Coupled-mode theory for stimulated Raman scattering in high-Q/Vm silicon photonic band gap defect cavity lasers

We demonstrate the dynamics of stimulated Raman scattering in designed high-Q/Vm silicon photonic band gap nanocavities through the coupled-mode theory framework towards optically-pumped silicon lasing. The interplay of other chi(3) effects such as two-photon absorption and optical Kerr, related free-carrier dynamics, thermal effects, as well as linear losses such as cavity radiation and linear material absorption are included and investigated numerically. Our results clarify the relative contributions and evolution of the mechanisms, and demonstrate the lasing and shutdown thresholds. Our studies illustrate the conditions for continuous-wave and pulsed highly-efficient Raman frequency conversion to be practically realized in monolithic silicon high-Q/Vm photonic band gap defect cavities.


Introduction
Silicon is increasingly being considered as the dominant platform for photonic integrated circuits, providing the advantages of dense CMOS electronics integration, transparency in the telecommunication C-band, as well as high-index contrast for compact integrated optical functionalities. Passive silicon photonic devices have already recently reported remarkable progress [1][2][3][4]. With strong modal confinement, nonlinear optical properties in silicon are enhanced and active functionalities in highly integrated silicon devices have been realized, such as all-optical switches [5], and all-silicon Raman amplification and lasing [6][7][8][9][10][11][12][13][14].
Photonic crystals (PhC), with periodic modulation in the index of refraction, permit unique opportunities in specific studies and applications [15,16]. Two-dimensional (2D) PhC slabs confine light by Bragg reflection in-plane and total internal reflection in the third dimension. Introduction of point and line defects into 2D PhC slabs create nanocavities and PhC waveguides with arbitrary dispersion control that can be designed from first principles. Such defect cavities have subwavelength modal volumes, on order 0.02(λ/n) 3 , corresponding to high field intensities per photon for increased nonlinear interaction. Through k-space design of cavity modes [17,18], recently cavities with remarkable high quality factors (Q) [1,2,[19][20][21][22] have been achieved, permitting for long photon lifetimes for light-matter interactions. The strong optical confinement and photon lifetimes in these cavities permit fundamental studies and integrated nanophotonics applications [23], such as channel add/drop filters [24], low-threshold quantum well lasers [25], cavity quantum electrodynamics [26], enhancement of optical nonlinearities [27], and ultrasmall nonlinear bistable devices [28,29].
Raman scattering in silica-based high-Q microcavities, such as microspheres [30], microdisks and microtoroids [31], have shown remarkable ultra-low lasing thresholds. In addition, Raman lasing in silicon waveguides has also been observed [6][7][8][9][10][11][12][13][14] where the bulk Raman gain coefficient g R is 10 3 to 10 4 times larger in silicon than in silica and twophoton absorption induced free-carrier absorption [32] addressed by pulsed or reversed biased p-i-n diode operation. To achieve significant amplification and ultimately lasing, the gain medium should be placed in a cavity with sufficiently high-Q, and ultrasmall modal volumes V m . The enhanced stimulated Raman amplification and ultralow threshold Raman lasing in high-Q/V m photonic band gap nanocavities was suggested [33].
Stimulated Raman scattering (SRS) in periodic crystals with slow group velocity was theoretically studied with a semiclassical model [34], and enhancement in line-defect photonic crystal slow-light waveguides was also proposed [35].
Here we employ a coupled-mode theory framework to study the various contributions on Raman scattering and lasing [36,37]. Coupled-mode equations are widely used in passive photonic devices such as optical waveguide direction couplers, channel add-drop filters [38,39] and also in the analysis of optical nonlinearities [28,[40][41][42]. The coupled-mode equations for stimulated Raman scattering in silicon-on-insulator waveguides [9,[43][44][45], fiber Bragg grating [46] and silica microsphere [47,48] have been studied. Following from the work of Ref [28,40,42], we derive, in this present paper, the coupled-mode equations for stimulated Raman scattering in high-Q/V m silicon photonic band gap nanocavities. The dynamics of coupling between the pump cavity mode and the Stokes cavity mode is explored in the presence of cavity radiation losses, linear material absorption, two-photon absorption, and free-carrier absorption. The refractive index shift from the Kerr effect, free-carrier dispersion, and thermal dispersion are also considered in the coupled-mode equations. These equations can be numerically integrated to describe the dynamical behavior of the system for the designed L5 photonic band gap nanocavities. Specific examples such as lasing threshold, both pump and Stokes seed in continuous wave (CW) operation, pulsed pump with CW Stokes seed, on-off gain (loss), and the interaction of pump pulse and Stokes pulse are investigated in detail.

Design concept and coupled-mode theory
Stimulated Raman scattering is an inelastic two-photon process, where an incident photon interacts with an excited state of the material (the LO and TO phonons of single-crystal silicon). The strongest Stokes peak arises from single first-order Raman phonon (threefold degenerate) at the Brillouin zone center. We have proposed a photonic band gap cavity with five linearly aligned missing air holes (L5) in an air-bridge triangular lattice photonic crystal slab with thickness of 0.6a and the radius of air holes is 0.29a, where the lattice period a = 420 nm [33]. The designed cavity supports two even modes, pump mode and Stokes mode, with spacing 15.6 THz, corresponding to the optical phonon frequency in monolithic silicon. Figure 1 shows the scanning electron micrograph (SEM) of the designed and fabricated L5 cavity coupled with photonic crystal waveguide.
where E p and E S are the electric fields of pump mode and Stokes mode respectively, are the third-order nonlinear polarizability, p ε and S ε are the dielectric constants. The third-order nonlinear polarization, ( ijkl χ is the third-order nonlinear electric susceptibility.
Assume the electric fields of pump mode and Stokes mode are, Then the third-order nonlinear polarization, and only the real part of amplitudes, we obtain where the Raman gain coefficient in the photonic band gap nanocavities, where n p,S are the refractive indices at the pump and Stokes wavelengths λ p and λ S respectively, and The Raman gain is assumed constant, without saturation or parametric instability. The effective modal volume V R for Raman scattering indicates the spatial overlap between the pump mode and the Stokes mode, The bulk gain coefficient, Note that in this classical formulation, the Raman gain coefficient in the photonic band gap nanocavities c S g is still equivalent to the bulk Raman gain coefficient B R g , since possible cavity quantum electrodynamics enhancements are not yet considered.
The electric field of input pump wave and input Stokes wave in the waveguide are, where the field amplitude is normalized by 1 τ are the loss rates into waveguide (in-plane) and into freespace (vertical). i κ is the coupling coefficient of input pump wave s p (t) or Stokes wave s S (t) coupled to the pump mode a p (t) or the Stokes mode a S (t) of the cavity, and The threshold pump power for the stimulated Raman lasing is obtained from Equations (20) and (21), The lasing threshold scales with V R /Q p Q S as illustrated in Equation (22) This framework has been reported earlier in Johnson et al. [40] and Uesugi et al. [42] for a single frequency in cavities. We further advance these investigations for the pump-Stokes interactions, as well as studying the lasing thresholds and dynamics under various conditions. The total loss rate for each cavity mode is: The linear material absorption lin τ 1 is assumed small since operation is within the bandgap of the silicon material. where the first terms represent the TPA due to two pump photons or two Stokes photons.
The second terms represent that one pump photon and one Stokes photon are absorbed simultaneously. Si β is the TPA coefficient of bulk silicon. The effective mode volume indicates the spatial overlap between the pump mode and the Stokes mode, and . We note that the (β Si /V i,TPA ) TPA term is the effective silicon-air contribution, summing over the cavity modal distributions within the solid and negligible contribution from air, and neglecting crystal anisotropy. The bulk TPA coefficient is also assumed to be frequency-independent [50], and without surface modification for simplicity. The fc τ is the effective free-carrier lifetime accounting for both recombination and diffusion.
Time constants of radiative and Auger recombination, as well as from bulk defects and impurities, are assumed to be significantly slower than the free-carrier recombination and diffusion lifetime [50]. We note that while free-carrier lifetime can vary with carrier density and carrier density can vary spatially with intensity in the cavity, an effective lifetime is used here for simplicity. A quiescent carrier density of N 0 = 10 22 m -3 is used in the initial condition for silicon.
In equations (23) and (24), The mode-averaged temperature difference between the photonic crystal cavity and its environment T ∆ is governed by [40] Equations (23-25), (31), (36), and (42) therefore describe the dynamic behavior of SRS in photonic crystal nanocavities, and is numerically integrated in our work to describe the dynamical behavior of pump-Stokes interactions in our L5 photonic crystal cavity system that supports the desired two-mode frequencies at the appropriate LO/TO phonon spacing.

Lasing threshold
We now consider the case of lasing threshold. Around the lasing threshold, the Stokes gain equals the losses, and the Stokes mode energy 2 S a is much smaller than the pump mode energy 2  Table 1.
By comparing the curve in the absence of TPA and FCA ( 0 = Si β ) and the curve with ), it is observed that TPA increases the threshold pump mode energy but the effect of TPA is relatively weak. The effect of TPA-induced FCA is much more dramatic as shown for different free-carrier lifetimes fc τ . The lasing threshold increases when fc τ is larger. There is a minimum Stokes Q S required for lasing, as seen in the solutions plotted in Figure 3. If Q S is lower than a critical value for certain fc τ , there is no solution numerically and physically this translates to an absence of a lasing threshold regardless of the pump intensity. For increasing fc τ , the critical value of Q S increases monotonically as can be seen in Figure 3. The solid and dotted curves show the lasing and shutdown thresholds, respectively [44]. The shutdown threshold is the pump power in which the lasing output power returns to zero due to increasing TPA and FCA.
For the L5 cavity studied in the present paper, Q S = 21,000, the maximum fc τ is around 0.175 ns, and the threshold pump mode energy is 29 fJ. For air-bridged silicon photonic band gap nanocavities, fc τ = 0.5 ns [42], which is much higher than the maximum fc τ . In order to get lasing for this cavity, instead of using CW pump signal, pulse pump signal can be used to reduce the TPA-induced FCA for loss reduction.
We now solve for the input-output characteristics of photonic band gap defect cavity laser by considering equation (24) in steady state. Figure 4 shows the laser input-output characteristics with different free-carrier lifetimes fc τ . Q S = 30,000 and Q S = 60,000 are considered for comparison. Cavities with higher Q S have higher output Stokes signals, lower lasing threshold pump mode energies and higher shutdown thresholds. The required corresponding pump power in the input waveguide can also be calculated based on equation (23) in steady state.

Lasing dynamics
We now consider the dynamics of the Raman lasing interactions. Equations (23)(24)(25), (31), (36), and (42) are numerically integrated with a variable order Adams-Bashforth-Moulton predictor-corrector method (Matlab ® ode113 solver). All the parameters used in calculation are presented in Table 1. Figure 5 shows the dynamics of Raman amplification with 60 mW CW pump wave and 10 µW CW Stokes seed signal, free carrier lifetime is 0.5 ns. In the beginning when free carrier density is low, Raman gain is greater than loss and there is amplification. When free carrier density increases, FCA dominates the loss and Stokes signal is suppressed.
The temperature difference then increases significantly. The cavity resonance is red shifted and the pump mode energy goes down. From the numerical results, the Kerr effect is predominantly weak. The FCD effect dominates at first when the temperature difference is low, with a resulting blue shift. Eventually thermal effect dominates, with a resulting red shift. Consider the case of a different carrier lifetime at 0.1 ns. Figure 6 shows the calculated results with free carrier lifetime of 0.1 ns. With lower free carrier lifetime, the free carrier density and the temperature difference are lower, so that the net Raman gain is greater than zero. The oscillation of Stokes mode energy near t = 0.5 ns is due to the dispersion induced Stokes resonance frequency shift.
In order to get lasing from this cavity, instead of using CW pump signal, pulse pump signal with pulse width narrower than the free carrier lifetime is used to reduce the TPAinduced FCA, so as to reduce loss and increase net gain. Figure 7 shows the dynamics of Raman amplification with pump pulse of 60 mW peak power, pulse width T FWHM = 50 ps and 10 µW CW Stokes signal. The free carrier density and the temperature difference are significantly reduced, and a strong Stokes pulse is generated by the pump pulse. The resonance frequency shift is also significantly reduced by the pump pulse operation.
Consider now the case of on-off gain and loss in our cavity system, where the probe signal changes between the pump pulse on and off when the probe frequency is on-(off-) resonance with the Stokes frequency [9]. Pulsed pump beam with 60 mW peak power and T FWHM = 50 ps, and CW probe beam with 1 mW power are used. Figure 8 shows the on-off gain and on-off loss. When the probe is on the Stokes frequency, an increase in the probe signal due to the SRS is observed, and the on-off gain is around 8 dB. When the probe frequency is detuned from the Stokes frequency (no Raman gain), the loss in the probe signal due to the pump pulse generated free carriers is observed, and the on-off loss is around 20 dB. Figures 9 and 10 show the dynamics of the system.
We also consider the interaction of both pump and Stokes pulses with comparable peak power, with numerical results shown in Figure 11. Pump peak power is 60 mW and Stokes peak power is 20 mW. Both pump pulse and Stokes pulse have T FWHM = 50 ps.
The Stokes pulse is amplified by the pump pulse. Due to the high free carrier density induced FCD effect and high Q S , there is an observed oscillation in the amplified Stokes pulse.

Conclusions
In this work we have derived the coupled-mode equations for stimulated Raman scattering in high-Q/V m silicon photonic band gap nanocavities towards optically-pumped silicon lasing. Both the lasing threshold and the lasing dynamics are numerically studied in the presence of cavity radiation losses, linear material absorption, two-photon absorption, and free-carrier absorption, together with the refractive index shift from the Kerr effect, free-carrier dispersion, and thermal dispersion. With increasing the cavity Q factors and decreasing the free carrier lifetimes, the reduction in the threshold pump energy is solved numerically, considering all mechanisms and realistic conditions. With CW pump operation, the Stokes signal is suppressed due to strong TPA-induced FCA.
With pulse pump operation, the TPA-induced FCA is significantly reduced and Stokes net gain increases, which shows that compact Raman amplifiers and lasers based on high-Q/V m silicon photonic band gap nanocavities are feasible.