arXiv subjects

Nathan Harper

Publications and source records attributed to Nathan Harper.

5 recordsLinked to original sources

arXiv2025

Generation of UV-Vis correlated photon pairs using PP-LaBGeO5

The SHG and SPDC output of a periodically poled bulk LBGO crystal is measured. The SHG efficiency is measured to be (3.52 +/- 0.04) x 10^-3 %/W. The SPDC output is temporally characterized by a Hanbury-Brown-Twiss interferometer; a maximum CAR of 244 and pair generation rate of (1.45 +/- 0.47) x 10^6 pairs s^-1 mW^-1 is measured. An EMICCD spectrometer is used to measure the spectral characteristics of the PP-LBGO source. Of the four gratings tested, one operational grating (poling period = 2.10 um) of the PP-LBGO device outputs a temperature tunable spectrum of entangled photon pairs centered at 532 nm with signal and idler photons being tunable from 507 to 556 nm, respectively. The work paves the way for future waveguided entangled photon sources reaching UV wavelengths using periodic poling.

physics.optics↗

arXiv2023

Experimental Upper Bounds for Resonance-Enhanced Entangled Two-Photon Absorption Cross Section of Indocyanine Green

Resonant intermediate states have been proposed to increase the efficiency of entangled two-photon absorption (ETPA). Although resonance-enhanced ETPA (r-ETPA) has been demonstrated in atomic systems using bright squeezed vacuum, it has not been studied in organic molecules. We investigate for the first time r-ETPA in an organic molecular dye, indocyanine green (ICG), when excited by broadband entangled photons in near-IR. Similar to many reported virtual state mediated ETPA (v-ETPA) measurements, no r-ETPA signals are measured, with an experimental upper bound for the cross section placed at $6 \times 10^{-23}$ cm$^2$/molecule. In addition, the classical resonance-enhanced two-photon absorption (r-TPA) cross section of ICG at 800 nm is measured for the first time to be $20(\pm13)$ GM, suggesting that having a resonant intermediate state does not significantly enhance two-photon processes in ICG. The spectrotemporally resolved emission signatures of ICG excited by entangled photons are also presented to support this conclusion.

quant-ph↗

arXiv2023

Entangled Photon Correlations Allow a Continuous-Wave Laser Diode to Measure Single Photon, Time-Resolved Fluorescence

Fluorescence lifetime experiments are a standard approach for measuring excited state dynamics and local environment effects. Here, we show that entangled photon pairs produced from a continuous-wave (CW) laser diode can replicate pulsed laser experiments without phase modulation. As a proof of principle, picosecond fluorescence lifetimes of indocyanine green are measured in multiple environments. The use of entangled photons has three unique advantages. First, low power CW laser diodes and entangled photon source design lead to straightforward on-chip integration for a direct path to distributable fluorescence lifetime measurements. Second, the entangled pair wavelength is easily tuned by temperature or electric field, allowing a single source to cover octave bandwidths. Third, femtosecond temporal resolutions can be reached without requiring major advances in source technology or external phase modulation. Entangled photons could therefore provide increased accessibility to time-resolved fluorescence while also opening new scientific avenues in photosensitive and inherently quantum systems.

quant-ph↗

arXiv2023

Tunable and efficient ultraviolet generation in nanophotonic lithium niobate

On-chip ultraviolet sources are of great interest for building compact and scalable atomic clocks, quantum computers, and spectrometers; however, few material platforms are suitable for integrated ultraviolet light generation. Of these materials, thin-film lithium niobate is the most competitive due to its ability to be quasi-phase-matched, optical confinement, and nonlinear properties. Here, we present efficient (197 $\pm$ 5 %/W/cm$^{2}$) second harmonic generation of UV-A light in a periodically poled lithium niobate nanophotonic waveguide. We achieve on-chip ultraviolet powers of 30 $\mu$W, demonstrating the potential for compact frequency-doubling of common near-IR laser diodes. By using a large cross section waveguide (600 nm film thickness), we achieve insensitivity to fabrication errors, and can attain first-order quasi-phase-matching with relatively long poling periods (>1.5 $\mu$m). The device also demonstrates linear wavelength tunability using temperature. By varying the poling period, we have achieved the shortest reported wavelength (355 nm) generated through frequency doubling in thin-film lithium niobate. Our results open up new avenues to realize ultraviolet on-chip sources and chip-scale photonics.

physics.optics↗

arXiv2022

Single Photon Scattering Can Account for the Discrepancies Between Entangled Two-Photon Measurement Techniques

Entangled photon pairs are predicted to linearize and increase the efficiency of two-photon absorption, allowing continuous wave laser diodes to drive ultrafast time-resolved spectroscopy and nonlinear processes. Despite a range of theoretical studies and experimental measurements, inconsistencies persist about the value of the entanglement enhanced interaction cross section. A spectrometer is constructed that can temporally and spectrally characterize the entangled photon state before, during, and after any potential two-photon excitation event. For the molecule Rhodamine 6G, which has a virtual state pathway, any entangled two-photon interaction is found to be equal to or lower than classical, single photon scattering events. This result can account for the discrepancies between the wide variety of entangled two-photon absorption cross sections reported from different measurement techniques. The reported instrumentation can unambiguously separate classical and entangled effects and therefore is of importance for the growing field of nonlinear and multiphoton entangled spectroscopy.

quant-ph↗