Simultaneous in-plane and vertical low threshold laser operation in a deep-etched InP-based two-dimensional photonic crystals

C. Cojocaru*a,c, F. Raineria, G-H. Duanb, R. Raja, A. Levensona aLaboratoire de Photonique et de Nanostructures (CNRS-UPR 20), Route de Nozay,

91460 Marcoussis, FRANCE b OPTO+, Alcatel Research &Innovations, Route de Nozay - 91460 Marcoussis, FRANCE

c Universitat Politècnica de Catalunya, Departamento de Física i Enginyeria Nuclear, Colom 11, 08222 Terrassa, SPAIN

ABSTRACT Simultaneous emission in the guided (in-plane) and vertical (perpendicular to the photonic crystal periodicity) directions is demonstrated in a deep-etched defect-free two-dimensional InP-based semiconductor photonic crystal. A laser threshold of an average intensity of 15 kW/cm2 is measured at room temperature for a pump diameter corresponding only to 9 periods of the photonic crystal. The laser characteristics as a function of pump level are particularly discussed.

Keywords: photonic crystals, semiconductor micro-lasers, integrated optics

1. INTRODUCTION Since the beginning of the semiconductor lasers, an immense effort has been devoted to the lowering of laser threshold and size with the aim of increasing their potential in integrated active optics. Undoubtedly, two-dimensional (2D) photonic crystals (PC) lasers hold a lot of promise in this context. So far, two types of 2D PC lasers have been experimentally demonstrated. The first one, called defect-mode laser, is based on trapping light in optical defects created in an uniform periodic pattern. Its principle relies on the possibility of achieving a high quality-factor resonator within these defects.1,2 In this work we explore a second type of 2D PC laser, called Bloch mode lasers, whose principle is based on the increase of the density of optical modes at the photonic band edges.3-6 This offers the possibility of reducing the laser threshold by taking advantage on the lowering of the group velocity which can be interpreted as a 2D feedback mechanism.7,8 We particularly investigate here the deep-etched configuration, where the photonic crystal and its lower refractive index substrate are deeply etched. This configuration is very well suited for integrated optics. We demonstrate for the first time simultaneously laser emission in both guided (in-plane) and vertical (perpendicular to the PC periodicity) directions.

2. SAMPLE AND EXPERIMENTAL SET-UP

We have designed a triangular-lattice of circular air holes drilled in a low index contrast InP/GaAsInP/InP monomode waveguide. Deep circular air holes are drilled by an optimized process of electron beam lithography, reactive ion etching and inductively coupled plasma etching. By deeply etching the 2D PC into a low index contrast waveguide, the optical guided mode is confined into few µm. Special attention has to be paid to their fabrication: very deep and anisotropic etching process is necessary to avoid the scattering into the waveguide cladding. Figures 1a and 1b show a top view and a section view, respectively, of the fabricated 2D PC. The sample exhibits 4,5µm deep air holes with a reasonably uniformity of the hole shape, depth and diameter, with an aspect ratio higher than 12.

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Figure 1. Scanning electron microscope image of the fabricated deep-etched triangular lattice InP-based 2DPC

(a) top view; (b) section view

The period a of the lattice varies between 490 and 620 nm with a constant filling factor f=0.2. These particular parameters are chosen so that a very low group velocity mode occurs at the Γ point around the emitting wavelength of the active medium (here λgap). The calculated TE photonic bands diagram of the structure is shown in Figure 2. The flatness of the band near this region attests for a low group velocity and as a consequence, for a high density of optical modes. The laser emission is expected at the wavelength (i.e. around 1,1µm) of the mode denoted by circle and pointed by an arrow on Figure 2.

Figure 2. Photonic band structure of the 2D PC triangular lattice with a period of 570 nm and a filling factor of 20%. calculated by a two-dimensional plane-wave expansion method. The circle points the photonic band region where laser emission is expected.

For each period, both ΓM and ΓK orientated structures are fabricated during the same run of technological processes. A general view of the sample is represented in Figure 3. For the laser effect described here the active material is GaxAs1-

xInyP1-y, (x=0.2 and y=0.42), with λgap=1.18 µm.

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The sample is optically pumped at room temperature, in the normal direction to the photonic crystal surface. 120 fs pump pulses at 810 nm were delivered by a Ti:Sapphire laser with 80 MHz repetition rate. The pump beam is focused down to 5µm spot diameter on to the sample surface via an achromatic microscope objective, covering ∼9 units cells of the photonic crystal. The substrate of the sample is cleaved very close (< 15 µm) to the region where the 2DPC is situated (see figure 3) to enable the detection of the in plane emission. Guided and vertical emitted light are simultaneously collected using microscope objectives and coupled into optical spectrum analysers via monomode optical fibres.

3. EXPERIMENTAL RESULTS

As the pump intensity reaches the threshold value, laser emission is detected simultaneously in the vertical and guided directions. For the 2D PC having a lattice period a1=570 nm, a sharp laser peak is observed at 1178 nm for both detection directions. The full width at half maximum of this peak is 1.6nm. Figure 4 shows a spectrum of the laser peak (Figure 4a) and the spatial distribution of the laser emission (Figure 4b) that occurs in this structure for a pump power of 15.8 kW/cm2, both corresponding to the laser emission in the vertical direction.

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Figure 4. (a) Laser emission spectrum (b) Spatial profile of the laser emission

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The measurement of the laser wavelength shows very good agreement with the computed photonic diagram of the structure (a/λ=0.49 which gives λ=1163nm) and confirms the very good quality of our sample. We have measured the in-plane laser emission spectra for different 2D PC periods: a1=490 nm, a2=530 nm and a3=570 nm. The operating wavelengths are found to be λ1=1115.7 nm (curve a in fig.5), λ2=1143.8 nm (curve b in fig.5), and λ3=1178 nm (curve c in fig.5). As the lattice constant is decreased, the laser operating wavelength diminishes. This is perfectly explained by the dependence of the low group velocity mode wavelength, at the origin of the laser effect, on the lattice constant and filling factor.

Figure 5. Laser spectra corresponding to the guided configuration. The three peaks plotted in this figure correspond to different periods of the 2D PC structure: (a) a1=490 nm, (b) a2=530 nm and (c) a3=570 nm.

The output peak power versus the pump intensity is plotted on Figure 6a for vertical emission, and on Figure 6b for the in-plane emission in ΓK direction. We have obtained similar results for the ΓM orientated samples. The laser threshold is measured for pump intensities around 15 kW/cm2 for both vertical and in-plane detection directions. The laser emission is observed to be almost linearly polarized in the in-plane direction, with the electric field parallel to the air holes (TE polarization). Note that this is the same polarization as the one considered in the numerical simulations (see Figure 2). On another hand, the vertical emission is elliptically polarized for both the ΓM and ΓK orientated structures. In all directions of the detection, laser operation exhibits saturation behaviour.

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Figure 6. Laser peak power as a function of the pump intensity for a 2D PC with a constant lattice of 570 nm and f=20%. (a) laser emission detected in vertical direction; (b) laser emission detected in the ΓK in-plane direction. We have included several laser emission spectra corresponding to different experimental point on these two curves.

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In order to have an insight of the origin of this saturation we have measured the spectra of the laser emission at different pump levels. Some examples are provided in Figures 6a and 6b. The first main feature is that no appreciable red shift is observed. This clearly shows that thermal effects are negligible. A second observation is that the laser spectra broadens as the pump power is increased. This is the usual power broadening. Finally, for powers higher than 60kW/cm2 a second peak appears as shown in figure 7. Whether this second peak is due to higher order transverse modes or to the lift of the degeneracy at the Γ point deserves further investigations.

Figure 7. Laser spectra corresponding to a lattice constant of 490nm and filling factor of 20%, when the pump intensity was 60kW/cm2.

4. CONCLUSIONS

We have demonstrated both guided and vertical room temperature band-edge laser operation from a non defective deep-etched 2DPC. The small laser threshold of 15 kW/cm2 is obtained for only 9 photonic crystal unit cells, meaning that it could be further decreased by increasing the number of excited periods. We experimentally investigated the spectral evolution of the laser emission as a function of the pump power. No appreciable red shift is measured, meaning that deep etched 2D PCs are able to efficiently manage the thermal problems. The demonstration of simultaneous emission is obtained in the guided (in-plane) and vertical (perpendicular to the PC periodicity) directions at the same wavelength opens an easy road to the exploitation of 2DPC integrated all optical circuits.

REFERENCES

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* e-mail:crina.maria.cojocaru@upc.es; phone 34 93 7398745; fax: 34 93 7398000