Electrical pumping of a third-order mode semiconductor laser

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Optics & Laser Technology 36 (2004) 669 – 676 www.elsevier.com/locate/optlastec

Electrical pumping of a third-order mode semiconductor laser N.G. Semaltianos∗ , A. De Rossi, B. Vinter, V. Berger, M. Calligaro, V. Ortiz THALES Research and Technology, Domaine de Corbeville, Orsay 91404, France Received 2 October 2002; received in revised form 10 December 2003; accepted 2 February 2004

Abstract AlGaAs-based quantum well laser structures with third-order waveguide mode emission at 775 nm are a promising route toward compact twin-photon sources at 1:55
m based on the principle of modal phase matching between the pumping frequency and fundamental modes at half frequency in III–V semiconductor waveguides. Following the demonstration and characterization of an optically pumped third-order mode semiconductor laser, in this paper we present data of the corresponding structure under conditions of electrical pumping. By pumping electrically and optically the same sample made for current injection, identical transverse far-;eld angular laser mode pro;les are measured and with very low parasitic losses. Although they do not follow the third-order mode emission pattern as it is expected, however this means that the di=erent way of pumping, that of the electrical one as compared to optical pumping is not responsible for the absence of third-order mode emission. Furthermore, since the undoped optically pumped laser sample correctly emits on the third-order mode, it is concluded that the cladding layers of the structure still need to be optimized in doping and thickness, in order to reduce the internal losses for the third-order mode. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Semiconductor lasers; Laser diodes; Design of speci;c laser systems; Beam characteristics; Pro;le intensity, and power, spatial pattern formation; Nonlinear waveguides

1. Introduction The high nonlinear susceptibility of GaAs as well as the possibility to integrate quantum well (QW) laser sources with nonlinear interactions provides with the unique opportunity of using this material for waveguided nonlinear parametric processes such as parametric Buorescence (PF). In the area of quantum cryptography or in quantum optics experiments, PF is a useful method for the fabrication of a so-called twin-photon source [1–3]. However, the whole scheme and setup used so far for the production of twin photons are, in general, complicated, involving an external laser beam with frequency !p which pumps a bulk nonlinear crystal and creates twin down generated photons at frequency !i = !s = !p =2 (!i and !s are the idler and signal frequencies). The commercialization of a quantum communication system requires the fabrication of a compact twin-photon source and this can be realized by making use of the idea that the pump source which is a QW laser emitter, lies inside ∗

Corresponding author. Tel.: +1-33-630659872;. E-mail addresses: nikolaos [email protected], [email protected] (N.G. Semaltianos). 0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.02.001

the nonlinear semiconductor waveguide. The critical issue in such kind of nonlinear device is that in order to obtain eKcient PF the interacting waves must be phase matched. Among the phase matching schemes used so far, an alternative phase matching scheme which is also well suited for molecular beam epitaxy (MBE) grown materials and which overcomes the de;ciency of III–V semiconductors of not being birefringent, is the modal dispersion phase matching (MDPM). In this method, phase matching is obtained by carefully designing the multilayer waveguide so as the effective indices ne= of three di=erent guided modes i; j and k to satisfy the MDPM condition: ne= ; i (!p ) = 12 [ne= ; j (!i ) + ne= ; k (!s )];

(1)

where !p is the pump frequency and !i = !p =2; !s = !p =2 are the idler and signal frequencies [4,5]. Fig. 1 illustrates the MDPM in the sample structure used in this study and which will be described in detail later; in this speci;c laser design, photons on the third-order mode [i=transverse electric (TE2 )] at p = 775 nm can generate through phase matched PF, idler and signal photons at i = s = 2 p = 1:55
m on the two fundamental modes [j = TE0 and k=transverse magnetic (TM0 )]. This is because, as seen in Fig. 1, nTE2 (775 nm) is equal to half the

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λ =1.55 µm

3.4 Al0.30Ga0.70As

3.2 TE

0

TM2

0

2

3.1

3.0

TM

TE

75 n nm λ =775

effective index

3.3

TE

TM

1

1

AlAs

2.9 0.8

1.0

1.2

1.4

1.6

1.8

wavelength ( µm) Fig. 1. E=ective indices of the sample as a function of wavelength. The two dash–dotted lines show the indices of the Al0:30 Ga0:70 As (upper curve) and AlAs (lower curve). The triangle joining TE2 at 775 nm (left open dot) and TE0 and TM0 curves at 1:55
m (two open dots on the right) illustrates the phase matching condition: nTE2 (775 nm) = ( 12 )[nTE0 (1550 nm) + nTM0 (1550 nm)]. Note that in this PF scheme, the two down generated photons are cross-polarized, which is the best situation for quantum optics applications.

sum of nTE0 (1550 nm) and nTM0 (1550 nm) and, thus, the phase matching condition expressed in Eq. (1) is satis;ed. In addition, the overlap integral between the third-order mode and the fundamental one, which gives the eKciency of this nonlinear process, is optimized [5]. With the purpose of making a compact twin photon source at 1:55
m based on the above idea, we have undertaken a program of work in trying to fabricate a QW laser diode operating at 775 nm on the third-order laser mode. As a ;rst step in the evolution of our work, we have already demonstrated operation of the optically pumped laser device [6,7]. The laser mode was measured to be third-order at room temperature in agreement with theoretical stimulations and its temperature and pumping power dependencies were fully characterized [8]. In this paper we would like to present our preliminary development e=orts in trying to fabricate the corresponding electrically pumped laser device. We have grown a sample structure compatible with the corresponding optically pumped structure, but which could be pumped electrically and achieved lasing action at 775 nm at room temperature. We have carried out a detailed characterization under pulsed current pumping of the laser operation of devices with di=erent ridge widths in terms of optical spectral characteristics, output power versus applied current, dependence of threshold, lasing peak position and of DC current–voltage characteristic curves on sample temperature. Measurements of the transverse angular distribution of the far-;eld laser radiation,

however, revealed that the laser mode is not a third-order mode as it is expected. By pumping optically as-grown (unprocessed) sample bars cleaved from the same wafer, which was made for current injection, an identical far-;eld laser radiation pattern is measured. This means that the ‘di=erent’ way of pumping, that of the electrical one (as compared to optical pumping), is not responsible for not obtaining clear laser emission on the third-order mode. By reviewing analytically the detailed physical processes of carrier injection, electron–hole pair generation and recombination which are occurring in the two ways of pumping, electrical and optical, it is pointed out that a measurement of the far-;eld pattern of the as-grown sample before processing is suKcient to reveal the laser mode of the sample structure which will be used later for current injection. Since the undoped optically pumped sample correctly emits on the third-order laser mode, it is concluded that the cladding layers of the structure still need to be optimized in doping and thickness, in order to reduce the internal losses for the third-order mode. 2. Sample structure and experimental setup A schematic diagram of the sample structure is shown in Fig. 2. In analogy to the optically pumped structure [6], the sample was grown by MBE and its active region O Al0:11 Ga0:89 As active QW surrounded consists of a 100 A O Al0:50 Ga0:50 As barrier layers (B) followed by by 1285 A O Al0:25 Ga0:75 As satellite layers (G) on both sides. 1450 A

laser ridge w p++ contact digital grading

cladding

Au wire contact Cr/Au non-alloyed cont. Si3N4 insulator GaAs:Be, 1000Å p- or n-AlAs/GaAs n- or p-AlAs/GaAs superlat. 8500Å

G

p-Al0.25Ga0.75As, 1450Å

B

p-Al0.50Ga0.5 0As, 1285Å

B

n-Al0.50Ga0.50As, 1285Å

G

n-Al0.25Ga0.75As, 1450Å

QW n-Al

cladding

digital grading

n-- contact

Ga

0.11

As, 100Å

0.89

n-AlAs/GaAs superlat. 8500Å

n-AlAs/GaAs GaAs:Si, 1000Å

n+-GaAs substrate AuGe/Ni/Au-alloyed cont. Fig. 2. Schematic diagram of the sample structure.

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671

Fig. 3. Schematic diagram of the experimental setup used in the present work.

In order to be able to pump the laser device electrically, contacts for current injection were deposited at the bottom and O thick on the top of the sample consisting of GaAs 1000 A − 18 −3 ++ layers, n doped (1 × 10 cm ) using Si and p -doped (1018 –1019 cm−3 ) using Be, respectively, and also all layers in the active region of the sample including the QW were doped at a level of 1017 cm−3 . The cladding layers were O thick superlattices separated from the AlAs/GaAs 8500 A contact layers by AlAs/GaAs digital grading structures for the bottom and top, respectively. The whole structure was grown on n+ GaAs substrate. Laser bars for optical pumping with lengths of around 1.5–1:8 mm were cleaved from the as-grown (unprocessed) wafer. Electrically pumped lasers were processed on the wafer for gain-guiding with di=erent widths (w) for the current aperture of 8; 14; 26 and 50
m (named as devices 1; 2; 3 and 4). Si4 N3 was used as the insulator and non/alloyed Cr/Au contacts were deposited on the top surface. A AuGe/Ni/Au-alloyed contact was evaporated on the substrate backside. The devices were cleaved to laser bars with lengths between 0.41 and 2 mm. A schematic diagram of the experimental setup used in the present work is shown in Fig. 3. For the di=erent types

of measurements, two di=erent experimental arrangements were used, within the area enclosed by the dotted line. In the con;guration denoted by (I) in the ;gure, the DC current– voltage (I –V ) characteristics at di=erent temperatures as well as the output optical spectra of the devices under either optical or electrical pumping conditions were measured. In the other con;guration denoted by (II), the transverse angular distribution of the far-;eld laser radiation from the devices under either optical or electrical pumping conditions as well as the average output power versus input current density (P–J characteristics) were measured at room temperature. 3. Experimental results and discussions 3.1. Current–voltage characteristics First I –V characteristics of the devices were measured under DC forward bias conditions and typical curves at room temperature are shown in Fig. 4(a). The di=erential resistance of the devices calculated from the slope of the

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N.G. Semaltianos et al. / Optics & Laser Technology 36 (2004) 669 – 676 1 mm × 8 µm 1 mm×14 µm 1 mm×26 µm 1 mm×50 µm

0.08 1.4

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1.2

0.06 DC J(k A/cm2 )

DC current (A)

0.10

0.04

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0.02

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(a)

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2 3 voltage (V)

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0.00 8 6

ideality factor η proportionality constant

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1 mm×50 µm

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6

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3.2. Laser operation characterization

turn-on voltage (V)

-10 series resistance (Ω)

ln (DC current)(A)

-5

190 K (upper left-hand-side inset in Fig. 4(b)). As the temperature is lowered, the apparent ‘ideality factor’ increases beyond 2. This shows at temperatures below around 200 K the increasing importance of the series resistance e=ect, the measured voltage for a certain current, is higher than the true voltage across the diode depletion region due to the voltage drop across the series resistance of the access layers. This is also seen in a plot of the series resistance and turn-on voltage versus temperature shown in the lower right-hand-side inset of Fig. 4(b), extracted from the corresponding I –V characteristics showing that the series resistance increases more than twice for a decrease of the temperature from 200 to 100 K as compared to an increase of the series resistance by only 1.5 times in the region from 300 to 200 K. Furthermore, the increase of the turn-on voltage with temperature is greater than the corresponding increase of the band gap with temperature.

1.0

(b)

-20

100

1

200

250

300

temperature (K)

290K 260K 240K

0

150

2

3

4

voltage (V) Fig. 4. (a) I –V characteristics at room temperature measured under DC bias conditions for laser devices with 1 mm cavity length and ridge widths of 8, 14, 26 and 50
m. Inset shows the corresponding current density (current per unit area) versus voltage (J –V characteristics). (b) ln I –V characteristics for 1 mm×50
m device measured at several temperatures down to 80 K. Insets show the corresponding series resistance and turn-on voltage as well as the ideality factor versus temperature.

high voltages linear part of the curves was around 2 Q and the turn on voltage estimated from the intersect with the voltage axis at I = 0 A of the extrapolated straight lines was around 1:6 V, corresponding approximately to the band gap of the QW Al0:11 Ga0:89 As material at room temperature (Eg = 1:565 eV) for all devices. By plotting the current density (J ) (current per unit area) versus voltage, a single curve is obtained for all devices (inset in Fig. 4(a)). From a logarithmic plot of the current versus voltage (Fig. 4(b)), the ideality factor  in the diode characteristic curve equation, I = I0 eqV=kT , is estimated equal to 2 for low currents below 20
A (corresponding to voltages of the order of 1 V). This means that the recombination current dominates in this region of applied currents. The same value of the ideality factor is obtained in the low currents region of the I –V characteristics measured at low temperatures down to

Lasing action from the devices was examined under pulsed current operation (100 ns; 10 kHz pulses). The output spectrum from the device was recorded for every pump current applied by using a charge coupled device (CCD) detector. Typical spectra at room temperature for devices 1–4 are shown in Figs. 5(a)–(d), respectively, for different applied currents. For device 1 having the thinnest ridge width of 8
m (Fig. 5(a)) for low current of around 0:17 A the electroluminescence (EL) related to the excitonic recombination transition between the fundamental state of conduction band electrons to the fundamental state of valence band heavy holes of the QW (e1 –hh1 ), starts appearing as a broad band centered at 776 nm (1:598 eV). As the current increases further, the EL from the QW grows in intensity but for an applied current of 0:3 A another broad band starts appearing at 722 nm (1:717 eV) as a shoulder in the low wavelength side of the QW band and this corresponds to EL related to the band edge transition from the G layers. By increasing the current further to 0:9 A the EL from the G layers becomes stronger than the EL from the QW and dominates the spontaneous emission spectra. The assignment of the peaks observed in the recorded spectra to the corresponding transitions mentioned above, results from calculations of the QW energy levels using a simple model [9,10]. The calculated energy at 290 K of the e1 – hh1 transition of the QW is 1:598 eV and the band edge excitonic transition of the G layers is 1:738 eV. These energies are very close to the experimentally measured ones mentioned above. Lasing emission from this device at room temperature is not observed, neither from the QW nor from the G layers even for the highest applied current of 7 A. The con;nement provided by the gain is not enough to con;ne in such a narrow width and the mode spreads in the adjacent nonpumped, thus absorbing region. However, lasing from the QW for this device is only observed at

N.G. Semaltianos et al. / Optics & Laser Technology 36 (2004) 669 – 676 10 10 10

5

4

QW

(a)

3

10 10

10 10 10 10

intensity (arb. units)

1 mm×8 µm

G

10 10 10 10 10 10 10 10 10

2

7A 2A 0.9 A 0.3 A 0.17 A

1

7 6 5 4

1.98 A (×0.036) G 1.92 A 1.84 A 1.53 A 1.50 A 1.32 A 0.37 A

QW

1 mm×14 µm

(b)

3 2 1 6

1 mm×26 µm

QW 5 4 3

(c) 0.52 A 0.46 A 0.37 A

2 1 0

10 8 10

10 10 10 10 10 10

7 6 5

QW

1 mm ×50 µm

0.8 A (× 3.04e-05) 0.705 A (×0.085) 0.7 A (× 0.18) 0.6 A 0.51 A 0.46 A

(d)

4 3 2

700

720

740

760

780

800

wavelength (nm) Fig. 5. Spectra recorded for di=erent pulsed pumping currents (100 ns; 10 kHz) below and above threshold from laser devices: (a) 1 mm×8
m, (b) 1 mm × 14
m, (c) 1 mm × 26
m and (d) 1 mm × 50
m.

temperatures below around 230 K. Above this temperature lasing is observed ;rst from the G layers, and then, as the pumping current increases further lasing from the QW starts appearing in the spectra at 756 nm. Fig. 5(b) shows spectra recorded from device 2 with ridge width of 14
m. In this case for small currents below about 0:37 A the EL from the QW is stronger than the EL from the G layers. As the current increases, the EL from the G layers grows up in intensity and for an applied current of around 1:5 A (corresponding to a threshold

673

G =10:7 kA=cm2 ) a weak peak starts current density of J(thr)(2) emerging from the high wavelength side of the G layers EL peak, at around 730 nm. Note that the EL from the QW does not change much in the same region of applied currents. As the current increases further up to 1:84 A this peak grows in intensity and narrows in width. For currents greater than 1:84 A (corresponding to a threshold current density QW of J(thr)(2) = 13:1 kA=cm2 ) lasing action related to the QW starts appearing. The lasing spectrum consists of the main peak at 775 nm and weaker side peaks at 778 and 772 nm. For this device 2, lasing action is ;rst observed from the G layers rather than from the QW and this is probably due to the fact that because of the narrow ridge width the threshold current density required to obtain an inverted population in the QW is higher than the corresponding current density required to obtain an inverted population in the G layers. For device 3 with ridge width of 26
m (Fig. 5(c)) lasing related to the e1 –hh1 transition of the QW is observed at 776 nm for an applied current of 0:46 A corresponding QW to a threshold current density of J(thr)(3) = 1:8 kA=cm2 . For higher current of around 0:52 A another weaker peak appears at 769 nm and this is probably stimulated emission related to the transition from the lowest conduction band electron state to the highest light hole valence band state of the QW (e1 –1h1 ) due to the change of the peak gain distribution with pumping power. For device 4 with ridge width of 50
m (Fig. 5(d)) the lasing peak appears at 775 nm for an applied current of QW 0:7 A corresponding to threshold current density of J(thr)(4) = 1:4 kA=cm2 . For higher currents lasing from the G layers appears in the spectra. By testing a number of di=erent chips the lowest threshold for device 4 was measured to be equal to 1:13 kA=cm2 . Next we have measured the threshold current densities versus sample temperature and data in the temperature region 140–300 K are shown in Fig. 6(a) for device 1 and in Fig. 6(b) for device 4. The threshold current densities were determined by recording the spectra with the CCD detector at each pumping current and then calculating the integrated intensity of the QW laser peak. Note that using the standard power meter detector will result in overestimation of the threshold, since in our specially designed device, lasing is also observed from the G layers, simultaneously or even before the lasing from the QW (especially for device 1 at high temperatures). From the linear part of the curves in Figs. 6(a) and (b) at high temperatures, the characteristic temperatures T0 in the empirical relation which expresses thresholds in the exponential form versus temperature (Jthr (T ) = J0 eT=T0 ) are estimated equal to 48 and 260 K for devices 1 and 4, respectively. The lower characteristic temperature of device 1 compared to device 4 indicates that the threshold for this device is more sensitive to temperature changes which may be due to the fact that the heating e=ects for device 1 which has narrower ridge are more important than for device 4 which has wider ridge.

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N.G. Semaltianos et al. / Optics & Laser Technology 36 (2004) 669 – 676 100

50

2.00 mm×50 µm 1.80 mm×50 µm 1.50 mm×50 µm 1.30 mm×50 µm 1.00 mm×50 µm 0.80 mm×50 µm 0.41 mm×50 µm

7 6 4

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(a)

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30

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300x10 -1

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ηd

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threshold current density (kA/cm

2

)

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1.02.0x10-3

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10

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intensity (arb. units)

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threshold current density (kA/cm )

1.5 mm×50 µm

Fig. 7. P–J characteristics for laser devices with di=erent cavity lengths between 0.41 and 2 mm measured under pulsed current operation (170 ns; 10 kHz). Inset shows inverse di=erential quantum eKciency versus cavity length.

50 4.3 °C 22.7 °C 30.2 °C 40 °C 50.5 °C 60 °C 70.2 °C 80 °C 90.3 °C

40 30 20 10

(c) 0 740

760

780

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peak position (nm)

800 795 790 785 780 lasing pk. pos.

775 770

e1-hh1 (theory )

(d) 0

20

40

60

80

100

temperature (°C)

Fig. 6. Threshold current densities versus temperature from 140 K up 300 K for: (a) a 1 mm × 8
m device and (b) a 1 mm × 50
m device. (c) Lasing spectra recorded from a 1:5 mm × 50
m device at several temperatures between 4:3◦ C and 90:3◦ C. (d) Circle points correspond to lasing peak position versus temperature extracted from the spectra in (c), while solid line indicates the energy of the e1 –hh1 transition of the QW calculated theoretically using a simple model.

The high-temperature operation of the laser devices was tested, from room temperature up to around 100◦ C and typical spectra are shown in Fig. 6(c) for a 1:5 mm × 50
m device. The laser peak position versus temperature shown in Fig. 6(d) follows the theoretically calculated e1 –hh1 transition energy for the QW in the whole temperature region. The observed lasing peak is always higher in energy by

∼8 meV compared to the e1 –hh1 transition due to the band ;lling e=ect. A di=erence (of around ∼13 meV) between the lasing peak position and the e1 –hh1 transition was also observed previously, in the case of the optically pumped structure, at temperatures below around 52◦ C [8]. At temperatures above 52◦ C the laser peak was observed at energies higher than the peak related to the e1 –hh1 transition due to the broadening of the observed transitions with temperature. Finally, we have measured the parasitic losses due to free carrier absorption in the cladding layers of our laser sample structure under electrical pumping operation. Data of the average output power for laser device 4 (having the lowest threshold) versus input current density (P–J characteristics) are shown in Fig. 7 measured for devices with di=erent cavity lengths between 0.41 and 2 mm mounted epilayer side down, by applying current pulses of 170 ns duration and frequency of 10 kHz. The maximum power coupled was about 50
W and the con;guration of the detector was kept the same for all the devices with di=erent lengths so as to maintain the same coupling eKciency. From the slope of the initial straight line parts, just above threshold, of the corresponding P–I curves (where I is the applied current), we can estimate the di=erential external quantum eKciency de;ned as d = (dP=h)=(dI=q) and a plot of −1 d versus cavity length L is shown in the inset of Fig. 7. Then, according −1 to the relation −1 d = i [1 + (p =ln(1=Rm )L], assuming a mirror reBectivity of Rm = 0:3, the slope of the straight line gives the parasitic losses equal to p = 1:5 cm−1 (the parasitic losses also include losses due to free carrier absorption in the contact layers, roughness, etc. [9]). The losses

N.G. Semaltianos et al. / Optics & Laser Technology 36 (2004) 669 – 676

measured here are quite low compared to a usual GaAs/AlGaAs laser diode where parasitic losses of around 10 cm−1 are measured [9].

1.2

675

1.5 mm×50 µm, I=1.02 A I T E (θ)

1.0

2

I TE (θ) 0

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I TE (θ) 1

0.6

3.3. Laser mode

0.2 0.0 1.2

signal (arb. units)

Finally, we have measured the transverse angular dependence of the far-;eld laser radiation from our devices. A typical measured pro;le (normalized to unity at the maximum signal) is shown in Fig. 8(a) for a 1:5 mm × 50
m laser device pumped electrically with pulsed pumping current of 1:02 A (182 ns; 10 kHz) (circle points). Zero degree angle corresponds to the direction perpendicular to the output facet of the laser device. The pattern consists of two main lobes with maxima at ±20◦ (inner lobes) and two weaker ones with maxima at around ±60◦ (outer lobes). For comparison on the same graph, curves are shown for the transverse angular pro;les of the fundamental (TE0 ) (dashed line), ;rst-order (TE1 ) (solid line) and third-order (TE2 ) (dotted line) laser modes, calculated theoretically using the theory of Rayleigh–Sommerfeld di=raction integrals [11] as explained in detail in Ref. [8]. It is readily seen that the measured pro;le does not follow any of the theoretically calculated patterns. To examine the laser mode under optical pumping of the same sample structure used for current injection, laser bars were cleaved from the as-grown (unprocessed) wafer with cavity lengths in the order of 1.5–1:8 mm and pumped transversely, optically using a pulsed Nd:YAG laser (532 nm; 6 ns; 10 Hz). A typical angular pro;le of the output radiation normalized to unity at the maximum signal) is shown in Fig. 8(b) under pumping power density of 374:4 kW=cm2 (crossed squares). Note that the threshold for this sample was measured to be equal to 125:3 kW=cm2 which is almost six times higher than the threshold measured for the corresponding sample made without any contact layers for optical pumping only [6]. It is immediately seen that this pro;le is identical to the one measured under electrical pumping of the processed sample bars (Fig. 8(a)). In pro;les measured from bars cleaved from a region close to the edge of the wafer (crossed circles in Fig. 8(b), the two wide angle lobes (at ±60◦ ) appear very weak. This is probably due to a slight decrease of the width of the waveguide at those regions, which in turn leads to a change in mode con;nement. It must be emphasized that as it has already been mentioned in Section 3.2 of the paper, for high enough pumping powers a peak at around 726:3 nm appears in the optical spectra, corresponding to lasing from the G layers. The threshold for the G peak lasing for this sample was measured equal to 382 kW=cm2 ; thus, care was taken such that the angular pro;les shown in Fig. 8(b) could have been measured without the G peak present in the spectra. This avoids any contributions to the measured far ;elds from lasing originating from the G layers where the TE1 mode has the higher overlap [8]. For comparison, in

0.4

(a) as grn, L=1.64 mm, 374.4 kW/cm2 as grn (edge), L=1.64 mm, 233.8 kW/cm 2

1.0 0.8 0.6 0.4 0.2 0.0 1.2

(b)

2

opt. pump., L=1.56 mm, 30 kW/cm

1.8 mm×50 µm, I=1.05 A opt. pump., L=2 mm, 233.83 kW/cm2

1.0 0.8 0.6 0.4 0.2

(c)

0.0 -100

-50

0

50

100

angle (°C) Fig. 8. (a) Dashed, solid and dotted lines correspond to theoretically calculated transverse angular pro;les of the far-;eld laser radiation using the theory of Rayleigh–Sommerfeld di=raction integrals for the fundamental (TE0 ), ;rst (TE1 ) and third (TE2 )-order laser modes, respectively. Circle points correspond to a pro;le measured from a 1:5 mm × 50
m device pumped electrically with pulsed current of 1:02 A (182 ns; 10 kHz). (b) Crossed squares and circles correspond to angular pro;les measured from a bar with cavity length L = 1:64 mm cleaved from the center and edge, respectively, of the as-grown (unprocessed) wafer made for current injection, but pumped optically with a pulsed Nd:YAG laser (532 nm; 6 ns; 10 Hz). Solid squares correspond to data under optical pumping measured from a laser bar with cavity length L = 1:56 mm cleaved from the corresponding wafer made for optical pumping only, and solid line indicates ;tting to the data the expression 0:846ITE2 ()+0:055ITE0 ()+0:098ITE1 () taken from Ref. [8]. (c) Circle points correspond to a pro;le measured from a 1:8 mm × 50
m device pumped electrically with pulsed current of 1:05 A (182 ns; 10 kHz) while square points correspond to a 2 mm × 50
m device processed normally for electrical pumping but pumped optically.

Fig. 8(b) is also shown a pro;le measured from the sample made for optical pumping only (solid squares) taken from Ref. [8]. We have also pumped optically (on top of the metal contact surface layers) a processed sample made for current injection and angular pro;les are shown in Fig. 8(c) under electrical pumping as well. The same characteristic pattern is observed in all cases. The fact that the angular mode pro;le from the structure made for current injection under optical pumping is identical to the pro;le measured

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from the same structure when it is pumped electrically simply means that this ‘di=erent’ way of pumping that of the electrical one is not responsible for the absence of the third-order mode emission in the measured patterns. To understand better the di=erence, if any, between the two ways of pumping, optical or electrical, it is now interesting to review analytically the detailed physical processes of carrier injection, electron–hole pair generation and recombination which are occurring in the two ways of pumping of the same sample structure made for current injection. Considering ;rst electrical pumping of the sample, before current injection there is a number of holes in the valence band in the p-type cladding layers due to thermal ionization of acceptors and a number of electrons in the n-type cladding layers due to thermal ionization of donors. When current is applied, extra holes are injected in the p-type region from the positive contact and the opposite for electrons. These carriers di=use under the inBuence of the external electric ;eld into the QW where they recombine and emit laser radiation. Thus in the electrically pumped case carriers are not created in the active region but they are coming from the contacts. In the case of optical pumping, however electron– hole pairs are created in the cladding layers. The holes in the p-type region will add up to the already existing holes but the electrons created in the p-type region will initially drift towards the active region of the sample and accumulate into the QW and the opposite for the electrons. Lasing is not observed yet because the carrier concentration is not high enough to create an inverted population in the QW. As the pumping power increases under conditions of continuous photopumping an excess amount of holes is created in the p-type region and an excess amount of electrons in the n-type region which eventually di=use into the QW creating an inverted population and emit laser radiation. Thus in the optically pumped case, initially for low pumping powers there is a small current from the p-type to the n-type region which is later compensated by the current from the n-type to the p-type region like in the electrically pumped case. Also, the number of free carriers in the cladding layers is initially higher but the di=erence compared to the carrier population which already exists in the cladding layers is negligible as it is also con;rmed by theoretical stimulations of the structure. The ;nal conclusion is that the two ways of pumping, electrical and optical are equivalent in terms of the physical processes of carrier injection, electron– hole pair generation and recombination. This can explain the identical patterns which we measure for the far-;eld of the laser mode in the two cases of pumping and shows that the laser mode pro;le does not depend on how the material is pumped. Furthermore, since the undoped sample made for optical pumping only, emits correctly laser radiation on the third-order mode, as compared to the doped sample made for current injection, it is concluded that the absence of the third-order mode is due to large free carrier losses for this mode in the cladding layers. This in turn indicates that the cladding layers of the structure still need to be optimized in

doping and thickness, in order to reduce the internal losses for the third-order mode. 4. Conclusion In conclusion, we have grown and characterized under electrical as well as optical pumping a structure which corresponds to the optically pumped third-order mode semiconductor laser which we had previously demonstrated. The far ;eld of the laser device was measured to consist of two ‘inner’ lobes with maxima at ±20◦ and two ‘outer’ ones with lower intensities and maxima at ±60◦ . Under both electrical and optical pumping of the same structure made for current injection, identical transverse angular pro;les are measured for the output laser radiation which means that the di=erent ways of pumping, that of the electrical one as compared to optical pumping, is not responsible for the absence of the third-order mode emission in the measured pro;les. These data as well as previous data obtained on the sample made for optical pumping only, indicate that the cladding layers of the structure still need to be optimized in doping and thickness, in order to reduce the internal losses for the third-order mode. Acknowledgements This work was performed within the framework of the European Community (EC) project ‘QUCOMM’ (Long Distance Photonic Quantum Communications). One of the authors (N.G.S.) acknowledges a Marie Curie Research Fellowship of the EC, Grant No. IST-1999-80034 under the program ‘QLOPORT’ (Quantum Well Laser Optical Parametric Oscillator at Room Temperature). References [1] Kwiat PG, Mattle K, Weinfurter H, Zeilinger A, Sergienko AV, Shih Y. Phys Rev Lett 1995;75:4337. [2] Bouwmeester D, Pan J-W, Mattle K, Eibl M, Weifurter H, Zeilinger A. Nature 1997;390:575. [3] Pan JW, Bouwmeester D, Daniell M, Weinfurter H, Zeilinger A. Nature 2000;403:515. [4] Wagner HP, Wittmann S, Schmitzer H, Stanzl H. J Appl Phys 1995;77:3637. [5] JVager M, Stegeman GI, Flipse MC, Diemeer M, MVohlmann G. Appl Phys Lett 1996;69:4139. [6] De Rossi A, Semaltianos NG, Chirlias E, Vinter B, Ortiz V, Berger V. Appl Phys Lett 2002;80:4690. [7] Semaltianos NG, Vinter B, De Rossi A, Berger V, Ortiz V. J Appl Phys 2002;92:1262. [8] Semaltianos NG, De Rossi A, Vinter B, Berger V, Ortiz V. J Appl Phys 2002;92:2242. [9] Rosencher E, Vinter B. Optoelectronics. Cambridge: Cambridge University Press; 2002. [10] Weisbuch C, Vinter B. Quantum semiconductor structures. Boston: Academic Press; 1991. [11] Born M, Wolf E. Principles of optics. Oxford: Pergamon Press; 1980.