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We present continuous wave laser emission in a photonic crystal microcavity operating at 1.5 µm at room temperature. The structures have been fabricated in an InP slab including a single layer of self-assembled InAs/InP quantum wires (QWrs) as active material. Laser emission in air suspended membranes with thresholds of effective optical pump power of 22 µWand quality factors up to 55000 have been measured.
©2009 Optical Society of America
1. Introduction
The development of high-density optoelectronic integrated circuits may benefit from the large integration capacity of micro/nanolasers. These light sources exhibit low power consumption and low footprint and can be operated at higher temperatures and at higher modulation rates than conventional lasers [1]. Due to their ability to tailor the light in the wavelength scale, photonic crystals (PCs) have attracted strong attention [2, 3] enabling the development of two dimensional PC slabs with photonic crystal microcavities (PCMs) [4]. These photonic microstructures confine the light in a volume of the order of a cubic wavelength and present high quality factors (Q) [5, 6] being therefore ideal candidates for the fabrication of more efficient light sources [1, 7]. In the last years, quantum wells (QWs) or quantum dots (QDs) have been used mainly as the gain medium in PCM lasers. Our approach in this work is to use a single layer of self-assembled InAs/InP quantum wires (QWrs) [8] as active material. Due to the reduced dimensionality of the QWrs compared with QWs we should expect several advantages like a lower threshold for laser emission [9, 10], reduced temperature sensitivity [11], reduction of the linewidth enhancement factor [12], reduced surface recombination [13] (which is very important in structures like PCM with many material-air boundaries), good lateral carrier confinement and in-plane polarization anisotropy [8]. Also, InAs/InP QWrs offer the possibility to tune the spontaneous emission beyond 1.6 µm [14, 15], which is very interesting for practical applications such as gas sensing and molecular spectroscopy. Finally, compared with QDs, quantum wires allow a good compromise between their lateral confinement and the activemedium volume and can also be fabricated isolated using patterned GaAs substrates [16] or by self-assembled methods on InP [17] for single QWr lasing applications.
Low temperature self-tuning lasing operation around 0.9 µm has been demonstrated using a single layer of low density InAs/GaAs QDs in a PCM [18]. Similarly, lasing at ~1.2µm in a PCM has been obtained using a single layer of high density InAs/GaAs QDs [19]. To demonstrate room temperature (RT) lasing around 1.3 µm in a QD based GaAs PCM, it was necessary to stack five self-assembled InAs/GaAs QD layers as active material [20, 21, 22]. Meanwhile, laser emission at 1.5 µm at RT using pulsed optical excitation has been demonstrated both, in a compact “2.5 D” PC Γ-point laser [23] and, in an air suspended PCM [24], containing a single layer of InAs/InP QDs as gain medium. Finally, Nozaki et al. [25] showed for the first time RT continuous wave (CW) lasing at 1.5 µm with thresholds around 1.2 µW of absorbed pump power (Peff) using an InP PCM containing multiple GaInAsP/InP quantum wells.
In this work, we show RT lasing at 1.5 µm using CW non-resonant optical pumping of a PCM air suspended membrane containing just a single layer of InAs/InP self-assembled QWrs. Our devices are based on L7-type [26] PCMs with high quality factors Q~55000 and threshold values of (Peff~22 µW).
2. Design and fabrication
2.1. Epitaxial material
The starting material consists of a single layer of self-assembled InAs/InP QWrs grown by solid source molecular beam epitaxy and embedded in a 237-nm-thick InP slab. An In0.53Ga0.47As layer of 700 nm thickness is used as sacrificial layer for the membrane realization. Fig. 1 (a) shows an atomic force microscope image (AFM) of an uncapped sample containing QWrs. The QWrs morphology is clearly elongated along the [11̄0] crystal direction with average width, height and pitch period of 15 nm, 3.6 nm and 18 nm, respectively. Their length exceeds 1 µm in most cases. These QWrs exhibit a 23% linear polarization anisotropy along their elongated direction [8]. Fig. 1 (b) contains a RT emission spectrum of the gain medium detected with a Ge cooled detector (detector peak sensitivity at 1.63 µm) and shows that the emission is broad and finds its maximum at 1.56 µm. More details about the growth procedure and optical properties of these QWrs can be found elsewhere [8, 14].
Fig. 1. a) Atomic force microscope (AFM) image of an uncapped sample of self-assembled InAs/InP QWrs. b) Room temperature photoluminescence spectrum of the quantum wires sample without photonic crystal structure.
Fig. 2. Normalized field patterns of the fundamental mode of the L7 PCM calculated by 3D-FDTD. a) Hz-field pattern. b) Ex-field component. c) Ey-field component. d) |E|2 electric field intensity. The parameter of the simulation are: the lattice constant, a=440 nm, the hole radius rate, r/a=0.27 and the thickness of the slab, d=237 nm.
2.2. Photonic crystal structure
We have selected a L7-type cavity as PCM removing seven holes along the ΓK direction of the triangular lattice. The L7 cavity belongs to the Ln group whose modes can be classified as either even or odd respect to a plane along the longitudinal direction of the cavity [27]. The parameters of the structure are the lattice constant, a=440 nm, the hole radius rate, r/a=0.27 and the thickness of the slab, d=237 nm. This cavity presents a low energy mode at ωa/2πc≃ 0.283 with high Q=70000, small modal volume V=1.2(λ/n)3, and effective index neff=2.78 calculated using the guide-mode expansion method [28]. In addition, the fundamental mode emission is well isolated (Δλ≃20 nm) from the higher order modes and can potentially exhibit high β values as it has been demonstrated in PCMs with InAs/GaAs QDs as active material [29]. Fig. 2 shows the magnetic field component perpendicular to the 2D periodic structure (Hz), the in-plane components of the electric field (Ex,Ey), and the electric field intensity (|E|2) at the centre of the slab calculated by three dimensional finite differences in the time domain (3D-FDTD) [30]. The fundamental mode has a well defined linear polarization perpendicular to the longitudinal direction of the cavity (ΓK direction of the triangular lattice) [31].
Fig. 3. Scanning electron microscope image of the fabricated L7-type photonic crystal microcavity. The lattice parameter is a=440 nm and the value of r/a~0.27.
2.3. Fabrication
A 120 nm thick SiOx layer was deposited by plasma enhanced chemical vapor deposition. Processing of the PC structures was done by electron beam lithography on a polymethylmethacrylate (PMMA) layer on top of the SiOx. Each cavity is surrounded by eleven rows of holes to minimize the in-plane losses. Reactive ion beam etching (RIBE) was used to open the holes in the SiOx by a CHF 3/N 2 mixture. The hard mask pattern was transferred to the semiconductor material by reactive ion etching (RIE) by a CH 4/H 2 mixture combined with O 2 plasma cycling. After the process, a thick layer of SiOx remains on the top of the sample. It was eliminated in hydrofluoric acid water diluted HF(1) : H 2 O(5) during 90 s. For the obtention of the PC membrane the sample was submerged in a HF(1) : H 2 O 2(1) : H 2 O(20) solution and time controlled. For more details about the fabrication process see Ref. [32]. Figure 3 shows a scanning electron microscope image of one of the fabricated PCMs. The holes present a circular shape with a high homogeneity in position and size as is observed.
3. Optical characterization
The optical characterization was performed by microphotoluminescence (µ-PL) spectroscopy at room temperature. The sample temperature was kept constant slightly above RT (26±0.1 °C). The structures were optically pumped with a 785 nm CW laser diode. An objective lens 20× (0.40 NA) is used to focus the excitation spot (diameter~1.5 µm) over the sample. The emitted light was collected by the same objective lens and focused in a single mode optical fiber coupled to a 0.85 m focal length double spectrometer. A cooled InGaAs photodiode array was used as detector.
Figure 4 (a) shows the non-linear behavior of the integrated emission intensity versus the absorbed pump power (L-L curve) for a PCM oriented along the [11̄0] direction (parallel to the QWrs). This geometry has been investigated before using InGaAs QWrs stacked on V-groove patterns on GaAs with the result of laser emission between 2 and 70 K under pulsed excitation [16, 33]. To this respect, we must note that our PCMs fabricated along the perpendicular QWrs direction ([110]) do not show laser emission which we tentatively attribute to the enhanced surface recombination at the material/air surfaces [34]. On the other hand, laser emission was consistently observed in parallel PCMs fabricated in the same sample. The absorbed pump power was obtained by the method described in Ref. [35]. The reflectivity of the InP was estimated ~30%, and the absorption coefficient α=12900 cm -1 [36] giving an absorption fraction ~24%. The curve shows a pronounced change on the slope with a clear kink. From the linear fit of the L-L curve above the kink, a threshold pump power of Pth=22 µW is obtained. This value compares well with values obtained in PCM containing InAs/InP QDs operating at 1.5 µm under pulsed excitation [23, 24]. Also with the ones obtained in PCMs with GaInAsP/InP QWs [25, 35] and microdisks with InAs/InP QWrs [13] in the CW pumping regime. Figure 4 (b) shows a logarithmic plot of the spectra measured below threshold for effective excitation powers between 0.08Pth and 0.8Pth. At the highest power, we estimate Q=55000 from lorentzian fit to the data with the minimum linewidth that we can measure with confidence in our experimental setup. The measured quality factor is among the highest obtained in active InP PCMs [25, 35, 37].
Fig. 4. a) Integrated photoluminescence intensity versus effective excitation power (blue dots). Red line is the linear fitting for the data measured above the kink. Dash line indicates the threshold power (Pth=22µW). b) Logarithmic plot containing several emission spectra measured below threshold (dots). Continuous lines stand for the corresponding lorentzian fits.
Figure 5 shows the evolution of the emission peak wavelength as a function of the excitation power. The spectral peak position shows a initial blue-shift for low excitation power attributed to the variation of the refractive index with carrier density as reported for air suspended PCM microlasers [38]. As the excitation power increases, due to the CW excitation, the thermal effects become important and produce an opposite change in the refractive index resulting in a red-shift of the emission peak. This behavior has been observed in GaAs PCM microlasers operating at room temperature in the CW excitation regimen [21] and it is absent in PCM in the pump pulsed regime where the thermal effects are overcome and only the blue-shift is observed.
Fig. 5. Emission peak wavelength versus effective excitation power. Red line shows the threshold power.
3.1. Rate equations analysis
We have analyzed further the emission characteristics of our QWrs based PCM laser solving the microcavity laser equations [18, 39]:
Here, N stands for the carrier density, P is the photon density, Rp is the incident pump rate, (τr,τnr,τc) are the radiative, nonradiative and cavity photon lifetimes, respectively, Γ is the confinement factor, and G(N) is a linear gain function with g standing for the differential gain coefficient and Ntr for the transparency carrier density.
Figure 6 shows the L-L curve in double log scales and the evolution calculated using the indicated spontaneous emission factors β. The value of τc was obtained directly from the measured Q=55000. The radiative lifetime τr=2 ns is characteristic of these QWrs at 77 K [34] and Γ~0.02 can be estimated for our PCM lasers from the cavity mode volume and the overlap with the active medium. The non-radiative rate is given by with the propagation distance da and velocity vs for surface recombination and the Auger recombination coefficient given by their nominal values in InP [39]. We had varied Ntr, β and g looking for the best fit to our data. Since these parameters are not independent, the value of β can not be determined without knowing the material gain g×Ntr. In our case, given the high Q value, Ntr=2±0.5×1017 cm-3 can be estimated from the position of the threshold with reasonable accuracy in a broad range of (β,g) from (0.02,10×g 0) to (0.2,100×g 0), where g 0=3.0×10-16 cm2. Assuming a material gain≈1500 cm-1 as for an InGaAs QW (lower gain is not expected for the QWrs) [39], we deduce a spontaneous emission factor β=0.06 for our PCM laser based in a single layer of QWRs. As expected for one dimensional systems, the transparency carrier density calculated this way is one order of magnitude smaller than in samples with QWs [38, 39] but still substantially larger than the value reported for PCM lasers with InAs/GaAs QDs [18, 40]. Our analysis suggests, within the uncertainties just mentioned, a differential gain g=7.7×10-15 cm2 and a spontaneous emission factor much larger than in conventional lasers.
Fig. 6. Log-log plot of the integrated emission intensity versus the excitation pump power. Blue dots are measured data. Red lines are the calculated curves extracted for the indicated β values with g=7.7×10-15 cm2 and Ntr=2×1017 cm-3.
4. Summary
We have demonstrated room temperature laser emission at 1.5 µm using a single layer of self-assembled InAs/InP QWrs embedded in a L7 PCM with a quality factor Q~55000. Our laser operates under non resonant continuous wave optical pumping with an effective pump power threshold of 22 µW. The characteristics of our laser as extracted from the analysis of the laser rate equations are in between QD and QW PCM lasers operated under similar conditions.
Acknowledgments
L.J. Martínez thanks an I3P fellowship and I. Prieto thanks a FPI fellowship BES-2006-13649. The authors thanks D. Gerace and L.C. Andreani for the gme simulation and J. Canet-Ferrer, G. Muñoz-Matutano and J. Martínez-Pastor for the time resolved photoluminescence data. The authors would like to acknowledge support from CONSOLIDER-Ingenio (CSO2006-00019 (QOIT)) 2010, CAM Nanocomic (S-0505ESP0200) and Naninpho-QD (TEC2008-03756-C03-01)).
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