https://doi.org/10.3390/mi12121492
“A novel compact laser absorption spectrometer is developed for colorimetric detection. We demonstrate the realization of the system as well as example measurements of phosphate in water samples based on the malachite green (MG) method. A phosphate concentration range of 1 mg/L1 mg/L to 31.25 μg/L31.25 �g/L (which corresponds to a molar concentration range of 10.5 μmol/L10.5 �mol/L to 329 nmol/L329 nmol/L) is investigated. This photometer demonstrates the ease of integration of organic distributed feedback (DFB) lasers and their miniaturizability, leading the way toward optofluidic on-chip absorption spectrometers. We constructed an optically pumped organic second-order DFB laser on a transparent substrate, including a transparent encapsulation layer, to have access to both emission directions of the surface-emitting laser. Using the two different surface emission directions of the laser resonator allows monitoring of the emitted light intensity without using additional optical elements. Based on these advances, it is possible to miniaturize the measurement setup of a laser absorption spectrometer and to measure analytes, such as phosphate.”
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2. Experimental
The measurement of the absorbance of a colorimetric reaction is tied to the sample geometry. The absorbance is exponentially coupled to the optically penetrated distance. For this purpose, a laser is set up and its emission is attenuated when it passes through a phosphate-selective color-forming reagent.
2.1. Design and Fabrication
In our work, the emission source is an organic second-order DFB laser in a sandwich construction. Optical emission is achieved through a thin layer of an organic semiconductor polymer. Organic semiconductors are conjugated molecules, with the semiconducting properties arising from the overlap of molecular orbitals [14]. Organic semiconductors, which intrinsically offer a four-level laser architecture, achieve their gain by the recombination of excited singlet exciton states. While excited singlet states contribute to optical gain, other excited states, such as triplet excitons and polarons, are considered detrimental to lasing [15]. Both triplet excitons and polarons are the major product of charge recombination [15,16]; thus, efficient organic lasers mostly employ pulsed optical pumping [17,18] (see section Optical Setup below). In addition to the pump configuration used in this work, it is also possible to pump the organic laser with a pulsed inorganic light emitting diode or laser diode [19,20,21].
The organic semiconductor materials used have high absorption for the pump radiation [16], which enables a thin nanometer-scale organic layer to act as a planar waveguide. The superposition of frequency-selective diffraction at a spatial corrugation of the boundary surface and the guiding planar layer leads to a DFB resonator structure.
The process steps for manufacturing the organic DFB laser are illustrated in Figure 1b. A schematic cross section of the device is shown in Figure 1a. The three functional laser layers DFB substrate, organic emission layer and encapsulation layer are stacked on a commercially available microscope glass slide, which from now on is denoted as an auxiliary substrate (see Figure 1b Step 1).
Figure 1. Laser design: (a) Three-dimensional illustration of a surface-emitting DFB laser with a standard glass cuvette transmitted by the bottom emission of the laser (not to scale): (a) encapsulation layer; (b) active organic layer and core of the planar waveguide; (c) auxiliary substrate (microscope glass slide); (d) DFB resonator substrate; (e) top emission; (f) bottom emission (b) illustration of the fabrication process steps of the organic second-order DFB laser (not to scale); 1. Auxiliary substrate; 2. Apply MD700 film; 3. Positioning of master on MD700; 4. UV-exposure through master; 5. Removing master; 6. Apply active organic emission layer; 7. Encapsulate with MD700 (Materials: a, MD700; b, conjugated polymer; c, glass; g, fused silica.
The DFB substrate carries a sinusoidal one-dimensional grating with a period of Λ=400 nm�=400 nm. In order to create the DFB substrate, a replica of a master grating was molded into a Fluorolink® MD700 (PFPE-urethane methacrylate) layer on top of an auxiliary substrate. Here, the master grating used is a segment of a patterned fused silica wafer produced by laser interference lithography. MD700, diluted with 2% by weight of the photoinitiator Darocur® 1173, was deposited on the auxiliary substrate (Figure 1b Step 2). The master was lightly pressed into the MD700 with the patterned surface pointing downwards (Figure 1b Step 3). The ultraviolet (UV)-curable MD700 was exposed for 60 s through the master using a mercury vapor lamp with a power density of 800 mW⋅cm−2800 mW⋅cm−2 (Figure 1b Step 4). Subsequently, the master can be removed and later be reused, the crosslinked MD700 now carries a negative replication of the master grating (Figure 1b Step 5). The depth of the sinusoidal structure in the MD700 with dst=38.16 nm±0.37 nm���=38.16 nm±0.37 nm agrees well with the structure depth of the glass master with dst=38.74 nm±0.62 nm���=38.74 nm±0.62 nm. The active organic emission layer consists of a guest–host system of MEH–PPV (ADS100RE) and F8BT (ADS233YE). The guest, poly(2-methoxy,5-(2′-(ethyl)hexyloxy)-p-phenylene vinylene), and the host, poly(9,9-dioctylfluorene-alt-benzothiadiazole), were dissolved in toluene at an experimentally optimized weight ratio of 6%:94%. In a spin coating process, the active organic material was applied to the DFB substrate at a spin speed of 750 rpm in a toluene-saturated atmosphere (Figure 1b Step 6). This results in an approximately 320 nm320 nm thick layer that forms the optical waveguide of the laser cavity. To reduce photooxidation and increase the life expectancy of the laser, the organic layer is encapsulated with an additional MD700 layer (Figure 1b Step 7). As illustrated by the beam lobes in Figure 1a, all layers have a high transmittance in the visible spectrum and laser emission can be radiated in bottom and top directions.”
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2.5. Optical Setup
Figure 3 and Figure 4 outline the optical pump setup and emission measurement setup of the organic DFB laser in our laboratory. With a cuvette being placed between the organic laser and the bottom emission detection (Figure 4d)), we also used this setup as our organic laser absorption spectrometer. Sketched in blue is the UV pump radiation required to optically pump the organic DFB laser. The pump pulse of a frequency tripled passively Q-switched ND:YAG laser (FTSS355-Q2; CryLas, Berlin, Germany) with a wavelength of λpump=355 nm�����=355 nm exhibits a pulse duration of 1.9 ns1.9 ns. All measurements are carried out at a pump pulse repetition rate of 1 Hz1 Hz. The use of neutral density (ND) filters as a variable ND filter mounted on a stage combined with a revolver equipped with discrete ND filters allows variation of the pump energy. A beam splitter (92:8) was positioned in the pump beam behind the ND filters for pump pulse energy monitoring. Pump energy calibration of the monitor photodiode was performed using an energy sensor (Pe10b; Gentec, Quebec, QC, Canada). A focusing unit consisting of a collimator and two plano-convex lenses was used to create an elliptical pump spot with variable beam diameter in length (ωy=180 μm��=180 �m to 1500 μm1500 �m) and fixed diameter in width (ωx=150 μm��=150 �m). The organic DFB laser was arranged at an angle (α=82∘�=82°) in the pump beam. The oblique pump configuration at a steep angle allows the measurement of organic laser emission at small distances without affecting the pump radiation. In addition, the steep angle prevents the sample fluid from being exposed to the UV pump source. The actual pump spot on the surface of the organic laser thus also exhibited an elliptical shape but with transformed dimensions (ωy=1497 μm��=1497 �m, ωx=1078 μm��=1078 �m), with the longer beam waist oriented orthogonally to the grating lines of the DFB resonator. The organic DFB laser emission in both directions is outlined in red in Figure 3. The detection of the emission of the organic DFB laser as well as the monitoring of the pump emission was done with amplified Si Photodetectors (PDA10A; fixed gain; Thorlabs, Newton, MA, USA). In order to calibrate the pump energy monitor diode, the organic DFB laser was removed from the setup and an energy sensor was used instead. The photodetectors were read out with an oscilloscope (MSO9254A; Agilent, Santa Clara, CA, USA). To avoid discrepancies in bottom and top emission detection, nominally identical photodiodes and connectors were used.
Figure 3. Schematic of the optical pump setup (with focus unit) and the emission measurement setup of an organic DFB laser with two different measurement channels (top emission and bottom emission).
Figure 4. Image of the optical emission measurement setup; (a) pump laser, filter and monitoring, (b) focus unit, (c) organic DFB laser, (d) cuvette, (e) photodetector top emission, (f) photodetector bottom emission, (g) translation stage organic DFB laser, (h) translation stage laser detection unit.
As mentioned above, we used an organic DFB laser as emission source for absorption spectroscopy. A DFB resonator generates feedback based on a periodically distributed change of the refractive index and the optical gain. Resonance for the guided mode is enabled in close vicinity of the Bragg wavelength (λBragg������) given by the Bragg condition [14,28].
Λ=λBragg⋅N2⋅neff(λ)�=������⋅�2⋅����(�)
where Λ� is the period of the spatial modulation, neff(λ)����(�) is the effective refractive index of the guided mode, and N� is the order of diffraction. Thus, the Bragg condition describes the wavelength-selective resonance based on the diffraction of a guided mode at a periodically changing interface [29]. A change in lasing wavelength can be achieved by changes in periodicity [21,30,31] or guidance of the mode. One common way to change the guiding of the mode is to vary the thickness of the core layer of the planar waveguide [14,32,33]. The laser emission wavelength can thus be varied through changing the thickness of the organic layer that forms the core layer of the waveguide.
Variation of the emission wavelength offers the possibility to optimize the laser to the color forming reaction for measuring a specific analyte. Figure 5a plots the absorbance of MG+P in the visible spectrum for three different phosphate concentrations. In the visible spectrum, MG+P shows two dominant absorption peaks. The two peaks λ1=439 nm�1=439 nm and λ2=634 nm�2=634 nm, mark possible measurement wavelengths. Both absorption peaks could potentially be used to determine the phosphate concentration. Our absorbance measurements were performed at λ2=634 nm�2=634 nm because this peak offers the highest measurement sensitivity. Our organic laser was designed for an emission wavelength close to the absorption maximum λ2=634 nm�2=634 nm. The resulting emission spectrum of the organic laser with a peak wavelength of λLas=631 nm����=631 nm is shown in Figure 5a.
Figure 5. Laser emission characteristics and spectral absorption of colorimetric reaction: (a) Absorbance spectra of MG+P for different phosphate concentrations measured with a UV-VIS spectrometer. Concentrations: A=0.5 mg/LA=0.5 mg/L; B=1 mg/LB=1 mg/L; C=2 mg/LC=2 mg/L. Additionally, a normalized organic laser emission spectrum is shown. The emission wavelength of the laser defines the measurement wavelength and is thus optimized for a local absorption maximum of MG+P. Absorbance of MG+P shows a significant dependence on the phosphate concentration in the vicinity of the measuring wavelength; (b) Emission characteristics of a fabricated organic DFB laser for both top and bottom emission: optical output pulse energy versus optical pump energy density and emission spectra (inset). Almost identical laser threshold energy density of about 150 μJ⋅cm−2150 �J⋅cm−2 in top and bottom emission direction is observed.
Optical alignment is performed with two five-axis translation stages (i5000; Luminos, Ottawa, Canada). The organic DFB laser is mounted on one stage (Figure 4g) and the detection instruments for recording the top and bottom organic laser emission on another (Figure 4h). In order to record both the optical spectrum and the intensity, the detection unit is mounted in a cage system (SR 30mm; Thorlabs, Newton, USA) on the translation stage. The fibers of the fiber-coupled spectrometer can easily be replaced by photodiodes without change in alignment. Furthermore, the detection units for top and bottom emission are mechanically connected to each other and can be moved symmetrically around the organic DFB laser. The emission spectrum was used as orientation for the alignment of the optical measuring equipment. The shift of spectral emission as a function of divergence angle of the organic DFB laser must be taken into account when aligning the measuring equipment [34]. Using two multimode fibers (d=600 μm�=600 �m) and a spectrometer, the fibers fixed to each other are aligned to the pump spot. Using rotational alignment of the fibers around the pump spot, fine tuning in the adjustment of the spectral emission is achieved. The inset in Figure 5b shows emission spectra in top and bottom direction, measured with a spectrometer (USB 2000; Ocean Optics, Orlando, FL, USA). The emission spectra in the different emission directions show hardly any spectral differences. The peak wavelengths are λBottom=631.42 nm�������=631.42 nm and λTop=631.38 nm����=631.38 nm, and the spectral bandwidths are ΔλFWHM−Bottom=1.325 nmΔ�FWHM−Bottom=1.325 nm and ΔλFWHM−Top=1.115 nmΔ�FWHM−Top=1.115 nm. The emission in the top direction is approximately 0.830.83 times smaller than the bottom emission. To avoid spectral differences due to measurement set up, both spectra were measured with the same spectrometer. Therefore, the spectrum of the pulsed laser was measured in each emission direction one after the other. The emissions in top and bottom direction are, thus, not associated with the same pump pulse. Any deviation in the pump pulse causes deviations in emission intensity. Both measurement fibers showed no discernible deviation in alignment to the surface normal of the organic laser. Fibers were positioned at a distance of 25 mm25 mm to the laser.
Examination of the laser characteristics is a common option to prove laser activity [35]. Thus, the optical output energy is plotted in Figure 5b for different pump energies. Laser action occurs above the laser threshold, where the slope efficiency is dramatically increased by the transition from spontaneous emission to stimulated emission. The threshold is reached at the same pump energy density for top (DTop=149.90 μJ⋅cm−2±12.3 μJ⋅cm−2����=149.90 �J⋅cm−2±12.3 �J⋅cm−2) and bottom (DBottom=147.14 μJ⋅cm−2±15.6 μJ⋅cm−2�������=147.14 �J⋅cm−2±15.6 �J⋅cm−2) emissions. The emission ratio of the organic DFB laser in top and bottom emissions appears to differ during measurement of the laser characteristics (laser output energy as a function of pump energy). The ratio of the slope efficiencies of top and bottom emissions is 0.88 ± 0.035, which deviates slightly from the theoretical prediction of 1 [24]. In order to achieve different pump energy densities, the optical power of the pump pulse must be varied. This energy variation is done by mechanical ND filters. It is assumed that small mechanical interventions in the pump beam guidance, which occur by mechanically shifting the ND filters, lead to a local change of the pump spot. The local change in pump position on the organic laser could lead to a discrepancy in measurement acquisition. The photodiodes aligned in advance with the pump position then no longer measure in the center of the organic DFB laser beam. Measurements carried out after realigning the diodes gave an emission ratio of 1, as predicted by Streifer et al. [24].
2.6. Data Acquisition and Processing
As mentioned previously, the emission intensity is measured with photodiodes. The acquisition of the pulsed DFB laser signals with free beam photodiodes was performed at a distance of 40 mm, which compensates for the difference between the active area of the photodiode and the area of the fiber core. It is known from literature that the pulse duration of organic laser emission correlates closely with the pump pulse duration, which here is 1.9 ns [36].
Figure 6a,b show the photovoltage UDiode������ of the photodiodes over time for an organic laser pulse. The observed pulse duration is three to five times longer than the pump pulse and varies in duration.
Figure 6. Single shot measurements of the emitted organic laser pulses in top and bottom direction. Observed pulse duration is three to five times longer than the pump pulse and varies in duration. (a) with a cuvette filled with DI water in the optical path of the bottom emission. This serves as a reference measurement; (b) with a cuvette filled with a highly absorbing sample (MG+P with 0.5 mg/L0.5 mg/L phosphate) in the optical path of the bottom emission.
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