(a) Chemical structure of PDI-C6; (b) scheme of DFB laser device (Λ: grating period; d: grating depth; h: thickness of active film; w 1(2): minimum (maximum) value of film thickness considered in “Model n eff av”) based on PS doped with PDI-C6 as active laser material.
SEM image (a) and AFM profile (b) of a DFB grating engraved on SiO2 with Λ = 368 nm and d = 105 nm.
Laser emission spectra for DFB devices with d = 65 nm based on active films of different thickness h. The PL spectrum of one of the films in a region without grating is also included.
(a) Experimental DFB wavelengths, λDFB, for devices with different grating depths, d, and theoretical Bragg wavelengths, λBragg, for mode TE0 calculated with “Model h” (full line) and “Model n eff av” (dotted and dashed lines for d = 30 and d = 220 nm, respectively), as a function of film thickness, h. Calculations with “Model h” for mode TE1 have also been included. Results obtained with “Model h + d/2” are not displayed since they are very similar to those of “Model n eff av”; (b) electric field profiles of the TE0 and TE1 modes in waveguide structures (“Model h”) of h = 300, 600, and 1200 nm (from left to right). x is the distance from the film surface and A, F, and S refer to air, film, and substrate, respectively.
Experimental DFB wavelengths, λDFB, versus index contrast. Solid lines are guides to the eye.
Output intensity (left axis) and linewidth (right axis) versus excitation pump energy for a DFB device with d = 65 nm and h = 776 nm.
Film thickness dependence for devices with h < 1000 nm and various grating depths, d, of (a) DFB thresholds, I th DFB; (b) total (integrated area) PL intensity: symbols are experimental data and the full line is the corresponding linear fit; (c) normalized thresholds (left axis, symbols are experimental data and thin solid lines are guides to the eye), defined as DFB thresholds multiplied by total PL intensity and penetration depth (right axis, thick full line) in substrate (x s) and air (x c).
DFB thresholds, I th DFB, for devices with h < 1000 nm and various grating depths, d, versus index contrast. Solid lines are guides to the eye.
DFB intensity at λ 0, relative to that at λ 1, for devices with h > 1000 nm, as a function of grating depth, d.
Output intensity versus excitation pump energy for DFB devices with: (a) the same grating depth, d = 220 nm, and different film thickness, h; and (b) Similar h (∼400 nm) and different d.
(a) Emission spectra above and below threshold (full and dashed lines, respectively) for DFB devices with different d/h ratios; (b) effect of changing the excitation spot diameter on the shape of the DFB spectrum (data shown correspond to the device with d = 220 nm and h = 290 nm).
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