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CARS spectroscopy and imaging

CARS Micro-Spectroscopy Kit

Coherent anti-Stokes Raman spectroscopy, also called Coherent anti- Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy used primarily in chemistry, physics and related fields.

Application notes


Coherent anti-Stokes Raman spectroscopy, also called Coherent anti- Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy used primarily in chemistry, physics and related fields. It is sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy, typically the nuclear vibrations of chemical bonds. Unlike Raman spectroscopy, CARS employs multiple photons to address the molecular vibrations, and produces a signal in which the emitted waves are coherent with one another. As a result, CARS is orders of magnitude stronger than spontaneous Raman emission. CARS is a third-order nonlinear optical process involving three laser beams: a pump beam of frequency pump, a Stokes beam of frequency Stokes and a probe beam at frequency probe. These beams interact with the sample and generate a coherent optical signal at the anti-Stokes frequency

CARS=pump-Stokes+probe.

The CARS signal CARS is resonantly enhanced when the difference between the pump pump and Stokes Stokes frequencies matches a vibrational transition vib of the molecule.



Minimally invasive technique
Non-photobleaching signal for live cell studies
Broadband tuning range 700 - 4000 cm-1
Higher sensitivity (stronger signals) than spontaneous Raman microscopy - faster, more efficient imaging for real-time analysis
CARS signal is at high frequency (lower wavelength) - minimal fluorescence interference
High spatial resolution
Integrated with Ekspla tunable laser PT259
System easy transformable to micro-spectrometer for fluorescence measurements based on two-photon excitation
E-CARS, F-CARS, P-CARS spectroscopy
Species selective microscopy
3D sample imaging
Dynamic live cell imaging
Long-term live cell processes monitoring
Non-destructive medical, biological research


Time-resolved UV/Vis/NIR spectroscopy

Nanosecond luminescence spectrometer

Nanosecond luminescence spectrometer is an excellent tool for laser spectroscopy and consists of nanosecond Nd:YAG laser and optical parametric oscillator (OPO) combined in a single optical parametric system, monochromator, data acquisition unit (DAQ), digitizer, optics for excitation beam steering and luminescence signal collecting, and optical signal detectors.

Measurement options
- Registration of luminescence spectra
- Registration of excitation spectra
- Measurements of luminescence time response parameters

Nanosecond luminescence spectrometer
is an excellent tool for laser spectroscopy and consists of nanosecond Nd:YAG laser and optical parametric oscillator (OPO) combined in a single optical parametric system, monochromator, data acquisition unit (DAQ), digitizer, optics for excitation beam steering and luminescence signal collecting, and optical signal detectors.

Tuning range of excitation wavelength 420 - 680 & 740-2300 nm (plus 210-420 nm with optional OPO second harmonic generator)
Luminescence wavelength 300 - 1700 nm
Excitation energy up to 20 mJ (at OPO tuning curve max)
Excitation pulse duration 3 - 6 ns
Registration system time response 10 ns - 10 ms
Excitation pulse repetition rate 10 Hz


Picosecond time-resolved spectrometer
Several options are available for picosecond excitation of the sample:
- lamp-pumped Nd:YLF laser harmonics (523 nm, 349 nm, 262 nm) or optical parametric generator (210-2300 nm) pumped by this laser, with pulse duration of 8 ps and pulse repetition rate of 10-20 Hz
- lamp-pumped Nd:YAG laser harmonics (532 nm, 355 nm, 266 nm) or optical parametric generator (210-2300 nm) pumped by this laser, with pulse duration of 20 ps and pulse repetition rate of 10-20 Hz
- diode-pumped Nd:YAG laser harmonics (532 nm, 355 nm, 266 nm), with pulse duration of 80 ps and pulse repetition rate of 1 kHz.
A streak camera or fast photomultiplier with digital oscilloscope can be used for registration of time-resolved picosecond luminescence.
Measurements of time-resolved absorption can be arranged in pump-probe scheme. Either output of optical parametric generator or laser-exited Xe luminescence can serve as a pump beam. Relaxation kinetics is obtained from a single pulse by streak camera or by scanning the delay between pump and probe pulses with motorized delay line.

1. Streak camera option This alternative enables single shot measurement of time- and spectrally- resolved spectra.
Temporal resolution 30 ps
Spectral resolution 0.1 nm
Spectral range 400 - 960 nm

2. Fast photomultiplier (PMT) option This alternative enables single shot measurement of a time-resolved trace at specific wavelength.
Temporal resolution 200 - 300 ps
Spectral resolution 0.1 nm
Spectral range 400 - 960 nm


SFG Vibrational Spectroscopy

SFG Spectrometer


Picosecond sum-frequency generation (SFG) spectrometer from EKSPLA is Ideal laser spectroscopy tool for in-situ investigation of surfaces and interfaces. SFG spectrometer operates from 4300 to 625 cm-1 and provides < 6 cm-1 spectral resolution. The heart of the system is a picosecond Nd:YAG laser generating 25 ps pulses that pump an OPG/DFG delivering from 260 to 20 J per pulse across the IR range

Basic operation principles of SFG spectroscopy In SFG experiment a pulsed tunable infrared IR (wVIR) laser beam is mixed with a visible VIS (wVIS) beam to produce an output at the sum frequency ( wSFG = wVIR + wVIS ).



SFG spectrometer includes:
Picosecond Nd:YAG laser
Optical parametric generator / amplifier / difference frequency generator (OPG/OPA/DFG)
Beams delivery optics
Monochromator
PMT or CCD signal detectors
Data acquisition system
Control software
Guiding beam for system alignment

Optional accessories:
Reference channel
Six axis sample holder (manual or three axes (X, Y and ) PC controlled)
Sample area purge box

Double resonance model
Both IR and VIS wavelengths are tunable in double resonance SFG option. This two-dimensional spectroscopy is more selective than single resonant SFG and applicable even to media with strong fluorescence. Double resonant SFG allows investigation of vibrational mode coupling to electron states at a surface. In double resonance SFG spectrometer model second OPG/OPA is used to generate tunable VIS beam in 210 - 680 nm range.

Software
All the spectrometer units (laser, optical parametric generator / amplifier / difference frequency generator (OPG/OPA/DFG), monochromators, photodetectors) are controlled from computer using LabVIEWTM environment. Spectrometer software features SFG spectra acquisition, azimuth scan, XY-mapping, surface dynamics investigation. OPG/OPA/DFG and monochromator calibration options are available.

SFG spectrometer options
Spectral resolution down to 2 cm-1 Using narrowband OPO/OPA/DFG with synchronously pumped parametric oscillator decreases SFG spectral resolution down to 2 cm-1.

Powerful and versatile toll for in-situ investigation of surfaces and interfaces:
Intrinsically surface specific
Selective to adsorbed species
Sensitive to submonolayer of molecules
Applicable to all interfaces accessible to light
Nondestructive
Capable of high spectral and spatial resolution

Features
High S/N ratio due to the superb laser source stability
Better than 6 cm-1 spectral resolution
Cost-effective picosecond system approach
Complete PC control
Acquisition of SFG spectra in wide wavelength range: 4300 - 625 cm-1
Monitoring of surface dynamics
Azimuth scan, XY-mapping of the sample (optional)
Models with 10, 20 or 50 Hz pulse repetition rate
Double resonance SFG model with tunable visible beam
Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes and other systems
Studies of surface structure, chemical composition and molecular orientation
Remote sensing in hostile environment
Investigation of surface reactions under real atmosphere, catalysis, surface dynamics
Studies of epitaxial growth, electrochemistry, material and environmental problems


SHG Spectrometer
Second harmonic generation (SHG) is an effective technique for surface probing. By SHG, monolayer adsorbtion can be detected. With different input/output beam polarizations the SHG can yield information on the average orientation of molecular adsorbates. Surface symmetry measurements can be performed rotating the sample around the surface normal. By using tunable lasers, SHG is applicable for surface-specific monolayer spectroscopy. SHG option could be ordered as an extention to Sum Frequency Generation (SFG) spectrometer.

SH excitation wavelength 1064 nm 532 nm
SH excitation pulse duration 20-30 ps
Resonant SH excitation wavelength 420-2300 nm


THz (Terahertz) spectroscopy

T-SPEC series real-time Terahertz Spectrometer






T-SPEC series real-time Terahertz Spectrometer offered by EKSPLA is a powerful tool for investigative applications of pulsed terahertz waves. With simple and robust design, it is easy-to-use and adaptable to individual requirements.





Fig.1. Example of THz-TDS experimental setup

Build Your own THz spectrometer As a totally flexible and cost effective solution, Ekspla offers the Terahertz Spectroscopy Kit. Four standard confgurations are available, optimized for transmission, reflection,imaging or pump-probe measurement. All can be easily interchanged and modified. Any other optional configuration can be ordered initially or as a future upgrade.

Basic THz Spectroscopy Kit includes:
- photoconductive antenna,
- THz emitter and detector,
- pump laser beam guiding optics,
- motorized slow delay line with controller,
- THz beam guiding mirrors,
- sample holder,
- lock-in amplifier,
- Labview based software for data acquisition.

Optionally:
- femtosecond laser,
- purging box, removes water absorption lines
- personal computer.

Wide spectral range up to 3.5 THz (116 cm-1)
Excellent spectral resolution better than 5 GHz (0.17 cm-1)
High power S/N ratio >106:1 at 0.4 THz
Real-time data acquisition up to 10 spectra/s
"No bearing" design of fast delay line - virtually unlimited lifetime
Transmission and reflection modes
High spatial resolution THz imaging
Complete PC control
User-friendly software for transmission or/and absorption measurements, imaging
THz transmission, reflection spectroscopy
Optical pump - THz probe spectroscopy
Chemical material characterization
Medical and biological nondestructive research
Semiconductor wafer inspection
Polymeric compounding
Explosives detection


THz emitter and detector for 800 nm wavelength lasers


Introduction
The terahertz (THz) and sub-THz frequency region (100 GHz - 10 THz) of the electromagnetic spectrum bridges the gap between the microwaves and infrared. The "THz gap" is attractive because of many possible applications of terahertz radiation: absorption or reflection spectroscopy, imaging of biological and other objects, THz tomography, ultrafast pump-probe spectroscopy.
Ekspla introduces the main building blocks for any THz system - THz emitter and detector. THz emitter and/or THz detector consists of a microstrip photoconductive antenna fabricated on low-temperature grown GaAs (LT-GaAs) substrate pumped by ultrafast laser with shorter than 150 fs pulse duration. THz radiation is collected and collimated by integrated Si lens, mounted on X-Y stage. Photoconductive antenna geometry, parameters of the Si lens, as well as the properties of LT-GaAs epitaxial layers are optimized for highest THz radiation output efficiency while preserving optimal bandwidth. As a result, typical emitted THz radiation power exceeds 10 W when pumped by mode-locked Ti:S laser with 100 mW output power and 150 fs pulse duration. FWHM bandwidth of detection system exceeds 700 GHz with usable spectral range of 0.1-3 THz.

THz imaging
THz radiation has an ability to penetrate deep into many organics materials, which makes THz imaging attractive for imaging of biological samples. Image of the sample can be obtained by raster-scanning of the sample trough the focused THz beam. Sub-millimeter resolution was reported in scientific literature.

THz Time Domain Spectroscopy



Fig. 1. THz time domain spectrosopy optical layout
The most typical application of THz emitter and detector is THz Time Domain Spectroscopy (THz-TDS). THz-TDS setup is shown in Fig. 1. Subpicosecond pulses of THz radiation are detected after propagation through a sample and an identical length of a free space. A comparison of the Fourier transforms of these pulse shapes gives the absorption spectra of the sample under investigation.

Pump-Probe THz Experiments
Femtosecond lasers let to investigate ultrafast nonequilibrium dynamics in semiconductors. For this aim, optical-pump-optical-probe techniques are usually employed. In such experiments, an intense optical pump pulse is used to excite free carriers in a sample, while a weaker probe beam monitors changes in its optical properties. In contrary to the optical probe, terahertz probe pulses are non-resonant with the band gap of semiconductor under investigation and, because of this, can be used as direct probes of free-carrier dynamics avoiding numerous experimental artefacts typical for optical-pump-optical-probe systems.

THz Spectroscopy Kit
Our "THz spectroscopy kit" contains all the components necessary to build THz-TDS system. The standard kit consists of photoconductive antenna THz emitter and detector, pump laser beam guiding optics, motorized delay line and bias power supply, THz beam guiding mirrors, sample holder and lock-in amplifier. All the components are assembled and tested on the baseplate of 6080 cm dimensions. The configuration of the kit can be easily modified, for example, sample holder can be mounted on motorized X-Y stage for imaging experiments.



Fig. 2. THz pulse waveform (a) and spectrum (b)
Typical examples of data collected are shown in Fig. 2. The THz pulse waveform and its Fourier spectrum were measured without sample inserted between emitter and detector in ambient air or argon atmosphere. The distance between emitter and detector is 30 cm.

- Photoconductive antenna THz emitter and detector
- Build-in hyper-spherical high-resistivity silicon lens
- 0.1-3 THz spectral range
- Sub-picosecond temporal resolution
- THz time domain spectroscopy
- THz imaging
- Optical pump-THz probe spectroscopy

THz Spectroscopy - THz Emitter & Detector for 1um LASERs




THz emitter from LT GaBiAs epitaxial layer has on its surface a coplanar Hertzian dipole type antenna structure with a width of 70 m, the width of the photosensitive gap is 20 m. GaBiAs layer is mesa-et ched in order to achieve high dark resistance and to simplify the laser beam alignment. High photosensitivity of the material allows to use for the excitation very low average power generated by, e.g., femtosecond fiberlasers. The efficiencyof the optical to THz power conversion reaching 0.0007 is larger than for other photoconductive THz devices.



Average THz power as a function on the bias on GaBiAs emitter measured by the Golay cell

THz Detector
THz detectors manufactured from newly developed GaBiAs epitaxial layers can be used for in TDS systems activated by 1060 nm and shorter wavelength laser pulses. High electron mobility (~5000 cm2/Vs) in the layer guaranties excellent sensitivity of the device; due to the shorter than 0.5 ps electron lifetime the detector is sensitive in the frequency range from 200 GHz to 5 THz. Detector is mounted together with 15 mm diameter hemispherical lens from high-resistivity silicon in an opto-mechanical holder with in-plane micro positioning capability and SMA connector.

Emitter parameters
Optical wavelength 1030 nm
Input optical power 5-25 mW
DC voltage < 40 V
Repetition rate < 100 MHz
Dark resistance > 30 M
Photoexcited gap of the chip 20 m

Detector parameters
Optical wavelength 1030 nm
Input optical power 5-25 mW
Detected THz bandwidth (at 70 fs laser pulse duration) 5 THz
Repetition rate < 100 MHz
Maximum of the THz spectrum at 800 GHz
Signal-to-noise ratio > 55 dB

Low temperature (LT) grown GaBiAs dipole structure
Optimized for lasers with 1030 nm and shorter wavelength (cost effective)
Wide spectral range 0.2-5 THz
Build-in hemispherical high-resistivity silicon lens
THz time domain spectroscopy
THz imaging
Optical pump - THz probe spectroscopy