Time Domain Thermoreflectance Measurements in the DAC

Contact: Alexander Goncharov
Location: Cyclotron Building

The recently built (with help of the Uni. of IL group, D. Cahill) TDTR system is designed for measurements of lattice thermal conductivity of materials, including in the DAC (Figure 5 on the left). The output of a Ti:Sapphire laser oscillator, which consists of ultrashort pulses (800 nm, <100 fs, ca 80 MHz repetition rate) is split into pump and probe beams. The pump beam is modulated at 10 MHz by an electro-optic modulator and passes through an optical delay line before being focused on the surface of the sample by a microscope.

The probe beam is mechanically chopped and focused by the same objective. Small changes in the reflected probe intensity are measured by an RF lock-in amplifier that is synchronized to the modulation frequency of the pump. The two output channels of the RF lock-in are measured by two computer-based audio frequency lock-in amplifiers that are synchronized to the optical chopper. The noise level of the measurement is <1 ppm/Hz 1/2 . This system has been recently used in several pilot measurements of metals and high-temperature superconducting materials to determine the pressure dependence of the electron-phonon coupling. It has been also tested to measure sound velocities (ps ultrasonics) of materials in the DAC using either optical detection of acoustical echo or time domain acousto-optical interference.