{"id":15100,"date":"2019-05-13T21:03:03","date_gmt":"2019-05-13T19:03:03","guid":{"rendered":"http:\/\/depeppers.es\/mc\/?p=15100"},"modified":"2019-09-21T11:16:27","modified_gmt":"2019-09-21T09:16:27","slug":"raman-spectroscopy-and-monocrom-products","status":"publish","type":"post","link":"https:\/\/testwebc3.com.es\/en\/raman-spectroscopy-and-monocrom-products\/","title":{"rendered":"Raman spectroscopy and Monocrom products"},"content":{"rendered":"
ABSTRACT<\/strong><\/h5>\n

This application note gives a brief introduction into Raman spectroscopy and related applications. It gives an overview over the physics, the requirements on the laser source and the detector properties necessary to detect a proper spectrum. The most critical Figures Of Merit will be discussed and last but not least it follows a presentation of Monocrom products that can be used to serve Raman spectroscopy applications.<\/p>\n

1. RAMAN SPECTROSCOPY – AN OVERVIEW<\/strong><\/h5>\n

Raman spectroscopy is a characterization technique concerning the irradiation of a sample with a light source and the analysis of its scattered light, in particular those photons that are scattered in-elastically (elastic scattering is known as Rayleigh scattering). The photons that suffer inelastic scattering interact with the matter either by gaining or loosing energy in the form of phonons. The portion of photons with a resulting energy lower than the incident beam constitute the Stokes radiation, while those photons that gain energy are part of the Anti-Stokes radiation. The possible elastic and inelastic scattering transitions are illustrated in the upper part of Fig. 1.1. Both, Stokes and Anti-Stokes, transitions have different probabilities. The latter is quite raw, but the signal can be enhanced via coherent excitation.<\/p>\n

In fact, it is the frequency shift (expressed in wavenumbers, generally in units of cm\u22121<\/sup>) what is measured in Raman spectroscopy, and its value is independent from the excitation wavelength \u03bb<\/span><\/em>i<\/em>n<\/sub>. However, in practice there are several factors that make one or another excitation wavelength more or less appropriate, including the sample itself.<\/p>\n

In principle, Stokes and Anti-Stokes radiation can be analyzed in order to measure the Raman shift. However, Stokes shows inherently a higher intensity since the relative thermal population of energy levels are defined by the Boltzmann factor:<\/p>\n\n\n

\"\"<\/figure>\n\n\n\n
\"\"<\/figure>\n\n\n\n

Figure 1.1: Illustration of the energy shift suffered by photons \ninelastically scattered when the incident photon excites the material (a\n phonon is absorbed) or de-excites it (a phonon is released). [1]<\/em><\/p>\n\n\n\n

In Eq. 1.1 the ni<\/sub><\/em> represents the population of a specific state i<\/em>, kB<\/sub><\/em> is the Boltzmann constant, h<\/em> is the Plank constant and together with the photon frequency Vi<\/sub> = cvac<\/sub><\/sup>\/\u03bbin<\/sub><\/em> it gives the energy of that state. The last variable missing is T<\/em>, the temperature in Kelvin. Due to the intrinsically low quantum efficiency of this interaction(1 in 106<\/sup> \u2212 108<\/sup> events), a high brightness light source is mandatory. Brightness Br<\/em> can be expressed via:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Here Plas<\/sub><\/em> denotes the laser power and Mo<\/sub><\/em>2<\/em> <\/sup>describe the beam quality in both axes x and y, respectively. Cross sections for Raman scattering reach from 10\u221231<\/sup>cm\u22122<\/sup> \u221210\u221225<\/sup>cm\u22122<\/sup>,\n which is still five orders of magnitude lower than Rayleigh scattering \n[3]. It is easy then to understand why although the Raman Effect was \nobserved for the first time in 1928 [4, 5], the development of Raman \nspectroscopy was not possible in a reliable way until the 60\u2019s, when the\n first lasers appeared. Additionally, the intensity of the Raman \nscattered light Iscat<\/sub><\/em> is proportional to the incident beam intensity I0<\/sub><\/em> and to the fourth power of its wavelength \u03bbin<\/sub><\/em> [6]:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 1.2: Relationship between the three main elements involved in\n Raman Spectroscopy. The analyte and the laser source are closely \ninterrelated. Besides, the detector is mainly a consequence of the \nwavelength, although the range of the detector plays an important role \non the wavelength choice. Detector and analyte do not condition to each \nother directly.<\/em><\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

So, in principle lower wavelengths should be the best option. \nHowever, things are never that easy, so let\u2019s consider this idea further\n in detail in the following paragraphs. It is important to have in mind \nthe three main elements involved in Raman Spectroscopy:<\/p>\n\n\n\n

\u2022 The analyte<\/p>\n\n\n\n

\u2022 The laser source<\/p>\n\n\n\n

\u2022 The detector<\/p>\n\n\n\n

The relationship and mutual influence of\n these three elements are illustrated in Fig. 1.2.1.1 The analyte Raman \nSpectroscopy is a material characterization technique that uses the \ninteraction between laser light and the rotational-vibrational \nenergy-level structure of molecular compounds, as a way to provide \nfingerprints of it. Therefore, Raman spectroscopy essentially represents\n a tool to identify certain materials and some relevant aspects of its \nmolecular and lattice structure, although it can be also used for \nquantification. A good example of the application of Raman Spectroscopy \nis the identification of solid carbon materials, which show distinctive \nspectroscopic features in Raman scattering depending on whether the \natoms are arranged to form diamond, graphite, amorphous carbon or even \nfullerenes, carbon nanotubes or graphene. The material under \ncharacterization can be either in solid, liquid or gaseous state, \nalthough for obvious reasons, the gaseous samples are not easily \ndetected with conventional techniques. However, there are several \nRaman-based techniques that are able to obtain an enhanced scattering \nresponse when applying complementary strategies. This is the case of \nSurface Enhanced Raman Spectroscopy (SERS), where the sample is placed \nover a novel metal\u2013coated substrate (like silver or gold). This way, the\n signal detected can be amplified by several orders of magnitude, \nallowing even single-molecule detection if the metal layer is \nnanostructured [3, 6, 8]. Another well established technique consists of\n irradiating the sample at a wavelength close to one of its electronic \ntransitions, leading to a truly photon absorption (in Raman scattering \nthe incident photon is not really absorbed by the sample) and a \nsubsequent highly enhanced Raman response. This is called Resonance \nRaman (RR) spectroscopy [6, 7]. Also worth to mentioned is Coherent \nAnti-Stokes Raman Spectroscopy (CARS), which is based on third-order \nsusceptibilities. In order to get a significant signal, high laser \nintensities are required to ensure a two-photon absorption. In the \nliterature can be found some more signal enhancement techniques \nlike\u2014among others\u2014Coherent Stokes Raman Spectroscopy (CSRS), \nPhoto-Acoustic Raman Spectroscopy (PARS) or Stimulated Raman Gain \nSpectroscopy (SRGS) [9].<\/p>\n\n\n\n

1.2 The laser source<\/strong><\/h5>\n\n\n\n

As explained above, due to the low \nquantum efficiency of Raman scattering, there are two conditions for a \nproper light source to promote observable Raman scattered photons. One \nis high brightness Br<\/em> (see Eq.1.2)\u2014so laser is obviously the \nbest choice\u2014, and the other is an as low as possible wavelength (see Eq.\n 1.3). UV and visible lasers in the NUV-blue-green region are a good \noption for inorganic compounds [7]. But when it comes to organic matter \nand biological samples, many compounds show a strong fluorescence in the\n visible region when irradiated with these wavelengths. Therefore, the \nRaman signal can be easily \u201cburied\u201d under an intense, broad fluorescent \nemission background (and noise). A quick path to avoid this is shifting \nthe incident wavelength of choice out of the visible and NUV region \ntowards the MUV or the NIR, but then we must consider other aspects like\n the influence of the detector. Usually, the range of Raman shift that \ngoes from 100 cm\u22121<\/sup> up to 4000 cm\u22121<\/sup> covers almost \nthe entire set of Raman-active species [7, 9]. However, when we \ntranslate this range into nanometers, the observable range turns to be \nonly a 26nm wide spectral window if \u03bbin<\/sub><\/em>\n = 248 nm, which implies a big challenge from the grating resolution and\n detector perspectives (UV-enhanced silicon detectors are a solution, \nbut sensitivity is still low). At the other extreme, if \u03bbin<\/sub><\/em>\n = 1064 nm, the window of observation is about 700 nm, which means that \ngermanium or InGaAs detectors are necessary. The relation between the \nexcitation wavelength and the width of the Raman spectrum is<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 1.3: Popular Raman excitation\n laser lines and their corresponding spectral observation window for the\n Raman scattering (Stokes radiation). The horizontal length of the side \nrectangles represent the spectral range in nanometers corresponding to a\n Raman shift going from 100 cm\u22121<\/sup> to 4000 cm\u22121<\/sup>, \nwhile the height illustrates the relative scattering intensity according\n to the excitation wavelength (notice that the vertical left axis is in \nlogarithmic scale). Additionally, relative spectral response of \ntraditional detector technologies are superimposed to illustrate the \nlink between laser source and detector.<\/em><\/p>\n\n\n\n

shown in Fig. 1.3. Moreover, MUV wavelengths can induce unwanted \nchanges in many samples (ionization, polymerization or bond-breaking \ntransitions) [6].<\/p>\n\n\n\n

1.3 The detector<\/strong><\/h5>\n\n\n\n

CCD cameras with Si-based detectors \nshows an excellent sensitivity specially in the red-NIR region, so the \noptimum combination seems to be a laser source in the range 400 \u2212 550nm \nwith a regular room temperature CCD camera. But having fluorescence in \nmind, the use of MUV lasers presents to main drawbacks: there are not \nyet cheap and widely available options in the laser industry below 300 \nnm, and the sensitivity of CCD cameras in the NUV-violet is not that \ngood. If we decide to move towards the NIR lasers, Raman scattered \nradiation will show lower and lower intensity as the irradiation \nwavelength increases. In addition, Si-based detectors are no longer a \ngood option when the Raman scattered light is over 1 micron, so other \ntype of detectors (like those based in InGaAs) must be used instead. So \nall in all, it is a question of identifying the best trade-off. In this \nsense, the progressive adoption of Raman spectroscopy by the industry \nhas converged towards the \u201chappy\u201d combination of 785nm plus CCD cameras,\n being nowadays the gold standard. This has enabled, for example, the \ncreation of affordable portable or even handheld Raman spectrometers, \nwhich contrast with the old, bulky and expensive laboratory equipment \nfrom the 80\u2019s. Lasers at 785nm can be diode-based (so very efficient, \ncompact, excellent emission characteristics, cheap and widely \navailable), while silicon photo-sensitivity stays within acceptable \nlevels (Raman shifts over 3000 cm\u22121<\/sup> are still detectable \nunder these conditions) and fluorescence is overcome in many cases. \nNonetheless, many cases still require different wavelengths and \ndifferent detectors.<\/p>\n\n\n\n

CCD cameras with Si-based detectors shows an excellent sensitivity specially in the red-NIR region, so the optimum combination seems to be a laser source in the range 400 \u2212 550nm with a regular room temperature CCD camera. But having fluorescence in mind, the use of MUV lasers presents to main drawbacks: there are not yet cheap and widely available options in the laser industry below 300 nm, and the sensitivity of CCD cameras in the NUV-violet is not that good. If we decide to move towards the NIR lasers, Raman scattered radiation will show lower and lower intensity as the irradiation wavelength increases. In addition, Si-based detectors are no longer a good option when the Raman scattered light is over 1 micron, so other type of detectors (like those based in InGaAs) must be used instead. So all in all, it is a question of identifying the best trade-off. In this sense, the progressive adoption of Raman spectroscopy by the industry has converged towards the \u201chappy\u201d combination of 785nm plus CCD cameras, being nowadays the gold standard. This has enabled, for example, the creation of affordable portable or even handheld Raman spectrometers, which contrast with the old, bulky and expensive laboratory equipment from the 80\u2019s. Lasers at 785nm can be diode-based (so very efficient, compact, excellent emission characteristics, cheap and widely available), while silicon photo-sensitivity stays within acceptable levels (Raman shifts over 3000 cm\u22121<\/sup> are still detectable under these conditions) and fluorescence is overcome in many cases. Nonetheless, many cases still require different wavelengths and different detectors.<\/p>\n\n\n\n

1.4 Figures of merit of lasers concerning Raman spectroscopy<\/strong><\/h5>\n\n\n\n

Apart from the wavelength of the laser \nsource, there are other important figures to keep in mind from the joint\n perspective of the laser manufacturer and the Raman equipment \nintegrator, which are listed below:<\/p>\n\n\n\n

\u2022 Beam quality: For samples where composition or structure spatial distribution is analyzed, TEM00<\/sub> beams maximize spatial resolution.<\/p>\n\n\n\n

\u2022 Polarization: Laser beam must be \nlinearly polarized in certain branches of Raman Spectroscopy where the \npolarization degree of the molecules is investigated.<\/p>\n\n\n\n

\u2022 Spectral linewidth: around 10pm or \nless is required to guarantee an acceptable resolution of the Raman \nspectra (the smallest difference in cm\u22121<\/sup> between Raman features that can be resolved)<\/p>\n\n\n\n

\u2022 Spectral purity: Whichever side modes \nconcerning the excitation wavelength need to be suppressed, so the main \npeak must prevail over them at a level > 60dB. This level of purity \nis usually acceptable at 1 \u2212 2nm around the main peak, although this \ndistance gets reduced as Raman shift goes into the sub \u2212 100 cm\u22121<\/sup> level.<\/p>\n\n\n\n

\u2022 Frequency stability: Since the \nacquisition time is usually in the order of seconds or tens of seconds, \nit is important to keep the excitation wavelength still enough (< \n10pm drift over time and operation temperature).<\/p>\n\n\n\n

\u2022 Output power stability: Typical power \nrange goes from 10 to 1000mW, depending on the analyte and the \nexcitation wavelength as mentioned above. The power stability is \nimportant mainly for quantification purposes and it is linked to the \nintegration time necessary to obtain a spectrum.<\/p>\n\n\n\n

\u2022 Isolation against optical feed-back: \nIn the particular case of confocal microscopy configurations \n(over-coupled excitation and backscattered beams), even a small portion \nof light backscattered into the laser source can cause power \ninstabilitiesor even laser degradation. Optical isolators must be used.<\/p>\n\n\n\n

1.5 Hot applications of Raman Spectroscopy<\/strong><\/h5>\n\n\n\n

Raman spectroscopy is gaining presence \nin bio-science given its non-invasive nature. Lasers for Raman \nspectroscopy can range many wavelengths, but usually they are in the \nUV-VIS-NIR part of the electromagnetic spectrum. Also, remote \nidentification of explosives can be carried out with Q-Switched lasers \nby using diode pumped solid state lasers 2nd, 3rd or 4th harmonic \ngeneration wavelengths. Raman spectroscopy can be found\u2014for instance\u2014in:<\/p>\n\n\n\n

– Art & Archaeology<\/p>\n\n\n\n

– Bio-science and Medical Diagnosis<\/p>\n\n\n\n

– Polymers and Chemical Processes<\/p>\n\n\n\n

– Semiconductor & Solar Industry<\/p>\n\n\n\n

– Geology and Mineralogy<\/p>\n\n\n\n

– Pharmaceutical Industry<\/p>\n\n\n\n

– Environmental Science<\/p>\n\n\n\n

– Raman Microscopy<\/p>\n\n\n\n

– Forensic Analysis<\/p>\n\n\n\n

– Gemology Teaching<\/p>\n\n\n\n

– Quality Control as well as General Research<\/p>\n\n\n\n

1.5.1 Take a closer look on food safety<\/strong><\/h5>\n\n\n\n

Based on signal enhancement techniques \nlike SERS it is possible to design on-line monitoring systems for food \nsafety and food quality control. In the past decades, since the food \nindustry is more profit driven and the globalization of the marked is \nprogressing, the consumers are more and more concerned about the food \nquality in mass production [10]. These concerns are based on real \nevidences, which proof a link between harmful food and health diseases. \nSome of them are analyzed and named by [12, 13]. The connection between \nhealth and food quality works in both directions, harmful food can cause\n diseases and healthy food can improve mental and physical body strength\n [10, 11]. These examples can be taken as motivation to ensure high \nquality food products. A reliable quality control can\u2014thanks to Raman \nspectroscopy (especially SERS) as well as other spectroscopy \nmethods\u2014easily be implemented into the production process. An overview \nconcerning food safety and quality control can be found in [14, 16] and \n[15] describes, for instance, how pesticides on fruit surfaces can be \ndetected.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 2.1: Picture of the fiber coupled version of the S-series \npackage. Also available as free-space version. Monocrom offers a broad \nvariety of wavelength and output power combinations.<\/em><\/p>\n\n\n\n

2. MONOCROM PRODUCTS THAT SERVE RAMAN SPECTROSCOPY APPLICATIONS<\/strong><\/h5>\n\n\n\n
2.1 Low power, single frequency diode laser<\/strong><\/h5>\n\n\n\n

Our S-series (depicted in Fig. 2.1) is \nideal for the most common Raman spectroscopy set-ups. It comes with \nfree-space or with SM-fiber output. Monocrom offers a broad spectrum of \nwavelength and output power combinations. Single frequency versions are \navailable with a linewidth as low as a few tens of MHz and a high side \nmode suppression ratio (SMSR) of typically 50 dB. On the other hand side\n the SM-fiber coupling capabilities deliver superb beam quality, which \nis mandatory for a high brightness Br<\/em> (see Eq. 1.2) and a high \nspatial resolution. The latter is important in Raman microscopy \napplications. Moreover, it opens the possibility to use PM-fibers for \npolarization depending Raman spectroscopy. Since most Raman spectroscopy\n applications need a stable output power Plas<\/sub><\/em> and a stablewavelength  \u03bbin<\/sub><\/em>\n over the integration time necessary to acquire a complete spectrum the \nS-series comes with athermoelectric cooler (TEC). The footprint of the \nstandard package is 100\u00d7100mm2<\/sup> but other packages can be manufactured on request.<\/p>\n\n\n\n

2.2 High energy solid state laser with\/without frequency conversion<\/strong><\/h5>\n\n\n\n

Our high energy diode pumped solid state\n laser (HESSL), which can be seen in Fig. 2.2, open the possibility to \nstep into all coherent Raman spectroscopy applications as well as remote\n Raman spectroscopy. The linewidth ( \u03bbin<\/sub><\/em> < 0.1nm) of the fundamental wavelength ( \u03bbin<\/sub><\/em> = 1064nm) transforms to < 1.77 cm\u22121<\/sup>\u2014 still narrow enough for the most requirements. The laser system is available in repetition rates 1 < Vrep<\/sub><\/em> < 500 Hz achieving up to Epulse<\/sub><\/em>\n = 1 J. Due to the high available output power the laser is suitable for\n remote Raman spectroscopy. Our high energy solid state laser  delivers a\n high power (< 2%@8 h) as well as pulse-to-pulse (< 1%rms) \nstability. Depending on the desired application the pulse width can be \nchosen between 4 ns < \u03c4pulse<\/sub><\/em> < 25 ns. It has to be kept in mind that \u03c4pulse<\/sub><\/em> influences Epulse<\/sub><\/em>\n and vice versa. Thanks to the high fundamental pulse energy, frequency \nconversion to the 2nd, 3rd or 4th harmonic are easily possible with high\n Epulse<\/sub><\/em>, harm. The footprint of the laser system is 900 \u00d7 500mm2<\/sup> and can be adapted to customers needs.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 2.2: Picture of the high energy solid state lasers<\/em><\/p>\n\n\n\n

References<\/strong><\/h5>\n\n\n\n

[1] J. Heath, N. Taylor: \u201cRaman Microscopy\u201d, John Wiley & Sons Ltd, 2017<\/p>\n\n\n\n

[2] E Smith, G. Dent: \u201cModern Raman Spectroscopy – A practical Approach\u201d, John Wiley & Sons Ltd, 2005<\/p>\n\n\n\n

[3] X. Liu: \u201cOrganic Semiconductor Lasers and Tailored Nanostructures\n for Raman Spectroscopy\u201d, Dissertation, Karlsruher Institute for \nTechnology, 2015<\/p>\n\n\n\n

[4] C. Raman, K. Krishnan: \u201cA new type of secondary radiation\u201d, Nature, Vol. 121, 501-502, 1928<\/p>\n\n\n\n

[5] G. Landberg, L. Mandelstam: \u201cEine neue Erscheinung bei der \nLichtzerstreuung in Kristallen\u201d, Naturwissenschaften, Vol. 16, 557-558, \n1928<\/p>\n\n\n\n

[6] P. Vandenabeele: \u201cPractical Raman Spectroscopy\u201d, John Wiley & Sons Ltd, 2013<\/p>\n\n\n\n

[7] E. Smith and G. Dent: \u201cModern Raman spectroscopy: a practical approach\u201d, John Wiley & Sons Ltd, 2005.<\/p>\n\n\n\n

[8] K. Kneipp: \u201cSurface-enhanced Raman scattering\u201d, Physics Today, Vol. 60, 40\u201346, 2007<\/p>\n\n\n\n

[9] P. J. Larkin: \u201cIR and Raman Spectroscopy – Principles and Spectral Interpretation\u201d, Elsevier, 2011<\/p>\n\n\n\n

[10] Z. You: \u201cApplication of Infrared Raman Spectroscopy in Analysis \nof Food Agricultural Products\u201d, AIDIC Servizi S.r.l., Chemical \nEngineering Transactions, Vol. 59, 763-768, 2017 [11] J. Depciuch et \nal.: \u201cApplication of Raman spectroscopy and infrared spectroscopy in the\n identification of breast cancer\u201d, Applied Spectroscopy, Vol. 70(2), \n251-263, 2016<\/p>\n\n\n\n

[12] A. A. Kadik et al.: \u201cApplication of IR and Raman spectroscopy \nfor the determination of the role of oxygen fugacity in the formation of\n n\u2013\u00d1\u2013\u00ce\u2013\u00cd molecules and complexes in the iron-bearing silicate melts at \nhigh pressures\u201d, Geochemistry International, Vol. 54(13), 1175-1186, \n2016<\/p>\n\n\n\n

[13] J. Yu et al.: \u201cRecent applications of infrared and Raman \nspectroscopy in art forensics: a brief overview\u201d, Applied Spectroscopy \nReviews, Vol. 50(2), 152-157, 2015<\/p>\n\n\n\n

[14] Z. Zhang: \u201cRaman Spectroscopic Sensing in Food Safety and Quality Analysis\u201d, University of Nebraska-Lincoln, 2017<\/p>\n\n\n\n

[15] J. Chen, D. Dong, S. Ye: \u201cDetection of pesticide residue \ndistribution on fruit surfaces using surface-enhanced Raman spectroscopy\n imaging\u201d, The Royal Society of Chemistry, Vol. 8, 4726-4730, 2018<\/p>\n\n\n\n

[16] Y. S. Li, J. S. Church: \u201cRaman spectroscopy in the analysis of \nfood and pharmaceutical nanomaterials\u201d, Elsevier, Journal of food and \ndrug analyzes, Vol. 22, 29-48, 2014<\/p>\n","protected":false},"excerpt":{"rendered":"

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