Guide to QCL Microscopy
What is a QCL or quantum cascade laser ?
In short, it is a tunable Mid-IR laser - but let's go into more detail.
A quantum cascade laser is a heterogeneous diode laser and as with any diode laser, radiation is generated by applying voltage to it. In contrast to a classical homogeneous diode laser that produces only a single wavelength, a QCL can emit light in a range of wavelengths in the mid and far infrared. To do this, the QCL is placed in a so-called external cavity, in which the emitted wavelength can be selected by tilting a grating. This process is called "tuning".
In a QCL, distinct semiconductor layers are stacked on top of each other to form the diode. The total length of the laser diode reaches several millimeters, although each individual layer is only a few nanometers thick. Depending on the thickness of the individual layers, a stack of layers can either form an “active region” or a doped electron injector. Typically, a QCL consists of a series of pairs of active regions and injectors.
How can we select the spectral range of a QCL?
Basically, QCL spectroscopy utilizes the fact that quantum objects (e.g. electrons) must adopt discrete states in a potential well. These potential wells and therefore the energy of quantum objects in it can be defined by the design of the individual layers of the heterogenous semiconductor structure of the QCL. Thus, by specifically designing of each active region of a QCL, we can determine which wavelength will be emitted during the transition of the electron.
If a QCL is meant to be used for spectroscopy in a broad spectral range, the design of the active regions must be adjusted accordingly. For example, the so called “bound to continuum” designs will create a quasi-continuous spectrum of photons in a broad spectral range. By placing the QCL in an external cavity, the emission wavelength of the laser can be set to any wavelength that’s within the spectrum of the active region, creating a tunable mid infrared source.
How does a quantum cascade laser create photons?
In a QCL, the radiation is not generated by the classical recombination of an electron from the conduction band with a hole of the valence band. If an appropriate bias voltage is applied to the QCL, the electrons in the conduction band of the semiconductor will start to "cascade" through the stack of active regions and injectors. Each electron will perform a laser transition in an active region, creating a mid-infrared photon.
Because the electrons remain in the conduction band after each laser transition, each electron will generate multiple photons, one for each active region of the QCL. Over the recent years, the average output power of tunable QCLs increased and the accessible spectral range broadened significantly. The performance improvements at their core are based on improved quantum mechanical calculations describing the energy states of electrons in active regions and injectors.
The difference between QCL and FT-IR spectroscopy
To understand why QCL opens a new door to an exciting path of infrared spectroscopy, we must first take a closer look at the gold standard: FT-IR. Keep in mind, that quantum cascade lasers fundamentally differ from the conventional thermal sources which are used for FT-IR.
A traditional, thermal source emits photons over a broad spectral range. Therefore, the number of photons per wavelength is quite small and consequently, the detectors for FT-IR spectroscopy must be highly sensitive.
Especially in microscopy, this often requires liquid nitrogen for cooling. Additionally, the detectors need to be very fast to be able to record interferograms in a spectrometer. A QCL, on the other hand, will emit all its photons on approximately the same wavelength. That means that the spectral power density of a QCL is typically orders of magnitudes higher than that of a thermal source.
This quasi-monochromatic nature enables the use of relatively slow, uncooled detectors, that would not be suitable for FT-IR spectroscopy.However, when combined with a QCL, we can take advantage of the full dynamic range of these detectors and detect signals at a single wavelength, resulting in excellent signal-to-noise ratios.
This brings us back to the initial question: why use QCLs for spectroscopy?
Sure, the better signal-to-noise ratio is a big plus but most importantly, a QCL microscope does not need to record a full IR spectrum for every pixel of the chemical image - unlike its FT-IR counterpart.
In certain scenarios, such as real-time infrared imaging, this makes the QCL-based instrument much more efficient than its FT-IR-based counterpart.
What is a QCL microscope?
Just like a conventional FT-IR microscope, a QCL IR microscope allows for chemical imaging of microscopic samples by acquiring spatially resolved spectral information. But instead of spending the time to capture a full IR spectrum at each measurement point, it enables you to focus on a specific spectral range and thereby increases the imaging speed by an order of magnitude.
In short, you get a QCL IR imaging microscope, when you replace the standard broad band thermal source and the spectrometer used for FT-IR with a quantum cascade laser and switch the N2-cooled FPA-detectors of an imaging FT-IR microscope with a room temperature microbolometer array.
Not every QCL microscope is the same
The widefield setup is one of various designs of QCL microscopes. Here, a larger sample area is illuminated at once and the transmitted or reflected radiation is then detected by a microbolometer array. And since QCL sources offer a much higher spectral power density, we can use these detectors to acquire infrared images at video framerates.
But of course, implementing QCL technology into an IR microscope comes with some technical challenges. First and foremost, the coherent nature of the laser source leads to something called coherence artefacts. These fringes and speckles in IR images and spectra are generally considered harmful for chemical imaging. Indeed, it is not trivial to separate the chemical information of the sample from the physical information describing the phase relationship of the scattered photons.
However, Bruker has addressed this (and more) with HYPERION II. It is finally possible to collect beautiful, coherence-artefact free chemical IR images in any measurement mode.
The advantages of QCL microscopy:
FT-IR micro spectroscopy can be significantly augmented by QCL-based IR microscopy, because of the complementary nature of both techniques. Although modern QCLs cover the entire spectral fingerprint region, the full Mid-IR region is, however, not yet accessible. This becomes particularly apparent since a broad spectral range is an innate strength of FT-IR and extending the range of QCL systems comes at a significant cost.
On the other hand, the main drawback of an FT-IR microscope remains the limited acquisition speed, which in turn is not a problem for a QCL-based widefield setup. By combining the two techniques, disadvantages can be avoided, and their advantages combined to increase the efficiency of both.
This is precisely why scientists need to carefully compare FT-IR and QCL in real-world scenarios. This is the only way to sustainably transfer and improve established IR applications. Likewise, it is essential that new QCL methods are verified by FT-IR to make them more reliable and robust.
The advantages of QCL microscopy:
- Reduced acquisition time by focusing on a specific spectral range
(e.g. a single absorption peak) - Real-time infrared imaging at a single wavelength
(at video framerates) - ROI selection in real-time by chemical data instead of visuals only
- Ultra-fast creation of large IR overview images with high chemical contrast
The limitations of QCL microscopy:
- Spectral range is limited compared to FT-IR (only MIR accessible)
- High power IR laser emission is dangerous to eyes and skin. That is why Bruker has developed a class 1 laser enclosure with safety interlocks to protect the user from harmful laser radiation.
QCL Microscopy Explained in 5 Minutes
Frequently Asked Questions about QCL IR
What is a laser?
A laser is a device that emits coherent radiation created by stimulated emission. (LASER: light amplification through stimulated emission of radiation)
What does QCL stand for?
QCL stands for quantum cascade laser.
What is a quantum cascade laser?
A quantum cascade laser (QCL) is a heterogeneous diode laser that can emit radiation in a range of wavelengths, specifically in the mid infrared (MIR) region of the electromagnetic spectrum.
What can QCLs do?
QCLs can be used as a source of infrared radiation in IR spectroscopy. Such QCL IR instruments do not require an interferometer but offer a limited spectral range.
What is a QCL IR microscope?
A QCL IR microscope can be used to create chemical images of microscopic samples. It acquires spectral information in every pixel of a chemical image, revealing the distribution of the chemical components of the sample.
What is QCL IR imaging?
For QCL IR imaging, a QCL microscope is designed as a widefield setup, where a microbolometer array is recording full images of a microscopic sample at video framerates. By using a array detector, very high data acquisition rates can be achieved.
Is QCL better than FT-IR?
It
fully depends on the specific experiment since both techniques have
unique benefits and limitations. While QCL has a limited spectral range
but provides high imaging speeds, FT-IR offers the full MIR spectral
range at limited acquisition speed.
Where are QCLs applied?
Currently they are most used in QCL IR microscopes, spectrometers and in systems for trace gas detection.
Can I combine FT-IR and QCL?
Bruker will soon offer a seamless integration of both techniques in one intrument. This way you gain both benefits by avoiding any limitation.