Przemyslaw P. Markowicz

Observation of stimulated emission by direct three-photon excitation

Multiphoton processes, predicted theoretically in 1931, were for a long time considered to be mainly of academic interest. This view changed when it was shown that a two-photon absorption process could, because of a quadratic dependence of excitation on intensity, produce a spatially confined excitation useful for three-dimensional data storage and imaging. Two-photon absorption has received considerable attention recently because of the development of highly efficient two-photon-sensitive materials, leading to numerous technological applications. These successes have created interest in exploring applications based on three-photon excitations. For a three-photon process, a longer excitation wavelength such as those common in optical communications can be used. Also, the cubic dependence of the three-photon process on the input light intensity provides a stronger spatial confinement, so that a higher contrast in imaging can be obtained. Here we report the observation of a highly directional and up-converted stimulated emission as an amplified spontaneous emission, produced in an organic chromophore solution by a strong simultaneous three-photon absorption at 1.3 µm. This achievement suggests opportunities for a three-photon process in frequency-upconversion lasing, short-pulse optical communications, and the emerging field of biophotonics.

Phase-sensitive time-modulated surface plasmon resonance polarimetry for wide dynamic range biosensing

A novel polarimetry scheme is proposed to improve the performance of phase-sensitive Surface Plasmon Resonance (SPR) biosensors. The scheme uses s-polarized light, not affected by SPR, as a reference beam, while information on the phase of the p-polarized component is obtained from an analysis of phase-polarization state of light of mixed polarization. We utilize temporal modulation of the beam reflected under SPR by a photo-elastic modulator and show that, under certain birefringent geometry, the signals at the 2nd and 3rd harmonics of modulated frequency can provide ultra-sensitive phase-based response to changes of the refractive index (thickness) of thin films on gold. We also show that the proposed configuration significantly improves detection limit compared to conventional intensity-sensitive SPR, yet enables to maintain wide dynamic range of measurements, which is normally difficult with phase-sensitive SPR schemes. Biosensing applications of the proposed scheme are illustrated in a biological model reaction of avidin - biotin binding on gold.

Second-harmonic and sum-frequency imaging of organic nanocrystals with photon scanning tunneling microscope

Second-harmonic generation and sum-frequency generation with photon scanning tunneling microscopy and shear-force detection are used to map the nonlinear optical response and the surface topograph of N-(4-nitrophenyl)-(L)-prolinol crystals with a subdiffraction-limited resolution. The domain-size dependence of the spatial feature is obtained, which shows the local orientational distribution of the optical near field radiated by nonlinear nanocrystals and reveals the difference between nanoscopic and macroscopic second-order optical nonlinearities of molecular crystals.

DNA-Ormocer based biocomposite for fabrication of photonic structures

We report microfabrication of high quality photonic structures such as two-dimensional photonic crystals and beam splitters from a high DNA load, photosensitive Ormocer nanocomposite. This nanocomposite combines the high dye loading capacity of DNA with the photopatternability and hardness of the Ormocer. The fabrication is performed with the two-photon lithography method. Detailed studies of the deoxyribonucleic acid distribution in the fabricated structures are conducted with Raman microscopy. We also demonstrate that the deoxyribonucleic acid based nanocomposite films cast on glass substrates are of high enough quality to support amplified spontaneous emission from dyes intercalated in the deoxyribonucleic acid.

Enhancement of two-photon emission in photonic crystals

We report the influence of photonic stopgaps on two-photon excited emission from highly efficient nonlinear chromophores infiltrated into high-quality photonic crystals. We have observed a sharp decrease (filter effect) in emission within the frequency range and direction of the stopgap as well as sharp enhancement of the two-photon excited emission associated with the stopgaps edge. This effect may be important for the development of low-threshold upconversion lasers.

Ultrashort 1.5-µm laser excited upconverted stimulated emission based on simultaneous three-photon absorption

Simultaneous three-photon excited stimulated emission at (cavityless lasing) approximately 610 nm has been achieved in an organic chromophore solution pumped by approximately 1.5-microm ultrashort coherent radiation. The stimulated emission can be observed only in the forward and backward directions and is characterized by its high directionality and spectral narrowing. The divergence angle for the backward stimulated emission is considerably smaller than that for the forward stimulated emission, which resembles optical phase conjugation.

Enhancement of third-harmonic generation in a polymer-dispersed liquid-crystal grating

We report the observation of significant enhancement of one-step third-harmonic generation in a one-dimensional photonic crystal pumped by a near-infrared laser beam tuned to the low-frequency edge of the first photonic band gap. The third-harmonic phase matching can be controlled by changing the angle of incidence of the fundamental radiation, allowing tunability of the third-harmonic wavelength. The observed phenomenon was modeled theoretically using the transfer-matrix method. The enhancement is attributed to the combined action of phase-matching between the pump and harmonic waves and pump-field localization within the photonic crystal.

Nanoscopic study of second-harmonic generation in organic crystals with collection-mode near-field scanning optical microscopy

Collection-mode near-field scanning optical microscopy (NSOM) is used to map nanoscopic second-harmonic generation (SHG) in N -(4-nitrophenyl)- (L) -prolinol crystals. A spatial resolution of 98 nm is achieved. Near-field polarization-dependent SHG measurement is performed, and a local effective SHG susceptibility of 224+/-18 pm/V is obtained.

Direct four-photon excitation of amplified spontaneous emission in a nonlinear organic chromophore

We report four-photon pumped amplified spontaneous emission in an organic chromophore. The phenomenon is observed when intense infrared laser radiation illuminates a nonlinear chromophore solution. As a result of this illumination, a strong and highly directional radiation in the visible wavelength range is generated in both the forward and the backward directions, with an angular divergence similar to that of the pump beam.

Cubic nonlinear optical effects in deoxyribonucleic acid (DNA) based materials containing chromophores

Third-order nonlinear optical properties were investigated for the system incorporating the Disperse Red 1 dye and the deoxyribonucleic acid - cetyltrimethylammonium complex (DNA-CTMA). The interaction of the dye with the DNA chains does not lead to major changes of the nonlinear optical effects due to the DR1 dye. Polarization dependences of the nonlinear absorption in this system reveal, however, some indications that the interactions may lead to ordering of the dye molecules against the DNA chains.