Laser performance parameters

The most commonly used wavelength in Raman spectroscopy is 785 nm. It offers the best balance between scattering efficiency, influence of fluorescence, detector efficiency and availability of cost-efficient and compact, high-quality laser sources.

There is a number of different kinds of lasers available at 785 nm. They offer different performance and cost characteristics, which means a careful selection can be important to find the best solution for a particular Raman spectroscopy set-up.

Critical performance parameters are:

1. Spectral bandwidth
   • This should be less than a few 10s of pm in order not to limit the resolution of the system. In some high resolutions applications a linewidth of much less than that can be required
2. Spectral purity (or side-mode suppression ration – SMSR):
   • Of the illumination source should be at least better than 60 dB in the spectral region where Raman peaks are to be detected
3. Wavelength stability:
   • Must be low, in the order of few pm, both over time and temperature in order not to limit the resolution of the system
4. Mode:
   • For high resolution imaging applications it is important with a high quality TEM00 transversally single-mode beam profile. (For probe-based systems multimode beams work, as long as they can be efficienly coupled into fibers of 50-200 µm core sizes.)

Technologies for lasers around 785 nm

There are basically 4 different types of lasers available at 785 nm, all having their own strengths and weaknesses when it comes to their use or Raman spectrscopy:

1. 785 nm diode lasers:
   • Standard Fabry-Perot type semiconductor lasers typically have spectral bandwidths of over 1nm are therefore usually not suitable for Raman spectroscopy applications.
2. 785 nm DFB diode lasers:
   • These are single transversal mode semiconductor emitters with a DBR (distributed bragg reflector) structure integrated with the gain structure on the chip itself. They can be purchased as a single laser chip or packaged ready for use. They can offer very narrow linewidths and even single-frequency performance but are limited in output power to a few 10s of mW. SMSR is normally only 30-50 dB even several nm form the main peak, meaning that a spectral clean-up filter must be used to make them suitable for Raman spectroscopy. The compact size makes them suitable for small hand-held systems.
   • 
3. 785 nm Narrow Linewidth Diode lasers (NLD) or frequency stabilized diode lasers:
   • Frequency stabilized diode lasers are based on high power Fabry-Perot semiconductor diode lasers with an external grating structure (typically a VBG – Volume Bragg grating) for frequency locking the emission to a narrow linewidth. The approach works with both single transversal (TEM00) and multi transversal mode emitters and results in laser emission with linewidths of a few 10s of pm down to single-frequency performance. Output powers range from just over 100 mW for single transversal mode lasers to Watt level lasers for multi mode. Like with the DBR lasers it is usually required to combine these lasers with a spectral clean-up filter to achieve sufficient SMSR for high quality Raman spectroscopy results. The Cobolt 08-NLD series is based on this technology and include both single-transversal mode TEM00 lasers with up to 120 mW power and multimode lasers with up to 500 mW output power. All Cobolt 08-NLD lasers have dichroic spectral clean-up filters integrated in the design and comes with an integrated optical isolator as standard, making the lasers insensitive to optical feedback. All Cobolt 08-NLD lasers can also be fiber-coupled into single-mode polarization maintaining (SM/PM) fibers or multimode fibers.
   • 
4. 785 nm frequency stabilized lasers with Enhanced Spectral Purity (ESP):
   • The frequency stabilized laser diodes performance mentioned in 3. can be further improved. Not in terms of reducing the linewidth but in terms of increasing the spectral purity. The spectral purity is important in high resolution Raman since the Raman signal is inherently weak. If the background spectrum is too high then the Raman signal cannot be seen. With the dichroic spectral clean-up filters in the standard Cobolt 08-NLD lasers it is possible to achieve a spectral purity of >60dB is at around 1-2 nm from the main peak, which is sufficient for detection of Raman shifts in the finger-print spectral region of 200-40000 cm^-1. However, low-frequency Raman applications, in the region <200 cm^-1, require a high side-mode suppression ratio (SMSR) a few 100 pm from the main peak. The Cobolt 08-NLD ESP lasers are based on a patent pending technology which enables a spectral purity of >60 dB as close as 300 nm away from the laser peak. These lasers are have Enhanced Spectral Purity (ESP) are very suitable for Raman spectroscopy with spectral shifts in the low-frequency or THz region.
   • 

Raman light sheet microscopy: a fast micro-spectroscopy imaging technique

Raman spectroscopy provides information about the chemical composition of materials, cells and tissues. The Raman process is induced by scattering of the excitation light with the vibration of the molecules contained in the sample. This provides an intrinsic label-free contrast mechanism from the sample that is rich in biochemical content. Taking 2D information of the molecular distribution that conforms the sample is thus, very attractive to the biomedical science community. However, the efficiency of the Raman scattering process is very low and, therefore, long acquisition times (that goes from few minutes to several hours) are required to obtain a single 2D image. This makes practically impossible the use of Raman imaging for in vivo studies and/or volumetric chemical information. In this post we present a recently published approach for fast 2D Raman imaging [1], based on a digital scanned light sheet microscopy (DSLM) system that uses interferometric tunable filters and a compact Cobolt 06-MLD Laser operating at 638 nm.

Figure 1. Raman LiShMS setup. GM: x-y Galvo-Mirror; EO: exciting objective; CO: collecting objective; SL: scan lens; TL: tube lens TF: tuning filter.

Raman imaging without using a spectrometer

A DSLM system can be adapted such that the digitally scanned plane of light (see Figure 1) is used to excite the Raman process at a single plane of the sample. The generated 2D Raman information is then imaged onto a CMOS camera through the collection arm of the microscope. Without any modification, such system will record an image where each pixel represents the integrated Raman information without any spectral resolution. To spectrally resolve the Raman information, an interferometric tunable filter (TF) is utilized. By changing the angular orientation of the TF, its sharp edged transition is continuously shifted.

Figure 2. Raman LiShMS imaging in different organic solvents using a 638 nm Cobolt Laser at 20 mW, a) spectral knife-edge traces, b) Raman spectra after the derivation process, and, c) Corresponding Raman spectra of the same solvents, taken with a confocal micro-Raman system. Reproduced from Rocha-Mendoza et al., Biomed. Opt. Express 6(9), 3449-3461 (2015).

This allows recording on every pixel a cumulative intensity distribution of the vibrational Raman bands as the TF angular position is changed (see inset of Fig. 1 and Fig. 2a). Afterwards, a simple differentiation of the images is used to recover the Raman spectrum (see Fig. 2b). Since this procedure is performed pixel-by-pixel on the entire image, a 3D spectrally resolved Raman map is obtained for each illuminated plane. The obtained Raman spectra are in good agreement with the spectra taken under a confocal micro Raman system (Fig. 2c).

Getting faster 3D hyperspectral Raman image

The feasibility of the technique for imaging solid 3D samples was demonstrated using a compact CW Cobolt laser at 638 nm for studying the contents of lipids and proteins in living biological samples, such as C. elegans nematodes (Figure 3). It is worth mentioning that the required time to extract all this 3D-Raman information (300 × 300 × 180 µm3) was about 5 minutes.

Figure 3. 3D Raman Light Sheet micro-spectroscopy of C. elegans nematodes. a) Bright-field and b) integrated Raman images of a sample with polystyrene (PS) beads as reference Raman markers. 3D orthogonal views of spectrally resolved Raman images of a C. elegans at c) 2910cm-1 and d) 2960 cm-1, respectively. Scale bars: 100 µm. Adapted from Rocha-Mendoza et al., Biomed. Opt. Express 6(9), 3449-3461 (2015).

Summary

A compact Cobolt laser was utilized in a light sheet scheme to perform rapid 2D and 3D Raman imaging spectrally resolved in the C-H region. The image acquisition speed is up to 4 orders of magnitude faster than the acquisition speed of confocal micro-Raman micro-spectroscopy [1].

Authors
Israel Rocha-Mendoza1,2, Jacob Licea-Rodriguez1,3, Monica Marro1, Omar E. Olarte1,4, Emilio J. Gualda1, and, Pablo Loza-Alvarez1*
1 ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Spain
2 CICESE; 3Cátedras CONACYT-CICESE, Ensenada, México; 4 Universidad ECCI, Bogotá, Colombia

References
1. I. Rocha-Mendoza, J. Licea-Rodriguez, M. Marro, O. E. Olarte, M. Plata-Sanchez, and P. Loza-Alvarez. “Spontaneous Raman light sheet microscopy using cw-lasers and tunable filters”, Biomed. Opt. Express. 6(9), 3449-3461 (2015).

New editorial editorial in Wiley’s Physics Best April 2020.

Low frequency Raman spectroscopy: How narrow-line diode lasers facilitate pharmaceutical inspection

Read the full editorial here

Recently there is also a lot of interest in accessing the low-frequency Raman spectroscopy region (<10 cm-1 to 200 cm-1). Raman shifts in this frequency range give access to lattice vibrations of molecular crystals and have the potential to more directly probe intermolecular interactions in solid materials. The low-frequency Raman region probes the same low-energy vibrational and rotational modes of molecular structures as terahertz spectroscopy (300 GHz – 6 THz). The THz region of Raman spectra contain important structural information about the molecules or crystal lattices under investigation. In the pharmaceutical industry, for instance, this structural information can help to determine the crystallinity, and therefore solubility, of pharmaceuticals. Specifically, low-frequency Raman spectroscopy provides an avenue to probe polymorphic structures of pharmaceutical systems before and after tableting, as well as drug identification and quantitation for crystalline materials, both of which are critical quality attributes in pharmaceutics.

Being able to determine the structural form of the Active Pharmaceutical Ingredients (API) is a primary objective in the pharmaceutical industry during drug development, manufacturing and quality control. APIs exhibit polymorphism, which is characterized as having identical chemical compositions but different solid-state structures that may affect the bioavailability and therapeutic index, which could lead to compromised efficiency of any final drug product [2-3]. Probing the low frequency Raman region has several advantages over existing techniques such as ease of collecting data (compared with tradition methods such as X-ray powder diffraction and differential scanning calorimetry), collecting spectrum rich in structural information as well as being able to discriminate crystalline forms.

Narrow-line diode lasers for low frequency Raman

Laser sources suitable for Raman spectroscopy at 785 nm can be fabricated from AlGaAs-based semiconductor devices. Depending on the emitter size and geometry they can be designed to emit single-transversal mode beams (lower power) or multi-transversal mode beams (higher power). Semiconductor lasers have a broad gain spectrum and typically have bandwidths of over 1 nm and with long spectral tails stretching several 10s of nm from the main peak. They can be made to emit spectrally single-mode radiation by introducing distributed wavelength selective gratings (DBR/DFB) into the structure. DBR/DFB lasers at around 780 nm are available at up to a few 10s of mW output power. As an alternative for higher power levels, it is possible to turn the broad-band emission from a semiconductor laser into narrow-band by building an external cavity with a separate wavelength-selective cavity element. In this way the stimulated emission from the laser will be frequency-locked to the spectral distribution of the feed-back from the external cavity element. A conventional way of constructing such frequency-locked semiconductor laser is to build an external cavity using a Volume Bragg Grating element (VBG).

The excellent spectral purity of the Cobolt 08-NLDM ESP 785 nm laser is shown by comparing its spectral peak (red) with a standard frequency-locked diode laser with an external dichroic filter (orange) and with a standard frequency-locked diode laser without external filter (blue). The Cobolt 08-NLDM ESP 785 nm achieves > 60 dB SMSR at < 0.3 nm (or < 5 cm−1) without any external filtering.

The output beam from the 785 nm semiconductor emitter is collimated and before going into the VBG element, which reflects a fraction of the light with a narrow spectral distribution back into the semiconductor. A draw-back for both DFB/DBR and conventional VBG frequency-locked laser devices is that a fair amount of broad-band Amplified Spontaneous Emission (ASE) from the semiconductor is still emitted from the laser device. This limits the SMSR ratio to around 40-50 dB up to several nm’s away from the main peak. In order for such lasers to be useful for Raman spectroscopy they have to be spectrally filtered with external clean-up filters. In the case of low-frequency Raman spectroscopy it is not enough to use standard dichroic filters with a typical bandgap of 1-2 nm, but necessary to use more narrow spectral filtering, typically by adding a second external VBG element. This additional VBG filter adds cost to the system and can be challenging to match spectrally to the specific output wavelength of the laser.

In order to overcome this draw-back, we present here an alternative and patent pending design for frequency-locking a semiconductor laser. Instead of using a partially transmissive VBG as in the conventional frequency locked diode laser, a highly reflective VBG element is used as the wavelength selective component in the external cavity. An intra-cavity polarizing element and a polarizing beam splitter are used to control the level of feed-back from the VBG to the emitter and the output coupling out of the cavity. In this way, only the stimulated emission is coupled out of the cavity and the broad-band non-stimulated emission is leaking out of the VBG element. The resulting spectral purity is similar to what is achieved with an external VBG clean-up filter, but with the use of only one single VBG element.

Authors: Dr Peter Jänes & Dr Håkan Karlsson, Cobolt, a part of HÜBNER Photonics.

Numerous different wavelengths of light are commonly used in Raman spectroscopy, ranging from the ultraviolet (UV) through the visible, and into the near-infrared (near-IR). Choosing the best illumination wavelength for a given application is not always obvious. Many system variables must be considered to optimize a Raman spectroscopy experiment, and several of them are connected to the wavelength selection.

Why is the choice of laser wavelength important for Raman Spectroscopy?

To start with, the Raman signal is inherently very weak. It relies on the photon-phonon interaction in the sample material, which is typically a one-in-a-million event. In addition, the Raman scattering intensity is inversely proportional to the 4th order of the illumination wavelength, which means that illumination at longer wavelengths results in a decreased Raman signal.

The detector sensitivity is also dependent on the wavelength range. CCD‘s are commonly used for detection of the Raman signal. The quantum efficiency of these CCD devices rolls off fairly quickly beyond 800 nm. For illumination beyond 800 nm, it is possible to use InGaAs array devices, but those are associated with higher noise levels, lower sensitivity and higher cost. The wavelength dependence of the Raman signal strength and the detection sensitivity all seem to point towards the use of shorter wavelength illumination (UV and visible) as opposed to longer wavelengths (in the near-IR). However, there is still a challenge to overcome with shorter wavelength illumination: Fluorescence emission. Many materials emit fluorescence when excited with UV-visible light, which can swamp the weak Raman signal. Even so, the most commonly used wavelength in Raman spectroscopy is 785 nm. It offers the best balance between scattering efficiency, influence of fluorescence, detector efficiency and availability of cost-efficient and compact, high-quality laser sources. However, the use of visible lasers in the blue and green (in particular at 532 nm) is increasing.

What performance characteristics should be considered when selecting a laser for Raman spectroscopy experiments?

In addition to wavelength, there are a number of important performance parameters that should be taken into account when choosing the best laser sources for a Raman spectrometer. Key performance parameters are:

Six important laser performance parameters

1. Laser wavelength:
   • The most commonly used wavelength in Raman spectroscopy is 785 nm. It offers the best balance between scattering efficiency, influence of fluorescence, detector efficiency and availability of cost-efficient and compact, high-quality laser sources. However, the use of visible lasers in the blue and green (in particular at 532 nm) is increasing. It also depends on the material under investigation.
2. Spectral linewidth:
   • This sets a limit to the spectral resolution of the recorded Raman signal (i.e. how small of a difference in Stokes shift can be detected). For most fixed-grating systems, the laser linewidth should be a few 10 pm or less in order to not limit the spectral resolution of the system. However, high resolution systems may require linewidths much less than that, sometimes even less than 1 MHz.
3. Frequency stability:
   • The laser line must stay very fixed in wavelength during recording of the spectrogram in order not to deteriorate spectral resolution. Typically, the laser should not drift more than a few pm over time and over a temperature range of several degrees C.
4. Spectral purity:
   • Detecting the Raman signal normally requires a spectral purity of >60dB from the laser source (ie how well side-modes to the main laser line are suppressed). For many cases, it is sufficient if the level of spectral purity is reached at around 1-2 nm from the main peak. However, low-frequency Raman applications require a high side-mode suppression ratio (SMSR) of a few 100 pm from the main peak.
5. Beam quality
   • In confocal Raman imaging applications, it is necessary to use diffraction limited TEM00 beams for optimum spatial resolution. However, for probe-based quantitative Raman analysis, the requirement is not as tight. It is normally sufficient with a beam quality that allows for efficient coupling into multi-mode fibres, e.g. with 50-100 μm core diameters.
6. Output power and power stability
   • Typical laser output powers range from around 10 mW in the UV, up to several 100 mW in the near-IR. The output power requirement is related to the wavelength, the type of material(s) that will be investigated, as well as the sampling frequencies and imaging speeds.The output power of the laser should not fluctuate with more than a few %, also in a varying ambient temperature.

The figure above illustrates the importance of point 4. and shows the difference in spectral purity of 785 nm diode based lasers.

The compactness, robustness, reliability, lifetime and cost structure are also very important parameters to be considered in the selection of the optimum illumination source for a Raman system. Raman instrumentation has progressed into becoming a standard analytical tool in many scientific and industrial applications. Users expect to run routine experiments or process monitoring measurements for years without the need for service or exchange of the laser source. In a growing number of cases, the instrument must also operate in harsh, industrial environments.

For these reasons, most Raman systems nowadays are equipped with solid-state based laser sources rather than gas lasers. Today, compact solid-state lasers with proven operation lifetimes of several 10,000 hours which meet the most advanced optical performance requirements are available in all wavelength ranges commonly used for Raman spectroscopy.

The different types of laser technology which can meet all the above mentioned 6 requirements can be grouped into 4 categories:

1. Diode-pumped lasers: SLM (single-longitudinal mode)
2. Single-mode diode lasers: DFB (distributed feedback) or DBR (distributed Bragg reflection)
3. VBG frequency stabilized diode lasers
4. Tunable single frequency lasers

Read the full editorial here:

Learn more watching our webinar on How to select a laser for Raman spectroscopy:

CW tunable lasers and their use in TERS

Photonics research undertakes considerable efforts to continuously refine nanoimaging techniques driven by the desire to characterize electronic and vibronic properties of new materials with nanometer resolution.

Tip-enhanced Raman spectroscopy (TERS) is an approach that has been well recognized and relies on strongly localized enhancement of Raman scattering of laser light at the point of a near atomically sharp tip. However, not least due to the lack of sources delivering laser light tunable throughout the visible spectral range, the vast majority of TERS experiments so far has been limited to single excitation wavelengths. A recent study demonstrates excitation-dependent hyperspectral imaging, exemplified on carbon nanotubes by implementing a tunable continuous-wave optical parametric oscillator (C-WAVE) into a TERS set-up.

Excitation-Tunable Tip-Enhanced Raman Spectroscopy

The three main components of a TERS set-up a: A laser light source for excitation, an atomic force microscope (AFM) equipped with a carefully selected sharp metallic tip, and a Raman spectrometer recording the inelastically scattered radiation. In essence, the physical principle behind TERS relies on so-called localized surface plasmons that are excited by the laser light in the microscope tip. These plasmons generate a strongly localized electromagnetic field, which not only enhances the incoming and Raman scattered radiation by orders of magnitude, but also ensures a highly localized excitation of the sample under study. Thus, by recording tip-enhanced Raman spectra intensities as a function of the tip position, TERS allows for nanoimaging with a spatial resolution down to below 10 nm.

The figure above illustrates the sequence of events and results of a TERS experiment carried out at a single laser excitation wavelength (633 nm) on a film of a CNT mixture. In a first step, a so-called composed Raman spectrum is recorded by placing the microscope tip at a particular (x,y) position in close proximity to the CNT film. The resulting composed Raman spectrum encompasses the RBM peaks of several CNTs (all of them in electronic resonance to the excitation wavelength). In a second step, the microscope tip is retracted, and the so-called far field spectrum is recorded without the tip-enhanced Raman contribution to the signal. By subtracting the far-field spectrum from the composed spectrum, the pure tip-enhanced Raman spectrum is obtained. Eventually, from the pure tip-enhanced Raman spectrum, the tube species underneath the tip position can be unambiguously identified (CNT with (7,5) chirality in the example shown in Figure 4a). For a spatial image of the particular CNT, the outlined procedure is repeated: The tip position is scanned step-wise over the sample surface, and at each point the intensity of the pure tip-enhanced Raman peak is determined. Figure 4b shows the result of such a scan and images the position of a (7,5) CNT in a 2 x 2 μm2 area. As can be clearly perceived, the CNT is around 800 nm long and bent in a step-like shape.

The full beauty of the experimental approach now unfolds when realizing that the imaging capability of the set-up is no longer limited to a subset of CNTs that happen to be in electronic resonance to a particular excitation wavelength. This hyperspectral principle of nanoimaging is at the very heart of the excitation-tunable tip-enhanced Raman spectroscopy (e-TERS) methodology very recently reported by Kusch and co-workers [5]. By using four different excitation wavelengths, they are able to identify and image a total of nine different CNT species within one and the same sample area as show in the images below.

The experimental demonstration of excitation-tunable tip enhanced Raman spectroscopy comes in tandem with the availability of novel tunable laser light sources based on OPO technology. From the general laser technology point-of-view, the performance characteristics of OPOs make them competitive alternatives to conventional lasers and related technologies for the generation of widely tunable cw radiation. From the experimental methodology point-of-view, we expect e-TERS to open new experimental horizons for studying the electronic and vibronic properties of matter on the nanometer scale.

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The authors gratefully acknowledge enlightening discussions and support by Patryk Kusch from the group of Stephanie Reich at the Free University Berlin.

[5] N. S. Mueller, S. Juergensen, K. Höflich, S. Reich, and P. Kusch, Excitation-Tunable Tip-Enhanced Raman Spectroscopy, J. Phys. Chem C, accepted