In the past, research groups working in spectroscopy tended to concentrate on particular regions of the electromagnetic spectrum, and hence on certain types of atomic and molecular processes (electronic transitions, rovibrational motion, rotation, etc.). Interdisciplinary fields such as molecular astrophysics and atmospheric chemistry are becoming increasingly multi-wavelength, with the probes selected to answer the fundamental scientific questions and not with respect to spectroscopic boundaries. At the same time, new means of generating light and powerful tools to fabricate and control spectroscopic instruments have been developed. We are actively designing new spectrometers that work across very wide wavelength regions based either on parametric generation or sum/difference frequency mixing. At wavelengths from the UV through IR we utilize parametric conversion, while in the far-IR or THz region we use differency frequency mixing. Each is described in turn below.
Optical Parametric Oscillators and Generators
In optical parametric processes, non-linear crystals are used to "split" a single photon into two longer wavelength photons, called the signal (shorter wavelength) and idler (longer wavelength) waves. The tuning range is limited in principle only by the phase matching and transparency of the OPO medium. A plethora of non-linear phase matching schemes, summarized in the figure, can be employed, but only a few designs have been well characterized spectroscopically. Optical parametric oscillators, or OPOs, place the non-linear crystals within cavities while optical parametric generators (OPGs) or amplifiers (OPAs) utilize either parametric fluorescence or amplify externally injected fields. Especially lacking are approaches which combine rugged designs that can easily incorporate new technologies and narrowband operation. Given this uncertain state of affairs, we have begun a collaborative effort with Casix and Coherent on the design of compact OPOs and OPGs that is being supported by an NSF MRI grant co-sponsored by the Atmospheric Chemistry and Materials Science Offices. Our workhorse design at present is a 355 nm-pumped type II phase matched BBO cavity outlined at the top of the figure at left and pictured on the top of this page. Unlike earlier designs, our type II cavities possess low thresholds (I(Th) = 20-30 MW/cm2) and high conversion efficiencies (>30%). Spectroscopically, the Type II OPO is also nicely suited to wide scanning, with a nearly constant linewidth of only 0.7-1.5 /cm without any frequency selective elements in the cavity. The excellent beam quality of the type II OPO also leads to among the highest doubling efficiencies ever measured in the 220-280 nm region, as summarized in the power tuning curves presented below. The insert presents a photoacoustic UV spectrum of nitric oxide, which demonstrates the resolution obtained. We stress that all of the mirrors and substrates shown in this modular design are standard, low cost, Nd:YAG optics. It is thus straightforward to extend the tuning range of the OPOs into the IR.
It is possible to operate OPOs near the transform limit with sufficiently narrow feedback elements, but tuning such cavities is cumbersome. We feel, therefore, that injection seeding offers a much more spectroscopically useful means of narrowing OPO output, as has been demonstrated by many researchers. Unlike previous approaches, however, we have found that our Infinity pump lasers can drive nanosecond (ns) OPG/OPAs. In our design, passing only the idler wave into the OPA stage results in excellent beam quality and small angular divergence. Since the cavity is completely eliminated, the mode competition and frequency pulling effects offsets commonly seen in seeded OPOs are absent, and we have been able to scan the OPG over the entire range of an external cavity diode laser (~400 /cm) without optical feedback. Idler or signal seeding may be used (P(seed)<1-10 mW), and the line width is <500 MHz. Thus, only effects such as non-linear index of refraction terms should spectrally broaden the output, and we ultimately expect the current OPG line width of <500 MHz (c.f. Fig. 7, bottom left) to approach the pump-limited bandwidth of ~250 MHz. The continuous nature of the open loop scanning is demonstrated by the 814.7 nm water photoacoustic spectrum below, in which the Voight profile line width (doppler + pressure broadening) is much, much greater than the OPG line width.
This material is based upon work supported by the National Science Foundation under Grant No. 9724500.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
At long wavelegnths, the so-called Manley-Rowe condition limits the conversion efficiency of non-linear optical schemes. Thus, methods such as laser sideband generation on THz gas lasers have long served as the workhorse spectrometers above 1 THz. Sideband systems have complete coverage between 0.6-3/4 THz, and ~50-75% coverage up to 6-8 THz at power levels of ~1 µW above 4-5 THz. They consume considerable power and supplies, however, and are stabilized only with great care. We have therefore turned to an all-solid-sate alternative to the far-IR gas laser sideband spectrometer based on optical photomixing. Photomixers work by placing THz antennae onto the surfaces of optoelectronic substrates which convert the time varying electron concentration in the bulk material, produced by the interaction of two laser frequencies above the semiconductor band gap, to radiation propagating into free space. Pioneering work in this area began with Prof. David Auston's group at Rice, who drove photomixers with ultrafast laser pulses, a field known as THz Time Domain Spectroscopy, or THz-TDS. Photomixers can also be used as continuous wave (cw) THz sources, and the initial foray into this area at MIT Lincoln Labs and NIST, Gaithersburg used large frame Ti:Sapphire or dye lasers to drive the photomixers. Recently, we have constructed a compact, lightweight, low cost instrument using either 852 nm distributed Bragg reflector (DBR) or external cavity diode lasers and LTG GaAs mixers fabricated at LL and UC Santa Barbara.
Initial photomixer concepts centered on small area devices at the terminals of THz planar antennae. The small area leads to high roll-off frequencies, but also to low optical power damage thresholds. Using a recently developed injection seeded 0.5 W two-color amplifier, we have experimentally demonstrated the first traveling wave THz photomixer, one based on angle tuned phase matching (see the figure below for an outline of the system). This breakthrough device has ~3 THz of bandwidth since it is limited primarily by the LTG-GaAs recombination time, unlike traditional small-area photomixers. The output is >1 µW from 0.3 to 3 THz, and is adequate to perform high resolution spectroscopy.
To carry out such studies, we have recently implemented a fiber-based system that includes absolute frequency locking and calibration to a part in 100 million by using an ultralow thermal expansion (ULE) coefficient cavity. The system is outlined below, and its continuous tunability and frequency stability is illustrated by the accompanying submillimeter spectrum of acetonitrile. The sweep time is 10 seconds, and the acetonitrile line width is <2 MHz. Doppler and pressure broadening contributes >1.5 MHz of the total, and the transition frequencies are measured to better than 50 kHz. Present applications of this spectrometer include high precision measurement of the THz spectra of astrophysically important molecules and the VRT spectra of weakly bound clusters.
Selected Recent Publications "A Simple, High Performance Type II BBO OPO'' Sheng Wu, Geoffrey A. Blake, Zhou Sung, & Ju Ling 1997, Applied Optics 36, 5898.
"Spectroscopic Applications and Frequency Locking of THz Photomixing with Distributed-Bragg-Reflector Diode Lasers in Low Temperature Grown GaAs'' Pin Chen, Geoffrey A. Blake, Michael C. Gaidis, Elliott R. Brown, Kevin A. McIntosh, Steve Y. Chou, Michael I. Nathan, & Fred Williamson 1997, Appl. Phys. Lett. 71, 1601.
"A New Microsampling Visible-Infrared Spectrometer Based on Optical Parametric Oscillator Technology'' Zhou Wang, George R. Rossman, & Geoffrey A. Blake 1998, Spectroscopy 13, 44.
"Two-Frequency Operation of an Injection-Seeded Semiconductor Laser Amplifier at 850 nm'' Shuji Matsuura, Pin Chen, Geoffrey A. Blake, John C. Pearson, Timothy J. Crawford, & Herbert M. Pickett 1998. Int. J. Infra. & MM Waves 19, 849.
"A Nanosecond Optical Parametric Generator/Amplifier Seeded by an External Cavity Diode Laser'' Sheng Wu, Vadym A. Kapinus, & Geoffrey A. Blake 1999, Opt. Commun. 159, 74.
"A Traveling-Wave THz Photomixer Based on Angle-Tuned Phase Matching'' Shuji Matsuura, Geoffrey A. Blake, Rolf A. Wyss, J.C. Pearson, Christoph Kadow, Andrew W. Jackson, & Arthur C. Gossard 1999, Appl. Phys. Lett. 74, 2872.
"A Tunable, Cavity-Locked Diode Laser System for Terahertz Photomixing'' Shuji Matsuura, Pin Chen, Geoffrey A. Blake, John C. Pearson, & Herbert M. Pickett 2000, I.E.E.E. Micro. Th. & Tech. 48, 380.
"Multicrystal Harmonic Generator that Compensates for Thermally Induced Phase Mismatch'' S. Wu, Geoffrey A. Blake, S. Sun, & J. Ling 2000, Opt. Commun. 173, 371.
Links to Other Instrumentation Groups & Sites
Brian J. Orr, Macquarie
Sandia OPO Web Page
Sandia NLO Database Program (for Windows) Time and Frequency Domain THz Groups
Links to Useful Commercial/Instrumentation Web Sites
Alpes Lasers (Swiss QC Laser Manufacturer)
Applied Optoelectronics (US Licensee of Lucent QC technology)
Infrared Multilayer Laboratory (Univ. of Reading, IR Filters)
OCLI Corporation (dielectric filters, OEM primarily)
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