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Abstract
We present an effective method for direct fiber coupling of a quantum dot (QD) that is deterministically incorporated into a cylindrical mesa. For precise positioning of the fiber with respect to the QD-mesa, we use a scanning procedure relying on interference of light reflected back from the fiber end-face and the top surface of the mesa, applicable for both single-mode and multi-mode fibers. The central part of the fiber end-face is etched to control the required distance between the top surface of the mesa and the fiber core. Emission around 1260 nm from a fiber-coupled InGaAs/GaAs QD is demonstrated and its stability is proven over multiple cooling cycles. Moreover, a single photon character of emission from such system for a line emitting above 1200 nm is proven experimentally by photon autocorrelation measurements with an obtained value of the second order correlation function at zero time-delay well below 0.5.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The development of fiber-coupled single-photon sources (SPS) for the telecommunication bands will enable long-distance secure data transfer in fiber-based quantum communication networks [1]. Many attempts of building an efficient SPS using single QDs as emitters have been already undertaken [2–15], but an effective QD to optical fiber coupling remains a challenge. There are several concepts reported in the literature to couple QDs to optical fibers or waveguides. These include embedding QDs into a photonic crystal waveguide [2], the application of photonic microstructures [3], tapered fibers [4–6], integrated arrays of optical fibers [7,8] and an optical horn antenna [9]. A simple coupling system using two collimating lenses was presented in [10], however, it requires active alignment at low temperature using piezoceramic actuators. Because of its simplicity and potentially high extraction efficiency, different configurations of direct QD to optical fiber coupling systems have also been studied, including those employing nanofiber tips [11], photonic crystal cavities attached to a fiber tip [12], bidirectional fiber-based couplers [13] and optical Fabry-Perot microcavities attached rigidly to pumping and collecting fibers [14]. It is worth underlining that most of the realizations of the QD to fiber coupling concepts have been demonstrated so far for wavelengths shorter than 1.1 µm which do not match the optical communications windows. Fiber-coupling at 1.3 µm has been shown in [12] using an optical fiber tip functionalized with semiconductor photonic crystal cavities, however, from high spatial density (300 QDs/µm2) InAs QDs. Moreover, a coupling at 1.37 µm from a single InAs/InP QD incorporated into nanobeam attached to a fiber taper has been also reported recently [15]. The two approaches are technologically challenging and therefore difficult to implement in mass applications.
Recent numerical studies [16–18] show that direct coupling of a QD to an optical fiber may assure the coupling efficiency reaching 23% at 1.3 µm for the optimal structure design consisting of an epitaxial QD with Bragg reflector underneath, embedded in a cylindrical mesa for tailoring the confinement and far field distribution of electromagnetic radiation. Moreover, for high coupling efficiency the collecting single-mode (SM) fiber must have high numerical aperture (about 0.4) and the core end-face must be located at predetermined distance from the top surface of the mesa, depending on the mesa dimensions, number of pairs of the Bragg reflector and the operation wavelength. In this work we propose a universal, with respect to both spectral range and material system, method for direct fiber coupling of the light emitted by a quantum emitter embedded in an optimized photonic structure, in which all parameters influencing a coupling efficiency are controlled with high accuracy. The technical feasibility of our approach is proven exemplary by coupling emission from a single long-wavelength (1255-1265 nm) InGaAs QD embedded in a mesa into an SM fiber.
One of the challenges in the proposed coupling method is a precise alignment of the fiber with a core diameter of a few microns with respect to the mesa containing a QD, which is positioned deterministically at the mesa center with an accuracy of about 50 nm. The fiber alignment must be carried out at room temperature which excludes the use of the photoluminescence (PL) from the InGaAs QD as a feedback signal. A possible solution could be to use the PL signal from the substrate covered with a gold mask as reported in [19], however, the alignment accuracy of about a few micrometers achieved in such an approach is insufficient for positioning an SM fiber. To obtain high positioning accuracy of the fiber end-face with respect to the mesa in vertical and lateral directions, we have developed a special spectral interferometry method. Similar approaches were used for optical measurements of surface topography [20,21], however, with the involvement of the reference arm and the focusing objective which excludes their applications for direct fiber positioning prior to gluing to the sample surface. The proposed method does not require any focusing optics nor the reference arm as it is based on the Fizeau interferometer, in which the interference of the broad-band light reflected from the fiber end-face and the top surface of a semiconductor microstructure provides a suitable feedback signal. The method has been initially presented in our recent conference communication [22]. Since then, it has been significantly improved and allows now for performing two-dimensional back-reflection interference scans of the microstructures with single QDs using either SM or multi-mode (MM) fibers. The absolute precision of the interference positioning method was validated against the PL 2-D scans obtained at room temperature from a quantum well (QW) embedded in the mesa. We also show experimentally that the proposed scanning method can localize mesas of sub-micrometer diameters. A key advantage of the proposed scanning method is the fact that it can be used at room temperature and provides 50 nm alignment precision with respect to the center of the mesa, which makes it very suitable for both, the SM and the MM fiber-coupling of photonic nanostructures with emission normal to the sample surface such as QD-microlenses [23], QD-micropillars [24], and QDs integrated into circular Bragg reflectors [25]. For direct fiber coupling of the QDs, we have used specially developed high numerical aperture (HNA) fibers (NA = 0.42, core diameter equal to 2.5 µm for SM fiber and 15 µm for MM fiber) with minimized residual thermal stress thanks to a multistep refractive index profile. We show an example of a QD emitting in the range of 1255-1265 nm coupled to an SM fiber and report on the stability of the PL signal over multiple cooling cycles. Finally, we present the results of photon autocorrelation measurements conducted on a single well-resolved quantum dot line at 1203 nm. The obtained second order correlation function shows clear antibunching at zero time-delay with g(2)(0) value well below 0.5, which proves the suitability of the proposed fiber-coupling approach for the realization of practical single-photon sources.
2. Arrangement of the fiber coupling system
In Fig. 1 we present a set-up used for PL measurements from the fiber-coupled QD based on single-mode fibers. The equivalent system configuration based on multimode fibers can be realized in a similar way. As it is shown in the red frame in Fig. 1, the photonic structure with QD is glued directly to an optical fiber end-face. As an optical excitation source we used a semiconductor laser (λpump = 805 nm, pulse duration 50 ps and repetition rate 80 MHz) connected to a Y-type fiber coupler directing the laser pulses to the QD. The fiber-coupler output port is connected to a standard SM fiber (Corning SMF-28e) spliced (insertion loss IL < 0.2 dB) to an HNA fiber optimized in terms of residual thermal stress for safe applications at cryogenic temperatures. The semiconductor structure with a QD is mounted on a cold finger in a cryocooler allowing for the minimal operation temperature of about 38 K, whereas the HNA fiber is sealed by an epoxy glue in a specially designed vacuum fiber feedthrough. The light emitted by the QD is coupled into the HNA fiber and transmitted through the fiber-coupler to a monochromator equipped with an InGaAs linear array detector.
Fig. 1. Fiber-based system for PL measurements from the cryo-cooled QD directly glued to the HNA fiber end-face.
To increase the coupling efficiency, we have used a specially developed fiber with GeO2 concentration in the core of about 40% mol ensuring high numerical aperture. High dopant concentration, however, increases the risk of preform breaking when it is cooled down from glass transition temperature (about 1000 K) to room temperature because of the mismatch of thermal expansion coefficients between the core and the cladding. Moreover, for the same physical reasons, highly doped fibers are more susceptible to breaking during cooling down to cryogenic temperatures. Thermal stress, for the sake of simplicity, expressed in terms of equivalent von Mises stress, reaches a peak value at the interface between the pure silica glass cladding and GeO2 doped core, see Fig. 2(a). To reduce thermal stress, we have used a three-step refractive index profile as shown in Fig. 2(b). By performing a series of simulations using the finite element method, we have determined the three step core parameters ensuring minimal thermal stress. As shown by our simulations, for the diameters of successive core layers in relation of 1:2:3 and GeO2 dopant concertation in successive layers of 40, 13, 5 mol%, the thermal stress is reduced by half, compared to the single step profile. The required single mode operation of the fiber was obtained by reducing the core diameter down to 2.5 µm. In Figs. 2(c) and 2(d), we show, respectively, the refractive index profile in the three step fiber preform used for fabrication of the SM and MM HNA fibers and the results of measurements of the numerical aperture, which according to one percent power level criterion for an SM fiber is equal to NA = 0.42, (0.40 for an MM fiber) while the cut-off wavelength λcut-off = 1050 nm. In Fig. 3 we show the cross sections of the fabricated SM and MM fibers obtained in the scanning electron microscope. The gray rings of different brightness visualize regions with different GeO2 concentrations.
Fig. 2. Simulated von Mises stress and GeO2 concentration profile of the single step (a) and three step (b) HNA fiber. Refractive index profile measured in the core of the fabricated fiber preform; zero level corresponds to refractive index of the cladding (c). Comparison of far field intensity distributions measured at the distance of 5 mm from the fiber end-face for Corning SMF-28e fiber (blue trace) and the fabricated HNA SM fiber (red trace) (d).
Fig. 3. Cross-sectional scanning electron microscopy (SEM) images of the fabricated high aperture SM (a-b) and MM (c-d) fibers. Gray rings of different brightness visualize regions with different GeO2 concentrations.
The HNA SM/MM fiber was spliced with a standard SM/MM fiber using TEC (Thermally-Expanded-Core) technique. The two fibers with different diameters (2.5/9 µm in case of SM HNA/Corning SMF-28e fiber and 15/50 µm in case of MM) are spliced using lower currents and longer times at each step of the standard splicing. Then, the core diameters in the spliced fibers are evened by controlled heating of the splice area causing the GeO2 dopant from the HNA core to diffuse to the cladding, eventually creating a gradual low loss splice (see Fig. 4), with insertion loss between 0.12 and 0.2 dB in both directions for SM and MM splice.
Fig. 4. Low loss splice between the SMF-28e fiber (left) and HNA SM fiber (right) made using thermally expanded-core technique.
As a quantum emitter we used self-assembled InGaAs/GaAs QDs grown by metal-organic chemical vapor deposition (MOCVD). To redshift their emission spectrum to the second telecommunication window, the QDs are covered with lower In content InGaAs strain reducing layer [26–28]. To obtain higher extraction efficiency, the QDs are incorporated into cylindrical semiconductor mesas fabricated deterministically (with a QD positioned in its center with a precision of about 50 nm [29]) by in-situ electron beam lithography and plasma etching. To simplify the mesa localization and avoid its breaking when accidentally touched with the fiber, only a small square area around the mesa is etched off, creating a rectangular aperture of the size of about 10-18 µm. A scanning electron microscopy (SEM) image of the mesa with clearly visible roughness of the side-wall and an optical microscope image of the mesa inside the aperture are shown in Fig. 5. The influence of the surface roughness of the mesa sidewalls can be twofold: carrier losses from the QDs due to non-radiative surface recombination lowering the internal quantum efficiency of the emitter (present also in the case of atomistically flat surfaces, e.g., epitaxial top mesa surface) and optical losses due to scattering of emitted radiation. Impact of both on the coupling efficiency is expected to be negligible in our case. The former due to deterministic positioning of the QD in the center of the mesa and typical sizes of the mesas around 1 µm resulting in distance to the mesa sidewalls exceeding 500 nm, and the latter due to upward directionality of the emission.
Fig. 5. SEM image of the QD-mesa (a). Optical microscope image of the mesa inside a square aperture – top view (b).
Figure 6(a) shows the scheme of the microstructure with a QD (treated as a dipole), the HNA SM fiber and all the relevant structure parameters used in the calculations carried out using commercial implementation of finite element method for solving Maxwell equations – JCMSuite [16]. For the fabricated structure coupled to the SM HNA fiber the values of the relevant parameters are: 21 pairs or layers creating the distributed Bragg reflector, mesa diameter of 525 nm, and mesa height equal to 480 nm. Moreover, the QD was positioned on the symmetry axis of the mesa, 450 nm below its top surface. The numerical simulations conducted for the fabricated structure parameters revealed that the maximum coupling efficiency to SM HNA fiber equal to 18.6% is obtained when the fiber end-face is placed at the distance of 200 nm from the top surface of the mesa, with a ± 50 nm tolerance corresponding to a 0.5% decrease in the coupling efficiency. This figure is by about 4.6% lower than the coupling efficiency reported in [16] because of differences between designed and actually fabricated structure. In Figs. 6(b) and 6(c), we show respectively the coupling efficiency as a function of the mesa-fiber distance and the light intensity distribution calculated for the measured structure parameters.
Fig. 6. Scheme of the mesa with embedded QD, and the HNA fiber (a). Calculated coupling efficiency as a function of fiber-mesa distance for the measured structure parameters with a maximum of 18.6% at a mesa-fiber distance of 200 nm (b). Calculated light intensity distribution, z = 0 corresponds to the position of the QD (c).
3. Alignment procedure
A scheme of the interferometric positioning system is shown in Fig. 7. A fiber-coupled supercontinuum (SC) source emitting in a wide spectral range is connected to one of the arms of a Y-type fiber coupler. The other arm is connected to a standard SM/MM fiber spliced with an SM/MM HNA fiber terminated with a zirconia ferrule. The ferrule is mounted into a holder attached to a tilt stage allowing for angular alignment in two directions. The sample with a QD-mesa structure is placed on top of another positioning system composed of XY manual stage for coarse lateral positioning, XYZ stage with piezo actuators for precise vertical and lateral movements and a tilt stage allowing for angular alignment of the sample until it is parallel to the plane of XY displacement.
Fig. 7. Scheme of the interferometric set-up for positioning of the SM/MM HNA fiber terminated with a zirconia ferrule to the center of the mesa.
Light transmitted through the HNA fiber is partially reflected (4%) from the fiber end-face and the top surface of the sample. The beams reflected from these two surfaces creating a Fizeau interferometer are collected by the same HNA fiber and directed with the third arm of the coupler to a compact spectrometer, which allows for observing the spectral interference fringes in real time. The interference signal is used as a feedback for (SM and MM) fiber positioning in both vertical and lateral directions thanks to the fact that the spectral position of interference fringes is directly related to the gap between the fiber end-face and the top surface of the semiconductor structure.
Parallel adjustment of the sample surface with respect to the XY displacement plane is proven by constant spectral position of the interference fringes when the flat part of the semiconductor structure is moved with respect to the fiber. When the HNA SM fiber passes over the aperture, a modification in the structure of the spectral interference fringes is observed as it is shown in Fig. 8 for the apertures containing mesas of different diameters. If the illuminated area is flat (i.e. area outside the aperture, see Fig. 8(a), at the bottom of the aperture, see Fig. 8(c) or on the top surface of large mesa shown in Fig. 8(e), row 1), then the distance d between the reflecting surface and the fiber end-face can be determined unambiguously from the following relation d=λ2/2Δλ, where Δλ is the spectral width of the interference fringe. Therefore, for the scanning fiber positioned over the flat areas of the semiconductor structure providing regular, high contrast spectral fringes, we could calculate the distance between the fiber end-face and the reflecting surface, which is displayed in Fig. 8(a). However, if the scanning HNA SM fiber is positioned over the edge of the aperture or the mesa (see Figs. 8(b) and 8(d)), then the light reflected back from the areas located at different distances from the fiber end-face differs in phase, which results in irregular shape and lower contrast of the spectral interference fringes. This effect is especially well pronounced when the scanning fiber is placed over the aperture edge, Fig. 8(b). Noteworthy, although the irregular spectral fringes contain the information about an average distance of the illuminated surface from the fiber end-face, the exact surface profile cannot be retrieved in a simple way. Similarly, in case the mesa is much smaller than the fiber core, then even if the fiber core is placed over the mesa, most of the light coupled back to the fiber is reflected from the bottom surface of the etched aperture surrounding the mesa. This situation causes only a contrast decrease without a significant change in the structure of the fringes compared to the fiber position over the bottom of the aperture (see Figs. 8(d) and 8(e), row 2).
Fig. 8. Interference signals registered while scanning the sample with a mesa using SM fiber (rows 1-2) and MM fiber (row 3) for different positions of the scanning fiber: outside the aperture (a), over the aperture edge (b), over the aperture bottom (c), over the mesa edge (d), and over the mesa center (e). Row 1 shows scans for the mesa of diameter D = 2 µm with clearly visible fringes phase shift. Row 2 shows scans for the mesa of D = 0.5 µm with greater decrease in fringes contrast. Row 3 shows scans conducted for the mesa of D = 0.5 µm using MM fiber, which shows similar behavior as for SM scanning fiber. The distance d between the fiber end-face and the sample was determined only for the fiber placed over the flat areas.
Instead of searching for the exact procedure for reconstructing the aperture height profile from the registered spectral fringes, which is not necessary to localize the mesa center, we simply used a visualization method which relies on averaging the interference signal in the tens of nanometers wide spectral window. As it is shown in Fig. 9, this approach allows for approximate depicting of the aperture height profile. Because of different behavior of the interference fringes during scanning the apertures with mesas of different sizes, the cross-section scans obtained by averaging the interference signal have different shapes. For a larger mesa with a diameter D = 2 µm, the averaged interference signal better approximates its actual shape (see Fig. 9(a)). For a smaller mesa, the registered signal does not necessarily reflect the mesa’s height profile, however, its center can be still identified from the symmetry of the captured signal (see Fig. 9(b)).”
Fig. 9. Interference signals averaged in a limited spectral window depicting topography of the scanned apertures with mesas obtained for the scanning SM fiber and mesas of diameter D = 2 µm (a) and D = 0.5 µm (b) and for the scanning MM fiber and mesa of D = 0.5 µm (c).
To perform scanning with the use of the HNA MM fiber, we connect it with a standard SM fiber using a fiber connector. As a result, predominantly axially symmetrical modes are exited in the MM fiber. Although during scanning with MM fiber, the light reflected back from the sample may be coupled into higher order spatial modes, most of them are filtered out at the MM/SM connection. As a result, when scanning with an MM fiber, the interference fringes behave very similar as for the SM fiber. The contribution related to higher order modes interference, visible when the light scattered back from the edges of aperture or mesa is coupled back to higher modes of the MM fiber, does not disturb significantly the interference signal and actually helps to localize the mesa with a better precision. As can be seen in Fig. 8 row 3, when the MM fiber crosses the mesa sidewall and part of the scattered light is coupled back into higher order modes, the interference fringe related to the fundamental mode is distorted in a narrow spectral range around 915 nm by the intermodal interference. This distortion is the strongest around the center of the mesa. Therefore, for properly selected spectral averaging window (910-920 nm), it is possible to obtain the interference signal profile with similar spatial resolution as for the SM fiber (see Fig. 9(c)), in spite of the fact that the core diameter of the HNA MM fiber (about 15 µm) is much greater than the mesa diameter (0.5 µm).
Remote computer control of the piezo positioner allowed us to perform 2-D scans of the semiconductor structures using the proposed method. The averaged spectral intensity maps obtained using the SM fiber for mesas of diameter D = 2 µm and D = 0.5 µm compared to optical microscope images of the same structures are shown in Fig. 10. The presented results clearly prove that by observation of the interference signal during the XY scanning, it is possible to locate the mesa center. We have estimated the positioning repeatability in a series of tests conducted for the SM and MM fibers for mesas of different diameters. For smaller mesas (D ≤ 2 µm) the alignment repeatability is close to 50 nm for both, the SM and the MM fibers. For cylindrical mesas of greater diameters, the alignment repeatability gradually decreases to about 150 nm because the flatness of the top mesa surface smoothens the feedback interference signal and degrades the positioning precision.
Fig. 10. Optical microscope images of the apertures with mesas of diameter D = 2 µm (a) and D = 0.5 µm (c). 2-D maps of the same structures obtained by collecting the feedback interference signal with the SM HNA scanning fiber (b, d). A zirconia ferrule with the HNA fiber glued to the semiconductor structure with QD (e).
A rough alignment of the fiber in a ferrule with respect to the center of the mesa is carried out at the distance of a few micrometers between the fiber and the top surface of the semiconductor structure. After such preliminary alignment, the semiconductor structure is moved up to reduce the gap between its top surface and the fiber to a few hundreds of nanometers. In the next step, the fiber is realigned to the center of the mesa and eventually set in a physical contact with the sample surface. The physical contact is necessary to keep the distance between the sample and the fiber end-face stable upon temperature changes during the cooling cycles. In such position, the ferrule with the coupling fiber is fixed to the semiconductor structure with UV cured ceramic glue of low coefficient of thermal expansion (CTE = 14 ppm/°C), Fig. 10(e).
The undertaken attempts of gluing the fiber at the required distance from the top mesa surface (controlled by the interferometric feedback signal) have shown that after curing the ceramic glue by a UV LED and during the successive cooling cycles, the final fiber distance changes in an unpredictable way and ranges between 40 and 800 nm due to glue hysteresis and different CTE of the glue, zirconia ferrule, fiber and the semiconductor microstructure. To stabilize the coupling fiber with respect to the mesa at low temperature, the fiber must be glued in physical contact with the semiconductor structure. Therefore, to ensure the required distance between the mesa and the fiber core, the central part of the fiber of diameter of about 10 µm is etched to the required depth by using the focused xenon ion beam, as it is shown in Fig. 11(a). To confirm the stability of the distance between the fiber core and the sample, the fiber etched by 350 nm (in zirconia ferrule) was glued in a physical contact to a BK7 glass plate and cooled down multiple times in liquid nitrogen (77 K). As it is shown in Fig. 11(b), during these experiments practically no change in the broad spectral interference fringe visible in the back reflected signal was observed. For comparison, in the same figure we show the calculated spectral intensity distributions for the gap of thickness of 300 and 400 nm (meaning a change by ± 50 nm from the initial distance). As the measured variations of the back reflected spectral signal in successive cooling cycles are small in comparison to the separation of the reference curves, we conclude that the gap remains constant with a precision of about 10 nm.
Fig. 11. SEM image of the fiber end-face etched by 350 nm (a). Brighter spot indicates location of the GeO2 doped core. Back reflected spectral interference signal measured at room temperature and at 77 K for four consecutive cooling cycles with the etched fiber glued in a physical contact to a BK7 glass plate (b). Black dotted and dashed lines correspond respectively to ± 50 nm change in the distance between the fiber core and the top mesa surface.
4. Positioning accuracy and stability tests
To determine the absolute positioning accuracy and confirm stability of the alignment during gluing process, we used the test semiconductor structures with a quantum well (QW) embedded in the mesas of different diameters between 1.5 and 3.6 µm. The QW emits at room temperature with maximum intensity at λ = 1080 nm. To register the PL signal, we replaced in the set-up shown in Fig. 7 the SC source by the laser diode (λ = 660 nm) and the spectrometer by a free-space InGaAs multichannel linear detector connected to a monochromator. In Fig. 12 we show the comparison of the 2-D scans of the mesa obtained using the spectrally averaged interference signal and the 2-D scans of the same structure acquired by registering the PL emission from the QW at room temperature.
Fig. 12. Side and top views of 2-D mesa topography acquired using the developed interference method (a,c) and the PL emission map collected at room temperature using the same HNA SM fiber (b,d). The size of the scanned square is 5×5 µm. The “mass centers” of the interference and PL emission maps are marked by white lines in (c,d).
Because of the low level of the PL emission signal compared to the interference signal, the PL emission scans are burdened with much greater noise. The absolute positioning accuracy was determined by comparing the positions of the “mass centers” of the interferometric and PL emission scans. For small mesas (D < 2 µm), the determined absolute difference in the positions of the two centers was equal to the positioning accuracy (about 50 nm). For larger mesas (D ≥ 2 µm), a constant offset ranging from a few tens to few hundreds of nanometers was observed between the two centers, which we attribute to nonuniform distribution of the PL emission intensity over the QW surface clearly visible in Fig. 12(d). The conducted experiments prove the high absolute precision (below 50 nm) of the developed interferometric positioning method, especially evident in the case of small mesas.
No change in the PL emission intensity from the QW (monitored at room temperature during the gluing process) confirmed stability of the collecting fiber. For nine tested samples with mesas of different diameters between 0.5 and 3.5 µm (with either QW or high-density QD ensemble) glued to the SM HNA fiber, almost constant PL signal (< 5% changes) was observed during multiple cool-down cycles, thus confirming a successful fixing of the fiber to the semiconductor structure.
Finally, the SM HNA fiber etched by 200 nm was glued using the proposed method to a mesa of diameter D = 0.5 µm with embedded InGaAs/GaAs QD. Using the all-fiber set-up shown in Fig. 1, we have registered the PL emission spectra for the structure with QD-mesa placed in the cryostat and cooled down to 40 K. In Fig. 13(a) we show the registered PL spectrum with a single QD emission lines clearly resolved in the range of 1250-1265 nm. Coupling stability and repeatability of the emission intensity were confirmed by multiple cooling cycles of the structure. Figure 13(b) shows normalized QD peak emission intensity measured via the fiber at 40 K for ten consecutive cooling cycles, for which we have obtained the relative standard deviation equal to 5.7%, most likely caused by a drift of the pump signal related to residual intermodal interference in the leading-in fiber.
Fig. 13. The PL spectrum of the single InGaAs/GaAs QD glued to the HNA SM fiber registered at 40 K with a single QD lines visible in the range of 1255-1265 nm (a). Normalized QD emission intensity for ten consecutive cooling cycles (b).
5. Single-photon emission
To verify the validity of the proposed approach for single-photon sources applications, photon autocorrelation measurements were conducted for a fiber-coupled single QD-mesa in the fiber configuration presented in Fig. 1. For this purpose, we used another, much brighter QD exhibiting a well-resolved emission line at 1203 nm at 40 K (base temperature of the used cryocooler), embedded in a mesa with diameter D = 1130 nm and glued to the HNA fiber using the developed procedure. The PL emission spectrum under non-resonant pulsed excitation of this QD at 40 K measured at the output of the fiber coupling system is shown in Fig. 14(a). After filtering of the selected emission line at 1203.1 nm from the output spectrum with a 0.32 m focal length monochromator (0.3 nm bandwidth – vertical dashed lines in Fig. 14(a)), the optical signal was coupled into a single mode fiber connected to a fiber 50/50 beam splitter. Each output of this beam splitter was connected to a superconducting NbN nanowire single-photon counting module with 20% quantum efficiency and dark counts below 100 Hz. The histogram of coincidences between the two detectors was further acquired using a multichannel picosecond event timer providing an overall temporal resolution of the Hanbury-Brown and Twiss setup of 80 ps.
Fig. 14. The PL spectrum of the single InGaAs/GaAs QD glued to the HNA SM fiber operated at 40 K (a) for which the autocorrelation measurements were conducted with the bandwidth of the spectral filter marked with vertical dashed lines (b). The fit (red) to the experimental data (blue) yields value of the second order correlation function at zero time-delay equal to g(2)(0) = $0.035_{ - 0.035}^{ + 0.073}$ after background subtraction (raw value: g(2)(0) = 0.32).
Excitation-power dependent PL spectra were measured (not shown here) and the average excitation power of 50 µW corresponding to 20% of the saturation power leading to the spectrum shown in Fig. 14(a) was selected for the autocorrelation measurements, the results of which are shown in Fig. 14(b). Substantial emission background of 1000 counts/s (compared to the dark counts of the detectors < 100 counts/s) is present in the spectrum and results in rather high level of the uncorrelated background in the coincidences histogram (gbg=0.3). This might be related to the strain reducing layer applied in the investigated sample to redshift the emission towards 2nd telecommunication window. It could provide a reservoir of shallow-bound charge carriers that might hop or tunnel into the QD and lead to additional photons not created directly by the optical excitation and therefore not exhibiting time-correlations with the radiative recombination trapped in the QD directly after excitation. This additional layer might also exhibit a low-energy states that spectrally overlap with the QD emission and therefore are not separable. These effects are related to the particular sample and are not important to evaluate the usefulness of the proposed direct fiber-coupling approach of for single-photon sources applications. Clear antibunching is observed in the histogram of coincidences at zero time-delay proving a single-photon character of emission from the QD under investigation. Experimental data were fitted (red solid line in Fig. 14(b)) as in [30] using the following function:
6. Conclusions
We have proposed and realized an effective method for direct coupling of single solid-state quantum emitters to an SM or MM optical fiber. This was possible thanks to overcoming several technical challenges, including fabrication of an SM or MM high aperture fibers with refractive index profile optimized for low temperature operation and etching the fiber end-face to predetermined depth to allow for the physical contact between the fiber and the surface of the semiconductor microstructure which is necessary to ensure stability during gluing process and temperature cycles. Moreover, the interferometric alignment method was developed for adjustment of the HNA optical fiber to the center of the semiconductor mesa with embedded single QD at room temperature. The alignment precision for mesas of diameter D < 2 µm was about 50 nm for both SM and MM fibers. According to the numerical simulations, the maximum coupling efficiency in our method may exceed 23% for optimal values of the structure parameters. We have demonstrated the feasibility of the proposed method by coupling a single InGaAs/GaAs QD deterministically embedded in the mesa to HNA SM fiber. We have measured the PL emission spectra in 10 consecutive cooling cycles for the fiber coupled QD. The relative standard deviation of the PL intensity was equal to 5.7%, which proves the coupling stability and repeatability. For a different fiber-coupled QD, showing a well-resolved and bright emission line at 1203 nm at 40 K, the photon autocorrelation measurements were conducted, for which a value of the second order correlation function at zero time-delay g(2)(0) equal to 0.035 was obtained after background subtraction, proving the applicability of the presented coupling method to single-photon sources.
It should be underlined that the proposed method can be used for coupling solid state quantum emitters of different types (i.e. not only QDs but also nitrogen-vacancy centers or transition-metal-dichalcogenide-based single photon emitters) to SM or MM fibers, independent of the emission wavelength (e.g. 0.85, 1.3, and 1.55 µm) and material.
Funding
Narodowe Centrum Badań i Rozwoju (2/POLBER-2/2016); European Regional Development Fund (Pro FIT, FI-SEQUR); Deutsche Forschungsgemeinschaft (CRC787); European Metrology Programme for Innovation and Research (EMPIR 17FUN06 SIQUST); Polish National Agency for Academic Exchange; Horizon 2020 Framework Programme.
Acknowledgments
We acknowledge the custom-designed picosecond pulsed laser provided by PicoQuant company.
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