Generating bursts of femtosecond laser pulses with a tunable delay and envelope in a folded Michelson interferometer.

Bursts of 16 femtosecond laser pulses are generated in a fourfold Michelson interferometer with a tunable delay and envelope. Solutions are given to solve the "forward problem" (bursts from a given parameter set) and "inverse problem" (obtain parameter set from a given burst). Three types of bursts are generated experimentally with envelopes suitable for applications in laser materials processing and the generation of terahertz radiation.

Surface structure evolution and Raman response for multipulse, few-cycle, laser damaged ZnSe.

Multiple 11-fs infrared, few-cycle laser pulses were applied to a polycrystal ZnSe surface to study the evolution of surface damage morphologies. The polycrystalline grain boundaries seem to be the initiation site of surface damage and formation of ripples, which evolve as the result of many laser pulses at the same site. Scanning electron microscopy and atomic force microscopy (AFM) were applied to characterize the surface. The crystalline change and material phase transition were examined by confocal Raman spectroscopy. The thermal expansion coefficient increased slightly in the ablated zone compared to the non-ablated zone according to an AFM thermal tip test. The results show the growth and organization of surface ripples and the change of thermal properties as the number of irradiations at each site increases.

Invariance of the r2-ln(F) relationship and attainable precision in ultrafast laser ablation experiments.

Pursuing ever-smaller feature size in laser-based lithography is a research topic of vital importance to keep this technique competitive with other micro-/nano-fabrication methods. Features smaller than the diffraction-limited spot size can be obtained by "thresholding", which utilizes the deterministic nature of damage threshold with ultrashort laser pulses and is achieved by precisely tuning pulse energies so that only the central portion of the focal spot produces permanent modification. In this paper, we examine the formulation commonly used to describe thresholding and show that the relationship between feature size (r) and laser fluence (F) is invariant with respect to the nature of laser absorption. Verified by our experiments performed on metal, semiconductor, and dielectric samples, such invariance is used to predict the smallest feature size that can be achieved for different materials in a real-world system.

Single-shot intense few-cycle pulse interaction with polycrystalline ZnSe.

The interaction of high-intensity few-cycle laser pulses with solids opens a new area of fundamental light-material interaction research. The applied research extends from extreme nonlinearity in solids to the next-generation high laser light damage resistance optical design. In this Letter, 11 fs infrared, carrier-envelope-phase (CEP) stable, two-cycle laser pulses were applied to investigate the process of laser-material interaction on the ZnSe surface. A systematic study of a few-cycle pulse laser-induced damage threshold on ZnSe was performed for a single-pulse regime (1-on-1). Laser damage morphologies were carefully characterized. Our experiment demonstrated the very beginning of laser-induced structures on the ZnSe surface by using the shortest infrared few-cycle laser pulse currently available with a stable CEP.

Reducing feature size in femtosecond laser ablation of fused silica by exciton-seeded photoionization: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 1994 (2020).OPLEDP0146-959210.1364/OL.385011.

Reducing feature size in femtosecond laser ablation of fused silica by exciton-seeded photoionization.

We demonstrate a method of laser ablation with reduced feature size by using a pair of ultrashort pulses that are partially overlapped in space. By tuning the delay between the two pulses, features within the overlapping area are obtained on the surface of fused silica. The observed dependence of the feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. This method could be used to enhance the resolution of laser-based lithography.

CREOL, the College of Optics and Photonics: a historical perspective.

To accompany Applied Optics' Institutional Focus Issue on CREOL, M. J. Soileau's guest editorial tells the story of how the College of Optics and Photonics (CREOL) at the University of Central Florida quickly grew from its inception to become one of the premier programs in optics and lasers.

Measurement of the optical damage threshold in fused quartz.

The damage thresholds of five different types of quartz glass used for the production of spectroscopic cuvettes for liquids were determined with single temporal and spatial mode nanosecond pulses at 532 nm. One of the glasses had a damage threshold of ≃420 J/cm(2), which was more than twice that of the other glasses.

Polarization dependence of absorption and laser induced surface damage thresholds in wire grid polarizers.

We report a theoretical and experimental study of the absorption of wire grid polarizers vs orientation and wavelength. Measurement of absorption and surface damage thresholds are in good agreement with the theory.

Self-protecting semiconductor optical limiters.

We present a detailed characterization of passive, picosecond optical-power-limiting devices using tightly focused beams in thick semiconductor samples. This study of limiting in ZnSe with 30-psec, 532-nm pulses shows that the resulting internal self-action (two-photon absorption plus free-carrier self-defocusing) protects the bulk material from optical damage. Simple scaling relations were determined from our results that link the limiting energy and the dynamic range to the focusing geometry and sample dimensions. These relations were used to design a monolithic optical limiter, optimized to have maximum dynamic range and minimum limiting energy. This device limits at an input energy of 10 nJ (300 W) and has a dynamic range greater than 10(4).

Effect of linear absorption on self-focusing.

We report the effect of linear absorption on self-focusing. Our experimental results show that power loss due to linear absorption before focus is responsible for the increase in the threshold power for self-focusing and that the critical power is independent of the linear absorption.

Time-resolved self-defocusing in Cd(0.23)Hg(0.77)Te and InSb.

We describe an experimental study of self-defocusing in Cd(0.23)Hg(0.77)Te and InSb at room temperature using a hybrid CO(2) laser at 10.6 microm. The time-resolved transmitted temporal pulse profiles showed the effects of refractive-index changes of up to Deltan ~ 0.1 caused by the electronically induced optical nonlinearity. Thermally induced contributions due to dn/dT ~ -1 x 10(-3) K(-1) in CdHgTe were also measured.

Self-defocusing in CdSe induced by charge carriers created by two-photon absorption.

We observe self-defocusing of picosecond, 1.06-microm pulses in CdSe. The effective nonlinear refraction can be 2 orders of magnitude larger than that of CS(2). We obtain good agreement with the theory presented here, which assumes that the self-refraction is caused by charge carriers created by two-photon absorption.

Self-focusing in CS(2) at 10.6 microm.

We have measured the nonlinear refractive index, n(2), of CS(2) at 10.6 microm by observing the onset of whole-beam selffocusing. We find that n(2) is (2.2 +/- 0.7) x 10(-10) esu, which is over an order of magnitude larger than n(2) in the visible.

Energy band-gap dependence of two-photon absorption.

We present measurements of the two-photon absorption coefficients beta(2) of 10 different semiconductors having band-gap energies between 1.4 and 3.7 eV. We find that beta(2) varies as E(g)(-3), as predicted by theory. In addition, the absolute values of beta(2) agree with theory, which includes the effect of nonparabolic bands, the average difference being less than 26%. This agreement permits confident predictions of two-photon absorption coefficients of other materials at other wavelengths.

Laser-induced damage measurements in CdTe and other II-VI materials.

Results of laser-induced damage measurements in CdTe and other selected II-VI materials are reported. These studies were conducted using pulsed 1.06-microm radiation from a Nd:YAG laser. The laser pulse width was varied from approximately 40 to 9000 psec (9 nsec). The laser-induced surface breakdown irradiance measured for CdTe over this pulse width range scaled as t(p)(-1/2)[t(p) is the laser pulse width (FWHM)]. This indicates that laser-induced damage in this material is due to linear absorption by a thin surface contamination layer.

Mechanisms of laser-induced failure in antireflection-coated LiNbO(3) crystals.

Substrate preparation before coating and coating cleanliness are the dominant factors affecting laser-induced damage to antireflection-coated LiNbO(3). Careful control of these factors resulted in an order of magnitude improvement in the damage threshold to 15-20 GW/cm(2) at 30 nsec across the entire surface.