Terahertz spectrometers Laser spectroscopy systems THz spectrometers

Basic Operation Principles

The terahertz (THz) and sub-THz One of the most common method THz-TDS TECHNOLOGY frequency region (100 GHz – 10 THz) of coherent THz generation and ADVANTAGES of the electromagnetic spectrum detection is approached in terahertz ▶▶ Broadband spectral coverage bridges the gap between the time-domain spectroscopy (THz-TDS) in THz range microwaves and infrared. THz waves setup. It consist of femtosecond ▶▶ High dynamic range penetrate dielectric materials like laser, optical delay line and two ▶▶ Complete amplitude and paper or plastic, are reflected by photoconductive antennas operating phase detection materials with free electrons like as THz emitter and detector. Sub- metals and are absorbed by molecules picosecond pulses of THz radiation are ▶▶ Picosecond time resolution with certain vibration levels within the detected after propagation through ▶▶ Optical pump-THz probe terahertz band. Terahertz absorption a sample and an identical length of experiment possibility or reflection spectroscopy, imaging a free space. A comparison of the of biological and other objects, THz Fourier transforms of these pulse tomography, and optical pump-THz shapes gives the absorption spectra of probe spectroscopy are all hot topics the sample under investigation. in recent scientific conferences with APPLICATIONS AREAS possible applications in semiconductor, ▶▶ Material characterization medical and security industries. ▶▶ Non-destructive testing

▶▶ Pharmaceutical industry

▶▶ Medical imaging

▶▶ Homeland security

▶▶ Semiconductor industry

Simplified scheme of THz time-domain spectroscopy setup

2.0 3.5 1.8 RDX absorption 3.0 PETN absorption 1.6

1.4 2.5

1.2 2.0

ance, cmˉ¹ 1.0

ance, cmˉ¹ 1.5 0.8

Absorb

0.6 Absorb 1.0 0.4 0.5 0.2

0 0 00.5 1.01.5 2.02.5 3.03.5 00.5 1.01.5 2.02.5 3.03.5 Frequency, THz Frequency, THz

Absorption spectra of explosives in terahertz range measured by Ekspla T-SPEC spectrometer

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Real-Time Terahertz Spectrometer T-SPEC series

FEATURES

▶▶ Wide spectral range up to 5 THz

▶▶ High dynamic range >90 dB @ 0.4 THz

▶▶ Real-time data acquisition up to 10 spectra/s

▶▶ Excellent spectral resolution up to 1 GHz

▶▶ “No bearing” design of fast delay line – virtually unlimited lifetime

▶▶ Transmission and reflection modes

▶▶ High spatial resolution Real-time Terahertz Spectrometer thus spectrometer obtains excellent THz imaging offered by Ekspla is a powerful tool spectral resolution up to 1 GHz. The ▶▶ Complete PC control for investigative applications of pulsed fast scan line is designed without ▶▶ User-friendly software terahertz waves. With simple and bearings and uses a magnetically robust design, it is easy-to-use and coupled drive which makes it adaptable to individual requirements. extremely reliable and significantly The unique design of microstrip extends the lifetime. photoconductive antenna fabricated T-SPEC spectrometer has housing APPLICATIONS on low-temperature grown GaAs with mounted gas inlets. It can be ▶▶ Chemical material substrate ensures broadband spectral used as purging box, when experiment characterization coverage and high dynamic range. requires special environmental ▶▶ Carrier lifetime and mobility The system is designed with two conditions, like nitrogen or dried in semiconductors delay lines: fast and slow. Fast scan air. The spacious sample area allows line allows real time data acquisition easy integration of additional ▶▶ Dielectric properties and with 10 spectra/s speed and 116 ps equipment, like cryostat or heater. complex refractive index time window. Average of collected On a special demand we can provide ▶▶ Metamaterials investigation

spectra can increase dynamic range the integration of such equipment, ▶▶ Medical and biological up to 90 dB at pulse maximum and ensuring good fit, spectrometer nondestructive research extend spectral range up to 5 THz. box sealing, vibration isolation and ▶▶ Thickness measurements Additional slow delay line allows operation automation. combination of multiple time windows;

1.2 -20

1.0 -40 0.8

0.6 -60

V 0.4

-80 Signal, 0.2

0.0 FFT Amplitude, dBm -100 -0.2 Noise floor -0.4 -120

0 20 40 60 80 100 120 0 1 23456 7 Delay, ps Frequency, THz Typical performance of T-SPEC series Real-time THz Spectrometer (measured in ambient atmosphere)

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Specifications Spectrometer is equipped with two Model T-SPEC 800 T-SPEC 1000 standard spectroscopy modules General specifications for transmission and reflection Spectral range >4.5 THz >3.5 THz configurations. Each module contains Dynamic range >90 dB @ 0.4 THz >70 dB @ 0.4 THz motorized sample manipulator. This Acquisition rate 10 scans/s allows measurements of multiple Spectral resolution: samples one by one, without physical fast scan 8.6 GHz access to the spectrometer. Reflection combined mode (fast + slow) ~1 GHz module has convenient vertical Scan range: architecture, where THz beams reaches fast scan 116 ps the sample from bottom and reflects combined mode (fast + slow) 928 ps backwards. The measured samples Beam diameter on the sample ~ 2 mm @ 0.4 THz can be replaced quickly just by laying Configurations Transmission / normal reflection them down on the sample holder. No Computer interface USB 2.0 adjustment is needed either when Dimensions 560 × 520 × 202 mm changing samples or when changing modules. PUMP LASER REQUIREMENTS Our T-SPEC series spectrometer Pump laser Integrated ELMO with Integrated FF50 second harmonic module is the perfect choice for broadband Laser output type Free space THz imaging. It allows scan of Wavelength 780 nm 1064 nm up to 25×25 mm sample with Pulse duration <100 fs <160 fs spatial resolution of approx. 1 mm. Output power >80 mW >60 mW Measurements contain information Pulse repetition rate 100 MHz 30 MHz about the target, revealing both structural and spectroscopic information.

THz image of tablet containing L-Tartaric acid and metallic part taken in transmission geometry. Tablet is almost transparent at 0.8 THz frequency and become visible close to absorption peak at 1.1 THz frequency

T-SPEC spectroscopy modules: reflection geometry (left) and transmission geometry (right)

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Fiber-coupled Terahertz Spectrometer T-FIBER series

FEATURES

▶▶ Pump pulse fiber delivery

▶▶ Real-time measurements

▶▶ Unlimited lifetime of delay line

▶▶ Flexible design

▶▶ THz imaging capability

▶▶ Complete PC control

▶▶ Excellent value for money

APPLICATIONS

▶▶ Time-resolved broadband THz spectroscopy

▶▶ Production processes monitoring

▶▶ Hazardous substances detection

▶▶ Paint and coatings layers Fiber-coupled Terahertz Spectrometer lifetime practically unlimited. Delay thickness measurements T-FIBER, offered by Ekspla, features line allows real time data acquisition ▶▶ Food and agricultural products flexible and robust design. It has with 10 spectra/s speed and 116 ps quality inspection integrated femtosecond fiber time window. laser with two fiber output ports. Fiber-coupled THz emitter and ▶▶ Medical imaging Comparing to common Ti:S oscillators, detector make switching between fiber lasers are smaller, cheaper, more experiment geometries easier than reliable and feature parameters that before. Due to its compact size and are perfect for terahertz generation. reliability during transportation, Femtosecond laser, delay line T-FIBER spectrometer is dedicated for and signal registration electronics laboratory use as well as for real field are integrated in a single compact applications. We are also happy to housing with footprint only 40×40 cm. customize it according to special OEM Minimal set of free space optics used customer requirements. in spectrometer allows stable long time operation. Special “no bearing” EMT-10-FC Sample DET-10-FC design of fast delay line makes its

Sample area schematic layout (basic version) EMT-10-FC DET-10-FC

Sample

EMT-10-FC DET-10-FC (a) Rotation stage (b) (c) Rotation stage EMT-10-FC DET-10-FC

Goniometer schematic layout: side view (a), top view in reflection geometry (b), top view in transmission geometry (c)

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Specifications Basic setup of T-FIBER spectrometer Model T-FIBER basic version T-FIBER with goniometer includes optical rail, with fiber-coupled General specifications THz emitter, detector, two PE lenses Spectral range >3 THz and sample holder. This setup provides Dynamic range >60 dB @ 0.4 THz >65 dB @ 0.4 THz transmission geometry and it is Acquisition rate 10 scans/s extremely simple in alignment. Spectral resolution 8.6 GHz As an option goniometer stage Scan range 116 ps can be supplied. This inexpensive Transmission / tunable angle Configurations Transmission module allows operation in multiple reflection / scattering most common geometries, including Incidence angles range – 18.5 – 90° transmission and tunable angle (in reflection mode) Detection angles reflection from approx. 18.5° up to – 37 – 286° (in scattering mode) 90°. It also can be used for unique Computer interface USB THz scattering experiments, because Main unit dimensions 400 × 400 × 158 mm sample and detector angles can be Spectroscopy setup footprint 670 × 70 mm 450 × 300 mm changed independently. This module gives also better focusability of THz PUMP LASER beam. Model LightWire FF50 Laser output type Fiber Wavelength 1064 nm Pulse duration <160 fs Output power >40 mW Pulse repetition rate 40 MHz

We are happy to tailor our products for your application

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THz Emitter and Detector for Free Space Coupling

THz emitter and THz detector consists FEATURES

of a microstrip photoconductive ▶▶ Based on LT-GaAs or antenna (PCA) fabricated on GaAs GaBiAs photoconductive material substrate. Depending on pump laser ▶▶ Optimized for wavelengths wavelength either low temperature around 800 nm or 1060 nm grown GaAs (LT-GaAs) or GaBiAs is ▶▶ Wide spectral range and low used as photoconductor. On its surface noise a coplanar Hertzian type dipole antenna structure is formed using ▶▶ Sub-picosecond temporal resolution AuGeNi metallization. Photoconductive antenna geometry, as well as the ▶▶ Technical passport and test properties of photoconductor epitaxial report included layers are optimized for highest THz radiation output efficiency, while preserving optimal bandwidth. As a APPLICATIONS result, typical emitted THz radiation power exceeds 10 µW, when pumped ▶▶ Time-resolved broadband THz spectroscopy by laser with 30 mW output power and <150 fs pulse duration. FWHM ▶▶ Optical pump – THz probe bandwidth of detection system spectroscopy exceeds 700 GHz with usable spectral ▶▶ THz imaging range of 0.1 – 5 THz.

THz emitter or detector is illuminated manufactured from high-density screws are used for Si lens positioning by laser beam from panel side. Laser silicon, mounted on X-Y stage. EKSPLA onto PCA center. SMA sockets on beams must be focused between two offers two standard types of these back side of the housing are used electrodes (Fig. 1). The gap between lenses: for collimated or diverging for connecting DC or AC bias to THz metallic contacts is similar to laser spot THz beam output. In second case emitter and lock-in amplifier input to diameter in detector case and larger – PCA is placed in aplanatic point of THz detector. Any of three M6 holes in emitter case. THz radiation silicon lens, which reduces THz beam can be used for THz emitter mounting is collected by integrated lens, spherical aberrations. Adjustment on optical table.

X-Y stage Spring element Adapter for Si lens 20 20 90° 13 4 6 50 50 Si lens 13

20 20 SMA socket 20 place 250 250 Glue 550 2500 550 Printed circuit 250 250 5000 5000 board (PCB) THz chip a) b)

Fig. 1. Microstip antena drawings: (a) emitter, (b) detector Fig. 2. THz emitter and detector crossection

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Micrometer adjustment screws 60

SMA socket

60 Si lens a) b) Microstrip photoconductor antenna

4 threads M6

Fig. 3a. THz emitter/ Fig. 3b. THz emitter/ detector housing detector housing

(model with collimated 27 (model with diverging THz output) c) THz output)

Specifications ¹⁾ Emitters Detectors 3) Model EMT-08 EMT-10 DET-08 DET-10

Photoconductive antenna Photoconductive material LT-GaAs GaBiAs LT-GaAs GaBiAs Dimensions of the wafer 5 × 1.5 mm Thickness 600 µm Antenna type strip line dipole Bias voltage 50 V max, 40 V typical ±12 V Central THz frequency ~0.5 THz 0.5 THz Detected THz bandwidth – up to 5 THz

Integrated focusing lens Material HRFZ-silicon Geometrical form hyper-hemi-sphere THz beam output collimated or diverging – X-Y adjustable stage range ±3 mm

Pump beam parameters Excitation wavelength 800±40 nm 1060±40 nm 800±40 nm 1060±40 nm Average power ¹⁾ <50 mW <20 mW <50 mW <20 mW Pulse duration <150 fs Pulse repetition rate 20-100 MHz Beam profile near to Gaussian Beam diameter ²⁾ ~2 mm

1) Subject to laser pulse duration, repetition rate and 2) Recommended value, if used with EKSPLA THz 3) Integrated preamplifier. beam size at the surface of wafer. emitter/detector mount of THz spectroscopy kit.

Ordering information Description Model Notes THz emitter for 800 nm wavelength EMT-8 Includes Si lens and coaxial cable with BNC connector THz detector for 800 nm wavelength DET-8 Includes Si lens and coaxial cable with BNC connector THz emitter for 1060 nm wavelength EMT-10 Includes Si lens and coaxial cable with BNC connector THz detector for 1060 nm wavelength EMT-10 Includes Si lens and coaxial cable with BNC connector THz emitter/detector mouting stage MNT Includes pump beam focusing lens on XYZ stage TMS-100M bias power supply TMS-100 30 – 70 V DC or square-wave output

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Fiber-coupled THz Emitter and Detector

FEATURES

▶▶ Based on unique GaBiAs photoconductive material

▶▶ Optimized for wavelengths around 1060 nm

▶▶ Technical passport and test report included

APPLICATIONS

▶▶ Time-resolved broadband THz spectroscopy

▶▶ Optical pump-THz probe spectroscopy

Fiber-coupled THz emitters and housing compatible with standard ▶▶ Suitable for all-in-fiber system detectors offered by EKSPLA are 1” optical holders. Performance of solution designed for broadband operation each device is checked and technical and can be used in standard time- passport, including testing report, is domain (THz-TDS) setup (Fig. 1). provided for customer.

EMT-10-FC DET-10-FC Integrated silicon lens Sample THz emitters and detectors are supplied with integrated hyper- hemispherical lenses, made from Delay line LightWire high-resistivity silicon, attached to PCA FF50 to increase the radiation efficiency of THz waves into free space. EKSPLA Fig. 1. THz time-domain spectroscopy setup offers two standard types of these lenses: for collimated or diverging Unique GaBiAs material used in THz beam output. Advantage of photoconductive antennas (PCA) collimated THz beam output is features excellent sensitivity for simple setup, because no additional wavelengths up to approx. 1060 nm optical components between THz and electron lifetime shorter than 1 ps. emitter and detector are required As a result ultra short half-cycle THz for experiment. However, this design pulses with broad spectra up to 4 THz features bigger spherical aberrations can be generated and detected. Pump of THz beam, which affects focusing. beam fiber delivery eliminates time- In second case design of lens assumes consuming adjustments and ensures positioning of the PCA in aplanatic maximum flexibility of experiment. As point, which significantly reduces an example, this feature allows fast spherical aberrations. As a result nearly and convenient switching between diffraction limited spot of THz beam different geometries: transmission, can be achieved. reflection, etc. THz emitters and detectors are mounted into compact

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Photoconductive antenna (PCA) Photoconductive antennas are particularly designed for THz emitter or THz detector. The substrate of 20 GaAs contains messa-etched epitaxial active layer 20 90° 13 4 of GaBiAs in order to achieve high dark resistance. 6 50 50

High photosensitivity of the material allows use of 13 20 20 low average power optical pulses for excitation. On 20 its surface a coplanar Hertzian type dipole antenna structure is formed using AuGeNi metallization (Fig. 2). 250 The gap between metallic contacts is similar to laser spot 250 550 2500 550 diameter in detector case and larger – in emitter case. 250 250 5000 5000 Photoconductive chip is mounted on PCB and placed inside metallic housing of device. SMA sockets on back a) b) side of the housing are used to connect DC or AC bias to Fig. 2. Microstip antena drawings: (a) emitter, (b) detector THz emitter or lock-in amplifier input to THz detector. (all dimensions are in micrometers)

Specifications Fiber delivery Model Emitter EMT-10-FC Detector DET-10-FC Standard configuration requires pre-chirped pulse with 0.5 – 2 ps Photoconductive antenna duration on fiber input. Typically such Photoconductive material GaBiAs pulse parameters are available directly Dimensions of the wafer 5 × 1.5 mm from fiber lasers before compressor Thickness 600 µm stage. In this case pulse is compressed Antenna type strip line dipole while propagating through optical Photosensitivity up to 1100 nm fiber with negative dispersion. Bias voltage 50 V max, 40 V typical – Another solution can be applied Detected THz bandwidth ¹⁾ – >4 THz for femtosecond pulses – near zero Integrated focusing lens dispersion optical fiber. In such fiber Material HRFZ-silicon femtosecond pulse keeps its form and Geometrical form hyper-hemi-sphere pulse duration doesn’t differ much on THz beam output collimated or diverging – input and output of fiber. In both cases Fiber delivery it works only for particular wavelength, Fiber length 3±0.2 m so it should be specified while Fiber connector FC/APC ordering. Laser radiation is delivered Wavelength ²⁾ 1064 nm through FC/PCA fiber connector. It is Maximum optical pulse energy focused onto PCA using lenses fixed 1 nJ (30 mW at 30 MHz) (on fiber input) inside the housing of THz emitter and EKSPLA LightWire FF50 Recommended pump source THz detector. (with double output option)

1) Pumped by 130 fs, 20 mW, 30 MHz pulses. 2) Other wavelengths are available on request.

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

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Terahertz Registration System TRS-16

Advanced electronics used in T-SPEC and T-FIBER terahertz spectrometers is also available as stand-alone product named TRS-16. It can be used for home-made opto-electronic or electro-optic pulsed THz spectroscopy systems. The complete set includes electronics module and fast scan delay line based on voice coil, with possibility to connect second slow delay line based on stepper motor. It also includes powerful software package used for system control and data acquisition. Similar version of dedicated software is also used in all Ekspla terahertz spectrometers.

ELECTRONICS MECHANICS The electronic module of TRS-16 system consists of two The fast scan delay line is designed without bearings main parts: delay lines control and signal registration. and uses a magnetically coupled drive which makes it Fast delay line based on voice coil allows real time data extremely reliable and significantly extends the lifetime. acquisition at 10 scans/s rate with 116 ps scanning window. A special mechanical design makes movement of Use of the optical linear encoder ensures the extremely low retroreflector straight along one axis, which results perfect jitter, thus registration system can achieve high dynamic beam pointing of the beam passing through the delay range up to 90 dB. The registration part has embedded line. The maximum line scanning rate is matched with its 16 bit analog to digital converter, operational preamplifier resonance frequency. This feature substantially reduces and the THz emitter bias modulator. TRS-16 system also energy consumption, vibrations and heat generation. provides bias for the photoconductive THz emitter in range from 10 to 95 V DC and for the operational preamplifier ±12 V DC. Operational preamplifier is also included into the kit.

Specifications Specifications

Analog to digital converter 16 bit Maximum scan window 116 ps Detector antenna Speed 40 dB up to 300 kHz 10 Hz preamplifier gain (at max scan window) Dimensions (L×W×H) 230 × 170 × 55 mm Dimensions (L×W×H) 214 × 75 × 120 mm

SOFTWARE The Terahertz Registration System TRS-16 comes with dedicated software capable to: ▶ register THz signal, get FFT spectrum, absorbance and transmittance in real time (10 scans/s); ▶ make a raster scan image using standard stepper motor XY stage; ▶ find the absorption, transmission, index of refraction and thickness of the sample; ▶ THz signal continuous data-flow recording.

Raster scanning with Spectrometer 2D software

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Accessories

THz emitter/detector mounting stage Bias power supply TMS-100 for THz emitter

Front view of TMS-100 power supply

Features ▶▶ Low noise ▶▶ DC or square-wave output (needs external TTL input) ▶▶ Cable for connection to THz emitter included

Specifications

Emitter (or detector) Square-wave modulation frequency 10 Hz – 100 kHz mounted on its stage Output voltage 30 – 70 V (adjustable) Max current 1 mA Output socket BNC Separately from THz emitter and detector EKSPLA provides Mains 100/220 V AC, 50/60 Hz convenient mounting stage compatible with both devices. Dimensions (W×L×H) 130 × 190 × 39 mm It contains lens, mounted on adjustable XY holder, compact stage for shifting lens along optical axis and rail, on which both: lens and emitter/detector are mounted. This helps with pump beam guiding to the gap between electrodes located on photoconductive antenna, as well as adjusting Operational preamplifier for THz detector beam diameter on active surface. Convenient fixing screws allow fast mounting and removing each part of this simple setup.

Specifications External view of preamplifier Beam height 105 mm X, Y axis travel range 3 mm Specifications Z axis travel range 13 mm Preamplifier 1) Lens diameter 25.4 mm Preamplifier type current-voltage converter Lens focal diameter 50 mm Conversion coefficient > 10⁶ AR/AR at selected wavelength Lens anti-reflection coating (standard: 800 / 1030 / 1064 nm) Preamplifier head dimensions 60 × 12 × 15 mm

Power supply Preamplifier power supply 155 × 65 × 80 mm dimensions Preamplifier power supply output + 15 V, - 15 V voltage Preamplifier power supply 220 V or 110 V line voltage

1) Preamplifier head is equipped with SMA connector matched with EKSPLA THz detector.

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THz Spectroscopy Kit

EKSPLA “THz spectroscopy kit” are assembled on the baseplate of Basic THz Spectroscopy contains all the components necessary 60×80 cm dimensions. Four standard kit includes:

to build THz-TDS system. The standard configurations are available, optimized ▶▶ THz emitter and detector kit consists of photoconductive for transmission, reflection, imaging or ▶▶ Pump laser beam guiding optics antenna THz emitter and detector, pump-probe measurement. All can be ▶▶ Motorized slow delay line with pump laser beam guiding optics, easily interchanged and modified. Any controller motorized delay line with controller, other optional configuration can be bias power supply TMS-100, THz ordered initially or as a future upgrade. ▶▶ THz beam guiding mirrors beam guiding optics, sample holder THz spectroscopy kit is controlled by ▶▶ Sample holder

and lock-in amplifier. All components dedicated software. ▶▶ Lock-in amplifier

▶▶ Labview based software for data acquisition M1 L2 L1 Pumping laser ~800 nm, <150 fs, >60 mW, 80 MHz Optional Items: ID 1 ▶▶ HWP1 Femtosecond laser P2 ▶▶ Delay lineHLR1 Purging box, removes water ID 2 M3 HWP2 ID 5 M5 absorption lines

▶▶ Motorized X,Y sample stage P1 ID 3 M2

Purging box

ID 6 ID 4 THz Sample THz L3 Emiter L5 X-Ystage L6 Detector L4 M6

M4

Optical layout of THz spectroscopy kit in transmission configuration

Our custom THz systems examples

LIDAR operating in terahertz range was Optical pump – terahertz probe spectroscopic used to scan remote objects at 7 m technique allows investigation of carrier distance. dynamics in semiconductors, nanocrystals, THz LIDAR installed in Lomonosov Moscow State and other materials. University, Russia Optical pump-THz probe system installed in The Institute of Physics, Chinese Academy of Sciences, Beijing, China

12 Savanoriu AV. 237, lt-02300 Vilnius, Lithuania tel +370 5 2649629 fax +370 5 2641809 e-mail [email protected] www.ekspla.com Why EKSPLA? ● Know-how in laser technology and nonlinear optics ● In house design and manufacturing ● Close partnership with scientific community ● Network of photonics companies in Vilnius ● ISO 9001 certified

Savanoriu Av. 237 LT-02300 Vilnius Lithuania Find local distributor at www.ekspla.com Ph.: +370 5 264 96 29 Fax: +370 5 264 18 09 [email protected] www.ekspla.com Revision number – 160322