Fabrication design Hybrid plano-concave microcavity

By using a quarter-wavelength DBR with a multilayer 2D material on top (Figure 2a), we designed our system (2D material + DBR stack) to have a maximum reflectivity at the center wavelength of 637 nm. The selected wavelength of our system falls within the typical emission rates of the zero-phonon line (ZPL) of SPEs in hBN (500–800 nm). A quarter-wavelength thickness is conveniently chosen for the hBN where its value falls between experimentally achievable thicknesses of multilayer 2D materials [6].

Figure 2: Fabrication steps of hybrid microcavity. (a) hBN layer positioned on top of DBR. (b) Concave polymer shape is fabricated by direct laser writing process. (c) A silver layer is added on top of polymer.

A 3D concave shape polymer then could be fabricated on top of the 2D material (Figure 2b) by a direct laser writing system (e.g., Photonic Professional, Nanoscribe GmbH) by use of a 2PP process.

Afterwards an 80 nm silver layer could be added, by thermal evaporative deposition, on top of the concave shape polymer to ensure a high reflectivity inside our microcavity. When designing the concave shape polymer a small rectangular aperture at its edge must be taken into account in the fabrication step (Figure 2b,c) to prevent the accumulation of the photopolymer resist, inside the solidified concave polymer, when the sample is developed (SU-8 developer) and cleaned (IPA) to remove any remaining photoresist and developer, respectively, after the 2PP process is finished.

Analytical design Geometrical parameters of the plano-concave microcavity

When a polymer layer is added inside a bare microcavity, as in our case, two fundamental Gaussian beams are formed inside the air gap and polymer layer, respectively (Figure 3) [16].

Figure 3: Cross-section of hybrid plano-concave microcavity shows the geometrical parameters and the two Gaussian modes inside.

The spotsize W02 (Figure 3) of the fundamental Gaussian mode (TEM00 ) inside the cavity has to be as small as possible, since this means a small modal volume and consequently, a high Purcell factor [17].

By setting an arbitrary range of values for the length of the second Gaussian beam L2 and radius of curvature R2 of our plano-concave microcavity, Figure 4 shows the spotsizes W02 and W2 corresponding to different pair of values (R2, L2) for a hybrid plano-concave cavity. The spotsizes W02 and W2 are calculated by [18]:

(1)

and

(2)

respectively, where g = 1 – L2/R2 is the stability range for our plano-concave cavity and λ0 = 637 nm is the wavelength of the fundamental Gaussian mode, n2 = 1.52 is the refractive index of the polymer layer. The length of the second Gaussian beam is defined as L2 = L1 + Lpol + ∆z, where L1 is the length of the Gaussian beam in air, Lpol is the polymer thickness and ∆z is calculated by the ABCD law [16]:

(3)

where the complex numbers q1,2 = z1,2 + jzR,1,2 are known as the q-parameters for the Gaussian beams, where z2 = L2 − Lp, z1 = L1 and zR,1,2 is the Rayleigh length for each beam. For a Gaussian beam passing through a plane dielectric interface, we have A = B = C = 0, and D = n2/n1, where n1 = 1 is the refractive index of the air gap, therefore, by substituting in Equation 3, q2 = (n2/n1)q1. This leads to z2 = (n2/n1)z1 and W01 = W02. Finally, by defining ∆z = z2 − z1 we get:

(4)

As a threshold for R2 we set R2 ≥ L2 in accordance with the stability range where 0 ≤ g ≤ 1. Although work has been done to include the lensing effect of a curved “n1/n2” interface (see supplementary material of [19]), the planar surface (R1 = ∞) approximation values (Table 1) fall within the desired range with our FDTD simulations.

Figure 4: Spotsizes W02 and W2 for different values of R2 and L2.

We take a transversal cut through a fixed value of L2 (Figure 5) and observe the dependence of W02 and W2 to the radius of curvature (R2) of a plano-concave cavity. To achieve a high Purcell factor, and a small NA, R2 must be as small as possible (small W02), while maintaining the lower boundary condition (R2 ≥ L2), therefore the optimal values of R2, for any arbitrary L2, will reside near the vicinity of the minima of the W2 function (Figure 5), setting the boundary values for R2, for any given L2, at R2 ≈ 2L2.

Figure 5: Transverse cut of Figure 4 through length L2 = 5.03 μm to show dependence of R2 with spotsizes. As the values of R2 diminishes, while maintaining a constant L2, the functions for W02 (blue) and W2 (red) start to diverge, arriving at the limit of the paraxial approximation (stability regime).

Selecting the R2 parameter closer to the divergence of the W2 function (R2 = L2) could result in unstable resonators that will not hold a stable Gaussian mode inside. Theoretical work has been done with R2 ≈ L2[20], where a non-paraxial analysis is performed, although diffraction losses have to be considered for an accurate description of the experimental limits of stability [21]. In the unstable regime (R2 < L2) extensive work has also been done [22,23].

Electric field distribution and resonant modes of the plano-concave microcavity

A λ0/4n thickness layer of hBN (n = 1.72) was positioned on top of a 15-pair layer DBR with tantalum oxide (Ta2O5) and silicon oxide (SiO2) as the high- and low-index layers, respectively, on a (HL)15 configuration to ensure an electric field antinode at the surface of the hBN layer, making the hBN + DBR system a L(HL)15 dielectric stack. A transfer matrix model [24] was used to calculate the electric field distribution inside the hBN + DBR system (Figure 6).

Figure 6: Electric field distribution of a hBN + DBR system on a L(HL)15 configuration. Maximum electric field intensity is found at the surface of the hBN layer. Vertical lines (blue) represent the boundaries between each dielectric layer.

The full transfer matrix S of our microcavity is defined as:

(5)

where L and I represent the transfer and interface matrix, respectively, of the silver (Ag), polymer (pol), air, hBN and DBR layer. The transfer matrices Lpol and Lair are defined as [25]:

(6) (7)

where G1,2 = arctan(L1,2λ0/n1,2πW01,02) is the Guoy phase shift in the air (n1 = 1) and polymer layer, respectively, where W01 = W02.

The transmittance of the microcavity is calculated, from the matrix elements of S, to find its fundamental TEM resonant modes (Figure 7). We found the desired TEM modes at R2 = 8.1 μm and L2 = L1 + Lpol + ∆z = 5.03 μm, where L1 = 3.09 μm, Lpol = 0.4 μm and ∆z = 1.54 μm, which gives a physical cavity length of L = L2 − Δz = 3.49 μm. These values fall within the stability range R2 ≈ 2L2.

Figure 7: Transmittance of plano-concave cavity shows the fundamental TEM modes at 595 nm, 636 nm and 684 nm.

Numerical design Resonant modes of hybrid plano-concave microcavity

For the FDTD simulations, we used the Ansys Lumerical FDTD software. The polymer, and DBR stack were treated as lossless and non-dispersive materials [15]. A transmittance T = 8% at 637 nm is measured for our cavity, with an in-plane dipole inside, for a silver layer thickness of 80 nm. Identical values for the geometrical parameters previously mentioned (R2, L2, L1), except for R1 = 7.7 μm, were taken for the FDTD simulations, where an in-plane dipole emitter sits at the surface of the hBN layer to ensure a higher Purcell factor since the dipole interacts with an electric field antinode [26]. The Purcell factor was calculated by using the classical definition [27]:

(8)

where Pcav and Pfree is the power dissipated for the dipole inside the microcavity and in free space, respectively. A Purcell factor of FP ≈ 6 was achieved for the TEM mode at the DBR center wavelength. A Q-factor of Q = 731.4 ± 102.7 was also calculated in our simulations where the resonant modes of the microcavity (Figure 8) are shown in good agreement (Table 1) with the resultant modes from the analytical model (Figure 7).

Figure 8: Purcell factor of plano-concave microcavity. Fundamental TEM Gaussian modes are found at 595 nm, 636 nm and 684 nm. Inset shows transverse section of fundamental Gaussian mode at 637 nm.

Table 1: Geometrical parameters and fundamental TEM mode values of the designed hybrid plano-concave microcavity.

Parameter Analytical
(μm) FDTD
(μm) R2 8.1 8.1 physical cavity length, L 3.49 3.49 L1 3.09 3.09 L2 5.03 5.03 hBN thickness λ0/4n λ0/4n polymer thickness 0.4 0.4 1st TEM00 0.595 0.616 2nd TEM00 0.636 0.637 3rd TEM00 0.684 0.684 R1 ∞ 7.7