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Opt Lett. 2013;38(11):1772-4.
Liu W, Zhu L, Wang L, Fang C.
Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, USA.
Abstract
We demonstrate the versatile broadband wavelength tunability of frequency upconverted multicolor cascaded four-wave-mixing (CFWM) signals spanning the continuous wavelength range from UV to near IR in a thin type-I BBO crystal using 35 fs, 800 nm fundamental and chirped IR supercontinuum white light pulses. Two sets of spatially dispersed CFWM laser sidebands are concomitantly generated from two incident pulses as well as their second-harmonic-generation and sum-frequency-generation pulses in a crossing geometry. The tunable cascaded signals with ultrabroad bandwidth can be readily achieved via spatially rotating the BBO crystal to different phase-matching conditions and temporally varying the time delay between the two incident near-IR pulses.
Additional Information
Light conversion in nonlinear materials represents a vibrant and fruitful field, which has contributed to myriad advances in molecular spectroscopy, high throughput bioimaging, optical communication, photonics development, and material sciences. Our work in generating tunable broadband laser pulses started from a serendipitous discovery of bright laser sidebands when we focused two femtosecond amplified laser pulses onto a thin BBO crystal and tried to measure the incident pulse duration via second harmonic generation (SHG). We found more bright and colorful signals than expected, which stirred much interest followed by intense investigations. A range of nonlinear materials (e.g., glass, sapphire, calcium fluoride, quartz, lithium niobate, even water) have since provided us many testing grounds to explore these sidebands, across the wide spectral range from ultraviolet to near infrared. The innovative design of our versatile and flexible setup (see Figure) affords the opportunity to make parametric changes and characterize the nascent sideband pulses both in the spectral and temporal domain using various diagnostic techniques. We not only found SHG-induced cascaded four-wave mixing signals [Opt. Lett. 37(18), 3783-3785 (2012)], but also identified bright and colorful broadband up-converted multicolor arrays (BUMAs) at various phase-matching conditions in quadratic media [Opt. Lett. 38(11), 1772-1774 (2013)]. These sideband lasers are gifted with multifold advantages: background free and filter free, good stability, spatial separation from fundamental beams, great temporal compressibility, ultrabroad bandwidth, convenient optical control, and wavelength tunability.
We have implemented these nascent laser sidebands in ultrafast molecular spectroscopy, such as the emerging femtosecond stimulated Raman spectroscopy (FSRS) that is currently being developed in our laboratory. A stable broadband tunable pulse is highly desirable to improve the conventional white light continuum to act as the probe beam, and extend the detection limit to the challenging yet important low-frequency spectral region. Using one of the sidebands as the probe pulse, we obtained sharp vibrational features from an organic solvent mixture covering an unprecedented spectral range of ca. 100¾4,000 cm-1 [Appl. Phys. Lett. 103, 061110 (2013)]. Most importantly, time-resolved spectral measurement to track chemistry in action is now possible. In addition, we have shown that using a similar optical setup, cascaded third-harmonic generation occurs on an amorphous medium-air interface [Opt. Lett. 38(17), 3304-3307 (2013)]. The time-resolved signal intensity oscillations (i.e., quantum beats) infer the atomic motions on the femtosecond timescale that can potentially reveal phonon dynamics. We expect this versatile setup will find many more exciting applications across disciplines.
Liu, W.; Zhu, L.; Wang, L.; and Fang, C. (2013) Cascaded four-wave mixing for broadband tunable laser sideband generation. Opt. Lett. 38(11):1772-1774. DOI: 10.1364/OL.38.001772.
