0 IEE 2001 E1ectronic.r Letters Online No: 200101YS DOI: 10.1049/el:20010195 M. Notomi, A. Shiiiya and I. Yokohama (NTT Basic Reverircli Laborutoriw. 3-1 Marinosulo- Wakamiya, Atsugi, 243-0198 Jupan) E-mail: notoini@will.brl.nll.co.jp K. Yamada, J. Takahashi, and C . Takahashi (NTT Telecon7,nuizicntion Energy Laboratories, 3- I il4urinusrrtu- CVulk“ya. Alsugi, 243-0198

2 Junuory 2001

Jupun)

References

LIN, s.Y., CHOW,E., JOHNSON, s.G., and JOANNOPOULOS, J . I X ‘Demonstration of highly cfficient waveguiding in a photonic crystal slab at ihe 1.5-pm wavclcngth’, Opt. Lett., 2000, 25, pp. 1297-1299 BABA, T., FUKAYA, N., and YONBKURA, I.: ’Observation of light propagation in photonic crystal optical waveguides with bends’, Electron. Lett., 1999, 35, pp. 654-655

KOLODLIEJSKI, L.A.: ‘Guided modes in photonic crystid slabs’, Pl7j~s. Rev. B, 1999, 60, pp. 5751-5758 CHUTINAN, A., and NODA, s.: ‘Waveguides and waveguide bcnds in two-dimensional photonic crystal slabs’, Pliys. R p e . E, 2000, 62, pp. 4488-4492

JOHNSON, S.G., FAN, S. , VILLENEUVE, P.K., JOANNOPOULOS,.I.D., and

atures of 80K are measured. To the best of our knowledge, this is the highest peak power ever reported for a quantum cascade laser. Considering the number (N,, = 25) of active stages, this corre- sponds to a power-per-stage as high as 881nWIstage.

a Typical injector b High energy injeciioii scheme

151

High peak power (2.2W) superlattice quantum cascade laser

G. Scamarcio, M. Troccoli, F. Capasso, A.L. Hutchinson, D.L. Sivco and A.Y. Cho

The iniproved optical power performance of superlattice quantum cascade lasers with a novel injector design allowing the tunnelling of electrons into high energy states of thc excited miniband is reported. At 8 . 4 ~ and temperatures 5 120K, peak powcrs > 2W per facet are nicasured, coiiesponding to a record powcr of 88mWlstage. A slope erficiency of I60inWIA over a current range six iimes larger than lhc laser threshold is observed.

Quantum cascade (QC) lasers with superlattice (SL) active regions exploit high oscillator strength electronic transitions across the minigap between the first two conduction minibands of a periodic potential [1 - 41. The emission wavelength is determined by the minigap energy E,,,,. Population inversion is assured by the fast intraminiband electronic relaxation, if the first miniband width A, is significantly larger than the thermal energy. The high current capability associated with miniband transport leads to high output powers at wavelengths in the inid-infrared. The best performance reported to date has been attained with chirped SL active regions [5, 61, or with undoped SLs carefully designed to maximise the overlap between the states of laser transition and rcduce phonon bottleneck effects in the second miniband [4].

In SL QC lasers the active regions of the waveguide core con- sists of undoped SL active regions (alternated with injectors). This choice leads to low threshold current densities and high optical power performance [2 ~ 61. The injectors are chirped SLs selec- tively doped to completely screen the active SL from the external electric field and designed to form a miniband at a voltage drop per stage -(Al + Et!,Jq. This voltage corresponds to the alignment condition schematically shown in Fig. la, and to the onset of large current transport. The injector miniband is designed for elec- tronic tunnelling at an energy close to the lowest state in the sec- ond miniband of the active SL. This usual configuration is intended to lacilitate the accumulation of electrons at the bottom of the second miniband and maximise the population inversion at the minigap [2]. In contrast, we have recently shown that the use of injectors designed for electron tunnelling into high energy states of the second miniband gives rise to a smaller differential resist- ance and a larger slope efficiency in the spontaneous light-current characteristics of mesa devices [7]. Fig. Ib schematically illustrates this configuration. In this Letter, we report on an SL QC laser with a band structure designed for high energy injection. Peak powers of 2.2W per facet at a wavelength h = 8 . 4 ~ and temper-

ELECTRONICS LETTERS 1st March 2001 Vol. 37

vonege, v E@ Fig. 2 Current volliige charactetiytics cfD26G7 and D2397 at 10K

D2667 D2397

.- ~

_ _ - - Inset: Calculated conduction band diagram of sample D2667 at the flat-band voltage condition

The inset of Fig. 2 shows the conduction band profile of the investigated GaInAslAlInAsllnP QC laser structure (sample D2667) calculated with a selr-consistent solution of the Poisson and Schrodinger equations. The shaded areas represent the con- duction minibands. The waverunction square moduli of the rele- vant energy levels are indicated. For the sake of comparison and to identify the effects related with the different injection, we used in our structure the optiinised intrinsic Ga,,,,In~.5,As/Al~,481no.szAs active SL reported in [4] (sample D2397). Its layer thickness sequence is 213.71114.11114.61114.6/114,611/4,611/4,51114,3 (nm) going from right to left in Fig. 2, with the AlInAs barriers in bold type- face. The layers thickness in the modified injectors of sample D2667 are, starting from the 4nm thick injection barrier, 4.01~10.913.911.113.711 .513.311.912.912.112.1/2.612.213.011.813.3, where the underlined numbers indicate layers n-doped to 1 x 101Rc~n-3. Twenty-five stages were grown by molccular beam epi- taxy on InP substrate and embedded between two GaIiiAs layers Fi-doped to 5 x 10’6cm-3, with thickness of 400mn (bottom, with respect to the growth direction) and 220nm (top). These layers, together with the active region, form the waveguide core and enhance the refractive index contrast with respect to the cladding layers. Index guiding is provided on the lower side by the InP sub- strate and on the upper side by two AlInAs layers, both 1 . 2 ~ 1 thick and n-doped to 1 x lOI7cm and 2 x 10’7cm-3. A GaInAs layer 500nm thick and doped to 7 x 1018cm-3 for plasmoii- enhanced optical confinement was grown on top of the structure [2].

In a two-teiminal device like the QC laser, current and voltage cannot be controlled independently. This obvious constraint pre- vents reaching a high current density regime while preserving an

No. 5 295

electric-field-free active SL. With the aim of partially overcoming this limitation, the layer sequence in the high-energy injector was tailored to achieve a Pat-band condition under an applied voltage per stage of (A, + .!& + 6)Iq = 0.330V, as shown schematically in Fig. Ih and in more detail in Fig. 2. This voltage is higher than the onset for strong injection by the offset 6 35meV, close to the GaAs-like phonon energy of GalnAsiAlInAs heterostructurcs. A merit of this new injector design is that it allows Lis to practi- cally keep the flat-band condition in the active SL and to have a flat miniband in the injector, even at applied voltages much larger than the onset for injection. This avoids field-induced wavefunc- tion localisation both in the injector and in the active SL and pre- serves the characteristic miniband transport regime for a wider range of applied voltages, therefore leading to smaller differential resistance values. In fact, comparison between the current-voltage characteristics of two devices processed from wafers D2667 and D2397 (see Fig. 2) shows that the onset voltage for strong injec- tion is close to the expected value of 7.1V (0.285V per stage) for both devices, whereas above this voltage the specific differential resistance of laser D2667 (1Q-cm2) is a factor of 2.5 lower than that of laser D2397.

curlent denslfy, kAlcm2 0 5 10 15 20 I I

0.5 1 // . 200

300

OO 5 10 15 20 rz5zE cumnl , A __

Fig. 3 Peak collected optical power measured f i u m one fcrcet D2667 agoinst current a f various kern sink rem[Jeratzire.s

of Inser

Inset: High resolution FTIR spectrum of same dcvice

Fig. 3 shows the optical power-current characteristics of a laser processed from sample D2667, collected at temperatures in the range 8CL300K. The inset shows a typical high resolution spec- trum collected at 2.5A and 10K. The emission wavelength, cen- tered at h = 8.4pn1, is in good agreement with the calculated minigap energy (1 51 meV). Ridge waveguide laser cavities 3Xpm wide and 2.25” long were obtained by wet chemical etching and cleaving. Facets were left uncoated. The devices were indium sol- dered onto copper holders and mounted in an He-flow cryostat. Spectral analysis was performed with a Fourier-transform infrared spectrometer. The optical power was measured by focusing the laser radiation onto a calibrated HgCdTe detector operating at room temperature, with a near unity collection efficiency.

One striking feature of the curves reported in Fig. 3 is the almost linear increase of the output power in a current range approximately six times larger than the laser threshold at the low- est temperatures. This characteristic, related with the new injector design, allows the achievement of peak output powers above 2W for temperatures below 120K, above 1W up to 200K and of 0.15W at 300K. At variance with typical trends in QC lasers, 110 sign of power saturation is observed up to the maximum current achievable with our setup ( I = 17A) corresponding to a current density of 19.9Ncmz. This can be explained considering the signif- icantly lower differential resistance of sample D2667, and attain- ment of the flat-band condition at an applied voltage -12% larger than the onset for strong injection. In fact, these two features allow us to keep the intended band alignment for a wider range of currents, thus moving to larger current regimes the detrimental effects of field penetration on the optical gain.

At XOK, the slope efficiency (160mWIA) and the threshold cur- rent density (3.3kA/cm2) are comparable with those of laser D2397. This is expected since both devices share the same active region and similar waveguides, and hence have comparable mate- rial gain and losses. From the temperature dependence of the

threshold current density we found a characteristic temperature To = 120K, 25%) smaller than the value found for D2397. The better temperature performance of the latter device may be partly ascribed to the smaller lateral size (l6pn1) of the ridge waveguide.

Acknow1edgment.s: We gratefully acknowledge C. Gmachl for help in measurements and useful discussions, A. Tredicucci for provid- ing unpublished data on sample D2397 and the critical reading of the manuscript, and A.M. Sergent for expert technical assistance. This work was partly supported by INFM project PRA9X- SUPERLAS and MURST - CIPE cl. 26 - PSWP2. The work per- formed at Bell Laboratories was partially supported by DARPAi US Army Research Office under grant No. DAAD19-00-C-0096.

0 IEE 2001 I 2 January 2001 Electronics Letrers Online No: 20010221 D 01: 10. 1049/e1:200 1022 I G. Scamarcio and M. Troccoli (I’VFM, Dipartinzento Inleroleneo d i Fisica, Univerritd e Polilecnico di Bari, viu Amenclolri 173, 70126 Bur;, Itoly) E-mail: scamarcio~fisica.uniba.iI F. Capasso, A.L. Hutchinson, D.L. Sivco and A.Y. Cho (Bell Lrrboratories. Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA)

References

SCAMARCIO, G., CAPASSO. F., SIRTORI, C., FAIST, J . , HUTCHINSON, A.[-., SIVCO. D.L., and ~ 1 1 0 , A.Y.: ‘High-power infrared @-micrometers wavelength) superlattice lasers’, Scimce, 1997, 276, pp, 173-116 CAPASSO, F., TRPDICUCCI, A . , GMACHL. C., SIVCO, U.I.., RIJTCHINSON, A.L , CHO. A.Y , alld SCAMARCIO, G.: ‘High- performance superlattice quantum cascade lasers’, IEEE J . Se/. topic^ Qucintun? Electron., 1999, 5, pp. 792-807 TREDICUCCI, A , CAPASSO, F., GMACHL, C., SIVCO, D.L., HUTCHINSON. A.L., CHO, A Y . , FAIST, J., and SCAMARCIO, C.: ‘High- power interminiband lasing in intrinsic superlattices’, Apol. P/zy.s. Lett., 1998, 72, pp. 2388-2390

HUTCHINSOY, A.L., and CIIO, A Y . : ’High-performance quantum cascade Iascrs wilh electric-field-free uiidoped superlattices’, I Pliutonics Teclinol. Lett., 2000, 12, pp. 260-262

HUTCHINSON, A.L., and CHO. A.Y.: ‘High-performance interminiband quantum cascadc lasers with graded superlattices’, Apppl. ’ Phys. Lett.. 1998, 73, pp. 2101-2103 SCHRENK, W., FINGER, N., GIANORDOIS, S. , GORNIK, E., and STRASSER, G.: ‘Continuous wave operation of distributed feedback AIAs/GaAs superlallice quantum-cascade laser’, Appl. Phys. Leti., 2000, 77, pp. 3328-3330

GMACHL. c., CAPASSO, F., SIVCO, D.L., CHO, AY., and STRICCOI.~, M.: ‘Electronic distribution in superlattice quantum cascadc lasers’, AppL PhJ1.s. Lett., 2000, 77, pp. 1088-1090

TRPDICUCCI, A., CAPASSO, F., GMACHL, C., SIVCO, D.I..,

TREUICUCCI. A., CAPASSO, F., GMACHL, C., SIVCO, D.I..,

TROCCOLI, M., SCAMARCIO, G., SPAGNOLO, V., ‘TREIIICUCCI. A.,

Monolithically integrated multi-wavelength laser by selective area growth with metal organic vapour phase epitaxy

T. Van Caenegem, D. Van Thourhout , M. Galarza, S. Verstuyft, 1. Moerman, P. Van Daele, R. Baets, P. Demeester, C.G.P. Herben, X. J.M. Leij tens and M.K. Smit

A multi-wavelength laser (MWL) is fabricated by means of selective area growth (SAG) with metal organic vapour phase epitaxy (MOVPE). The MWL cousists of an asmy of xnplifiers monolithically integrated with a trausmissive (de-)multiplexer and, to the aalhors’ knowledge, is the first device of this kind realised with only two growth steps making use of SAG MOVPE.

Introduction: The increasing demand for complex photonic inte- grated circuits (PICs) which can be used, for example, for wave- length division multiplexing (WDM) [I] purposes, has led to the development of various monolithic integration techniques. These

296 ELECTRONICS LETTERS 1st March 2001 Vol. 37 No. 5