Quarter-wave stack

Figure 143 illustrates the high spectral selectivity of the microcavity structure. The EL spectrum of the Eu complex-based conventional LED from Fig. 136a is compared with the EL spectrum of the same organic layers system placed in a microcavity formed by the MgAg metal/100% mirror (150 nm) and a dielectric half mirror (a quarter-wave stack... 

FIGURE 4.3. (a) Schematic structure of a microcavity LED with an Alq emitting layer. The top mirror is the electron-injecting contact and the bottom mirror is a three-period dielectric quarter-wave stack (QWS) with Si02(n = 1.5) and SixNy(n = 2.2). (b) Electroluminescence spectrum from a cavity LED compared with that from a noncavity LED. The noncavity LED possesses the same layer structure as shown in (a), but has no QWS. 

In these experiments, two different substrates were employed one quartz substrate with a three-period Si02/Si3N4 quarter-wave stack and the other plain quartz. Thirty nanometers of ITO was deposited on both substrates by pulsed-laser deposition of a commercial ITO target in 1 mtorr of O2 at a substrate temperature of 250° C.32 This was followed by depositing 60 nm of TAD and 60 run of Alq,... 

Ri denotes the front reflectance and R2 that of the rear side. The most commonest method is, as already discussed, the superposed deposition of two or more quarter-wave stacks, with shifted centre wavelength and suitable transmittance peak attenuation, on the front surface of the glass. 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

Figure 14.21 A quarter-wave stack of alternating-layers of high and low refractive index solids, wh. and each of optical thickness one quarter of a wavelength, A/4...

H) and low (L) refractive indices, and l, illuminated by light falling perpendicular to the surface (Figure 14.21). The arrangement is called a quarter-wave stack. For a quarter-wave stack deposited on a substrate in the sequence ... 

A quarter wave stack on a glass substrate ( = 1.545), in air, is required to reflect light of 650 nm from a laser. The materials chosen have refractive indices of 2.15 (tantalum pentoxide, Ta20s) and 1.35 (cryolite, Na3AlF6). What are the real (physical) thicknesses of the layers ... 

The width of the omnidirectional frequency range can be approximated for quarter-wave stacks as [249]... 

Figure 1. Frequency dependence of group-velocity dispersion for Fibonacci stacks of 6-stage (a), of 4-stage (c) and for periodic structures (b), (d) of the same length as corresponding Fibonacci stack. Normalization frequency too = 2.45 10 s" corresponds to quarter-wave eonstituent layers.

Deterministic non-periodic Fibonacci structures have a specific feature not present in periodic ones. Quarter-wave Fibonacci multilayers can contain more layers than periodic ones on the same length scale because they can have more layers with high index of refraction which are geometrically thinner. This allows to increase the dispersion of the structure and to decrease geometrical compression length for optical chirped pulses. Theoretical analysis has shown that this diminish can amount to as much as 10 times (compared to periodic stacks) if the refraction index contrast is high. Therefore these structures can minimize the size of compressors for chirped optical pulses. 

Interference mirrors are dielectric thin film coatings where low- and high-refractive index layers alternate. The optical thickness of each of the layers is equal to quarter-wavelength QJAn). They are denoted as distributed Bragg mirrors or distributed Bragg reflectors (DBR), sometimes simply as Bragg mirrors. Other names include quarter-wave mirrors (QWM), quarterwave stacks (QWS) and highly reflective (HR) layers. 

Using these values, a nine-stack quarter-wave filter containing over 300 monolayers was constructed (Fig. 3.13). Agreement between the measured and theoretically modelled filter responses was found to be... 

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