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Contrast enhancement on crystalline silicon in polarized reflection mode tip-enhanced Raman spectroscopy
Optics Communications 274 (2007) 231–235 www.elsevier.com/locate/optcom
Contrast enhancement on crystalline silicon in polarized reflection mode tip-enhanced Raman spectroscopy Quang Nguyen b
a,*
, Razvigor Ossikovski a, Joachim Schreiber
b
a LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France HORIBA Jobin Yvon SAS, Raman Division, 231 rue de Lille, 59650 Villeneuve d’Ascq, France
Received 31 August 2006; received in revised form 22 January 2007; accepted 25 January 2007
Abstract Tip-enhanced Raman spectroscopy in reflection mode makes possible the nanoscale characterization of non-transparent samples, such as silicon, inaccessible in transmission mode. However, a particular feature of this technique is the superposition of the far-field Raman signal with the near-field one generated in the tip vicinity sometimes resulting in a low near-field-to-far-field contrast. By using a polarized configuration and orientation optimization of a (0 0 1) crystalline Si sample we were able to enhance significantly the contrast through reducing the far-field contribution, reaching a value of about 40. This contrast enhancement method can be applied in principle to any crystalline sample. 2007 Elsevier B.V. All rights reserved. PACS: 07.79.Fc; 78.30.j; 42.25.Ja Keywords: Tip-enhanced Raman spectroscopy; Polarization
1. Introduction The capability of obtaining simultaneously Raman spectroscopy and surface topography information with nanoscale spatial resolution is of great importance for both basic research and applications. Consequently, the development and application of near-field Raman spectroscopy is an active research topic of a number of groups [1–10]. In order to increase its spatial resolution, Raman spectroscopy is either combined with an aperture near-field scanning optical microscopy (‘‘aperture’’ configuration) [1] or with an atomic force microscopy (‘‘apertureless’’ configuration) [2–4] since strong electric field enhancement can be achieved in the vicinity of a sharp metallic [11] or dielectric [12] tip. In the latter case, the Raman scattering process is further enhanced when the tip is coated with a noble metal in a way similar to surface-enhanced Raman scattering *
Corresponding author. Tel.: +33 1 69333219; fax: +33 1 69333006. E-mail address: [email protected] (Q. Nguyen).
0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.01.057
(SERS). The coated tip produces a local amplification resulting from both the ‘‘antenna effect’’ [11,12] and the ‘‘local SERS effect’’ [2–10], due to the excitation of surface plasmons (polaritons) at the tip apex. This technique is generally referred to as tip-enhanced Raman spectroscopy (TERS). The apertureless configuration presents several advantages with respect to the aperture one such as higher excitation intensity and better spatial resolution, the latter depending only on the dimensions of the coated tip apex. However, one of its particular features is that the measured signal contains two contributions: a far-field one – usually large in the case of bulk samples – coming from the illuminated sample area together with a near-field one generated in the tip vicinity. The former is determined by the laser spot size, while only the latter, being defined by the tip apex dimensions, contains nanoscale-resolved information. Clearly, a way to increase the near-field-to-far-field contrast is to reduce the far-field contribution. A common solution is the use of transmission mode whereby a
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high-numerical-aperture objective is used producing a laser spot size of about 300 nm [3,4,7,8]. The undesired far-field contribution is further decreased if one measures a thin film sample [2–4,6], an individual nanoobject (e.g., a carbon nanotube [13]) or if one takes advantage of resonant or nonlinear effects such as locally induced coherent antiStokes Raman scattering (CARS) [14]. However, the transmission mode is clearly inapplicable to non-transparent samples. The main purpose of this paper is to demonstrate how a near-field-to-far-field contrast increase can be achieved in apertureless oblique reflection mode TERS by taking advantage of radiation polarization effects. In particular, the inherently low contrast observed on crystalline Si can be improved by reducing the far-field contribution through the polarization control of both the incident and scattered radiations as well as through sample orientation. By implementing experimentally the calculation results of the farfield response of c-Si as a function of the polarization conditions further minimization of the far-field contribution can be achieved and the contrast ratio, thus optimized. 2. Experiment We used a Raman spectrometer (HR800 from HORIBA Jobin Yvon) optically coupled in an oblique (or off-axis) backscattering configuration to an atomic force microscope (AFM; XE-100 from PSIA) through a custom-made coupling system. Such geometry is capable of measuring any kind of samples, including thick and non-transparent ones [5,8–10]. Fig. 1 shows the experimental configuration as well as a picture of the coupling system. The latter makes use of a long-working-distance objective (50·, NA = 0.45; Olympus) positioned with the aid of four manual translation stages in x, y, z and axial (focal) directions. The objective is used for both illumination and collection. The angle of incidence is about 70. A tunable Ar laser (514-nm wavelength used) from Melles Griot is focused on the sample surface to form an excitation spot of about 1.5-lm lateral size. The laser power was kept below 2 mW in order to avoid sample heating and tip apex deterioration. The polarization of the incident and scattered radiations can be manually varied by using, respectively, a half-wave plate
Fig. 1. Configuration of the near-field Raman experiment in oblique reflection mode. A picture of the optical coupling is inserted. The 50· excitation-collection objective tilted at 70 is seen.
Fig. 2. Scattering geometry. The axes of the sample reference frame (x y z) are oriented along [1 0 0], [0 1 0] and [0 0 1] axes of the sample. The scattering plane, as well as the incidence angle u0, are defined by the sample normal and the direction of propagation of light. The laboratory reference frame (x0 y 0 z0 ) has its y 0 -axis opposite to the direction of propagation of light and its x 0 -axis normal to the scattering plane. The incident, ei, and scattered, es, polarizations defined by the half-wave plate and the analyzer, respectively, have their azimuths referenced to z 0 -axis. The angle between the projection of y 0 onto the xy-plane and the y-axis is the sample azimuth h.
and an analyzer mounted on graduated rotation stages. The uncertainty of the azimuth setting is ±5. Fig. 2 exemplifies the scattering geometry of the experiment and defines the incident and scattered polarization states, ei and es, determined by the half-wave plate and the analyzer azimuths, respectively. Veeco MSCT silicon nitride AFM probes for contact mode operation were used because of their suitable geometry (tip placed at the very end of the cantilever). All tips were coated by thermal evaporation with a 20-nm Cr layer followed by a 80-nm Au one. The samples studied were 1 in. · 1 in. pieces of c-Si wafers with (0 0 1) crystallographic orientation. When placed on the AFM holder the samples could be manually rotated to an arbitrary azimuth with a ±5 uncertainty. In all measurements, the integration time was 1 s. 3. Results and discussion It has been both theoretically [15] and experimentally [10] established that the incident laser radiation must be polarized parallel to the tip axis for maximum TERS enhancement if the polarization of the scattered radiation is not analyzed with an analyzer (generally called ‘‘depolarized’’ configuration although the term ‘‘unanalyzed’’ would be more correct). Under these conditions, Fig. 3a shows the Raman spectrum of the 520 cm1 Si–Si phonon line of a (0 0 1)-oriented c-Si sample obtained with the tip withdrawn as well as the spectrum with the tip in contact with the sample. Calculated from the usual formula [10] C I near =I far ¼ I tot =I far 1 where Ifar, Inear and Itot are, respectively, the far-field (tip withdrawn), the near-field and the total (tip in contact) scattered intensities, the near-fieldto-far-field contrast C equals 0.15. (It should be noted that the last formula underestimates the contrast by a constant
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233
Fig. 3. (a) Raman intensities with tip in contact and tip withdrawn resulting in a contrast of 0.15 on a (0 0 1) c-Si sample. Incident polarization at p; no analyzer in the scattered beam. (b) Far- and total-field intensities (left axis) and contrast (right axis) as functions of the incident polarization. In all measurements the analyzer azimuth was set so as to minimize the far field contribution.
value since it neglects the ‘‘shadowing effect’’ on the farfield contribution when the tip is in contact. However, this underestimation, amounting to a uniform downshift in the values of C, has no impact on the results and conclusions drawn.) Other groups having performed near-field Raman experiments on the same material but using Ag-coated tips have reported contrasts ranging from 0.15 to 0.35 [5,9]. Agcoated tips are known to exhibit stronger TERS amplification than Au-coated ones due to their plasmon resonance being closer to the excitation wavelength used [10,16]. However, their main drawback is their rapid oxidation in air environment making them unsuitable for industrial applications. Consequently, we used Au-coated tips only in our experiment. To further increase the contrast, Mehtani et al. [10] used a polarized configuration. They introduced a halfwave plate – analyzer combination in the incident and scattered beam, respectively, in order to suppress the far-field signal while still letting through a part of the near-field signal and obtained a contrast of about 9 with an Au-coated tip and a 514-nm excitation. The efficiency of the polarization approach in the increase of contrast is due to the fact that the far- and near-field contributions have different polarizations. Motivated by the above results and willing to explore more deeply the polarization effects in TERS, we investigated experimentally the contrast as a function of the incident polarization. The azimuth of the linear polarization of the laser beam (or the incident polarization ei, see Fig. 2) was varied between 0 (p-polarization) and 90 (s-polarization) with the aid of the half-wave plate inserted in the laser beam. Likewise, the polarization state es of the scattered beam was analyzed with the analyzer inserted in the output optical path. At each incident polarization, the analyzer azimuth was set at the azimuth value minimizing the farfield intensity. Fig. 3b shows the total and far-field intensities, as well as the contrast. Unlike in the depolarized configuration, the maximum contrast in Fig. 3b is not found at p-incident polarization because at p-polarization the near field and the far field have close polarizations and are
almost equally suppressed by the analyzer. Nor does the maximum contrast occur at s-polarization since the near field is much less enhanced at this polarization [6]. Instead, the highest contrast of 3 was observed at an intermediate incident polarization (45) and an analyzer azimuth of 90. Again, the increase in contrast with respect to the depolarized configuration is to be attributed to the different polarizations of the near- and the far-field contributions to the scattered intensity and consequently, to the ability to minimize the latter with the analyzer. Another important parameter not yet taken into account is the influence of the sample orientation or azimuth. Indeed, for a crystalline material like Si the polarization states of both the far and near field depend on the sample azimuth. In the far-field case, this fact is exemplified by the well-known expression giving the Raman scattered intensity I for cubic crystals [17,18], X 2 I/ jeTs Rj ei j ð1Þ j
in which ei and es are, respectively, the incident and scattered electric fields already defined in Fig. 2 (the superscript T stands for transpose). The quantities Rj ; j ¼ 1; 2; 3, are the Raman tensors of the three (TO1, TO2 and LO) Si–Si phonon modes at 521 cm1 along the crystallographic axes [1 0 0], [0 1 0] and [0 0 1], respectively, 0 1 0 1 0 1 0 0 0 0 0 1 0 1 0 B C B C B C R1 ¼ @ 0 0 1 A ; R2 ¼ @ 0 0 0 A ; R 3 ¼ @ 1 0 0 A 0 1 0 1 0 0 0 0 0 ð2Þ Since a change in azimuth of the sample through rotation about its normal amounts to a rotation of the sample reference frame (x y z) at an angle h about the z-axis (see Fig. 2), the tensors Rj (2) should be transformed accordingly and the scattered intensity I becomes a function of the sample azimuth h. The detailed calculations can be found in Ref. [19]. In order to study the effect of sample rotation and validate the model calculations, we measured the far-field
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scattered intensity of a (0 0 1)-oriented c-Si sample as a function of its azimuth with the incident polarization set at 0 and at two analyzer azimuths, 0 and 90. Fig. 4a shows a fairly good agreement between the experiment and the simulations based on Eq. (1) despite the fact that neither the finite aperture angle nor the possible presence of depolarization in the objective were modeled. (The negligible influence of the last two effects can be explained, at least qualitatively, by the relatively moderate objective NA, equal to 0.45.) The 90-period oscillation of the scattered intensity observed on both calculated and measured curves is to be directly related to the (0 0 1) crystallographic orientation of Si. Its extrema may be used as a sample azimuth reference. For instance, minimum intensity is achieved with analyzer at 90 and sample azimuth at 45 (or 135, 225, 315). Going back to Fig. 3b, one finds that the total-field intensity has a maximum at 70 incident polarization. Therefore, at this incident polarization, contrast maximization can be achieved by minimizing the far-field contribution through both sample rotation and a suitable analyzer azimuth setting. Fig. 4b shows calculations of the far-field intensity performed with Eq. (1) as a function of the sample azimuth h for five combinations of two different incident polarization azimuths and three analyzer azimuths. As easily derived from the figure, the absolute minimum in the far-field intensity is attained for an incident polarization of 70 at 100 analyzer azimuth and 83 sample azimuth. By applying the above calculated values to the experiment, we were able to suppress the far field almost totally, thus obtaining a contrast of nearly 40 (Fig. 5). Note that complete far-field suppression on c-Si cannot be achieved in the half-wave plate – analyzer polarized Raman configuration since the scattered intensity is always partially depolarized because of the incoherent superposition of the three Si–Si phonon modes as seen from Eq. (1). Moreover, since an increase in contrast would generally result not only from far field reduction but also from near field enhancement, the latter being dependent on a number of factors, e.g., material, size, shape and orientation of the tip apex, the highest achievable contrast values will vary with the tips used. We have experimentally found values
Fig. 5. Maximum contrast of nearly 40 obtained at incident polarization of 70, analyzer at 100 and sample azimuth at 83.
ranging from 9 to 40 with a mean of 12 for 10 different Au-coated tips. The important point is that, for all tips used, the highest contrast value was each time attained for the same set of polarization parameters (incident and scattered polarizations ei and es, sample azimuth h) minimizing the far field contribution calculated with Eq. (1). Therefore, this approach can be applied as a ‘‘contrast boost’’ method each time a crystalline material – whatever its crystal symmetry class – exhibits low contrast values (e.g., in the case of a bulk sample, of a laser wavelength being far from the tip plasmon resonance or of a lowenhancement-factor tip). The possibility of increasing the contrast values suggests potential applications of this technique to the analysis of crystalline materials. For instance, the characterization of strain induced in thin Si layers by underlying Six Ge1x layers, of great interest for the microelectronic industry and addressed by several groups [20] by means of near-fieldand micro-Raman spectroscopy, can be performed in a more efficient way with the help of the above technique by reducing the strong far field coming from the (0 0 1)-oriented c-Si substrate and thus, increasing the contrast.
Fig. 4. Sample azimuth dependence of the far-field Raman intensity: (a) experiment and calculation with 0 (p) incident polarization and analyzer consecutively set at 0 and at 90; (b) calculations at two incident polarizations P and three analyzer azimuths A.
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4. Conclusions
References
We report a polarized configuration of an oblique reflection mode TERS equipment consisting of a Raman spectrometer optically coupled with an AFM. The instrument was applied to the near-field characterization of c-Si under various polarization conditions. The inherently low near-field-to-far-field contrast on this material was improved by using appropriate settings of the polarization components thus reducing the far-field contribution to the signal. We calculated the far-field scattered intensity by taking into account the c-Si crystal properties and its orientation and applied experimentally the values obtained in order to further minimize the far-field contribution. In this way, we were able to obtain a contrast value of nearly 40. The contrast-increasing polarization approach described can be applied in principle to any low-contrast crystalline material.
[1] C.L. Jahncke, M.A. Paesler, H.D. Hallen, Appl. Phys. Lett. 67 (1995) 2483. [2] M.S. Anderson, Appl. Phys. Lett. 76 (2000) 3130. [3] N. Hayazawa, Y. Inouye, Z. Sekkat, S. Kawata, Opt. Commun. 183 (2000) 333. [4] R.M. Sto¨ckle, Y.D. Suh, V. Deckert, R. Zenobi, Chem. Phys. Lett. 318 (2000) 131. [5] W.X. Sun, Z.X. Shen, Mater. Phys. Mech. 4 (2001) 17. [6] N. Hayazawa, A. Tarun, Y. Inouye, S. Kawata, J. Appl. Phys. 92 (2002) 6983. [7] A. Hartschuh, E.J. Sa´nchez, X.S. Xie, L. Novotny, Phys. Rev. Lett. 90 (2003) 095503. [8] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, Phys. Rev. Lett. 92 (2004) 096101. [9] R.E. Geer, N. Meyendorf, G.Y. Baaklini, B. Michel, Proc. SPIE 5766 (2005) 134. [10] D. Mehtani, N. Lee, R.D. Hartschuh, A. Kisliuk, M.D. Foster, A.P. Sokolov, J.F. Maguire, J. Raman Spectrosc. 36 (2005) 1068. [11] R. Bachelot, P. Gleyzes, A.C. Boccara, Opt. Lett. 20 (1995) 1924. [12] F. Zenhausern, M.P. Boyle, H.K. Wickramasinghe, Appl. Phys. Lett. 74 (1994) 2641. [13] N. Anderson, A. Hartschuh, S. Cronin, L. Novotny, J. Am. Chem. Soc. 127 (2005) 2553. [14] T. Ichimura, N. Hayazawa, M. Hashimoto, Y. Inouye, S. Kawata, Phys. Rev. Lett. 92 (2004) 220801. [15] L. Novotny, R.X. Bian, X.S. Xie, Phys. Rev. Lett. 79 (1997) 645. [16] P.I. Geshev, S. Klein, T. Witting, K. Dickmann, M. Hietschold, Phys. Rev. B 70 (2004) 075402. [17] E. Anastassakis, Y.S. Raptis, J. Appl. Phys. 57 (1985) 920. [18] G.H. Loechelt, N.G. Cave, J. Mene´ndez, J. Appl. Phys. 86 (1999) 6164. [19] R. Ossikovski, Q. Nguyen, G. Picardi, Phys. Rev. B 75 (2007) 045412. [20] Y. Saito, M. Motohashi, N. Hayazawa, M. Iyoki, S. Kawata, Appl. Phys. Lett. 88 (2006) 143109.
Acknowledgements One of the authors, Q. Nguyen, is sponsored by a CIFRE contract with HORIBA Jobin-Yvon. We also thank Prof. A.P. Sokolov from Akron University for helpful discussions, as well as Dr. N. Hayazawa from RIKEN Institute for kindly providing us with Ref. [20]. Special thanks are due to Dr. G. Picardi for valuable inputs and careful manuscript reading.
Contrast enhancement on crystalline silicon in polarized reflection mode tip-enhanced Raman spectroscopy Quang Nguyen b
a,*
, Razvigor Ossikovski a, Joachim Schreiber
b
a LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France HORIBA Jobin Yvon SAS, Raman Division, 231 rue de Lille, 59650 Villeneuve d’Ascq, France
Received 31 August 2006; received in revised form 22 January 2007; accepted 25 January 2007
Abstract Tip-enhanced Raman spectroscopy in reflection mode makes possible the nanoscale characterization of non-transparent samples, such as silicon, inaccessible in transmission mode. However, a particular feature of this technique is the superposition of the far-field Raman signal with the near-field one generated in the tip vicinity sometimes resulting in a low near-field-to-far-field contrast. By using a polarized configuration and orientation optimization of a (0 0 1) crystalline Si sample we were able to enhance significantly the contrast through reducing the far-field contribution, reaching a value of about 40. This contrast enhancement method can be applied in principle to any crystalline sample. 2007 Elsevier B.V. All rights reserved. PACS: 07.79.Fc; 78.30.j; 42.25.Ja Keywords: Tip-enhanced Raman spectroscopy; Polarization
1. Introduction The capability of obtaining simultaneously Raman spectroscopy and surface topography information with nanoscale spatial resolution is of great importance for both basic research and applications. Consequently, the development and application of near-field Raman spectroscopy is an active research topic of a number of groups [1–10]. In order to increase its spatial resolution, Raman spectroscopy is either combined with an aperture near-field scanning optical microscopy (‘‘aperture’’ configuration) [1] or with an atomic force microscopy (‘‘apertureless’’ configuration) [2–4] since strong electric field enhancement can be achieved in the vicinity of a sharp metallic [11] or dielectric [12] tip. In the latter case, the Raman scattering process is further enhanced when the tip is coated with a noble metal in a way similar to surface-enhanced Raman scattering *
Corresponding author. Tel.: +33 1 69333219; fax: +33 1 69333006. E-mail address: [email protected] (Q. Nguyen).
0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.01.057
(SERS). The coated tip produces a local amplification resulting from both the ‘‘antenna effect’’ [11,12] and the ‘‘local SERS effect’’ [2–10], due to the excitation of surface plasmons (polaritons) at the tip apex. This technique is generally referred to as tip-enhanced Raman spectroscopy (TERS). The apertureless configuration presents several advantages with respect to the aperture one such as higher excitation intensity and better spatial resolution, the latter depending only on the dimensions of the coated tip apex. However, one of its particular features is that the measured signal contains two contributions: a far-field one – usually large in the case of bulk samples – coming from the illuminated sample area together with a near-field one generated in the tip vicinity. The former is determined by the laser spot size, while only the latter, being defined by the tip apex dimensions, contains nanoscale-resolved information. Clearly, a way to increase the near-field-to-far-field contrast is to reduce the far-field contribution. A common solution is the use of transmission mode whereby a
232
Q. Nguyen et al. / Optics Communications 274 (2007) 231–235
high-numerical-aperture objective is used producing a laser spot size of about 300 nm [3,4,7,8]. The undesired far-field contribution is further decreased if one measures a thin film sample [2–4,6], an individual nanoobject (e.g., a carbon nanotube [13]) or if one takes advantage of resonant or nonlinear effects such as locally induced coherent antiStokes Raman scattering (CARS) [14]. However, the transmission mode is clearly inapplicable to non-transparent samples. The main purpose of this paper is to demonstrate how a near-field-to-far-field contrast increase can be achieved in apertureless oblique reflection mode TERS by taking advantage of radiation polarization effects. In particular, the inherently low contrast observed on crystalline Si can be improved by reducing the far-field contribution through the polarization control of both the incident and scattered radiations as well as through sample orientation. By implementing experimentally the calculation results of the farfield response of c-Si as a function of the polarization conditions further minimization of the far-field contribution can be achieved and the contrast ratio, thus optimized. 2. Experiment We used a Raman spectrometer (HR800 from HORIBA Jobin Yvon) optically coupled in an oblique (or off-axis) backscattering configuration to an atomic force microscope (AFM; XE-100 from PSIA) through a custom-made coupling system. Such geometry is capable of measuring any kind of samples, including thick and non-transparent ones [5,8–10]. Fig. 1 shows the experimental configuration as well as a picture of the coupling system. The latter makes use of a long-working-distance objective (50·, NA = 0.45; Olympus) positioned with the aid of four manual translation stages in x, y, z and axial (focal) directions. The objective is used for both illumination and collection. The angle of incidence is about 70. A tunable Ar laser (514-nm wavelength used) from Melles Griot is focused on the sample surface to form an excitation spot of about 1.5-lm lateral size. The laser power was kept below 2 mW in order to avoid sample heating and tip apex deterioration. The polarization of the incident and scattered radiations can be manually varied by using, respectively, a half-wave plate
Fig. 1. Configuration of the near-field Raman experiment in oblique reflection mode. A picture of the optical coupling is inserted. The 50· excitation-collection objective tilted at 70 is seen.
Fig. 2. Scattering geometry. The axes of the sample reference frame (x y z) are oriented along [1 0 0], [0 1 0] and [0 0 1] axes of the sample. The scattering plane, as well as the incidence angle u0, are defined by the sample normal and the direction of propagation of light. The laboratory reference frame (x0 y 0 z0 ) has its y 0 -axis opposite to the direction of propagation of light and its x 0 -axis normal to the scattering plane. The incident, ei, and scattered, es, polarizations defined by the half-wave plate and the analyzer, respectively, have their azimuths referenced to z 0 -axis. The angle between the projection of y 0 onto the xy-plane and the y-axis is the sample azimuth h.
and an analyzer mounted on graduated rotation stages. The uncertainty of the azimuth setting is ±5. Fig. 2 exemplifies the scattering geometry of the experiment and defines the incident and scattered polarization states, ei and es, determined by the half-wave plate and the analyzer azimuths, respectively. Veeco MSCT silicon nitride AFM probes for contact mode operation were used because of their suitable geometry (tip placed at the very end of the cantilever). All tips were coated by thermal evaporation with a 20-nm Cr layer followed by a 80-nm Au one. The samples studied were 1 in. · 1 in. pieces of c-Si wafers with (0 0 1) crystallographic orientation. When placed on the AFM holder the samples could be manually rotated to an arbitrary azimuth with a ±5 uncertainty. In all measurements, the integration time was 1 s. 3. Results and discussion It has been both theoretically [15] and experimentally [10] established that the incident laser radiation must be polarized parallel to the tip axis for maximum TERS enhancement if the polarization of the scattered radiation is not analyzed with an analyzer (generally called ‘‘depolarized’’ configuration although the term ‘‘unanalyzed’’ would be more correct). Under these conditions, Fig. 3a shows the Raman spectrum of the 520 cm1 Si–Si phonon line of a (0 0 1)-oriented c-Si sample obtained with the tip withdrawn as well as the spectrum with the tip in contact with the sample. Calculated from the usual formula [10] C I near =I far ¼ I tot =I far 1 where Ifar, Inear and Itot are, respectively, the far-field (tip withdrawn), the near-field and the total (tip in contact) scattered intensities, the near-fieldto-far-field contrast C equals 0.15. (It should be noted that the last formula underestimates the contrast by a constant
Q. Nguyen et al. / Optics Communications 274 (2007) 231–235
233
Fig. 3. (a) Raman intensities with tip in contact and tip withdrawn resulting in a contrast of 0.15 on a (0 0 1) c-Si sample. Incident polarization at p; no analyzer in the scattered beam. (b) Far- and total-field intensities (left axis) and contrast (right axis) as functions of the incident polarization. In all measurements the analyzer azimuth was set so as to minimize the far field contribution.
value since it neglects the ‘‘shadowing effect’’ on the farfield contribution when the tip is in contact. However, this underestimation, amounting to a uniform downshift in the values of C, has no impact on the results and conclusions drawn.) Other groups having performed near-field Raman experiments on the same material but using Ag-coated tips have reported contrasts ranging from 0.15 to 0.35 [5,9]. Agcoated tips are known to exhibit stronger TERS amplification than Au-coated ones due to their plasmon resonance being closer to the excitation wavelength used [10,16]. However, their main drawback is their rapid oxidation in air environment making them unsuitable for industrial applications. Consequently, we used Au-coated tips only in our experiment. To further increase the contrast, Mehtani et al. [10] used a polarized configuration. They introduced a halfwave plate – analyzer combination in the incident and scattered beam, respectively, in order to suppress the far-field signal while still letting through a part of the near-field signal and obtained a contrast of about 9 with an Au-coated tip and a 514-nm excitation. The efficiency of the polarization approach in the increase of contrast is due to the fact that the far- and near-field contributions have different polarizations. Motivated by the above results and willing to explore more deeply the polarization effects in TERS, we investigated experimentally the contrast as a function of the incident polarization. The azimuth of the linear polarization of the laser beam (or the incident polarization ei, see Fig. 2) was varied between 0 (p-polarization) and 90 (s-polarization) with the aid of the half-wave plate inserted in the laser beam. Likewise, the polarization state es of the scattered beam was analyzed with the analyzer inserted in the output optical path. At each incident polarization, the analyzer azimuth was set at the azimuth value minimizing the farfield intensity. Fig. 3b shows the total and far-field intensities, as well as the contrast. Unlike in the depolarized configuration, the maximum contrast in Fig. 3b is not found at p-incident polarization because at p-polarization the near field and the far field have close polarizations and are
almost equally suppressed by the analyzer. Nor does the maximum contrast occur at s-polarization since the near field is much less enhanced at this polarization [6]. Instead, the highest contrast of 3 was observed at an intermediate incident polarization (45) and an analyzer azimuth of 90. Again, the increase in contrast with respect to the depolarized configuration is to be attributed to the different polarizations of the near- and the far-field contributions to the scattered intensity and consequently, to the ability to minimize the latter with the analyzer. Another important parameter not yet taken into account is the influence of the sample orientation or azimuth. Indeed, for a crystalline material like Si the polarization states of both the far and near field depend on the sample azimuth. In the far-field case, this fact is exemplified by the well-known expression giving the Raman scattered intensity I for cubic crystals [17,18], X 2 I/ jeTs Rj ei j ð1Þ j
in which ei and es are, respectively, the incident and scattered electric fields already defined in Fig. 2 (the superscript T stands for transpose). The quantities Rj ; j ¼ 1; 2; 3, are the Raman tensors of the three (TO1, TO2 and LO) Si–Si phonon modes at 521 cm1 along the crystallographic axes [1 0 0], [0 1 0] and [0 0 1], respectively, 0 1 0 1 0 1 0 0 0 0 0 1 0 1 0 B C B C B C R1 ¼ @ 0 0 1 A ; R2 ¼ @ 0 0 0 A ; R 3 ¼ @ 1 0 0 A 0 1 0 1 0 0 0 0 0 ð2Þ Since a change in azimuth of the sample through rotation about its normal amounts to a rotation of the sample reference frame (x y z) at an angle h about the z-axis (see Fig. 2), the tensors Rj (2) should be transformed accordingly and the scattered intensity I becomes a function of the sample azimuth h. The detailed calculations can be found in Ref. [19]. In order to study the effect of sample rotation and validate the model calculations, we measured the far-field
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scattered intensity of a (0 0 1)-oriented c-Si sample as a function of its azimuth with the incident polarization set at 0 and at two analyzer azimuths, 0 and 90. Fig. 4a shows a fairly good agreement between the experiment and the simulations based on Eq. (1) despite the fact that neither the finite aperture angle nor the possible presence of depolarization in the objective were modeled. (The negligible influence of the last two effects can be explained, at least qualitatively, by the relatively moderate objective NA, equal to 0.45.) The 90-period oscillation of the scattered intensity observed on both calculated and measured curves is to be directly related to the (0 0 1) crystallographic orientation of Si. Its extrema may be used as a sample azimuth reference. For instance, minimum intensity is achieved with analyzer at 90 and sample azimuth at 45 (or 135, 225, 315). Going back to Fig. 3b, one finds that the total-field intensity has a maximum at 70 incident polarization. Therefore, at this incident polarization, contrast maximization can be achieved by minimizing the far-field contribution through both sample rotation and a suitable analyzer azimuth setting. Fig. 4b shows calculations of the far-field intensity performed with Eq. (1) as a function of the sample azimuth h for five combinations of two different incident polarization azimuths and three analyzer azimuths. As easily derived from the figure, the absolute minimum in the far-field intensity is attained for an incident polarization of 70 at 100 analyzer azimuth and 83 sample azimuth. By applying the above calculated values to the experiment, we were able to suppress the far field almost totally, thus obtaining a contrast of nearly 40 (Fig. 5). Note that complete far-field suppression on c-Si cannot be achieved in the half-wave plate – analyzer polarized Raman configuration since the scattered intensity is always partially depolarized because of the incoherent superposition of the three Si–Si phonon modes as seen from Eq. (1). Moreover, since an increase in contrast would generally result not only from far field reduction but also from near field enhancement, the latter being dependent on a number of factors, e.g., material, size, shape and orientation of the tip apex, the highest achievable contrast values will vary with the tips used. We have experimentally found values
Fig. 5. Maximum contrast of nearly 40 obtained at incident polarization of 70, analyzer at 100 and sample azimuth at 83.
ranging from 9 to 40 with a mean of 12 for 10 different Au-coated tips. The important point is that, for all tips used, the highest contrast value was each time attained for the same set of polarization parameters (incident and scattered polarizations ei and es, sample azimuth h) minimizing the far field contribution calculated with Eq. (1). Therefore, this approach can be applied as a ‘‘contrast boost’’ method each time a crystalline material – whatever its crystal symmetry class – exhibits low contrast values (e.g., in the case of a bulk sample, of a laser wavelength being far from the tip plasmon resonance or of a lowenhancement-factor tip). The possibility of increasing the contrast values suggests potential applications of this technique to the analysis of crystalline materials. For instance, the characterization of strain induced in thin Si layers by underlying Six Ge1x layers, of great interest for the microelectronic industry and addressed by several groups [20] by means of near-fieldand micro-Raman spectroscopy, can be performed in a more efficient way with the help of the above technique by reducing the strong far field coming from the (0 0 1)-oriented c-Si substrate and thus, increasing the contrast.
Fig. 4. Sample azimuth dependence of the far-field Raman intensity: (a) experiment and calculation with 0 (p) incident polarization and analyzer consecutively set at 0 and at 90; (b) calculations at two incident polarizations P and three analyzer azimuths A.
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4. Conclusions
References
We report a polarized configuration of an oblique reflection mode TERS equipment consisting of a Raman spectrometer optically coupled with an AFM. The instrument was applied to the near-field characterization of c-Si under various polarization conditions. The inherently low near-field-to-far-field contrast on this material was improved by using appropriate settings of the polarization components thus reducing the far-field contribution to the signal. We calculated the far-field scattered intensity by taking into account the c-Si crystal properties and its orientation and applied experimentally the values obtained in order to further minimize the far-field contribution. In this way, we were able to obtain a contrast value of nearly 40. The contrast-increasing polarization approach described can be applied in principle to any low-contrast crystalline material.
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Acknowledgements One of the authors, Q. Nguyen, is sponsored by a CIFRE contract with HORIBA Jobin-Yvon. We also thank Prof. A.P. Sokolov from Akron University for helpful discussions, as well as Dr. N. Hayazawa from RIKEN Institute for kindly providing us with Ref. [20]. Special thanks are due to Dr. G. Picardi for valuable inputs and careful manuscript reading.