NONLINEAR OPTICS: Technological advances brighten horizons for organic nonlinear optics

Nonlinear optical materials are finding use in a variety of applications, including optical modulators, switches, signal-processing elements, frequency conversion, and optical-correlation devices.

Th 0608lf04f1

Nonlinear optical materials are finding use in a variety of applications, including optical modulators, switches, signal-processing elements, frequency conversion, and optical-correlation devices.

NASSER PEYGHAMBARIAN, LARRY DALTON, ALEX JEN, BERNARD KIPPELEN, SETH MARDER, ROBERT NORWOOD, AND JOSEPH W. PERRY

Organic nonlinear materials have been extensively studied over the last two decades and dramatic progress in this area has taken place during the last four years.1-4

Recent advances in organic nonlinear optical (NLO) materials, including materials with exceptional electro-optic, two-photon-absorption, and third-harmonic-generation properties, have included supramolecular assemblies prepared using chromophoric building blocks with optimized molecular properties. These advances have led to unprecedented electro-optic coefficients with r33 of greater than 300 pm/V (as a comparison, r33 for lithium niobate is 30 pm/V). The usefulness of some of these innovative organic materials has been demonstrated in electro-optic (EO) modulators with low operating voltage and low insertion loss, in ultrashort-pulse diagnostic applications, and in imaging applications.

High-performance electro-optic materials

It is well known that phenyl (Ph)- and perfluorophenyl (PhF)-containing molecules have a strong tendency to cocrystallize and form “face-to-face” stacks of alternating hydrocarbon and perfluorinated moieties. These give rise to improved mechanical strength and thermal stability because of the complementary multipolar and van der Waals interactions between Ph and PhF rings.

We have exploited this property to develop a new class of highly efficient dendritic NLO molecular glasses through the functionalization of Ph-dendron and PhF-dendron on the head and tail of the second-order NLO chromophores. By taking advantage of the site isolation effect and the strong attractive forces provided by these dendrons, enhanced polar order during the poling process has been realized (see Fig. 1).

Th 0608lf04f1
FIGURE 1. A schematic drawing illustrates a possible rationale involving the self-assembly of dendrons containing hydrogen (purple color) and fluoro (gray color) to explain the high EO coefficient and alignment stability.
Click here to enlarge image

The resulting amorphous molecular EO materials possess excellent film-forming properties, and good optical quality and dielectric strength. In addition, they can be purified easily and have well-defined structures.

By further doping these dendronized chromophore glass materials with additional highly efficient NLO chromophores, poled films have been obtained that exhibit record-high and stable EO coefficients (r33 > 300 pm/V at the 1.3 µm telecommunication wavelength), which we believe arise from an enhanced poling efficiency through the organized dendritic host. These values are approximately 10 times greater than that of the best inorganic crystal, LiNbO3. When implemented together with very polarizable NLO chromophores, these supramolecular modifications could lead to further improvement in EO activity.

Ultrahigh EO activity should enable the design and fabrication of innovative devices with low drive voltage and compact size. It also offers the possibility of realizing direct integration of optical devices on silicon (Si) chips. Such a combined materials-and-devices effort could provide a new paradigm for information technology with ultrahigh speed and very large bandwidth.

High-performance electro-optic devices

The basic problem of achieving high poling efficiency in an EO device structure has been a long-standing challenge and approaches to solving this problem have focused on conducting polymers, which have poor optical properties. Consequently, a sol-gel buffer has been developed that possesses a combination of good optical properties in the operating temperature range, where it essentially acts as a low-loss dielectric with high conductivity relative to the EO films in the poling temperature range, enabling high-efficiency poling.5 In several systems, this buffer made it possible to extend the breakdown range of EO polymers by a factor of 2 or 3, thereby increasing the EO coefficient by similar factors (see Fig. 2).

Th 0608lf04f2
FIGURE 2. A comparison of poling results obtained from a single-layer EO polymer film and similar thickness film on a sol-gel buffer shows a factor-of-two improvement in poling efficiency.5
Click here to enlarge image

This development applies equally well to conventional waveguide devices and the more recent Fabry-Perot geometries. The technological impact of a factor of 2 to 3 on the “in-device” EO coefficient is enormous, which is evident by the level of effort that has been required on the chemistry side to achieve these kinds of improvements.

Hybrid polymer/sol-gel waveguide modulators have been successfully fabricated and packaged with optical fibers to demonstrate the feasibility of combining high EO activity of organic materials and low optical loss (absorption and insertion) of a glass waveguide in the same system for integrated EO devices. The hybrid approach combines a photo-patternable hybrid organic-inorganic sol-gel material with active NLO chromophores for fabricating low-loss, highly reliable modulators at 1.55 µm. This approach also enables simple processing, low cost, and ease of fiber coupling and packaging.

Single-mode EO active waveguides with low loss (less than 1 dB/cm at 1.55 µm) were fabricated by reverse channel patterning of the sol-gel. The insertion loss of the packaged device (7 to 8 dB) is the lowest among all EO polymer-based modulators. An EO polymer was developed by the Alex Jen group at the University of Washington that (under the technique described in Fig. 2) provides an r33 of approximately 120 pm/V. A low drive voltage of 3.9 V was obtained for this hybrid sol-gel/polymer modulator, a performance level already on par with commercial low-loss lithium niobate (LiNbO3) modulators (see Fig. 3).6

Th 0608lf04f3
FIGURE 3. A Mach-Zender modulator was fabricated using a hybrid waveguide sol-gel/polymer (left). It yielded a low drive voltage of 3.9 V (right) and performance comparable with commercial low-loss lithium niobate (LiNbO3) modulators.6
Click here to enlarge image

null

Strong, broad-bandwidth two-photon absorption

Over the past decade, interest has grown dramatically in two-photon absorption (TPA) and applications in three-dimensional (3-D) fluorescence imaging, dynamic range compression, optical limiting, and 3-D microfabrication. Of particular interest in photonics applications has been the use of two-photon induced polymerization for the fabrication of 3-D photonic crystals. To realize the full potential of these applications, compounds with large TPA cross sections, δ, are of key importance.

We have shown that attaching electron donors or combinations of donors and acceptors to conjugated organic chromophores, to form D-π-D or D-A-D molecules, results in an order-of-magnitude increase in the peak δ values that arises largely from the effects of intramolecular charge transfer. Compounds based on distyrylbenzene conjugated bridges typically show peak δ values in the range of 900 to 2000 Göppert Mayer units (GM, where 1 GM = 10-50 cm4 seconds) with peak TPA wavelengths of about 730 to 830 nm.

Because extended conjugation length and the distance between donor and acceptors can have a direct impact on charge transfer, transition energies, and transition moments, we recently investigated donor terminated molecules with extended conjugation length, five-ring phenylene vinylene systems, and different donor/acceptor substitution patterns. We have shown that these molecules exhibit two near-IR TPA bands with peak excitation wavelengths of about 820 and 970 nm and corresponding peak δ values of 1300 to 2700 GM and 3600 to 5300 GM, respectively.7 It is of interest that these compounds also show a high fluorescence quantum efficiency (about 0.7) in toluene solution.

Importantly, the molecule with the largest separation of the terminal donors and the acceptors gave the largest δ for the low-energy band, indicating that the donor-acceptor charge-transfer interaction is playing a major role with respect to the optical properties of these extended systems. An important point for applications is that, because of the large cross sections and the proximity of two TPA bands, the TPA spectra show very broadband activity, with δ of 1000 GM over the range of at least 720 to more than 1000 nm, which essentially covers the complete spectral range of modelocked Ti:sapphire lasers. As a result, these chromophores can be used effectively in TPA applications that require excitation wavelength agility.

Third-harmonic generation and applications

Organic nonlinear optical materials enable applications that utilize frequency conversion such as second-harmonic generation (SHG), a process in which light at frequency ω is converted into light at frequency 2ω. While SHG is limited to materials or interfaces that lack inversion symmetry, third-harmonic generation (THG), which produces a beam at 3ω, can in principle be observed in any isotropic bulk material.

To date, applications of THG have been limited by a paucity of materials that generate strong THG radiation. Recently, we developed organic films that exhibit efficient third-harmonic generation, and demonstrated several applications, including short-pulse diagnostics and imaging through scattering media. The push-pull chromophore [2-tricyanovinyl 3-hexyl-5(4-N.N’ diphenyl-4dibutyl) vinyaniline-thiophene] doped at 20 wt. % into a polymer matrix of polystyrene and processed into a 10‑µm-thick film produced 17 µW of green light when excited by a 250 mW laser beam from an optical parametric oscillator, emitting pulses at a rate of 82 MHz with a duration of 90 fs at a wavelength of 1550 nm (see Fig. 4, left).

Th 0608lf04f4
FIGURE 4. The third-harmonic-generation (THG) signal of a 1550 nm laser beam was produced in a thin organic film containing a push-pull chromophore (left). Frequency-resolved optical gating (FROG) using a portable, low-cost fiber spectrometer traces an asymmetric short pulse obtained by THG (right)
Click here to enlarge image

The strong THG signals allowed the demonstration of interferometric autocorrelation for the characterization of short pulses with a standard nonamplified silicon detector, and frequency-resolved optical gating (FROG) with a portable fiber spectrometer (see Fig. 4, right).8 The excess of THG signal produced in these organic films required the use of attenuation filters to prevent saturation of the low-cost portable spectrometer.

These new materials also enabled the demonstration of real-time, time-gated, direct imaging through scattering media using third-harmonic generation in the eye-safe and telecommunication-compatible near-infrared spectral region (1550 nm). Imaging through scattering media with an attenuation of 14 mean-free paths has been demonstrated successfully.9

ACKNOWLEDGMENT

The authors would like to acknowledge support from NSF CMDITR Science and Technology Center (DMR-0120967) and the DARPA MORPH program.

REFERENCES

1. N.P. Peyghambarian, R.A. Norwood, Optics and Photonics News 16(2) 30 (2005); see also Optics and Photonics News 16(4) 28 (2005).

2. A. Jen, J. Luo, T. Kim et al., Proc. SPIE 5935, 5935061 (2005).

3. J. Luo, M. Haller, H. Ma et al., J. Phys. Chem. B 108(25) 8523 (2004).

4. L. Dalton, B. Robinson, A. Jen et al., Proc. SPIE 5935, 5935021 (2005).

5. C. DeRose, Y. Enami, C. Loychik et al., Submitted for publication.

6. Y. Enami, C. DeRose, C. Loychik et al., Submitted for publication.

7. S.-J. Chung et al., J. Am. Chem. Soc. 127(31): 10844 (2005). See also references therein.

8. G. Ramos-Ortiz et al., Appl. Phys. Lett. 85, 179 (2004).

9. G. Ramos-Ortiz et al., Opt. Lett. 29, 1 (2004).

NASSER PEYGHAMBARIAN is chair of photonics and lasers and ROBERT NORWOOD is a research professor at the College of Optical Sciences, University of Arizona, Tucson, AZ 85721. LARRY DALTON and ALEX K-Y JEN are chemistry professors at the University of Washington, Seattle, WA. BERNARD KIPPELEN and SETH R. MARDER are associate director and director of the center for organic photonics and electronics and JOSEPH W. PERRY is a chemistry professor at the Georgia Institute of Technology, Atlanta, GA; e-mail: [email protected].

More in Optics