Abstract:
A system for monitoring and controlling optical energy. A system is disclosed having: an optical system with a surface for receiving an optical beam; a coating applied to the surface, wherein the coating includes an optical nanoporous dielectric thin film having an array of tilted nanoscale rods configured to reflect a scatter beam at a determined angle and pass a remaining portion of the optical beam to the surface; a satellite detector arranged to detect an intensity of the scatter beam; and a control system that receives and processes scatter beam data from the satellite detector to determine an intensity of the optical beam impacting the surface of the optical system.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority to co-pending provisional application, “Oriented backscattering wide dynamic-range optical radiation sensor and the application there of,” Ser. No. 61/912,598, filed on Dec. 6, 2013, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates generally to material coatings for optical devices, and more particularly to a coating that provides anti-reflection properties while deflecting a controlled fraction of light flux for monitoring and control purposes. 
         [0004]    2. Related Art 
         [0005]    There exist any number of high performance optical materials, e.g., solar cells, sensors, lenses, glass, mirrors, etc., that have surfaces that manipulate or exploit optical radiation, such as ultraviolet (UV) or light energy. For example, a solar cell has a surface made from a semiconducting material such as silicon that converts light energy into electricity. In a further example, sensors such as photo resistors output a resistance value based on an amount of light energy incident upon the sensor surface. In yet another example, glass lenses utilize refraction to focus light beams. 
         [0006]    One of the challenges with such materials involves the ability to accurately measure the beam intensity impacting the material surface. The need for measuring intensity can be important in various applications, e.g., dirt or other contaminants can limit the amount of light entering the material, which reduces the efficacy or impacts the operation of the device. Current approaches for measuring light on an optical surface often involve the use of beam splitters and/or optical attenuators. Unfortunately, devices such as those employing optical attenuators must absorb a significant amount of energy, which adversely impacts the device used to perform the evaluation. Such devices often convert optical energy to heat energy, thereby damaging the attenuator material. 
       SUMMARY OF THE INVENTION 
       [0007]    Disclosed herein is a novel type of coating that combines unique optical properties, such as serving as an antireflection coating with the ability to deflect a controlled fraction of the light flux for monitoring and control. 
         [0008]    In a first aspect, the invention provides a system for monitoring and controlling optical energy, comprising: an optical system having a surface for receiving an optical beam; a coating applied to the surface, wherein the coating includes an optical nanoporous dielectric thin film having an array of tilted nanoscale rods configured to reflect a scatter beam at a determined angle and pass a remaining portion of the optical beam to the surface; a satellite detector arranged to detect an intensity of the scatter beam; and a control system that receives and processes scatter beam data from the satellite detector to determine an intensity of the optical beam impacting the surface. 
         [0009]    In a second aspect, the invention provides a method for monitoring and controlling optical energy, comprising: providing an optical system having a surface; providing a coating applied to the surface, wherein the coating includes an optical nanoporous dielectric thin film having an array of tilted nanoscale rods; receiving an optical beam directed at the coating; reflecting a scatter beam at a determined angle and passing a remaining portion of the optical beam to the surface; detecting an intensity of the scatter beam at a satellite detector; and processing scatter beam data from the satellite detector to calculate an intensity of the optical beam impacting the surface. 
         [0010]    In a third aspect, the invention provides an attenuation system, comprising: an optical system having a surface for receiving an optical beam; and a coating applied to the surface, wherein the coating includes an optical nanoporous dielectric thin film having an array of tilted nanoscale rods configured to reflect a scatter beam at a determined angle and pass an attenuated portion of the optical beam to the optical system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
           [0012]      FIG. 1  depicts a monitoring system in accordance with embodiments of the invention. 
           [0013]      FIG. 2  depicts an image of a nanoporous dielectric thin film material having tilted nanoscale rods in accordance with embodiments of the invention. 
           [0014]      FIG. 3  depicts an image of a two layer nanoporous dielectric thin film material having tilted nanoscale rods in accordance with embodiments of the invention. 
           [0015]      FIG. 4  depicts experimental and simulated results showing peak intensity for the material of  FIG. 2 . 
           [0016]      FIG. 5  depicts a table showing peak intensity positions for nanorods at different tilt angles. 
       
    
    
       [0017]    The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements. 
       DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 1 , a monitoring and control system  10  is shown that generally includes an optical energy receiver  18  having a scattering medium  16  applied to its surface  19 . Optical energy receiver  18  may comprise any type of optical material that receives, manipulates or exploits optical energy, e.g., a solar cell, a sensor, a detector, a lens, glass, a mirror, etc. Optical energy receiver  18  may optionally be implemented with a device  20  that for example, outputs electricity, heat energy, a control signal, etc., to a control system  26 . The combination of the scattering medium  16 , optical energy receiver  18  and/or device  20  is generally referred to herein as an optical system  30 . Also included in monitoring and control system  10  is a satellite detector  24  for detecting a deflected flux or scatter beam  22  that is indicative of the amount of radiation of a main beam  14  incident upon the surface  19  of optical energy receiver  18 . Main beam  14  may originate from any source  12  and comprise any type of optical energy, including the sun, ambient lighting, reflective lighting, manmade lighting, LED lighting, etc. 
         [0019]    Scattering medium  16  employs an optical nanoporous dielectric thin film material (i.e., coating) comprising an array of highly tuned tilted nanoscale rods, such as that shown in  FIGS. 2 and 3 . The scattering medium  16  provides a non-absorbing beam manipulation that is controlled by the material composition, and the unique nanoscale structure of the nanoscale thin film coating. By choosing material species with zero absorption at a particular wavelength band, while maintaining the unique highly ordered internal structure of nanoscale thin film coating, beam manipulation that is virtually free of absorption is realized. The nanoporosity, refractive index, and thickness of the nanoscale thin film coating can be tailored to meet the design specifications of receivers  18  with different dynamic ranges. 
         [0020]    The fabrication process of the nanoporous dielectric thin film materials is purely additive and compatible with state-of-the-art optical energy detectors. For example, tilted low-n Alumina nanorods fabricated by glancing angle deposition may be utilized. Oblique-angle deposition, also known as glancing angle deposition (GLAD), is a general thin film deposition method for depositing nano-scale porous materials. Being a physical deposition process, oblique-angle deposition utilizes surface-diffusion and self-shadowing effects to form nanometer size rods on a specular substrate surface. Such a deposition process is known to be applied to a variety of optical thin film materials. Tailored- and low-refractive index thin film materials, fabricated by glancing angle deposition, may include a widely tunable refractive index, and have compatibility with a variety of bulk material species, and can thus be readily applied for fabricating multilayer structures. 
         [0021]    Several layers of a coating can therefore be used, including using a periodic multilayer design to enhance the scatter beam  22 . A multilayer scattering medium  16  arranged with a designed separation can form constructive interference such that the detection peak of the scatter beam  22  can be narrowed in angular width and enhanced in intensity. The implementation can be optimized using a genetic algorithm. 
         [0022]      FIGS. 2 and 3  show cross-sectional-views scanning electron microscopy (SEM) images of a one and the two-layer alumina ARC on a Si reference sample, respectively. As shown in  FIG. 3 , the deposition angle α is defined as the angle between the Si substrate normal and the direction of incident alumina vapor flux. In this example, the deposition rate of Layer-1 and Layer-2 were maintained at 0.15-0.2 nm·s−1 during e-beam evaporation. 
         [0023]    Once implemented, the disclosed system  10  can perform real-time sensing of the optical energy of the scatter beam  22 , while the main beam  14  is transmitted through to the surface  19  of the optical energy receiver  18  for a primary application. No extra beam splitter is required to bend the beam  14  for detection. The satellite detector  24  can capture and determine an intensity of the scatter beam  22 , which is proportional to the intensity of the main beam  14 . Because the scattering medium  16  is non-absorbing, it does not interfere with the operation of the optical energy receiver  18 . Further, because the energy in the scatter beam  22  is significantly lower than the main beam  14  (e.g., three or so orders of magnitude less), satellite detector  24  may be implemented with a relatively high intensity detector relative to detector used by optical energy receiver  18 . 
         [0024]    Scattering medium  16  also allows for a greater dynamic range, beyond the 60-70 dB of current sensors. A multistage design for example could provide a dynamic range similar to that of the human eye, e.g., 140-200 dB. 
         [0025]    In a further embodiment, scattering medium  16  comprising a multilayered nanoporous dielectric thin film may be employed as a beam attenuator. Since the attenuation of an optical energy beam relies on non-absorbing scattering, the disclosed type of beam attenuator does not suffer from attenuator damage due to high optical density. An optical energy attenuator using scattering medium  16  is based on scattering. Therefore, no energy or heat accumulates in the scattering medium  16 . 
         [0026]    In the embodiment shown in  FIG. 1 , the satellite detector  24  can be utilized as part of a feedback loop in which a measured amount of intensity of scatter beam  22  is fed back to and utilized by control system  26  to make system level adjustments. For example, if the optical system  30  provided a sensor that outputs a signal based on a detected amount of incident radiation from main beam  14 , the optical system  30  could be adjusted or biased based on the amount of scatter beam  22  detected by satellite detector  24  over time. Accordingly, as changing conditions impact the optical energy receiver  18 , the sensitivity or performance of the optical system  30  could be adjusted. 
         [0027]    In another example, solar cells with an antireflection coating using scattering medium  16  could be provided along with satellite detector  24  to monitor surface contamination via control system  26  and indicate when the surface must be cleaned to maintain the solar cell efficiency. Such a coating could be used for any system controlling contamination or for cleaning displays, such as displays in systems such as Google Glass, or even more conventional glasses, sunglasses, outdoor displays, windows, etc. Possible applications also include highly sensitive, high speed, wide dynamic range optical energy sensors used for smart lighting, medical imaging, machine automation, and surveillance. 
         [0028]    As noted, scattering medium  16  includes an array of obliquely aligned nanorods that provide asymmetric backscattering, i.e., medium  16  will generate a scatter beam  22  when a main beam  14  is received, without absorbing any of the energy. The behavior of the scatter beam  22  relative to the main beam  14  can be readily determined based on the design of the scattering medium  16 . For instance, in the illustrative example shown in  FIG. 2 , the scattering medium comprises nanoporous alumina with a layer thickness of 550 nm. The effective refractive index (neff) of the nanoporous alumina layer is neff=1.07 at λ=410 nm. The depicted alumina nanorod array is arranged to have tilt angle of 126° (36° with respect to the substrate plane). Based on simulation and/or experimentation, it is possible to ascertain the angle at which peak intensity of the scatter beam will occur. For example, as shown in  FIG. 4 , a measured (left side) and simulated (right side) scattering distribution is shown for the scattering medium of  FIG. 2 . In this example, the simulated reflectance peak occurred at −119°, which is in excellent agreement with the measured scattering intensity peak of −115°.  FIG. 5  further illustrates simulated scattering peak position and diffraction peak position of tilted alumina nanorod arrays with tilt angles of 18°, 27°, 36°, and 45°. With a satellite detector  24  positioned at the appropriate angular position, a proportionality factor between the main beam intensity and the scatter beam intensity can be readily ascertained. 
         [0029]    Given the predictable behavior of the nanorods arrays, scattering medium  16  and satellite detector  24  can thus be designed, implemented, and tuned to predictably deflect and capture a proportional amount of the main beam  14  at a determined angle α relative to the surface  19  of the optical energy receiver  18 . It is understood that any number of factors may impact the overall design and function of monitoring and control system  10 , including thickness and properties of the scattering medium  16 , tilt angle of the nanorods, placement of the satellite detector  24 , etc. 
         [0030]    As shown  FIG. 1 , control system  26  may for example comprise a computing system having a processor, programmed memory and input/output that can read in scatter beam data from the satellite detector  24 , process the scatter beam data based on a predetermined proportionality factor, and output a result based on or proportional with with an intensity of the main beam  14 . For example, it might be determined that the scatter beam  22  has an intensity that is a multiple of 0.0025 relative the main beam  14  (i.e., a proportionality factor of 0.0025). Accordingly, once the scatter beam  22  is read by the satellite sensor  24 , the main beam intensity can be determined and outputted by control system  26  in real-time. For example, the main beam intensity (MB) may be calculated as MB=SB/(0.0025), where SB is the scatter beam intensity. Control system  26  may also be implemented as purely hardware, e.g., a circuit, or a combination of hardware and embedded software. 
         [0031]    Control system  26  may utilize the scatter beam data for any purpose. For example, control system  26  could utilize the data: to calibrate the proportionality factor between the main beam and the scattered beam, e.g., at a weak input signal; to frequency lock the scatter signal  22  to the main beam signal  14  for weak signal measurement; for measuring the light absorption in the device structure by comparing the scattered and transmitted (main beam) signals, etc. 
         [0032]    The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims.