Microscopy Lab (pre 170905)

Lab contact: +81(0)98-966-8874
Location: Lab 2 A674
 

The Microscopy Lab setups relies on a MHz Ti:Sa oscillator system (Femtolasers Femtosource XL 650, 800 nm, 4 MHz, 45 fs, 650 nJ) which is used to generate a great variety of wavelengths in the far infrared (3 mm – 16 µm), the near infrared (800 nm, 1 – 1.4 µm) and UV (400 nm, 266 nm). This, combined with a high repetition rate, allows us to perform high signal-to-noise ratio pump-probe and THz Time-Domain Spectroscopy experiments giving us the ability to conduct a very large variety of projects and to access the carrier dynamics of materials, optoelectronic devices or chemical compounds. Below, we present a little bit about the history of the Microscopy Lab, some introductory tutorials about the techniques we use, and a few technical details about the lab’s capabilities.

 

HISTORY

The Microscopy Lab was the first lab established by the Femtosecond Spectroscopy Unit, and thus the first ultrafast lab at OIST. Built in a record time of three months from the day the Unit was established, we first setup shop in a temporary location in Lab 1 in January 2012 (See video 1 below), while Lab2 was mere scaffolding. It has since moved to its permanent location in Lab 2 in February 2014 (See video 2 below).

VIDEO 1

VIDEO 2

 

PUMP-PROBE EXPERIMENTAL TECHNIQUE

The pump-probe technique is one of the most common methods to obtain information on the ultrafast electron dynamics (sub-ps to ns) in a medium. The main idea is the following. The beam containing the pulses delivered by an ultrafast (mode-locked) laser is split in two:

(1) a pump beam with a high optical fluence (or power density) that is used to excite the sample generating photocarriers to a non-equilibrium state

(2)  a probe beam with a weak optical fluence and an adjustable time delay (controlled by a motorized delay stage) that overlaps the pump beam on the sample.

One measures the pump induced change in the transmission or reflection of the probe beam as a function of the time delay, i.e. as a function of the time delay between the arrival of the pump and probe pulses. This measurement contains information on the relaxation of the electrons in the medium. In practice, the pump beam is modulated (mechanical chopper, acousto-optic modulator, etc…) and the change in the probe intensity is measured with a photodetector which output voltage is demodulated by a lock-in amplifier synchronized with the pump modulation frequency. The most usual technique is the one color pump-probe (or degenerate pump-probe) for which pump and probe are at the same wavelength (ex: 800 nm – 800 nm). Different wavelengths can also be used for the pump and probe (two color pump-probe) in order to excite carriers and probe their state at a different energy (ex: 400 nm – 800 nm, 800 nm – THz, etc…). These wavelengths can be generated, e.g. via nonlinear crystals or optical parametric amplifiers/ oscillators.

In the microscopy lab, we use a MHz laser system which provides lower peak power (compared to a kHz amplifier system) but the high repetition rate allows faster modulation and averaging on the photodetector on an important number of pulses thus leading to high signal-to-noise pump-probe signals.

 

TERAHERTZ TIME-DOMAIN SPECTROSCOPY

THz Time-Domain Spectroscopy or THz-TDS is a technique that allows one to measure a complex THz electric field, i.e. amplitude and phase. It is now a widely used tool to investigate materials parameters (complex refractive index, conductivity, mobility and others) in the far infrared spectral range (1 THz = 4 meV = 300 µm) without the need of using Kramers-Kronig transformations. To achieve this, the beam from a mode-locked laser is split in two parts:

(1) a generation beam used to produce THz pulses via a photoconductive antenna or optical rectification in a nonlinear crystal, and

(2) a gating beam used to detect the THz electric field via a photoconductive antenna or Pockels effect in a nonlinear crystal.

  • Generation via photoconductive antennas

A photoconductive antenna consists, at its core, two biased electrodes deposited on a semiconductor (e.g. GaAs). By exciting the gap between the DC biased electrodes, the semiconductor becomes conductive on a very short time scale generating a current pulse that radiates a picosecond long quasi-monocycle electric-field transient in the far infrared.

  • THz optics for transmission measurements

To collect the generated THz radiation, a set of 90° off-axis parabolic mirrors are used. The studied sample is placed at the focal point after the second parabolic mirror. The transmitted THz pulses are then collected and focused on the detection system and overlaps with the gating beam.

  • Electro-optic sampling

In the presence of THz electric field, the electro-optic crystal (e.g. ZnTe) becomes birefringent inducing a change in the polarization of the gating beam (measured by an optical balance composed of quarter-wave plate, a Wollaston prism and photodiode balance). This change is directly proportional to the amplitude of the THz electric field (Pockels effect).  By moving a delay stage on the gating beam path, it is possible to sample the THz electric field amplitude at different time delays and then build the THz Time-Domain waveform. The spectrum is obtained via a Fourier transform.

 

Microscopy Lab THz setup

In the microscopy lab, we use interdigitated photoconductive antennas excited by sub-15fs NIR pulses (Femtolasers XS) in a reflective configuration and electro-optic detection to achieve 0.1 – 20 THz (3 mm – 16 µm) bandwidth. For more information, see P.J. Hale et al., Optics Express 22(21), 26358-26364 (2014).

 

THz temporal waveform and FFT spectrum

 

EQUIPMENT AND EXPERIMENTAL SETUPS

  • Oscillator system

Femtolasers Femtosource XL 650, central wavelength 800 nm, repetition 4 MHz, 45 fs pulse duration, 650 nJ.

Autocorrelation and spectrum

  • Short pulse module

Femtolasers XS based on a nonlinear fiber, sub-15 fs, 150 nJ

Autocorrelation and spectrum

  • Broadband THz Time-Domain Spectroscopy
  • Optical Pump-Terahertz Probe (OPTP)
  • MHz Optical Parametric Amplifier
  • UV generation
  • Micro Pump-Probe
  • 4K Pulsed Tube Cryostat with 4 optical access

 

​Page last updated on February 15 2015 (TH).