Terahertz Spectroscopy: Time-domain and
Ultra-fast Spectroscopy

Ingrid Wilke's terahertz (THz) and ultra-fast Spectroscopy lab is focused on biological and medical applications of time-domain THz methods; applications of time-domain THz methods in accelerator physics; single-shot femtosecond electron beam bunch length measurements; and dielectric and superconducting THz properties of transition metal oxide thin films.

In the last decade time-domain terahertz transmission spectroscopy (TDTTS) has become a powerful method for studying properties of various materials from dielectrics to semiconductors and superconductors. TDTTS operates with sub-picosecond pulses of electromagnetic radiation, which in the frequency domain implies the coverage of a very broad range spanning from tens of gigahertz to a few terahertz. Thus, explains Wilke, TTDTS bridges a large frequency gap between microwave and conventional infrared spectroscopy.

Technical Description:
A time-domain THz-transmission spectrometer is typically powered by a femtosecond Ti:sapphire laser and operates according to the pump-probe scheme. A schematic of the experimental arrangements is displayed in Fig. 1. The pump-probe principle is characterized by the splitting of the initial Ti:sapphire laser beam into two parts – the pump beam and the probe beam. The pump beam hits an emitter, which in response to the optical pulse releases a sub-picosecond pulse of THz-radiation. The probe beam gates the detector whose response is proportional to the amplitude and the sign of the electric field of the THz-pulse. By varying the delay between pump and probe pulses the whole time profile of the THz-pulse is traced. The complex transmittance of a sample is then placed in the focus of the THz-beam and given as the ratio of the Fourier transforms of a THz-pulse transmitted through the sample and a reference, for example a freely propagating THz-pulse.

Wilke's lab employs time-domain Thz-spectroscopy to investigate the electro-magnetic properties – electrical conductivity, dielectric properties – of thin films and bulk materials. A recent focus of the lab's research has been the investigation of superconducting thin films. TDTTS measurements of superconducting thin films are motivated by a basic understanding of quasi-particle excitations and pairing mechanisms as well as an assessment of the performance of high-temperature superconductors in passive electronic devices operating at microwave and THz-frequencies.

Fig. 1

Relativistic electron beam diagnostics
Time-domain THz methods are also a unique new method to measure the length and shape of single relativistic electron bunches in linear accelerators. Accelerators employed in next generation TeV linear electron-positron colliders for high energy physics, or used as drivers for new femtosecond X-ray free electron lasers (FELs), require dense relativistic electron bunches with bunch lengths shorter than a picosecond. Precise measurements of the electron bunch length and its longitudinal charge distribution are necessary to monitor the preservation of the beam quality while the electron bunch train travels through the beam pipe, as well as to tune and to operate a linear collider or a FEL (Fig.2).

Fig. 2. Experimental arrangements for electron bunch length measurements by electro-optic sampling with chirped optical pulses. The electron bunch length is measured by using an electro-optic crystal of ZnTe placed inside the vacuum pipe at the entrance of the undulator. The shaded parts indicate the vacuum housing of the electron beam.

Ultra-fast spectroscopy
A new area of research underway in Wilke's lab is the interaction of femtosecond optical laser pulses with biological cells, in particular the generation of pores in the cell membrane as well as the basic understanding of this process. Femtosecond optoporation offers great potential for targeted transfection of cells with high transfection efficiency, opening up possible applications in drug delivery and genetic engineering fields.

Contact Information:
Ingrid Wilke
Assistant Professor of Physics
Rensselaer Polytechnic Institute
110 8th Street
Troy, NY 12180-3590

(518) 276-6318