Design and Optimization of Focal Plane Arrays ...

Design and optimization of focal plane arrays integrated with dielectric microspheres Farzaneh Abolmaali,1* Nicholaos I. Limberopoulos,2 Augustine M. Urbas,3 and Vasily N. Astratov1,2,* 1

Department of Physics and Optical Science, Center for Optoelectronics and Optical Communications, University of North Carolina at Charlotte, Charlotte, NC 28223-0001, USA 2 Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB, OH 45433, USA 3 Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, OH 45433, USA *E-mails: [email protected], [email protected]

dielectric microspheres, especially the spheres with high index of refraction such as n~1.9, have significantly shorter focal lengths and, correspondingly, larger AOVs compared to the commercial microlens arrays. We did modeling and detailed system analysis to interpret light collection efficiency and AOV of Mid-IR FPAs [8]. Our work can be considered as an alternative approach to the FPAs design based on integration of individual mesas with dielectric microspheres. It should be noted that the spectral response of individual pixels can be enhanced by using surface plasmonic gratings [16], nanoparticles [17], nanoantennas [18], subwavelength hole arrays [19], microstructures surfaces [20], and photonic crystals [21]. These techniques enhance the absorptivity of the pixels (usually, over a narrow spectral bandwidth); however, the incident power falling outside the detectors’ mesas is either lost or, in some cases, creates artifacts in the imagery by generating electrical currents in the active circuitry [2].  –Š‹• ™‘”ǡ —•‹‰ ˆ‹‹–‡ †‹ˆˆ‡”‡…‡ –‹‡ †‘ƒ‹ ȋ Ȍ ‘†‡Ž‹‰ǡ ™‡ †‡˜‡Ž‘’ ‘—” ƒ’’”‘ƒ…Š „ƒ•‡† ‘ ‹–‡‰”ƒ–‹‘ ‘ˆ • ™‹–Š †‹‡Ž‡…–”‹… ‹…”‘•’Š‡”‡•Ǥ ‡ ‘’–‹‹œ‡† ‘—” †‡•‹‰• ˆ‘” ƒ…Š‹‡˜‹‰ ƒš‹ƒŽ • ˆ‘” •’Š‡”‡• ™‹–Š •’‡…‹ˆ‹… †‹ƒ‡–‡”ǡ ‹†‡š ‘ˆ ”‡ˆ”ƒ…–‹‘ǡ ƒ† ‘†‹ˆ‹‡†–”—…ƒ–‡†•Šƒ’‡Ǥ 

Abstract— In this work, we propose and optimize several designs of mid-IR focal plane arrays (FPAs) where individual pixels are integrated with dielectric microspheres to provide maximal light collection efficiency in combination with large angleǦǦofǦview (AOV). The designs are developed for backilluminated FPAs with progressively shorter effective focal length to increase AOV by more than an order of magnitude compared to commercial microlens arrays. Mid-wave infrared photodetector; Keywords— Microsphere lens; Photonic Nano-jet; Microlens array; Focal plane array; Front-side illuminated design; back-side illuminated design; Angle of view; Finite difference time domain

I.

INTRODUCTION

Infrared focal plane arrays (FPAs) have a dynamic journey since the primary 2-D arrays were manufactured in the late 70s. Today’s fixed and portable mission necessities are motivating the development of these thermal imaging systems to sufficiently high resolution, pixel size minimization [1], and increasing the operational temperature for a wide variety of space science, security, surveillance, military, and commercial applications. All these factors require highly efficient collection of light into a small active region of detector (mesas) which in principle can be achieved by using microstructure surfaces such as microlens arrays [2, 3]. But in practice, such commercially available optics provide imaging with relatively small angle of view (AOV) around ~1 degree. This is determined by the fact that the microlenses with a dome shape have only one refracting surface with rather limited refractive index contrast [4]. Much larger AOVs are required for developing imaging sensors, optical perception systems, and, in general, systems for imaging in a broad range of distances and directions. Recently, we experimentally demonstrated [5-8] the enhanced sensitivity of such structures using “photonic jets”, sharply focused beams with subwavelength transversal width produced by dielectric microspheres [9-15]. The

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II.

MODELING DESIGN GUIDELINES

The main design guideline for achieving larger AOV is connected with a reduction of the effective focal length. It can be achieved by reducing the sphere radius (R), increasing of the sphere refractive index (n), and truncating the sphere with sufficiently high n. Another important design guideline is that the focusing plane should coincide with the plane of photodetectors to provide efficient coupling into the photodetectors’ mesas. Since the analytical approach is limited by the paraxial approximation, here we used a full wave finite difference time domain (FDTD) modeling to design the photodetectors integrated with microspheres and to compare their

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performance with the commercially available microlens array. In our modeling design, we calculated the maximal AOV provided by a standard silica microlens array and compared it with the case of a slightly truncated microsphere with higher refractive index. Both designs were considered for the backside illuminated structure. We assumed that the semiconductor slab has index 3.5, and the thickness of the slab was adjusted to provide a sharp focusing at the back surface. The geometrical parameters of both designs were specifically selected to be compatible with typical 60 ȝm pitch sizes in close-packed arrays in mid-IR FPAs. To simplify calculations, we consider a 2-D cylindrical geometry which has all essential features of 3-D case. We selected photodetector mesa (pixel) diameter d=10 ȝm which corresponds to the smallest sizes used in practice in mid-IR detectors. The illumination was provided by plane waves at O=1.5 —m, however the investigated optical properties are not strongly wavelength dependent, and the results are generally applicable in the mid-IR range 3-5 —m. I.

FIG.1. (a-b) Microlens array integrated with the back-illuminated optical detector focal plane array with 200 ȝm substrate. (c) Electric field map calculated by FDTD simulation at Ȝ=1.5 ȝm. The position of microlens, substrate and detector are shown with white dashed lines. (d) Electric field map calculated at Ȝ=1.5 ȝm for ș=0 ° and ș=1° in the region marked by red dashed lines in (c), it can be seen that for ș=1°, the focused light beam is at the edge of the detector

STANDARD MICROLENS ARRAY

II.

We started our modeling for a microlens array with the set of parameters typical for the commercially available arrays [4]. As illustrated in Figs. 1(a,b), we selected fused silica microlens array with a radius of curvature of Rc=120 —m. It was assumed that the dome-shaped lenses with the base diameter of 95 —m are fabricated at the surface of a slab with the thickness of 20 —m and with the index of 1.44. The location of microlens, substrate and detector are shown by dashed lines in the calculated EM maps in Fig 1(c). Since here we considered the case of back-illuminated structure, the beam propagates through the semiconductor slab with the thickness of 200 —m and then reaches to detector mesas. In order to focus light in mesa’s location at the back surface of the substrate, the air gap of about 174 —m between the microlens array and the semiconductor slab should be created. It makes the system even more sensitive to any slight deviations of the angle of incidence from the normal (ș=0°). As shown in the lower image of Fig. 1(d), for ș=1° the focused beam shifts away from the mesa’s center and reaches its edge. Consequently, for angles of incidence beyond ș=1°, the beam is blocked by the edge of the mesa, which means that AOV is close to 1° in this case. It can be viewed that such arrays provide efficient collection of light only in an extremely narrow cone around the normal direction which can be a limiting factor for many applications.

SLIGHTLY TRUNCATED MICROSPHERES

Due to the perfect spherical shape, the microspheres have intrinsically shorter focal distances compared with different dome-shaped lenses with the same radius of curvature. It causes the microsphere arrays to provide larger AOVs in contrast with the microlens arrays. The description of the methods which can be used for aligning individual microspheres with the photosensitive mesas of FPAs goes beyond the scope of this work. Just to provide basic ideas which can be used for such assembly, the microspheres can be first assembled using microhole arrays with air suction [5]. After that, the ordered array of microspheres can be transferred on the surface of FPA and fixed using a layer of epoxy or photoresist. It is important to align individual microspheres with the centers of the photodetector mesas. Such alignment can be performed using virtual imaging of the device mesas through the microspheres. Recently, the microsphere-assisted imaging was used for studies various nanoplasmonic [22-27] and biomedical structures [28, 29]. Due to the reciprocity principle, observation of the image of the mesa centered with the sphere means that the sphere is perfectly aligned with the mesa [8].

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ACKNOWLEDGMENT This work was supported by the Center for Metamaterials, an NSF I/U CRC, award No. 1068050. Also, this work was sponsored by the Air Force Research Laboratory (AFRL/RYD, AFRL/RXC) through the AMMTIAC contract with Alion Science and Technology and the MCF II contract with UES, Inc. REFERENCES [1]

FIG. 2. (a) 60 ȝm polystyrene microsphere truncated at a 5 ȝm depth in contact with a 20 ȝm substrate in the back-side illumination design. (b,c) Electric field maps calculated at Ȝ=1.5 ȝm for ș=0° and ș=8°, respectively.

[2] [3]

Selecting microspheres with diameters in a range of 60 —m which matches the pitch sizes of many FPAs used in the mid-IR range, makes the focal distances of such spheres too short for focusing light at the back surface of the semiconductor slab with the thickness of 20 —m. To slightly increase the effective focal length of such lenses, we considered truncated (~5ȝm) sphere, as schematically illustrated in Fig. 2(a). This design was developed for plastic or polystyrene microspheres which have index, n=1.56, at mid-IR wavelengths. In practice, such truncation can be achieved due to the low melting temperature of the polystyrene microspheres by partially melting in the region where they contact the heated substrate [30, 31]. In this case, comparison with Fig. 1(d) shows that AOV=8° which significantly exceeds the angle of view provided by a standard microlens array. In the case of front illuminated design, AOVs can be additionally increased due to the fact that the focusing should be provided close to the spherical surface.

[4]

[5]

[6]

[7]

[8]

[9]

SUMMARY

[10]

To simultaneously increase the light collection efficiency and angle-of-view (AOV) of MWIR FPAs in our recent work we suggested using “photonic jets” produced by dielectric microspheres. It is important to note that our designs, generally allow reducing the size of the photosensitive mesas which results in smaller dark currents and can potentially result in higher operating temperatures of MWIR FPAs. In this work, using numerical modeling, we optimized our designs for achieving maximal AOVs for the microsphere with index n=1.56, diameter 60 ȝm, and a modified truncated shape. We demonstrated that the light collection efficiencies can be enhanced in the backsideilluminated design which shows almost an order of magnitude higher AOVs compared to standard microlens arrays.

[11]

[12] [13] [14]

[15] [16]

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P. B. Catrysse and T. Skauli, “Pixel scaling in infrared focal plane arrays,” Appl. Opt. 52, C72-C77, 2013. A. Rogalski, “Progress in focal plane array technologies,” Progress in Quantum Electronics 36, 342–473, 2012. C. A. Keasler and E. Bellotti, “A numerical study of broadband absorbers for visible to infrared detectors,” Appl. Phys. Lett. 99, 091109, 2011. SUSS Micro Optics SA, Neuchatel, Catalogue 2007, Microlens Arrays, http://www.amstechnologies.com/fileadmin/amsmedia/downloads/20 67_SMO_catalog.pdf V. N. Astratov, K. W. Allen, N. I. Limberopoulos, A. Urbas, and J. M. Duran, “Photodetector focal plane array systems and methods,” U.S. patent 9,362,324, June 2016, http://www.freepatentsonline.com/9362324.html. K. W. Allen, J. M. Duran, G. Ariyawansa, N. I. Limberopoulos, A. M. Urbas, and V. N. Astratov, “Photonic Jets for Strained-Layer Superlattice Infrared Photodetector Enhancement,” IEEE Proceedings of National Aerospace and Electonics Conference, Dayton, Ohio, 25–27 June 2014, pp. 32–33. K. W. Allen, “Waveguide, Photodetector, and Imaging Applications of Microspherical Photonics,” Ph.D. dissertation (University of North Carolina at Charlotte), 2014, Chap. 5, pp. 123–136. E-print http://arXiv:org/abs/1503.03374. K. W. Allen, F. Abolmaali, J. M. Duran, G. Ariyawansa, N. I. Limberopoulos, A. M. Urbas, V. N. Astratov, “Increasing sensitivity and angle-of-view of mid-wave infrared detectors by integration with dielectric microspheres,” Appl. Phys. Lett. 108 (24), 241108, 2016. V. N. Astratov, Photonic Microresonator Research and Applications, Springer Series in Optical Sciences Vol. 156, edited by I. Chremmos, O. Schwelb, and N. Uzunoglu (Springer, New York), 2010, Ch. 17, pp. 423-457. V. N. Astratov, A. Darafsheh, M. D. Kerr, K. W. Allen, N. M. Fried, A. N. Antoszyk, and H. S. Ying, “Photonic nanojets for laser surgery,” SPIE Newsroom, March 12, 2010. A. Darafsheh, M.D. Kerr, K.W. Allen, N.M. Fried, A.N. Antoszyk, H.S. Ying, and V.N. Astratov, “Integrated microsphere arrays: Light focusing and propagation effects,” in Optoelectronic Integrated Circuits XII, edited by Louay A. Eldada, El-Hang Lee, Proc. of SPIE, vol. 7605, 9pp., Feb. 2010, paper 76050R. A. Darafsheh and V. N. Astratov, “Periodically focused modes in chains of dielectric spheres,” Appl. Phys. Lett. 100, 061123, 2012. A. Darafsheh, N. Mojaverian, N. I. Limberopoulos, K. W. Allen, A. Lupu, and V. N. Astratov, “Formation of polarized beams in chains of dielectric spheres and cylinders,” Opt. Lett. 38, 4208–4211, 2013. K. W. Allen, A. Darafsheh, F. Abolmaali, N. Mojaverian, N. I. Limberopoulos, A. Lupu, V. N. Astratov, “Microsphere-chain waveguides: Focusing and transport properties,” Appl. Phys. Lett. 105 (2), 021112, 2014. I. Mahariq, V. N Astratov, H. Kurt, Persistence of Photonic Nanojet Formation Under the Deformation of Circular Boundary, J. Opt. Soc. Am. B 33, 535-542, 2016. G. Gu, J. Vaillancourt, P. Vasinajindakaw, and X. Lu, “Backsideconfigured surface plasmonic structure with over 40 times photocurrent enhancement,” Semicond. Sci. Technol. 28, 105005, 2013.

[17] D. M. Shaadt, B. Feng, and E. T. Yu, “Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles,” Appl. Phys. Lett. 86, 063106, 2005. [18] L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics 2, 226-229, 2008. [19] T. Matsui, A. Argrawal, A. Nahata, and Z. V. Vardeny, “Transmission resonances through aperiodic arrays of subwavelength apertures;” Nature (London) 446, 517, 2007. [20] C. A. Keasler and E. Bellotti, “A numerical study of broadband absorbers for visible to infrared detectors,” Appl. Phys. Lett. 99, 091109, 2011. [21] K. T. Posani, V. Tripathi, S. Annamalai, N. R. Weisse-Bernstein, S. Krishna, R. Perahia, O. Crisafulli, and O. J. Painter,Nanoscale “quantum dot infrared sensors with photonic crystal cavity,” Appl. Phys. Lett. 88, 151104, 2006. [22] V. N. Astratov, K. W. Allen, N. Farahi, Y. Li, N. I. Limberopoulos, D. E. Walker, A. M. Urbas, V. Liberman, and M. Rothschild, “Optical nanoscopy with contact microlenses overcomes the diffraction limit,” SPIE Newsroom, February 1, 2016. [23] V. N. Astratov, A. V. Maslov, K. W. Allen, N. Farahi, Y. Li, A. Brettin, N. I. Limberopoulos, D. E. Walker, A. M. Urbas, V. Liberman, and M. Rothschild, “Fundamental limits of superresolution microscopy by dielectric microspheres and microfibers,” Proc. SPIE 9721, 97210K, 2016. [24] A. Darafsheh, G. F. Walsh, L. Dal Negro, and V. N. Astratov, “Optical super-resolution by high-index liquid-immersed microspheres,” Appl. Phys. Lett. 101, 141128, 2012. [25] A. Darafsheh, N. I. Limberopoulos, J. S. Derov, D. E. Walker, Jr., and V. N. Astratov, “Advantages of microsphere-assisted super-resolution imaging technique over solid immersion lens and confocal microscopies,” Appl. Phys. Lett. 104, 061117, 2014. [26] K. W. Allen, N. Farahi, Y. Li, N. I. Limberopoulos, D. E. Walker, Jr., A. M. Urbas, V. Liberman, and V. N. Astratov, “Super-resolution microscopy by movable thin-films with embedded microspheres: Resolution analysis,” Ann. Phys. (Berlin) 527, 513–522, 2015. [27] K. W. Allen, N. Farahi, Y. Li, N. I. Limberopoulos, D. E. Walker, Jr., A. M. Urbas, and V. N. Astratov, “Overcoming the diffraction limit of imaging nanoplasmonic arrays by microspheres and microfibers,” Opt. Express 23, 24484–24496, 2015. [28] L. Li, W. Guo, Y, Yan, S. Lee and T. Wang, “Label-free superresolution imaging of adenoviruses by submerged microsphere optical nanoscopy,” Light: Science & Applications 2, e104, 2013. [29] H. Yang, N. Moullan, J. Auwerx, and M. A. M. Gijs, “SuperResolution Biological Microscopy Using Virtual Imaging by a Microsphere Nanoscope,” Small 10, 1712-1718, 2014. [30] J. Li and Y. Zhang, Porous Polymer Films with Size-Tunable Surface Pores Chem. Mater. 19, 2581-2584, 2007. [31] P. Mao, A. K. Mahapatra, J. Chen, M. Chen, G. Wang, and M. Han, “Fabrication of Polystyrene/ZnO Micronano Hierarchical Structure Applied for Light Extraction of Light-Emitting DevicesACS,” Appl. Mater. Interfaces 7, 19179-19188, 2015.

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