Laser secondary neutral mass spectrometry (laser SNMS) is one of the most sensitive ways to analyze the chemical composition of surfaces, providing high spatial resolution. An ideal laser source for this would have a continuous tuning range from the UV through the visible spectrum to the near-IR, allowing it to detect a large variety of different species. In addition, it would meet laser SNMS requirements of nanosecond pulses with energies up to the millijoule level, a narrow spectral line width, and repetition rate of 1kHz for high throughput. Principally, Ti:Sapphire lasers fulfill these ideal requirements, but neither the fundamental laser frequencies nor the higher harmonics cover the visible spectrum region between 520nm and 670nm.
At the University of Münster, we have created a Ti:Sapphire laser system that reduces that region, closing the gap between UV and near-IR. Using computer-controlled wavelength tuning and without the need to exchange optics, crystals, or whole converter units, we have achieved nearly continuous wavelength tuning from 240nm to 1020nm.1 A module for difference frequency generation (DFG) has been developed to provide tunable output through most of the visible spectral range. This converter unit mixes the frequency-doubled output of the Ti:Sapphire laser with the fundamental output of a Nd:YAG laser.
DFG is a nonlinear optical three-wave interaction process similar to sum-frequency generation, which is well known for higher harmonic generation. To achieve a high efficiency, the induced polarization wave in the nonlinear material and the generated electromagnetic field at the output frequency must be kept in phase. This phase-matching condition may be fulfilled by using birefringent materials and aligning the wave vector to a certain angle in reference to the optical axis. For tunable lasers, this angle must be adapted to the input wavelength by rotating the nonlinear crystal. Additionally, mixing the pulsed output of two independent lasers requires the synchronization of the pulses, where the pulse build-up time of the gain-switched Ti:Sapphire laser is a function of wavelength. Hence, two parameters must be adapted in a tunable DFG module: the phase-matching angle of the nonlinear crystal and the pulse trigger of the laser.
The DFG module essentially consists of four parts: a Q-switched Nd:YAG laser as source of the signal wave, a beta-barium borate (BBO) crystal mounted on a motorized rotary stage, photodiode sensors for monitoring the input and output power of the module, and a computer to control the phase-matching angle and the triggering of the two laser pulses. For this control unit, a delay generator is directly integrated into the computer as a peripherial component interconnect (or PCI) card.
The closed-loop control system that tunes the DFG unit works in four steps. First, the desired output wavelength is entered and the computer calculates and sets the accurate input wavelength of the frequency-doubled Ti:Sapphire laser. Next, initial values for the phase-matching angle and the trigger delay of the DFG unit are imported from a look-up table. After that, the phase-matching angle and trigger delay are varied until a maximum DFG output is found. Lastly, the look-up table is updated with the experimental values of the phase-matching angle and trigger delay that yielded maximum DFG output.
At a pulse-repetition rate of 1kHz, the computer-controlled DFG unit has output energy to 295μJ at 641nm, which corresponds to an efficiency of 30%. The tuning range of this particular converter module is 520nm to 700nm. With a spectral line width of the input fields at 20GHz (or 11pm), the output line width is 40GHz (or 55pm). The pulse duration is around 10ns over the entire tuning range.
In conclusion, a high-repetition-rate Ti:Sapphire laser system with an almost continuous tuning range from the UV to the near-IR has been prototyped for laser SNMS investigations. The automated wavelength tuning through the entire output spectrum will allow the detection of a large variety of different species with little setup time for wavelength switching. In the future, further scaling of the output energy up to 500μJ in the visible should be possible by increasing the frequency-doubled input power.
The laser SNMS system was developed as a German Federal Ministry of Education and Research (BMBF) project. We thank the University of Münster, the BMBF, and the Association of German Engineers (VDI) for the support of our work. Special thanks to Professor H. F. Arlinghaus for determining the system and parameter requirements for Laser SNMS.
Figure 1. A snapshot of non-filtered DFG output shows a wide-spectrum visible component.
Figure 2. The fully equipped setup includes two FHG units for the wavelength range from 190nm to 240nm (marked 3 and 4 in the schematic). The DFG module components are highlighted by a white background.
Figure 3. The output spectrum of the laser system shows both experimental proof of principle (hatched areas) and the computer-controlled prototype (solid lines).