Experiments

3-1 Principle of Femtosecond Time-Resolved Measurement

Ultrashort pulse laser has become a useful tool to investigate ultrafast carrier dynamics. The pulse-width of the laser can achieve femtoseconds (10^-15 sec) and it is short enough to measure scattering/relaxation processes in materials.

When a short pulse is incident into the surface of a semiconductor, electrons will transit from valence band to conduction band then relax back to valence band through various relaxation regimes and processes, e.g., carrier-carrier or carrier-phonon scattering as described in Chapter 2. The process of relaxation changes dielectric constant (ε) in the material, and then affects its absorption/reflection rate. From the Fermi’s golden rule, the change of reflection rate is proportional to the number of carriers. The carrier density is estimated using the simple relation: n

n

with the change of refractive index from the Drude model, where

m o

nw Ne

n=− ²/2 ^{2 *}ε

∆ , n is the refractive index of sample, N is carrier density, e is elementary charge, m^* is the effective mass of the conducting electrons, andε^o is the permittivity of free space. Through a weaker (probe) pulse laser detecting the reflection/transmission continuously, we can understand the phenomenon of carrier dynamics.

The most common technique is pump-probe reflection/transmission measurement.

The spot size of the probe beam on the sample is smaller than that of the pump to ensure measuring a uniform photoexcited region. We control the optical path length of pump beam to reach the goal of temporal analysis.

the dotted line represents the temporal change of reflection/transmission rate of the sample after absorbing the pump pulses, and then the probe pulses incidence onto the sample in a delay time t to probe the signal of reflection/transmission. The third part of the figure is the voltage signal transferred from a detector modulated with a chopper. Io(t) is the DC value which represents the intensity of reflection rate R;

∆I(t) is the AC value which represents the change of reflection ∆R. If dividing ∆I(t) by Io(t), we can get a dimensionless signal of ΔR/R. Continuously controlling the time delay t, we will fully measure the temporal change of ∆R/R.

In addition, if the relaxation time of carriers is longer than the time between two laser pulses, we will not be able to observe the complete relaxation. For example, the repetition rate of the laser is 75 MHz, there are 13.3 nanoseconds (ns) between two laser pulses. Suppose the relaxation time of the sample is longer than 13.3 ns, then the sample will absorb the energy of another pulse before it relaxes. The sample we used has no such a situation.

t (delay)

Probe pulses

N (t)

time

time

time 13 ns

open

I₀ (t)

Optical chopper

@ 3000 Hz at pump pulses

∆^{I (t)}

detecter & lock-in amplifier

@ 3000 Hz

∆

Reflective probe pulses

∆

I(t) / I₀(t) = R(t) / R₀(t)

Fig. 3-1 The illustration for the principle of pump-probe technique.

3-2 Experimental System of Femtosecond

Time -Re solved Measurement

3-2.1 Ultrafast Laser System

A commercial Kerr-lens-mode-locked Ti:Sappire laser (Coherent Mira900) is used as the light source which is pumped by a diode-pumped frequency-doubled Nd:YVO4

laser (Coherent Verdi-8) with 527 nm with maximum power of about 8W. The structure of Ti:Sappire laser is showed in Figure 3-2. The output wavelength is tunable from 700 nm to 900 nm using a prism sequence and a slit inside the laser cavity with a repetition rate of 75 MHz. Power of the output laser beam is less than 1W.

Fig. 3-2 The optical beam path within Coherent Mira-900.

3-2.2 Autocorrelator

An autocorrelation system is for measuring the pulse-width of an ultrafast laser. A schematic of intensity autocorrelator is showed in Figure 3-3. A beamsplitter separates the laser beam into two beams, and one of them is temporally delayed by a translation stage. We put a beta barium borate (BBO) nonlinear optical crystal at the overlapped focal point of these two beams. The autocorrelation signal comes from second harmonic generation (SHG) of BBO is detected by a silicon detector. The detector receives the frequency-doubled intensity of two laser beams which is determined by autocorrelation function:

dt t

I t I

S(

τ

τ

where τ is the time delay, and I(t) is the instantaneous intensity.

Figure 3-4 shows the intensity of measurement [31]. The pulse-width is 230fs obtained by Guassian fitting.

Fig. 3-3 Sketch diagram of the intensity autocorrelator.

-800 -600 -400 -200 0 200 400 600 800 12

14 16 18 20 22

Autocorrelation signal (a.u.)

Delay time (fs) 230 fs

Fig. 3-4 The autocorrelation trace.

3-2.3 Experimental Setup of Pump-probe Measurement

The schematic diagram of pump-probe setup performed at room temperature is shown in Figure 3-5. The mode-locked Ti:Sappire laser described in Section 3-2.1 is the light source. After a variable neutral density filter (ND1) which is for attenuating the power of the laser beam, a 50/50 beamsplitter (BS) splits the beam into pump and probe beams. Another variable neutral density filter (ND2) is used to attenuate the power of probe beam and keep the pump probe ratio 160:1.

A chopper with 25 KHz is used to control pump beam and modulate its frequency for Lock-in Amplifier receiving the signal. A time delay stage with 1.25 µm resolution is applied to change the optical path length of pump beam, resulting in a relative time delay between pump and probe pulses. A half-wave (λ/2) plate rotating the polarization of probe beam by 90^o is for avoiding the optical coherent artifacts.

Pump and probe beams meet each other on the sample. Each of them focuses separately by two convex lenses with focal lengths of 10 and 5 cm. The spot size of focused pump beam is about 300 µm. A CCD camera is adopted to ensure the spatial overlap of pump and probe beams. A silicon photodiode detector (HAMAMATSU C5460) is for receiving the reflected light of probe beam and passing it to the lock-in amplifier (Stanford Research System, Model SR830). The final data as a function of temporal delay of pump and probe beams is then showed on a PC monitor.

λ

Fig. 3-5 Femtosecond time-resolved photoreflectance measurement system.

3-3 Zero Delay Point of the System

To find the zero delay point of pump and probe beams is a very important step in this experiment. We focused the two beams respectively on a BBO crystal, and then observed frequency-doubled light after the overlapping point. The position of time delay stage where BBO has the strongest intensity is defined to be zero delay point.

Type-Ι phase matching condition of SHG is given by ∆k =k(ω₂)−2k(ω₁)=0,

ordinary refractive index, and ne is the extra-ordinary refractive index.

3-4 InGaAsN Single Quantum Well Structures

The samples we used in our measurement are InGaAs(N)/GaAs single quantum wells which were grown by Tansu et al.. Several dilute-nitride In0.4Ga0.6As single quantum wells on GaAs substrate were developed by low-pressure metal-organic chemical vapor deposition (LP-MOCVD) method during the past few years.

Trimethylgallium, trimenthylaluminium, and trimethylindium are used as group-Ⅲ sources. AsH3 and PH3 are used as group-Ⅴ precursor and U-dimethylhydrazine is N precursor. The doping sources are SiH4 and dielthylzine for n and p dopants, respectively. The active layers are 60Å In0.4Ga0.6As1-xNx QW, sandwiched with various barrier materials: 3000 Å in GaAs cofining layers, 75 Å in GaAs0.85P0.15

tensile barriers, and 30 Å in GaAs0.67P0.33 buffer layers. In and N contents of the InGaAsN material were determined from high-resolution x-ray diffraction and secondary mass ion spectroscopy. The N composition x in the structures are 0% and 2%, respectively. Figure 3-6 is the schematic diagram of the samples. The growing details of InGaAs(N) can be consulted in Refs. [26][27].

Fig. 3-6 Cross-sectional schematic conduction band diagram of the In0.4Ga0.6As1-xNx

SQW (Ref. [26]).

Figure 3-7 is the high resolution transmission electron microscopy (HRTEM) image of the interface quality of In0.4Ga0.6As0.98N0.02 SQW [28]. The rough interface seems to have dark island regions which is about 2-3 nm.

Fig. 3-7 HRTEM image of In0.4Ga0.6As0.995N0.005/GaAs SQW.

3-5 Method and Steps of Pump-probe Measurement

After setting up the experimental system, the steps of the experiment are as follows:

1. Turn on the laser: Turn the laser on and tune the wavelength to 820/880 nm

then adjust it to mode-locking. Use the spectrometer to observe the spectrum of laser beam and ensure the stability of it.

2. Stick the sample on: Obstruct the laser beam before sticking on the sample to prevent reflected light will be incident back into the laser cavity. Then stick the sample on a three-dimensional stage.

3. Measure the spectrum at room temperature: Ensure the overlapping of pump and probe beams hourly during the measurement. Control the polarization of pump and probe beams perpendicular to each other to avoid interference.

Measure ΔR under different pumping powers.