Frequency-Doubling of Femtosecond Pulses in “Thick” Nonlinear Crystals With Different Temporal and Spatial Walk-Off Parameters

We present a comparative study on frequency-doubling characteristics of femtosecond laser pulses in thick nonlinear crystals with different temporal and spatial walk-off parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at 1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of β-BaB2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance bandwidth for the SHG process, under different temporal and spatial walk-off conditions in the two crystalsPeer ReviewedPostprint (author's final draft

MENGGUNAKAN METODE DISKUSI UNTUK MENINGKATKAN KEMAMPUAN MENULIS PADA SISWA KELAS IV SD NEGERI NEUHEUN KABUPATEN ACEH BESAR

Banda Ace

Pan-cancer analysis of transcripts encoding novel open-reading frames (nORFs) and their potential biological functions.

Uncharacterized and unannotated open-reading frames, which we refer to as novel open reading frames (nORFs), may sometimes encode peptides that remain unexplored for novel therapeutic opportunities. To our knowledge, no systematic identification and characterization of transcripts encoding nORFs or their translation products in cancer, or in any other physiological process has been performed. We use our curated nORFs database (nORFs.org), together with RNA-Seq data from The Cancer Genome Atlas (TCGA) and Genotype-Expression (GTEx) consortiums, to identify transcripts containing nORFs that are expressed frequently in cancer or matched normal tissue across 22 cancer types. We show nORFs are subject to extensive dysregulation at the transcript level in cancer tissue and that a small subset of nORFs are associated with overall patient survival, suggesting that nORFs may have prognostic value. We also show that nORF products can form protein-like structures with post-translational modifications. Finally, we perform in silico screening for inhibitors against nORF-encoded proteins that are disrupted in stomach and esophageal cancer, showing that they can potentially be targeted by inhibitors. We hope this work will guide and motivate future studies that perform in-depth characterization of nORF functions in cancer and other diseases

Nonlinear interaction of structured optical beams

The general principles of Maxwell’s electromagnetic theory and quantum mechanics were well established much before the invention of lasers. However, after the first report of laser in 1960 and subsequent advancement in the field of laser technology, these theories have been revisited to understand the effects of higher order interactions between intense laser light beams and matter in terms of nonlinear susceptibilities. As such, the study of behavior of light in nonlinear media, in which the dielectric polarization responds nonlinearly to the electric field of the light, has given birth to a new branch of optics called nonlinear optics. Typically nonlinear optical effects are studied using laser beams with Gaussian intensity distribution. However, in recent times, structured coherent optical beams including optical vortices, hollow Gaussian beam, and Airy beam have found wide range of applications in variety of fields in science and technology. All existing techniques used to date to generate such beams suffer from different limitations including lower output power and restricted wavelength range. On the other hand, interactions of such beams with nonlinear media are mostly unexplored. During my PhD we have studied the nonlinear interaction of optical beams with different spatial structures. We have used second order nonlinear interactions such as second harmonic generation (SHG), where two photons from same laser get annihilated to produce a new photon of double energy, and sum frequency generation (SFG), where two photons of two different lasers get annihilated to produce a new photon of energy equal to the sum of the energies of the annihilated photons. The study also includes the nonlinear generation of structured beams such as Laguerre Gauss beams (optical vortices), a new class of vortex beam known as “perfect” vortex beams, hollow Gaussian beam, and Airy beam in different spectral and temporal domains. Most of the lasers (but not all) produce electromagnetic radiation in Gaussian intensity profile. However, due to unavailability of suitable laser gain medium, the nonlinear optical effects play pivotal role in generating coherent optical radiation with wavelength inaccessible to lasers. Using an ultrafast femtosecond laser at 1064 nm we have studied the second order nonlinear interactions such as SHG and SFG in different nonlinear crystals to produce ultrafast coherent radiation at 532 nm and 355 nm in Gaussian intensity profiles. Such beams have variety of applications, including spectroscopy, material processing, pumping of optical parametric oscillators and generation of structured optical beams. The efficiency of nonlinear optical processes varies proportional to the square of the length of the nonlinear crystal and the intensity of the laser beam. However, use of longer crystal length and increase of laser intensity through tight focusing do not necessarily increase the overall efficiency of the nonlinear process. There is always an optimum focusing condition for efficient nonlinear interactions, [1] have predicted such optimum condition for SHG of continuous wave (cw) or long-pulse lasers. However, the optimum focusing condition in the presence of temporal walk-off arising from the use of ultrafast lasers can be different from that of the cw and long-pulse lasers [2, 3]. We have also investigated the optimum focusing condition for single-pass SHG and SFG of ultra-short femtosecond pulses for generating the optical beams at 532 and 355 nm in Gaussian intensity distribution respectively. We have also done a comparative SHG performance study of the crystals having different temporal and spatial walk-off parameters. We have further investigated the effect of the ratio of confocal parameters (beam focusing condition) and power ratio of the interacting pump beams in SFG process. Knowing the effect of Gaussian beam in nonlinear frequency conversion processes, we have studied the interaction of orbital angular momentum (OAM) of the laser beams with nonlinear medium. Unlike Gaussian beams, optical vortex beams, spatially structured beams with helical wave-front [4], carry photons with OAM. These beams have doughnut shaped intensity profile with zero intensity at the point of phase singularity. Optical vortices are characterized by its topological charge (order), or winding number, l and are found to carry OAM of l} per photon. Using second order nonlinear crystals we have studied the frequency doubling characteristics of high-power, ultrafast, optical vortex beams by generating optical vortices of order up to 12 at 532 nm and 266 nm wavelengths. We have experimentally verified the OAM conservation law, the OAM of the generated photon is equal to the sum of the OAMs of the annihilated photons, in SHG process. We have also demonstrated a new scheme to generate optical vortices of orders l = 1 to 6 by using only two spiral phase plates (linear optical elements to generate optical vortices of a fixed order) of phase winding 1 and 2. We further observed that the efficiency of vortex SHG process decreases with the order of the vortex. We attributed such effect to the increase of the area of vortex beam with its order. However, it was not possible to overrule the contribution (if any) of OAM in the SHG process as the area and order of the vortex are not mutually independent parameters. The decrease of SHG efficiency of optical vortices with order restricts the study of nonlinear interaction of vortices to a certain order. Additionally, the dependence of beam area with its order does not provide clear information about the contribution of vortex order (OAM) in nonlinear frequency conversion process. However, a recent advancement in the field of structured beam has produced a new class of vortex beam, known as “perfect” vortex. These beams have area independent of the vortex order. Using such vortices we have experimentally verified that the vortex SHG efficiency does not depend upon the order (OAM) of the optical vortices. We have also studied the nonlinear frequency conversion of such beams to produce "perfect" vortex beam of order up to 12 with power as high as 1 W for all orders. We verified OAM conservation of "perfect" vortices in SHG process. Due to the OAM conservation in SHG process, the OAM of the frequency doubled vortex beam is twice that the pump beam. But what will happen to the output beam if the interacting photons in the nonlinear process have opposite OAMs? To study such effect we have studied the SFG (SHG is the special case of SFG process) process of two pump beams having equal vortex orders but opposite in sign (direction of the helical phase variation). As expected, due to OAM conservation law the output beam was found to have no OAM (l = 0). However, the output beams have no light (dark) region at the center of the beam similar to the vortex beam. This is a new class of structured beam known as hollow Gaussian beam (HGB) [5] and our method gives a new way of generating HGB through nonlinear processes. The increase of annular ring radius of these beams with the order of the input vortex beams signifies that HGBs also have certain orders. However, there is no experimental or theoretical means to determine the order of such beams. We have devised a new way to determine the order of hollow Gaussian beams. To broaden our study and to address other structured beams we have generated Airy beam and characterized its properties. Unlike other structured beams, Airy beam has peculiar properties such as beam shape invariance with propagation (non-divergence), propagation along curved trajectory in free space (self-acceleration), and self-restoration (selfhealing) of beam shape even after obstruction by small objects. Using intracavity cubic phase modulation of an ultrafast singly resonant optical parametric oscillator (SRO), we have generated ultrafast beam in 2- D Airy intensity distribution with wavelength tunability across the near-IR wavelength range. In addition to the Airy beam, the SRO produces Gaussian output beam in the near- to mid-IR wavelength range across 1.4 - 1.7 mm with power as much as 1.54 W.by N. Apurv ChaitanyanPh.D

Hollow Gaussian beam generation through nonlinear interaction of photons with orbital-angular-momentum

Hollow Gaussian beams (HGB) are a special class of doughnut shaped beams that do not carry orbital angular momentum (OAM). Such beams have a wide range of applications in many fields including atomic optics, bio-photonics, atmospheric science, and plasma physics. Till date, these beams have been generated using linear optical elements. Here, we show a new way of generating HGBs by three-wave mixing in a nonlinear crystal. Based on nonlinear interaction of photons having OAM and conservation of OAM in nonlinear processes, we experimentally generated ultrafast HGBs of order as high as 6 and power >180 mW at 355 nm. This generic concept can be extended to any wavelength, timescales (continuous-wave and ultrafast) and any orders. We show that the removal of azimuthal phase of vortices does not produce Gaussian beam. We also propose a new and only method to characterize the order of the HGBs.by N. Apurv Chaitanya, M. V. Jabir, J. Banerji, G. K. Samant

Ultrafast optical vortex beam generation in the ultraviolet

We report on the generation of ultrafast vortex beams in the deep ultraviolet (DUV) wavelength range at 266 nm, for the first time to our knowledge. Using a Yb-fiber-based green source in combination with two spiral phase plates of orders 1 and 2, we were able to generate picosecond Laguerre–Gaussian (LG) beams at 532 nm. Subsequently, these LG beams were frequency doubled by single-pass, second-harmonic generation in a 10 mm-long \u1d6fd-BaB2O4β-BaB2O4 crystal to generate ultrafast vortex beams at 266 nm with a vortex order as high as 12, providing up to 383 mW of DUV power at a single-pass, green-to-DUV conversion efficiency of 5.2%. The generated picosecond UV vortex beam has a spectral width of 1.02 nm with a passive power stability better than 1.2% rms over >1.5  h>1.5  h.Peer Reviewe

Ultrafast optical vortex beam generation in the ultraviolet

We report on the generation of ultrafast vortex beams in the deep ultraviolet (DUV) wavelength range at 266 nm, for the first time to our knowledge. Using a Yb-fiber-based green source in combination with two spiral phase plates of orders 1 and 2, we were able to generate picosecond Laguerre–Gaussian (LG) beams at 532 nm. Subsequently, these LG beams were frequency doubled by single-pass, second-harmonic generation in a 10 mm-long \u1d6fd-BaB2O4 crystal to generate ultrafast vortex beams at 266 nm with a vortex order as high as 12, providing up to 383 mW of DUV power at a single-pass, green-to-DUV conversion efficiency of 5.2%. The generated picosecond UV vortex beam has a spectral width of 1.02 nm with a passive power stability better than 1.2% rms over >1.5  h.by N. Apurv Chaitanya, S. Chaitanya Kumar, Kavita Devi, G. K. Samanta, and M. Ebrahim-Zade

All periodically-poled crystals based source of tunable, continuous-wave, single-frequency, ultraviolet radiation

We report on all periodically-poled crystals based singly-resonant optical parametric oscillator (SRO) generating continuous-wave, single-frequency, tunable UV radiation with maximum output power of 336mW in 18.5 MHz line-width at 399.2nm. It has tunability of 18nm.by A Aadhi et. al