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IR Raman spectroscopy with semiconductor devices for excitation and detection
Volume 62, number 5
OPTICS COM MU N IC A TIO N S
1 June 1987
IR R A M A N S P E C T R O S C O P Y W I T H S E M I C O N D U C T O R D E V I C E S FOR EXCITATION AND DETECTION A. Rubens B. DE CASTRO and Paulo R.B. P E D R E I R A IFG f f UNICA MP, 13081 (~ampinas, SP Brazil Receixed 22 January 1987
We describe a Raman spectrometer with no moving parts based on a GaAIAs single longitudinal mode diode laser for excitation and a cooled Si diode array fopr detection. The measured performance was a factor of three worse than calculated. The calculated performance can be enhanced by more than three orders of magnitude with a few practical improvements and more powcrRd laser sources.
Raman spectroscopy is a sensitive probe for physical and chemical analysis [ l ]. It has not been used in routine assays mainly because the light source for excitation is bulky, the photometric equipment requires precision alignment and is mechanically fragile, data acquisition is slow and the whole system is costly. However, most of these disadvantages can be cirumvented by using solid state techniques. Technical goals for a R a m a n field-instrument would be as follows: spectral sensitivity covering the visible and near IR range, low dark noise, compact size, low power consumption, no moving parts, computer controlled for ease of automated operation, fast data aquisition capability, moderate cost. We report on the performance of a Raman spectrometer using a GaAIAs single longitudinal mode laser as source of excitation and a Si diode as the detector. While the system we built is not sufficiently fast for practical applications, it illustrates quantitatively a number of problems, some of which we solved. Fig. 1 is a block diagram of the spectrometer. Three different types of GaA1As single longitudinal mode lasers have been obtained for this work: (a) RCA large-optical-cavity constricted double heterostructure [2] samples (10 mW at 820 n m ) : (b) NEC buried coarctate mesa structure [3] samples (30 m W at 760 nm); (c) a commercial EOC laser purchased from General Optronics (30 m W at 860 nm). The spectrum shown here was obtained with the latter one. 348
•
S A M P L E CELL
LASER ]
DFSPERSION DIODE A R R A Y
2ON TROL 8,
1
STORAGE
,
L
T VIDEO ~ TERMINAL"
3RAPHIE DISPLAY
TO VAX
Fig. 1. Block diagram of the spectrometer. The GaAIAs laser diode junction plane {and the polarization of the electric field) are orthogonal to lhc scattering plane. The diode array is cooled to 40 C with a solid stale heat pump.
All of these lasers had to be injection current tuned for single mode operation. Mode stability was maintained for periods of over one hour. The laser emission covers in all cases a large solid angle. Collimating and delivering this light for Raman scattering in liquids in the 90 degree geometry poses some difficulties. We tried a Zeiss Neofluar microscope objective ( N A = 0 . 7 5 ) placed at a short distance from the laser facet. This arrangement collected more than 60% of the laser output, generating a narrow parallel beam, but was unacceptable because the reflections induced multi-mode lasing.
0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 62, number 5
OPTICS COMMUNICATIONS
Very high quality line foci have been obtained exploiting spherical aberration in off-center spherical reflectors [ 4 ]. We attempted a similar agreement to focus the laser light in the center of a long cylindrical cell, but this required abandoning the 90 degree scattering geometry and there appeared undesirable reflections of laser light into the detection system. However, by using a rectangular sample cell with flat faces and placing the laser faces very close to the bottom of the cell, the same spherical reflector (radius of curvature 30 mm; edges 30 r a m × 5 0 m m ) could be used in a most convenient way to collect 100% of the laser output and deliver about 60% of this power in a narrow waisted beam along the axis of the sample cell. The efficiency was limited by surface roughness of the reflector machined out of low cost brass (afterwards found to be somewhat porous), and reflectivity losses in the optical coating (Cu overlaid with SiO). The whole sample/lager assembly, including some bulky XYZ positioners, is only 12 cm × 20 cm × 3 0 cm. Given the use of a very low power IR light source, alignment is tricky. We found it useful to fill the sample cell with dilute soap water, which scatters brightly. The beam can then be seen with an IR viewer. After the system is aligned, the cell .with soap water is replaced with the sample cell and one begins the Raman exposure. Dispersion would ideally be provided by a large aperture "corrected" concave holographic grating [ 5 ]. We had a standard Jobin Yvon type IV concave holographic grating with N A = 0 . 2 2 which we used succesfully in the visible. In the near IR this grating suffered from severe astigmatism and other aberrations. We did ascertain via detailed computer simulations that a practical concave holographic grating with large NA can be designed and manufactured to image the spectral range 860 to 960 nm in a focal plane array detector with adequate resolution and negligible losses due to astigmatism. We have not yet bad it manufactured, hence in this work we used a classical plane grating (600 g/mm, blazed at 750 nm) in a Czerny-Turner mount. The detector was a E G G Reticon RL1024S Si diode array thermo electrically cooled to - 40 ° C with a Marlow Industries solid state heat p u m p model SP 1056. The heat p u m p was driven by a proportional temperature controller. The diode array was operated in the boxcar mode.
I June 1987
Clocks and other digital signals were generated under microcomputer control by a parallel I/O buffer chip. The video signals (even and odd) were processed analogically with DC restore [6], then sampled and held during the time needed for 12 bit AD conversion and finally stored in m e m o r y (16 bit wide, 1024 channels) at a rate of 13 kHz, limited in the case by the time needed to write in the parallel buffer chip generating the array control signals. This could be shortened by one order of magnitude employing fancier techniques, but the overall readout time of 80 ms is adequate for our purposes. All interface and control electronic boards were, as a consequence of a very tight budget, built in this laboratory. The control microcomputer was also built in the laboratory with VLSI components of the 6800 family. It has 16 kbytes of dynamic RAM, floppy disk, many ports of parallel I/O, a graphic display system and a serial link to a VAX 11/780 computer. The performance of the detection system was estimated in two ways. Total read-out RMS noise was measured to be about 2000 electronic charges. The theoretical limit is set by diode capacitance recharge noise and would be 700 electronic charges. We attribute our high noise to non ideal substrate supply voltage and non ideal noise performance in the video front-end. On the other hand, using our concave holographic grating and a krypton laser for excitation in the visible, and defining suitable spectral figures of merit, we could compare our spectra to those obtained by Surbeck and co-workers [7]. We find that the performance of our system is quite comparable to theirs. Fig. 2 shows a spectrum of toluene obtained with less than 8 m W of excitation at 860 nm. It was smoothed according to the rule s(k) = [ u ( k - 2 ) +u(k+ 2)]/8+ [u(k- 1) +u(k) +u(k+ 1)]/4, k = 3 , 4 .... 1022 where u(k) is an unsmoothed spectral intensity and s(k) is the corresponding smoothed point. The coefficients were chosen for convenience in assembler programming only. The effect of this averaging on resolution is totally negligible in our case, while the theoretical enhancement of the signal to RMS noise ratio is 2.14. This spectrum should be quantitatively discussed. Assuming 8 m W of radiant power at the beam waist (diameter 0.2 m m ) , the excitation photon flux is 1.1 × 10-~°/cm2 s. The absolute scattering cross section for the toluene lines near 1000 cm ~ can be 349
Volume 62, number 5 2000
OPTICS COMMUNICATIONS
.
i
1500 i t z
o£D )- 1 0 0 0
j I
i
,,
Ld 500
800
1000 & k ( c m ')
1200
Fig. 2. Raman spectrum of toluene, recorded in the range 920 to 9?0 nm. Total exposure time was 300 s. Excitation was lcss than
8 mW cw at 860 nm. Resolutionwas 8 cm ~,limited by slit width. 1 memory count corresponds to 760 electronic charges.
deduced from the known absolute scattering crosssection for the benzene line at 992 cm 1 [8] and a comparison of benzene and toluene recorded under the same conditions. We take the value 6 × 10 3, cm 2. The linear magnification of the collection optics was 1.6; the slit was effectively 2.5 m i n X 0 . 3 ram. The sample active region is therefore a cylinder 1.6 m m high 0.094 m m in diameter, and the sampled volume is less than 400 picoliters. The collection solid angle (calculated inside the liquid) is 23X 10 4 st tad. The calculated n u m b e r of R a m a n photons available for detection is 33000/s. Total losses in collection lenses, windows, grating and the reduction in detector sensitivity in the IR result in an estimated attenuation of 2.3 times; also, this flux is spread out over approximately 10 sensors. Hence the expected sensor discharge rate is 1650 electrons/s. Now, the total exposure time for the toluene spectrum shown was 3000 s, and the measured peak intensity corresponds to 1.5XI06 electrons, which is only 1/3 of the expected figure. We think this indicates n o n - o p t i m a l collection and non-uniform illumination of the active region. The signal to noise ratio is also m a n y times lower than theoretically expected. The excess noise might be related to the procedures for acquisition and subtraction of spectral background. This actual performance (which is several times worse than theoretical expectations), can be enhanced in m a n y ways. Delivery of laser power to 350
1 June 1987
the sample could be made more efficient by a factor of 1.6 with improved reflector surface. Laser light intensity could be increased by two orders of magnitude replacing the single laser diode with a high power laser array. Developmental models recently offered for sale give cw outputs of 500 m W but are not single longitudinal mode; single longitudinal mode operation at over 60 m W cw output power has been demonstrated in laboratory operation [9]. It is feasible to double the acceptance angle of the monochromator, which would increase the collection solid angle fourfold. A total detector noise of 900 electrons has been reported by Surbek et al. [7], hence with i m p r o v e m e n t s in the video preamplifiers another factor of 2 can be obtained. The expected overall i m p r o v e m e n t factor is 1200. We are currently exploring these various avenues. We thank Dr. D. Botez for kindly supplying the RCA C D H laser samples, Dr. K. Endo for the NEC BCM units, Dr. J.H. Nicola for lending us a precision sample cell and Mr. R.W. Sprogis for assembling the electronic boards. The reflector was machined at the I F G W machine shop, polished by Ms. A.C. Silva Almeida and Dr. G. Mendes, coated by Ms. C. Diogo and Dr. Z.P. Arguello. We acknowledge financial help from FAPESP, C N P Q and FINEP.
References [ 1] E. Mazur, Rev. Sci. lnstrum. 57 (1986) 2507: N.A. Marie}, C.K. Mann and T.J. Vickers, Appl. Spectr. 39 (1985) 268. [2] D. Botez, Appl. Phys. Lett. 36 (1980) 190. [ 3 ] K. Endo, H. Kawano,M. Nido, Y. Kuwamura,T. Furuse and 1, Sakuma, Proc, Ninth Intern. Semiconductor Laser Conf. (Rio de Janeiro, Brazil, Aug 1984) p. 38. [4] 1.N. Ross and E.M. Hodgson, J. Phys. E: Sci. lnstrum. 18 (~985) 169. [5] H.H. Schlcmmer and M. Machler, J. Phys. E: Sci. lnstrum. 18 (1985) 914. [6] G.R. Hopkinson and D.H. Lumb, J. Phys. E: Sci. Instrum. 15 (1982) 1214. [7] H. Surbeck. W. Hug, M. Cremaud, M. Bridoux, A. Deflbntrine and E. da Silva, Optics Comm. 38 (1981) 57. [8] T.C. Damen, R.C.C. Leite and S.P.S. Porlo, Phys. Rev. Lett. 14 (1965) 9. [9] D.E. Ackley.Appl. Phys. Left. 42(1983) 152.
OPTICS COM MU N IC A TIO N S
1 June 1987
IR R A M A N S P E C T R O S C O P Y W I T H S E M I C O N D U C T O R D E V I C E S FOR EXCITATION AND DETECTION A. Rubens B. DE CASTRO and Paulo R.B. P E D R E I R A IFG f f UNICA MP, 13081 (~ampinas, SP Brazil Receixed 22 January 1987
We describe a Raman spectrometer with no moving parts based on a GaAIAs single longitudinal mode diode laser for excitation and a cooled Si diode array fopr detection. The measured performance was a factor of three worse than calculated. The calculated performance can be enhanced by more than three orders of magnitude with a few practical improvements and more powcrRd laser sources.
Raman spectroscopy is a sensitive probe for physical and chemical analysis [ l ]. It has not been used in routine assays mainly because the light source for excitation is bulky, the photometric equipment requires precision alignment and is mechanically fragile, data acquisition is slow and the whole system is costly. However, most of these disadvantages can be cirumvented by using solid state techniques. Technical goals for a R a m a n field-instrument would be as follows: spectral sensitivity covering the visible and near IR range, low dark noise, compact size, low power consumption, no moving parts, computer controlled for ease of automated operation, fast data aquisition capability, moderate cost. We report on the performance of a Raman spectrometer using a GaAIAs single longitudinal mode laser as source of excitation and a Si diode as the detector. While the system we built is not sufficiently fast for practical applications, it illustrates quantitatively a number of problems, some of which we solved. Fig. 1 is a block diagram of the spectrometer. Three different types of GaA1As single longitudinal mode lasers have been obtained for this work: (a) RCA large-optical-cavity constricted double heterostructure [2] samples (10 mW at 820 n m ) : (b) NEC buried coarctate mesa structure [3] samples (30 m W at 760 nm); (c) a commercial EOC laser purchased from General Optronics (30 m W at 860 nm). The spectrum shown here was obtained with the latter one. 348
•
S A M P L E CELL
LASER ]
DFSPERSION DIODE A R R A Y
2ON TROL 8,
1
STORAGE
,
L
T VIDEO ~ TERMINAL"
3RAPHIE DISPLAY
TO VAX
Fig. 1. Block diagram of the spectrometer. The GaAIAs laser diode junction plane {and the polarization of the electric field) are orthogonal to lhc scattering plane. The diode array is cooled to 40 C with a solid stale heat pump.
All of these lasers had to be injection current tuned for single mode operation. Mode stability was maintained for periods of over one hour. The laser emission covers in all cases a large solid angle. Collimating and delivering this light for Raman scattering in liquids in the 90 degree geometry poses some difficulties. We tried a Zeiss Neofluar microscope objective ( N A = 0 . 7 5 ) placed at a short distance from the laser facet. This arrangement collected more than 60% of the laser output, generating a narrow parallel beam, but was unacceptable because the reflections induced multi-mode lasing.
0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 62, number 5
OPTICS COMMUNICATIONS
Very high quality line foci have been obtained exploiting spherical aberration in off-center spherical reflectors [ 4 ]. We attempted a similar agreement to focus the laser light in the center of a long cylindrical cell, but this required abandoning the 90 degree scattering geometry and there appeared undesirable reflections of laser light into the detection system. However, by using a rectangular sample cell with flat faces and placing the laser faces very close to the bottom of the cell, the same spherical reflector (radius of curvature 30 mm; edges 30 r a m × 5 0 m m ) could be used in a most convenient way to collect 100% of the laser output and deliver about 60% of this power in a narrow waisted beam along the axis of the sample cell. The efficiency was limited by surface roughness of the reflector machined out of low cost brass (afterwards found to be somewhat porous), and reflectivity losses in the optical coating (Cu overlaid with SiO). The whole sample/lager assembly, including some bulky XYZ positioners, is only 12 cm × 20 cm × 3 0 cm. Given the use of a very low power IR light source, alignment is tricky. We found it useful to fill the sample cell with dilute soap water, which scatters brightly. The beam can then be seen with an IR viewer. After the system is aligned, the cell .with soap water is replaced with the sample cell and one begins the Raman exposure. Dispersion would ideally be provided by a large aperture "corrected" concave holographic grating [ 5 ]. We had a standard Jobin Yvon type IV concave holographic grating with N A = 0 . 2 2 which we used succesfully in the visible. In the near IR this grating suffered from severe astigmatism and other aberrations. We did ascertain via detailed computer simulations that a practical concave holographic grating with large NA can be designed and manufactured to image the spectral range 860 to 960 nm in a focal plane array detector with adequate resolution and negligible losses due to astigmatism. We have not yet bad it manufactured, hence in this work we used a classical plane grating (600 g/mm, blazed at 750 nm) in a Czerny-Turner mount. The detector was a E G G Reticon RL1024S Si diode array thermo electrically cooled to - 40 ° C with a Marlow Industries solid state heat p u m p model SP 1056. The heat p u m p was driven by a proportional temperature controller. The diode array was operated in the boxcar mode.
I June 1987
Clocks and other digital signals were generated under microcomputer control by a parallel I/O buffer chip. The video signals (even and odd) were processed analogically with DC restore [6], then sampled and held during the time needed for 12 bit AD conversion and finally stored in m e m o r y (16 bit wide, 1024 channels) at a rate of 13 kHz, limited in the case by the time needed to write in the parallel buffer chip generating the array control signals. This could be shortened by one order of magnitude employing fancier techniques, but the overall readout time of 80 ms is adequate for our purposes. All interface and control electronic boards were, as a consequence of a very tight budget, built in this laboratory. The control microcomputer was also built in the laboratory with VLSI components of the 6800 family. It has 16 kbytes of dynamic RAM, floppy disk, many ports of parallel I/O, a graphic display system and a serial link to a VAX 11/780 computer. The performance of the detection system was estimated in two ways. Total read-out RMS noise was measured to be about 2000 electronic charges. The theoretical limit is set by diode capacitance recharge noise and would be 700 electronic charges. We attribute our high noise to non ideal substrate supply voltage and non ideal noise performance in the video front-end. On the other hand, using our concave holographic grating and a krypton laser for excitation in the visible, and defining suitable spectral figures of merit, we could compare our spectra to those obtained by Surbeck and co-workers [7]. We find that the performance of our system is quite comparable to theirs. Fig. 2 shows a spectrum of toluene obtained with less than 8 m W of excitation at 860 nm. It was smoothed according to the rule s(k) = [ u ( k - 2 ) +u(k+ 2)]/8+ [u(k- 1) +u(k) +u(k+ 1)]/4, k = 3 , 4 .... 1022 where u(k) is an unsmoothed spectral intensity and s(k) is the corresponding smoothed point. The coefficients were chosen for convenience in assembler programming only. The effect of this averaging on resolution is totally negligible in our case, while the theoretical enhancement of the signal to RMS noise ratio is 2.14. This spectrum should be quantitatively discussed. Assuming 8 m W of radiant power at the beam waist (diameter 0.2 m m ) , the excitation photon flux is 1.1 × 10-~°/cm2 s. The absolute scattering cross section for the toluene lines near 1000 cm ~ can be 349
Volume 62, number 5 2000
OPTICS COMMUNICATIONS
.
i
1500 i t z
o£D )- 1 0 0 0
j I
i
,,
Ld 500
800
1000 & k ( c m ')
1200
Fig. 2. Raman spectrum of toluene, recorded in the range 920 to 9?0 nm. Total exposure time was 300 s. Excitation was lcss than
8 mW cw at 860 nm. Resolutionwas 8 cm ~,limited by slit width. 1 memory count corresponds to 760 electronic charges.
deduced from the known absolute scattering crosssection for the benzene line at 992 cm 1 [8] and a comparison of benzene and toluene recorded under the same conditions. We take the value 6 × 10 3, cm 2. The linear magnification of the collection optics was 1.6; the slit was effectively 2.5 m i n X 0 . 3 ram. The sample active region is therefore a cylinder 1.6 m m high 0.094 m m in diameter, and the sampled volume is less than 400 picoliters. The collection solid angle (calculated inside the liquid) is 23X 10 4 st tad. The calculated n u m b e r of R a m a n photons available for detection is 33000/s. Total losses in collection lenses, windows, grating and the reduction in detector sensitivity in the IR result in an estimated attenuation of 2.3 times; also, this flux is spread out over approximately 10 sensors. Hence the expected sensor discharge rate is 1650 electrons/s. Now, the total exposure time for the toluene spectrum shown was 3000 s, and the measured peak intensity corresponds to 1.5XI06 electrons, which is only 1/3 of the expected figure. We think this indicates n o n - o p t i m a l collection and non-uniform illumination of the active region. The signal to noise ratio is also m a n y times lower than theoretically expected. The excess noise might be related to the procedures for acquisition and subtraction of spectral background. This actual performance (which is several times worse than theoretical expectations), can be enhanced in m a n y ways. Delivery of laser power to 350
1 June 1987
the sample could be made more efficient by a factor of 1.6 with improved reflector surface. Laser light intensity could be increased by two orders of magnitude replacing the single laser diode with a high power laser array. Developmental models recently offered for sale give cw outputs of 500 m W but are not single longitudinal mode; single longitudinal mode operation at over 60 m W cw output power has been demonstrated in laboratory operation [9]. It is feasible to double the acceptance angle of the monochromator, which would increase the collection solid angle fourfold. A total detector noise of 900 electrons has been reported by Surbek et al. [7], hence with i m p r o v e m e n t s in the video preamplifiers another factor of 2 can be obtained. The expected overall i m p r o v e m e n t factor is 1200. We are currently exploring these various avenues. We thank Dr. D. Botez for kindly supplying the RCA C D H laser samples, Dr. K. Endo for the NEC BCM units, Dr. J.H. Nicola for lending us a precision sample cell and Mr. R.W. Sprogis for assembling the electronic boards. The reflector was machined at the I F G W machine shop, polished by Ms. A.C. Silva Almeida and Dr. G. Mendes, coated by Ms. C. Diogo and Dr. Z.P. Arguello. We acknowledge financial help from FAPESP, C N P Q and FINEP.
References [ 1] E. Mazur, Rev. Sci. lnstrum. 57 (1986) 2507: N.A. Marie}, C.K. Mann and T.J. Vickers, Appl. Spectr. 39 (1985) 268. [2] D. Botez, Appl. Phys. Lett. 36 (1980) 190. [ 3 ] K. Endo, H. Kawano,M. Nido, Y. Kuwamura,T. Furuse and 1, Sakuma, Proc, Ninth Intern. Semiconductor Laser Conf. (Rio de Janeiro, Brazil, Aug 1984) p. 38. [4] 1.N. Ross and E.M. Hodgson, J. Phys. E: Sci. lnstrum. 18 (~985) 169. [5] H.H. Schlcmmer and M. Machler, J. Phys. E: Sci. lnstrum. 18 (1985) 914. [6] G.R. Hopkinson and D.H. Lumb, J. Phys. E: Sci. Instrum. 15 (1982) 1214. [7] H. Surbeck. W. Hug, M. Cremaud, M. Bridoux, A. Deflbntrine and E. da Silva, Optics Comm. 38 (1981) 57. [8] T.C. Damen, R.C.C. Leite and S.P.S. Porlo, Phys. Rev. Lett. 14 (1965) 9. [9] D.E. Ackley.Appl. Phys. Left. 42(1983) 152.