國立台灣大學林清富教授實驗室

研究領域摘要

主題五: 光發射光電元件

研究人員: 趙家忻

英文摘要:

In 1879, Thomas Edison invented the light bulb that utilized black-body radiation for illumination. 1 This invention is still in common use today. However the

illumination efficiency remains low because the visible spectrum (400nm-700nm) of black-body radiation occupies only 5% of the total radiation energy even at

the high temperature of 2200 oC. Increasing illumination efficiency is very attractive for the concern of energy consumption. Unfortunately, black-body

radiation is deemed to be of a nature that is difficult to change. Recent advance in research on photonics opens up the possibility of light manipulation using

photonic crystals. 2-8 Metallic photonic crystals thus have been shown to modify black-body radiation for the enhancement of emission in infrared (IR)

spectrum. 8, 9 However, the physics of the enhancement is still not clear due to the complication of photonic crystals. The modification of black-body radiation

for the enhancement of the visible spectrum has neither been shown because of the difficulty in their fabrication. Here we show another way to enhance black-

body radiation in the desired spectrum. Using photonic boxes with a size of about 200 nm, our experiment demonstrates that black-body radiation has

significantly enhanced light emission at around 400 nm with a narrow spectral width at a much lower temperature than conventional situation. Because of the

much simpler structure, its physics can be easily understood. Other visible spectra can also be enhanced by simply increasing the size of photonic boxes.

Between the resonance wavelengths, there are no existing EM waves in the box. The forbidden spectrum, analogous to the terminology for photonic crystals, is

called “band gap”.2 Therefore, photonic boxes have many “band gaps”. In addition, there is a maximum wavelength beyond which no corresponding EM

wave exists in the box. The maximum wavelength is given by lcutoff = na, called the cut-off wavelength. Therefore, photonic boxes have an extremely wide

band gap, which is almost infinite in principle. If the box has other shapes, for example, cylindrical shape or other geometric shapes, we can also analogously

apply EM theory to calculate its resonance wavelengths. Their characteristics of appearing cut-off wavelength and suppressing long-wavelength spectrum are

similar to those of the cubic photonic box.

If a body consists of many photonic boxes that do not have the same size, the resonance wavelengths will form a band with its density of states (DOS) undulated

by the size distribution function. Then the band gap between the resonance peaks shrinks and may disappear if the size distribution function is broad. However,

the widest band gap that extends from the largest cut-off wavelength (corresponding to the largest box) to infinite wavelength should still exist. Fig. 1 illustrates

the DOS of a body containing many photonic boxes. The cases of a unique size and various sizes are shown. On the other hand, the photon statistics that favors

long-wavelength spectrum 11 will cause the DOS at the cut-off wavelength to dominate light emission. The enhanced emission of black-body radiation at the

resonance wavelength using photonic boxes is verified by the following experiment. The resonance wavelengths are designed to be in the ultraviolet (UV) to

blue light regime to show that the concept of photonic boxes can be easily applied to other wavelengths of visible light or IR radiation.

The photonic boxes were examined using atomic force microscopy (AFM). Fig. 3 shows the AFM image. The AFM is able to reflect each individual photonic

box because the top surface is not flat, indicating that the evaporated Pt at step 5 (Fig. 2) does not exactly have the same thickness as the SOG. However,

because most boxes still have Pt surrounding the dielectric, the function of the photonic boxes is preserved. Because the IIPL method naturally results in

periodic patterns, the fabricated boxes are aligned periodically although it is not necessary to align them in this way for the enhanced spectrum at the resonance

wavelength. The AFM image shows that the patterns have periods of 432 nm and 487 nm respectively along two perpendicular directions. The shape is not

rectangular because the UV lithography cannot produce sharp corners, causing the resonance wavelength to be different from the theoretical design. However,

the characteristics of photonic boxes are still preserved. Fig. 3 also shows that the size exhibits variation, so there are various cut-off wavelengths and the

enhanced emission should have broadened spectrum.

At the temperature of 983K, the peak intensity at 390 nm is about 5 times larger than the background level at 470 nm with an enhancement factor of larger than

5000. The slightly reduced contrast is due to the fact that the photonic boxes are partially destroyed. The reason is two-fold. First, because the top layer of Pt is

only about 10 nm, this Pt film might break at high temperature as a result of the surface tension. Secondly, the substrate Si and metals Cr, Au, and Pt might form

alloys at high temperature. Then the photonic-box structures are reshaped. Using materials that sustain high temperature should improve the operational

situation.

The spectral width is 90 nm, which is about the same for all temperatures. This is another important feature characterized by the DOS function of the photonic

boxes according to Eq. (1). As discussed above, it is due to the size variation of the photonic boxes, so the spectral width does not vary with the temperature.

The temperature-independent behaviors are different from those in the usual black-body radiation, but similar to the behaviors of metallic photonic crystals.

However, the emission peak and the spectral width of the black-body radiation of the metallic photonic crystals are due to the periodic nature. 8, 9, 13, 14 In

comparison, with photonic boxes, the black-body radiation has the peak wavelength located at the resonance wavelength of the largest-number boxes and the

spectral width governed by the size variation. If the photonic boxes have a single size, we expect that the emission should have a very narrow line. Our

investigation of photonic boxes gives a much clearer explanation on the physics of the black-body radiation modified by photonic structures.

There is also an enhanced spectrum in the vicinity of 800 nm. This is because some of the neighboring boxes have no Pt between them due to the imperfect

photolithography, causing two neighboring boxes to be connected together. As a result, one side of the box becomes twice as large as the design size and the

resonance wavelength is doubled. Although the number of those connected boxes is small, the favored long-wavelength spectrum of photon statistics 11 leads

to an emission at around 800 nm much more enhanced than that at around 400 nm.

In conclusion, we demonstrate that photonic boxes with a size of about 200 nm are able to enhance the blue light of black-body radiation. The enhanced blue

light has a peak at 390nm and a spectral width of about 90 nm, which are both temperature-independent. The enhancement factor is more than 5000. The

physics can be easily explained by the modified density of states as a result of photonic boxes. Black-body radiation can also be modified to enhance other

spectrum using photonic boxes of different sizes or shapes.

Fig. 1 Density of states of the photonic boxes. Unique box size will exhibit a delta function,

and various sizes of boxes will broaden the density of states.

Fig. 2 Fabrication steps of the photonic boxes.

Fig. 3 AFM image of the fabricated photonic boxes.

Fig. 4 Measured spectra of radiation from heated photonic boxes at different temperatures and theoretical fitting with formula (1) at T=983K.

Fig. 5 Traditional tungsten bulb has only about 5% energy conversion coefficient with visible light range under 2500K.

Fig. 6 Box size dominates the peak wavelength.

Fig. 7 SEM image of the photoresist to fabricate metallic photonic boxes

Fig. 8 SEM image of the metallic photonic boxes

Fig. 9 Image of the metallic photonic boxes before heating

Fig. 10 Image of the metallic photonic boxes at high temperature

中文摘要:

此研究之目的為研究金屬三維共振腔結構之黑體輻射的發光特性。傳統黑體輻射的能量分佈在可見光區域僅只有5%（溫度在2200°C時），意即

有大部份能量的光浪費掉而無法作用在照明的用途。我們利用金屬三維共振腔的概念製作奈米金屬光子盒，可以將光侷限在一個金屬包附的三

維共振腔內，設計對應的尺寸，限制光發出的波長。我們的研究發現，當使用尺寸為200奈米的金屬光子盒，可以使其發出最長為400奈米的光

譜，並且能大輻降低長波長波段能量的光。當溫度僅達到1000°C時，在400奈米的光強度比長波長波段的光強度高上5倍之多。若設計不同尺寸

的金屬光子盒，在加熱到高溫後，也可以使其對應不同波長的光。此一概念大幅改變傳統黑體輻射的概念，使金屬光子盒在較低的溫度就可以

達到可見光的輻射，未來在照明產業上的應用極具潛力。

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