Photonic-crystal surface-emitting lasers (PCSELs), distinguished by their single-mode high-power characteristics that rival bulky gas and solid-state lasers, are regarded as the next generation of semiconductor lasers^[1,2]. Unlike conventional semiconductor lasers that suffer from a large beam divergence angle^[3–6], external optical components are unnecessary for PCSELs, which reduces the cost and complication of the whole system. They are therefore expected to be applied in a wide range of fields, including smart manufacturing, optical communication, display and lighting, and sensing^[7–10]. However, due to the distributed feedback nature of photonic crystals, they often require a large cavity size to achieve sufficiently low cavity loss, which determines the threshold mode gain of the PCSELs. This consequently leads to a high operating current of PCSELs, rendering them unsuitable for applications that demand low power consumption and high-speed performance^[11].

Despite some recent reports on high-speed PCSELs^[12–15], their threshold currents are generally quite high, typically exceeding 200 mA and sometimes surpassing 1 A. To reduce the threshold currents of PCSELs, single-lattice photonic-crystal resonators with various hole shapes were utilized by Lu et al., and a small threshold current of 49 mA was then realized with a cavity size of 125 µm^[16]. Further, Noda et al. achieved a minimum threshold current of 21 mA by employing a double-lattice photonic-crystal resonator, which enhances optical feedback by introducing a lattice separation of approximately half the lattice constant^[17]. Many other solutions such as photonic-crystal heterostructures^[18], selective area mixing^[19], and topological band inversion^[20,21] have also been proposed to reduce the threshold current. However, due to the challenges faced in their fabrication processes, the corresponding electrically pumped devices with a low threshold have not yet been achieved experimentally.

Here, we adopt the triple-lattice photonic crystal as the resonator, which has been conceptually demonstrated to enable a lower threshold current in our previous research^[22]. This structure can be viewed as the superposition of three identical lattice groups, working together in a manner of constructive interference. It allows higher coupling coefficients and optical feedback, compared to their single-lattice or double-lattice counterparts. Based on this configuration, a threshold current as low as 12 mA is presented in this paper. Both the steady-state characteristics and high-speed frequency response of the triple-lattice PCSELs were studied.

Figure 1(a) schematically illustrates the structure of our triple-lattice PCSELs, where the active layer and photonic-crystal (PC) layer are sandwiched between two doped cladding layers. Such an epitaxial configuration favors only a fundamental transverse mode^[23]. The optical modes oscillating in the photonic-crystal resonator will be extracted from the substrate, which can be explained by the coupled-wave model sketched in Fig. 1(b). Light waves with various wave vectors propagate in-plane and couple with each other to form a two-dimensional (2D) standing-wave field, the energy of which could be simultaneously coupled to the vertical direction via first-order Bragg diffraction^[23–26]. This is the origin of the surface emission of PCSELs.

To quantize the surface emission and edge leakage of the resonant modes, the vertical and in-plane radiation loss are utilized in the analysis of PCSELs. They constitute together the total radiation loss of resonant modes, e.g.,  αtotal=α⊥+α∥. The in-plane radiation loss and internal loss should be kept as low as possible for the design of low-threshold energy-efficient PCSELs, while a moderate vertical radiation loss should be carefully selected.

Figure 1(c) presents the in-plane structure of the triple-lattice photonic crystal used in our device. The separations between holes along the x- and y-axes are both set as half of the lattice constant. Such a configuration of the unit cell is meant to enhance the optical feedback to the maximum extent. As shown in Fig. 1(c), total radiation losses of the PCSELs with various types of photonic-crystal resonators are calculated as a function of cavity sizes, which are defined as the side lengths of the square photonic-crystal region. Compared to common single-lattice photonic crystals with circular or triangular holes, the triple-lattice photonic crystal tends to have a lower mode loss, or threshold current density. It favors the design of low-threshold small-size PCSELs. In the calculation, all photonic crystals have the same air filling factor of 10%, and hole depth of 200 nm. The calculation of radiation loss is based on three-dimensional (3D) coupled-wave theory. In this theory, 3D analysis was considered. Two-dimensional standing waves parallel to the plane of the photonic crystal are calculated, and the vertical epitaxial structure of the laser is also taken into account. As for the specific magnitudes of in-plane and vertical radiation loss, they are derived using the fundamental waves [Rx,Sx,Ry,Sy] obtained from the aforementioned theory.

Figure 2 shows the structure of our fabricated 940 nm triple-lattice PCSELs. The chip has a side length of 500 µm, with light emitted from the substrate through a circular output aperture with a diameter of 120 µm, as shown in Fig. 2(a). The epi-side has a circular mesa with a diameter of 100 µm, which is surrounded by a 25 µm wide trench for the confinement of injection current.

Figure 2(b) shows the top-view scanning electron microscope (SEM) image of the triple-lattice photonic crystal. The lattice constant of the photonic crystal is 274 nm, approximately equal to the lasing wavelength within the material. It ensures that resonance occurs at the Γ2-point in k-space. The background material of the photonic crystal is GaAs, and the holes were etched to a depth of 220 nm.

The cross-sectional structure of the PCSELs after regrowth is sketched in Fig. 2(c). The embedded photonic crystal is adjacent to the active layer, with an electron-blocking layer between them to suppress the carrier leakage into the photonic-crystal layer. Air holes with a depth above 166 nm were retained in a GaAs background material after regrowth, obtaining a large refractive-index contrast for high optical feedback. The non-uniformity of the photonic crystal in hole depth and diameter after regrowth is primarily due to the imperfection of hole shapes in the preceding dry etching process, as well as the differences in the coordination numbers of holes originated from such a triple-lattice photonic crystal configuration.

During our fabrication, epitaxial layers consisting of an n-Al0.7Ga0.3As cladding layer, InGaAs/AlGaAs multi-quantum wells, a p-Al0.5Ga0.5As carrier blocking layer, and a p-GaAs layer were grown on the n+-GaAs substrate subsequently. With a 120 nm silica film serving as the hard mask, the photonic crystal pattern was fabricated within a square region with a side length of 100 µm, utilizing electron-beam lithography followed by dry etching. Later, a p-Al0.4Ga0.6As cladding layer and a p+-GaAs contacting layer were regrown on top of the photonic crystal layer. The opening of the holes was encapsulated during this regrowth process, leaving air holes embedded inside the background material^[27–29]. On the epi-side of the device, a circular trench with a depth of 1.3 µm was fabricated by dry etching to form the mesa. Then, a 300 nm silica film was deposited on the top surface of the device as an electrical insulating layer, with an 80 µm diameter circular contact window opened for current injection by reactive ion etching. Finally, the Ti-Pt-Au p-electrode and Ni-Au/Ge-Ni-Au n-electrode were formed on the top and bottom surfaces of the device, respectively, by a lift-off process. The output window, featuring a diameter of 120 µm, was preserved for surface emission, slightly larger than the p-mesa structure. The PCSELs were later cleaved into square chips, each with a side length of 500 µm, and then bounded to thermally conductive submounts with p-side down for testing. The temperature of these submounts was controlled by a thermoelectric cooler.

As shown in Fig. 3(a), we measured the light-current (L-I) characteristics of the triple-lattice PCSELs at various operating temperatures under continuous wave (CW) operation. A low threshold current of 29 mA was obtained at 10°C. However, the threshold current of the device gradually increases with the temperature, due to the carrier escape and the decrease of material gain. At room temperature, the threshold current is approximately 36 mA. Furthermore, the threshold current of the PCSELs is greatly affected by the recombination lifetime of carriers within the quantum well. A longer radiative recombination lifetime will be beneficial for reducing the threshold current of the device, which can be achieved by reducing the width of the quantum well and the defect concentration within the well. The low slope efficiency of the PCSEL is due to the small vertical radiation loss inherent in the symmetric triple-lattice photonic crystal structure^[30,31]. With increasing temperature from 10°C to 30°C, the slope efficiency decreases from 0.06 to 0.04W·A−1.

Figure 3(b) shows the threshold current of PCSELs as a function of operating temperature on a logarithmic scale. The characteristic temperature of threshold current can be evaluated by Eq. (1), which is about 67 K within the temperature range from 10°C to 30°C^[32]: Ith(T)=Ith(T1)exp[(T−T1)/T0].(1)

The lasing spectra of the triple-lattice PCSELs were also measured at various injection currents under room temperature, as shown in Fig. 4(a). The spectrum shows no apparent lasing peak at an injection current of 33 mA, which is below the threshold. When the current increases to 39 mA, lasing peaks appear at 934.15 nm, which corresponds to the band edge A of the PCSELs^[33]. The peak wavelength has a red shift of 0.34 nm when the injection current increases to 80 mA. Besides, a new lasing peak exists at 932.40 nm, which corresponds to the band edge B. The measured wavelength separation between them is about 2.09 nm with a side-mode suppression ratio (SMSR) of 5 dB, due to the small threshold discrimination between the two band edge modes, A₀ and B₀.

It is consistent with the calculated results of the PCSELs as shown in Fig. 4(b), which shows the total radiation loss of the eigenmodes as a function of wavelength. For simplification, the photonic-crystal holes used in our calculation were treated as the equal-volume cylinders extracted from the SEM result presented in Fig. 2. Among all resonant modes, the fundamental mode at band edge A has the lowest threshold gain, approximately 2.2cm−1, while the competing mode at band edge B has a threshold gain of about 2.5cm−1. The lasing peak at band edge A in the spectrum is actually composed of multiple envelopes, representing the fundamental mode and higher-order modes with slight wavelength differences. This may be related to the inhomogeneity in the structural fabrication process. Note that the internal loss is not discussed here. The wavelength interval between mode A and mode B is 1.99 nm. Additional modes shaded in yellow in Fig. 4(b) are artifacts due to the numerical algorithm based on the finite-difference method, which are not considered for lasing^[34]. Modes A and B correspond to two distinct band edges, exhibiting markedly different electromagnetic field distributions. In contrast, modes A₀ and A₁ represent the fundamental mode and the higher-order mode associated with the same band edge position. While sharing similar field distributions, they possess different in-plane propagation vectors, and consequently different group velocities. Figure 4(c) shows the temperature-dependent lasing spectra of the PCSELs measured at an injection current of 80 mA. The wavelength tuning coefficient is about 0.13nm·K−1. Band edges C and D are unlikely to achieve lasing because their threshold gains are nearly two orders of magnitude higher than those of band edges A and B.

Figure 5(a) shows the near-field patterns of the fabricated triple-lattice PCSELs. When the injection current is below threshold, a homogeneous near-field pattern was displayed, indicating an even current injection. With the increase of injection currents to 39 mA (above threshold), the near-field pattern shows a more coherent characteristic, which is further enhanced at a current of 80 mA. The corresponding far-field patterns are shown in Fig. 5(b). A double-lobe pattern was demonstrated due to the multi-mode operation of the device as explained in Fig. 4. In Fig. 5(c), we present the far-field intensity profiles of the PCSELs along the x- and y-directions at 80 mA, with divergence angles of 3.84° and 1.63° estimated from the 1/e2 width, respectively. This is far lower than the divergence angle of traditional semiconductor lasers, which is commonly larger than 10°.

As shown in Fig. 6(a), the fabricated triple-lattice PCSELs are bounded on AlN submounts for frequency response measurement at 10°C. The light was collected into a high-speed detector by a multi-mode fiber, and then treated by a vector network analyzer. The measurement results at four different injection currents are shown in Fig. 6(b). The −3dB bandwidth at 50, 100, 150, and 200 mA are 2.6, 4.4, 5.4, and 5.8 GHz, respectively. Of course, this is based on the top-and-bottom electrode structure, which typically exhibits a large capacitance that limits the bandwidth. If coplanar electrodes are used, the bandwidth is expected to be further increased. Nonetheless, the achieved bandwidth of 5.8 GHz exceeds that reported in Refs. [12,15], where the measured −3dB bandwidths are both under 4 GHz. This enhancement can be mainly attributed to the substantially smaller cavity sizes employed in this work, in contrast to those reported in the literature, which range from 200 to 500 µm.

To further reduce the threshold current of PCSELs, we investigate three different mesa sizes, e.g.,  50, 75, and 100 µm, to examine their influence as illustrated in Fig. 7(a). Their threshold currents are, respectively, 12, 27, and 36 mA under room temperature, due to the changes of carrier concentration. To our knowledge, 12 mA is the lowest reported threshold current for PCSELs up to now. Figure 7(b) also shows that the low threshold current corresponds to a reduced slope efficiency. In Fig. 7(a), the L-I curve of the device with a 50 µm mesa diameter shows multiple kinks. This may be because heat accumulation becomes more pronounced at this modified size, leading to an uneven temperature distribution within the device. This affects the refractive index of the materials in the resonant cavity, resulting in greater instability in the device’s operating modes.

As in the aforementioned explanation, the low slope efficiency of the triple-lattice PCSELs is mainly due to the structural symmetry when the lattice separation d=0.5a. It tends to have a zero vertical radiation loss (or infinite quality factor), although a little vertical radiation was still obtained in our calculation due to the finite in-plane size of our photonic-crystal resonator. An adjusted lattice separation will solve this problem by breaking the symmetry protection of the photonic-crystal structure. Theoretically, as the lattice separation decreases from 0.5a to 0.45a, the corresponding extracting efficiency of the triple-lattice PCSELs will rise from 5% to 89%, along with the increase of total radiation loss. It should be noted that the in-plane radiation loss holds around 1cm−1 without a significant increase. The increase in the total radiation loss is primarily attributed to the changes in vertical radiation. The measured light-current curves of the fabricated PCSELs with various lattice separations are shown in Fig. 7(c) under CW operation at room temperature. The PCSEL with a separation of 0.45a shows a significant improvement in slope efficiency and output power, which are 0.15W·A−1 and 5 mW, respectively, along with a slight increase of threshold current.

In conclusion, we have demonstrated a novel class of very low-threshold 940 nm PCSEL featuring a −3dB bandwidth of 5.8 GHz based on the triple-lattice photonic-crystal resonators, which have significantly enhanced optical feedback. With a cavity length of 100 µm, a minimum room-temperature threshold current of 12 mA was demonstrated under CW operation. To our best knowledge, this is the lowest reported threshold current for PCSELs, rivaling that of the traditional high-speed distributed feedback (DFB) Bragg edge-emitting lasers. Furthermore, the divergence angle of our triple-lattice PCSELs is approximately one order of magnitude smaller compared to the latter. We believe that our work will contribute to the development of PCSELs for the application of high-speed optical communication.