Doug Wake, Director of the MRL Laser Lab aligns a beam




Director
	Douglas Wake
   	390D MRL	
        333-8876
        Email:  wake@uimrl7.mrl.uiuc.edu

	Ray Strange (Laser Technician)
	Email:  strange@uimrl7.mrl.uiuc.edu


The MRL Laser Facility offers principal investigators access to time-resolved and nonlinear spectroscopic techniques without the large equipment expenditures and long lead times necessary for mastering complicated techniques. Users are taught what measurements are possible and trained to safely and effectively use state of the are ultrafast electronic/optical techniques and equipment. The Facility is equipped with a blend of commercial instruments and customized apparatus. There is a bias toward the commercial where this choice offers greater flexibility, ease of use, and reliability, in order to best serve the changing user population.

A mode-locked YLF laser is the principal source of driving picosecond experiments. The fundamental infrared pulses are internally doubled to 527 nm. The average power of this green-wavelength source is 2.5 watts comprised of 40 ps pulses at 76 MHz. The YLF laser can simultaneously pump two dye lasers that emit wavelength--tunable pulses from 560 nm to 900 nm. The dye laser pulse rate is variable from a full 76 MHz to 150 kHz by cavity dumping the outputs. The pulse widths are adjustable from 600 fs to 7ps. The laser pulses are used to create electronic excitations in materials under study. The dynamics of these excitations may be followed in time by probing the induced absorption with the second pulse (time delayed), by measuring the luminescence emitted after excitation, or by measuring the reflectivity or Raman spectra at appropriate time intervals. The lab is equipped to measure all of these characteristics.

The luminescence capability is obtained with the time-correlated photon counting technique using a microchannel plat-photomultiplier (MCP-PMT). Using a constant diffraction discriminator to detect the arrival of the luminescence-photon pulse, one records histograms of the arrival times to generate a time trace of the luminescence following excitation. A 0.5 m Spex spectrometer disperses the luminescence before the MCP-PMT to select an individual wavelength. By sequentially recording many wavelengths and inverting the data set, one obtains full luminescence spectra as a function of time with 40 ps time resolution.

Induced absorption, time-resolved Raman, and reflectivity require a second laser pulse to measure the sample properties as a function of time, but the distinct advantage is better time resolution, improved by an order of magnitude. These techniques are also complementary to photoluminescence, all yielding information from a particular point of view of the relaxation processes in the material under study. The induced absorption signal is commonly measured with a simple photodiode behind the sample with optical filtering to screen out the excitation pulse. Signal to noise is greatly improved by chopping the excitation beam and using a lock-in amplifier to measure the difference in the probe beam between excitation on and off. Time-resolved Raman spectra and dispersed by a Spex triple spectrometer and allowed to accumulate on a cooled CCD camera with the shutter held open for several minutes. The time resolution is determined by the time delay between the excited and Raman-probe pulses which is held constant for a single given exposure. The laser beams may also be focused on the sample to additionally obtain spatial resolution of the dynamical transport of excitations within the system under study with 3-micron spatial resolution. Picosecond imaging has been applied to the techniques of luminescence and induced absorption in the Laser facility in the study of GaAs quantum wells. Klein and Cooper conduct semiconductor experiments in the Laser Laboratory.

The picosecond lasers are destabilized by vibrations of only a few microns in amplitude. Consequently, the picosecond work is isolated from laboratory vibrations coupled through the floor by resting the experiments on three connected optical tables floating on a cushion of air. The majority of these experiments undertaken in the Facility makes use of two liquid He optical cryostats that control sample temperature from 2 to 300 K. One of these cryostats is configured for liquid He immersion and control of the sample temperature down to 1.5 K. The two cryostats can be used simultaneously for separate experiments.

At times a cw source is more appropriate to determine the characteristic of a sample and evaluate a candidate sample prior to an extensive time-resolved study. The Facility has a cw Ar ion laser and an adaptable 599 dye laser for this purpose. Mode-locking of this pair is also possible and has recently been extended to nearly 500 nm, but for time-resolved use it has been largely superseded by the YLF laser.

Non-linear optical experiments on new materials are carried out with the Facility's flash-lamp-pumped Q-switched Nd-YAG laser. Second and third harmonics of the 1064 nm fundamental are generated in materials under suitable conditions with high peak power (megawatt) nanosecond pulses. Apparatus has been developed that enables the temperature and high voltage applied to the sample to be varied in situ. The recent addition of a gas Raman shifter makes additional wavelengths available for these experiments. Since the harmonics are generated coincident with the driving fundamental pulse, the signal may be detected with a photomultipier tube signal displayed on an oscilloscope or, for low-level signals, acquired over many pulses by averaging with a box car synchronized to the YAG laser.

An excimer laser deposition system for the growth of high temperature superconductor films is also located in the Laser Facility. High power uv pulses are focused onto targets of high temperature compounds in two vacuum growth chambers. The evaporated material reforms on a substrate in the chamber as a high quality superconductor film, e.g., BKB or YBCO.

Other ancillary equipment includes: fast oscilloscopes, computer based data acquisition systems, power meters, calibration lamps, etc. Many of these are useful to other researches in the MRL and are loaned out on a short term basis as needed.

Research scientist Douglas Wake is charged with management and development in the Facility. He is aided by technician Ray Strange who also has responsibilities in the SQUID magnetometer and microfabrication facilities.