Integration of an opto-chemical detector based on group III-nitride nanowire heterostructures R. Kleindienst,1,* P. Becker,2 V. Cimalla,3 A. Grewe,1 P. Hille,2 M. Krüger,1 J. Schörmann,2 U. T. Schwarz,3,4 J. Teubert,2 M. Eickhoff,2 and S. Sinzinger1 1
Technische Universität Ilmenau, Institut für Mikro- und Nanotechnologien (IMN MacroNano), Fachgebiet Technische Optik, P.O. Box 100565, 98648 Ilmenau, Germany 2
Justus-Liebig-Universität Gießen, I. Physikalisches Institut, Gießen, Germany
3 4
Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Germany
Albert-Ludwigs-Universität Freiburg, Institut für Optoelektronik, Freiburg, Germany *Corresponding author: roman.kleindienst@tu‑ilmenau.de Received 31 October 2014; accepted 11 December 2014; posted 15 December 2014 (Doc. ID 226073); published 28 January 2015
The photoluminescence intensity of group III nitrides, nanowires, and heterostructures (NWHs) strongly depends on the environmental H2 and O2 concentration. We used this opto-chemical transducer principle for the realization of a gas detector. To make this technology prospectively available to commercial gasmonitoring applications, a large-scale laboratory setup was miniaturized. To this end the gas-sensitive NWHs were integrated with electro-optical components for optical addressing and read out within a compact and robust sensor system. This paper covers the entire realization process of the device from its conceptual draft and optical design to its fabrication and assembly. The applied approaches are verified with intermediate results of profilometric characterizations and optical performance measurements of subsystems. Finally the gas-sensing capabilities of the integrated detector are experimentally proven and optimized. © 2015 Optical Society of America OCIS codes: (170.0110) Imaging systems; (170.3010) Image reconstruction techniques; (170.3660) Light propagation in tissues. http://dx.doi.org/10.1364/AO.54.000839
1. Introduction
Compact and robust gas-monitoring devices for reliable operation at room temperature and above are of special interest for a wide range of industrial and safety applications. In order to increase the sensitivity of such devices, nanoscale transducer structures have been investigated, taking advantage of a higher surface-to-volume ratio. In this context, optical readout methods are highly promising as they allow parallel probing of large ensembles of nano objects without manipulating or processing single nanostructures. Furthermore, optical readout techniques 1559-128X/15/040839-09$15.00/0 © 2015 Optical Society of America
feature significant advantages for certain applications as, e.g., measurements in potentially hazardous environments that prohibit any form of electricity at the point of measurement.m In recent works an optical approach using semiconducting group III-nitride (III-N) nanowire heterostructures (NWHs) as opto-chemical transducers has been proposed [1–3]. As shown schematically in Fig. 1, suitable NWHs can be optically excited by light of energy above the semiconductor band gap to emit photoluminescence (PL) light. Changes in the environmental gas composition lead to modified surface adsorption and thus affect the PL signal intensity. This is exemplarily presented in Fig. 2, which shows the transient PL response of InGaN/ GaN NWHs under variation of the ambient O2 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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Fig. 3. (a) Schematic and (b) STEM image of nanowires with alternating InGaN and GaN sections (reproduced by permission of IOP Publishing) [4]. (c) SEM image of the NWH ensemble. Fig. 1. Schematic of the excitation-luminescence principle with EPh;exc > EPh;PL .
concentration using a HeCd gas laser for PL excitation. Similar to other oxidizing gases such as NO2 or O3 , a quenching of the PL signal is observed [3]. On the contrary, for Pt-coated NWHs an increase of the PL intensity is found when reducing gases such as H2 and hydrocarbons are adsorbed [2]. In both cases, the observed effects have been explained by modifications of the rate for nonradiative surface recombination [1,3]. Detection limits in the range of several ppm (hydrocarbons, O2 ) and several hundred ppb (H2 , NO2 , O3 ) have been reported [2,3]. For an application of these outstanding properties in commercial devices, all components of the measurement setup need to be integrated within a compact and robust sensor. In this work we demonstrate the possibility of the miniaturization of a large-scale laboratory setup into an integrated, highly efficient optical microsystem that combines a laser diode for excitation, a photodiode for detection, and the NWHs as active gas-sensing elements via highly efficient beam shaping and collector optics. As transducer structures, InGaN/GaN NWHs with several alternating layers of different III-N materials [schematically shown in Fig. 3(a)] were grown by molecular beam epitaxy using a self-assembled process [4]. Figures 3(b) and 3(c) depict a transmission electron microscopy image of a single NWH structure and a scanning electron microscopy image of the NWH ensemble, respectively. The NWHs feature strong carrier confinement that is reflected by a high thermal stability of the PL signal that allows for the detection of gas concentration changes even at elevated temperatures up to 250°C. In Section 2 our concept for integrating a sensor based on InGaN/GaN NWHs for detecting O2 concentration changes is introduced. Afterward we go into
Fig. 2. Temporarily resolved PL response of uncoated InGaN/ GaN NWHs on O2 concentration changes in N2 at 90°C [1]. 840
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the details of the realization and characterization of the sensor system. We start with an explanation of the optical excitation and detection system design in Section 3. Here standard design goals were extended by additional boundary conditions targeting on an enhanced system performance while maintaining the manufacturability. Especially for the calculation of the optical excitation beam path, this led to an application-driven design approach. The implementation of all optical functionalities into the optical integration platform is presented from the technological point of view in Section 4. With characterizations of the shape accuracy, surface quality, and optical performance in Section 5, the applied design and fabrication approaches were proven for the excitation as well as detection subsystem. Finally in Section 6 the overall sensor functionality as an interaction of excitation and detection is demonstrated with exemplary measurements of the O2 concentration changes. 2. System Integration Concept
Besides the main aspect of miniaturization, the sensor integration concept is intended to retain the excellent properties of group III-nitride-based NWHs for reliable gas concentration change measurements. For this purpose the following requirements for setting up the sensor were defined: (a) Scattered and reflected excitation radiation need to be spatially separated from the PL signal at the position of the PL detector to achieve a sufficiently high signal-to-noise ratio [5]. (b) To guarantee long-term stable measurement conditions and high detection reliability only the NWHs are in contact with the test medium. The detection periphery, i.e., the excitation light source, the PL detector, and other electronics, is completely isolated from the investigated environment [2]. In Fig. 4 a conceptual layout of the miniaturized sensor system with a size of approximately 30 mm × 60 mm × 30 mm satisfying both requirements is shown. The linear polarized excitation radiation at 405 nm is emitted by a laser diode (output facet size 2.0 μm × 0.4 μm) with a divergence of 10° (FWHM) in slow axis and 22° (FWHM) in fast axis [6]. With two optical surfaces the light is shaped to an aberration-minimized focus at the gas-sensitive
Fig. 4. Concept for integrating the gas-sensitive NWHs with opto-electronic detection periphery.
nanowires at the top of the system. Applying a sufficiently high power density, the NWHs generate a PL signal with a Lambertian emission characteristics and a bandwidth of about 50 nm (FWHM) around the center wavelength of 500–520 nm (depending on the In concentration). Concentric with the excitation spot and the center of PL, the detection subsystem is introduced. Experiments have shown that PL intensity changes can even be detected using a Si photodiode (active area 3.6 mm × 3.6 mm) [7] without any optics. However, with additional collector optics, an increased PL signal can be achieved, as a higher portion of the PL signal is concentrated onto the detector. Furthermore, in the imaging configuration, scattered excitation radiation is further discriminated. For spectral separation a dielectric and an absorption filter are integrated within the detector subsystem. In the next section the design goals are derived from the integration requirements and preliminary investigations. Afterward the approaches for the design of the excitation and the collector optics are explained in detail. 3. Optical Design
For efficiency reasons the optoelectronic components are connected to each other via the NWHs using free space optics. To guarantee for a robust and compact sensor setup though the excitation source and optics, the gas-sensitive nanowires and the detection subsystem are combined within an optical integration platform. While the surfaces for shaping of the excitation beam are directly provided by the integration platform, the PL collector optics is part of the supplemental detection subsystem. Details of the design approaches for both, the optical excitation as well as the detection system, are given subsequently. A.
Optical Design of the Excitation System
Because of the suppression of nonradiative Shockley– Read–Hall recombination, the PL signal is growing superlinearly with increasing excitation density. For this reason an optical system is required that focuses the highly divergent excitation radiation at a high numerical aperture with minimized aberrations onto the NWHs. To reduce false light within the system and to increase the SNR (signal-to-noise
Fig. 5. Optical design approach based on an ellipsoid for transmission efficiency-enhanced excitation beam shaping [5].
ratio) of the gas sensor only high-efficiency beamshaping approaches were taken into account that comply with the concept constraints introduced in Section 2. In preparation for the fabrication process, exclusively analytical specifications of optical surfaces were considered. These allow for direct programming of our ultraprecision micromilling tool and thus highest fabrication accuracy. Transcription errors occurring from transferring design into fabrication data are eliminated, and systematic fabrication errors can be compensated in real time [8]. As illustrated in Fig. 5 we used an ellipsoidal mirror as a starting point for the design, which perfectly images a point source from focal point 1 into its second focal point. With an extension of only 2.0 μm × 0.4 μm the laser output facet is small compared to the desired overall size of the sensor. Thus if aligned in focal point 1 the ellipsoidal mirror generates an image of the laser facet, i.e., focus, with negligible aberrations at focal point 2 where the NWHs are located. The excitation radiation is coupled into the substrate and shaped within the material polymethylmethacrylate (PMMA, Plexiglas, GS218) to the desired intensity distribution. However, an analysis of the schematic in Fig. 5 reveals that any distance, dimension, or angle within the microoptical system is influenced by each of the four degrees of freedom of this starting design: RoC (radius of curvature), conic (conic constant between −1 and 0), ε (tilt of the ellipse axis), as well as γ (tilt of laser diode). This parameter range was initially restricted by a limitation of the general dimensions of the integration platform like the thickness defined by the position of the laser output facet and the surface of the ellipsoidal mirror, the overall sensor thickness, the position of the source relative to the image plane, and the space left underneath the NWHs for the PL detection subsystem. 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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The final configuration was accomplished by optimizing the system based on an ellipsoidal mirror with respect to diffraction limited focusing capability and highest transmission efficiency without using any anti-reflection (AR) coating. In particular spherical aberration is compensated by coupling the excitation beam into the substrate through an appropriately shaped surface [9,10]. Except for minimal field aberrations the beam quality is maintained, and the highest possible energy density at the NWHs for the given focusing NA is achieved by this means. The overall transmission efficiency is enhanced by reducing reflection losses at the substrate–gas interface at the top of the system. Therefore the excitation beam is polarized parallel with respect to the folding plane and coupled out of the PMMA substrate into the gaseous environment around the Brewster angle. Considering the specific angular intensity distribution of the incident beam, represented by an appropriately sampled ray bundle, the system parameters were finally optimized with respect to maximum transmission. The diagram in Fig. 6 shows the average reflection losses versus the marginal ray angle of incidence. Complying with the concept constraints and design requirements, a minimum of only 0.28% at 36.36° was found for a RoC of 8.05 mm, a conic of −0.47, an ellipse tilt of 42.12°, and a source tilt of 23.2° [5]. In Fig. 7 the excitation beam path is visualized with rays traced through a solid model of the microoptical system with a functional size of 23 mm× 20 mm × 21 mm. After passing the curved coupling surface, the excitation radiation is focused with the ellipsoidal mirror section onto the gas-sensitive nanowires at the top of the system. The PL signal intensity is measured with the detection subsystem directly below. B.
Optical Design of the Detection Subsystem
The PL signal is detected using a Si photodiode with a sensitive area of optical density (OD) 3.6 mm × 3.6 mm. Scattered excitation radiation is suppressed with a dielectric edge filter (OD 2) followed by an absorption edge filter (OD 4) in front of the
Fig. 6. Average reflection losses versus marginal ray angle of incidence for ray bundles with polarization parallel to the folding plane. 842
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Fig. 7. Solid model of the integration platform in PMMA with visualized excitation beam path.
photodiode. In this configuration additional fluorescence induced by the excitation radiation in the absorption filter is minimized and the excitation radiation is cancelled almost completely. The weak PL signal detected with this simple photodiode-filter combination (see measurements Section 6) directly below the PL center can be further improved in terms of the signal peak intensity while maintaining the SNR introducing additional collector optics. For this purpose we have chosen a miniaturized high NA 4-f setup (Fig. 8) inserted into the cavity underneath the NWHs. The first lens collimates a large portion of the PL signal, which is then focused by the second lens onto the sensitive area of the photodiode. Because of its angle sensitive properties, the dielectric filter is placed as specified by the supplier within the collimated beam path between both lenses. The angularly insensitive absorption edge filter is space-economically integrated into the convergent beam behind the focusing lens. The PL intensity distribution with a FWHM diameter of about 30 μm at the photosensitive area based on ray-tracing simulations is depicted in Fig. 8. Estimations resulted in a detection intensity increased by a factor of 5 compared to detection without collector optics. Besides an enhanced spectral filtering behavior, this imaging setup provides an improved discrimination of scattered excitation and false light. Furthermore, in this configuration the detection is significantly more tolerant against position changes of the luminescent point caused, e.g., by misalignment of the excitation source. The simulation in Fig. 8 (green beam path) shows that even for a displacement of the excitation focus of 1 mm the corresponding image is produced on the photosensitive area. After briefly describing our fabrication and assembly approach in the next section, experimental results demonstrating the optical performance of both the excitation and the detection subsystem, as well as the overall detection capability of the integrated sensor, are presented in Sections 5 and 6.
Fig. 8. Model of the collector optics and ray-tracing-based simulation of the PL intensity distribution at the sensitive area of the photodiode.
4. Realization of the Optical Integration Platform and the Detection Subsystem A.
Fabrication of the Optical Integration Platform
The flexibility and accuracy required for the fabrication of the optical integration platform in UVoptimized PMMA (Plexiglas GS218) is provided by ultraprecision micromilling. In our optimized process the analytical surface specifications are directly used for controlling the machining axes. This helps to avoid accuracy loss due to data conversions. Compared to fixed fabrication data, milling paths are computed instantaneously during the milling process, which allows for flexible compensation of positioning deviations during fabrication. Thus with an appropriately chosen design approach and an advanced control over the micromilling process, the highest fabrication accuracy can be achieved while keeping the data volume at a minimum [8]. In principle, the integration platform was processed in two steps: First rough machining was used to generate the basic shape comprising the mount for the excitation laser, the section of the ellipsoidal mirror, the curved coupling surface, the plane surface for coupling into the NWHs, and the cavity for the detection subsystem. Afterward, a finishing process (using diamond tools) [Figs. 9(a)–9(d)] was applied to obtain the shape accuracy and surface quality specifically required for the optically functional surfaces [11]. With a total machining time of about 60 h from the blank substrate to the integration platform (Fig. 10), this fabrication approach is suitable for prototyping or tooling purposes. An economic replication can be realized with a sol gel process in SiO2 providing high transmission and long-term stability, even prospectively for applied shorter excitation wavelengths [12].
Fig. 9. (a) Ultraprecision micromilling of the excitation source platform, (b) the ellipsoidal mirror section, (c) the concave coupling surface, and (d) the surface for coupling out of the substrate into the NWHs.
B. Assembly of the Detection Subsystem
The collector optics was set up by stacking the lenses and filters with suitable spacers within an aluminum tube. The objective is finally combined with the photodiode and integrated within the cavity below the NWHs. An increased degree of integration can be reached by arranging the lenses and filters directly within the cavity. However, to suppress false light an additional process for applying a high OD layer at the cavity walls is required. 5. Profilometric and Optical Performance Characterizations
In this section experimental results proving the optical performance of the excitation system and the collector optics are presented. Furthermore, the
Fig. 10. Optical integration platform in UV optimized PMMA (GS2018). 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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Fig. 11. Setup for (a) excitation spot measurement and (b) integration system during experiment.
fabrication process is evaluated with interferometryand AFM-based profilometric characterizations of the integration platforms optical surfaces. A.
Optical Excitation System Fig. 13. (a) Difference of design data and profilometric measurement. (b) Illustration of applied strategy for milling the ellipsoidal mirror.
For our experiments the emission characteristics of the excitation laser diode were emulated by focusing a laser beam with λ 405 nm with the corresponding asymmetrical NA at the intended position of the laser output facet. At the future position of the NWHs the focus produced by the excitation system is investigated with a CCD chip in contact with the integration platform. Figure 11(a) shows a schematic and a photograph of the setup. The excitation beam path and the beam shaping capability is visualized by the long-term exposure of the setup in Fig. 11(b). The focus captured at the top of the system (Fig. 12) possesses a 1∕e2 -diameter of 24 μm in the horizontal direction (folding plane) and of 14 μm in the vertical direction, which is in good agreement with the simulated dimensions of 21 μm and 13 μm, respectively. However, a small portion of energy is shifted into sidelobes, which are clearly visible in the horizontal intensity profile (Fig. 12, red curve). The reason for those can be found in profilometric characterization results of the central section of the ellipsoidal mirror, presented below [5]. The overall deviation of the measured surface from the design data in Fig. 13(a) shows a low average form difference of
Technische Universität Ilmenau, Institut für Mikro- und Nanotechnologien (IMN MacroNano), Fachgebiet Technische Optik, P.O. Box 100565, 98648 Ilmenau, Germany 2
Justus-Liebig-Universität Gießen, I. Physikalisches Institut, Gießen, Germany
3 4
Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Germany
Albert-Ludwigs-Universität Freiburg, Institut für Optoelektronik, Freiburg, Germany *Corresponding author: roman.kleindienst@tu‑ilmenau.de Received 31 October 2014; accepted 11 December 2014; posted 15 December 2014 (Doc. ID 226073); published 28 January 2015
The photoluminescence intensity of group III nitrides, nanowires, and heterostructures (NWHs) strongly depends on the environmental H2 and O2 concentration. We used this opto-chemical transducer principle for the realization of a gas detector. To make this technology prospectively available to commercial gasmonitoring applications, a large-scale laboratory setup was miniaturized. To this end the gas-sensitive NWHs were integrated with electro-optical components for optical addressing and read out within a compact and robust sensor system. This paper covers the entire realization process of the device from its conceptual draft and optical design to its fabrication and assembly. The applied approaches are verified with intermediate results of profilometric characterizations and optical performance measurements of subsystems. Finally the gas-sensing capabilities of the integrated detector are experimentally proven and optimized. © 2015 Optical Society of America OCIS codes: (170.0110) Imaging systems; (170.3010) Image reconstruction techniques; (170.3660) Light propagation in tissues. http://dx.doi.org/10.1364/AO.54.000839
1. Introduction
Compact and robust gas-monitoring devices for reliable operation at room temperature and above are of special interest for a wide range of industrial and safety applications. In order to increase the sensitivity of such devices, nanoscale transducer structures have been investigated, taking advantage of a higher surface-to-volume ratio. In this context, optical readout methods are highly promising as they allow parallel probing of large ensembles of nano objects without manipulating or processing single nanostructures. Furthermore, optical readout techniques 1559-128X/15/040839-09$15.00/0 © 2015 Optical Society of America
feature significant advantages for certain applications as, e.g., measurements in potentially hazardous environments that prohibit any form of electricity at the point of measurement.m In recent works an optical approach using semiconducting group III-nitride (III-N) nanowire heterostructures (NWHs) as opto-chemical transducers has been proposed [1–3]. As shown schematically in Fig. 1, suitable NWHs can be optically excited by light of energy above the semiconductor band gap to emit photoluminescence (PL) light. Changes in the environmental gas composition lead to modified surface adsorption and thus affect the PL signal intensity. This is exemplarily presented in Fig. 2, which shows the transient PL response of InGaN/ GaN NWHs under variation of the ambient O2 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
839
Fig. 3. (a) Schematic and (b) STEM image of nanowires with alternating InGaN and GaN sections (reproduced by permission of IOP Publishing) [4]. (c) SEM image of the NWH ensemble. Fig. 1. Schematic of the excitation-luminescence principle with EPh;exc > EPh;PL .
concentration using a HeCd gas laser for PL excitation. Similar to other oxidizing gases such as NO2 or O3 , a quenching of the PL signal is observed [3]. On the contrary, for Pt-coated NWHs an increase of the PL intensity is found when reducing gases such as H2 and hydrocarbons are adsorbed [2]. In both cases, the observed effects have been explained by modifications of the rate for nonradiative surface recombination [1,3]. Detection limits in the range of several ppm (hydrocarbons, O2 ) and several hundred ppb (H2 , NO2 , O3 ) have been reported [2,3]. For an application of these outstanding properties in commercial devices, all components of the measurement setup need to be integrated within a compact and robust sensor. In this work we demonstrate the possibility of the miniaturization of a large-scale laboratory setup into an integrated, highly efficient optical microsystem that combines a laser diode for excitation, a photodiode for detection, and the NWHs as active gas-sensing elements via highly efficient beam shaping and collector optics. As transducer structures, InGaN/GaN NWHs with several alternating layers of different III-N materials [schematically shown in Fig. 3(a)] were grown by molecular beam epitaxy using a self-assembled process [4]. Figures 3(b) and 3(c) depict a transmission electron microscopy image of a single NWH structure and a scanning electron microscopy image of the NWH ensemble, respectively. The NWHs feature strong carrier confinement that is reflected by a high thermal stability of the PL signal that allows for the detection of gas concentration changes even at elevated temperatures up to 250°C. In Section 2 our concept for integrating a sensor based on InGaN/GaN NWHs for detecting O2 concentration changes is introduced. Afterward we go into
Fig. 2. Temporarily resolved PL response of uncoated InGaN/ GaN NWHs on O2 concentration changes in N2 at 90°C [1]. 840
APPLIED OPTICS / Vol. 54, No. 4 / 1 February 2015
the details of the realization and characterization of the sensor system. We start with an explanation of the optical excitation and detection system design in Section 3. Here standard design goals were extended by additional boundary conditions targeting on an enhanced system performance while maintaining the manufacturability. Especially for the calculation of the optical excitation beam path, this led to an application-driven design approach. The implementation of all optical functionalities into the optical integration platform is presented from the technological point of view in Section 4. With characterizations of the shape accuracy, surface quality, and optical performance in Section 5, the applied design and fabrication approaches were proven for the excitation as well as detection subsystem. Finally in Section 6 the overall sensor functionality as an interaction of excitation and detection is demonstrated with exemplary measurements of the O2 concentration changes. 2. System Integration Concept
Besides the main aspect of miniaturization, the sensor integration concept is intended to retain the excellent properties of group III-nitride-based NWHs for reliable gas concentration change measurements. For this purpose the following requirements for setting up the sensor were defined: (a) Scattered and reflected excitation radiation need to be spatially separated from the PL signal at the position of the PL detector to achieve a sufficiently high signal-to-noise ratio [5]. (b) To guarantee long-term stable measurement conditions and high detection reliability only the NWHs are in contact with the test medium. The detection periphery, i.e., the excitation light source, the PL detector, and other electronics, is completely isolated from the investigated environment [2]. In Fig. 4 a conceptual layout of the miniaturized sensor system with a size of approximately 30 mm × 60 mm × 30 mm satisfying both requirements is shown. The linear polarized excitation radiation at 405 nm is emitted by a laser diode (output facet size 2.0 μm × 0.4 μm) with a divergence of 10° (FWHM) in slow axis and 22° (FWHM) in fast axis [6]. With two optical surfaces the light is shaped to an aberration-minimized focus at the gas-sensitive
Fig. 4. Concept for integrating the gas-sensitive NWHs with opto-electronic detection periphery.
nanowires at the top of the system. Applying a sufficiently high power density, the NWHs generate a PL signal with a Lambertian emission characteristics and a bandwidth of about 50 nm (FWHM) around the center wavelength of 500–520 nm (depending on the In concentration). Concentric with the excitation spot and the center of PL, the detection subsystem is introduced. Experiments have shown that PL intensity changes can even be detected using a Si photodiode (active area 3.6 mm × 3.6 mm) [7] without any optics. However, with additional collector optics, an increased PL signal can be achieved, as a higher portion of the PL signal is concentrated onto the detector. Furthermore, in the imaging configuration, scattered excitation radiation is further discriminated. For spectral separation a dielectric and an absorption filter are integrated within the detector subsystem. In the next section the design goals are derived from the integration requirements and preliminary investigations. Afterward the approaches for the design of the excitation and the collector optics are explained in detail. 3. Optical Design
For efficiency reasons the optoelectronic components are connected to each other via the NWHs using free space optics. To guarantee for a robust and compact sensor setup though the excitation source and optics, the gas-sensitive nanowires and the detection subsystem are combined within an optical integration platform. While the surfaces for shaping of the excitation beam are directly provided by the integration platform, the PL collector optics is part of the supplemental detection subsystem. Details of the design approaches for both, the optical excitation as well as the detection system, are given subsequently. A.
Optical Design of the Excitation System
Because of the suppression of nonradiative Shockley– Read–Hall recombination, the PL signal is growing superlinearly with increasing excitation density. For this reason an optical system is required that focuses the highly divergent excitation radiation at a high numerical aperture with minimized aberrations onto the NWHs. To reduce false light within the system and to increase the SNR (signal-to-noise
Fig. 5. Optical design approach based on an ellipsoid for transmission efficiency-enhanced excitation beam shaping [5].
ratio) of the gas sensor only high-efficiency beamshaping approaches were taken into account that comply with the concept constraints introduced in Section 2. In preparation for the fabrication process, exclusively analytical specifications of optical surfaces were considered. These allow for direct programming of our ultraprecision micromilling tool and thus highest fabrication accuracy. Transcription errors occurring from transferring design into fabrication data are eliminated, and systematic fabrication errors can be compensated in real time [8]. As illustrated in Fig. 5 we used an ellipsoidal mirror as a starting point for the design, which perfectly images a point source from focal point 1 into its second focal point. With an extension of only 2.0 μm × 0.4 μm the laser output facet is small compared to the desired overall size of the sensor. Thus if aligned in focal point 1 the ellipsoidal mirror generates an image of the laser facet, i.e., focus, with negligible aberrations at focal point 2 where the NWHs are located. The excitation radiation is coupled into the substrate and shaped within the material polymethylmethacrylate (PMMA, Plexiglas, GS218) to the desired intensity distribution. However, an analysis of the schematic in Fig. 5 reveals that any distance, dimension, or angle within the microoptical system is influenced by each of the four degrees of freedom of this starting design: RoC (radius of curvature), conic (conic constant between −1 and 0), ε (tilt of the ellipse axis), as well as γ (tilt of laser diode). This parameter range was initially restricted by a limitation of the general dimensions of the integration platform like the thickness defined by the position of the laser output facet and the surface of the ellipsoidal mirror, the overall sensor thickness, the position of the source relative to the image plane, and the space left underneath the NWHs for the PL detection subsystem. 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
841
The final configuration was accomplished by optimizing the system based on an ellipsoidal mirror with respect to diffraction limited focusing capability and highest transmission efficiency without using any anti-reflection (AR) coating. In particular spherical aberration is compensated by coupling the excitation beam into the substrate through an appropriately shaped surface [9,10]. Except for minimal field aberrations the beam quality is maintained, and the highest possible energy density at the NWHs for the given focusing NA is achieved by this means. The overall transmission efficiency is enhanced by reducing reflection losses at the substrate–gas interface at the top of the system. Therefore the excitation beam is polarized parallel with respect to the folding plane and coupled out of the PMMA substrate into the gaseous environment around the Brewster angle. Considering the specific angular intensity distribution of the incident beam, represented by an appropriately sampled ray bundle, the system parameters were finally optimized with respect to maximum transmission. The diagram in Fig. 6 shows the average reflection losses versus the marginal ray angle of incidence. Complying with the concept constraints and design requirements, a minimum of only 0.28% at 36.36° was found for a RoC of 8.05 mm, a conic of −0.47, an ellipse tilt of 42.12°, and a source tilt of 23.2° [5]. In Fig. 7 the excitation beam path is visualized with rays traced through a solid model of the microoptical system with a functional size of 23 mm× 20 mm × 21 mm. After passing the curved coupling surface, the excitation radiation is focused with the ellipsoidal mirror section onto the gas-sensitive nanowires at the top of the system. The PL signal intensity is measured with the detection subsystem directly below. B.
Optical Design of the Detection Subsystem
The PL signal is detected using a Si photodiode with a sensitive area of optical density (OD) 3.6 mm × 3.6 mm. Scattered excitation radiation is suppressed with a dielectric edge filter (OD 2) followed by an absorption edge filter (OD 4) in front of the
Fig. 6. Average reflection losses versus marginal ray angle of incidence for ray bundles with polarization parallel to the folding plane. 842
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Fig. 7. Solid model of the integration platform in PMMA with visualized excitation beam path.
photodiode. In this configuration additional fluorescence induced by the excitation radiation in the absorption filter is minimized and the excitation radiation is cancelled almost completely. The weak PL signal detected with this simple photodiode-filter combination (see measurements Section 6) directly below the PL center can be further improved in terms of the signal peak intensity while maintaining the SNR introducing additional collector optics. For this purpose we have chosen a miniaturized high NA 4-f setup (Fig. 8) inserted into the cavity underneath the NWHs. The first lens collimates a large portion of the PL signal, which is then focused by the second lens onto the sensitive area of the photodiode. Because of its angle sensitive properties, the dielectric filter is placed as specified by the supplier within the collimated beam path between both lenses. The angularly insensitive absorption edge filter is space-economically integrated into the convergent beam behind the focusing lens. The PL intensity distribution with a FWHM diameter of about 30 μm at the photosensitive area based on ray-tracing simulations is depicted in Fig. 8. Estimations resulted in a detection intensity increased by a factor of 5 compared to detection without collector optics. Besides an enhanced spectral filtering behavior, this imaging setup provides an improved discrimination of scattered excitation and false light. Furthermore, in this configuration the detection is significantly more tolerant against position changes of the luminescent point caused, e.g., by misalignment of the excitation source. The simulation in Fig. 8 (green beam path) shows that even for a displacement of the excitation focus of 1 mm the corresponding image is produced on the photosensitive area. After briefly describing our fabrication and assembly approach in the next section, experimental results demonstrating the optical performance of both the excitation and the detection subsystem, as well as the overall detection capability of the integrated sensor, are presented in Sections 5 and 6.
Fig. 8. Model of the collector optics and ray-tracing-based simulation of the PL intensity distribution at the sensitive area of the photodiode.
4. Realization of the Optical Integration Platform and the Detection Subsystem A.
Fabrication of the Optical Integration Platform
The flexibility and accuracy required for the fabrication of the optical integration platform in UVoptimized PMMA (Plexiglas GS218) is provided by ultraprecision micromilling. In our optimized process the analytical surface specifications are directly used for controlling the machining axes. This helps to avoid accuracy loss due to data conversions. Compared to fixed fabrication data, milling paths are computed instantaneously during the milling process, which allows for flexible compensation of positioning deviations during fabrication. Thus with an appropriately chosen design approach and an advanced control over the micromilling process, the highest fabrication accuracy can be achieved while keeping the data volume at a minimum [8]. In principle, the integration platform was processed in two steps: First rough machining was used to generate the basic shape comprising the mount for the excitation laser, the section of the ellipsoidal mirror, the curved coupling surface, the plane surface for coupling into the NWHs, and the cavity for the detection subsystem. Afterward, a finishing process (using diamond tools) [Figs. 9(a)–9(d)] was applied to obtain the shape accuracy and surface quality specifically required for the optically functional surfaces [11]. With a total machining time of about 60 h from the blank substrate to the integration platform (Fig. 10), this fabrication approach is suitable for prototyping or tooling purposes. An economic replication can be realized with a sol gel process in SiO2 providing high transmission and long-term stability, even prospectively for applied shorter excitation wavelengths [12].
Fig. 9. (a) Ultraprecision micromilling of the excitation source platform, (b) the ellipsoidal mirror section, (c) the concave coupling surface, and (d) the surface for coupling out of the substrate into the NWHs.
B. Assembly of the Detection Subsystem
The collector optics was set up by stacking the lenses and filters with suitable spacers within an aluminum tube. The objective is finally combined with the photodiode and integrated within the cavity below the NWHs. An increased degree of integration can be reached by arranging the lenses and filters directly within the cavity. However, to suppress false light an additional process for applying a high OD layer at the cavity walls is required. 5. Profilometric and Optical Performance Characterizations
In this section experimental results proving the optical performance of the excitation system and the collector optics are presented. Furthermore, the
Fig. 10. Optical integration platform in UV optimized PMMA (GS2018). 1 February 2015 / Vol. 54, No. 4 / APPLIED OPTICS
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Fig. 11. Setup for (a) excitation spot measurement and (b) integration system during experiment.
fabrication process is evaluated with interferometryand AFM-based profilometric characterizations of the integration platforms optical surfaces. A.
Optical Excitation System Fig. 13. (a) Difference of design data and profilometric measurement. (b) Illustration of applied strategy for milling the ellipsoidal mirror.
For our experiments the emission characteristics of the excitation laser diode were emulated by focusing a laser beam with λ 405 nm with the corresponding asymmetrical NA at the intended position of the laser output facet. At the future position of the NWHs the focus produced by the excitation system is investigated with a CCD chip in contact with the integration platform. Figure 11(a) shows a schematic and a photograph of the setup. The excitation beam path and the beam shaping capability is visualized by the long-term exposure of the setup in Fig. 11(b). The focus captured at the top of the system (Fig. 12) possesses a 1∕e2 -diameter of 24 μm in the horizontal direction (folding plane) and of 14 μm in the vertical direction, which is in good agreement with the simulated dimensions of 21 μm and 13 μm, respectively. However, a small portion of energy is shifted into sidelobes, which are clearly visible in the horizontal intensity profile (Fig. 12, red curve). The reason for those can be found in profilometric characterization results of the central section of the ellipsoidal mirror, presented below [5]. The overall deviation of the measured surface from the design data in Fig. 13(a) shows a low average form difference of
