Spectrochemists now have several ways to generate blue light with semiconductor laser diodes, but which one is best for analytical spectroscopy?
emiconductor laser diodes have the potential to be used for new applications in analytical instrumentation. The GaN laser diode, which has only been commercially available since 1999, extends the operating wavelengths of laser diodes from the “red” and nearIR to the “deep blue” range. In this article, we describe the properties of GaN laser diodes and compare them with alternative means for generating blue and nearUV radiation.
What’s available? The use of tunable lasers in routine analytical instruments is still very rare. The dye laser was the first type of tunable laser that could cover wide ranges in the optical spectrum. Although it has been exten-
sively and successfully used in research laboratories, its application in routine instruments was fraught with problems, such as lack of robustness and stability, large sizes, high costs, and difficulty in automating procedures. On the other hand, tunable solid-state lasers are attractive choices for routine analytical spectroscopy. In particular, semiconductor lasers are reliable and relatively inexpensive, and their micrometer dimensions make them interesting light sources for miniaturized analytical instruments and techniques. Lead-salt laser diodes for the mid-IR spectral range—the fingerprint region of molecular spectroscopy—have been around for more than 30 years. They have been used successfully and are available in commercial instruments. However, lead-salt laser spectrometers must be operated at cryogenic temperatures and often show wavelength drift due to aging ef-
Kay Niemax - Alexander Zybin – Institute of Spectrochemistry and Applied Spectroscopy (Germany) David Eger – Soreq Nuclear Research Center (Israel) M A R C H 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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ers from these schemes are moderate. Typical powers between tens of nanowatts and several microwatts are generated, depending on the fundamental powers of the laser diodes, which are sufficient for sensitive absorption experiments but not for fluorescence measurements. Moreover, the detection limits for absorption spectroscopy could be improved if higher laser powers were available. Therefore, when the first GaN laser diodes with a wavelength of ~400 nm were reported, it was an important event for spectroscopists. Unfortunately, at ~$2000 (USD), GaN laser diodes are still expensive because they are produced in small quantities. Therefore, an important question is frequently asked by possible users: Does the GaN laser diode meet my requirements for analytical spectroscopy?
3 mW
2 mW
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Wa 405.0 vele ngt h
(nm 405.2 )
46
Cu
n rre
t (m
A)
FIGURE 1. The longitudinal mode spectra of a free-running GaN laser diode versus current (power). The spectra were recorded with a high-resolution échelle spectrometer equipped with a charge-coupled device detector.
fects. Although there are several research groups using lead-salt lasers for molecular gas analysis, the drawbacks of these laser diodes have prevented wider application. They will probably be replaced by quantum cascade lasers (QCLs), a new class of semiconductor lasers for the mid-IR range (1). Although QCLs are semiconductor lasers, they operate under a different principle. Whereas laser diodes generate radiation by electron-valence band hole recombinations in bipolar semiconductor devices, quantum jumps among conduction band energy levels in a unipolar multilayer quantum structure produce the stimulated emission of radiation in a QCL. By tailoring the production of the QCL, the wavelength can be set within the range 3.5–17 µm. Significant progress in the development of these novel lasers has made it possible to operate QCLs in a pulsed mode at room temperature and in continuous-wave (cw) mode at liquid nitrogen temperature. Diode lasers that generate near-IR (AlGaAs/GaAs) and red (InP/InGaAsP) light are very reliable devices. They deliver sufficient cw output power for spectroscopy; have narrow line widths; and can be easily tuned by temperature and current. They are being used for overtone molecular spectroscopy (2), atomic spectroscopy (3), and especially diode laser atomic absorption spectrometry (DLAAS) (4). In the past, the blue and UV laser radiation necessary for electronic transitions in atoms or molecules has been generated by frequency doubling and sum frequency generation of diode laser light in nonlinear optical media. However, the output pow136 A
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A look at GaN laser diodes GaN laser diodes (type NLHV500A) are currently available from only one source, Nichia Chemical Industries (Japan). In our studies with these new lasers, we did not open the hermetically sealed housing, nor was an external cavity or optical feedback applied. The diodes were operated with a commercial power supply, which regulates the temperature and the diode current with a precision of ~1 mK and 10 µA, respectively. Presently, GaN laser diodes can deliver wavelengths from ~390 to ~420 nm at room temperature. The nominal maximum output power is 5 mW, but short operations of up to 10 mW are allowed. Just above the threshold, multilongitudinal mode operation is observed. However, at 2 mW, one particular mode already dominates (Figure 1), although about half of the laser power is still in side modes. In this case, about three-quarters of the power was concentrated in the main mode at 3 mW, while
403.83
32.18oC
32.51oC
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31.52oC
37.76 oC
403.81
31.01
404.0
Wavelength (nm)
Intensity
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403.5
oC
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36.04-37.45 oC
24.0-26.45 oC 21.03 oC
403.0 40
41
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44 45 Current (mA)
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FIGURE 2. The ranges of quasi single-mode operation of the GaN laser diode between 21 °C and 38 °C. Inset shows an enlarged view of the ranges between 31 °C and 32.5 °C.
4.0
3.5
Dispersion (pm/mA)
it was >80% near the maximum nominal power (> 4 mW). At 5 mW, the total power measured was 4.1 mW in the main mode. For comparison, there are near-IR AlGaAs/GaAs laser diodes in which
