**3. Nanostructured antireflection coatings for optical sensor applications**

### **3.1 Overview of nanostructured AR coating technology**

Reducing optical reflections from surfaces, which is important to many optical applications including optical lenses, windows, photovoltaic devices, and photodetectors, has commonly been achieved through coating, or texturing, the surfaces of interest. Nanostructures that minimize reflection loss have been investigated for the development of high performance antireflection (AR) coatings. Such nanostructured AR coatings having optimal index profiles can demonstrate broadband AR properties, particularly with air as the ambient medium.

Glass, a dielectric material widely used in a variety of optical applications including lenses, windows, and as a cover or encapsulation for semiconductor optoelectronic devices, is basically transparent to wavelengths longer than 400 nm. However, due to Fresnel reflection losses glass reflects about 4% of the incident light from its front surface, or ~8% from both surfaces. This undesired reflection in certain applications can degrade the efficiency of the underlying device (e.g., sensor or photovoltaic cell), reduce its signal-to-noise ratio (e.g., in the case of a photodetector), and cause glare (e.g., from electronic displays). For such applications, it is important not only to reduce reflectance but also to improve the transmittance through the surface. This requires that the coating material be nonabsorbent, and the coating surface be specular.

AR coatings have conventionally been composed of single layers having optical thicknesses equal to one quarter of the wavelength (λ/4) of interest. Ideally, such a single-layer λ/4 AR coating should have a refractive index *nλ*/4 given by [18]:

 *n*λ/4 ≈ √ \_\_\_\_\_\_\_\_\_\_\_\_ *nsubstrate* <sup>×</sup> *nair* (10)

However, due to the unavailability of materials having the precise desired refractive index, the performance of λ/4 AR coatings often deviates from the optimum, which is especially the case for low-index substrates such as glass. For example, an ideal single-layer λ/4 AR coating on a glass surface in an air ambient would require a material with a refractive index of (1.46)1/2 ≈ 1.21. Due to the unavailability of optical materials with very low (*n* < 1.4) refractive indexes, near-perfect graded-index AR coatings for glass substrates have not been practically achievable.

Recently, however, a new class of optical thin film materials consisting of tunable nanostructures has enabled the realization of very low refractive index materials [9, 19]. Using these nanostructured materials, AR coatings can greatly minimize reflection losses and enhance the sensitivity and performance of detection and imaging systems.

The multilayer AR coatings feature step-graded refractive indexes which decrease in discrete steps from that of the substrate (e.g., ~1.5 for SiO2 or ~3.5 for Si) to a value of close to that of air (e.g., 1.18). These AR coatings with specular surfaces comprising multiple discrete layers of non-absorbing materials exploit thin film interference effects to reduce reflectance and maximize transmittance. Such discrete multilayer AR coatings, which have been shown outperform continuouslygraded AR coatings, offer very effective antireflection performance [20].

**Figure 12(a)** shows the measured and calculated refractive index of deposited SiO2 vs. deposition angle following a formula developed by Poxson et al. [20]. This plot demonstrates the potential to tune the refractive index of a given material to virtually any value between its bulk value and that of air (~1.0) through controlling the deposition angle. The SEM images in **Figure 12(b)** show the gradual decrease in density of SiO2 nanocolumns transitioning from a dense bulk film deposited at an angle of 0° to a nanostructured film deposited at 75°.

#### **3.2 Growth of nanostructured AR coating layers**

The nanostructured optical layers are fabricated using a scalable physical vapor deposition (PVD) self-assembly process, which allows them to be processed on almost any type of substrate. This process involves the formation of a highly directional vapor flux, which can be implemented through melting various optical coating materials. Surface diffusion and self-shadowing effects during the growth process enable the formation of the nanostructured thin films.

As illustrated in **Figure 13**, random growth fluctuations on the substrate produce a shadow region that incident vapor flux cannot reach and a nonshadow region where incident flux deposits preferentially, creating rod-like structures with lower effective refractive indexes [21]. The deposition angle, the angle between the normal to the sample surface and the incident vapor flux, results in the formation of nanostructures tilted relative to the sample surface. This process offers advantages such as tunability of the refractive index, flexibility in choice of materials, simplicity of the growth process, and ability to optimize the coatings for any substrateambient material system.

Since the gaps between the nanostructures are typically much smaller than the wavelengths of visible and IR light, the nanostructured layer acts as a single homogenous film having a refractive index intermediate in value between that of the ambient air and of the nanostructure material that decreases in density with increasing gap size. Nanowires and other types of nanostructures grown by the self-assembly process provide a pathway for fabricating high-quality broadband AR coatings for a variety of nanosensor applications.

#### **Figure 12.**

*(a) Refractive index of oblique-angle-deposited SiO2 as a function of deposition angle. (b) SEM images showing a gradual decrease in density of SiO2 nanocolumns for nanostructured AR coatings transitioning from a dense bulk film at 0° deposition angle to a nanostructured film at 75° deposition angle [20].*

**83**

**Figure 14.**

**Figure 13.**

*Nanostructure Technology for EO/IR Detector Applications*

**3.3 Nanostructured AR coatings for visible EO applications**

**Figure 14** presents the measured refractive index dispersion curve as a function of wavelength from a nanostructured SiO2 layer deposited at a highly oblique angle [22, 23]. This low-index nanostructured SiO2 film was deposited on a Si substrate and measured by ellipsometry. Also shown is a comparison of experimental reflectivity data with theoretical calculations. These results illustrate that the nanostructures grown by the self-assembly process provide a pathway for fabricating high quality broadband AR coatings for a variety of sensing and imaging applications. **Figure 15** demonstrates the use of SiO2 and TiO2 nanostructures to achieve high performance, step-graded AR coatings on AlN substrates [10, 23]. In the following sections recent efforts are summarized to extend this technology to different substrates and lenses and to other bands of interest spanning the visible to the infrared

We have fabricated and tested various step-graded AR structures comprising layers of nanostructured SiO2 using the PVD self-assembly process [17, 21, 24].

*(a) Schematic of self-assembly process for synthesis of nanostructured films, showing (a) initial formation of material islands at random locations across the substrate, followed by (b) formation of self-shadowed regions and nanocolumnar growth when the material vapor flux arrives at a non-normal deposition angle θ to the substrate [21].*

*(a) Refractive index dispersion curve of low-index SiO2 nanostructure thin film on a Si substrate, with (b)* 

*comparison of the measured and calculated reflectivity spectra [22, 23].*

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

for next-generation sensors.

*Nanostructure Technology for EO/IR Detector Applications DOI: http://dx.doi.org/10.5772/intechopen.85741*

**Figure 14** presents the measured refractive index dispersion curve as a function of wavelength from a nanostructured SiO2 layer deposited at a highly oblique angle [22, 23]. This low-index nanostructured SiO2 film was deposited on a Si substrate and measured by ellipsometry. Also shown is a comparison of experimental reflectivity data with theoretical calculations. These results illustrate that the nanostructures grown by the self-assembly process provide a pathway for fabricating high quality broadband AR coatings for a variety of sensing and imaging applications.

**Figure 15** demonstrates the use of SiO2 and TiO2 nanostructures to achieve high performance, step-graded AR coatings on AlN substrates [10, 23]. In the following sections recent efforts are summarized to extend this technology to different substrates and lenses and to other bands of interest spanning the visible to the infrared for next-generation sensors.

#### **3.3 Nanostructured AR coatings for visible EO applications**

We have fabricated and tested various step-graded AR structures comprising layers of nanostructured SiO2 using the PVD self-assembly process [17, 21, 24].

**Figure 13.**

*Nanorods and Nanocomposites*

ambient material system.

variety of nanosensor applications.

angle of 0° to a nanostructured film deposited at 75°.

**3.2 Growth of nanostructured AR coating layers**

process enable the formation of the nanostructured thin films.

**Figure 12(a)** shows the measured and calculated refractive index of deposited SiO2 vs. deposition angle following a formula developed by Poxson et al. [20]. This plot demonstrates the potential to tune the refractive index of a given material to virtually any value between its bulk value and that of air (~1.0) through controlling the deposition angle. The SEM images in **Figure 12(b)** show the gradual decrease in density of SiO2 nanocolumns transitioning from a dense bulk film deposited at an

The nanostructured optical layers are fabricated using a scalable physical vapor deposition (PVD) self-assembly process, which allows them to be processed on almost any type of substrate. This process involves the formation of a highly directional vapor flux, which can be implemented through melting various optical coating materials. Surface diffusion and self-shadowing effects during the growth

As illustrated in **Figure 13**, random growth fluctuations on the substrate produce a shadow region that incident vapor flux cannot reach and a nonshadow region where incident flux deposits preferentially, creating rod-like structures with lower effective refractive indexes [21]. The deposition angle, the angle between the normal to the sample surface and the incident vapor flux, results in the formation of nanostructures tilted relative to the sample surface. This process offers advantages such as tunability of the refractive index, flexibility in choice of materials, simplicity of the growth process, and ability to optimize the coatings for any substrate-

Since the gaps between the nanostructures are typically much smaller than the wavelengths of visible and IR light, the nanostructured layer acts as a single homogenous film having a refractive index intermediate in value between that of the ambient air and of the nanostructure material that decreases in density with increasing gap size. Nanowires and other types of nanostructures grown by the self-assembly process provide a pathway for fabricating high-quality broadband AR coatings for a

*(a) Refractive index of oblique-angle-deposited SiO2 as a function of deposition angle. (b) SEM images showing a gradual decrease in density of SiO2 nanocolumns for nanostructured AR coatings transitioning from* 

*a dense bulk film at 0° deposition angle to a nanostructured film at 75° deposition angle [20].*

**82**

**Figure 12.**

*(a) Schematic of self-assembly process for synthesis of nanostructured films, showing (a) initial formation of material islands at random locations across the substrate, followed by (b) formation of self-shadowed regions and nanocolumnar growth when the material vapor flux arrives at a non-normal deposition angle θ to the substrate [21].*

#### **Figure 14.**

*(a) Refractive index dispersion curve of low-index SiO2 nanostructure thin film on a Si substrate, with (b) comparison of the measured and calculated reflectivity spectra [22, 23].*

#### **Figure 15.**

*(a) Cross-sectional SEM image of TiO2 and SiO2 step-graded index nanostructure coatings that approximate a modified quintic profile. The graded-index coating consists of three TiO2 nanostructured layers and two SiO2 nanostructured layers and (b) deposition angles, thicknesses, and refractive indexes for each layer in the graded index coating [10, 23].*

These multilayer nanostructured AR coatings have been deposited on both sides of glass substrates, and the corresponding transmittance characterized as a function of wavelength and incident angle. The nanostructured AR coatings have likewise been successfully demonstrated on the curved surfaces of converging and diverging optical lenses, which are key components that manipulate the optical pathways in optical and infrared systems.

#### *3.3.1 Glass substrates*

**Figure 16** compares the measured broadband performance of an uncoated glass slide to one coated on both sides with a multilayered, nanostructured SiO2 coating, where the transmittance is characterized as a function of wavelength and light incidence angle [10]. The inset in **Figure 16** shows a representative cross-sectional SEM image of the two-layer structure. The nanostructured coatings were prepared in an electron beam evaporator using different deposition angles to form distinct layers with step-graded refractive index profiles.

The transmittances of the coated and uncoated glass slides were measured using an angle-dependent transmittance measurement setup consisting of a xenon lamp light source and Ando AQ6315A optical spectrum analyzer calibrated for detection of transmitted photons over the 400–800 nm wavelength range. The measured peak broadband transmittance at normal incidence of the uncoated glass slide was 92%, corresponding to ~4% reflection loss at each glass/air interface.

The peak transmittance was 98.3% for the double-sided, nanostructure-coated glass, implying an average broadband reflection loss of less than 1% at each glass/ air interface. As shown in **Figure 16**, the transmittance through the nanostructured SiO2 coated glass was also significantly higher than that through the uncoated glass across a wide range of incident angles. While the transmittance of the uncoated glass slide falls below 80% at an incident angle of 65°, the glass slide with the double-sided coating still maintains a transmittance above 95% at this angle of light incidence.

The transmittances of coated and uncoated glass slides have also been measured at normal incidence as functions of wavelength using a JASCO V-570 spectrophotometer. As shown in **Figure 17**, the average measured broadband transmittance between 350 nm and 1800 nm increases from 92.2% for the uncoated glass to 98.6% for the double-sided nanostructured coated glass [9]. This transmittance exceeds 97.8% at all wavelengths between 440 and 1800 nm, corresponding to a glass-air

**85**

*3.3.2 Optical lenses*

**Figure 17.**

*substrate [9].*

**Figure 16.**

their benefits for optical system applications.

*Nanostructure Technology for EO/IR Detector Applications*

interface reflectivity below 1.1% over this relatively wide spectrum. These optimized nanostructured AR coatings have been shown to outperform ideal quarterwavelength MgF2 coatings over all visible wavelengths and incident angles [21].

*Wavelength-dependent transmittance measurement of step-graded, nanostructured SiO2 AR coating on glass* 

*Incident angle dependent broadband transmittance through glass slide coated on both sides with a step-graded, nanostructured SiO2 AR coating, along with that for an uncoated glass slide. A representative cross-sectional* 

*SEM image of the dual-layer nanostructured coating is shown in the inset [10].*

Light passing through an uncoated lens will suffer reflection losses at both the input and output lens interfaces. These losses add up and can limit the performance of detector devices. Demonstration of AR coatings on curved lens surfaces extends

We have designed and optimized step-graded index profiles for optical lenses, as well as for ZnSe IR lenses which are covered in the following section. Multilayer

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

#### **Figure 16.**

*Nanorods and Nanocomposites*

optical and infrared systems.

layers with step-graded refractive index profiles.

corresponding to ~4% reflection loss at each glass/air interface.

*3.3.1 Glass substrates*

**Figure 15.**

*index coating [10, 23].*

These multilayer nanostructured AR coatings have been deposited on both sides of glass substrates, and the corresponding transmittance characterized as a function of wavelength and incident angle. The nanostructured AR coatings have likewise been successfully demonstrated on the curved surfaces of converging and diverging optical lenses, which are key components that manipulate the optical pathways in

*(a) Cross-sectional SEM image of TiO2 and SiO2 step-graded index nanostructure coatings that approximate a modified quintic profile. The graded-index coating consists of three TiO2 nanostructured layers and two SiO2 nanostructured layers and (b) deposition angles, thicknesses, and refractive indexes for each layer in the graded* 

**Figure 16** compares the measured broadband performance of an uncoated glass slide to one coated on both sides with a multilayered, nanostructured SiO2 coating, where the transmittance is characterized as a function of wavelength and light incidence angle [10]. The inset in **Figure 16** shows a representative cross-sectional SEM image of the two-layer structure. The nanostructured coatings were prepared in an electron beam evaporator using different deposition angles to form distinct

The transmittances of the coated and uncoated glass slides were measured using an angle-dependent transmittance measurement setup consisting of a xenon lamp light source and Ando AQ6315A optical spectrum analyzer calibrated for detection of transmitted photons over the 400–800 nm wavelength range. The measured peak broadband transmittance at normal incidence of the uncoated glass slide was 92%,

The peak transmittance was 98.3% for the double-sided, nanostructure-coated glass, implying an average broadband reflection loss of less than 1% at each glass/ air interface. As shown in **Figure 16**, the transmittance through the nanostructured SiO2 coated glass was also significantly higher than that through the uncoated glass across a wide range of incident angles. While the transmittance of the uncoated glass slide falls below 80% at an incident angle of 65°, the glass slide with the double-sided coating still maintains a transmittance above 95% at this angle of light

The transmittances of coated and uncoated glass slides have also been measured at normal incidence as functions of wavelength using a JASCO V-570 spectrophotometer. As shown in **Figure 17**, the average measured broadband transmittance between 350 nm and 1800 nm increases from 92.2% for the uncoated glass to 98.6% for the double-sided nanostructured coated glass [9]. This transmittance exceeds 97.8% at all wavelengths between 440 and 1800 nm, corresponding to a glass-air

**84**

incidence.

*Incident angle dependent broadband transmittance through glass slide coated on both sides with a step-graded, nanostructured SiO2 AR coating, along with that for an uncoated glass slide. A representative cross-sectional SEM image of the dual-layer nanostructured coating is shown in the inset [10].*

#### **Figure 17.**

*Wavelength-dependent transmittance measurement of step-graded, nanostructured SiO2 AR coating on glass substrate [9].*

interface reflectivity below 1.1% over this relatively wide spectrum. These optimized nanostructured AR coatings have been shown to outperform ideal quarterwavelength MgF2 coatings over all visible wavelengths and incident angles [21].

#### *3.3.2 Optical lenses*

Light passing through an uncoated lens will suffer reflection losses at both the input and output lens interfaces. These losses add up and can limit the performance of detector devices. Demonstration of AR coatings on curved lens surfaces extends their benefits for optical system applications.

We have designed and optimized step-graded index profiles for optical lenses, as well as for ZnSe IR lenses which are covered in the following section. Multilayer step-graded AR coating index profiles were created and optimized for glass lenses by controlling the refractive index profiles and thicknesses of the individual layers. Nanostructured SiO2 layers of the desired refractive index were deposited on the entire surface of the optical lens by the self-assembly process. **Figure 18** compares the transmittance of uncoated and nanostructured SiO2 multilayer AR-coated optical lenses [10].

The nanostructured AR coating significantly improves the optical transmittance through the lens from 94% to nearly 100%, which is maintained over the entire visible and majority of the NIR spectra. AR-coated lenses can thus transmit a virtually unattenuated optical signal to a sensor over a broader spectrum through eliminating unwanted reflections, enabling more effective detector devices with significantly higher responsivities. This approach can be expanded to benefit various IR components, significantly improving the detection, sensing, and imaging capabilities of electro-optical and infrared systems.

#### *3.3.3 Flexible substrates*

Broadband and high-performance AR coatings have been demonstrated on flexible substrates such as polycarbonate films. Polycarbonate is a useful material for commercial products including display filters, plastic lenses, and face shields, and is commonly utilized for defense applications as well. Polycarbonate also provides high impact resistance and has an excellent flammability rating.

Nanostructured multilayer SiO2 AR coatings having optimized step-graded index profiles have been deposited on both sides of polycarbonate sheets. **Figure 19** compares the optical transmittance spectra of uncoated and AR-coated polycarbonate sheets [9]. The expanded transmittance spectrum over the visible (~400–800 nm) band is plotted in the inset of **Figure 19**. As seen in the inset, the uncoated polycarbonate sheet shows approximately 90% transmittance over the visible spectrum. For the AR-coated polycarbonate sheet, however, the transmittance rises to nearly 100%, and this enhancement is observed for the entire visible and part of the NIR band. Such high AR performance demonstrated over the entire visible spectrum makes the nanostructure AR coatings potentially beneficial for certain electro-optical applications including CMOS image sensors.

#### **Figure 18.**

*Measured wavelength-dependent transmittance of nanostructured SiO2 coated lens compared to an uncoated lens. The AR coating provides nearly 100% transmittance, which is maintained over a wide spectrum of light [10].*

**87**

over IR bands.

**Figure 19.**

*3.4.1 MWIR applications*

*3.4.2 LWIR applications*

at different deposition angles.

IR signal losses over the LWIR band.

*Nanostructure Technology for EO/IR Detector Applications*

**3.4 IR applications of nanostructured AR coatings**

Infrared detection technology plays a critical role in various terrestrial and space applications. In order to extend the application of the AR coatings to MWIR and LWIR spectral bands, nanostructured AR coatings have been fabricated on silicon wafers by sequentially growing multiple nanostructured Si and SiO2 layers with desired graded-index profiles through control of the deposition angle. These AR coatings minimize the reflection loss from approximately 30% to less than 1.5%

*Optical transmittance spectrum of transparent polycarbonate film (5 mil thickness) before and after application of nanostructured AR coating. The AR coating provides nearly 100% transmittance at 610 nm [9].*

The measured wavelength-dependent reflectance of multilayer AR-coated and uncoated Si substrates are shown plotted in **Figure 20** [9]. The multilayer AR coating has been synthesized with graded-index profiles optimized for the 3–5 μm MWIR band. As seen in the figure, the average measured reflectance for the multilayer AR-coated Si wafer is less than 1.5%, while the average measured reflectance for the uncoated silicon <211> wafer is ~35% over the 3–5 μm MWIR spectral band.

The measured wavelength-dependent reflectance of an uncoated Si substrate and AR-coated Si <211> substrate having an index profile specifically designed for the 6–11 μm LWIR spectral band is plotted in **Figure 21** [25]. This dual-layer AR coating was synthesized by sequentially growing two nanostructured Si layers each

The average reflectance from 6 to 11 μm for the two-layer AR-coated Si wafer is less than 1.5%, while that of the uncoated Si <211> wafer is ~28%. These results clearly demonstrate that the multilayer AR coating significantly minimizes incident

Infrared optical components such as windows and lenses typically reflect a significant portion of the incoming IR signal to be detected. The incorporation of AR coatings on these IR components can greatly enhance overall system performance. We have demonstrated the nanostructured AR coatings on various components for

improved IR performance, including ZnSe lenses for LWIR applications.

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

#### **Figure 19.**

*Nanorods and Nanocomposites*

electro-optical and infrared systems.

*3.3.3 Flexible substrates*

step-graded AR coating index profiles were created and optimized for glass lenses by controlling the refractive index profiles and thicknesses of the individual layers. Nanostructured SiO2 layers of the desired refractive index were deposited on the entire surface of the optical lens by the self-assembly process. **Figure 18** compares the transmittance of uncoated and nanostructured SiO2 multilayer AR-coated optical lenses [10]. The nanostructured AR coating significantly improves the optical transmittance through the lens from 94% to nearly 100%, which is maintained over the entire visible and majority of the NIR spectra. AR-coated lenses can thus transmit a virtually unattenuated optical signal to a sensor over a broader spectrum through eliminating unwanted reflections, enabling more effective detector devices with significantly higher responsivities. This approach can be expanded to benefit various IR components, significantly improving the detection, sensing, and imaging capabilities of

Broadband and high-performance AR coatings have been demonstrated on flexible substrates such as polycarbonate films. Polycarbonate is a useful material for commercial products including display filters, plastic lenses, and face shields, and is commonly utilized for defense applications as well. Polycarbonate also provides

Nanostructured multilayer SiO2 AR coatings having optimized step-graded

**Figure 19** compares the optical transmittance spectra of uncoated and AR-coated polycarbonate sheets [9]. The expanded transmittance spectrum over the visible (~400–800 nm) band is plotted in the inset of **Figure 19**. As seen in the inset, the uncoated polycarbonate sheet shows approximately 90% transmittance over the visible spectrum. For the AR-coated polycarbonate sheet, however, the transmittance rises to nearly 100%, and this enhancement is observed for the entire visible and part of the NIR band. Such high AR performance demonstrated over the entire visible spectrum makes the nanostructure AR coatings potentially beneficial for

*Measured wavelength-dependent transmittance of nanostructured SiO2 coated lens compared to an uncoated lens. The AR coating provides nearly 100% transmittance, which is maintained over a wide spectrum of light [10].*

index profiles have been deposited on both sides of polycarbonate sheets.

high impact resistance and has an excellent flammability rating.

certain electro-optical applications including CMOS image sensors.

**86**

**Figure 18.**

*Optical transmittance spectrum of transparent polycarbonate film (5 mil thickness) before and after application of nanostructured AR coating. The AR coating provides nearly 100% transmittance at 610 nm [9].*

#### **3.4 IR applications of nanostructured AR coatings**

Infrared detection technology plays a critical role in various terrestrial and space applications. In order to extend the application of the AR coatings to MWIR and LWIR spectral bands, nanostructured AR coatings have been fabricated on silicon wafers by sequentially growing multiple nanostructured Si and SiO2 layers with desired graded-index profiles through control of the deposition angle. These AR coatings minimize the reflection loss from approximately 30% to less than 1.5% over IR bands.

#### *3.4.1 MWIR applications*

The measured wavelength-dependent reflectance of multilayer AR-coated and uncoated Si substrates are shown plotted in **Figure 20** [9]. The multilayer AR coating has been synthesized with graded-index profiles optimized for the 3–5 μm MWIR band. As seen in the figure, the average measured reflectance for the multilayer AR-coated Si wafer is less than 1.5%, while the average measured reflectance for the uncoated silicon <211> wafer is ~35% over the 3–5 μm MWIR spectral band.

#### *3.4.2 LWIR applications*

The measured wavelength-dependent reflectance of an uncoated Si substrate and AR-coated Si <211> substrate having an index profile specifically designed for the 6–11 μm LWIR spectral band is plotted in **Figure 21** [25]. This dual-layer AR coating was synthesized by sequentially growing two nanostructured Si layers each at different deposition angles.

The average reflectance from 6 to 11 μm for the two-layer AR-coated Si wafer is less than 1.5%, while that of the uncoated Si <211> wafer is ~28%. These results clearly demonstrate that the multilayer AR coating significantly minimizes incident IR signal losses over the LWIR band.

Infrared optical components such as windows and lenses typically reflect a significant portion of the incoming IR signal to be detected. The incorporation of AR coatings on these IR components can greatly enhance overall system performance. We have demonstrated the nanostructured AR coatings on various components for improved IR performance, including ZnSe lenses for LWIR applications.

#### **Figure 20.**

*Measured wavelength-dependent reflectance of multilayer AR-coated and uncoated Si <211> wafers. The multilayer AR coating on Si <211> wafers offers significantly lower reflectance over the 3–5 μm MWIR spectral band [9].*

#### **Figure 21.**

*Measured LWIR wavelength-dependent reflectance of two-layer AR coating structure on Si <211> substrate with that for an uncoated substrate. The AR coating significantly reduces reflection over this range compared to the uncoated Si substrate [25].*

#### **Figure 22.**

*Measured wavelength-dependent reflectance of uncoated and single-layer nanostructure AR-coated ZnSe lenses. The AR-coated ZnSe lens demonstrates an average reflectance of 0.6%, compared to 15.2% for the uncoated ZnSe lens, over the 8–10 μm spectral band [9].*

**89**

*Nanostructure Technology for EO/IR Detector Applications*

Single-layer nanostructured AR coatings have been applied to ZnSe lenses that significantly enhance performance by minimizing the reflection of incident IR signals at the lens surfaces. **Figure 22** compares the measured reflectance spectra of ZnSe lenses with and without the AR coatings [9]. The AR-coated ZnSe lens reflects

In this chapter we have first discussed recent efforts towards the design, modeling, and experimental development of next-generation carbon nanotube-based bolometers for IR sensing and imaging. The goal in the development and advancement of this technology is to enable high performance, high frame rate, uncooled microbolometers for MWIR and LWIR bands. We have presented recent results of growing variously orientated SWCNT and MWCNT films that demonstrate the promise of using CNTs for developing high performance and small pixel microbo-

The growth and application of nanostructured layers for developing high quality

antireflection coatings likewise offers an innovative approach for minimizing reflection losses in state-of-the-art sensors and optical windows for both defense and commercial applications. The step-graded multilayer antireflection technology has been shown to be both broadband and omnidirectional in nature. These nanostructure-based AR coatings have demonstrated high performance over spectral bands spanning the visible to the IR with the potential to include larger area

Optical sensing technology is critical for various defense and commercial applications including optical communication and IR imaging. Advances in detector materials and technologies have been achieved over a broad spectrum using Si and SiO2 based nanostructures and novel carbon nanotube-based materials. These technological advances are opening doors for new approaches to apply device design methodologies that can offer enhanced performance and low-cost optical sensors and systems benefitting a wide range of infrared and electro-optical

The authors thank Dr. Whitney Mason of DARPA/MTO for technical discussions and guidance. We would like to thank Ms. Susan Celis and Mr. Oscar Cerna of DARPA for their ongoing support. This research was developed with Phase II SBIR Programs from the Defense Advanced Research Projects Agency (DARPA) and the Army Research Laboratory (ARL). The views, opinions and/ or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. Distribution Statement "A" (Approved for Public Release,

on average ~0.6% of the incident signal over the 8–10 μm band, compared to reflectance of ~15% for the uncoated lens. The performance of the nanostructured AR coatings may be further improved by implementing multilayer rather than

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

single-layer structures.

lometer arrays.

applications.

**Acknowledgements**

Distribution Unlimited).

**4. Summary and conclusions**

substrates to benefit next-generation sensors.

Single-layer nanostructured AR coatings have been applied to ZnSe lenses that significantly enhance performance by minimizing the reflection of incident IR signals at the lens surfaces. **Figure 22** compares the measured reflectance spectra of ZnSe lenses with and without the AR coatings [9]. The AR-coated ZnSe lens reflects on average ~0.6% of the incident signal over the 8–10 μm band, compared to reflectance of ~15% for the uncoated lens. The performance of the nanostructured AR coatings may be further improved by implementing multilayer rather than single-layer structures.
