Michael P. Hasselbeck

Femtosecond infrared pulses tunable from 9 to 18??µm at an 88-MHz repetition rate

Femtosecond mid-infrared laser pulses that are continuously tunable in the wavelength range from 9 to 18mum are demonstrated. These nearly bandwidth-limited pulses are generated by phase-matched difference-frequency mixing within the broad spectrum of 20-fs pulses from a mode-locked Ti:sapphire laser in GaSe. A direct determination of the pulse duration at 11.5mum gives a value of 140 fs. The average mid-infrared power of 1muW is ~100 times greater than that for infrared generation by non-phase-matched optical rectification.

Advances in laser cooling of thulium-doped glass

Recent developments in cooling thulium-doped heavy-metal fluoride glass are presented. Thulium-doped fluorozirconate (ZBLANP) is cooled to 19 K below ambient with a multiple-pass pump scheme. This represents over an order of magnitude increase from the previously reported single-pass geometry. The results agree with a simple model for anti-Stokes fluorescence cooling that includes considerations of quantum efficiency and parasitic heating mechanisms. Issues relating to a practical optical refrigerator are examined, including a general model for the effects of multiple pump passes.

Efficient Terahertz Emission from InAs Nanowires

Abstract : We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is 15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection.We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

Effects of epitaxial lift-off on interface recombination and laser cooling in GaInP∕GaAs heterostructures

Photoluminescence of GaAs passivated with GaInP is studied over the temperature range 7–450K. Different photocarrier recombination mechanisms are identified as the temperature changes. An interface recombination velocity of less than 0.6cm∕s is measured at 300K. Lift-off processing inhibits but does not preclude laser cooling of GaAs.

Resonant cavity-enhanced absorption for optical refrigeration

A 20-fold increase over the single path optical absorption is demonstrated with a low loss medium placed in a resonant cavity. This is applied to laser cooling of ytterbium-doped fluorozirconate glass resulting in 90% absorption of the incident pump light. A coupled-cavity scheme to achieve active optical impedance matching is analyzed.

Differential luminescence thermometry in semiconductor laser cooling

We demonstrate a non-contact, spectroscopic technique to measure the temperature change of semiconductors with very high precision. A temperature resolution of less than 100 μK has been obtained with bulk GaAs. This scheme finds application in experiments to study laser cooling of solids. We measure a record external quantum efficiency of 99% for a GaAs device.

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

Cooling of Yb:YLF using cavity enhanced resonant absorption

Using a cavity resonant absorption scheme we demonstrate record laser cooling for the rare-earth doped crystalline solid Yb:YLF. A temperature drop of nearly 70 degrees is obtained with respect to the ambient. Our preliminary results indicate that minimum achievable temperature in this material/sample is 170 K, as measured using a modified differential luminescence thermometry technique. This indicates outstanding potential for Yb:YLF as a cryogenic laser cooler material.

Sensitive ultrashort pulse chirp measurement

The chirp of an ultrashort laser pulse can be extracted with high accuracy from a modified spectrum auto-interferometric correlation waveform by using a new time domain algorithm that allows signal averaging. We display results revealing high sensitivity to chirp even with signal-to-noise levels approaching unity. Correction algorithms have been developed to accommodate signal distortion arising from bandwidth limitations, interferometer misalignment, and nonquadratic detector response.

Emission of electromagnetic radiation by coherent vibrational wavesin stimulated Raman scattering

Electromagnetic radiation is emitted by the vibrational and collective modes of an opaque solid as the result of impulsive stimulated Raman scattering. Raman scattering of near-infrared femtosecond laser pulses produces coherent longitudinal optical phonon and plasmon oscillations in the semiconductor InSb. These oscillations radiate into free space at THz frequencies and are directly detected. The THz spectra exhibit features consistent with Raman selection rules including interference of allowed and forbidden Raman scattering.