1. Introduction

There are many benefits of quantum dot (QD) microdisk lasers: simple fabrication, low-threshold, high β-factor, high Q-modes, high temperature stability, high integrability, planar direction of emission. Using InAs/InGaAs QD active region has an additional advantage of a deep carrier localization and an emission wavelength in the 1.3 μm spectral range. Single-mode operation of QDs microdisk laser is highly desirable for practical applications. However, gain spectrum width in Stranski-Krastanow InAs/InGaAs QDs ranges from 50 to 160-200 nm [1,2]. Meanwhile, the free spectral range (FSR) in a 10-μm diameter GaAs-based microdisk resonator is only 12 nm. As a result, a laser spectrum of a QD microdisk typically comprises a series of whispering gallery mode (WGM) resonances [3]. A straightforward way to achieve a single-mode lasing is to reduce a microresonator diameter down to 2-3 µm. For example, a side mode suppression ratio (SMSR) of 20 dB has been demonstrated with QD microrings as small as 2.7 µm [4]. However, microresonators of such small size are hard to handle and may suffer from an excessive sidewall roughness as well as surface recombination. Also the WGM Q-factor decrease exponentially with decreasing the cavity size [5]. We have recently demonstrated that the use of a dense array of InGaAs QDs provides single-mode lasing even in microdisks of a relatively large size (9 µm in diameter) [6]. Nevertheless, a more general method, i.e. independent of an active region kind, of mode control is required for suppressing undesirable modes and simultaneously keeping a high Q factor of a microdisk resonator.

The change of emission wavelength and scattering of emitted laser light was observed in a mushroom microdisk laser with GaAs/AlGaAs quantum well (QW) with subwavelength-sized notch etched at the resonator edge [7]. The use of holes etched at the periphery of a 2.56 μm in diameter InGaAs/InGaAlAs/InP QW mushroom microdisk laser results in improvement of extraction efficiency and change of emission wavelength (so-called photonic crystal disk laser) [8]. Single mode lasing was observed in lasers based on microgear cavity with an etched grating on a sidewall [9]. A suppression of some optical modes was observed in large (80μm in diameter) QD ring lasers with small notches (50 nm width and 500 nm depth) introduced at the side walls [10].

In this work we developed a novel method for optical modes decimation by etching sub-micron local pits or grooves on the top surface of QD microresonators with the focused ion beam (FIB) technique.

2. Experiment

An epitaxial structure was grown by molecular beam epitaxy on semi-insulating GaAs (100) substrate. The active region comprised five layers of InAs/In_0.15Ga_0.85As QDs separated by 30-nm-thick GaAs spacers. The active region was placed in the middle of 220-nm-thick GaAs waveguiding layer confined on both sides by 20-nm-thick Al_0.3Ga_0.7As barriers. A 400-nm-thick Al_0.98Ga_0.02As cladding layer was grown beneath the waveguiding layer. Microdisks (MD) and microrings (MR) were fabricated using photolithography and reactive ion etching. We should notice, that due to the photolithography imperfections the diameters of microdisks under study were varied. For example, microdisk diameter according to SEM data with nominal diameter of 6 μm may vary from 5.9 μm to 6.1 μm. Thus emission wavelengths of the same WGM also may vary from disk to disk in the range of several nanometers. Further, the Al_0.98Ga_0.02As layer was transformed into an (AlGa)_xO_y layer by the selective oxidation process to ensure the optical confinement on the substrate side. The outer diameter D was varied in different structures in the 6–12 μm range. Figure 1 shows a scanning electron microscopy (SEM) image of 6-μm microdisk.

 

Fig. 1 Scanning electron microscopy image of 6-μm microdisk with schematically shown interaction of focused Ga⁺ ion beam with microdisk top surface.

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Focused ion beam (FIB) was applied to mill pits or grooves on top of the resonators as shown schematically in Fig. 1. A focused 30kV Ga⁺ ion beam with 5pA current was used. The FIB column is placed in Cross Beam system from Carl Zeiss (Neon 40). Spatial distribution of light intensity inside the initial microresonators was studied by means of near-field scanning optical microscopy (NSOM).

For optical studies CW-operating YAG:Nd laser (λ = 532nm) was used. The samples mounted in a flow cryostat were measured at 78K. A piezoelectrically adjustable Olympus LMPlan IR objective x100 with NA 0.8 was used to focus the incident laser beam to a spot of approximately 2μm in diameter and to collect the μPL signal from a microresonator. The collected μPL was then dispersed via 1000-mm monochromator Horiba FHR and measured with cooled InGaAs CCD array. The overall spectral resolution was 0.04 nm for 1200 mm⁻¹ grating.

3. Experimental results

Typical NSOM images taken from MR structures without FIB-etched pits are presented in Fig. 2 for two resonant wavelengths (1279.8 nm and 1291.3 nm). WGM patterns correspond to TE_1,32 and TE_2,29 modes. The radial intensity distribution of the TE_1,32 mode is characterized by the only maximum which is located at a distance of 200 nm off the resonator edge. The TE_2,29 mode has two radial maxima and the first one is located very similar to the TE_1,32 mode maximum, whereas the second one is shifted towards the resonator center to a distance of 600 nm off the edge. These data are in good agreement with our modeling results.

 

Fig. 2 NSOM images of WGM modes of different orders in microring resonator with 6 μm outer diameter and 2 μm inner diameter.

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An effect of FIB etching on the modal structure can be explained as follows. A FIB-etched pit formed at 600 nm off the resonator edge does not disturb the TE_1,32 mode, while it affects the TE_2,29 mode. A pit etched closer to the edge, at 200 nm, affects both modes. However, since the TE_1,32 mode is fully localized close to this distance, such pit primarily disturbs the TE_1,32 mode while its influence on the TE_2,29 is low.

First, we drilled two pits of approximately 100 nm in diameter and 20 nm in depth shifted to the distance of 200nm off the edge and one pit in the center of the 6μm microdisks. Micro-photoluminescence spectra measured at 78K with the microdisk resonators with one pit etched in the center of the disk and two pits placed at opposite sides of the disk (shifted on 200 nm from edge) before and after FIB treatment are compared in Fig. 3(a) and 3(b). The spectra presented at Fig. 3 are obtained at the pump intensity near the threshold (~1.5P_th for the sample after FIB). SEM images of the resonators are shown in inserts. The initial spectrum contains a number of resonant lines corresponding to different WGMs of which the dominant one is that, whose spectral position (1193.3 nm) is closest to the QD spectral maximum (1195 nm). Other (side) modes are observed at ~1182, ~1184, ~1205 and ~1206 nm in the spectra. Placing of the FIB pit in the center of microdisk does not affect the modes and their lasing characteristics. Generally speaking this geometry corresponds to the case of the microring resonator. So we can conclude that FIB treatment itself does not change the resonator parameters.

 

Fig. 3 μPL spectra of 6μm microdisk laser with one pit etched in the center of the disk (a) and two pits placed at opposite sides of the disk (b) obtained at 78K before and after etching pits by FIB. The pump intensity ~1.5Pth of the FIB-treated microdisk.

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The emission spectrum of the microdisk with two pits placed at 200nm from edge is remarkably thinned out after pit etching by FIB. The pits act as scattering centers suppressing those modes that have spatial position of intensity maxima near the pits. Only two modes, at 1182.4 and 1204.8 nm, now dominate the spectrum, whereas the intensities of the other lines are lower by at least 17 dB. It should be noted that the spectral separation of these two lines corresponds well to the FSR, so they can be identified as neighboring azimuthal modes of the same radial order. We attribute the dominant line of the initial spectrum to a WGM of the first radial order and the strongest lines in the spectrum after the FIB treatment to WGMs of the second or, probably, the third radial order.

The light-light curves were investigated to estimate the lasing threshold (Fig. 4(a)). After FIB etching the threshold power was increased by only a factor of 2 and became P_th = 0.3 mW. A near-threshold linewidth (FWHM) of the lasing mode is ~40 pm (λ/Δλ > 30’000) remains unchanged after FIB treatment being limited by the optical system resolution. The lines, corresponding to suppressed modes have increased FWHM (130-300 pm). We can suppose that the threshold increases because of the non-radiative recombination enhancement at the etched pits.

 

Fig. 4 Optical output power (a) and linewidth of the lasing mode as a function of excitation power for the microdisk before (triangles) and after (circles) FIB etching.

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Figure 5 shows a temperature induced evolution of the microdisk with two etched pits. The lasing was observed up to 210K. The lasing wavelength redshifts to 1240 nm at 210K with the hopping, following bandgap shift of the QDs. The observed quenching of the lasing with temperature is due to the non-radiative recombination of carriers on defects formed by pits. To keep the lasing up to room temperature additional technology steps after FIB treatment (such as surface passivation) are required.

 

Fig. 5 μPL spectra of 6μm microdisk at different temperatures. Excitation power P = 0.7 mW.

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Next we introduced etched pits with the same parameters to a MR with an outer/inner diameter of 11.4/4.4 μm. PL spectra obtained before and after FIB etching are shown in Fig. 6(a). A noticeable discrimination of WGMs is observed again.

 

Fig. 6 μPL spectra of 11.4 / 4.4μm microring laser obtained at 78K before and after etching pits by FIB (a) and μPL spectra of 5.9μm microdisk laser obtained at 78K before and after etching the groove by FIB.

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Next we etched the groove along a radius of a 5.9-μm microdisk from its center towards the rim. The groove is about 150 nm in width, 10 nm in depth and 2 μm in length. PL spectra obtained at 78K before and after FIB etching are shown at Fig. 6(b); a top view SEM image is presented in the insert. The groove position was chosen to predominately disturb higher-order radial modes. The spectrum is thinned out after the groove milling: one mode at 1188.2 nm (we believe it is of the first radial order) becomes prevailing over the others, whereas five lines of comparable intensities are in the initial sample.

4. Conclusion

It is demonstrated that the treatment of a circular microresonator surface with a focused ion beam can result in a decimation of the whispering gallery modes. The method does not affect the linewidth of the lasing mode and leads to an insignificant increment of the lasing threshold. Depending on location and shape of the etched pits or grooves, intensity of different modes can be suppressed / enhanced.