Part II Theory
5.3 Experimental Techniques
5.3.1 Basic Setup
In the previous section, we discussed the concepts and cross section of Raman scattering. Experimentally, many techniques are necessary to maximize the signal- to-noise ratio and they are the topics of this section. Two common types of Raman spectroscopy are dispersive (grating-spectrograph-based) and interference-based Fourier-transform (FT) Raman spectroscopy, but dispersive spectroscopy is the simplest and most general technique. Only incoherent Raman scattering will be discussed here.
A basic dispersive Raman system setup is illustrated in Fig. 5.6, where the light from a laser is Raman-scattered by the sample and then filtered, collected, and characterized by frequency to produce the Raman spectra. The choice of each component and their detailed parameters depends on the applications and often need to be compromised.
5.3.1.1 Lasers
The choice of lasers for Raman spectroscopy is usually determined by the sensi- tivity and fluorescent reduction. Usually visible light provides good sensitivity with the convenience of the easier alignment without IR detection cards. How- ever, some samples will fluorescent violently at these wavelengths, so near-IR wavelengths, which induce much less fluorescence, may be required. Lasers with wavelengths longer than 1000 nm are used almost exclusively for FT Raman spectrometry. The limitation is comes from the silicon-based CCD detector (even when back-thinned for better red response), which has a sensitivity range from about 400 to 1000 nm.
Laser
Spectrometer
Detector
Sample
θ
Ω
Figure 5.6: A schematic Raman system using 90◦ geometry
Many types of lasers are suitable for Raman systems. Some of the most widely used are gas lasers such as Ar+ and Kr+ ion lasers, which provide a set of high intensity lines (e.g., 457, 488, 514.5, and 406, 647, 752 nm, respectively) up to 2000 mW (combined) at reasonable costs. They are not efficient and usually require big power supplies and water cooling. A cheap alternative is a He-Ne laser (632.8 nm), which has relatively low power up to 50 mW.
Laser diodes (up to 500 mW, 650–800 nm) are becoming popular, owing to their compact design and low cost. There are some fundamental drawbacks though:
(a) only a single-mode diode will work because of its broad gain curve; (b) the temperature of the diodes must be regulated to eliminate mode hopping; (c) the long-term wavelength stability is not easy to achieve; (d) optical feedbacks, which may cause frequency instability, must be minimized; (e) a good bandpass filter is necessary due to the spontaneous emission tails; and (f) an optical design that
takes into account beam divergence is necessary. Even with these drawbacks, diode laser systems have been quite successful. Some of the problems, such as stability, can also be alleviated by using an external cavity with the diodes.
All the lasers mentioned above are continuous (CW) ones. For non-dynamics studies, there is no advantage to using pulsed lasers, which cause sample damage or nonlinear effects. But some lasers, such as Nd:YAG lasers are able to produce high-frequency (MHz) low-peak power pulses, which could be considered as quasi- continuous. The neodymium ions in a YAG laser are pumped by flashtube to a short-lived excited-state, which quickly decays to to a relatively long-lived state.
The decay from this state will produce the well-known 1064 nm laser (shown in Fig. 5.7. The output from a Nd:YAG laser can also be frequency doubled through nonlinear process to produce a 532 nm laser. The Nd:YAG lasers can also operate in the so called “Q-switched mode”, in which they can produce intense (250 MW) 10–25 nanosecond pulses. Although not useful for normal dispersive Raman spectroscopy, the pulsed lasers can be quite useful for time-resolved dynamics studies, as discussed later. Dye lasers pumped by YAG lasers can provide a wide tunable range, and this is also important to dynamics studies. This is the one of the laser systems that was used for the Raman studies in this work.
The Ti:sapphire (650–1100 nm) CW laser is probably the most versatile laser for use with Raman systems, due to its tunability, and it can have an output up to 2000 mW. It has a quasi-continuum background that needs to be filtered. It is probably the laser of choice when cost is not considered. Ultrafast Ti:sapphire lasers in femtosecond or picosecond pulsed modes are also used for time-resolved dynamics studies.
Last, but not least, the diode-pumped solid-state (DPSS) laser is a recent entry to Raman spectroscopy. It usually uses Nd:YAG crystal pumped by diodes and can produce up to 15 W at 532 nm. DPSS lasers generally have a higher beam quality and can reach very high powers while maintaining a relatively good beam quality in single mode. As a drawback, DPSS lasers are also more sensitive to temperature and can only operate optimally within a small range, so an environment control
1064 nm
4 3/2
411/2
F
I
Pump
Figure 5.7: A schematic energy levels for Nd ions in a Nd:YAG laser
system is necessary. Some DPSS lasers only cost as much as diode lasers and significantly less than other lasers with the same output performance and quality.
High quality DPSS lasers at $1–2K are available through many manufactures. One additional benefit of DPSS lasers pumped by Nd:YAG is that they can share the same set of notch or edge filters as the Nd:YAG laser. A DPSS laser of this type is also used for the the Raman studies in this work.
5.3.1.2 Spectrometers and Filters
To separate the frequency components of the scattered light, spectrometer(s) or a combination of spectrometer(s) and filter(s) are used. The first consideration of a spectrometer for Raman scattering is the low stray light. Because the intensity of the laser can be very strong and a significant percentage of the light is scattered elastically, the spectrometers and filters must be very effective at rejecting the laser line, otherwise the background from stray light will overwhelm the Raman signal. Secondly, to maximize the signal, a lowf number, and thus larger light collection solid angle, is preferred. Thirdly, the grating line density must be chosen
depending on the detector system and the wavelength range of the application.
There is always a balance between spectral coverage and spectral resolution. Many modern spectrometers have multiple gratings mounted on a turret, which can precisely change gratings remotely.
Double monochromators were standard in past. They are very effective at rejecting the stray light because the two monochromators are in series. They do suffer from some drawbacks, including a highf number, a narrow focal plane, and a very high dispersion, which is good for photomultiplier tubes (PMT), but not very useful for array detectors. Triple monochromators solve the problem of high dispersion by using a pair of the three monochromators in a subtractive configuration as a bandpass filter. They are similar to double monochromators in high stray light rejection (good) and a highf number (bad). But with so many gratings and mirrors, they suffer significantly in the signal intensity.
Use of single spectrometer is becoming more popular because of advances in filter making and spectrometer design. Single spectrometers have the obvious advantage of high throughput and lowf number, but the stray light is usually a big challenge. Modern holographic notch filters or edge filters (Semrock Ultrasteep is one type of filter used for this work) are able to achieve optical density (OD)
> 6 rejection at laser lines and the transition can be less than 90 cm−1. The actual performance of the filters depends on the choice of wavelength and it can become quite expensive (at over one thousand dollars a piece) if it is necessary to work at several different wavelengths. There are always variations of the filter transmission near the transition edges and they need to be corrected, especially for broad and slow-varying signals very close to the edge. Some spectrometers are specially designed for Raman spectroscopy to address the conflicting challenges of lowf numbers, low stray light, and flat focal plane. In an imaging spectrometer, the aberration is corrected by nonspherical mirrors or special grating designs. In a holographic spectrometer, diffraction gratings are made by the same technique as holographic filters, which allows more radical designs.
5.3.1.3 Detectors
Photomultiplier tubes (PMT) and intensified photodiode arrays (IPDA) were stan- dard equipment for Raman spectroscopy applications, but charge-coupled device (CCD) detectors are quickly replacing them everywhere except in special cases.
A PMT is usually much less expensive, and has the advantage of a wide opera- tion range at the UV end, but it can operate only in a single-channel (frequency) mode. An IPDA has the same advantage for UV operation and is easily gated down to 5 ns, which can effectively reject most fluorescence, which is a slow process compared to Raman scattering.
Nowadays CCDs are the de facto standard for Raman detectors. They have high yield, two-dimension capacity, and very low dark current. (They can be quite expensive, however.) To reduce the dark current, they need to operate at low temperature (e.g., < 70◦C). This was previously achieved by liquid nitrogen cooling, but lately it has become possible to use thermoelectric devices to achieve similar performance. This is the detector that was used for this work. New coatings and designs help extend the effective range well into the IR and UV. CCDs can also be gated for time-resolved- or fluorescence-related applications. The signals from 2D CCD detectors are usually binned along one dimension for Raman spectra.
When taking long exposures, the spectra are collected either through hardware binning or software binning. The first has limitations on the maximum intensity but generates less readout error, while the latter gives more error and is much slower.