University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2004 • 134-140
Development of Quantum-Cascade Laser Based Biosensor Technology
Abstract--The principal objective of this project is the development of new types of sensitive, selective, real-time gas sensors based on continuous wave quantum and interband cascade lasers for various chemical sensing applications ranging from medical diagnostics to monitoring spacecraft air quality. Tunable laser absorption spectroscopy in the mid-infrared spectral region is a sensitive analytical technique for trace gas quantification. During the past year, researchers developed and validated a quartz-enhanced photoacoustic spectroscopy-based formaldehyde sensor using an interband cascade laser provided by NASA-JPL.
The recent realization of distributed feedback (DFB) interband cascade lasers (ICLs)1 has made it possible to access wavelengths between 3.0 and 4.5 mm,2,3 a spectral region which has been difficult to cover with intraband quantum cascade lasers (QCLs). ICLs employ transitions between the conduction and valence bands as in bipolar diode lasers, but instead of losing an electron to the valence band, it is recycled through interband tunneling into the conduction band of the next cascade stage. This is made possible with the type-II broken gap alignments in InAs/GaInSb quantum well structures. Because the conduction and valence bands have opposite dispersion curvatures, fast phonon scattering loss is circumvented in ICLs, resulting in a more efficient operation with a low threshold current density. The emission wavelength of ICLs can be tailored in a wide spectral range, particularly on the shorter wavelength side due to a large band offset between their constituent materials. Presently continuous wave (cw) operation of ICLs is obtained at cryogenic temperatures. However, there are no theoretical limitations preventing near-room temperature operation. Such devices will be realized as technical issues are resolved. Continuous wave (cw) operation with thermoelectric rather than cryogenic cooling has already been demonstrated for QCLs.4,5
The DFB-ICL used in this work emitted near 3.5 mm. This spectral region is important for various gas sensing applications because it corresponds to a spectral range of the C-H stretch vibration of aldehydes (e.g., formaldehyde symmetric stretch) and alkanes (e.g., methane). Formaldehyde (H2CO) is of particular interest since it is a hazardous and carcinogenic substance, which is released from chemical binders present in numerous manufactured items and hence its presence in the environment cannot be avoided. The Occupational Safety and Health Administration (OSHA) has issued general industrial standards with an upper limit of 0.75 ppm for long term exposure (8 hours time-weighted average) and 2 ppm for short term exposure (15 minutes).6 NASA has also established spacecraft maximum allowable concentration levels for crew exposure to H2CO for extended periods of time.7H2CO has been identified as a potential biomarker in breath analysis of human subjects. For example, in exhaled breath from breast cancer patients, concentration levels of 1.2 ppmv were observed.8 Furthermore, H2CO is an important and reactive component present in all regions of the atmosphere arising from the oxidation of biogenic and anthropogenic hydrocarbons. Tropospheric H2CO concentration measurements provide a means of validating photochemical model predictions concerning hydrocarbon oxidation that are used to advance ozone chemistry.9,10
To quantify H2CO concentrations, several different chemical11,12 and physical detection methods are used. Chemical analyzers, which employ coloration of a formaldehyde-sensitive reagent are sensitive at ppbv levels, but they suffer from cross interference effects by other aldehydes and require long sampling times (i.e., minutes or more). To overcome these limitations, laser based spectroscopic sensors have been developed. Several different tunable, cw laser sources have been employed to access H2CO absorption lines, such as lead-salt lasers,9 difference frequency generation (DFG)13-15 sources, and CO overtone gas lasers.16 Optical parametric oscillators (OPOs)17,18 and solid-state lasers19 are also capable of addressing H2CO absorption lines. The best H2CO detection sensitivity reported9 (< 50 pptv) was achieved using lead salt diode laser based absorption spectroscopy in a multipass optical cell with an effective optical pathlength of 100 m.
Currently, ICLs provide optical output powers of ~ 10 mW, a value which is expected to increase in the future. While the sensitivity of direct absorption spectroscopy does not fundamentally depend on laser power (neglecting the shot-noise limit), other approaches can make use of high spectroscopic source power to lower the detection limits. Photoacoustic spectroscopy (PAS) is one such approach. PAS sensitivity scales linearly with the available laser power. A novel modification of PAS, called quartz-enhanced PAS (QEPAS) also permits matching the size of the laser source and the absorption detection module (ADM), both with a ~ 1 cm characteristic linear dimension.
In this work, we report the development and performance characteristics of a formaldehyde sensor using a cw DFB-ICL and QEPAS. QEPAS is based on the photoacoustic signal build-up in a high-Q piezoelectrically active quartz crystal instead of a low-Q gas filled resonator as in traditional PAS. It has advantages of a very small sample volume (~ 1 mm3) required for analysis and high immunity to ambient acoustic noise.20 The sensitivity of QEPAS to a particular trace species is strongly dependent upon the V-T relaxation rate of this species in a certain host gas. If the rate is too slow, the thermal response giving rise to the detected pressure waves cannot follow the modulation frequency. A theoretical analysis of this rate is difficult and hence an experimental investigation is required to evaluate the sensitivity of the QEPAS to specific chemical species. In this work we performed such studies for H2CO detection in dry nitrogen, dry nitrogen with 5% of SF6 and normal 50% humidity room air.
ICL Parameters and H2CO Absorption Line Selection
The ICL utilized in this work is able to operate continuous wave at temperatures up to
170 K. The laser bias voltage is ~ 7-8 V and the threshold current varies from
3 to 22 mA as the temperature changes from 78 K to 170 K. The laser
frequency can be tuned continuously from 2833 cm-1
(T = 78 K) to 2816 cm-1 (T = 170 K) in a
single frequency mode by means of current and temperature tuning. The temperature tuning
rate is 0.178 cm-1 / K. At a fixed heat-sink temperature of 78 K, the
laser emits up to 12 mW and the output is tunable from 2831.8 cm-1 to
2833.7 cm-1 by current with a tuning rate of 0.0327 cm-1/mA.
The low threshold current operation of ICLs requires a current source with relatively high
resolution. The linewidth of a similar IC laser operating at 3.3 mm was measured to
be < 20 MHz.21
With the available ICL operating at 78 K it is possible to access four significant formaldehyde absorption lines (see Fig. 1). For H2CO measurements in ambient air at concentrations < 50 ppbv, a line at 2833.191 cm-1 (line # 4) is favorable because it is free from interference by water and methane. However, this line was not selected because the ICL power at this frequency is approximately two times smaller than the powers at the spectral positions of other available absorption lines. This results in a weaker QEPAS signal, because the QEPAS signal scales with excitation power. Line #1 at 2832.146 cm-1 is a superposition of two lines and provides a smaller QEPAS signal than the absorption line #2 at 2832.483 cm-1 selected for this study. Line #2 was accessed with an ICL drive current of 42 mA. The optimum H2CO absorption line at 2831.642 cm-1 used in.9,13-15 is also accessible with this ICL at heat sink temperatures > 78 K.
Figure 1. HITRAN 2000 simulation of four H2CO absorption lines at a total pressure of 200 Torr and 1 ppmv concentration. The arrow indicates the ICL current tuning range. Increasing current causes frequency to decrease.
Technical Plan and Equipment
The H2CO sensor is depicted schematically in Fig. 2. The
QEPAS ADM consisting of a crystal resonator/transducer and acoustic micro-resonator has
been described in earlier publications.20,22,23 Briefly, a watch tuning fork
(TF) is used as the photoacoustic transducer. Two glass tubes (2.54 mm in length and
320 mm inner diameter) are arranged on both sides of the TF forming an acoustic
microresonator, which enhances the QEPAS signal by a factor of ~ 10. The piezoelectric
current generated by the excitation of the TF by photoacoustic wave action is converted to
a voltage by a transimpedance amplifier (feedback resistor = 10 MW). ADM is placed
into a gas cell with a volume of ~ 1 cm3. Outer dimensions of the cell with
electrical feedthroughs, pressure gauge, swagelock fittings and transimpedance amplifier
mounted on the ISO-KF flange are 2.5" × 1.5" × 2.5".
Figure 2. Schematic of ICL based QEPAS H2CO sensor architecture: (L) Lens, (BS) Beamsplitter, (RC) Reference Cell, (SW) Sapphire windows, (Perm) Permeation, (Preamp) Preamplifier, (Harm.) Harmonic. The reference frequency supplied to both lock-in amplifiers from the function generator is half the resonance frequency of the QEPAS tuning fork. A transimpedance amplifier connected to the TF is not shown. The dashed line indicates an ultra pure PFA (perfluoroalkoxy) tubing.
The ICL is mounted inside a liquid nitrogen (LN2) cooled cryostat (Cryo Industries, Inc). The ICL radiation (wavelength modulated at half the TF frequency of f ~ 32 kHz) is focused into ADM with a ZnSe lens (focal length 12.5 mm, 25 mm diameter). The QEPAS signal is subsequently demodulated by a lock-in amplifier (Stanford Research Model SR830 DSP) at frequency f (2nd harmonic detection) and processed by a laptop computer. A CaF2 beamsplitter directs five percent of the ICL beam through a short reference cell (5 cm long) filled with para-formaldehyde (which sublimes to H2CO with a vapor pressure of 1.45 Torr at 300 K). A photodetector signal is demodulated by a second lock-in amplifier at the 3rd harmonic of the f/2 reference and used as a feedback to lock the laser frequency to the H2CO absorption line via a proportional adjustment of the ICL current offset.
A gas standard generator (Kin-Tek model 491M) based on a permeation tube was used to provide H2CO concentrations ranging from 0.5 to 25 ppmv in a diluting gas (i.e., N2 or air). The QEPAS cell pressure was maintained constant by means of a pressure controller (MKS Instruments Type 659), which also measured the gas flow (fixed at 75 sccm for all measurements with a needle valve located after the QEPAS cell). Since formaldehyde is a sticky molecule with a high dipole moment (2.3 D), ultra pure PFA (perfluoroalkoxy) tubing was employed in the gas flow system. This type of tubing has a smooth interior surface, thereby reducing adsorption of H2CO to the tubing walls. The QEPAS signal showed no dependence on the flow rate.
Experimental Activity
For good performance, it is essential to optimize two QEPAS parameters, namely the sampled
gas pressure and the laser current modulation depth.20,22,23 For this sensor
the optimum gas pressure inside the QEPAS cell was found to be 200 Torr, with a TF
resonance frequency of f = 32760 Hz and a Q-factor of 16725. The
optimum ICL current modulation amplitude at 200 Torr sample gas pressure was 4 mA
corresponding to 0.13 cm-1 and roughly matching approximately two times
the collision-broadened FWHM (0.06 cm-1 at 200 Torr) of the selected
absorption line, similar to the previous QEPAS studies.24 The addition of 5% SF6
to the N2 diluting gas enhanced the QEPAS signal by a factor of two.
Ambient air was also found to result in a factor of 1.5 higher QEPAS signal when it was
used as a diluter in place of dry N2. This effect is most likely due
to its water content and the related increase of V-T energy transfer rate (the signal
enhancement is not due to H2O absorption as this is one order
of magnitude lower than that of H2CO).
A spectral scan performed across the H2CO absorption line at 2832.5 cm-1 (optimum selected line #2 as stated above in section 2) at the experimental parameters is shown in Fig. 3. The H2CO concentration was set to 17.7 ppmv, and ambient air was employed as the diluting gas. The ICL frequency was scanned in 20 MHz steps. The lock-in time constant was 1 second with a 3-second delay between two consecutive measurements. The minimum detection sensitivity of the sensor was evaluated from the 1 s-noise of the background and was determined to be 2.7 × 10-8 cm-1 × W × (Hz)-1/2 for an IC laser power of 3.4 mW inside the cell (limited by the required spatial filtering of the applied laser beam and optical losses from the ZnSe lens and the sapphire windows of the QEPAS cell) and a peak formaldehyde absorbance of 6.6 × 10-5 cm-1. This photoacoustic figure of merit is in reasonable agreement with the results of a recent near-infrared QEPAS based trace gas measurements.23
Figure 3. 2nd harmonic QEPAS scan of the H2CO absorption line at 2832.48 cm-1 (line #2 depicted in Fig.2). The H2CO concentration was 17.7 ppmv, the QEPAS cell pressure was 200 Torr. Ambient air was used as diluting gas.
Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the H2CO concentration (see Fig. 4). The ICL was locked to the formaldehyde line at 2832.483 cm-1. The gas standard generator was used to produce H2CO concentrations in steps from 2 to 20 ppmv with ambient air as the diluting gas. The QEPAS signal for each concentration step was measured every 30 s for a total time duration of 10 minutes. The lock-in time constant was set to t = 10 s for these measurements. The results are depicted in Fig. 4a. Data for each step were averaged and a calibration curve obtained (Fig. 4b) using the formaldehyde concentrations derived from data sheets of the permeation tube. The results confirm that the QEPAS signal is proportional to the H2CO concentration.
Figure 4. H2CO monitoring for different concentrations in flow conditions. (a) Experimental data (dots) and calculated values (line). (b) Calibration curve obtained from measured QEPAS signals and known H2CO concentration values from data supplied by permeation tube vendor (Kin-Tek).
The fundamental limit of the QEPAS-based spectrometer is determined by the thermal noise of the TF, which can be calculated theoretically25 as
 |
(1) |
where 
is the RMS voltage noise
observed at the transimpedance amplifier output (with a feedback resistor Rfb = 10 MW), Df is the detection
bandwidth, kB is the Boltzmann constant, T = 300 K is the
temperature, and R = 68 kW is the measured TF
electrical resistance at the resonant frequency and 200 Torr gas pressure. If the lock-in
amplifier time constant is t, then Df
= 1/pt. The noise in one quadrature component is
 |
(2) |
The noise calculated using the Eqs. (1), (2) at the conditions corresponding to the
measurements presented in Fig. 4 is 
=
0.62 mV. The scatter of points in Fig. 4 gives = 0.68 mV,
which agrees with the theoretical thermal noise limit within the uncertainty of
measurements. Thus, noise is not related to the laser radiation and, therefore, the
detection limit is expected to be improved proportional to the laser power.
Conclusions
In summary, we have demonstrated the feasibility of using a novel cw 3.53 mm DFB ICL as a spectroscopic source for a QEPAS based sensor to
detect formaldehyde concentrations at sub-ppmv levels. The measured detection sensitivity
of the sensor is 2.2 × 10-8 cm-1 × W × (Hz)-1/2 which
is in agreement with other QEPAS-based gas analyzers.20,23 This detection
sensitivity corresponds to 0.6 ppmv H2CO concentration in air
with 3.4 mW of laser power delivered to the QEPAS ADM and a time resolution of
10 seconds. It is expected that future ICL devices will be capable of emitting ~100
mW or more optical power. Assuming that such ICL performance is achieved and all the
emitted power is delivered to the ADM, the QEPAS based H2CO
detection sensitivity would be ~ 25 ppbv. Further gain in performance is anticipated with
optimization of the microresonator design and by using lower frequency TFs. Thus, the
combination of a powerful thermoelectrically cooled ICL and QEPAS ADM provides the
technologies for compact field-deployable trace gas sensors.
Acknowledgments
We acknowledge the assistance of Dr. Anatoliy Kosterev and Markus Horstjann in providing
support and numerous useful discussions. The authors acknowledge Dr. R. Yang of NASA -JPL
for providing the interband cascade laser used in this work. Financial support of the work
performed by the Rice group was provided by the Institute for Space Systems Operations
(ISSO), the National Aeronautics and Space Administration (NASA), the Texas Advanced
Technology Program, the National Institute of Health via a sub contract from Physical
Sciences, Inc. the Office of Naval Research via a sub-award from Texas A&M University
and the Welch Foundation.
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Publications
Bakhirkin, Y. A., A. A. Kosterev, C. Roller, R. F. Curl, and F. K. Tittel.
"Mid-Infrared Quantum Cascade Laser Based Off-Axis Integrated Cavity Output
Spectroscopy for Biogenic NO Detection," Appl. Opt. 43 (2004);
2257-66.
Horstjann, M., Y. A. Bakhirkin, A. A. Kosterev, R .F. Curl, and F. K. Tittel.
"Formaldehyde Sensor Using Interband Cascade Laser-Based Quartz-Enhanced
Photoacoustic Spectroscopy," Appl. Phys. B 79 (2004): 799-803.
Kosterev, A. A., Y. A. Bakhirkin, F. K. Tittel, S. Blaser, Y. Bonetti, and L. Hvozdara.
"Photoacoustic Phase Shift as a Chemically Selective Spectroscopic Parameter," Appl.
Phys. B, Rapid Communications 78 (2004): 673-76.
Presentations
Bakhirkin, Y. A., M. Horstjann, A. A. Kosterev, and F. K. Tittel. "Interband Cascade
Laser Based Formaldehyde Sensor Using Quartz-Enhanced Photoacoustic Spectroscopy,"
Optoelectronics 2005, Photonics West, San Jose, CA, Jan. 22-27 2005.
Kosterev, A. A., Y. A. Bakhirkin, and F .K. Tittel. "Quartz-Enhanced Photoacoustic
Spectroscopy with Semiconductor Lasers: the Road to Ultracompact Trace Gas Sensor,"
Novosibirsk, Russia, 4th International Symposium on Modern Problems of Laser Physics,
(MPLP 2004), Novosibirsk, Russia, Aug. 22-27, 2004.
Tittel, F. K., Y. Bakhirkin, R. F. Curl, A. A. Kosterev, S. So, D. Weidmann, and G.
Wysocki. "Development of Quantum Cascade Laser-Based Sensor Technology for Trace Gas
Monitoring Applications," 5th Quantum Cascade Laser Workshop, Fraunhofer Institute
for Physical Measurement Techniques, Freiburg, Germany, Sept. 23-24, 2004.
Tittel, F. K., A. A. Kosterev, and Y. Bakhirkin. "Recent Advances in Quartz-Enhanced
Gas-Phase Photoacoustic Spectroscopy," 3rd IEEE Intl. Conference on Sensors, Vienna,
Austria Oct. 24-27, 2004.
Tittel, F. K., A. A. Kosterev, Yu. Bakhirkin, G. Wysocki, C. Roller, S. So, D. Weidmann,
and R. F. Curl. "Recent Advances of Trace Gas Sensors Based on Diode and Quantum
Cascade Lasers," 2004 Annual IEEE LEOS Meeting, Rio Mar, Puerto Rico, Nov. 7-11,
2004.
Tittel, F. K., Y. Bakhirkin, R. F. Curl, A. A. Kosterev, S. So, D. Weidmann, G. Wysocki.
"Recent Developments of Quantum Cascade Laser-Based Trace Gas Sensor Technology:
Opportunities and Challenges," Optoelectronics 2005, Photonics West, San Jose, CA,
Jan. 22-27, 2005.
Wysocki G., Y. A. Bakhirkin, A. A. Kosterev, S. So, D. Weidmann, and F. K. Tittel.
"Sensitive Trace Gas Detection with Quantum Cascade Lasers," IEEE/ LEOS Seminar,
Wroclaw, Poland, Dec. 22, 2004
Funding and Proposals
Tittel, F. K. "Quantum Cascade Laser Based Sensors for Chemical and Environmental
Analysis," Texas Advanced Technology Program, Rice University PI., Jan. 1, 2003-Aug.
30, 2005, $187,380.
--. "Photonic Technologies for Early Detection of Human Disease," NASA-National
Cancer Institute, April 1, 200-June 30, 2005, $367,515.
--. "Quantum Cascade Laser Photoacoustic Sensor for Chemical Warfare Agent
Detection," Pacific Northwest National Laboratory, Jan. 3, 2005-Dec. 31, 2005,
$75,000.
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Table of Contents
Institute for Space Systems Operations - Y2004 Annual
Report
Copyright © 2005
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