Cavity thermal emitters with short period of hole array

Although CTEs with RDHA could offer pure emission spectra without SPPs modes and Bragg scattered CMs, their emission intensities are much weaker due to

low densities of holes. In this section, novel CTEs with short period of hole array (SPHA) are proposed to overcome the intensity problem. High output intensities, high temperature stability, low FWHM of emission peaks and pure emission spectra without SPPs modes and Bragg scattered CMs could be achieved simultaneously.

Besides, non-ideal effects such as LCMs are weak either.

4.3.1 Experiments

The side view, top view and fabrication processes of CTEs with short period hole array (SPHA) are identical as those described in Sec. 4.1.1. The structure parameters of all samples O, P and Q are summarized in Table 4.4. The reflection spectra are measured along ΓK direction with angle ϕ normal to the surface as shown in Fig.

4.1 (b).

Table 4.4 The structure parameters and emission peaks of samples O, P and Q, d denotes the diameters of holes and h denotes the thickness of to silver film.

Sample a (μm) d (μm) tox(μm)

O 2.3 1.15 2

P 1.7 1 2

Q 1.7 1 1.6

4.3.2 Results and discussion

The design principle of CTEs with SPHA is very simple. In order to get high output intensities, holes are arranged in the highest packed form: hexagonal lattice. In order to reduce the FWHM of emission peaks and eliminate non-ideal effect such as LCMs, the diameters of holes were chosen as small as possible. Finally, the period of hole array is shrunk as small as possible in order to place more holes inside the top Ag layer to increase the overall output intensities. Interesting enough, once these three criteria are satisfied, high performance cavity thermal emitters with good high temperature stability, strong emission intensity, narrow-band emission peaks and pure emission spectra can be achieved simultaneously.

Figs. 4.15 (a) and (b) show the dispersion relations of reflection for samples O and P along the ΓK direction, respectively. The curves (k, A ,m) CM denote the CMs whose wave vectors satisfy Eq. (4.9) with (k, A ,m). Similarly, the curves (i,j) Ag/SiO2

denotes the SPPs modes whose wave vectors satisfy Eq. (4.10). For small period of hole array, G1 and G2 of Eq. (4.6) and (4.7) becomes large due to small values of a.

Substituting large G1 and G2 into Eq. (4.9) and (4.10) yields higher ω so that moves the SPPs modes and Bragg scattered CMs to the higher energy in the dispersion relation of reflection spectra. Compared Fig. 4.15 (a) with Figs. 4.3 (b) and 4.3 (d), when the period of hole array shrinks from 3 μm to 2.3 μm , all Bragg scattered CMs

and SPPs modes are shifted from the energies above 0.4eV at kx=0 as shown in Fig.

4.3 (d) to the higher energies above 0.48eV at kx=0 as shown in Fig. 4.15 (a); once the period of hole array continue shrinks to the 1.7μm as shown in Fig. 4.15 (c), all Bragg scattered modes and SPPs modes are shifted to the even higher energy above 0.6eV at kx=0. The reflection spectra becomes purer for smaller period, gradually approach the reflection spectra of CTE with RDHA by comparing Figs. 4.15 (a) and (b) with Fig. 4.9 (a). Besides, the non-ideal effect such as localized cavity modes as described in the previous section are weak either due to small hole size.

(a)

(b)

Fig. 4.15 The dispersion relation of reflection spectra for (a) sample O and (b) sample P. The period of hole array for sample O and P are 2.3 μm and 1.7 μm, respectively.

Figs. 4.16 (a) and (b) display the emissions spectra for samples O and P in the normal directions, (1,0) Ag/SiO2 denotes the (1,0) Ag/SiO2 degenerated modes whose wave vectors satisfy Eq. (4.12) with (i,j)= ( 1,0)± , (0, 1)± or ( 1, 1)± ± , respectively.

Compared Fig. 4.16 (a) with Fig. 4.3(b), when the period of hole array shrinks from 3 to 2.3 μm , the (1,0) Ag/SiO2 degenerated mode are shifted from 3.86 μm to 2.72 μm , the Bragg scattered (1,0,1) CMs composed of six degenerated cavity modes with

(k,A,m)= ( 1,0, m)± , (0, 1, m)± or ( 1, 1, m)± ± are shifted from 3.1 μm to 2.58 μm where blackbody radiation is too weak to be observed. For the emission spectrum of sample P whose period of hole array shrinks to 1.7 μm as shown in Fig. 4.16 (b), all SPPs and Bragg scattered CMs are shifted to the shorter wavelength where blackbody radiation are too weak to be observed although there are still reflection deeps observed in the reflection spectra for such modes as shown in Fig. 4.15 (b).

Compared the emission spectra of sample P shown in Fig. 4.16 (b) with the emission spectra of sample H shown in Fig. 4.14 (b), it can be found that the emission spectra of CTEs with SPHA can be as pure as CTEs with RDHA, but the output intensity of CTEs with SPHA are much stronger than CTEs with RDHA due to high density of holes.

(a)

(b)

Fig. 4.16 The emission spectra in the normal directionϕ=0^o for (a) sample O and (b) sample P. The period of hole array for sample O and P are 2.3 μm and 1.7 μm, respectively. The thicknesses of SiO2 of both samples are 2 μm.

The FWHM, (Δλ)/λ , Q factors and the output powers of (0,0,1) CM for samples O and P are (0.2 μm, 0.17 μm) , (0.035,0.03), (28.6,33) and (186mW/cm², 137mW/cm²), respectively. All of them are better than those of the traditional plasmonic thermal emitters (PTEs) could achieve as summarized in Table 4.3.

Another advantage of CTEs is that CMs of CTEs in the direction other than normal direction would not split into four branches across wide energy band as the SPPs modes of PTEs as shown in Fig. 4.3 (a) but would exhibit only slight blue shift in the

narrow energy band as shown in Fig. 4.15 (b), especially for large kx (large ϕ).

The dispersion curves and the emission peaks of the CTEs with SPHA are tunable simply by changing the thickness of SiO2 according to Eqs. (4.16) and (4.17).

Fig. 4.17 shows emission spectrum in the normal direction (ϕ=0^o) for sample Q with the SiO2 thickness of 1.6 μm, the wavelength of (0,0,1) CM is 4.69 μm. The FWHM,

(Δ λ) / λ, Q factor and output power of (0,0,1) CM are 0.119μm, 0.0253, 39 and 140mW/cm², respectively.

Fig. 4.17 The emission spectrum for sample Q. The thickness of SiO2 is 1.6 μm.

Finally, compared the emission spectra of sample P as shown in Fig. 4.16 (b) to sample E as shown in Fig. 4.14(a), the CTE with top thin silver film can not work well in the high temperature due to the stability of top thin film, the key point is that

the surface thin film can not be thicker than the skin depth of silver (~20nm) too much in order to let the light resonated in the cavity to leak out the thin silver film to outside.

However, such thin thicknesses can not offer good thermal stability in high temperature operation. For CTEs with SPHA, the surface reflective mirror of the Fabry-Perot resonance cavity is formed by the subwavelength circular holes, the minimum skin depth of subwavelength holes in normal directionϕ= is [71] 0^o

h 2 2 the wavelength of radiation modes guided in the subwavelength circular holes.

Fig. 4.18 shows the calculation of minimum skin depth of subwavelength holes in normal directionϕ= . Since the skin depths of subwavelength holes are much larger 0^o than the skin depth of silver, the thickness of surface silver film can be elevated from 15nm of sample E to the 100nm of sample P. Fig. 4.19 shows the comparison of reflection spectra for sample P and sample E at ϕ=12^o, almost the same reflection spectra are obtained except at the wavelength smaller than 2.5 μm where SPPs and Bragg scattered CMs of sample P appear. However, CTEs with SPHA could offer better thermal stability in the high-temperature operation where CTEs with top thin silver film could not achieved.

Fig. 4.18 The calculation of minimum skin depth of subwavelength holes to the wavelengths of lights guided inside the holes according to Eq. (4.23).

Fig. 4.19 The reflection spectra atϕ=12^o for sample E which is a CTE with 15nm top thin silver film and sample P which is a CTE with SPHA. The period and diameter of hole array of sample P are 1.7 μm and 1 μm, respectively.

In conclusions, high performance mid-infrared narrow-band CTEs with SPHA have been realized successfully. The SPPs modes and Bragg scattering CMs are shifted to the short wavelengths by short period where blackbody radiations are too weak to be observed. Small hole size eliminates non-ideal effects such as LCMs and FP-hole modes and offers narrower bandwidth emission peaks with small FWHM, The thickness of top silver film is thick enough to offer good thermal stability in high temperature operation. High density surface hole array offer strong emission intensity where CTEs with RDHA could not achieve. The FWHM, (Δ λ) / λ and Q factors are demonstrated to be all better than what traditional PTEs could achieve. The wavelengths of emission peaks are tunable by the thickness of the cavity.