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Features & Applications

FEATURES

Based on unique GaBiAs photoconductive material
Optimized for wavelengths around 1060 nm
Pump pulse fiber delivery
Technical passport and test report included

APPLICATIONS

Time-resolved broadband THz spectroscopy
Optical pump-THz probe spectroscopy
Suitable for all-in-fiber system solution

Description

Fiber-coupled THz emitters and detectors offered by EKSPLA are designed for broadband operation and can be used in standard timedomain (THz-TDS) setup (Fig. 1). Unique GaBiAs material used in photoconductive antennas (PCA) features excellent sensitivity for wavelengths up to approx. 1060 nm and electron lifetime shorter than 1 ps. As a result ultra short half-cycle THz pulses with broad spectra up to 5 THz can be generated and detected. Pump beam fiber delivery eliminates timeconsuming adjustments and ensures maximum flexibility of experiment.

As an example, this feature allows fast and convenient switching between different geometries: transmission, reflection, etc. THz emitters and detectors are mounted into compact housing compatible with standard 1” optical holders. Performance of each device is checked and technical passport, including testing report, is provided for customer.

INTEGRATED SILICON LENS

THz emitters and detectors are supplied with integrated hyperhemispherical lenses, made from high-resistivity silicon, attached to PCA to increase the radiation efficiency of THz waves into free space. EKSPLA offers two standard types of these lenses: for collimated or diverging THz beam output. Advantage of collimated THz beam output is simple setup, because no additional optical components between THz emitter and detector are required for experiment.

However, this design features bigger aberrations of THz beam, which affects focusing. In second case design of lens assumes positioning of the PCA in aplanatic point, which significantly reduces aberrations. As a result nearly diffraction limited spot of THz beam can be achieved.

PHOTOCONDUCTIVE ANTENNA (PCA )

Photoconductive antennas are particularly designed for THz emitter or THz detector. The substrate of GaAs contains messa-etched epitaxial active layer of GaBiAs in order to achieve high dark resistance. High photosensitivity of the material allows use of low average power optical pulses for excitation. On its surface a coplanar Hertzian type dipole antenna structure is formed using AuGeNi metallization (Fig. 2).

The gap between metallic contacts is similar to laser spot diameter in detector case and larger – in emitter case. Photoconductive chip is mounted on PCB and placed inside metallic housing of device. SMA sockets on back side of the housing are used to connect DC or AC bias to THz emitter or lock-in amplifier input to THz detector.

FIBER DELIVERY

Each fiber-coupled THz emitter and detector is delivered with specially design optical fiber. Standard configuration requires pre-chirped pulse with 0.5-2 ps duration on fiber input. Typically such pulse parameters are available directly from fiber lasers before compressor stage. In this case pulse is compressed while propagating through optical fiber with negative dispersion.

Another solution can be applied for femtosecond pulses – near zero dispersion optical fiber. In such fiber femtosecond pulse keeps its form and pulse duration doesn’t differ much on input and output of fiber. In both cases it works only for particular wavelength, so it should be specified while ordering. Laser radiation is delivered through FC/PCA fiber connector. It is focused onto PCA using lenses fixed inside the housing of THz emitter and THz detector.

Fig. 1. THz time-domain spectroscopy setup

Fig. 2. Microstip antena drawings: (a) emitter, (b) detector (all dimensions are in micrometers)

Specifications

Model	Emitter EMT-10-FC	Detector DET-10-FC
Photoconductive antenna
Photoconductive material	LT-GaBiAs
Dimensions of the wafer	5 x 1.5 mm
Thickness	600 µm
Antenna type	strip line	dipole
Photosensivity	up to 1100 nm
Bias voltage	50 V max, 40 V typical	-
Detected THz bandwidth ¹⁾	-	> 4 THz
Integrated focusing lens
Material	HRFZ-silicon
Geometrical form	hyper-hemi-sphere
THz beam output	collimated or diverging
Fiber delivery
Fiber length	3±0.2 m
Fiber connector	FC/APC
Wavelength ²⁾	1064 nm
Maximum optical pulse energy (on fiber input)	1 nJ (30 mW at 30 MHz)
Recommended pump source	EKSPLA LightWire FF50 (with double output option)

Pumped by 130 fs, 20 mW, 30 MHz pulses
Other wavelengths are available on request

ORDERING INFORMATION

Description	Model	Notes
Fiber-coupled THz emitter for 1060 nm wavelength	EMT-10-FC	Includes Si lens, optical fiber with FC/APC connector and coaxial cable with BNC connector
Fiber-coupled THz detector for 1060 nm wavelength	DET-10-FC	Includes Si lens, optical fiber with FC/APC connector and coaxial cable with BNC connector

T-FIBER Emitter & Detector