J. Phys. Chem. 1996, 100, 16293-16297
16293
Stimulated Raman Gain Spectroscopy of Thin Layers Using Dielectric Waveguides J. S. Kanger, C. Otto, and J. Greve* Applied Optics Group, Department of Applied Physics, BMTI and MESA Institutes, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ReceiVed: May 6, 1996X
In this paper we show the possibility of measuring Raman spectra of thin layers using waveguide stimulated Raman gain spectroscopy (WSRG). In WSRG, the thin layer is probed by the evanescent field of the pump and Stokes beams as they propagate as guiding modes in an optical dielectric waveguide structure. We have measured the Raman spectrum of a 30 nm thin polystyrene layer that was deposited on top of a Si3N4 waveguide. The measured gain of the strong 1000 cm-1 benzene ring stretch vibration was found to be 10-3, which is about 5 orders of magnitude higher than for a classical transmission geometry. Additionally, we show the advantage of this technique over spontaneous Raman scattering in waveguides in the case of strongly fluorescing waveguides.
Introduction Waveguides are more and more frequently used for the detection of Raman scattering of thin (mono) molecular layers. Spontaneous waveguide Raman spectroscopy started in 1976 when Levy et al.1 showed the possibility of measuring spectra of micron thick polystyrene layers and using them as optical waveguides. Later, monolayer sensitivity was obtained by depositing the thin layer on top of a waveguide and using the evanescent field for resonantly enhanced excitation.2-4 A drawback of the waveguide technique is the fluorescent background of the waveguide itself. It was only recently shown by Kanger et al.5 that monolayer sensitivity can also be reached for nonresonant excitation by using the proper waveguide materials and waveguide configurations. In 1983 Stegeman et al.6 started to develop waveguide coherent anti-Stokes Raman scattering (WCARS). As with spontaneous Raman, CARS also benefits from the waveguide structure through the large interaction volume. Large signal levels were predicted6 and measured7 for thick polystyrene waveguides and even for monolayers.8-10 The advantage of this technique over spontaneous Raman is the lack of fluorescence. The major drawback, however, is the high nonresonant background of the waveguide, which in the case of monolayer detection obscures weak Raman signals. Even when it is possible to suppress this background by means of destructive interference among pump, Stokes, and CARS modes, the technique remains very complicated.6,11 Recently, a third Raman technique was studied for the application of waveguides. Kanger et al.12 predicted that stimulated Raman spectroscopy in dielectric waveguide structures can give rise to large signal levels, and it follows from their analysis that it even has monolayer sensitivity. Stimulated Raman spectroscopy has the advantage of CARS in the sense that the fluorescence can be strongly suppressed,13 but it lacks the disadvantage of the high nonresonant background interference. Experimentally, it was shown that the predicted high signal levels can be obtained for micron thick polystyrene waveguides.14 In this paper we go one step further. We demonstrate that spectra of thin layers can be measured by probing the layers with the evanescent field of pump and Stokes * To whom correspondence should be sent. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01281-6 CCC: $12.00
modes that are guided by the waveguide on which the thin layer is deposited. In this paper we will first introduce the method of waveguide stimulated Raman spectroscopy (WSRG) followed by a theoretical derivation of the expected signal levels. Since WCARS and WSRG are strongly related, a comparison of the signal-tonoise ratios expected for WSRG and WCARS will be made and discussed. Secondly, in the experimental section the setup and the experiments are described. Finally, the results are discussed and the main conclusions of this research will be drawn. Method Gain Factors. In stimulated Raman gain spectroscopy a pump beam with frequency ωp and a Stokes beam with frequency ωs interact in a Raman active medium. Owing to this interaction, the Stokes beam will experience a gain in intensity due to stimulated emission if the energy difference between pump and Stokes photons equals the energy of a Raman active vibration. In general we can write for the Stokes intensity Is
Is ) Is(0) exp{gdl}
(1)
The total gain G ) gdl, which can be expected for a sample with a thickness dl and a nonlinear Raman cross section χ(3), is given by15
Gtransmission )
()
3ωs µ0 Im{χ(3)}dlIp npns 0
(2)
where Ip is the pump intensity and np and ns are the refractive index of the layer at the pump and Stokes wavelengths, respectively. The detection of thin layers with the use of stimulated Raman gain spectroscopy is not very efficient in the normal transmission or reflection geometry. If for example a pump beam with an intensity of 100 MW/cm2 is used, the gain of a monolayer of benzene (dl ≈ 0.5 nm, χ(3) ≈ 10-12 esu16) is only 10-8. This gain can be significantly increased if use is made of waveguide structures. In Figure 1A a schematic representation of a waveguide is given. The waveguide itself consists of a thin high refractive index layer (film) sandwiched between the cladding (often air) and the substrate, both having a smaller index of refraction than the film layer. Planar © 1996 American Chemical Society
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Kanger et al.
Figure 1. Schematic representation of a dielectric waveguide consisting of a cladding, film, and substrate, where a thin layer is deposited on top of the film: (A) electric field distribution f(z) of a TE0 mode; (B) approximation of the electric field distribution as used in the calculations.
waveguides can guide TE (transverse electric) modes as well as TM (transverse magnetic) modes. In the case of WSRG the pump and Stokes beams are guided by the optical dielectric waveguide. On top of the film layer the thin layer of interest is deposited as also shown in Figure 1A. Also shown in this figure is the electric field distribution of a propagating TE0 mode. The field penetrates outside the waveguide into the thin layer and can therefore probe the Raman vibrations of this layer. The gain can be derived from the Maxwell equations using coupled mode theory:12
Gwaveguide )
cm whereas the thickness dw of the waveguide can be on the order of 100 nm if a high refractive index material is chosen. This means that the ratio between the gain observed in a waveguide geometry and the gain observed in a transmission geometry is typically on the order of 105. Signal-to-Noise Calculations. Next, a comparison of signalto-noise ratios will be given for WSRG and WCARS. The signal in stimulated Raman gain is measured as a small increase of the Stokes or probe laser beam. Since the noise on the laser power will in general be much higher than the signal, it becomes necessary to use lock-in detection techniques. The pump beam is modulated by a chopper, and a lock-in amplifier for detection reduces the classical noise in the Stokes beam detection signal. The signal power Ps in this case is given by
Ps ) (1/2)GwaveguidePSt
The factor 1/2 is introduced owing to the duty cycle of the chopper, and PSt is the power of the Stokes beam at the detector. It has been shown17 that if the lock-in frequency is chosen carefully, the shot noise of the Stokes beam exceeds the classical noise at the lock-in frequency even when mode-locked dye lasers are used. Additional noise contributions, which are manifested in a fluctuation of the signal, have not been included. These contributions do reduce the maximum signal-to-noise ratio attainable but are unimportant near the detection limit. When the dark current and the Johnson noise are both neglected, the signal-to-noise ratio equals
3ωs L +∞ Pp∫-∞ Im{χ(3)}[fp(z)]2[fs(z)]2 dz (3) 4 H
In this equation L is the interaction length of the beams inside the waveguide and H is the width of the beams. The integral in this equation is often referred to as the overlap integral, since it expresses the spatial overlap of the electric fields of pump and Stokes beams with the nonlinear susceptibility χ(3) of the material of interest. The so-called mode distribution functions fp and fs give the complex amplitude distribution of the electric field of the guiding mode for the pump and Stokes field, respectively. These functions are normalized such that the power carried by a guiding mode is 1 W per meter wave front, that is,
∫-∞+∞[f(z)]2 dz )
2ωµ0 β
(4)
with β the propagation vector of the guiding mode. To get an estimation of the expected gain compared with the gain in a transmission geometry, we can assume a mode distribution function that is rectangular as shown in Figure 1B. Using eq 4 to get the values of f(z), we can then rewrite eq 3 into
Gwaveguide )
()
dl 3ωs µ0 Im{χ(3)} LI npns 0 dl + dw p
(5)
with dw being the thickness of the film. The ratio dl/(dl + dw) in this equation is the fraction of the Stokes beam that interacts with the thin layer. Comparing eqs 2 and 5, we can write for the ratio between the two gain factors
Gwaveguide L ) Gtransmission dl + dw
(6)
In a waveguide structure the interaction length L is typically 1
(7)
S ) N
()
x
〈is2〉
〈in 〉 2
x
) Gwaveguide
ηPSt 4pωsB
(8)
where η defines the quantum efficiency of the detector and B the bandwidth. Together with eq 3 this yields
(NS ) )
3ωs L +∞ P ∫ Im{χ(3)}[fp(z)]2[fs(z)]2 dz 4 H p -∞
x
ηPSt (9) 4pωStB
The signal-to-noise ratio for WCARS is derived for the case where the nonresonant background is fully suppressed. In this case the signal power on the detector is fully determined by the resonant contributions to the third-order susceptibility χ(3). The signal-to-noise ratio can be written as (neglecting dark current and Johnson noise and close to the detection limit as in the WSRG case)
S ) N
()
x x 〈is2〉
〈in 〉 2
)
ηPR 2pωcB
(10)
The WCARS signal power obtained for the waveguide geometry can be derived using coupled mode theory in a similar way as the signal levels in WSRG are calculated and is given by6
PR )
9ωc2 L 2 2 +∞ P P |∫ {χ(3)}[fp(z)]2[fs(z)][fc(z)] dz|2 (11) 256 H p St -∞
()
where ωc gives the frequency of the generated CARS light and fc the field distribution function of the mode in which this light is scattered. Combining the above equations leads to an expression for the WCARS signal-to-noise ratio
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J. Phys. Chem., Vol. 100, No. 40, 1996 16295
(NS ) ) 3ω L P∫ 16 (H) c
+∞ (3) 2 p -∞ Im{χ }[fp(z)] [fs(z)][fc(z)]
dz
x
ηPSt (12) 2pωcB
where the imaginary part of χ(3) can be used because in resonance the χ(3) is fully imaginary. The expressions for the signal-to-noise ratios of WSRG and WCARS look very similar. Except for a factor of 2x2ωs/ωc, the only difference is found in the overlap integral. If the detected CARS field would be of the same mode character as the Stokes field, fc would equal fs and the overlap integrals would in fact be identical. However, in the case of nonresonant background suppression, the integrand of the overlap integral should be asymmetric in the zdirection.6,11 This means that fs should be of a different mode character than fc. As a consequence, the thickness of the waveguide must be larger in order to be able to guide at least two modes. To compare the overlap integral for WCARS with the one for WSRG, a particular waveguide geometry plus a mode combination has to be chosen. For WCARS a waveguide geometry and mode combination is taken, which has been proven experimentally to give a satisfactory background suppression. This waveguide consisted of a 600 nm thin film with a refractive index of 2.0 on top of a substrate with a refractive index of 1.46. The mode combination that was used is TE1 for the pump and Stokes mode and TE2 for the CARS mode. For WSRG we take a waveguide with a 100 nm thick film, with a refractive index of 2.0, on top of a substrate with a refractive index of 1.46. Consequently, both beams must be of TE0 mode. It has been shown that this configuration gives the highest signal levels in WSRG.12 In this situation the overlap integral for the WSRG is roughly 20 times larger than the one for WCARS. The WCARS situation can be optimized by minimizing the thickness of the film layer. However, the waveguide should still be thick enough to carry at least two modes. For a film layer with a refractive index of 2 this means that the thickness should at least be 300 nm. In this situation a TE0 and a TE1 mode can propagate. The TE1 mode has the highest field at the surface, so the mode combination in which the pump field and the Stokes field are of TE1 character and the CARS field is of TE0 character is favorable. For this situation the overlap integral for WCARS is increased by 5 times, which means that the integral for WSRG is still 4 times higher. For the signal-to-noise ratios this results in
(NS )
/
WSRG
(NS )
CARS
ωs ≈ 101 ωc
g 8x2
(13)
This shows that the attainable signal-to-noise ratio for WSRG is at least 1 order of magnitude larger than for WCARS in the case of shot-noise-limited detection and background-free WCARS signals. Experimental Section Ridge Waveguides. In the WSRG experiments described in this paper we used a ridge waveguide. In a ridge waveguide the light is not only confined in the z-direction (see Figure 1) but also in the y-direction. This means the light is guided by a channel and propagates in the x-direction. This is accomplished by making a change in film thickness at the edges of the channel as seen in Figure 2B. In this way the effective refractive index (βc/ω) inside the channel can be made slightly higher than outside the channel, enabling confinement of the light in the y-direction. The advantage of using ridge or channel waveguides
Figure 2. (A) Experimental setup for WSRG: BS, beamsplitter; NF1 and NF2, holographic notch filters for pump light rejection; PD1, signal photodiode; PD2, reference photodiode; obj, objective. (B) A more detailed drawing of the ridge waveguide and coupling construction.
for WSRG are twofold. (i) Since the light is confined in two dimensions, long interaction lengths can be attained in combination with small beam diameters. With planar waveguides the interaction length is limited by the length of the foci of the pump and Stokes beams. (ii) The coupling of the pump and Stokes beams into the guiding modes of the waveguide can be achieved by end-fire coupling. In this technique the beams are made collinear and focused at the face of the channel. This method is much less complicated than prism coupling, which is needed to couple light into a guiding mode for a planar waveguide. The two beams need to be focused at the edge of the coupling prism at a precise angle that is determined by the propagation vector of the guiding mode and differs for the pump and Stokes beam. Waveguide Fabrication. A 3 in. silicon wafer was thermally oxidized at 1150 °C for approximately 15 h to grow a 2.5 µm thick SiO2 layer. This SiO2 layer (refractive index of 1.46) was used as a substrate for the waveguide. On this substrate a 130 nm thick Si3N4 layer was grown using LPCVD (low-pressure chemical vapor deposition). The refractive index of this material is 2.0. Channels of different widths were etched on the waveguide in 15 min. A standardized etch liquid (ammonium fluoride etchant AF, Merck) was used consisting of HF and NH4F (HF:NH4F ) 1:7). With an etching rate of 0.7 nm/min at room temperature, this resulted in a step of ∼10 nm. This is sufficient to confine the light in the channel. The width of the channel that was used in the experiments was 25 µm. For the spontaneous Raman experiments a planar Si3N4 waveguide was used that had the same thickness. The polystyrene layer was made by spin coating (4000 rpm; 40 s) a solution of polystyrene dissolved in toluene at a concentration of 0.5 wt % on top of the Si3N4 waveguides. Setup. The setup that was used in the stimulated Raman gain experiments is depicted in Figure 2. Two dye lasers are synchronously pumped with the second harmonic of a modelocked Nd:YLF laser (Coherent Antares). One of the dye lasers is equipped with a Rh6G dye and was tuned to 595.0 nm to
16296 J. Phys. Chem., Vol. 100, No. 40, 1996
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Figure 3. Experimental setup for spontaneous Raman in waveguides: GT, Glan Thompson prism; F1, F2, and F3, focusing and collection lenses; M1 and M2, mirrors for adjusting the coupling angle; NF, notch filter for pump light rejection. The length of the light strike in the waveguide that is imaged onto the detector is ∼5 mm.
serve as the pump laser source. The second dye laser (DCM) was used as the Stokes laser source and was tuned during the experiment from 631.6 nm (975 cm-1) to 635.7 nm (1075 cm-1). Both lasers were cavity dumped at 3.8 MHz. The temporal widths of the laser pulses were 5 ps as obtained from the autocorrelation trace. The laser beams were overlapped in time and then coupled into the channel waveguide by focusing at the end-face of the channels with a 20× objective. The average power of the pump and Stokes beams inside the waveguide was kept low (∼25 µW) in order to keep the peak intensity below 50 MW/cm2. After traveling approximately 2 cm inside the waveguide, the light was coupled out using a SrTiO3 prism. The light was then directed through a notch filter (NF1 in Figure 2) to reject the pump beam and focused onto a photodiode. To separate the Raman gain signal from the Stokes beam, the pump laser was modulated at 3.3 kHz and a lock-in amplifier was used, which was tuned to the chop frequency. The main sources of noise in the intensity of the Stokes beam are the fluctuations in the jet of the dye laser. To reject some of this noise, a reference signal was passed through a second notch filter and subtracted from the signal in a differential amplifier before it was analyzed with the lock-in amplifier. With this system it is not possible to obtain a shot-noise-limited detection. A more sophisticated detection scheme based on a high-frequency AMFM double modulation technique should be used in order to reach the shot-noise limit.17 The spectrum was measured in 300 points with an integration time of 2.5 s per point. The spontaneous Raman spectrum was measured with the same pump beam as in the stimulated Raman gain experiments. The light at 595 nm was coupled into the TE0 mode of the polystyrene -Si3N4-SiO2 waveguide structure by means of a SrTiO3 coupling prism. The experimental setup is schematically shown in Figure 3. The scattered Raman light, which is radiated into the cladding, is collected using a set of lenses and imaged onto the entrance slit of a Jobin-Yvon HR480 polychromator equipped with a 600 lines/mm grating. For detection a CCD camera was used. In this way a spectral range of 1200 cm-1 could be simultaneously detected. Spectra were recorded in 10 min.
Figure 4. Waveguide stimulated Raman gain spectrum of a 30 nm thick polystyrene layer on top of a 130 nm thick Si3N4 waveguide. The peak gain is 10-3. The total measuring time was 750 s.
regime (where the pulse widths of the beams are comparable with the relaxation time of the vibrations) the gain is smaller. For Gaussian shaped pulses the gain in this case (pulse width of ∼5 ps and a vibrational relaxation time of ∼5 ps) is reduced by a factor of 4.14 This means the expected gain should be 2.5 × 10-3, which shows a rather good agreement with the experimentally observed gain. The spontaneous Raman spectrum measured in the same waveguide configuration is shown in Figure 5. The spectrum shows a very high fluorescence background. The shot-noise of the background exceeds the signal of the polystyrene peak, which could therefore not be detected within the given measuring time. The signal-to-background ratio in this case will be smaller than 6 × 10-3. For the stimulated Raman gain spectrum the signal-to-background ratio is larger than 10. This means that a fluorescence background reduction of at least 3 orders is obtained with the waveguide stimulated Raman gain technique. The theoretical fluorescence suppression is given by the ratio of the lifetime of the fluorescence τF and the pulse duration t of the pump and Stokes pulses.19 The lifetime of the fluorescence, which has a maximum emission at 630 nm, was measured using a time-correlated single photon detection scheme. It was determined to be larger than a few nanoseconds. This means that the ratio τF/t > 103, which corresponds to the measurements.
Results The stimulated Raman gain spectrum of the thin polystyrene layer on top of the Si3N4 waveguide is shown in Figure 4. Both the strong benzene ring stretch vibration at 1000 cm-1 and the much weaker vibration around 1030 cm-1 are clearly visible. The maximum gain at the peak of the 1000 cm-1 vibration was measured to be 1 × 10-3. The steady state gain calculated from eq 2 is 1 × 10-2. It has been shown18 that in the transient
Conclusions Waveguide stimulated Raman gain spectroscopy seems to be a promising technique for the study of thin molecular layers. Very thin layers of polystyrene can easily be measured in this way, even on top of highly fluorescing waveguides. This makes the choice of a suitable waveguide much easier than for the waveguide spontaneous Raman technique. Also, in comparison
Spectroscopy of Thin Layers
J. Phys. Chem., Vol. 100, No. 40, 1996 16297 reasons it is expected that waveguide stimulated Raman gain spectroscopy will find wide application in the study of monolayers. Acknowledgment. This work was supported by the S.T.W., Technology Foundation, Grant TTN 11.2511. References and Notes
Figure 5. Waveguide spontaneous Raman spectrum of a 30 nm thick polystyrene layer on top of the 130 nm thick Si3N4 waveguide used in Figure 4. The polystyrene signal is not intense enough to be detected owing to the high fluorescence of the waveguide.
with WCARS, WSRG looks the better choice. Experimentally, WSRG is much less complicated than WCARS owing to self phase matching and the lack of nonresonant background while theoretically the signal-to-noise ratios that can be achieved are comparable with a slight advantage for WSRG. For these
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