtained by simply varying the temperature, as the emission wavelength of the laser for equal current densities in both section is 10.53pm at T = -73C and 10.45pm at T = 27C (corresponding to a tuning of 7 cm-'). Temperature may still be used in a system for fine tuning the emission wavelength. The use of both tuning techniques will be especially suited when exciting narrow absorption lines that might fit between two Fabry-Pkrot modes

Wavelength [wm] 10.5 10.4 10.3 10.2

1 .oo 5 s 3 0.75 E -8 0.50 3 8 0.25 v)

0

2.00

.75

.-

.25

.oo 950 960 970 980

Wavelength [cm"]

JMA3 Fig. 2. Laser tuning at T -lOC and constant optical power P = 3mW. (a) A few rep- resentative spectra for various currents. (b) cur- rents in both sections for each tuning energy.

0 2 4 6 8 10

P o p t P ~

JMA3 Fig. 3. Maximum tuning range as a function of optical power and temperature.

and thus never be excited. In this case one will thermally maintain an optical mode at the ab- sorption wavelength and then electrically se- lect which Fabry-Pkrot mode will lase.

Because the electrical tuning range is propor- tional to the dynamical range of the gain, it de- creases with increasing temperature and optical power. This dependence is shown in Figure 3. However lowering the temperature below - 1OC will not further increase the tuning range.

This tuning is obtained purely electrically by a suitable band structure design. As such, the modulation speed is only limited by the speed of the device. We believe that this broad electrical tunability will enable new room tem- perature spectroscopy techniques.

This work was supported by the ministry for education and science (OFES) through the European project UNISEL. *Also with Alpes Lasers, rue Champrdveyres 2, CH-2008 Neuchhtel, Switzerland **Swiss Institute of Technology, CH-1015 Lau- sanne, Switzerland; E-mail: Ursula.Oersterle@ epfZ.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 (h - 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 transitions and to the optical losses from intra- band free-carrier absorption, which are roughly proportional to A'.

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 A - 8 ~ r n . ~

Here we present the realization of a QC laser employing these so-called "chirped su- perlattices and operating at A - 17 Km, 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 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

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 T~~ = 0.5 ps, while for the uppermost state of miniband I we obtain T~ < 0.3 ps, with a relaxation time between them Of 711,~ = 3.6 PS.

Wavelength (pm) 16 17 16

560 560 600 620 Wavenumber (cm-1)

JMA4 Fig. 2. Emission spectrum of a laser 0.72 mm long and 22.5 p m 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 pm diameter operated with current pulses of 2 A (400 ns width, 82.4 kHz repetition rate) at 5 K.

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 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

Current Density (kA/cmZ) 10 20 30 9O

MONDAY MORNING / QELS’99 / 3

(a) I I

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.

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

Noncascaded 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 comparatively low threshold current densities, as well as in an inherently high optical output power as each electron above laser threshold creates photons in all N stages which it succes- sively traverses. The rapid progress and high

JMA5 Fig. 1. (a) Conduction band profile of the active region with its injector and relaxation (Bragg reflector) regions under an applied electric field of 65 kV/cm and the moduli squared of the relevant wavefunctions (labeled 1,2,3, and g for the ground state of the injector, “I” indicates the injection barrier). The actual layer thicknesses in nanometers are (from left to right, starting from the first AlInAs barrier layer): 0.5/5.0/0.8/4.8/ 0.9/4.4/1.0/4.0/1.1/3.6/1.2/3.2/1.2/3.0/l.6/ 3.0/3.8/2.1/1.2/6.5/1.2/5.3/2.3/4.0/1.1/3.6/ 1.2/3.2/1.2/3.0/1.6/3.0/1.0. The bold-face layers are Si-doped to 2.5 X IO” ~ m - ~ ; the un- derlined layers contain the electron states directly involved in the laser transition (wavy arrow). The shaded region in the injector represents a band- like manifold of states. A similar “miniband ex- tracts electrons from levels 1 and 2; the energy range with low density of states, denoted the “minigap”, prevents electrons from tunneling out of level 3. (b) Intensity profile of the fundamental waveguide mode and schematic of the waveguide composition.

performance of Q C - l a s e r ~ ~ , ~ strongly rein- forced the notion that cascading is essential for intersubband injection lasers.

Here, we demonstrate the first non- cascaded intersubband injection lasers based on a single active region. Several major advan- tages arise from this new structure: first, only few layers are necessary to build the active region core, which simplifies sample growth and preparation. Second, low operating volt- ages -2-3 V are achieved, which is essential for applications which have previously been optimized for interband diode lasers and their low voltage compliance requirements.

The laser structure has been grown in the InGaAs/AlInAs on InP material system using molecular beam epitaxy. An active region is embedded between an electron injector and a Bragg reflector region (see Figure I). These are similar to those of cascaded QC-lasers and are described in more detail in reference 2. How- ever, as only one active region is used, the requirements previously derived from bridg- ing many stages together are significantly re- laxed and each region is optimized separately. This, together with a perfected active region and a low-loss waveguide (sketched in Figure l), made the non-cascaded intersubband laser possible.

The pulsed light output-current character- istics of a 3.12-mm long and 15-pm wide stripe-laser are shown in Figure 2. The thresh- old current density is 25.6-kAcm-’ at 10-K in good agreement with the calculation. The peak power is 20-mW at 10-K and 4-mW at 110-K. A maximum slope efficiency per facet of