Laterally coupled photonic crystal surface emitting laser arrays

Scalable, high-speed, and compact light sources have become attractive in the research of photonics computing using concepts of artificial neural networks and machine learning. Photonic computing is a revolutionary breakthrough aimed at achieving hardware functionalities of complex computing.^1,2 In parallel with the development of learning and computing algorithms, semiconductor lasers are key hardware components to realize all-on-chip photonics computing due to their reliable, stable, CMOS-compatible, and compact features. Furthermore, laser array structures have been applied to increase the bandwidth of computation and information processing rate.^3,4 Applications of semiconductor laser arrays can be found in many areas such as material processing,⁵ optical communications,^6,7 optical sensing,⁸ and LiDAR.⁹ Driven by the market needs for more powerful information processing abilities, the investigation of laser arrays has become important.

The vertical-cavity surface-emitting laser (VCSEL) array structures have been extensively investigated for beam combining,¹⁰ long-wavelength lasers,¹¹ and high-speed laser arrays.¹² However, it remains challenging tasks to achieve coherent laser arrays with low beam divergence with the VCSEL architecture. Current photonic computing architectures that have high computing speed and energy efficiency, such as photonic neuromorphic computing,¹³ usually require coherent light sources to obtain stable phase information from elements of Mach–Zehnder interferometers.

Photonic crystal surface-emitting lasers (PCSELs) are a new type of semiconductor laser design that features in-plane optical feedback from a photonic crystal (PC) modulation. The lasing mode in a PCSEL cavity operates at the Γ point of momentum space and forms a surface-emitting beam with low beam divergence.^14,15 Such orthogonal electrical injection and optical feedback design offers huge freedom of optical cavity design and enables potentials of semiconductor lasers in speed, power, linewidth, beam quality, and reliability.¹⁶ Due to the nature of PC modulation, a single mode laser can be implemented in a broad-area cavity where high-power beam emission can be achieved. Recently, high-power continuous-wave (CW) lasers with power exceeding 50 W and brightness over 1 GW cm⁻² sr⁻¹ have been demonstrated based on the PCSEL architecture.¹⁷ Even higher output power can be achieved with higher current injection into a single PCSEL device and larger cavity size. However, larger current injection will cause inevitable thermal issues, resulting in efficiency reduction due to complex thermo-optical and electro-optical effects.¹⁸ Moreover, at higher injection currents, the spatial hole burning effects also degrade the mode/beam properties.^19,20

By engineering the in-plane feedback of PCSEL cavities, it is feasible to implement coupling between PCSELs in the lateral directions. The square lattice arrangement in the PC region and the distance between individual PCSELs are the important design optimization parameters to realize the laterally coupled PCSEL arrays. In addition, the optimal airhole size of core-clad regions to confine the lasing optical mode in individual PCSEL as well as to support the evanescent mode in the clad region to enhance lateral coupling are equally important. The lateral coupling control between two PCSELs has been demonstrated by placing a waveguide coupler to switch on/off coherent coupling between two PCSELs.^21,22 However, the coupler structure may occupy a large portion of device area and reduce the power density of the PCSEL array. In this paper, we investigate a passively coupled PCSEL array by placing PCSEL cavities closer to overlap their evanescent field in the lateral directions. The PCSEL cavities are designed with slightly different air hole sizes and energy bands in core/cladding regions.^23–25 The core region is a major part of the cavity that supports laser emission. The cladding region creates a band discontinuity to confine the optical mode in the core region. In the lateral direction, the modes from adjacent PCSELs leak into the coupling region as evanescent waves passively couple resulting in laterally coupled PCSEL arrays.

In this paper, we investigated the passive coupling between PCSELs to demonstrate the concept of laterally coupled coherent PCSEL array operation. We fabricated monolithic single PCSEL and PCSEL array devices based on a GaAs/AlGaAs heterostructure.^26,27 Experimental testing results show that spectral linewidth of 0.14–0.22 nm from 2-by-2 PCSEL arrays is observed, indicating feasible coherent beam combining from individual PCSELs in the array. Self-interference experiments were conducted to measure the coherence by the visibility of interference fringes, result shows strong coherency of the emitted beam from the PCSEL array. The measured beam coherency from PCSEL arrays is comparable to that from a single PCSEL. Therefore, our proposed coherent PCSEL array can be used for chip-scale photonic computing applications, such as high-power laser sources for photonics integrated circuit (PIC) and quantum computing chips.

Figure 1(a) shows the schematic view of a single PCSEL device designed on a GaAs-based heterostructure. In our PCSEL design, MBE was used for the base structure growth, then metal organic chemical vapor deposition (MOCVD) was employed for the epitaxial regrowth process on an N type GaAs wafer. The first growth includes the N contact, N cladding, MQWs, and GaAs PC layers. The cavity structures are then patterned and etched using electron beam lithography (EBL) and reactive ion etching (RIE) processes. A detailed explanation regarding the device fabrication and testing procedure involved during the process can be found here.²⁸ The PC cavity consists of the core and cladding regions for controlled lateral confinement and light leakage for lateral coupling. The air holes have radii of 62 and 55 nm in the core and cladding regions, respectively. The lattice constant is 308 nm for the lasing wavelength of 1040 nm.

Figure 1(b) shows the band diagrams with different air hole sizes. A shift in the band structure can be realized by changing the air hole filling ratio and keeping the lattice constant the same. Such a shift in the band structure creates a bandgap prohibiting the light from propagating into the cladding region, similar to the evanescent waves. The lateral confinement is controlled by two different lattice designs in the core/cladding regions, and we call this cavity the hetero-PC cavity. By controlling the number of PC periods in the cladding region, different light leakage can be implemented to tune the coupling strength. We use the 3D finite-difference time-domain (FDTD) method to simulate the optical modes with 70 periods of the PC core and 30 periods of cladding as shown in Fig. 1(c). It can be observed, in Fig. 1(c), that the detuning of band diagram creates a lateral evanescent field out of the core region. The optical mode is also optimized in the vertical direction for optimal alignment with the gain materials [Fig. 1(d)]. Such lateral confinement control can impact directly on the lateral light leakage, and the cavity Q is enhanced with stronger confinement as shown in Fig. 1(e). Therefore, the field overlap due to the evanescent fields leaking from two PCSELs can be controlled by the separation of two cavities.

Figure 2(a) is a cross-section schematic of our designed PCSEL array structures. Individual PCSEL mesa is formed by etching through most part of P-cladding layers of the coupler region for electrical isolation between different PCSELs. Each PCSEL has individual P metal contact for efficient current injection into the PC cavities. Figure 1(b) shows the schematic of the passively coupled PC cavities. As shown in Fig. 2(c), the electric field envelopes of the PCSEL modes in a 1-by-2 PCSEL array are simulated by the full structure 3D finite-difference time-domain (FDTD) method. In the vertical direction, shown in the top panel of Fig. 2(c), the lasing mode in a PCSEL cavity is mainly concentrated in the PC and MQW layers, and thus passive coupling is realized through the PC and MQW layers. With lateral confinement, the optical modes are localized at the two PC cavities as shown in the bottom panel of Fig. 2(c). The emitted beam passes through the GaAs substrate and aperture opening on the N metal contact layer. We can observe the overlapping of an evanescent field at the center of the coupling region between two cavities, as shown in Fig. 2(d), which can be utilized for passive coupling.

To embed the air hole structures for the PC cavity array into the PCSEL heterostructure, two-step growth was adopted with initial material growth from the substrate up to the PC layer. Figures 3(a) and 3(b) show the scanning electron micrography (SEM) images right after electron beam lithography (EBL) patterning and reactive ion etching (RIE) to form the PC air holes. Figure 3(a) shows the top view of a two-by-two PCSEL array, and Fig. 3(b) shows the zoomed-in images with air hole cross section in the bottom panel and surface profile on the top panel. The PC cavities are designed with circular air holes in a square lattice to achieve 2D light confinement in the PC layer. The core/cladding regions have different air hole sizes (radii in core/cladding regions are 62/55 nm), but the same lattice constant of 308 nm. After fabrication processes, light-current (LI) characteristics were tested for the PCSEL devices using a pulsed current source (Newport LDP 3830 precision) at 1 μs pulse width and 1% duty cycle, shown in Fig. 3(c). As shown in the inset of Fig. 3(c), the lasing threshold for both PCSEL array and single PCSEL is about 1 kA/cm². Output peak power of 280 mW is obtained from the PCSEL array device with each cavity side length of 100 μm at an injection current of 2.4 A, and the maximum output power achieved before thermal rollover for the 2-by-2 PCSEL array is ∼50% higher than that of a single PCSEL, which matches scaling relation according to the size of the emission area.

For a laser array, the beam combining of lasers at the far-field forms a certain pattern due to the interference of light beams from individual lasers. Figure 4(a) shows FDTD far-field simulation results of a 2-by-2 array, assuming a PCSEL beam waist of 100 μm with identical Gaussian beam profiles for each laser. The beam pattern features a brighter center mainlobe. Figure 4(b) shows far-field beam profiles emitted from single PCSEL (left) and 2-by-2 PCSEL array (right) at injection current of 10 times lasing threshold (10I_th), which are 200 mA for single PCSEL and 300 mA for PCSEL array, respectively. In the array design, the air holes are uniformly distributed in a square lattice in the whole region, and the PC cavities are defined by the larger air holes. The distances between the neighboring cavities are designed to be integer times of lattice constant. Therefore, our passively coupled PCSELs have identical phase difference between the individual cavities for coherent operation. It is worth further investigating phase control by varying the distance between PCSELs.

Furthermore, we characterize the coherent beam combining performances by testing the emission spectra and coherency of emitted beams, as shown in Fig. 5. The PCSEL arrays exhibit 0.22 nm linewidth compared to 0.075 nm for a single PCSEL with a side length of 100 μm [Figs. 5(a) and 5(b)]. Measurements of different 2-by-2 PCSEL array devices show linewidth was in the range of 0.14–0.22 nm. The narrow linewidth from the PCSEL array demonstrates that the individual PCSELs are operating at a similar wavelength for potential coherent operation. To further demonstrate the coherent beam combing from the arrays, we used the Michelson interferometer and carried out self-interference experiments to test the visibility of fringes.

Such interference visibility describes the coherency between the two beams.²⁹ 

The interference patterns were taken at injection current of 3I_th and are shown in Fig. 5(c). The degree of coherency obtained from light beams of PCSEL arrays is similar to that of a single PCSEL, as shown in Fig. 5(d). The coherency of a laser array relies on a stable relative phase between each PCSEL device.³⁰ At higher injection currents, the coherency of PCSEL array beams degrade due to the thermal impacts and mode competition. Further improvements can be made by introducing different thermal management schemes for active/passive cooling of the devices. Therefore, a PCSEL array for coherent beam combing that can be implemented by the lateral passive coupling between PCSELs and broad-area high-power coherent laser beam emission can be achieved.

In this paper, based on the PCSEL architecture, we experimentally demonstrated far-field beam combining and coherent operation of compact, laterally coupled laser arrays. The spectral linewidths of the PCSEL array are measured to be in the range of 0.14–0.22 nm, and coherency is between 70% and 90% at different injection currents. The beam profiles, linewidth, and coherency performances of a PCSEL array are similar to those of a single PCSEL device. This report has demonstrated PCSEL arrays in a square shape. Other shape configurations can also be implemented, such as the hexagonal shape PC cavity, which has the most compactness. Further work also includes the introduction of cooling devices and integration of more individual PCSELs in the array.

The authors acknowledge the support from ARO STTR (Semergytech, Inc.) and JDETO programs.

The authors have no conflicts to disclose.

C. Gautam, M. Pan, and Y. Chen contributed equally to this paper.

C. Gautam: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (lead); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (equal). M. Pan: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Y. Chen: Conceptualization (equal); Investigation (equal); Methodology (equal); Validation (equal); Writing – review & editing (equal). T. J. Rotter: Methodology (equal); Resources (equal); Validation (equal); Writing – review & editing (equal). G. Balakrishnan: Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). W. Zhou: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.