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2D photonic crystal (2D PC) structures consisting in a square lattice of Indium Phosphide (InP) microrods bonded on a Silicon/Silica Bragg mirror are experimentally investigated. We focus on slow Bloch modes above the light line, especially at the Γ-point where a vertical emission can be obtained. Stimulated emission around 1.5µm is demonstrated in such structures, at room temperature, for the first time. In addition the achieved threshold power lies within the range reported for surface emitting devices based on conventional lattices of holes. It is shown that the laser mode is laterally confined by a carrier induced refractive index change, under pulsed excitation. It is also demonstrated that this type of 2D PC is well suited for sensors integrated in microfluidic sytems.
©2009 Optical Society of America
1. Introduction
Photonic crystals are considered today among of the best platforms for the control of light at the wavelength scale. More specifically, two dimensional (2D) photonic crystals (PC) have been extensively used to fabricate microresonators which give the required optical feedback for laser operation. Two confinement schemes have been studied: photons can be either trapped in a microcavity [1–3] which is a defect in the periodical lattice, or slowed down by a distributed feedback effect in a perfect 2D PC [4–10]. In the latter case, slow Bloch modes (SBM) located at extremes of the band structure are exploited, and microlasers with very low threshold have been demonstrated [8,10]. In both schemes, the 2D PC is commonly a periodic lattice of holes drilled in a thin semiconductor membrane surrounded by a low optical index material (e.g. air or silica). Surprisingly, to our knowledge, microlasers based on a 2D lattice of rods have not been reported. Although this type of structures is less suitable for the fabrication of a 2D PC in a suspended membrane, they can be fabricated, using similar technology, in a semiconductor layer bonded on a low index dielectric (Fig. 1).
Fig. 1. 2D PC structures consisting in (a) a square lattice of high index microrods bonded on a low index layer and (b) a square lattice of air hole drilled in a high index layer.
The use of rods in place of holes could offer the following advantages. On the one hand, when the laser is optically pumped, the resulting carriers are confined in the rods and cannot diffuse outside the pumping area and an accurate control of the carrier injection can be achieved this way. This could be a tool to accurately control the area where stimulated emission occurs, even if the optical Bloch mode is not laterally confined.
On the other hand, considering optofluidic applications, the integration of 2D PC resonators with microfluidic devices has been envisaged and demonstrated [11–14]. In previously published works, where 2D lattices of holes are used, the optical resonator is located at the surface of the fluidic channel [12,14] or liquids are infiltrated in holes [11]. With a lattice of rods, the liquid can flow through the resonator itself and its interaction with the optical mode is reinforced, increasing the sensitivity of the sensor.
In this paper, a surface emitting microlaser, based on a 2D square lattice of InP rods lying on a silica layer is demonstrated. The influence of the environment on its optical properties is also evaluated. The paper is organised in 3 parts First, the design and fabrication of the samples are described. Then, micro-photoluminescence experiments under pulsed optical excitation are performed to characterize the stimulated emission regime, as well as the real and Fourier space properties of the PC structure. Finally, an evaluation of the sensitivity of the laser device to a change of the optical index of its environment is carried out, via the deposition of a polymer layer.
2. Design and fabrication
The studied structures consist in a square lattice of 30×30 InP rods lying on a Si/SiO2 substrate (Fig. 2(a)). The filling factor is 50% and the lattice parameter is 0.72µm, resulting in a 21×21µm structure. Four InAsP quantum wells are embedded into the InP rods in order to provide optical gain around λ 0=1.55µm. The 2D PC is designed to exhibit, at this wavelength, a SBM located at the Γ-point of the first Brillouin zone (A1 mode on Fig. 2(b)), leading to a surface emitting device. This resonator stands onto a Si/SiO2 Bragg mirror separated by a 3λ 0/4n SiO2 gap. This configuration allows for reducing the vertical optical losses, thus increasing the Q factor. More details about the design can be found in reference [15].
The samples were fabricated using standard technological processes. On the one hand, the III–V heterostructure used for the devices is grown by solid source molecular beam epitaxy. A 300 nm-thick sacrificial/etch-stop layer of In0.53Ga0.47As is grown on a 2-inch InP(001) wafer. Then, a 250 nm InP layer, including the quantum wells, is grown. On the other hand, the Si/SiO2 Bragg mirror is deposited on a Si wafer by low pressure chemical vapor deposition. The III–V heterostructure is transferred on top of the Bragg reflector, using SiO2–SiO2 wafer bonding. The InP substrate and etch-stop layer are removed by selective wet chemical etching. A silica hard mask is deposited onto the InP membrane. Then, the 2D PC is patterned in a maN2400 negative tone resist mask using e-beam lithography. The PC structure is then transferred by means of Reactive ion Etching into the InP membrane.
Fig. 2. (a) Cross-section of the studied structures (b) Reciprocal space and band diagram (TE polarization) for a square lattice of dielectric rods embedded in silica (filling factors 50%): the studied A1 SBM is circled and its electric field intensity distribution is also given.
Fig. 3. SEM images of the fabricated photonic crystal structure consisting of 30×30 microrods. The experimental lattice parameter is 720nm and the InP filling factor is approximately 46%.
In a 2D PC with finite lateral dimensions, photons are partly reflected at its boundaries and the resulting cycling of the SBM leads to cavity modes. The resonant wavelengths and Q factors for the two first modes have been calculated; we found λ 1=1.5651µm (Q1=8900) for the first mode, and λ 2=1.5664µm (Q2=3700) for the second one. Note that, due to a negative curvature of the considered band (Fig.2(a)), the first mode corresponds to the lowest resonant wavelength. For a specific mode, the threshold for stimulated emission depends strongly on its optical losses which are quantified by its Q factor. Therefore, we can expect a laser effect on the first mode, which exhibits the highest Q.
3. Optical characterization of the microlaser
These active photonic structures were characterized at room temperature under pulsed optical pumping at 780 nm, with a pulse width of 5.8 ns and a 1.7% duty cycle. The pumping beam was focused using a x20 microscope objective (NA=0.4). The light emitted above the sample was collected through the same objective, and analyzed by a spectrometer, with a spectral resolution of about 0.1 nm.
At any pump power, a single photoluminescence peak is observed. We think that, due to its high Q factor and a good overlap with the Gaussian pumping spot, only the first mode is efficiently excited: this point will be confirmed below. The emission spectrum, measured for a pumping power of about 5 mW, shows a sharp peak (Fig 4(b)) at 1501nm. The variation of the peak intensity as a function of the incident peak pump power is displayed in Fig. 4(a). From these experimental data, we can conclude that the structure exhibits a clear signature of lasing, with a threshold around 3.5 mW.
Fig. 4. (a) Light-in Light-out curve under pulsed optical excitation at room temperature(b) PL spectrum of the device above threshold (1.4Pth)..
This value is close to comparable structures based on a lattice of holes. As a comparison, Table 1 gives typical values we have measured in our group on microlasers based on 2D PC of holes, exploiting a SBM at the Γ point, and with a similar gain material (InAsP/InP quantum wells) and similar pumping conditions.
Table 1. Peak pump power at threshold for various type of 2D PC surface emitting microlasers.
SBM at the Γ point are known to be suitable for surface emitting devices. In order to investigate the emission pattern of our microlasers, we have performed far field experiments.
Direct imaging of the far field pattern is made possible by the use of Fourier optics, clearly described in [17]. The photoluminescence signal emitted by the microlaser near the vertical direction and collected through the microscope objective (NA=0.4) is imaged either in the Fourier (Fig 5 (a)) or in the real (Fig 5 (b)) space depending on the lens position in front of the InGaAs camera. The far field images are limited by the numerical aperture of the microscope objective, which leads to an angular range of light detection of +/-25°.
Fig 5. Experimental setup (F1 is the microscope objective focal length, F2 is the focal length of the achromatic lens L2): (a) Fourier space imaging of the light emitted from the sample; one emission direction corresponds to one point (kx,ky) on the image. (b) Real space imaging of the photoluminescence intensity emitted from the sample.
The “real space image” corresponds to the mode intensity distribution, filtered by its evanescent part, close to the PC membrane. From Fig. 6(a), we can then estimate a 10µm lateral extension of the laser mode. Its far-field pattern is shown on Fig. 6(b). The observed “doughnut” shape is characteristic of a non degenerated Γ-SBM [18]. Indeed, for symmetry reasons, this mode cannot couple with a vertical plane wave, but, due to its lateral confinement, it is able to emit light at slightly off-normal directions [19]. The mean emission direction is about 7° off-normal and the external radius of the doughnut radius is about 12°. Note that this value is close to that of the Airy disk corresponding to a 10µm aperture. Keeping in mind that the lateral size or our resonator is 21µm, it is clear that the laser mode is confined within a surface smaller than the whole structure.
Fig. 6. Angle resolved measurements: (a) photoluminescence emission intensity of the Bloch mode in real space and (b) far field pattern of the laser mode, above threshold.
To explain this lateral confinement, two reasons can be invoked. Imperfect e-beam lithography (e.g. proximity effects) can lead to a variation of the rod diameters through the PC lattice. Indeed, a close study of SEM images (Fig. 3) shows that the diameter slightly decreases (~10 nm) from the center of the structure to its boundaries. However, such a variation results in a wavelength shift of the Γ point smaller than 0.5nm. Moreover, smaller rods result in a PC with higher mode frequencies, which cannot act as a barrier for Bloch modes with negative curvature [15]. Another explanation is a carrier induced confinement in the area illuminated by the pump signal. We have shown in previous publications [20] that free-carrier optical generation can significantly blue-shift the wavelength of a SBM. In our case, the laser peak wavelength actually decreases with the pump power (Fig. 7), supporting this explanation. For pump power lower than 2.5mW, a 3.6nm/mW decreasing of the wavelength is observed, indicating a photo-carriers induced effect. Above threshold the carrier density is not completely clamped, as it has been previously observed for microlasers with sufficient β-factor [21,22], and the peak wavelength slightly decreases (0.15nm/mW).
More specifically, the confinement process can be understood as follows. Before threshold, the density of photo-generated carriers increases with pump power, leading to a decrease of the refractive index. As the rods are electrically isolated from each other, carriers cannot diffuse laterally, and this refractive index change is strictly localized under the pump beam. This way, a “photonic well” (Fig.8) is formed with a width, W, equal to that of the pump beam. The lateral extension of the resulting confined mode is then approximately W+2µ, where µ corresponds to the evanescent tail of the field distribution in the barriers. Following the envelope function formalism describe in [23], we get:
The parameter α is the curvature () at the Γ point extreme of the band structure (see Fig 2(a)) chosen for the laser SBM: it is derived by a parabolic fitting. δω is the Kerr induced shift of the laser mode frequency. In our case (λ~1500 nm, δλ~2nm), we find:
Taking into account that, with our ×20 microscope objective, the width, W, of the pump beam is 5–7 µm, the deduced lateral size of the laser mode is indeed close to the measured width of the “real space” image (Fig. 6(a)). It must be noticed that the photonic barrier, which is less than 4 nm, can only confine the fundamental mode, which can also explain the monomode behavior observed in the PL experiments.
Fig. 8. (a) Carrier induced optical confinement scheme in an optically pumped PC resonator. (b) Real space image of the field intensity repartition inside the PC structure.
4. Effect of the environment on the laser wavelength
In order to evaluate the sensitivity of our device to a variation of the refractive index of the superstrate, we have deposited a layer of polymer (PMMA) with a 1.489 refractive index.
The PMMA layer was deposited by spin coating using a low rotation speed to ensure a layer thickness (about 3µm) higher than the rod thickness. Care was taken to check that no air bubble was trapped by the PMMA in the PC array.
PL experiments (same conditions as in last section) have been performed on these samples. Stimulated emission has been obtained for similar threshold power, and the laser peak wavelength is redshifted by 30 nm (Fig. 9). We have shown, using 3D FDTD simulations that such a variation corresponds to a complete filling of the space in between the rods of the PC lattice.
Fig. 9. Measured photoluminescence spectra of the microlaser (a) without PMMA (b) with a PPMA layer on top of the pillar photonic crystal.
One of the key parameters determining the performances of an optofluidic sensor, based on an optical resonance, is the bulk refractive index sensitivity (BRIS) [24]. It is defined as the resonant wavelength shift for a unitary change of the refractive index (RIU) of the environment. In our case we get:
This value is comparable with BRIS obtained with other photonic devices (for a comparison see Table 1 in [25]), and very close to that obtained in [14] with a PC based nanocavity. Although its sensitivity is rather moderate, our device presents several advantages: this compact structure is surface addressable, insuring a good collection of the signal (e.g. in an optical fiber). Moreover, in rod based PC lattices, fluids can flow through the resonator itself, thus maximizing the interaction with the electromagnetic field. This opens the way to efficient optofluidic integration.
5. Conclusion
Optical properties, under stimulated emission, of a 2D PC rod lattice have been investigated. Surface emitting laser operation at room temperature has been demonstrated, for the first time, on this type of structures. The threshold power is comparable to those obtained with similar devices based on hole lattices. It has been shown that the use of rods allows for the control of the emitting area of a Bloch mode laser, through carrier induced refractive index change. It has been demonstrated that the laser wavelength is sensitive to the environment of the device, opening the way to a new type of integrated optofluidic devices where the liquid can flow through the resonator itself, reinforcing the sensitivity of the sensors.
Acknowledgments
The authors would like to thank Philippe Regreny for growing the epitaxial stacks and the NANOLYON facility for its technological assistance. Jean-Marc Fedeli and Lea Di Cioccio, from CEA-LETI, are acknowledged for Bragg reflector deposition and molecular bonding. This work was partly developed into the frame of the 6th PCRD “ePiXnet” european network of excellence, and benefits from the support of the Rhône-Alpes region.
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