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Quantum cascade lasers for mid-infrared spectroscopy
Vibrational Spectroscopy 30 (2002) 53±58
Quantum cascade lasers for mid-infrared spectroscopy L. Hvozdaraa, N. Penningtona, M. Kraftb, M. Karlowatzb, B. Mizaikoffa,* a
Applied Sensors Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332±0400, USA. b Institute of Analytical Chemistry, Vienna University of Technology, 1040 Vienna, Austria Received 31 August 2001; received in revised form 1 April 2002; accepted 3 April 2002
Abstract Spectroscopy in the mid-infrared (MIR) band (3±25 mm) is known to be a powerful tool for both, qualitative and quantitative analysis even in complex matrices. In combination with MIR transparent optical waveguides, this technology represents a reliable and sensitive real-time sensing principle with high application potential in medical sciences, long-term observation of environmental pollution and process control in chemical industry. Evanescent wave sensors are particularly suitable for addressing organic analytes in aqueous media, especially when using appropriate polymer coatings at the waveguide surface as analyte enrichment layers. The suitability for such systems to operate under harsh conditions was recently demonstrated with the prototype of a MIR underwater sensor system based on a Fourier transform infrared (FT-IR) spectrometer capable of tracing volatile organic compounds (VOCs) in a marine environment. The signal transduction is based upon attenuated total re¯ection (ATR), in combination with silver halide MIR transparent ®bers representing the active sensor head. Recent developments in the ®eld of quantum cascade lasers (QCLs) provide a viable concept of MIR laser sources, with the possibility of tailoring the emission wavelength within a broad range of frequencies. A QCL is a microfabricated, compact light source, with the option of near-room temperature operation [1] at a low voltage. These properties make QCLs a light source with a signi®cant potential for MIR sensor technology and particularly for integration into compact, remotely operated MIR target spectrometers in the near future. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mid-infrared sensors; Quantum cascade laser; Attenuated total re¯ection
1. Introduction Medicine, process control, environmental science and many other scienti®c and technological branches have a strong demand for online and continuously operating sensing systems. The mid-infrared (MIR) spectral range can be ef®ciently utilized for the construction of powerful sensing and detection schemes, as fundamental vibrational and rotational molecular frequencies are resonant with those corresponding to the MIR region *
Corresponding author. Fax: 1-404-894-7452. E-mail addresses: [email protected] (L. Hvozdara), [email protected] (B. Mizaikoff).
considered from 3 to 25 mm [2]. Essentially, all chemical species, and in particular organic compounds, exhibit strong absorption of light at MIR frequencies. MIR spectroscopy is a well-established nondestructive method for highly sensitive and selective concentration determination and identi®cation of chemical species. In the ®ngerprint region (approximately 7±20 mm), substance speci®c absorptions enable the distinct recognition of various chemical species and even of structural isomers. As such, MIR spectroscopic methods and among them predominantly Fourier transform infrared (FT-IR) spectroscopy, can be considered routine applications among standard laboratory techniques.
0924-2031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 ( 0 2 ) 0 0 0 3 8 - 3
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L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
Field applications of MIR techniques were until recently limited mostly due to technical factors, such as the availability of compact and ef®cient light sources, detectors, durable optical components and appropriate waveguides and ®ber-optics, respectively. Considerable advances in MIR technology during the last decade allow nowadays the construction of compact sensing devices based on MIR signal transduction principles, which are suitable for ®eld and remote online operation. 2. Quantum cascade lasers and their properties Bandstructure engineering, together with advanced epitaxial techniques, enables the design of arti®cial semiconductor crystals, so-called quantum heterostructures. Crystalline growth with mono-molecular layer resolution allows the modulation of the semiconductor bandstructure with various energetic schemes, according to the desired design. Light ampli®cation using inter-sub-band transitions in a quantum well system was ®rst proposed by Kazarinov and Suris in 1971 [3]. The ®rst quantum cascade laser (QCL) has been presented by Faist et al. in 1994 [4]. Three InGaAs/InAlAs coupled quantum wells offering three well-de®ned, discrete electron levels are applied as active cells of the laser. Graded injectors (superlattice structures) are bridging the active cells in a 25-period cascade sequence. These lasers exhibit laser emission up to Tmax 88 K, with maximum peak output of 8.5 mW per facet at a wavelength of l 4:16 mm. This signi®cant success started a dynamic development in the ®eld of unipolar. semiconductor lasers. In 1996, a ®rst laser operating at and above room temperature (Tmax 320 K) had been demonstrated [5]. The optical output at T 300 K is still 6 mW per facet. This laser demarks a fundamental breakthrough for the technological applicability of QCLs. No requirement for cryogenic cooling makes it the ®rst MIR semiconductor laser with high technological potential. Further milestones in QCL development have been marked by the introduction of a single mode circular-microresonator distributed feedback (DFB) QCL [6]. All these achievements have been reached [10,11] using a single material systemÐan InGaAs/InAlAs heterostructure matched to InP. The ®rst QCL based on a GaAs/AlGaAs material system has been published by Sirtori et al. in 1998 [7].
The laser exhibits a peak output power of 70 mW per facet and a threshold current density of 7.3 kA/cm2 at the temperature of 80 K. A QCL using a GaAs/AlGaAs graded superlattice as a gain medium, has been introduced by Strasser et al. in 1999 [8]. The laser exhibited emission at l 13 mm at temperatures up to T 80 K. Peak output powers in the range of 100 mW are recorded. The current activities in the ®eld of GaAs/ AlGaAs QCLs are mainly aimed towards the option of room temperature operation [9]. Furthermore, intensive research in the ®eld of terahertz (far-infrared) emission from the GaAs/AlGaAs quantum cascade structures is being performed [10,11]. Using the unipolar laser concept, it is possible to design lasers emitting at several frequency bands simultaneously. The common semiconductor materials enable the construction of lasers for frequencies between 3.4 mm [12] and 19 mm. Practically, every frequency within this range can be reached. The type of the laser resonator determines the shape of the emission spectrum. A characteristic multi-mode spectrum is obtained from a Fabry±Perot (F±P) resonator, as shown in Fig. 1(a). A microdisk resonator, as well as a DFB laser, usually emits on a single mode, as demonstrated in Fig. 1(b). Precise control of the laser emission wavelength and single mode operation is achieved using a DFB resonator [6,13]. Advanced resonator designs fabricated by means of focused ion beam (FIB) milling and cutting techniques resulting in an enhanced laser performance are reported recently [14,15]. A laser with a FIB cut coupled cavity resonator enables wavelength control, as well as switching from multi-mode to single mode performance [14]. QCLs with FIB-micromachined Bragg re¯ectors showed a signi®cant threshold current drop [15]. In summary, QCLs represent MIR light sources characterized by the following features: (i) small dimensions, (ii) high spectral density, (iii) long lifetime (>10,000 h), (iv) low energy consumption, (v) possibility of high frequency modulation, (vi) CW operation (up to 27 8C), (vii) long-term wavelength and power stability, (viii) the possibility of single mode operation, (ix) possibilities of tuning and mode locking [16]. 3. Mid-infrared chemical sensing In general, absorption spectroscopic sensors feature a radiation source (broadband emitter or laser), a
Fig. 1. (a) Multi-mode emission spectrum of a GaAs/AlGaAs QCL measured at T 77 K. The laser is exhibiting more than 40 longitudinal modes centered at 1000 cm 1 (l 10 mm). These spectra are typical for a F±P resonator geometry. The F±P lasers can be applied for the spectroscopic investigation of compounds with a broad absorption spectrum, as typical for liquid phase spectra. (b) Spectrum of a DFB laser fabricated of the same wafer as (a), measured at T 77 K. These lasers have a stable emission frequency and all the power of the emitted radiation is concentrated into a single emission mode. Their application potential is mainly precise spectroscopic analysis in the gas phase and similar laser spectroscopic applications.
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L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
de®ned absorption path, and a detector. In the simple case of MIR gas measurements, a free path in the atmosphere constitutes an absorption path, e.g. for atmospheric gas measurements. More frequently for laboratory applications, a single-pass or multi-pass optical cell is applied. Recently, a step towards miniaturizing such gas measurement systems has been presented by using a hollow waveguide acting as both, radiation guiding optics and miniaturized gas cell in combination with QCL technology [17]. This hollow waveguide concept appears as viable alternative to conventional MIR sensing systems using standard gas cells, especially with respect to the possibility of waveguide microfabrication and thus further miniaturization of the sensing system. For MIR absorption measurements in the aqueous phase, it is common to use the principle of attenuated total re¯ection (ATR), as schematically shown in Fig. 2 [18,19]. The optical element can be an ATR slab waveguide (ATR crystal), or a solid-state or hollow ®ber waveguide, respectively. Both, the ATR crystal and the solid-state waveguide can be covered with a
Fig. 2. Scheme of evanescent ®eld spectroscopy with a coated ®ber waveguide used as an optrode. A guided mode is exciting evanescent waves at the surface of the waveguide, which propagate along the surface constituting the evanescent ®eld around the waveguide. Analyte diffusion from the aqueous phase into a polymer coating attached to the surface of the waveguide, causes absorption speci®c attenuation of the evanescent ®eld. As a result, the guided mode is attenuated with the intensity loss being proportional to the concentration of the analyte in the evanescent ®eld of the waveguide.
polymer coating, which separates strongly IR absorbing water from the waveguide surface, and simultaneously enriches the analyte within the penetration depth of the evanescent ®eld [20±22]. This MIR chemical sensor concept has led to a variety of systems with applications ranging from the medical area to environmental surveillance. Recently, the ATR principle has been applied for the construction of a MIR sensing system coupled to a submersible FT-IR spectrometer [23]. The spectrometer is based on a redesigned commercial unit (Bruker vector [22]), with a SiC broadband emitter as a light source and a thermoelectrically cooled mercury-cadmium-telluride (MCT) detector. The optrode consists of a 70 cm long silver halide waveguide, which is stabilized on a U-shaped support [24]. The active sensing region of the ®ber (35 cm) is coated with poly(ethylene/propylene) copolymer as enrichment layer for VOCs dissolved in seawater [25]. A 100 ppb concentration of tetrachloroethylene in seawater has been detected and six different chlorinated hydrocarbons in the lower parts per million range have been distinguished in the broadband spectrum using this instrument. This system is currently being optimized with a miniaturized Stirling cooled MCT detector for ®eld trials. With respect to such applications, QCL technology represents a signi®cant step forward towards the miniaturization of spectroscopic sensing systems. This is of particular importance for the deployment of chemical sensing systems in harsh environments, such as the deep sea. The availability of QCLs creates the basis for a wide range of applications for laser spectroscopic methods in the MIR spectral range. Laser based sensing systems offer several advantages compared to conventional systems with black body sources: (i) sensitivity in the sub-parts per billion range, (ii) selectivity given by narrow spectral line width, (iii) dynamic range with real time response, which is linear over several orders of magnitude, (iv) low maintenance (no consumables), (v) small dimensions, (vi) possibility of hybrid integration, (vii) mechanical stability and robustness. Current works are focused on the design and development of QCL based sensing systems for the determination of target compounds, such as methane, aromatic hydrocarbons and chlorinated hydrocarbons in a marine environment. A scheme of a proposed
L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
57
Fig. 3. Scheme of a sensing system using a multi-pass ATR slab waveguide, a QCL and an array of microbolometers as detecting element. Absorption is detected as a vector of values simultaneously picked up from the microbolometers. The signal is evaluated by exponential ®t to the vector of values. The system is therefore not sensitive to intensity drifts of the light source (note that all three curves originate at a different point). The system has a high dynamic rangeÐsmall absorptions can be easily detected (curve A), as well as stronger absorptions (curve B). Very strong absorptions causing total extinction of the radiation at certain sections of the crystal (curve C) can still be detected, since absorption path in the ATR crystal is variable.
system is shown in Fig. 3. The key elements involve a QCL as a radiation source, a multi-pass ATR slab waveguide, an array of microbolometers or similar arrayed MIR detectors, and digital signal processing (DSP) electronics. All sensor components are hybridintegrated on one substrate. The analyte absorbance is detected as a vector quantity and evaluated from the exponential ®t to signals from the microbolometers. This system is not sensitive to drift effects of the input light intensity. Hence, ideally it does not require re-calibration. The dynamic range is broader than it would be in case of a single pass cell. Due to the multiplicity of microbolometers, the absorption ATR path is virtually variable (Fig. 3). Hybrid integration makes the system robust and reliable for long-term deployment at harsh conditions. Furthermore, it enables the integration of various other sensors for complementary data acquisition. 4. Conclusion The early results described in this contribution indicate that MIR spectroscopy and laser spectroscopy provides a viable method with a high application potential for chemical sensor technology. Further developments in our research group are aimed towards the construction of integrated laser spectroscopic chemical sensing systems using QCLs. A signi®cant
improvement of the sensitivity and the dynamic range, as well as of the overall stability and performance of such sensing systems is expected. Acknowledgements Financial support of the US Department of Energy, the European Union and the Austrian Federal Ministry of Education, Science and Culture (BMBWK) is gratefully acknowledged, as well as the excellent cooperation with our partners in the SOFIE project. Bruker Optik GmbH (Ettlingen/Germany) is thanked for continuous support during the spectrometer reconstruction. The authors acknowledge A. Katzir and his research group at the Tel-Aviv University (Israel) for their support with silver halide ®bers, and E. Gornik and his team at the Vienna University of Technology (Austria), J. Faist and his group at the University of Neuchatel (Switzerland) and Antoine Mueller, Alpes Lasers (Neuchatel, Switzerland) for QCL technology. References [1] D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, M. Hans, Appl. Phys. Lett. 78 (2001) 1964. [2] B. Mizaikoff, Proc. SPIE 3849 (1999) 7. [3] R.F. Kazarinov, R.A. Suris, Sov. Phys. Semicond. 5 (1971) 707.
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[4] J. Faist, F. Capasso, C. Sirtori, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553. [5] J. Faist, F. Capasso, C. Sirtori, D.L. Sivco, J.N. Baillargeon, A.L. Hutchinson, S.N.G. Chu, A.Y. Cho, Appl. Phys. Lett. 68 (1996) 3680. [6] J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D.L. Sivco, J.N. Baillargeon, A.Y. Cho, Appl. Phys. Lett. 70 (1997) 2670. [7] C. Sirtori, P. Kruck, S. Barbieri, P. Collot, J. Nagle, M. Beck, J. Faist, U. Oesterle, Appl. Phys Lett. 73 (1998) 3486. [8] G. Strasser, S. Gianordoli, L. Hvozdara, W. Schrenk, K. Unterrainer, E. Gornik, Appl. Phys. Lett. 76 (1999) 1345. [9] S. Barbieri, C. Sirtori, H. Page, M. Stellmacher, J. Nagle, Appl. Phys. Lett. 78 (2001) 282. [10] M. Rochat, J. Faist, M. Beck, U. Oesterle, M. Ilegems, Appl. Phys. Lett. 73 (1998) 3724. [11] J. Ulrich, R. Zobl, W. Schrenk, G. Strasser, K. Unterrainer, E. Gornik, Appl. Phys. Lett. 77 (2000) 1928. [12] J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, S.-N.G. Chu, A.Y. Cho, Appl. Phys. Lett. 72 (1998) 680. [13] W. Schrenk, N. Finger, S. Gianordoli, L. Hvozdara, G. Strasser, E. Gornik, Appl. Phys. Lett. 76 (2000) 253. [14] L. Hvozdara, A. Lugstein, N. Finger, S. Gianordoli, W. Schrenk, K. Unterrainer, E. Bertagnolli, G. Strasser, E. Gornik, Appl. Phys. Lett. 77 (2000) 1241.
[15] L. Hvozdara, A. Lugstein, S. Gianordoli, W. Schrenk, G. Strasser, K. Unterrainer, E. Bertagnolli, E. Gornik, Appl. Phys. Lett. 77 (2000) 1077. [16] F. Capasso, C. Gmachl, R. Paiella, A. Tredicuci, A.L. Hutchinson, D.L. Sivco, J.N. Baillargeon, A.Y. Cho, H.C. Chu, IEEE J. Select. Topics Quantum Electron. 6 (2000) 930. [17] L. Hvozdara, S. Gianordoli, G. Strasser, W. Schrenk, K. Unterrainer, E. Gornik, C.S. Murthy, M. Kraft, V. Pustogow, B. Mizaikoff, A. Inberg, N. Croitoru, Appl. Opt. 39 (2000) 6926. [18] J. Fahrenfort, Spectrochim. Acta 17 (1961) 698. [19] N.J. Harrick, Internal Re¯ectance Spectroscopy, Interscience, New York, 1967. [20] R. Krska, K. Taga, R. Kellner, Appl. Spectrosc. 47 (1993) 1484. [21] B. Mizaikoff, R. GoÈbel, R. Krska, K. Taga, R. Kellner, M. Tacke, A. Katzir, Sens. Actuators B 29 (1994) 58. [22] J.E. Walsh, B.D. MacCraith, M. Meany, J.G. Vos, F. Regan, A. Lancia, S. Artjushenko, Analyst 121 (1996) 789. [23] B. Mizaikoff, Measur. Sci. Technol. 10 (1999) 1194. [24] M. Kraft, M. Jakusch, B. Mizaikoff, A. Katzir, Proc. SPIE 28 (1999) 3849. [25] M. Kraft, B. Mizaikoff, Int. J. Environ. Anal. Chem. 78 (2000) 367.
Quantum cascade lasers for mid-infrared spectroscopy L. Hvozdaraa, N. Penningtona, M. Kraftb, M. Karlowatzb, B. Mizaikoffa,* a
Applied Sensors Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332±0400, USA. b Institute of Analytical Chemistry, Vienna University of Technology, 1040 Vienna, Austria Received 31 August 2001; received in revised form 1 April 2002; accepted 3 April 2002
Abstract Spectroscopy in the mid-infrared (MIR) band (3±25 mm) is known to be a powerful tool for both, qualitative and quantitative analysis even in complex matrices. In combination with MIR transparent optical waveguides, this technology represents a reliable and sensitive real-time sensing principle with high application potential in medical sciences, long-term observation of environmental pollution and process control in chemical industry. Evanescent wave sensors are particularly suitable for addressing organic analytes in aqueous media, especially when using appropriate polymer coatings at the waveguide surface as analyte enrichment layers. The suitability for such systems to operate under harsh conditions was recently demonstrated with the prototype of a MIR underwater sensor system based on a Fourier transform infrared (FT-IR) spectrometer capable of tracing volatile organic compounds (VOCs) in a marine environment. The signal transduction is based upon attenuated total re¯ection (ATR), in combination with silver halide MIR transparent ®bers representing the active sensor head. Recent developments in the ®eld of quantum cascade lasers (QCLs) provide a viable concept of MIR laser sources, with the possibility of tailoring the emission wavelength within a broad range of frequencies. A QCL is a microfabricated, compact light source, with the option of near-room temperature operation [1] at a low voltage. These properties make QCLs a light source with a signi®cant potential for MIR sensor technology and particularly for integration into compact, remotely operated MIR target spectrometers in the near future. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mid-infrared sensors; Quantum cascade laser; Attenuated total re¯ection
1. Introduction Medicine, process control, environmental science and many other scienti®c and technological branches have a strong demand for online and continuously operating sensing systems. The mid-infrared (MIR) spectral range can be ef®ciently utilized for the construction of powerful sensing and detection schemes, as fundamental vibrational and rotational molecular frequencies are resonant with those corresponding to the MIR region *
Corresponding author. Fax: 1-404-894-7452. E-mail addresses: [email protected] (L. Hvozdara), [email protected] (B. Mizaikoff).
considered from 3 to 25 mm [2]. Essentially, all chemical species, and in particular organic compounds, exhibit strong absorption of light at MIR frequencies. MIR spectroscopy is a well-established nondestructive method for highly sensitive and selective concentration determination and identi®cation of chemical species. In the ®ngerprint region (approximately 7±20 mm), substance speci®c absorptions enable the distinct recognition of various chemical species and even of structural isomers. As such, MIR spectroscopic methods and among them predominantly Fourier transform infrared (FT-IR) spectroscopy, can be considered routine applications among standard laboratory techniques.
0924-2031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 ( 0 2 ) 0 0 0 3 8 - 3
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L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
Field applications of MIR techniques were until recently limited mostly due to technical factors, such as the availability of compact and ef®cient light sources, detectors, durable optical components and appropriate waveguides and ®ber-optics, respectively. Considerable advances in MIR technology during the last decade allow nowadays the construction of compact sensing devices based on MIR signal transduction principles, which are suitable for ®eld and remote online operation. 2. Quantum cascade lasers and their properties Bandstructure engineering, together with advanced epitaxial techniques, enables the design of arti®cial semiconductor crystals, so-called quantum heterostructures. Crystalline growth with mono-molecular layer resolution allows the modulation of the semiconductor bandstructure with various energetic schemes, according to the desired design. Light ampli®cation using inter-sub-band transitions in a quantum well system was ®rst proposed by Kazarinov and Suris in 1971 [3]. The ®rst quantum cascade laser (QCL) has been presented by Faist et al. in 1994 [4]. Three InGaAs/InAlAs coupled quantum wells offering three well-de®ned, discrete electron levels are applied as active cells of the laser. Graded injectors (superlattice structures) are bridging the active cells in a 25-period cascade sequence. These lasers exhibit laser emission up to Tmax 88 K, with maximum peak output of 8.5 mW per facet at a wavelength of l 4:16 mm. This signi®cant success started a dynamic development in the ®eld of unipolar. semiconductor lasers. In 1996, a ®rst laser operating at and above room temperature (Tmax 320 K) had been demonstrated [5]. The optical output at T 300 K is still 6 mW per facet. This laser demarks a fundamental breakthrough for the technological applicability of QCLs. No requirement for cryogenic cooling makes it the ®rst MIR semiconductor laser with high technological potential. Further milestones in QCL development have been marked by the introduction of a single mode circular-microresonator distributed feedback (DFB) QCL [6]. All these achievements have been reached [10,11] using a single material systemÐan InGaAs/InAlAs heterostructure matched to InP. The ®rst QCL based on a GaAs/AlGaAs material system has been published by Sirtori et al. in 1998 [7].
The laser exhibits a peak output power of 70 mW per facet and a threshold current density of 7.3 kA/cm2 at the temperature of 80 K. A QCL using a GaAs/AlGaAs graded superlattice as a gain medium, has been introduced by Strasser et al. in 1999 [8]. The laser exhibited emission at l 13 mm at temperatures up to T 80 K. Peak output powers in the range of 100 mW are recorded. The current activities in the ®eld of GaAs/ AlGaAs QCLs are mainly aimed towards the option of room temperature operation [9]. Furthermore, intensive research in the ®eld of terahertz (far-infrared) emission from the GaAs/AlGaAs quantum cascade structures is being performed [10,11]. Using the unipolar laser concept, it is possible to design lasers emitting at several frequency bands simultaneously. The common semiconductor materials enable the construction of lasers for frequencies between 3.4 mm [12] and 19 mm. Practically, every frequency within this range can be reached. The type of the laser resonator determines the shape of the emission spectrum. A characteristic multi-mode spectrum is obtained from a Fabry±Perot (F±P) resonator, as shown in Fig. 1(a). A microdisk resonator, as well as a DFB laser, usually emits on a single mode, as demonstrated in Fig. 1(b). Precise control of the laser emission wavelength and single mode operation is achieved using a DFB resonator [6,13]. Advanced resonator designs fabricated by means of focused ion beam (FIB) milling and cutting techniques resulting in an enhanced laser performance are reported recently [14,15]. A laser with a FIB cut coupled cavity resonator enables wavelength control, as well as switching from multi-mode to single mode performance [14]. QCLs with FIB-micromachined Bragg re¯ectors showed a signi®cant threshold current drop [15]. In summary, QCLs represent MIR light sources characterized by the following features: (i) small dimensions, (ii) high spectral density, (iii) long lifetime (>10,000 h), (iv) low energy consumption, (v) possibility of high frequency modulation, (vi) CW operation (up to 27 8C), (vii) long-term wavelength and power stability, (viii) the possibility of single mode operation, (ix) possibilities of tuning and mode locking [16]. 3. Mid-infrared chemical sensing In general, absorption spectroscopic sensors feature a radiation source (broadband emitter or laser), a
Fig. 1. (a) Multi-mode emission spectrum of a GaAs/AlGaAs QCL measured at T 77 K. The laser is exhibiting more than 40 longitudinal modes centered at 1000 cm 1 (l 10 mm). These spectra are typical for a F±P resonator geometry. The F±P lasers can be applied for the spectroscopic investigation of compounds with a broad absorption spectrum, as typical for liquid phase spectra. (b) Spectrum of a DFB laser fabricated of the same wafer as (a), measured at T 77 K. These lasers have a stable emission frequency and all the power of the emitted radiation is concentrated into a single emission mode. Their application potential is mainly precise spectroscopic analysis in the gas phase and similar laser spectroscopic applications.
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L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
de®ned absorption path, and a detector. In the simple case of MIR gas measurements, a free path in the atmosphere constitutes an absorption path, e.g. for atmospheric gas measurements. More frequently for laboratory applications, a single-pass or multi-pass optical cell is applied. Recently, a step towards miniaturizing such gas measurement systems has been presented by using a hollow waveguide acting as both, radiation guiding optics and miniaturized gas cell in combination with QCL technology [17]. This hollow waveguide concept appears as viable alternative to conventional MIR sensing systems using standard gas cells, especially with respect to the possibility of waveguide microfabrication and thus further miniaturization of the sensing system. For MIR absorption measurements in the aqueous phase, it is common to use the principle of attenuated total re¯ection (ATR), as schematically shown in Fig. 2 [18,19]. The optical element can be an ATR slab waveguide (ATR crystal), or a solid-state or hollow ®ber waveguide, respectively. Both, the ATR crystal and the solid-state waveguide can be covered with a
Fig. 2. Scheme of evanescent ®eld spectroscopy with a coated ®ber waveguide used as an optrode. A guided mode is exciting evanescent waves at the surface of the waveguide, which propagate along the surface constituting the evanescent ®eld around the waveguide. Analyte diffusion from the aqueous phase into a polymer coating attached to the surface of the waveguide, causes absorption speci®c attenuation of the evanescent ®eld. As a result, the guided mode is attenuated with the intensity loss being proportional to the concentration of the analyte in the evanescent ®eld of the waveguide.
polymer coating, which separates strongly IR absorbing water from the waveguide surface, and simultaneously enriches the analyte within the penetration depth of the evanescent ®eld [20±22]. This MIR chemical sensor concept has led to a variety of systems with applications ranging from the medical area to environmental surveillance. Recently, the ATR principle has been applied for the construction of a MIR sensing system coupled to a submersible FT-IR spectrometer [23]. The spectrometer is based on a redesigned commercial unit (Bruker vector [22]), with a SiC broadband emitter as a light source and a thermoelectrically cooled mercury-cadmium-telluride (MCT) detector. The optrode consists of a 70 cm long silver halide waveguide, which is stabilized on a U-shaped support [24]. The active sensing region of the ®ber (35 cm) is coated with poly(ethylene/propylene) copolymer as enrichment layer for VOCs dissolved in seawater [25]. A 100 ppb concentration of tetrachloroethylene in seawater has been detected and six different chlorinated hydrocarbons in the lower parts per million range have been distinguished in the broadband spectrum using this instrument. This system is currently being optimized with a miniaturized Stirling cooled MCT detector for ®eld trials. With respect to such applications, QCL technology represents a signi®cant step forward towards the miniaturization of spectroscopic sensing systems. This is of particular importance for the deployment of chemical sensing systems in harsh environments, such as the deep sea. The availability of QCLs creates the basis for a wide range of applications for laser spectroscopic methods in the MIR spectral range. Laser based sensing systems offer several advantages compared to conventional systems with black body sources: (i) sensitivity in the sub-parts per billion range, (ii) selectivity given by narrow spectral line width, (iii) dynamic range with real time response, which is linear over several orders of magnitude, (iv) low maintenance (no consumables), (v) small dimensions, (vi) possibility of hybrid integration, (vii) mechanical stability and robustness. Current works are focused on the design and development of QCL based sensing systems for the determination of target compounds, such as methane, aromatic hydrocarbons and chlorinated hydrocarbons in a marine environment. A scheme of a proposed
L. Hvozdara et al. / Vibrational Spectroscopy 30 (2002) 53±58
57
Fig. 3. Scheme of a sensing system using a multi-pass ATR slab waveguide, a QCL and an array of microbolometers as detecting element. Absorption is detected as a vector of values simultaneously picked up from the microbolometers. The signal is evaluated by exponential ®t to the vector of values. The system is therefore not sensitive to intensity drifts of the light source (note that all three curves originate at a different point). The system has a high dynamic rangeÐsmall absorptions can be easily detected (curve A), as well as stronger absorptions (curve B). Very strong absorptions causing total extinction of the radiation at certain sections of the crystal (curve C) can still be detected, since absorption path in the ATR crystal is variable.
system is shown in Fig. 3. The key elements involve a QCL as a radiation source, a multi-pass ATR slab waveguide, an array of microbolometers or similar arrayed MIR detectors, and digital signal processing (DSP) electronics. All sensor components are hybridintegrated on one substrate. The analyte absorbance is detected as a vector quantity and evaluated from the exponential ®t to signals from the microbolometers. This system is not sensitive to drift effects of the input light intensity. Hence, ideally it does not require re-calibration. The dynamic range is broader than it would be in case of a single pass cell. Due to the multiplicity of microbolometers, the absorption ATR path is virtually variable (Fig. 3). Hybrid integration makes the system robust and reliable for long-term deployment at harsh conditions. Furthermore, it enables the integration of various other sensors for complementary data acquisition. 4. Conclusion The early results described in this contribution indicate that MIR spectroscopy and laser spectroscopy provides a viable method with a high application potential for chemical sensor technology. Further developments in our research group are aimed towards the construction of integrated laser spectroscopic chemical sensing systems using QCLs. A signi®cant
improvement of the sensitivity and the dynamic range, as well as of the overall stability and performance of such sensing systems is expected. Acknowledgements Financial support of the US Department of Energy, the European Union and the Austrian Federal Ministry of Education, Science and Culture (BMBWK) is gratefully acknowledged, as well as the excellent cooperation with our partners in the SOFIE project. Bruker Optik GmbH (Ettlingen/Germany) is thanked for continuous support during the spectrometer reconstruction. The authors acknowledge A. Katzir and his research group at the Tel-Aviv University (Israel) for their support with silver halide ®bers, and E. Gornik and his team at the Vienna University of Technology (Austria), J. Faist and his group at the University of Neuchatel (Switzerland) and Antoine Mueller, Alpes Lasers (Neuchatel, Switzerland) for QCL technology. References [1] D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, M. Hans, Appl. Phys. Lett. 78 (2001) 1964. [2] B. Mizaikoff, Proc. SPIE 3849 (1999) 7. [3] R.F. Kazarinov, R.A. Suris, Sov. Phys. Semicond. 5 (1971) 707.
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