Continuous-wave operation of photonic band-edge laser at 1.55 µm on silicon wafer

G. Vecchi, F. Raineri, I. Sagnes, A.M. Yacomotti, P. Monnier, R. Braive, S. Bouchoule,

A. Levenson and R. Raj Laboratoire de Photonique et de Nanostructures (CNRS UPR20), Route de Nozay, 91460

Marcoussis, France

ABSTRACT

We present continuous-wave laser operation at room temperature at 1.55 µm by optically pumping a photonic crystal structure containing an InGaAs/InP quantum well active layer. The active layer is integrated onto a Silicon chip by means of Au/In bonding technology. This metallic layer provides the reduction of heating by thermal dissipation into the substrate, and increases the quality-factor by reducing the radiative losses.

Keywords: photonic crystals, lasers, vertical emitting lasers.

1. INTRODUCTION Photonic-crystal (PhC) structures incorporating active layers open new perspectives for performing optical functionalities in compact integrated photonic circuits. A lot of effort has been devoted towards efficient PhC laser sources where the laser operation is based either on defect-nanocavity [1-2] or on band-edge light confinement effects [3-4]. PhC laser operation was mostly obtained under pulsed excitation, both optical [1-4] and electrical [5-6]. However, for future integrated photonic applications, continuous-wave (CW) laser operation is of paramount importance. Two-dimensional (2D) distributed feedback surface emitting laser, operating under CW current injection at room temperature was reported [7], where the active layer is separate from the grating layer. Here, the distributed feedback from the PhC to the lasing mode is weak, but this is especially suited to sustaining laser operation over a broad area. In order to realize more compact devices, it is necessary to exploit a stronger feedback from the corrugated structure, by etching completely the active medium with the photonic lattice. However, patterning air-holes through the quantum wells reduces significantly the carrier lifetime, leading to higher thresholds. The consequent increase in the required pumping makes the heat sinking under CW operation an even more challenging task. Recently, careful choice either of the active layer or the heat sink materials has allowed room temperature CW operation of PhC lasers in defect-cavity configuration. For instance, the superior carrier confinement properties of an InAs/GaAs quantum dot active medium were exploited to provide PhC laser emission at 1.3 µm in a suspended 2D PhC slab [8]. Alternatively, a suitable choice of bonding materials with efficient heat dissipation, such as sapphire and AlOx, allowed CW PhC lasing near 1.55 - 1.6 µm [9-10].

Here we report on the first, to the best of our knowledge, continuous-wave operation of a PhC band edge laser at telecom wavelength at room temperature [11]. The structure is composed of a III-V semiconductor layer on Silicon. The active InP/InGaAs PhC slab is transferred on to Silicon by Au/In metallic dry bonding [12]. The metallic layer provides us simultaneously with two advantages, a high-reflectivity giving an improved photon confinement inside the PhC resonator by interference mechanism [13], and a high thermal conductivity leading to an efficient thermal dissipation through the substrate. The present work is a timely contribution to the rapidly developing domain of hybrid structure devices. Such systems have recently received a lot of attention due to the exciting possibility that they offer for monolithically integrating Silicon and active III-Vs compact devices into a unique architecture [14-16].

*gabriele.vecchi@lpn.cnrs.fr; phone +33 1 69636226; fax +331 69636006; http://www.lpn.cnrs.fr

Photonic Materials, Devices, and Applications II, edited by Ali Serpengüzel,Gonçal Badenes, Giancarlo Righini Proc. of SPIE Vol. 6593, 659322, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.724170

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2. DESIGN AND FABRICATION OF THE STRUCTURE The 2D PhC structure was designed to have a Bloch mode with high Q factor near 1.55 µm, for strong light confinement in the range of wavelengths suitable for telecom applications. The in-plane wave vector was chosen to be in the proximity of the Γ point of the Brillouin zone for enabling near vertical device operation. The scheme of the hetero-structure is shown in Fig. 1(a).

Fig. 1. (a) Schematic hetero-structure. (b) SEM top image of one PhC sample.

The III-V material layer was epitaxially grown by MOCVD. It consists of a 265 nm thick InP slab incorporating four lattice-matched In0.53Ga0.47As QWs, emitting near 1.53 µm. The photonic mode of interest is intrinsically coupled to the radiative modes. One way to reduce these losses and, consequently, increase the photon lifetime in the resonator consists in placing a high reflectivity layer below the PhC structure. The distance between the bottom mirror and the active layer was judiciously chosen with the view to induce destructive interference of vertical radiative losses [13], in order to enhance the quality factor of the Bloch mode resonator. We chose SiO2 as the material spacer, to benefit from the high index contrast between the InP slab and SiO2 spacer, leading to a strong optical confinement within the active part of the PhC slab. The thickness of SiO2 layer is ≈ 785 nm. The metallic mirror is made of an Au/In alloy, and accomplishes further tasks, such as raising the collection efficiency of light from the top surface of the structure. However, the most important aim of the metallic mirror is to bond the III-V PhC layer on top of a Silicon substrate. As compared to previously proposed solutions to improve the photonic band edge laser performance, by using either highly reflective Bragg [13,17] or gold mirrors [18], the present configuration allows the simultaneous improvement of both the optical and thermal behavior of PhC-based devices, leading, in particular, to evacuate more efficiently the heat from the active layer towards the substrate. After having been transferred onto the silicon substrate by Au/In bonding, the InP layer was fully drilled by a 2D graphite PhC structure of air holes. The graphite lattice constant is 745 nm and the hole diameter is 310 nm. The silicon nitride mask for patterning the air holes is fabricated by e-beam lithography followed by reactive ion etching and the semiconductor was processed by inductive plasma etching. Fig. 1(b) shows the SEM top image of the fabricated PhC structures.

3. EXPERIMENTAL RESULTS We first studied the linear and the nonlinear response of the PhC experimentally. Experiments were performed at room temperature using a pump and probe set-up [17]. 130 fs-long pulses at 810 nm provided by a Ti:Sapphire laser are used for the pump, and the 150 fs-long probe pulses around 1550 nm from an optical parametric oscillator (OPO) are used for the linear characterization. For nonlinear experiments the temporally synchronized pump and probe beams are at normal incidence to the PhC surface and are linearly polarized. This is important particularly for the probe in order to optimally couple the light into the structure. The repetition rate of the pulses is 82 MHz. The pump and probe beams are focused down to 30 µm and 15 µm spot diameter, respectively, via a microscope objective. We collected the retro reflected beam in the direction perpendicular to the PhC through the same objective and focused it onto a single mode optical fiber

265 nm InP layer with four wells

BCB bonding layer

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connected to an optical spectrum analyzer (OSA). The pump wavelength was chosen to be 810 nm in order to excite the electronic states high into the conduction bands. Subsequently, we characterized the emission of the PhC band edge laser by pumping it optically with a CW diode laser emitting at 830 nm. The pump beam is focused at normal incidence on the sample surface using a 10X microscope objective. The spot size of CW pump is about 15 µm. The laser radiation emitted in the vertical direction is analyzed using the OSA, which has resolution of 20 pm. Fig. 2 shows the scheme of the experimental apparatus used for non-degenerate pump and probe characterization at normal incidence.

Fig. 2. Optical layout for pump-probe characterization at normal incidence.

3.1 Pump and probe reflectivity

In Fig. 3(a) we show the normalized reflectivity spectra as measured using ultra fast pump and probe pulses. The spectra are normalized by using the gold reflectivity spectrum as reference. First, the inset shows the linear (pump-off) reflectivity spectrum: no evidence of photonic resonance is visible in the range between 1547 nm and 1555 nm. Then, by pumping the system with 810 nm pulses at increasing fluence, a resonant dip appears in the reflectivity spectra, when the value of fluence is around 0.65 µJ/cm2. By increasing the pump further, the spectral dip is observed to become narrower and more pronounced.

Fig. 3. (a) Reflectivity spectra for different values of pump fluence. The inset shows the linear (pump-off) spectrum. (b) Wavelength shift versus the pump fluence.

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This observation is related to the presence of QW absorption. Indeed the resonance is initially broad due to absorption and its width progressively diminishes as the system approaches the transparency regime. This is evidenced when 810nm-pump beam is injected: the resonance width is reduced for increasing values of pump fluence, and, simultaneously, its central wavelength blue-shifts. The narrowing of the resonance is due to saturation of the absorption, while blue-shifts correspond to electronically-induced decrease of the refraction index. The reflectivity dip reaches its maximum contrast when the total material absorption at the resonant wavelength is saturated. This occurs in the present case for a very low fluence of about 0.84 µJ/cm2. At this point, the transparency condition is reached and the quality factor associated with the resonance gets to its maximum value. Fig. 3(b) reports the shift of the resonance central wavelength as a function of the pump fluence. From the linear fit, the intersection with zero pump axis would occur at the wavelength position of 1554.5 nm. As shown in the inset of Fig. 3(a), no dip is visible at that wavelength in the pump-off spectrum, due to the effect of QW absorption at the resonant wavelength.

Fig. 4. Reflectivity spectrum near to the transparency condition for the active material. The inset zooms in the resonant dip, and the corresponding Q factor is reported.

Fig. 4 reports the reflectivity spectrum in the pump-probe regime at the condition of transparency. The inset zooms in the very sharp photonic feature, which is responsible for CW lasing operation. The quality factor of the photonic resonance is determined from its spectral linewidth ∆λ at half height (Full Width Half Maximum, FWHM), leading to a value of Q = λ / ∆λ up to 13500, with λ = 1550 nm and ∆λ being about 115 pm. This is one of the highest Q factor every reported in literature associated with the leaky photonic modes in defect-free 2D photonic crystals [13,18].

We also studied the dependence of resonance shift with respect to temperature by placing the sample on top of a Peltier mounting stage. Fig. 5(a) displays the pump-probe reflectivity spectrum of the photonic resonance, recorded near the transparency condition for different values of the control temperature. The central wavelength we measured by changing the temperature from 23°C down to 13°C: the resonance blue shifted correspondingly from 1549.53 nm to 1548.78 nm, leading to 0.077 nm/°C temperature rate of change (from the linear fit in Fig. 5(b)). This value is similar to other results in literature [19] obtained from fully InP-based devices.

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Fig. 5. (a) Pump-probe reflectivity spectra under transparency condition at four different values

of room temperature. (b) Dependence of the central wavelength position on the temperature.

3.2 Photonic crystal laser emission

We detected the laser emission under different pumping regimes, pulsed, quasi-CW and CW, to compare the thermal effect in each case. The pulsed pump was supplied by the Ti:Sa ultra fast source. The output of a diode laser module with or without intensity modulation provided the quasi-CW and CW pumps, respectively. Measurements reported in this section were done without mounting the sample on the Peltier stage. Fig. 6(a) shows the spectrum of the laser emission, centered near 1550 nm, obtained in the quasi-CW regime, to reduce thermal load and increase, if necessary, the damage threshold. Indeed, we modulate the diode laser to have 300 µs of pumping with a period of 3 ms (i.e. 10% duty cycle). The value of FWHM of the emission is as small as 60 pm. Fig. 6(b) shows the far field radiation pattern as recorded on a CCD infrared camera. The lasing photonic mode displays a doughnut-shaped emission pattern [7]. This is attributed to the fact that the photonic mode does not couple to a radiative mode at exactly the Γ point [20-21].

Fig. 6. (a) Laser emission spectrum under quasi-CW pump regime. (b) Far field emission pattern of the PhC laser.

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Figure 7 shows the laser output intensity spectrum as measured both under quasi-CW and pulsed pumping.

Fig. 7. Laser emission intensity spectra under quasi-CW (dotted line) and pulsed (full line) pumping regime.

The vertical scale is logarithmic, and the intensity shift of spectra is related to the fact that we set differently the OSA mode acquisition for each pumping regime. The main information of Fig. 7 concerns the wavelength peak position: the laser is emitted at 1550.3 nm and at 1549.5 nm under quasi-CW and pulsed pumping condition, respectively. The relative red shift in wavelength occurring under quasi-CW pumping is related to the additional heating of the structure. Further wavelength red-shift was observed when turning off the modulation of the CW pump. Fig. 8 compares the output intensity spectra of CW PhC laser with and without modulation. Similar to the case of Fig. 7, the vertical axis has a log scale and the same shift in intensity is present. The emission peak in CW regime has red-shifted further to ≈ 1550.7 nm.

Fig. 8. Laser emission intensity spectra with quasi-CW (dotted line) and CW (full line) pumps.

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Unexpectedly, a second emission peak appears at longer wavelength, at 1552.5 nm. It could be related to the presence of another photonic mode. As a conclusive remark, from figures 7 and 8 it turns out that managing the heat dissipation is crucial for rendering the CW laser devices reliable.

3.3 Photonic crystal laser characterization

We performed the characterization of the PhC laser in CW regime, without any modulation of the pump diode, and setting the Peltier controller to have T = 12°C. A nitrogen gas flux was constantly maintained to avoid the condensation of air humidity on the sample surface.

Fig. 9. (a) PhC laser intensity spectra for different excitation power. (b) Reflectivity spectrum of the photonic resonance after sample CW irradiation. (c) Surface image

of the sample using a 100x microscope objective.

Fig. 9(a) shows the output laser spectra for different values of CW pump power, on a logarithmic intensity scale. The small fluctuations of the central wavelength could be related to the heating. In this prototypal PhC device CW-lasing is attained, however, care is taken to avoid heat-related damage when increasing the pump level above threshold. We limited the maximum pump (external) power to about 7 mW. In Fig. 9(b) we show the reflectivity spectrum at the transparency condition as measured after the CW laser characterization. Looking at the spectral characteristics, such as contrast and quality factor, and comparing it to the spectra taken before CW irradiation (like in Fig. 5(a)), we argue that no remarkable alteration is reported owing to lasing. The quality of the sample surface looks preserved after CW irradiation, as the optical check using a 100X microscope objective confirms. Fig. 9(c) shows a magnified image of the central area of the sample surface. Figure 10 plots the output laser versus the excitation power. Data corresponds to the values of peak power from spectra in Fig. 9(a). The laser threshold is evaluated to be about 6.1 mW. The inset of Fig. 10 shows the dependence of the laser linewidth (FWHM) on the excitation power: it decreases above the laser threshold.

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Fig. 10. (a) Laser output power at 1.55 µm as a function of the CW excitation power. (b) Linewidth of emission for different values of excitation power.

In conclusion, we have reported on CW PhC laser surface-emitting at 1.55 µm. The photonic crystal device exploits a high Q photonic mode at the dispersion band edge. The device consists of a hybrid structure, made of an active III-V PhC slab bonded on top of a Silicon chip. The device operation takes advantage of the metallic bonding layer both optically and thermally: as a bottom mirror it helps in increasing the Q factor of the resonator, while it enables more efficient heat dispersion towards the substrate. Both effects have contributed to lowering the threshold of laser operation, and to obtaining CW lasing at room temperature.

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