Terahertz (THz) radiation lies between the microwave and far-IR regimes of the electromagnetic spectrum. It is able to penetrate deep into nonconductive materials without inducing damage (such as with x-rays or ionizing radiation). This has many medical applications,1 thanks also to the strong absorption exhibited by polar molecules (such as H2O) that allows researchers to recognize organic materials and gain valuable information about protein structures. Another interesting application, particularly for homeland security, is the remote detection (up to 100m) of hidden weapons and explosives.2 However, difficulties in developing THz sources and suitable detectors mean that this spectral region from 30μm to 1mm (or frequencies of 0.3–10THz) is relatively unexplored.
Many approaches have been proposed to generate THz radiation. Among the most promising techniques is difference-frequency mixing of compact laser sources emitting intense pulses simultaneously at two wavelengths. This approach can be made much more reliable if a single laser can be made to emit two different wavelengths simultaneously. Nanosecond (Q-switching) or picosecond (mode-locking) intense laser pulses can be readily generated with many techniques. A state-of-the-art solution is the employment, inside the laser cavity, of a suitable saturable absorber. This optical component shows intensity-dependent absorption characteristics (light absorption rapidly reduces with increasing incident-light intensity) that favors the pulsed regime against the continuous emission of light. A necessary condition for effective frequency mixing is synchronization of the laser pulses at different wavelengths, which is usually guaranteed by the saturable absorber employed for either Q switching or mode locking.
Disordered-crystalline hosts doped with trivalent neodymium ions (Nd3+) have proved excellent candidates for two-color pulse generation in the nanosecond Q-switching regime. Indeed, these laser materials present closely spaced (yet well-defined) emission wavelengths arising from laser transitions of Nd3+ ions at specific sites. However, picosecond multiwavelength pulse operation offers a more favorable damage threshold for nonlinear crystals, thanks to an increase of this limit with decreasing laser-pulse duration. We have obtained promising results in the multiwavelength picosecond mode-locking regime with two new disordered laser crystals: neodymium-doped gadolinium aluminum gallium garnet, Gd3AlxGa5−xO12 (or Nd:GAGG), and neodymium-doped lutetium gadolinium gallium garnet, (LuxGd1−x)3Ga5O12 (or Nd:LGGG).
We used the conventional Czochralski method to grow Nd:LGGG and Nd:GAGG with x = 0.1 and 1, respectively. These materials present spectroscopic properties similar to the widely investigated gadolinium gallium garnet, Gd3Ga5O12, with absorption bands around 800nm suitable for direct diode pumping and a relatively high emission cross section around 1060nm (see Figure 1). We ran preliminary tests on both materials in diode-pumped continous-wave operation. The pump source was a 50×1μm2 broad-area emitter, 1W nominal-output-power laser diode with 803–808nm temperature-tunable output wavelength. At the maximum pump level we obtained 255 and 230mW maximum output power with slope efficiencies (with respect to absorbed pump power) of 55 and 61% in Nd:GAGG and Nd:LGGG, respectively (optimum output-coupler transmittivity of 5% in both cases).
Figure 1. Neodymium-doped lutetium gadolinium gallium garnet (Nd:LGGG) and gadolinium aluminum gallium garnet (Nd:GAGG) emission cross section (σem). λ: Wavelength.
We employed a semiconductor saturable absorber mirror to obtain picosecond passive mode locking. To investigate the multiwavelength operation, we inserted a single SF10 (dense flint) glass prism into the cavity. With the right resonator design,3 the dispersive prism can ensure an equal round-trip time for pulses with different central wavelengths. The saturable absorber automatically synchronizes multiple pulses, since intracavity intensity-dependent losses are minimized only if different pulses arrive simultaneously.
In Nd:LGGG we observed several different mode-locking regimes with single, double, or triple simultaneously emitted pulses at 1062.2, 1066.5, and 1067.1nm. The autocorrelation of the pulses suggests a duration of 4.2ps, while the average output power was approximately 40mW. In Nd:GAGG we obtained stable dual-wavelength mode locking with two lines centered at 1061.4 and 1062.7nm. The average output power was approximately 65mW and the pulse duration ~4.1ps (see Figure 2). The absence of interference fringes in the autocorrelation trace (a cross correlation of the two pulses) can be explained by the presence of phase noise or pulse jitter, typically on the order of a fraction of the pulse duration in this kind of lasers.
Figure 2. Autocorrelation trace (in arbitrary units, a.u.) of Nd:GAGG multiwavelength picosecond (ps) pulses. Inset: Optical spectrum. SHG: Second-harmonic generated.
In summary, disordered Nd3+-doped LGGG and GAGG laser crystals seem promising active media for compact picosecond-pulse multiwavelength sources. With an appropriate resonator design, a more compact and straightforward scheme than used for previous oscillators4,5 has produced reliable multiwavelength picosecond oscillation. A future development will focus on scaling up the average output power, which remains too low for efficient THz generation. The first step will be to investigate the behavior of these disordered-crystalline hosts under higher pump-power levels. Direct amplification in simple, commercially available fiber modules should provide straightforward scalability up to a few watts. At this average power level radiation in the range 0.13–1.3THz can be generated through difference-frequency mixing in a suitable nonlinear crystal.
Federico Pirzio, Antonio Agnesi, Giancarlo Reali
University of Pavia
Andrea Arcangeli, Mauro Tonelli
University of Pisa
Zhitai Jia, Xutang Tao
State Key Laboratory of Crystal Materials
2. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, THz imaging and sensing for security applications: explosives, weapons and drugs, Semicond. Sci. Technol. 20, pp. 266-280, 2005.
4. G. Q. Xie, D. Y. Tang, H. Luo, H. J. Zhang, H. H. Yu, J. Y. Wang, X. T. Tao, M. H. Jiang, L. J. Qian, Dual-wavelength synchronously mode-locked Nd:CNGG laser, Opt. Lett. 33, pp. 1872-1874, 2008.
5. G. Q. Xie, D. Y. Tang, W. D. Tan, H. Luo, S. Y. Guo, H. H. Yu, H. J. Zhang, Diode-pumped passively mode-locked Nd:CTGG disordered crystal laser, Appl. Phys. B 95, pp. 691-695, 2009.