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Abstract
Photonic crystal lasers with a high-Q factor and small mode volume are ideal light sources for on-chip nano-photonic integration. Due to the submicron size of their active region, it is usually difficult to achieve high output power and single-mode lasing at the same time. In this work, we demonstrate well-selected single-mode lasing in a line-defect photonic crystal cavity by coupling it to the high-Q modes of a short double-heterostructure photonic crystal cavity. One of the FP-like modes of the line-defect cavity can be selected to lase by thermo-optically tuning the high-Q mode of the short cavity into resonance. Six FP-like modes are successively tuned into lasing with side mode suppression ratios all exceeding 15 dB. Furthermore, we show a continuous wavelength tunability of about 10 nm from all the selected modes. The coupled cavity system provides a remarkable platform to explore the rich laser physics through the spatial modulation of vacuum electromagnetic field at submicron scale.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photonic crystal (PhC) lasers are promising candidates for integrated light sources in nano-photonic circuits due to their small mode volume and high Q factor [1–3]. Over the past few decades, ultrafast PhC lasers with direct modulation speed far exceeding 100 GHz have been demonstrated [4]. Electrically pumped PhC devices operated at room temperature have been realized with high-speed modulation bandwidth and low operation energy [5]. Ultracompact PhC lasers using III-V active regions have been successfully integrated with silicon photonics [6–8]. These results have drastically increased the application potential of PhC nanolasers. However, due to the small volume of active region, PhC lasers or in general all nano-/micro-cavity lasers suffer from low output power which limits their applications in driving large-volume photonic integration. One of the possible solutions is to extend the cavity length in one dimension to form a line-defect cavity, which can achieve higher output power while maintaining the narrow spectrum linewidth of PhC lasers [9,10]. The FP-like lasers with longer cavities have lower threshold gain and higher threshold injection density, accounting for the increases of the average photon density and output power [11–13]. Meanwhile, the slow-light-enhanced gain leads to a reduction in the pumping power density at the lasing threshold as well as an unconventional dependence of the threshold gain on the cavity length [9]. However, the line-defect cavities hold quasi-FP-like modes supporting multi-mode lasing with a small free spectral range (FSR) due to strong light dispersion in PhCs [14,15]. Therefore, the effective manipulation of mode oscillation to achieve single-mode lasing is important in controlling laser emission at submicron scale.
Several approaches have been proposed and developed in traditional semiconductor lasers to realize single-mode lasing, such as a larger free spectral range (FSR), external cavity feedback [16–18], parity-time (PT) symmetry effect [19–21], and spatial injection [22–24]. Alternatively, the modulation of mode Q factor [10,25,26] can be used to realize FP mode selection in a waveguide cavity by coupling to a high-Q cavity in microcavity lasers. By spectrally tuning the high-Q mode into resonance [27], the enhancement of the Q factor of FP modes has been observed which results in single-mode lasing.
In this work, we demonstrate for the first time a mode selection at submicron scale in a line-defect PhC laser through the modulation of mode Q factor. Coupled double-heterostructure (DHS) cavities with 2-period and 80-period PhC lattices have been employed [28]. The long DHS cavity with 80-period lattice constants forms a FP cavity. The short DHS cavity with 2 period lattice constants supports two modes with comparable Q factors and spatially separated mode profiles, which can be well selected by varying the spatial injection [24,29]. By spectrally tuning one of the two high-Q modes of the short cavity into resonance with the FP cavity, one of the FP-like modes is selected to lase with an increased Q factor due to the redistribution of vacuum field in between coupled cavities. Combining the closely-spaced multi-mode in the line-defect cavity and the two tunable high-Q modes in the short cavity, six FP-like modes can be successively selected to lase with single mode suppression ratios (SMSRs) exceeding 15 dB. The lasing wavelength of each mode can be tuned up to 0.8 nm by changing the input power in the two DHS cavities simultaneously. The entire wavelength tuning range of all six modes covers near 10 nm.
2. Method
Our platform for Q-factor modulation to realize single mode lasing is based on coupled photonic crystal DHS cavities. The cavities are fabricated in a 220 nm-thick suspended In0.74Ga0.26As0.57P0.43 membrane, which provides a luminescence peak around 1160 nm at 78 K. A single layer of 5 nm InGaAs quantum well (QW) is sitting in the middle of the membrane. The lattice constant of PhC is a0 = 450 nm with a filling factor of 0.29. The DHS cavities are defined by slightly modifying the lattice constant along a W1 PhC waveguide from a0 to a1 = 1.03×a0 (Fig. 1(a)). The FP and short DHS cavities consist of 80 and 2 periods of PhC lattices, respectively. In the coupled cavity system, the barrier between two cavities contains five periods of the original lattice constant. A continuous-wave (CW) laser working at 976 nm is used to optically pump PhC cavities in a micro-photoluminescence (PL) setup which is operated at 78 K. Elongated laser spots with an adjustable length are implemented by using cylindrical lenses to feed either the FP cavity or short DHS cavity. To modulate the vacuum field in between the coupled cavities, the short DHS cavity is pumped with a CW laser working at 637nm as the heat source. The collected area is at one of the side mirrors of the FP cavity, which is located far from the short DHS cavity to ensure that the collected signal is emitted from the FP cavity.
Fig. 1. a. Schematic of the coupled photonic crystal DHS cavities in an InGaAsP membrane. b, short DHS cavity with 2 periods of the lattice constant slightly larger than the original lattice constant (a1 = 1.03 x a0) works as the mode selector. c, FP cavity with 80 periods of lattice constant works as a multi-mode laser under uniform pumping with an elongated laser spot. d, The coupled-cavity laser appears with single-mode lasing while the high-Q mode of short DHS cavity is tuned into resonance with one of the FP modes.
As shown in Fig. 1(b), the short DHS cavity with 2-period of PhC lattice can support two modes with comparable Q factors and spatially separated mode profiles. The first bound state mainly localizes inside the cavity while the second bound state largely stays in the waveguide in the vicinity to the cavity [29]. The spatially separated mode profile indicates that both modes can be selected to be the dominating mode by spatial injection [24,27]. Therefore, the short DHS cavity provides two controllable optical windows to extend the wavelength tuning range of the FP laser. The change in the refractive index based on thermo-optic tuning redshifts the mode wavelength and thereby precisely controls the detuning [30,31]. Figure 2(c) shows the FP cavity with 80-period lattice constant, which supports multi-mode lasing with a small FSR due to the strong dispersion. Single mode lasing is achieved by thermo-optically shifting the high-Q mode of the short DHS cavity into resonance with one of the FP-like modes. Therefore, the short DHS cavity acts as a mode selector in this coupled cavity laser as shown in Fig. 1(d).
Fig. 2. Experimental laser behavior of the 80-period photonic crystal DHS cavity under elongated laser spot pumping. a, The emission spectra at different pumping power. b, The light-light curves and linewidth as functions of pumping power of three main modes.
3. Mode characteristics of single cavities
To demonstrate the mode selection in coupled cavities, control experiments have been conducted in both DHS cavities with 2- and 80-period modulated lattice constant. Figure 2(a) shows the evolution of the optical spectra of the mode intensity by increasing the pump power, which is measured in the 80-period DHS cavity under uniform pumping with an elongated laser spot. Each mode can be defined by the number of nodes in the sinusoidal field envelope alongside the defect cavity. The group refractive index near the band edge of the PhC cavity exceeds 20, which is closely related to the mode interval. Figure 2(b) displays the light-light curves and linewidth as a function of pumping power of three main modes. The measured thresholds for these main modes are all around 240 µW, revealing the level of comparability in multi-mode competition. The FP cavities always appear to be multi-mode lasing due to the broad gain spectrum and the decreasing FSR in the slow-light region in PhCs.
Figure 3(a) shows the evolution of the optical spectra of the mode intensity when the pumping power at the short DHS cavity increases. We observe that two modes achieve lasing simultaneously in the case of uniform pumping. The light-light curves and the linewidth as functions of pumping power of these two modes are shown in Fig. 3(b), from which we observed the clear threshold at the pumping power of 30 µW. The mode at longer wavelength (1507.7 nm) is the first bound state, in which the electromagnetic field is localized inside the cavity (in the inset of Fig. 3(c), the black dotted line is marked as the axial position of the short DHS cavity). The mode at shorter wavelength (1505 nm) is the second bound state, in which the electromagnetic field is distributed into the waveguide. We plot the mode intensity profile along the waveguide direction of short DHS cavity in Fig. 3(d). The local maxima of the first and second bound states are separated by 1.57 µm. The vacuum zero-point energy, which exists ubiquitously due to the quantization of electromagnetic fields in the resonant cavity, has the same distribution as the field of standing wave of the cavity. Governed by the spatial variation of the vacuum electromagnetic field, the single-mode lasing can be selected by adjusting the pumping spot position [29]. While the laser spot is overlapped with the center position of the mode profile, the corresponding mode can be selected. SMSRs over 25 dB and 20 dB are demonstrated for the first and second bound states, respectively.
Fig. 3. Mode behavior of the 2-periods short DHS cavity. a, The light emission spectra show that two modes achieve lasing simultaneously. b, c, The first or second bound state can be selected to lase with a high SMSR value while the pumping laser spot is placed at the center position of the mode profile. d, The mode intensity profile along the waveguide direction of short DHS cavity.
4. Mode characteristics of coupled cavities
3.1 Coupling process
We employ a two-beam pumping scheme to operate the two cavities separately by locating two laser spots at each of the cavities. Figure 4(a) shows the spectrum map versus pumping power on the short DHS cavity. The mode at 1504.63 nm red-shifts with increasing power is the second bound state of DHS cavity, and the mode red-shifts from 1508.77 nm is the first bound state. Several FP-like modes are clearly observed in the map with relatively small red shifts due to the heat leakage from the short DHS cavity. As shown in the region within the yellow box in Fig. 4(a), the strong coupling between second bound state and FP modes has been observed, where the wavelength of coupled modes reveals anti-crossing. Differently, the crossing behavior has been observed in the region within the green box in Fig. 4(a), indicating the weak coupling between the first bound state and FP modes. These experimental results show that the coupling between the second bound state and the FP mode is stronger than first bound state. It is due to the electromagnetic field distribution of the second bound state in the waveguide in the vicinity of the short DHS cavity, suggesting that a part of the mode field of the second bound state sits in the middle barrier in the coupled cavity system thereby increasing the coupling strength. Two crossing points remarked with number 1 and 2 are selected for analysis, the corresponding change in the Q-factor is up to 40% for the strong coupling (Fig. 4(b) and 4(c)) and up to 20% for the weak coupling (Fig. 4(d) and 4(e)), respectively. In Fig. 4(e), yellow dots show a sharp Q-factor increase of the FP mode, this is due to the coupled modes achieving to lase driven by the input power on the DHS cavity. Both lasing-induced linewidth changes and coupling-induced linewidth changes result in the dramatic change of the Q-factor. In photonic crystal waveguides, fabrication induced disorder leads to propagation losses and Anderson-like localization [32], which could affect the threshold characteristics and the coupling efficiency between the modes of the two cavities and hence needs to be treated with care.
Fig. 4. Q factor tuning of the FP-like modes when coupling to the first and second bound states. a, A spectrum map exhibits the coupling between the bound states of the short DHS cavity and the different FP modes, which corresponds to different pumping power at the short DHS cavity. b c, The wavelength and Q factor of the coupled modes as a function of the detuning at the anti-crossing point marked with number 1. d e, The wavelength and Q factor of the coupled modes as a function of the detuning at the crossing point marked with number 2.
3.2 Spectrum comparison
In order to intuitively explain the mode selection mechanism while two modes are in resonance, the scanning of the first bound state is shown in Fig. 5. The gray line in Fig. 5(a) is the multi-mode lasing spectrum of the coupled cavity when only the FP cavity is pumped. While the relative wavelength position of the coupled modes changes, the output spectrum of this system would be different. The shaded area in Fig. 5(a) is marked as the red-shift area of the high-Q DHS mode due to the change of the pump power on the short DHS cavity. By carefully adjusting the detuning, one of the FP modes is selected to lase with an SMSR exceeding 15 dB while the high-Q DHS mode is in resonance (Fig. 5(b)), the arrows point to the wavelength position of the high-Q DHS mode). If we further increase the detuning, the Q factor of two coupled modes cannot be efficiently modulated to suppress other modes, therefore the system recovers multimode lasing (Fig. 5(c)). The method of Q-factor modulation can be understood as reducing the threshold gain of the desired mode to increase the efficient consumption of the carrier. Compared to the other approach in which the mode selection is achieved by PT-symmetry breaking, the loss of the desired mode is decreasing while the system transfers to PT-symmetry breaking phase. There are also some novel approaches, such as spatial optical injection, which rely on the spatial overlap between spatial optical gain distribution and the mode profile of desired modes.
Fig. 5. The evolution of light emission spectrum while the wavelength of the high-Q DHS mode of the short DHS cavity is redshifted. a, Multi-mode lasing spectrum of the coupled cavity without pumping the short DHS cavity. The shaded area shows the wavelength range where the high-Q DHS mode is tuned. b, One of the coupled modes gets lasing with a high SMSR value in resonance. c, The system is multi-mode lasing while two cavities are out of resonance.
5. Stable single-mode lasing
As shown in Fig. 3, the first and second bound states of the short DHS cavity can be used to create two individual optical windows to extend the single-mode lasing range. The shaded areas in Fig. 6(a) and 6(b) display the wavelength shifting range of high-Q DHS mode. By thermo-optically tuning the high-Q DHS mode, six FP-like modes are selected to lase successively while they are in resonance, covering a broad tuning range near 10 nm due to the so-called Vernier effect [33,34]. Three modes can be selected to single-mode lasing with a SMSR over 15 dB for the first bound state window (Fig. 6(a)) and over 20 dB for the second bound state window (Fig. 6(b)). The combination of Q-factor modulation and Vernier effect in coupled cavities improves the flexibility of mode selection. In principle, mode selection based on Q-factor modulation can be employed to design the coupled PhC cavities with different structures to meet the practical demand, which is also suitable for other types of microcavities.
Fig. 6. Mode selection of coupled PhC cavities. a, Three modes are selected to lase successively with a SMSR value exceeding 15 dB while the first bound state is in resonance with one of the FP modes. b, Three modes are selected to lase successively with a SMSR value exceeding 20 dB in the second bound state window.
6. Wavelength tunable single-mode lasing
In Fig. 7(a), we show the continuous wavelength tunability over 0.8 nm for the selected single mode. The method is based on the manipulation of mode coupling by changing the pumping power on the 2-period and 80-period DHS cavities simultaneously. Meanwhile, the continuous wavelength tunability can be realized for each mode in both of the optical windows. We demonstrate the continuous tunability in the range of 10 nm by using this system (Fig. 7(b)). The mode selection attributes to the detuning between one of the FP-like modes and high-Q DHS mode which are further determined by the refractive index change in both cavities. The PhC cavity can achieve a wide range of tunability [35–39] due to its strong nonlinearity, thereby increasing the wavelength tunability of the lasing modes in the coupled system without special design. By combining the optical nonlinearity, Vernier effect, and Q-factor modulation, the coupled PhC cavities can provide high output power and tunable single-mode laser for on-chip integration in the application of optical interconnection. Considering the sensitivity of optical nonlinearity enhanced by slow-light effects, the coupled PhC cavities can offer promising platforms for a range of sensing applications, such as temperature sensing and refractive index sensing [40–42]. Furthermore, the coupled cavity system can find its applications in multi-channel sensors, while the coarse adjustment is realized via selecting different modes of the FP cavity for sensing and the fine adjustment is realized via the continuous shifting of the resonance point.
Fig. 7. Wavelength tunable of selected modes. a, Experimental result shows an extended tunability of 0.8 nm for a single mode while changing the input power on short DHS and FP cavities, simultaneously. b, Considering all the modes that can be selected, wavelength tuning close to 10 nm can be achieved.
7. Conclusion
In conclusion, we have proposed and demonstrated a tunable single mode laser based on coupled photonic crystal DHS cavities through the modulation of the Q factor of coupled modes. The coupling between 2-period cavity mode and FP-like mode has been observed as a result of the redistribution of the vacuum field. The short DHS cavity can be efficiently manipulated due to the spatial separated mode field profile, therefore extending the lasing mode selection window. While the high-Q DHS mode is in resonance with one of the FP-like modes, there are six FP-like modes that can be individually selected to lase with a SMSR exceeding 15 dB. Meanwhile, the continuous tunability in the range close to 10 nm in this system has been demonstrated. This coupled PhC scheme has improved the laser performance with increasing output power, single-mode lasing, and wavelength tunability, which can be critical for the future application of PhC nanolasers in integrated nano-photonics. The mechanism of coupled PhC cavities can be adopted for materials with higher thermal conductivity, such as InGaAsP/InP [43], InGaAlAs/InGaAlAs [5] QW membranes, and InP membranes with InAs QDs [2], to further realize room temperature operation and thereby enable our general idea applied to on-chip nanophotonic integration.
Funding
National Key Research and Development Program of China (2021YEB2800500); National Natural Science Foundation of China (61574138, 61905217, 61974131); Natural Science Foundation of Zhejiang Province (LGJ21F050001); Major Scientific Project of Zhejiang Laboratory (2019MB0AD01).
Acknowledgments
We thank Micro/Nano Fabrication Center of Zhejiang University and Westlake Center for Micro/Nano Fabrication for the facility support and technical assistance.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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