1. Introduction

Nonlinear optical effects are enhanced in micro/nano photonic structures with strong light confinement, such as photonic nanowires [1,2], whispering gallery mode resonators [3,4], photonic crystal (PhC) waveguides [5–7] and nanocavities [8–11]. Compared to conventional nonlinear optics in bulk crystals, such photonic structures have the potential to achieve strong nonlinear effects at lower powers and with greatly reduced device sizes, together with integratability into optical circuits. These advantages have been recently demonstrated in several works on nonlinear effects such as second/third order harmonic generation [2,4,6–10,12], by using external laser sources to introduce photons to the structures. These nonlinear optical wave mixers, whilst being useful for generating coherent light at wavelength regions inaccessible by conventional semiconductor lasers, are held back by the requirement of external laser sources and accompanying cumbersome optical setups, which tend to make the system voluminous, unwieldy and hence costly. Needless to say, this obstacle also holds true in conventional bulky light sources based on nonlinear frequency conversion.

In this context, semiconductor nanolasers with a function of self-frequency conversion (SFC) [13,14], in which both lasing action and nonlinear frequency conversion take place within the same laser material, can provide an ideal solution. Such SFC nanolasers will directly generate coherent light of various wavelengths within the nano-scale devices, potentially by current injection. Strong nonlinear effects could be available even within tiny resonators by adopting cavity designs with high quality (Q) factors and small mode volumes (V) [12,15–18]. So far, several monolithic light sources using SFC [19,20] have been studied, however, none of them have made use of the wavelength-scale optical confinement effect and have been closely investigated in terms of the effect of Q/V on their performances.

In this work, we demonstrate the hitherto-unexplored, nanocavity-based SFC lasers by means of monolithically-fabricated PhC nanocavity quantum dot (QD) lasers [21,22]. The nanocavities support continuous-wave (CW) lasing at the near infrared (NIR), which is up-converted to coherent visible (VIS) light by efficient intra-cavity second harmonic generation (SHG), facilitated by high Q/V and the large second order nonlinear susceptibility, χ⁽²⁾ ( = 170 pm/V at 1064 nm) [23], of GaAs-based nanocavities. The efficient conversion processes enable us to observe SHG using only a few intracavity NIR photons in average and show a further improvement by strengthening the optical confinement effect. Moreover, we establish a micro-scale integration of 26 different-color lasers, taking advantage of high integratability of our device and a broadband QD gain.

2. Device design, fabrication and experimental setup

A schematic of one of the PhC nanocavity lasers under investigation is shown in Fig. 1(a). The structure is based on an air bridge two dimensional PhC membrane consisting of a periodic air hole lattice. The lattice constant, a, is varied from 244 to 340 nm in the following experiments, and the total area of the single cavity occupies only 40 μm² on average. From these tiny lasers, both coherent VIS and NIR light can be generated without any additional assembly of optical elements.

 

Fig. 1 Structure of PhC nanocavity QD lasers investigated. (a) Schematic of the GaAs-based cavity structure, showing the simultaneous emission of coherent VIS and NIR light: the former is generated via the intra-cavity frequency doubling. The three missing airholes in the PhC lattice serve as the defect cavity, supporting the NIR lasing. The PhC slab contains 6 layers of InAs QDs, providing broadband gain in the NIR. In the following experiments, cavities with different lattice constant a spanning 244 to 340 nm are investigated. All the cavities are monolithically fabricated into the QD wafer. The total area of the cavity occupies only 40 μm² on average. (b) Scanning electron micrograph image of a L3 PhC nanocavity, forming an air bridge structure. The lower end of the structure is cleaved to clarify the air space under the slab, which is formed by removing an Al_0.7Ga_0.3As sacrificial layer by a wet etching process. (c) Electric field distribution of the fundamental cavity mode investigated. Air hole positions are overlaid in the plot. (d) PL spectrum of the QD wafer at 10 K, taken by optical pumping with a power of 12 μW. The wavelengths of 1040 and 1170 nm indicate the ground state emission peaks of the stacked QDs, grown by two different growth conditions.

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All the cavities are fabricated monolithically into the same semiconductor wafer by a simple combination of standard semiconductor fabrication technologies. The wafer was grown on a (100)-oriented semi-insulating GaAs substrate by molecular beam epitaxy and consists of a 200-nm-thick GaAs slab incorporating 6 layers of self-assembled InAs QDs, deposited on top of a 1000-nm-thick Al_0.7Ga_0.3As sacrificial layer that was later removed.

The airbridge cavities are monolithically fabricated by a single-step electron beam lithography process, followed by an inductively coupled plasma reactive ion etching and a wet etching by a hydrofluoric acid solution. A scanning electron microscope image of a fabricated nanocavity is shown in Fig. 1(b), exhibiting successful patterning of the device. The three missing air holes (forming the defect cavity) are patterned along the crystal orientation of [110].

The cavity design is based on the so-called modified L3 type [24], a plan view of which is overlaid with the calculated electric field distribution in Fig. 1(c). This fundamental cavity mode confines light within a very small V of 0.032 μm³ (for a = 314 nm) and possesses a high theoretical cavity Q of more than 150,000. This mode volume is thousands times smaller than that of previously-reported SFC lasers based on vertical cavity surface emitting lasers [20]. The other, higher order, cavity modes have sufficiently-low cavity Qs such that only single mode lasing was observed in the entire operational wavelength range. The air hole radius is fixed to 0.29a for all cavities. In order to achieve higher Q, the first and third air holes are shifted outwards by 0.2a.

Control over the lasing wavelength, λ, can be done simply by changing the PhC lattice constant, a. We observed that λ obeys a linear relation to a: λ = 295 + 2.8a nm, for a ranging from 244 to 340 nm.

Gain in the PhC nanocavity lasers is provided by 6 layers of InAs QDs grown in the GaAs slab. To obtain broadband gain, the bottom two QD layers were grown at a higher temperature, so that their ground state emission was shortened to 1040 nm (130 nm shorter than that of the upper 4 layers). Figure 1(d) shows a photoluminescence (PL) spectrum from the QD wafer at 10 K. The emission covers a wide range of the NIR spectrum, spanning 950 nm to 1260 nm, when including the contribution from the QD’s excited states. The peak wavelengths of the QD ground state emission are assigned by investigating the excitation power dependence of the PL spectrum.

The samples were mounted in a liquid helium flow cryostat kept at 10 K and characterized by a confocal micro-PL setup. The sample position was controlled by a combination of a motorized stage and a piezoelectric positioner. Excitation was provided by a CW diode laser oscillating at 808 nm, focused through a x100 objective lens. The excitation power is defined as that measured after the objective lens, and the estimated spot diameter on the sample surface is about 3 μm. The PL signal was collected by the same objective lens and analyzed by spectrometers equipped with nitrogen-cooled multi-channel detectors for VIS and NIR. VIS near field images were taken one by one using a color charge coupled device camera, together with a stack of shortpass filters, which allows transmission of the VIS (450-640 nm), while strongly rejects the NIR (660-1320 nm). In this measurement, the excitation powers are set far above the thresholds of the nanolasers: 1.5 mW for most of the samples and 3 mW for samples with emission wavelength λ > 620 nm and < 510 nm. All the images are taken with a fixed accumulation time of 1 sec.

3. Optical characterization of SFC nanolasers

3.1 Light-in versus light-out characteristic

As an initial optical characterization, the excitation power dependence of a sample with a PhC lattice constant of 314 nm was investigated by the optical pumping. The light-in versus light-out (L-L) plot for the NIR cavity mode emission at 1174 nm is shown in the upper panel of Fig. 2(a). The nanocavity mode shows a laser oscillation with a lasing threshold of 12 μW, determined from the inflection point of the L-L curve. Figure 2 (b) shows a transition of the cavity linewidth, showing a significant linewidth narrowing. From the linewidth at the lower power edge of the transition region (corresponding pumping power of 4 μW), a cold cavity Q factor of 15,000 is deduced. The inset of the upper panel in Fig. 2(a) shows a NIR lasing spectrum of the sample under a pumping power of 590 μW, exhibiting single mode lasing with high spectral purity. From the same cavity, at the same time, a sharp, single emission peak at 587 nm is observed, as shown in the lower panel of Fig. 2(a), along with the respective L-L curve. For pumping powers larger than 40 μW, which corresponds to the linear region of the NIR laser, the VIS emission shows a quadratic power dependence. Figure 2(c) shows a plot of the VIS light intensity, I_VIS, measured at the detector as a function of that of the NIR laser mode intensity, I_NIR. The plot again displays a clear quadratic relation between the intensities of the two emission lines, I_VIS∝I_NIR². In addition, the peak wavelength of the VIS emission is observed to be half of that of the NIR peak throughout the measurements. Therefore, the VIS emission is attributed to the intra-cavity SHG of the NIR light in the nanolaser. This is, to the best of our knowledge, the smallest solid state yellow laser yet achieved: the color is still inaccessible by conventional semiconductor laser diodes. Remarkably, we observed the SFC signal even near the NIR lasing threshold, where only a few photons exist in the cavity on average, as demonstrated in the following section. This observation encourages the application of these nanocavities to the study of few-photon nonlinear optics: an exciting subject in various research fields such as quantum information technology [25,26].

 

Fig. 2 Optical characterization of PhC nanocavity QD lasers. (a) (Top) L-L plot of the NIR lasing mode at 1174 nm. The cavity shows lasing with a threshold pumping power of 12 μW assigned by the inflection point in the curve. The lasing threshold power is indicated by a blue line in the plot. The transition region of the laser (blue shaded) lies in excitation power range from 4 μW to 40 μW. Inset shows a lasing spectrum taken with a pumping power of 590 μW. (Bottom) L-L plot for the VIS emission peak at 587 nm generated by the intra-cavity SHG. Inset shows the narrow visible emission peak taken with the same pumping for the NIR case. In the linear region of the NIR laser, which corresponds to an excitation power larger than 40 μW, the curve shows quadratic dependence, suggesting the occurrence of intra-cavity SHG. Interestingly, the VIS light is observable even near the lasing threshold, where only a few photons exist in the cavity on average. (b) Plot of the cavity linewidth as a function of the excitataion power. (c) Plot of the VIS emission intensity, I_VIS, as a function of the NIR peak Intensity, I_NIR. The plot shows quadratic dependence regardless of the operation point of the NIR laser. Data were taken at 10 K.

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3.2 Estimation of intra-cavity photon number and frequency conversion efficiency

We estimate the average cavity photon number at the lasing threshold of the fundamental NIR mode by simulating the laser output characteristics using a rate equation model [27]. The model is described for cavity photon number (N_ph) and carrier number (N), and formulated as follows,

 

Fig. 3 (Left) Comparison of calculated L-L curve with the experimetal results. β values used in the calculations are denoted beside the L-L curves. The best fit to the experimental results obtained by the curve with β = 0.11. From the L-L curve for β = 0.11, the average cavity photon number at the laser threshold, $N_{ph}^{TH}$, is found to be 3.4. (Right) Measured nonlinear frequency conversion efficiencies, η ∝I_VIS/I_NIR², for different ten SFC nanolasers with the same design parameters. The values are plotted as a function of the respective measured Q factors.

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It is worth noting that $N_{ph}^{TH}$ has a strong dependence on the value of β in the rate equation model: once we adopt β of ~0.1, $N_{ph}^{TH}$ is found to be around a few photons even when we change the values of other parameters. This fact is also confirmed in the other rate equation models [28].

Next, we estimate the external conversion efficiency, η, defined by η = ${P_{VIS}^P/\left(P_{NIR}^P\right)}^2$, where $P_{NIR}^P$ and $P_{VIS}^P$ are the estimated output power of the NIR lasing mode, and the detected power of the VIS emission measured after the spectrometer, respectively. P is the excitation power in the experiment. In order to estimate $P_{NIR}^P$, we firstly obtain the average cavity photon number of the NIR mode, $N_{ph}^P$ by using the relationship $I_{NIR}^P/I_{NIR}^{TH}=N_{NIR}^P/N_{NIR}^{TH}$, where $I_{NIR}^P$ and $I_{NIR}^{TH}$ are the measured intensities of the NIR lasing mode at the pumping power P and at the lasing threshold, respectively. With this relation, a $N_{ph}^P$ value of 285 is obtained for P = 370 μW. Then, the output power of the NIR lasing mode, $P_{NIR}^P$, is evaluated to be 5.1 μW by using the relation, $P_{NIR}^P=\hslash {\omega }_{NIR}\kappa N_{ph}^P$, where κ is the cavity photon leakage rate (∝1/Q). At the same pumping power, the power of the VIS emission, $P_{VIS}^P$, is experimentally measured to be 26 fW. Therefore, the external conversion efficiency η = ${P_{VIS}^P/\left(P_{NIR}^P\right)}^2$ is determined to be 0.1%/W: a high value in spite of the thin nonlinear material. It is worth noting that the estimated external conversion efficiency, η, is obtained from a comparison of the ideally-estimated NIR laser output power without any consideration on the detection process, with the experimentally-obtained VIS emission power, which is imperfectly collected by the objective lens and is damped significantly by passing through numerous optical elements in the setup. Especially, signal transmission efficiency after the collection objective lens is only ~10%. Therefore, if we take into account those losses, the estimated η should be at least one order of magnitude larger than the value presented above.

We also estimated η for the 9 different SFC nanolasers on the same wafer by following the same procedure above. The 9 different nanolasers have different measured cavity Qs, presumably due to unequal effects of fabrication errors on each cavity. Right panel of Fig. 3 summarizes the obtained values of η from these ten samples in total, plotted as a function of the respective experimental Q factors. We observe a quadratic increase of η with respect to Q, agreeing with theoretical prediction presented in the Appendix and with previous studies using passive resonantors [10,29]. This result demonstrates that, even in active SFC nanolasers, η can be rapidly improved by enhancing the optical confinement.

4. Micro-scale integration of multiple different-color SFC nanolasers

The demonstrated VIS nanolaser can be combined with the broad QD gain, so as to realize monolithic integration of many lasers with different color emission within a micro-scale semiconductor chip. We investigated PhC nanocavitiy QD lasers with 26 different values of a, ranging from 244 nm to 340 nm. These nanolasers are monolithically patterned within a tiny footprint of 10 × 385 μm², as shown in Fig. 4(a). A series of emission spectra for both the VIS and NIR range are shown in Fig. 4(b). In the NIR, the QD gain supports single mode lasing for a wide NIR range spanning 986 to 1254 nm, which is, to the best of our knowledge, the broadest lasing band using a single semiconductor wafer. These coherent NIR emissions are up-converted to the VIS by the intra-nanocavity SHG and result in multi-color emission seamlessly spanning 493.3 nm to 627.0 nm. As a whole, the integrated array of SFC lasers cover extremely broad wavelength range in VIS and NIR. This result highlights the advantage of active SFC nanolasers, which do not require many pumping lasers with differentwavelengths in order to produce various colors of light, in stark contrast to devices based on passive SHG. Figure 4(c) shows a series of near field images of the VIS emission from the nanocavities. Also in these images, we confirm the colorful emission from the SFC nanolasers: blue-green, green, yellow, orange and red.

 

Fig. 4 Multi-color coherent light sources monolithically integrated within a micron scale region. (a) Arrangement of the 26 lasers. They form a line with a spacing of 15 μm, and are hence integrated in a tiny area of 10x385 μm². From the left to right, the lattice constant a is increase from 244 to 340 nm with a constant step of 3.85 nm. Because the cavities are patterned sparsely, further dense integration is possible. (b) Normalized emission spectra from the nanolasers both at the VIS (left) and NIR (right). Each single peak corresponds to lasing spectra from a single nanocavity. The peak wavelengths show red shift as the lattice constant a increases. (c) Color near-field images of PhC nanocavities with various values of a from 244 nm to 340 nm. From the top left to bottom right, a increases with a constant step, resulting in a linear increase of the emission wavelength with an average increment of 5.4 nm. The pumping power is 1.5 mW, which is far above from the lasing thresholds, and is increased to 3.0 mW for samples with emission wavelength λ > 620 nm and < 510 nm. The bottom right box shows a 2 μm scale bar. Data were taken at 10 K.

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The left panel of Fig. 5 shows a microscope image of a PhC cavity with a lattice constant of 314 nm. The image is taken under 650 nm light illumination and plotted in a grey scale. White lines have been added to show the edge of the PhC lattice and the defect cavity. After recording this image, a color image of the PhC nanolaser was also taken, as shown in the right panel. Using the same white lines as in the left panel, we can estimate the near field pattern of the visible light within the cavity structure. The two main bright lobes are confirmed to be located just above and below of the defect formed by three missing air holes. This main two bright lobes, separated by a dark region at the defect cavity, bear a striking resemblance to those observed in SHG processes using gallium phosphide and silicon PhC nanocavities: the former study achieved SHG through inducing a nonlinear polarization perpendicular to the PhC membrane [9], and the latter has a large contribution of surface SHG [10]. In both cases, the field maximum of the NIR mode does not coincide with that for the observed VIS field. We also note that these VIS near field patterns may also be explained by a nonlinear polarization parallel to the PhC slab. Elucidation of the exact nanoscopic origin of the nonlinear process in the present study will require further investigation, such as characterization of the optical modes for the SHG light by experiments and numerical simulations.

 

Fig. 5 (Left) Microscope image of a PhC nanocavity with a = 314 nm, illuminated by 650 nm light, and plotted with a grey scale. White lines show the edge of the PhC lattice and the location of the defect cavity formed by the three missing air holes. (Right) Color near field image of the nanocavity, overlaid with the white lines, showing the distribution of the VIS light near field within the cavity.

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5. Conclusion and discussion

We have demonstrated SFC nanolasers by means of PhC nanocavity QD lasers. This novel platform is shown to be useful for the study of few-photon nonlinear optics, as well as for nanolasers in a broad VIS band, which open the way for realizing monolithic full-color lasers on a single chip. These devices based on a common III-V semiconductor can in principle be operated by current injection [22] at room temperature [21], which are highly desirable properties, and highlight the advantages of SFC nanolasers. To achieve these, further improvement of the laser in terms of the quantum dot quality and other device parameters will be required. We have experimentally shown that increase of Q factor is highly effective to improve η of SFC nanolasers, which provides a foundation for seeking sophisticated designs of such devices.

Further increase of η is envisaged by improvements in various areas, such as increasing Q/V [10,12,17,18] (which is of major importance due to η’s quadratic dependence on it), and employing resonant cavity modes simultaneously for both VIS and NIR [12,15,16,30]. The latter improvement may enable almost 100% absolute conversion efficiency with a very low NIR power [17,18] (see also Appendix). However, in our devices, the generated harmonic light could be significantly absorbed by the host material when trying to confine it for long times, and hence we should keep the structure-dependent Q factor for the corresponding optical mode lower than that limited by the material absorption. Even while keeping the low Q, improvement of η can be achieved by optimizing the VIS mode in terms of its mode volume, out-coupling efficiency, and spatial mode overlap with the fundamental cavity mode. One further possibility is the use of transparent material even for the harmonic mode, including rare-earth doped crystals with optical nonlinearity, like Nd³⁺:LiNbO₃ [13,14] and Er³⁺:GaN [31]. Also of interest is the possible use of nanolasers as monolithic, ultra-compact far- and mid-infrared light sources via difference frequency generation between two lasing modes. Such multi-mode lasing can be supported by the QD gain [32] that is also expected to possess a large χ⁽²⁾ [33]. The SFC nanolasers demonstrated here would find applications in display, spectroscopy, medical and biochemical sensing technology, where controllable narrow emission lines, together with monolithic, cost-effective fabrication, are necessary.